An Advanced Organic Technology to Produce Chromium-Free Goat Leather: Insights and Methodology to Use Caesalpinia spinosa as a Tanning Agent

Abstract

Chromium-based technique has been used over the decades and, up to now, is the most extended technology to produce leather around the world. However, the environmental problems related to its production, with special regard to the extremely high amounts of residual chromium in the soil and in the water, are reducing the use of leather, and it is becoming replaced with the use of synthetic materials. Consequently, the current leather production is becoming unsustainable due to the higher cost of treating the effluents derived from it. To eliminate the high consumption of this mineral, a Caesalpinia spinosa (Tara) tanning technique was developed and optimized on a semi-industrial scale. The reported results indicated that, being highly superior to the traditional methods, this was possible after an extensive Tara characterization, which reported, for example, (a) IRR reported strong vibration peaks in different positions, which suggested the presence of polyphenols as a principal component of the Tara powder. This assumption was confirmed by the tannic acid content, which reported values equal to 52.25 ± 1.74 mg/g of Tara powder. These results suggested that the Tara serves as a good tannin agent. This was proven by the mechanical characteristics of the leather produced by using Tara. For example, when 10% of Tara powder was applied to the formulation, the best results were obtained in all the mechanical characteristics of the leather. Thus, for the tensile strength, the average value was equal to 340.77 ± 4.78 N/mm². Moreover, the sustainability of the process was demonstrated by the good wastewater quality, which showed a null concentration of chromium hexavalent and total chromium. As expected, the biochemical oxygen demand (BOD) and the chemical oxygen demand (COD) increased considerably, reaching values of 1461.00 mgO₂/L and 3150.30 mgO₂/L, respectively. Finally, the economic feasibility of the project was obtained through the cost/benefit and Internal Return Rate (IRR) analysis. The results reported a cost/benefit ratio equal to USD 1.03 and an IRR equal to 31%, thus the investment can be recovered after 5 years of execution of the project. This novel zero-chrome tanning process solves the long-standing environmental problems that have been linked to making leather and meets all the needs of today’s sustainable leather industry.

1. Introduction

Leather is a key insight into human evolution and societal development because its use has allowed humanity to survive since the appearance of humans in the world. In this sense, leather was one of the first materials produced by civilization to protect themselves against the extreme weather, the insects, and other risks that can appear in the natural world. Moreover, the leather industry has been constantly growing in demand through the centuries, and nowadays, leather and leather products are among the most traded products worldwide that are based on a sustainable and easily accessible resource [1]. International trade is predicted to surpass USD 80 billion yearly, with expectations that new and growing nations will increasingly join as members due to population growth and urbanization [2].

Furthermore, the leather sector holds a significant socio-economic position, comprising approximately 36,000 firms, generating a revenue of EUR 48 billion [2], and producing over 558,000 tons of leather from bovine hides globally [3], so it is notable the advantages that the leather industry produces, especially in emerging economies, because it creates a way to create new jobs, especially impacting the economy of families [4].

The manufacturing of leather is a multifaceted industrial process. The entire method entails a series of intricate chemical reactions and mechanical processes. The execution of multiple pre- and post-treatment procedures yields a final product characterized by distinct properties: stability, aesthetics, water resistance, thermal resistance, elasticity, and permeability to perspiration and air [5].

The tanning procedure conventionally utilizes chromium salts as tanning agents. Approximately 85% of all leather is manufactured using chrome-based processing techniques [6]. In chromium tanning, the pelt treated with 6 to 8 wt% chromium sulfates, via a covalent interaction with the ionized carboxyl groups of the skin, is termed wet-blue [7,8].

Ultimately, to enhance color, texture, brightness, and other physical attributes, the wet-blue or wet-white undergoes post-tanning. The tanned hide undergoes retanning with supplementary tanning agents, including vegetable extracts, synthetic tannins, and resins. Subsequently, it is colored and treated with fatty liquors to enhance tactile softness and tear resistance [7,9].

The primary issue concerning leather production is the significant potential impact of tanning and related activities on air, surface and groundwater, soil, and other natural resources, stemming from the chemicals utilized, the raw materials employed, and the effluents, wastes, and off-gases produced during the process.

Conventional pre-tanning and tanning operations are estimated to provide around 90% of the total pollution generated by a tannery [10]. The pre-tanning process leads to fluctuations in pH and an elevation in chemical oxygen demand (COD), total dissolved solids (TDS), chlorides, and sulfates in tannery effluents [11].

The traditional dehairing method with sodium sulfide and lime contributes to 84% of biochemical oxygen demand (BOD), 75% of chemical oxygen demand (COD), and 92% of suspended solids (SS) from a tannery [12]. Consequently, measures for pollution control, waste creation and disposal, chemical safety, accidents, and the use of raw materials, water, and energy are imperative [13].

In this context, the most challenging operation is tanning, as the typical chrome tanning method successfully utilizes only 60–80% of the available chrome, leaving all sodium chloride residuals in the float. The chrome concentration in spent float ranges from 2500 to 3000 mgCr/L [14], whereas the Cl concentration is a minimum of 20,000 mg/L [15].

Waste chromium and chloride from leather production present a substantial disposal challenge. Global statutory restrictions have been established for the discharge and disposal of chromium. The permissible limit for total chrome discharge in effluent is established at 1.5 mg/L for sewage systems, while the limit for chloride ions is set at 3000 mg/L for discharge into water bodies. Therefore, optimizing chrome tanning processes to enhance chrome absorption while reducing residual chrome and chloride levels in floats, or substituting chrome with a more biodegradable alternative, is a significant priority for all tanners [10].

Consequently, the alteration in the production approach is crucial for generating more environmentally sustainable leather. This initiative considers the utilization of a plant-based substance as a substitute for chromium. The vegetable tanning process typically necessitates three weeks for the dye to permeate the hide. Additionally, the hides are immersed in sodium bicarbonate or sulfuric acid solutions for bleaching and the extraction of surface-bound tannins. Prior to drying, lignosulfate, maize sugar, and oils can be included into the leather, followed by additional finishing processes [16].

Therefore, the main disadvantages with the current vegetable tanning techniques are the time-consuming operation and the poor properties of the leather, which cannot be compared for the moment with the properties of the chromium leather, so it is necessary to search for ways of production that are able to solve the cited problems.

Several plants are reported in the literature as being used as tanning agents to replace chromium. Sumac, chestnut, myrobalan, valonia, mimosa [17], and Tara are hydrolyzable tannins commonly utilized in commercial applications for their tanning efficacy, quality, performance, and extraction efficiency. For example, the sumac leaves reported a polyphenol content between 22 and 35, and Chestnut (Castanea sativa) wood reported a polyphenol content ranging from 4 to 10, while the Tara pods reported the highest content of polyphenols, which varied from 30 to 35% [18,19].

Thus, if we recall the tanning process, it consisted of the crosslinking and the attaching of the polyphenols to the COOH or -NH₂ groups of the leather protein through hydrogen and covalent bonds. Therefore, the higher the polyphenol content, the more the skin undergoes the tanning process. For example, [20] used Anogessus leiocarpus leaves to produce leather in combination with chromium. They reported that the plant extract had a polyphenol content of 20%, making it viable for testing in leather production; however, because of its polyphenol content, the extract needs to be combined with chromium to enhance the leather’s characteristics. So, they produced a normal tanning process but they added 22% of plant extract during the tanning process. The final characteristics of the leather were as follows: (a) tensile strength equal to 10 ± 0.35 N/mm²; (b) percentage of elongation equal to 40 ± 0.35%, and (c) water uptake during water vapor permeability equal to 70.0 ± 0.05 mg/cm²/h. Therefore, the leather had a good quality when Anogeissus leiocarpus was used in combination with chromium. Also, the authors [21] prepared a vegetal tanning process without the addition of chromium by using Acacia xanthophloea as a tannin agent. The principal outcomes of the research were as follows: (a) the average tannin content of the Acacia xanthophloea was equal to 23.80%; and (b) the tensile strength and the percentage of elongation were equal to 29.22 ± 4.40 N/mm² and 42.40 ± 8.89%, respectively. Therefore, the authors considered that the process can be scaled up to industrial uses [21].

On the other hand, the tannins from Tara are well known in the leather industry and they are appreciated because of their light color and lightfastness. For this reason, demand for Tara tannins increased during the last decades, at the time leather production for automobile upholstery experienced its highest demand. The commercially available Tara tannin agents conferred a high resistance of the leathers, which is higher than those obtained with other vegetable tannins [22]. But still, in the literature, the Tara is used in combination with other metal salts or chromium, which reduces the sustainability of the process, and it is important to search for technologies in which it can take advantage of the properties of the Tara to produce a green, chromium-free leather.

In this regard, the present research proposes the use of Caesalpinia spinosa (Tara), a native plant from South America, as a green tanning technology. The research focuses on creating an effective and beneficial method for tanning using tannic and gallic extracts from Caesalpinia spinosa. Since it is an organic tanning agent, its impact on the environment is not aggressive, and in place of using chromium as a tanning agent, it significantly reduces the pollution generated by this industry, which is considered one of the most harmful production practices for the environment and the health of people exposed to the waste and effluents generated.

The goal is to create a clear method for using tannin and gallic extracts from Caesalpinia spinosa in tanning. This method will control the process, reduce any negative effects, and focus on achieving high productivity, low costs, good profits, minimal environmental harm, and consistent quality of the final product. This novel technique was executed because the Ecuadorian governmental organizations are seeking a viable use of the Tara which is cultivated in the province. So, this research and the execution of the following projects are going to considerably impact the population in Chimborazo, creating opportunities to use the Tara, which no longer exists.

2. Materials and Methods

2.1. Materials and Reagents

The materials and reagents used in the production of the leather are mentioned in Appendix A. Moreover, the machines used to produce chromium-free leather (drums for pre-tanning, tanning and post tanning operations, fleshing machine, router machine, abattoir, Sammying machine, splitting machine, mechanical plate, and toggling) were designed and constructed by students of the ESPOCH and these machines are installed in the specialized leather laboratory of this university.

2.2. Preparation of the Tara Powder

Tara powder production started with the selection of the pod. This is carried out by choosing the pods that have the proper color. To produce the powder used in the tanning process, it is necessary to select the brown pods. The green or yellow pods are discarded, and they are sent back to the soil to be used as fertilizer. After, the pods were manually cleaned with water to avoid the appearance of impurities. Subsequently, the pods were manually cracked to obtain the seed which is going to be used to produce the powder. Afterwards, the brown pods were dried at room conditions with a controlled temperature equal to 16 °C for three hours. Subsequently, the dried pods were powdered in a ball mill (ALPA IBM1830, Philadelphia, PA, USA). After, the powder was sent to a mechanical sieving process, and the sieved powder was sent again to a bread mill (Moulinex, Lyon, France) to reduce its size; finally, this powder is sieved again, and the final product is stored in a dark place for further use (see Figure 1).

Figure 1. Flowchart of the Tara powder production process.

2.3. Preparation of the Chromium-Free Leather

The production of the chromium-free leather was divided into 3 main stages, which are the pre-tanning, the tanning, and the post-tanning operations. Essentially, the first stage followed a conventional process.

2.3.1. Soaking, Liming, Fleshing, and De-Liming Are the Four Main Operations

In brief, the skin was first weighted to calculate the quantity of the chemicals (% in mass) that were added in the different steps. Then, for the soaking, the skin was immersed in a 200% water bath at 25 °C. This bath contained 1% tensoactive, 0.5% bactericide, and 0.1% sodium carbonate. Then, the soaked skins were liming by preparing two baths. The first bath contained 100% water at 25 °C, 1% NaOH, and 1.5% NaS. This bath was eliminated, and another bath was prepared, which contained (NH₄)₂SO₄ and 0.2% Bassocym C10. Finally, the pre-tanned skins were washed by using 200% water.

2.3.2. Tanning Process

Furthermore, the heart of the present research is the change in the tanning process to eliminate the consumption of chrome. Therefore, it was necessary to prepare the pre-tanned skins by changing the chemicals and the conditions in the pickling operation in contrast with the normal tanning process. Table 1 describes the applied process to prepare pre-tanned skins to increase the absorption of the Tara in the tanned process.

Table 1. Pickling operation to produce chromium-free goat leather.

Used Chemical	Content	Temperature
Water	60% m/m mass water/mass of skin	25 °C
Sodium Chloride	20 g/L
Formic Acid	4 g/L
Sertan BK	4%
Rotated for two hours
Settle for 12 h and rotate the drum for 2 min

% in mass of used chemicals per mass of skin.

2.3.3. Post-Tanning Process

Moreover, the pickling skins were tanned by the addition of Tara. It is important to highlight that different levels of Tara were used to optimize the tanning process. So, they were 10% (T1), 12% (T2), and 14% (T3) of Tara. Thus, 12 skins were prepared for each level, as required for the statical analysis (see Section 2.5.3). Hence, the green tanning was accomplished by following the procedure established in Table 2. As observed, chrome was not added in any of the processes, and only organic products were used during the pickling and tanning operation. After the tanning process, the tanned, dry leathers were stretched.

Table 2. Organic tanning process to produce chromium-free goat leather.

Used Chemical	Content	Temperature	Procedure
Water	60% mass water/mass of skin	25 °C
Sodium Chloride	20 g/L		Rotated for 60 min
Formic Acid	4 g/L		Rotated for 60 min
Settle for 12 h and rotate the drum for 20 min			Rotated for 60 min
Tara	10, 12, and 14% 1:10 diluted	25 °C
Glutaraldehyde Basifying Agent	4% 0.3% 1:10—3 parts		Rotated for 60 min 1 part each hour and the final part for 3 h
Drain the bath, stack, and rest for 24 h. Hang to dry.

% in mass of used chemicals per mass of skin.

To reduce the hardness, which is associated with the green tanning leather, it was necessary to fat-liquor the tanned leathers to improve the quality of the prepared samples. This process was also used for the first time, and it was developed by our research group. For this, Table 3 indicates the formulation and procedure employed to achieve the cited operation.

Table 3. Fat-liquoring process to produce chromium-free goat leather.

Used Chemical	Content	Temperature	Procedure
Water	80% mass water/mass of skin	30 °C	Rotated for 20 min
Tensoactive	0.2%
Formic Acid	0.2%
Bath elimination
Water	80% mass water/mass of skin	35 °C	Rotated for 60 min
Sodium Formate	1%
Sodium Bisulfite	2%		Rotated for 60 min
Bath elimination
Water	300%	40 °C	Rotated for 40 min
Bath Elimination
Water	200% mass water/mass of skin	60 °C	Rotated for 60 min
Phosphoric Ester	12%, 1:5 parts
Sulphochlorinated paraffin	4%, 1:5 parts
Bath Elimination
Water	200% mass water/mass of skin	18 °C	30 min
Stack the leather for 12 h

% in mass of used chemicals per mass of skin.

Next, a new bath was prepared, which had 2%, and the drum was rolled for 60 min; then, 100% water was increased to 70 °C, plus 4% chlorinated paraffin, 1% lanolin, and 4% sulfated fat, mixed and diluted in 10 times its weight. After this, the drum rotated for 60 min. Next, 0.75% formic acid was added and rolled again for 10 min. Subsequently, 0.5% formic acid was added, diluted to 10 times its mass, and partitioned into two parts, and each part was rolled for 10 min. The bath was removed. Finally, the prepared leather samples were stacked in the dark so that they could drain and dry for 8 days. The last operations were saw dusted, softened, and leather stacked. For these, the leather samples were moistened using a small portion of damp sawdust overnight (with the aim of allowing them to absorb water, which would improve their softness). Afterwards, the sawdust-covered leather samples were softened in a molliza. Finally, they were stretched in a toggling, in which they remained for 12 h. The produced chromium-free leather samples were labeled and stored to analyze their properties.

2.4. Characterization of the Tara Powder

Chemical and physical analysis were performed to analyze the quality of the obtained Tara powder. Therefore, the functional groups of the Tara powder were evaluated using an FT-IR spectrometer (type Vector 22, from Bruker, Billerica, MA, USA). Furthermore, the total polyphenolic and the tannic acid content, which is the expected tanning agent, was evaluated by following the Folin–Ciocalteu assay [23]; in brief, 0.5 mg of Tara Powder was taken into test tube and mixed with 2.5 mL of a 10-fold dilute Folin–Ciocalteu reagent and 2 mL of 7.5% sodium carbonate. The tubes were covered with parafilm and allowed to stand for 30 min at room temperature. Then, the absorbance was read at 760 nm spectrometrically (type C10082CA, from Hamamatsu Photonics, Hamamatsu, Tokio, Japan) against Gallic acid as a standard (concentration of 0.01, 0.02, 0.03, 0.04, and 0.05 mg/mL of gallic acid were prepared in methanol). Moreover, the humidity of the Tara powder was determined by the oven-drying ISO 18134-3 standard method [22]. Also, it was calculated that the ash content of the powder was determined by following the ISO 763:2003 standard method [23]. Finally, the yield of the production of the Tara powder (mass of Tara powder/mass of Tara pods), used as an indicator of the production efficiency, was calculated using the following equation:

Yield of Extraction (%) = me/mp × 100

(1)

where me is the weight of powder (in kg), mp is the weight of pods (in kg), and the yield of extraction is given in % (kg Tara Powder/100 kg Tara Pods).

2.5. Characterization of the Chromium-Free Leather

2.5.1. Mechanical Characterization

The mechanical test encompassed measurements of tensile strength and percentage elongation (IUP-6, 2011), apparent density [24], shrinkage temperature up to 100 °C [25], flex resistance (Flexometer method) [26]. The whole determinations were executed at the specialized laboratory of leather—ESPOCH—by following the instructions according to the normative for each determination. In this sense, for the mechanical analysis, the universal tensile machine, the abrasion machine, and the other applied machines were built by students following the regulation standards (see the machines in Figure A1). Moreover, all the prepared samples were analyzed, and conventional chromium leather pieces were also evaluated to compare the final characteristics of both products.

2.5.2. Sensorial Characterization

The evaluation of the sensory properties of leather—fullness, roundness, and fineness of grain—was carried out through sensory impact using a scale from five to one, corresponding to the following ratings: 5 = Excellent, 4 = Very Good, 3 = Good, 2 = Fair, and 1 = Poor. In each one of these sensory properties, the following evaluative behavior was followed:

Fullness: To check for fullness (increase in collagen fibers), the whole area being examined was lightly pressed with the fingertips, trying to keep the pressure as even as possible across the surface. Roundness: The assessment of roundness was carried out through visual observation, as well as through tactile appreciation to verify the ability that goat leather must present to undergo deformations during the transition from flat to spatial form, for example, when a specific item is being made, such as a shoe. The highest ratings are given to those leathers that, despite being full, can be easily molded.

Flower fineness: The fineness of the flower was evaluated through the senses of sight and touch. Assessing whether the superficial layer fully adhered to the skin’s collagen, forming a homogeneous skin-fibrillar interwoven complex was necessary. Special care was taken to observe whether the surface of the skin was too smooth or very rough and coarse. Plenitude: It was used to assess the fullness of goat hides. The fingertips (as well as the palms) were slid over the entire surface of the leather to detect any negative signs of roughness, deformities, or cracks. That is to say, the leather must be very full and pleasant to the touch, since if its purpose is the making of footwear, this quality requirement must be observed very rigorously to avoid any kind of discomfort for the future user.

In this sense, the sensorial characteristics of the leather were evaluated by two experts in the industry. The first evaluator was a leather producer in Guano, Ecuador, and the second one was an expert in the production of green leather located in Lima, Peru. They were both selected because of their wide knowledge in the leather industry, with a special focus on the use of plants as tannin agents. They did not know the production process of each leather. There is no standard for the sensorial analysis. Therefore, the two experts passed their hands over the different leather samples, and after a careful evaluation, they gave a mark according to the sensation that the leather caused.

2.5.3. Statical Analysis of the Results

To evaluate the sensory and physical qualities of goat skins tanned with varying amounts of eco-friendly materials from Tara, a Completely Randomized Design was applied. It was decided to test the samples (skins) at three levels of Tara percentage: 10, 12, and 14%. The procedure involved conducting 12 trials (12 repetitions per level) and the total experimental runs were calculated according to Total number of Experimental runs = Number of levels × Experimental units. Thus, Table 4 presents the 36 experimental runs conducted in the experiment.

Table 4. Design of the experiment—Total number of Experimental runs.

Percentage (%) of Tara (Number of Levels of the Sample)	Number of Experimental Runs (Using 3 Skins per Level in the T1, T2, and T3)
10	1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12
12	13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, and 24
14	25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, and 36
Observations: T1, T2, and T3 correspond to the three treatments (according to the three levels of the experiments: 10, 12, and 14% of Tara).

Moreover, the additive linear model used for the application of the Simple Completely Random Design described in the upper paragraph was the following:

$$\mathrm{Y}\mathrm{i}\mathrm{j}=\mathsf{\mu }+{\mathsf{\alpha }}_{\mathrm{i}}+{\mathsf{ϵ}}_{\mathrm{i}\mathrm{j}}$$

(2)

where Y_ij is the value of the parameter under determination, μ is the effect of the mean per observation, α_i is the effect of Tara’s different levels, and ϵ_ij is the effect of the experimental error [27]. Likewise, for the determination of the significance of the sensory variables, the Kruskal–Wallis test was also used, whose additive linear model was

$$H={\displaystyle \frac{12}{nT(nT+1)}}={\displaystyle \frac{\sum RT·1^2}{nR{T}_1}}+{\displaystyle \frac{\sum RT·2^2}{nR{T}_2}}+{\displaystyle \frac{\sum RT·3^2}{nR{T}_3}}+2\left(nT+1\right);$$

(3)

where H is comparison value calculated with the K-W test, nT is Total number of observations at each Tara level, and R is the identified range in each group [28]. We decided to use a post hoc analysis, in this case a Kruskal–Wallis test, because the sensorial parameters are qualitative analyses; therefore, they are understood to be non-parametric variables. Thus, they cannot be studied by a simple analysis of variance (ANOVA) test. Moreover, since we studied more than two respondents’ variables, it is necessary to include this type of test to enhance the comparison between the medians and determine which level of Tara is best in relation to the different analyzed variables. The scheme of the Analysis of Variance is described in Table 5.

Table 5. Scheme of the analysis of the variation (ANOVA).

Source of Variation	Degrees of Freedom
Total	35
Treatment	2
Error	33

2.6. Total Cost Assessment of the Chromium Leather Production

The total cost assessment was executed in two parts. First, it was necessary to perform a mass and energy balance of the whole operation to establish the consumption of energy and chemicals used during the tanning process, and with this data, the cost of production of the chromium-free leather was calculated by the costs in the Ecuadorian market to process 1000 kg/month of goat skins. Then, the cost and benefit rate, the NPV (Net Present Value), and the IRR (Internal Rate of Return) were calculated to establish the viability of the incorporation of the present research on an industrial scale for a 5-year project duration.

3. Results

3.1. Tara Chemical Characterization

FTIR spectrum was employed to identify the principal functional groups of the active compounds present in the Tara powder based on the peak values in the IR region. According to Figure 2, the spectra of the Tara powder is well-marked by intense absorptions in the 4000–3000, 1700–1500, 1400–1000, and 800–700 cm⁻¹ regions, which is key feature of the IR tannin spectra [26]. Therefore, in the first region, a strong absorption peak around 3500 cm⁻¹ was observed, with a wide and strong band positioning at 3250 cm⁻¹, which is assigned to the hydroxyl groups stretching vibrations, and due to the wide variety of hydrogen bonding between OH [19,29,30]. Then, in the second part of the spectrum, it presented a shark peak at 2980 cm⁻¹; this is associated with the symmetric and antisymmetric -C-H- stretching vibrations of CH₂ groups. In the third part of the spectrum, a deformation vibration appeared because of the carbon–carbon bonds in the phenol at the region of 1700 cm⁻¹. Moreover, some was observed at 1700 cm⁻¹, as well as C-O at 1020 cm⁻¹ [31,32]. Finally, at 764 cm⁻¹, a distortion vibration of C=C is observed in the benzene rings [33]; so, this is clear evidence of the present of tannins in the Tara powder, thus proving that tannins absorb intensely in the hydroxyl and carbonyl region [34].

Figure 2. FTIR spectra for the Tara powder.

In this sense, as the IR spectrum reported, the main component of the Tara are the polyphenols, in which the most predominant is the gallic acid. This gives us a hint over the possibility of using Tara as a tanning agent. Because the tanning process to convert the skin into leather consisted of the crosslinking and the attaching of the polyphenols to the COOH or -NH₂ groups of the leather protein through hydrogen and covalent bonds. Therefore, the higher the polyphenol content, the more the skin undergoes the tanning process. Thus, if the Tara can penetrate the skin, it can transform the collagen into a stable compound, which is the most important step to produce chromium-free leather.

The total tannic acid content was evaluated, and the results are reported in Table 6. Thus, the tannic acid content was equal to 52.25 mg/g of Tara powder, and it is a clear sign of the tanning capacity of the Tara powder. Tannins are generally defined as naturally occurring polyphenolic compounds of high molecular weight to form complexes with the proteins [35]. The tanning capacity of the Tara powder is related to the fact that the high acid content of tannins tends to complex the collagen and stabilize the skin structure (see the mechanism of reaction proposed by Ref. [34] and reported at Figure A2). This occurs under optimal conditions such as pH, percentage of Tara powder, amount of other chemicals added, etc., which are the controlled parameters studied in the present research. Moreover, the total gallic acid content in Tara extract was higher in comparison with other studied plants such as Mimosa, which reported a tannic acid content equal to 1.33 ± 0.06 [29]; the same author [26] reported a tannin acid content equal 0.15 g/kg of tannins in the Samanea Saman (Jacq) Merr. Moreover, ref. [36] reported a tannin content equal to 43.04 ± 0.57 mg/g of extract in Artemisia absinthium; the author [37] reported a tannic acid content equal to 41.37 mg TAE/g Tea when it was evaluated in Rooibos Tea. Consequently, the tannic acid level is superior to that of Tara extract when compared to other plants that may be utilized in the leather industry. This initial method is crucial in leather production as tannic acid compounds confer strength, flexibility, and resistance to decay in leather. They are also renowned for their ability to induce protein coagulation and interact with collagen fibers in animal hides. Tanners accomplish this via a procedure termed tanning, which affixes the collagen to the hide, transforming it into a resilient material that is challenging to decompose [38].

Table 6. Chemical characterization of the Tara powder.

Variable	Results
Polyphenolic Content, mg/g of Tara powder	56 ± 0.03
Tannic Acid content, mg/g of Tara Powder	52.25 ± 1.74
Non-Tannic Acid content, mg/g of Tara Powder	3.25 ± 0.23
Humidity, %	6.50 ± 0.80
Ashes content, %	2.28 ± 0.05
Yield of production, %	90.59 ± 0.06

Mean ± standard deviations of three measurements.

Finally, the yield of production to obtain Tara Powder was equal to 90.59% ± 0.06%, which is affordable for industrial production. The producers are going to leverage most of the Tara pods, and that way, they are going to obtain high rentability. This benefits the farmers and the leather producers, so the price of the Tara powder will be affordable to replace the chromium in the tanning process.

3.2. Mechanical Characterization of the Chromium-Free Leather

The most important parameters to analyze the performance of a tanning process are the mechanical characteristics because they are going to be fundamental when the leather is used for making shoes or clothes [4]. Therefore, the prepared leathers were compared with a normal chromium tanning process to evaluate the performances of the Tara in the tanning process. In this sense, when 10% of Tara powder was applied to the formulation, the best results were obtained in all the mechanical characteristics of the leather. Thus, for the tensile strength, the average value was equal to 340.77 ± 4.89 N/mm²; for the percentage of elongation, the results were equal to 64.68 ± 2.50%; the apparent density was equal to 208.73 ± 1.71 kg∙cm⁻³; the flex resistances were equal to 9.06 ± 0.76 mm; and the shrinkage temperature was equal to 87.92 ± 1.25 °C (see Figure 3). Likewise, the Tara tanning process reported better mechanical properties when in comparison to other tanning alternative technologies, as the ones reported by [4] who used 6% of Oxazolidine in wet-white tanning and obtained a shrinkage temperature equal to 84.2 °C, while the author [39] reported a shrinkage temperature equal to 85.8 ± 1.5 °C when polyoxymethylene diepoxy ether and urotropine were used to produce a salt-free pickling-chrome tanning technology; in addition, the author [40] prepared an eco-friendly tanning process based on amino acids and obtained a tensile strength equal to 160.02 N/mm² and a percentage of elongation equal to 24.78%. The high mechanical performance of the Tara tanning process is explained by [41], which explained the reaction mechanism of the polyphenol compounds and the collagen. Thus, the principal interaction between hydrolyzable plant polyphenols and collagen involves multiple hydrogen bonds. The ensuing reaction involves the crosslinking of tannin molecules through the pyrogallol moieties, resulting in synergistic combination reactions that form a matrix of cross-linked polyphenolic species. These species function collectively as a singular chemical entity, yielding highly stable combination reactions that enhance the mechanical properties of the vegetable leather.

Figure 3. Tensile strength (A), percentage of elongation (B), apparent density (C), flex resistant (D), and shrinkage temperature (E) of the chromium-free leather produced by adding different levels of Tara powder (inset).

3.3. Sensorial Characterization of the Chromium-Free Leather

The sensorial characteristics were evaluated to analyze the impact of the prepared leather in the senses of the consumers. Thus, the sensorial characteristics reported a better characteristic when they were compared to the traditional chromium tanning process. In this sense, it was not an appreciable trend in correspondence with the different levels of Tara powder. In this context, the author [42] explained that vegetable-tanned leathers have better sensorial characteristics because, when they are tanned with plant extracts, the reaction conditions must not change, especially the conditions within the reaction. This keeps the collagen molecules’ physical and chemical properties from changing, which makes the tanning process firmer and lets the leather keep its qualities (Figure 4).

Figure 4. Fullness (A), roundness (B), and fineness of grain (C) of the chromium-free leather produced by adding different levels of Tara powder.

Optimization of Dosage of Tara Powder in the Tanning Process

Different levels of Tara powder during the tanning process were compared to establish the optimal technique to produce high-quality chromium-free leather. Therefore, 10, 12, and 14% of Tara powder were added to the drum, and the mechanical resistance was statical analyzed to verify if there is significance between both parameters. All the mechanical characteristics reported a high significance (p-value ≤ 0.001), which indicates that the level of Tara powder drastically influences the final characteristics of the goat leather. In this sense, when 10% of Tara powder was added, the best mechanical characteristics were reported for all the samples. Therefore, the 10% Tara powder is the optimal level and, for suitable production, it is necessary to prepare the leather with this quantity of tanning agent. On the other hand, when more Tara powder was added, the mechanical characteristics decreased considerably; this is explained by the fact that the ease at which the intervening molecules can be displaced as collagen shrinks. If the molecules are water, this matrix can be displaced relatively easily. When species bound to collagen are added to the matrix and some supramolecular water is swapped out and interacts with the remaining water, the matrix is less likely to move, which can be seen as a rise in the temperature at which it breaks down [43].

Hence, adding more tannins to the collagen matrix makes it harder for molecules to move around, which means the leather loses its mechanical properties. However, according to the optimization results, an alternative and sustainable technology is achieved because the properties of the leather when 10% Tara powder is added surpass those achieved with the conventional chrome tanning process (Table 7).

Table 7. Optimization parameters of dosage of Tara powder in the chromium-free leather production.

Variables	Tara Powder Levels, %			E.E.	p-Value	Sign
	10%T1	12%T2	14%T3
Tensile Strength, N/cm2	3407.74 a	2493.85 b	2816.83 b	801.13	0.00747	**
Percentage of Elongation, %	64.68 a	53.78 c	59.12 b	12.56	0.004	**
Apparent Density, kg∙cm−3	208.73 b	212.30 a	213.81 a	23.47	0.0005	**
Flex Resistances, mm	9.06 a	7.23 c	8.66 b	0.20	0.00001	**
Shrinkage Temperature, °C	87.92 b	87.92 b	88.50 a	0.36	0.00423	**

a: Higher average (average comparison according to Tukey); b: Mean Average (average comparison according to Tukey); c: Lowest average (average comparison according to Tukey); E.E.: Standard error; p-value: Probability; Sign: Significances, **: high significance.

On the other hand, the water quality before and after the tanning process was evaluated to determine the sustainability of the production of chromium-free leather. Hence, the parameters that experienced a high increase were the biochemical oxygen demand (BOD) and the chemical oxygen demand (COD), which reported values equal to 225.50 mgO₂/L, 1461 mgO₂/L, 414.76 mgO₂/L, and 3150.30 mgO₂/L before and after the tanning process. Moreover, other water parameters such as nitrates, total nitrates, and sulfates were evaluated, but the results did not report appreciable changes. The most interesting result was that both water samples had 0 mg/L of hexavalent chromium and total chromium, which meant that the technology worked as planned (see Table A3). Unfortunately, the COD and BOD parameters are out of the Ecuadorian legislation range, which established a maximum concentration of BOD and COD levels equal to 250 and 500 mg/L [27]. Thus, it is necessary to establish a wastewater treatment to overcome and reduce levels of contamination until permissible values. Thus, the most economical and feasible method for the wastewater treatment according to our context consisted of (a) an equalization tank, (b) a primary physical screening method, (c) a clarifier, (c) an aeration time, (d) a coagulation/flocculation tank, and (d) a sedimentation tank, while the sludge generated was proposed to be treated using a gravity thickener. The wastewater treatment plant is designed to reduce the BOD and the COD content to 350 and 250 mg/L, respectively. These values accomplish the Ecuadorean law. In this sense, the principal parameters of the design of the wastewater treatment plant are reported in Table A5 and the flowchart is reported in Figure 5.

Figure 5. Wastewater treatment plant design to eliminate the contaminants in the tanning process water.

Moreover, one of the main problems in maintaining the sustainability of the proposed technology is the waste generation. In the production of chromium-free leather, around 60 kg of skin waste is produced, which represents around 70% of the total weight. This quantity is in accordance with the life cycle assessment (LCA) presented by [44]. Also, 100 kg of Tara residues is produced in the production of 1 ton of Tara powder. Thus, it is fundamental to propose different technologies that can recover the maximum possible to prevent the contamination. Therefore, the first approach is to prevent the production of waste in the powder manufacturing. Thus, we proposed to use the residues of the Tara seed and the thick powder in the production of green inhibitors for the corrosion protection and the production of dermatological creams. Thus, we expected to recover 100% percent of the waste. We first suggested using skin residues to make collagen and glue in leather production. The second by-product is already being studied in our laboratory and we expect to produce a patent for this technology, while the production of collagen is reported in other studies such as the ones proposed by Ref. [29] or by Ref. [45]. Therefore, these methodologies enable the recovery of at least 70% of the waste generated. Finally, the process can be adjusted to reduce the consumption of chemicals. Thus, the methodologies proposed by the authors [46] can be updated to use enzymes or chemicals to prevent the generation of solid waste. The presented ideas are going to help to reduce the production of waste by at least 90%. This ensures the sustainability of the process, and the leather can be considered as a green technology.

3.4. Total Cost Assessment of the Chromium-Free Leather

Table 8 indicates the total cost assessment of the proposed technology. Hence, it can be seen that the cost/benefit of producing the chromium-free goat leather is equal to 1.03, so for each dollar that the producers invest, they are going to have a recovery of 3 cents. Moreover, the amortization of the investment is going to be achieved after 5 years of production; considering that the project is going to be executed by local producers, this time is suitable for them. Moreover, the internal return rate (IRR) was equal to 31%; thus, the project is viable, and the local organizations can execute it. In this sense, the main benefit of the proposed technology is the reduction in the use of chemicals, as reported in Table A4. To process 90 kg of skin, less than 3 kg of chemicals are needed, and these chemicals are biodegradable, so the environmental impact of the Tara tanning process is not considerable in contrast with the currently applied chromium technology. Overall, the best way to reach the Sustainable Development Goals set out in the 2030 Agenda is to carefully handle compounds in a circular way, using Tara powder instead of chromium. Also, the capital expenditure (CAPEX) and operational expenditures (OPEX) for the installation, operation, and maintenances to treat the wastewater are included into the total cost assessment. The CAPEX was calculated according to the proposed design (see Appendix A), while the OPEX were calculated based on the prices of energy and human resources, and the chemical consumption are based on the Ecuadorean prices. Thus, Table A14 and Table A15 describe the CAPEX and OPEX analysis for the wastewater treatment plant installation and operation. Thus, the total price for the installation is equal to USD 25,258.50, while the operational cost per month will be equal to USD 739. These values are included in the total cost assessment to produce chromium-free leather.

Table 8. Total cost assessment to produce chromium-free leather.

Year	Inlet, USD	Capital Expenditures (CAPEX), USD	Benefits, USD	Depreciation, USD	Amortizations, %	Subscription, USD	Cash Flow, USD	Current Income, USD	Current Discharge, USD	Current Net Flow, USD
0	-	436,432.50	-	-	-	-	-	-	436,432	−436,432
1	3,525,715	3,439,077	86,638	9105.50	190,184	190,612	95,316	3,000,000	3,000,000	94,186
2	3,756,273	3,693,084	63,188	9105.50	188,948	1663.90	259,578	4,000,000	3,000,000	253,458
3	3,986,831	3,945,113	41,718	9105.50	184,129	6482.04	228,470	4,000,000	4,000,000	220,439
4	4,217,388	4,189,431	27,957	9105.50	165,356	25,255	177,164	4,000,000	4,000,000	168,909
5	4,853,077	4,617,656	235,421	9105.50	92,218	98,393	238,352	5,000,000	4,000,000	224,551
Total										525,111
Cost/Benefit, USD										1.03
NPV, USD										525,111
IRR, %										31

Moreover, a sensibility analysis was executed to maximize the cost–benefit for the leather production—in this sense, the leather price in dm², the Tara cost per kilogram, and the cost of wastewater treatment per liter (see Figure 6). Therefore, the only way to optimize the cost/benefit is related to the increase in the leather price, but, at higher price, the leather is not going to compete with other types of leather in the Ecuadorean market. Moreover, the price of the Tara does not have a huge impact on the final economic benefits of the chromium-free leather, while the wastewater treatment does not represent a considerable cost, because the volume of water to produce our leather is not considerably high. Thus, the maximum cost/benefit that we can achieve from a competitive price of the leather is equal to 1.03.

Figure 6. Sensibility analysis to determine the maximum cost/benefit in the production of chromium-free leather.

4. Conclusions

Many researchers have proposed clean production systems to lessen the contamination that global industrial activities produce. Hence, the green leather producing systems are vital to overcome the problematic situation that these activities are faced due to its extended contamination. However, at the moment, the contamination problem is still a complicated topic in leather production, and the key problem is the high amounts of chromium emissions in conventional pickling-chrome tanning process, which is a long-standing, old problem that has never been resolved satisfactorily in recent decades. In order to solve the contamination problem, paying attention to the factors of the safety of production, the cost of materials, production simplicity, a novel chromium-free tanning process integrating the advantages of used plants powders was developed and optimized. The main findings of the proposed technology were as follows:

The total tannic acid content of the prepared Tara powder was equal to 52.25 mg/g of Tara powder, and it was a clear sign of the tanning capacity of the Tara powder. Moreover, this tannic content was superior to other plants reported in the literature. Also, the yield of Tara powder production was equal to 90.59% ± 0.06%, which is suitable for industrial production. The better mechanical properties were obtained when 10% of Tara powder was added. The average tensile strength was 340.77 ± 4.89 N/mm², the percentage of elongation was 64.68 ± 2.14%, the apparent density was 208.73 ± 3.78 kg∙cm⁻³, the flex resistance was 9.06±0.76 mm, and the shrinkage temperature was 87.92 ± 5.17 °C. These results were considerably higher than the results reported by the traditional chromium tanning process. To make the chromium-free tanning process better, the leather’s mechanical properties were checked before and after 10, 12, and 14% of Tara powder were added. All of the mechanical properties showed a high level of significance (p ≤ 0.001). This means that the amount of Tara powder has a huge effect on the final properties of the goat leather. In this sense, when 10% of Tara powder was added, the best mechanical characteristics were reported for all the samples, so this level is the optimal point to produce the best-quality leather. The total cost assessment for the production of 1000 kg/month of goat skin reported a cost/benefit rate equal to 1.03, so for each dollar that the producers invest, they are going to have a recovery of 3 cents. Moreover, the amortization of the investment is going to be achieved after 5 years of production, considering that the project is going to be executed by local producers; this time is suitable for them. Moreover, the IRR was equal to 31%, so the project is going to be economically feasible. However, the production of chromium-free leather experienced some limitations as follows: (i) high COD and BOD values were evaluated in the wastewater, which are above the Ecuadorian permissible levels; (ii) high waste generated during the whole process; (iii) machine calibration; and (iv) uptake data lacking. However, some possible solutions were addressed through the paper in relation to these problems. Thus, some of them are summarized as follows: (i) a wastewater treatment plant was designed, and the economical evaluation, CAPEX-OPEX, suggested that the installation of the plant is viable without a high increase in the production price; (ii) solutions to the waste generated were suggested. Thus, the production of green inhibitors and skincare products is viable by using the by-products generated during the production of the Tara powder, while the production of collagen or glue is feasible by recovering the skin generated through the tanning process; (iii) more experiments are needed to propose a calibration procedure to adjust the production of the leather; (iv) more data can be obtained from different producers when the chromium-free technology is patented and distributed in the country.

Overall, the proposed technology is a novel approach to eliminating the consumption of chromium all over the production process. The optimal results in terms of quality, cost, and environmental safety production reported by the Tara powder are indicative of the potential replacement of the chromium in the leather production process.

Appendix A.1. Materials and Reagents Used in the Production of Chromium-Free Leather

In the production of chromium-free leather, 30 goat skins were purchased in local market (San Alfonso, Riobamba), Tara Powder (produced by the local association, the production process is explained at Section 2.2), Sodium Carbonate (Na₂CO₃, 95% purity, from Sigma-Aldrich, St. Louis, MO, USA), Tergitol (tensoactive, 98% purity, from Sigma-Aldrich), Tickopur r-33 (bactericide, 80% purity, from Sigma-Aldrich), Sodium Hydrosulfide hydrate (NaSH∙2H₂O, 60% purity, from Merck, Darmstadt, Germany), lime (Ca(OH)₂, 88% purity, Sigma-Aldrich), petroleum derivates (form local producer), Terg-a-zyme^® enzyme detergent (70% purity, Sigma-Aldrich), Ammonium Sulfate ((NH₄)₂SO₄, 80% purity, Merck), Bassocym C10 (Yielding Product, 85% purity, BASF, Ludwigshafen, Germany), Sodium Bisulfate (NaHSO₃, 95% purity, from Merck), Sodium Chloride (NaCl, 95% purity, from Merck), Sodium Formate (HCO₂Na, 97% purity, from Sigma-Aldrich), Sodium Bisulfite (NaHSO₃, 40% purity, from Sigma-Aldrich), Formic Acid (H₂CO₂, 95% purity, Sigma-Aldrich), Sulfuric Acid (H₂SO₄, 99% purity, Merck), Oxalic Acid dehydrate (H₂C₂O₄·2H₂O, from Merck, 99% purity), Nutrapol PPs (Sulphated animal fat, 80% purity, from Mathiesen, Huechuraba, Chile), Lanolin (95% purity, from Sigma-Aldrich), Seriol SC-B (cationic fat, 80% purity, from Quimser, Barcelona, Spain), Neutrigan^® MO (basifying agent, 90% purity, from Austral Chemicals, Quinta Normal, Chile), Sertan BK (pre-tanning agent-Organic, Quimser), sawdust (from local producer), Dispersel (dispersant, 60% purity, from Coselsa, Lima, Peru), Blancotan (synthetic tannin, 80% purit, from Silva Team), Ledoresin AC (acrylic resin, 90% purity, from Silva Team, San Michele Mondovì, Italy), Sertan F (Skirt filler, 50% purity, from Quimser), Aluminum Sulfate (Al₂(SO₄)₃·18H₂O, 98% purity, Merck) Sertan N-Pak (Neutralizing retanning, 95% purity, from Quimser), Rokanol^® LP2424 MB (fatty alcohol, 80% purity, from PCC, Brzeg Dolny, Poland), Sodium Bicarbonate (NaHCO₃, 99% purity, from Sigma-Aldrich), Caustic Soda (NaOH, 95% purity, from Merck), Sodium Sulfide nonahydrate (Na₂S·9H₂O, 98% purity, from Sigma-Aldrich), SERLAC (Hydrolacquer, 80% purity, from Quimser), Capafin (organic dye, 95% purity, from Quimser), Sertack (Silicone, 95% purity, from Quimser), Serlac 22 (Nitrocellulosic lacquers, 90% purity, from Quimser), mineral turpentine (organic dye solvent, 80 purity, from Quimser), Esther Phosphoric (C₆H₁₀O₃·xH₃PO₄, 95% purity, from Merck), Sulphochlorinated paraffin (50% purity, Quimser) Cuiextan BS (metallic complex, 90% purity, from Quimser), Hydrogen Peroxide (H₂O₂, 38% purity, from Sigma-Aldrich), Glutaraldehyde (H₈C₅O, 25% purity, from Merck), Seradye (Oxidation dyes, 95% purity, from Quimser).

Figure A1. Machinery used for the mechanical test of the chromium-free leather characterization.

Figure A1. Machinery used for the mechanical test of the chromium-free leather characterization.

Figure A2. Reaction mechanism between the polyphenols and the collagen in the vegetable tanning process.

Figure A2. Reaction mechanism between the polyphenols and the collagen in the vegetable tanning process.

Appendix A.2. Tanning of Goat Skins with Different Levels of Tara (12–14 and 16%)

1. 

SOAKING (Static)

• Initial Weighing: Weigh dry skins together with the wool.
• Bath: 300% Water.
• Products: 0.3% Non-ionic detergent and 0.01% Sodium hypochlorite.
• Conditions: 25 °C, 12 h, pH 12.5.
• Action: Drain bath.
• Final Step: Weigh wet skins with wool.

2. 

UNHAIRING BY PAINTING (Spreading)

• Products: 5% Water, 3.5% Lime, and 2.5% Sodium sulfide.
• Conditions: 40 °C, 12 h.
• Action: Remove wool and weigh skins.

3. 

DRUM UNHAIRING (Inox Drum at 4–8 RPM)

• First Bath: 100% Water (25 °C), 0.7% Sodium sulfide, 0.7% Sodium sulfide (repeated), and 0.5% Sodium chloride.

  ∘

  Time: 30 min.

• Second Bath: 50% Water (25 °C) and 1% Lime.

  ∘

  Time: Three intervals of 30 min with 1% lime each.

• Rest/Rotation: 3 h rest, followed by 20 h total (rotating 10 min every hour).
• Final Action: Drain bath and weigh skins.

4. 

DELIMING

• First Wash: 200% Water (25 °C) and 0.2% Sodium bisulfite for 30 min. Drain bath.
• Second Wash: 100% Water (35 °C), 1% Sodium bisulfite (30 min), 1% Sodium Formate, and Bating agent (0.1% for 60 min, then 0.02% for 10 min).
• pH: 8.
• Final Action: Drain bath and wash with 300% water and 0.5% Formic acid (1:10) for 40 min.

5. 

DEGREASING

• Bath: 100% Water (30 °C), 2% Surfactant, and 4% Degreaser (60 min).
• Action: Drain bath.
• Final Wash: 200% Water (35 °C) with 1% surfactant (40 min), then 200% water at room temperature (20 min).

6. 

PICKLING (1st and 2nd Stages)

• Bath: 60% Water (room temp) and 10% Sodium chloride (10 min).
• Acidification: Formic acid 1:10 (0.4% in first stage, 0.2% in second stage) added in three parts (30, 30, and 60 min).
• Final pH: 3.2–3.5 (Stage 1) and 4.5 (Stage 2).
• Rest: 12 h, then rotate for 10 min.

7. 

TANNING (Tara)

• Products: TARA (Caesalpinia spinosa) at levels of 12%, 14%, or 16%.
• Procedure: Added in three parts. Rotate for 1 h after each addition (3 h total).
• Fixation: Formic acid 1:10 (added twice at 0.3% each) for 1 h and 4 h, respectively.
• Final pH: 3.2–2.8.
• Final Steps: 100% Water wash (40 °C) for 30 min, drain bath, and horse up (rest) for 48 h.
• Process End: Wet finishing.

Table A1. Mechanical analysis of the chromium-free leather produced by adding different levels of Tara powder.

Level of Tara, %	Repetition	Tensile Strength, N/mm2	Percentage of Elongation, %	Apparent Density, kg∙cm−3	Flex Resistant, mm	Shrinkage Temperature up to 100 °C
10	1	336.30	64.67	208.74	7.43	85.00
10	2	334.09	62.10	211.45	7.92	89.00
10	3	340.13	65.67	209.41	7.67	89.00
10	4	345.54	65.53	208.70	8.91	88.00
10	5	347.59	62.73	210.23	10.40	88.00
10	6	342.62	67.00	209.34	9.41	89.00
10	7	344.51	65.37	208.66	9.90	88.00
10	8	338.53	68.40	209.47	8.42	88.00
10	9	342.56	61.00	207.33	9.16	86.00
10	10	337.59	61.17	204.31	10.15	87.00
10	11	332.30	67.77	207.42	8.66	89.00
10	12	3475.41	61.77	209.74	10.64	89.00
12	1	222.74	52.60	210.22	7.43	87.00
12	2	223.74	52.27	211.98	6.81	86.00
12	3	228.43	50.23	213.78	7.18	86.00
12	4	235.00	50.20	212.65	7.43	86.00
12	5	222.94	53.70	214.17	6.81	85.00
12	6	243.19	54.57	211.34	7.43	85.00
12	7	237.45	52.53	213.65	7.43	89.00
12	8	245.53	56.93	211.98	7.43	89.00
12	9	243.81	56.90	212.45	6.81	89.00
12	10	242.12	51.50	211.47	7.43	87.00
12	11	240.85	58.43	212.62	7.18	86.00
12	12	246.80	51.53	211.23	7.43	85.00
14	1	278.67	63.17	215.45	8.66	87.00
14	2	308.12	64.40	214.78	8.04	87.00
14	3	277.46	65.10	213.63	8.04	88.00
14	4	285.41	60.83	214.75	8.66	89.00
14	5	284.06	62.73	213.47	8.66	89.00
14	6	283.31	61.83	213.55	8.66	89.00
14	7	285.29	59.25	214.62	8.66	90.00
14	8	286.30	50.17	214.25	9.28	87.00
14	9	269.85	61.74	213.65	8.66	89.00
14	10	270.36	61.25	211.97	8.66	88.00
14	11	276.25	58.98	213.12	9.28	89.00
14	12	275.12	60.24	215.05	8.66	90.00

Table A1. Mechanical analysis of the chromium-free leather produced by adding different levels of Tara powder.

Level of Tara, %	Repetition	Tensile Strength, N/mm2	Percentage of Elongation, %	Apparent Density, kg∙cm−3	Flex Resistant, mm	Shrinkage Temperature up to 100 °C
10	1	336.30	64.67	208.74	7.43	85.00
10	2	334.09	62.10	211.45	7.92	89.00
10	3	340.13	65.67	209.41	7.67	89.00
10	4	345.54	65.53	208.70	8.91	88.00
10	5	347.59	62.73	210.23	10.40	88.00
10	6	342.62	67.00	209.34	9.41	89.00
10	7	344.51	65.37	208.66	9.90	88.00
10	8	338.53	68.40	209.47	8.42	88.00
10	9	342.56	61.00	207.33	9.16	86.00
10	10	337.59	61.17	204.31	10.15	87.00
10	11	332.30	67.77	207.42	8.66	89.00
10	12	3475.41	61.77	209.74	10.64	89.00
12	1	222.74	52.60	210.22	7.43	87.00
12	2	223.74	52.27	211.98	6.81	86.00
12	3	228.43	50.23	213.78	7.18	86.00
12	4	235.00	50.20	212.65	7.43	86.00
12	5	222.94	53.70	214.17	6.81	85.00
12	6	243.19	54.57	211.34	7.43	85.00
12	7	237.45	52.53	213.65	7.43	89.00
12	8	245.53	56.93	211.98	7.43	89.00
12	9	243.81	56.90	212.45	6.81	89.00
12	10	242.12	51.50	211.47	7.43	87.00
12	11	240.85	58.43	212.62	7.18	86.00
12	12	246.80	51.53	211.23	7.43	85.00
14	1	278.67	63.17	215.45	8.66	87.00
14	2	308.12	64.40	214.78	8.04	87.00
14	3	277.46	65.10	213.63	8.04	88.00
14	4	285.41	60.83	214.75	8.66	89.00
14	5	284.06	62.73	213.47	8.66	89.00
14	6	283.31	61.83	213.55	8.66	89.00
14	7	285.29	59.25	214.62	8.66	90.00
14	8	286.30	50.17	214.25	9.28	87.00
14	9	269.85	61.74	213.65	8.66	89.00
14	10	270.36	61.25	211.97	8.66	88.00
14	11	276.25	58.98	213.12	9.28	89.00
14	12	275.12	60.24	215.05	8.66	90.00

Table A2. Sensorial analysis of the chromium-free leather produced by adding different levels of Tara powder.

Level of Tara, %	Repetition	Fullness, Points	Roundness, Points	Fineness of Grain, Points
10	1	3	5	3
10	2	3	5	4
10	3	4	5	3
10	4	3	4	4
10	5	4	5	4
10	6	3	5	3
10	7	4	5	4
10	8	4	4	3
10	9	4	5	4
10	10	3	4	3
10	11	3	5	3
10	12	4	5	4
12	1	4	4	4
12	2	4	4	4
12	3	5	4	5
12	4	4	5	4
12	5	4	4	5
12	6	3	4	4
12	7	4	3	4
12	8	4	4	4
12	9	5	3	5
12	10	4	4	4
12	11	5	3	5
12	12	4	4	4
14	1	4	3	5
14	2	5	4	5
14	3	4	3	5
14	4	5	4	4
14	5	4	3	5
14	6	5	3	4
14	7	5	4	4
14	8	5	4	5
14	9	5	4	5
14	10	4	3	5
14	11	5	4	4
14	12	5	3	5

Table A2. Sensorial analysis of the chromium-free leather produced by adding different levels of Tara powder.

Level of Tara, %	Repetition	Fullness, Points	Roundness, Points	Fineness of Grain, Points
10	1	3	5	3
10	2	3	5	4
10	3	4	5	3
10	4	3	4	4
10	5	4	5	4
10	6	3	5	3
10	7	4	5	4
10	8	4	4	3
10	9	4	5	4
10	10	3	4	3
10	11	3	5	3
10	12	4	5	4
12	1	4	4	4
12	2	4	4	4
12	3	5	4	5
12	4	4	5	4
12	5	4	4	5
12	6	3	4	4
12	7	4	3	4
12	8	4	4	4
12	9	5	3	5
12	10	4	4	4
12	11	5	3	5
12	12	4	4	4
14	1	4	3	5
14	2	5	4	5
14	3	4	3	5
14	4	5	4	4
14	5	4	3	5
14	6	5	3	4
14	7	5	4	4
14	8	5	4	5
14	9	5	4	5
14	10	4	3	5
14	11	5	4	4
14	12	5	3	5

Table A3. Water quality parameters evaluated before and after the 14% Tara tanning process.

Parameter	Unit	Before Tanning	After Tanning
BOD	mgO2/L	225.50	1461.00
COD	mgO2/L	414.76	3150.30
Nitrates	mg/L	37.79	45.63
pH	-	6.73	6.88
Total Nitrates	mg/L	57.25	137.79
Sulfates	mg/L	521.87	411.25
Chromium hexavalent	mg/L	0	0
Total Chromium	mg/L	0	0

Table A3. Water quality parameters evaluated before and after the 14% Tara tanning process.

Parameter	Unit	Before Tanning	After Tanning
BOD	mgO2/L	225.50	1461.00
COD	mgO2/L	414.76	3150.30
Nitrates	mg/L	37.79	45.63
pH	-	6.73	6.88
Total Nitrates	mg/L	57.25	137.79
Sulfates	mg/L	521.87	411.25
Chromium hexavalent	mg/L	0	0
Total Chromium	mg/L	0	0

Table A4. Mass balances for the tanning process by using 10% Tara powder.

Formulation	Mathematical Calculation	Results
${\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}={\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{t}\mathrm{h}}_{\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{e}}\ast {\mathrm{N}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}}_{\mathrm{s}\mathrm{k}\mathrm{i}\mathrm{n}}$	${\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}=3\ast 30skin$	${\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}=90\mathrm{k}\mathrm{g}$
${\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{t}\mathrm{h}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}}{\mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e}}}$	${\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{90 \mathrm{k}\mathrm{g}}{13.33 \mathrm{h}}}$	${\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=6.75{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast {\mathrm{\%}\mathrm{f}}_{{\mathrm{H}}_2\mathrm{O}}}{100}}$	${\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{6.75\frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}\ast 40}{100}}$	${\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=2.70{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{S}\mathrm{D}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast {\mathrm{\%}\mathrm{f}}_{\mathrm{S}\mathrm{D}}}{100}}$	${\mathrm{S}\mathrm{D}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{6.75\frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}\ast 2}{100}}$	${\mathrm{S}\mathrm{D}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=0.13{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{S}\mathrm{f}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast {\mathrm{\%}\mathrm{S}\mathrm{f}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}}{100}}$	${\mathrm{S}\mathrm{f}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{6.75\frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}\ast 0.5}{100}}$	${\mathrm{S}\mathrm{f}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=0.0037{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast {\%f}_{Cs}}{100}}$	${\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{6.75\frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}\ast 10}{100}}$	${\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=0.675{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{E}\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast 4_{\mathrm{A}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}}}{100}}$	${\mathrm{E}\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=6.75{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}\ast 4$	${\mathrm{E}\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=2.70{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{A}\mathrm{A}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast {\mathrm{\%}\mathrm{f}}_{AA}}{100}}$	${\mathrm{A}\mathrm{A}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{6.75\frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}\ast 1.5}{100}}$	${\mathrm{A}\mathrm{A}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=0.10{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$

SD: Dispersant, Sf: Fenolic Acid, Cs: Caelsaspinia Spinosa, AA: Acid auxiliar.

Table A4. Mass balances for the tanning process by using 10% Tara powder.

Formulation	Mathematical Calculation	Results
${\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}={\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{t}\mathrm{h}}_{\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{e}}\ast {\mathrm{N}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}}_{\mathrm{s}\mathrm{k}\mathrm{i}\mathrm{n}}$	${\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}=3\ast 30skin$	${\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}=90\mathrm{k}\mathrm{g}$
${\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{t}\mathrm{h}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}}{\mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e}}}$	${\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{90 \mathrm{k}\mathrm{g}}{13.33 \mathrm{h}}}$	${\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=6.75{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast {\mathrm{\%}\mathrm{f}}_{{\mathrm{H}}_2\mathrm{O}}}{100}}$	${\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{6.75\frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}\ast 40}{100}}$	${\mathrm{W}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=2.70{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{S}\mathrm{D}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast {\mathrm{\%}\mathrm{f}}_{\mathrm{S}\mathrm{D}}}{100}}$	${\mathrm{S}\mathrm{D}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{6.75\frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}\ast 2}{100}}$	${\mathrm{S}\mathrm{D}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=0.13{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{S}\mathrm{f}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast {\mathrm{\%}\mathrm{S}\mathrm{f}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}}{100}}$	${\mathrm{S}\mathrm{f}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{6.75\frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}\ast 0.5}{100}}$	${\mathrm{S}\mathrm{f}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=0.0037{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast {\%f}_{Cs}}{100}}$	${\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{6.75\frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}\ast 10}{100}}$	${\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=0.675{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{E}\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast 4_{\mathrm{A}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}}}{100}}$	${\mathrm{E}\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=6.75{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}\ast 4$	${\mathrm{E}\mathrm{C}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=2.70{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$
${\mathrm{A}\mathrm{A}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{{\mathrm{S}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{s}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}\ast {\mathrm{\%}\mathrm{f}}_{AA}}{100}}$	${\mathrm{A}\mathrm{A}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}={\displaystyle \frac{6.75\frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}\ast 1.5}{100}}$	${\mathrm{A}\mathrm{A}}_{\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}=0.10{\displaystyle \frac{\mathrm{k}\mathrm{g}}{\mathrm{h}}}$

SD: Dispersant, Sf: Fenolic Acid, Cs: Caelsaspinia Spinosa, AA: Acid auxiliar.

The material balance was performed for the processing of 30 skins. Thus, the skin weight was first calculated based on the received skin for the primary operations. Then, the mass entrances to the drum for the tanning process were included in the water consumption (Water_flow), the mass of dispersant (SD_flow), the mass of phenolic acid (Sf_flow), the Tara powder mass (C_sflow), the basifying agent (ECs_flow), and the mass of acetic acid (AA_flow). Then, these quantities were divided by the operation time to obtain the flow.

Appendix A.3. Wastewater Treatment Plant Design

1. 

Equalization tank design

The equalization tank was designed accordingly for the water consumption of the tanning process, which is equal to 0.10 m³/day. The calculations for the design were as follows:

Two equalization tanks are necessary in case of maintenance.

Flow rate: $\mathrm{Q}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{w}\mathrm{w}}}{\mathrm{N}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{t}\mathrm{a}\mathrm{n}\mathrm{k}\mathrm{s}}}={\displaystyle \frac{0.10}{2}}=0.05{\displaystyle \frac{{\mathrm{m}}^3}{\mathrm{d}\mathrm{a}\mathrm{y}}}$

Volume: $V=Q\ast T$; assuming 2 h of equalization. $V=0.05\ast 2=0.10m^3$

Assuming a depth equal to 1 m (according to the design parameters and excluding 0.05 m free board) and a width of 0.25 m, the length of the tank is equal to

$$L={\displaystyle \frac{V}{H\times W}}={\displaystyle \frac{0.10}{1\times 0.25}}=0.40m$$

Air required. The air required is based on the necessities for the water quality. Therefore, it is necessary to use a mixing factor equal to a = 1.1 m³ aire/m³ water and a minimum air flux of diffuser, F_A = 12 m³/h [47]. Thus, the air flow is equal to

$$Q_a=V\times a=0.40\times 1.1=0.44\cong 0.50{\displaystyle \frac{m^3}{h}}$$

Therefore, the number of diffusers needed is equal to

$$N_d={\displaystyle \frac{Q_a}{N_d}}={\displaystyle \frac{0.50}{12}}=0.041\cong 1diffusor$$

The specifications of the equalization tank are summarized in Table A5.

Table A5. Technical specifications for the construction of the equalization tank to treat the leather wastewater.

Parameter	Value
Quantity	2
Flow Rate	0.05 m3/day
Detention Time	2 h
Volume	0.10 m3
Length	0.40 m
Width	0.25 m
Height	1 m
Free Board	0.05 m
Air Requirement	0.50 m3/h
Diffuser Quantity	1

Table A5. Technical specifications for the construction of the equalization tank to treat the leather wastewater.

Parameter	Value
Quantity	2
Flow Rate	0.05 m3/day
Detention Time	2 h
Volume	0.10 m3
Length	0.40 m
Width	0.25 m
Height	1 m
Free Board	0.05 m
Air Requirement	0.50 m3/h
Diffuser Quantity	1

2. 

Bar Screen Chamber

The wastewater flow is not considerably high, therefore, only two screens are going to be used; the first is a coarse screen and the second is a medium screen. Thus, some data for the coarse screen are going to be assumed according to the specifications reported by [48,49]. These assumptions are as follows: Length, L = 1000 mm, Width, W = 10 mm, Opening, O = 50 mm, Inclination, I = 45°, number of bar screen chambers, N = 4, velocity of the screen (v_s): 0.6 m/s, bear shape factor (B) = 2.42 for flat sheet bar, submerging depth (H) = 1 m. So, the flow rate of each chamber is equal to

$$Q={\displaystyle \frac{Q_{ww}}{N_c}}={\displaystyle \frac{0.10}{4}}=0.03{\displaystyle \frac{m^3}{d}}\times {\displaystyle \frac{1d}{86400}}={3.47\times 10}^{-7}{\displaystyle \frac{m^3}{s}}$$

Cross-section area of the channel, A_c:

$$A_c={\displaystyle \frac{Q}{v}}={\displaystyle \frac{{3.47\times 10}^{-7}}{0.6}}={5.78\times 10}^{-7}{m}^2$$

Gross-section of the channel, A_g:

$$A_g=A_c\times \left(1+{\displaystyle \frac{W}{O}}\right)={5.78\times 10}^{-7}+\left(1+{\displaystyle \frac{10}{50}}\right)=1.2m^2$$

Velocity above screen, v_as:

$$v_{as}=v_s\times {\displaystyle \frac{O}{O+W}}=0.6\times {\displaystyle \frac{50}{60}}=0.50{\displaystyle \frac{m}{s}}$$

Head loss, h_f:

$$h_f=B\times {\left({\displaystyle \frac{W}{O}}\right)}^{{\displaystyle \frac{3}{4}}}\times {\displaystyle \frac{{v_2}^2}{2g}}\times \sin I=2.42\times {\left({\displaystyle \frac{10}{50}}\right)}^{{\displaystyle \frac{3}{4}}}\times {\displaystyle \frac{{0.6}^2}{2\left(9.8\right)}}\times \sin 45=0.01m$$

Width of the channel, W_ch:

$$W_{ch}={\displaystyle \frac{A_g}{H}}={\displaystyle \frac{1.2}{1}}=1.2m\times {\displaystyle \frac{1000mm}{1m}}=1200mm$$

Number of bars, N_b:

$$N_b={\displaystyle \frac{W_{ch}}{W+O}}={\displaystyle \frac{1200}{50+10}}=20$$

While for the medium screen, the assumptions are as follows: Length, L = 1000 mm, Width, W = 10 mm, Opening, O = 20 mm, Inclination, I = 60°, number of bar screen chambers, N = 4, velocity of the screen (v_s): 0.6 m/s, bear shape factor (B) = 2.42 for flat sheet bar, submerging depth (H) = 1 m. The calculation is as follows:

Cross-section area of the channel, A_c:

$$A_c={\displaystyle \frac{Q}{v}}={\displaystyle \frac{{3.47\times 10}^{-7}}{0.6}}={5.78\times 10}^{-7}{m}^2$$

Gross-section of the channel, A_g:

$$A_g=A_c\times \left(1+{\displaystyle \frac{W}{O}}\right)={5.78\times 10}^{-7}+\left(1+{\displaystyle \frac{10}{20}}\right)=1.5m^2$$

Velocity above screen, v_as:

$$v_{as}=v_s\times {\displaystyle \frac{O}{O+W}}=0.6\times {\displaystyle \frac{20}{30}}=1.36{\displaystyle \frac{m}{s}}$$

Head loss, h_f:

$$h_f=B\times {\left({\displaystyle \frac{W}{O}}\right)}^{{\displaystyle \frac{3}{4}}}\times {\displaystyle \frac{{v_2}^2}{2g}}\times \sin I=2.42\times {\left({\displaystyle \frac{10}{20}}\right)}^{{\displaystyle \frac{3}{4}}}\times {\displaystyle \frac{{0.6}^2}{2\left(9.8\right)}}\times \sin 60=0.02m$$

Width of the channel, W_ch:

$$W_{ch}={\displaystyle \frac{A_g}{H}}={\displaystyle \frac{1.36}{1}}=1.36m\times {\displaystyle \frac{1000mm}{1m}}=1360mm$$

Number of bars, N_b:

$$N_b={\displaystyle \frac{W_{ch}}{W+O}}={\displaystyle \frac{1360}{50+20}}=19.42\approx 20$$

The principal features about the construction of the bar screen chamber are summarized in Table A6.

Table A6. Technical specifications for the construction of the bar screen chamber to treat the leather wastewater.

Parameter	Coarse Screen	Medium Screen
Quantity of Screen	4	4
Chamber Flow	29.98 L/day	29.98 L/day
Rate Length of Bar	1000 mm	1000 mm
Width of Bar	10 mm	10 mm
Opening Inclination	50	20
Cross-section Area of Channel	${5.78\times 10}^{-7}{\mathrm{m}}^2$	${5.78\times 10}^{-7}{\mathrm{m}}^2$
Gross Area of Channel	$1.2{\mathrm{m}}^2$	$1.5{\mathrm{m}}^2$
Head Loss	$0.01\mathrm{m}$	$0.02\mathrm{m}$
Submergence Depth	1 m	1 m
Width of Channel	$1.2\mathrm{m}$	$1.36\mathrm{m}$
Quantity of Bar	20	20

Table A6. Technical specifications for the construction of the bar screen chamber to treat the leather wastewater.

Parameter	Coarse Screen	Medium Screen
Quantity of Screen	4	4
Chamber Flow	29.98 L/day	29.98 L/day
Rate Length of Bar	1000 mm	1000 mm
Width of Bar	10 mm	10 mm
Opening Inclination	50	20
Cross-section Area of Channel	${5.78\times 10}^{-7}{\mathrm{m}}^2$	${5.78\times 10}^{-7}{\mathrm{m}}^2$
Gross Area of Channel	$1.2{\mathrm{m}}^2$	$1.5{\mathrm{m}}^2$
Head Loss	$0.01\mathrm{m}$	$0.02\mathrm{m}$
Submergence Depth	1 m	1 m
Width of Channel	$1.2\mathrm{m}$	$1.36\mathrm{m}$
Quantity of Bar	20	20

3. 

Aeration tank design

The wastewater flow is not considerably high, therefore, only two aeration tanks are going to be used. Moreover, the initial BOD (Y_i) is equal to 1461 mg/L and the desired final BOD (Y_f) may be equal to 220 mg/L, assuming a BOD reduction of 85%. Thus, some data for the aeration tank are going to be assumed according to the specifications reported by [48]. These assumptions are as follows: Mixed liquor suspended solids (MLSS) = 600 mg/L, food-to-microorganism ratio (F/M) = 0.12, Width, W = 1.5 m, Depth (H) = 2 m (excluding 0.5 m free board), α = 1.0 and K = 0.06, minimum air flux of diffuser, and A_F = 12 m³/hour. So, the flow rate of each aeration tank is equal to

$$Q={\displaystyle \frac{Q_{ww}}{N_c}}={\displaystyle \frac{0.10}{2}}=0.05{\displaystyle \frac{m^3}{day}}$$

Volume of the aeration tank

$$V={\displaystyle \frac{Q\times {BOD}_i}{\frac{F}{M}\times MLSS}}={\displaystyle \frac{0.05\times 1461}{1500\times 0.12}}=1m^3$$

Length, L

$$L={\displaystyle \frac{V}{W\ast H}}={\displaystyle \frac{1}{1.5\ast 2}}=0.33m$$

Aeration Period, A_r

$$A_r={\displaystyle \frac{V}{Q\ast 2.4}}={\displaystyle \frac{1}{0.05\ast 24}}=8.33hours\approx 9hours$$

Solid retention time, SRT

$$V\times MLSS={\displaystyle \frac{\propto \times Q\left(Y_f-Y_i\right)\times SRT}{1+K\times SRT}}=1\times 600={\displaystyle \frac{1\times 0.05\left(1461-220\right)\times SRT}{1+0.06\times SRT}}=23days$$

BOD load per hour, Y_D

$$Y_D=\left(Q\times 1000\right)\times {\displaystyle \frac{Y_i}{{T_A\times 10}^3}}=\left(0.05\times 1000\right)\times {\displaystyle \frac{1461}{50\times {10}^3}}=1.46{\displaystyle \frac{kg}{h}}$$

Air requirement for reduced the BOD, A_B

$$A_B=Y_D\times 115=1.46\times 115=167.90{\displaystyle \frac{m^3}{h}}$$

Air requirement for mixing, A_M

$$A_M=V\times 1.10=1\times 1.10=1.10{\displaystyle \frac{m^3}{h}}$$

Number of diffusers, N_D

$$N_D={\displaystyle \frac{A_B}{A_F}}={\displaystyle \frac{167.90}{12}}=13.99\cong 14$$

The principal features about the design of the aeration tank are summarized in Table A7.

Table A7. Technical specifications for the construction of the primary aeration tank to treat the leather wastewater.

Parameter	Value
Quantity	2
Flow Rate	0.05 m3/day
Aeration Period	9 h
Length	0.33 m
Width	1.5 m
Depth	2 m
Free Board	0.5 m
Solids Retention Time	23 days
Air Requirement	169 m3/hour
Air Flux of Diffuser	12 m3/hour
Number of diffusers	14

Table A7. Technical specifications for the construction of the primary aeration tank to treat the leather wastewater.

Parameter	Value
Quantity	2
Flow Rate	0.05 m3/day
Aeration Period	9 h
Length	0.33 m
Width	1.5 m
Depth	2 m
Free Board	0.5 m
Solids Retention Time	23 days
Air Requirement	169 m3/hour
Air Flux of Diffuser	12 m3/hour
Number of diffusers	14

4. 

Clarifier design

The wastewater flow is not considerably high, therefore, only 2 clarifier tanks are going to be used. Moreover, some data for the clarifier are going to be assumed according to the specifications reported by [48]. These assumptions are as follows: Mixed liquor suspended solids (MLSS_c) = 2000 mg/L, detention time (T_d) = 2 h, Bottom slope factor (S_F) = 0.083, weir loading (W_L) = 250 m³/m²·day, over flow rate (O_A) = 0.24 m³/m²·day. So, the flow rate of each clarifier is equal to

$$Q={\displaystyle \frac{Q_{ww}}{N_c}}={\displaystyle \frac{0.10}{2}}=0.05{\displaystyle \frac{m^3}{day}}$$

Recycling ratio, R

$$R={\displaystyle \frac{MLSS}{MLSS-{MLSS}_C}}={\displaystyle \frac{600}{2000-600}}=0.43$$

Total Inflow, Q_i

$$Q_i=Q\times \left(1+R\right)=0.05\times \left(1+0.43\right)=0.07{\displaystyle \frac{m^3}{day}}$$

Surface Area, A

$$A={\displaystyle \frac{Q_{ww}}{O_A}}={\displaystyle \frac{0.05}{0.24}}=0.21m^2$$

Diameter, D

$$D=\sqrt{{\displaystyle \frac{4A}{\pi }}}=\sqrt{{\displaystyle \frac{4\left(0.21\right)}{\pi }}}=0.52m$$

Volume, V

$$V={\displaystyle \frac{Q\times T}{O_A}}={\displaystyle \frac{0.05\times 2}{0.24}}=0.40m^3$$

Side wall height, H

$$H={\displaystyle \frac{V}{A}}={\displaystyle \frac{0.40}{0.21}}=1.90m$$

Bottom slope, S_L

$$S_L={H\times S}_F=1.90\times 0.083=0.15m$$

Center height, C_H

$$C_H=H+S_L=1.90+0.15=2.05m$$

The principal features about the design of the clarifier are summarized in Table A8.

Table A8. Technical specifications for the construction of the clarifier to treat the leather wastewater.

Parameter	Value
Quantity	2
Flow Rate	0.05 m3/day
Detection time	2 h
Over flow rate at average flowrate	0.24 m3/m2·day
Surface Area	0.21 m2
Diameter	0.52 m
Side Wall Height	1.90 m
Center Height Free Board	2.05 m
Weir Loading	250 m3/m2·day

Table A8. Technical specifications for the construction of the clarifier to treat the leather wastewater.

Parameter	Value
Quantity	2
Flow Rate	0.05 m3/day
Detection time	2 h
Over flow rate at average flowrate	0.24 m3/m2·day
Surface Area	0.21 m2
Diameter	0.52 m
Side Wall Height	1.90 m
Center Height Free Board	2.05 m
Weir Loading	250 m3/m2·day

5. 

Aeration tank design

The wastewater flow is not considerably high, therefore, only 2 aeration tanks are going to be used. Moreover, some data for the clarifier are going to be assumed according to the specifications reported by [48]. These assumptions are as follows: Mixed liquor suspended solids (MLSS_c) = 2000 mg/L, detention time (T_d) = 2 h, Bottom slope factor (S_F) = 0.083, weir loading (W_L) = 250 m³/m²·day, and over flow rate (O_A) = 0.24 m³/m²·day. So, the flow rate of each aeration tank is equal to

$$Q={\displaystyle \frac{Q_{ww}}{N_c}}={\displaystyle \frac{0.10}{2}}=0.05{\displaystyle \frac{m^3}{day}}$$

Recycling ratio, R

$$R={\displaystyle \frac{MLSS}{MLSS-{MLSS}_C}}={\displaystyle \frac{600}{2000-600}}=0.43$$

Total Inflow, Q_i

$$Q_i=Q\times \left(1+R\right)=0.05\times \left(1+0.43\right)=0.07{\displaystyle \frac{m^3}{day}}$$

Surface Area, A

$$A={\displaystyle \frac{Q_{ww}}{O_A}}={\displaystyle \frac{0.05}{0.24}}=0.21m^2$$

Diameter, D

$$D=\sqrt{{\displaystyle \frac{4A}{\pi }}}=\sqrt{{\displaystyle \frac{4\left(0.21\right)}{\pi }}}=0.52m$$

Volume, V

$$V={\displaystyle \frac{Q\times T}{O_A}}={\displaystyle \frac{0.05\times 2}{0.24}}=0.40m^3$$

Side wall height, H

$$H={\displaystyle \frac{V}{A}}={\displaystyle \frac{0.40}{0.21}}=1.90m$$

Bottom slope, S_L

$$S_L={H\times S}_F=1.90\times 0.083=0.15m$$

Center height, C_H

$$C_H=H+S_L=1.90+0.15=2.05m$$

The principal features about the design of the clarifier are summarized in Table A9.

Table A9. Technical specifications for the construction of the aeration tank to treat the leather wastewater.

Parameter	Value
Quantity	2
Flow Rate	0.05 m3/day
Aeration Period	9 h
Length	0.33 m
Width	1.5 m
Depth	2 m
Free Board	0.5 m
Solids Retention Time	50 days
Air Requirement	169 m3/hour
Air Flux of Diffuser	12 m3/hour
Number of diffusers	14

Table A9. Technical specifications for the construction of the aeration tank to treat the leather wastewater.

Parameter	Value
Quantity	2
Flow Rate	0.05 m3/day
Aeration Period	9 h
Length	0.33 m
Width	1.5 m
Depth	2 m
Free Board	0.5 m
Solids Retention Time	50 days
Air Requirement	169 m3/hour
Air Flux of Diffuser	12 m3/hour
Number of diffusers	14

6. 

Coagulation/flocculation tank design

The wastewater flow is not considerably high, therefore, only one coagulation and flocculation tank are going to be used. Moreover, some data for the tanks are going to be assumed according to the specifications reported by [48]. These assumptions are as follows: Rapid mixing for the addition of the coagulant, coagulant (iron chloride, 500 mg/L of wastewater), velocity gradient (G) = 800 s⁻¹, rapid mixing detection time: 120 s, rotational speed (v) = 45 rpm (0.75 rps), power (P) = 0.56 kW, efficiency = 80%, water viscosity (μ) at 18 °C = 1.053 × 10⁻³ Pa·s, water density ($\rho$) = 1000 kg/m³, mixer = turbine with six flat blades, and constant of blade turbine (K_T) = 63. So, the volume of the tank (V) is equal to

$$V_T={\displaystyle \frac{P}{\mu \times G^2}}={\displaystyle \frac{560}{0.001\times {800}^2}}=0.88m^3$$

Impeller Diameter, D_I

$$D_i=\left({\displaystyle \frac{P}{K_{T}v\rho }}\right)={\displaystyle \frac{560}{63\times 0.75\times 1000}}=0.41m$$

Tank Diameter, D_T

$$D_T={\displaystyle \frac{I_D}{Ratioimpellertotankdiameter}}={\displaystyle \frac{0.44}{0.33}}=1.33m$$

Tank Surface Area, A_T

$$A_T={\displaystyle \frac{{{\pi D}_T}^2}{4}}={\displaystyle \frac{\pi \times {1.33}^2}{4}}=1.40m^2$$

Tank Depth, H

$$H={\displaystyle \frac{V_T}{D_T}}={\displaystyle \frac{0.88}{1.40}}=0.63m$$

The following design parameters were chosen for the flocculation tank design: velocity gradient (G₁) = 40 s⁻¹, G₂ = 20 s⁻¹, and G₃ = 10 s⁻¹, type: paddle wheel, type of flow = cross–flow horizontal- shaft, retention time (T): 3600 s, speed gradient (G_T) = 10,000, number of tapered flocculation = 3, and the basin is to have a width (W) of 0.27 m to adjoin an existing basin. The paddle wheels are to have blades with a 6 in width and length (L) of 0.01 m. The outside blades should clear the floor by 0.30 m and be 0.30 m below the water surface and seven wheels. There are to be six blades per paddle wheel. The wall clearance is 0.30. So, the volume of the tank (V) is equal to

$$V_C=Q\times T=1.15\times {10}^{-4}\times 3600=0.42m^3$$

Area of the profile section, A_s

$$A_S={\displaystyle \frac{V}{W}}={\displaystyle \frac{0.42}{0.27}}=1.55m^2$$

Tank height, H

$$H={\displaystyle \frac{A_S}{W}}={\displaystyle \frac{1.55}{0.27}}=5.76m$$

Separation between wheels, s

$$s={\displaystyle \frac{W-(numberofblades\times L)}{numberofblades}}={\displaystyle \frac{0.27-(6\times 0.01)}{6}}=0.04m$$

Power in each compartment, P

First compartment, P₁

$$P_1=\mu V{G}^2=0.002\times 0.42\times {40}^2=13.44W$$

Second compartment, P₂

$$P_2=\mu V{G}^2=0.002\times 0.42\times {20}^2=3.36W$$

Third compartment, P₃

$$P_3=\mu V{G}^2=0.002\times 0.42\times {10}^2=0.84W$$

Total Power, P_t

$$P_t=P_1+P_2+P_3=13.44+3.36+0.84=17.64W$$

The principal features about the design of the flocculation/coagulation tank are summarized in Table A10.

Table A10. Technical specifications for the construction of the coagulation/flocculation tank to treat the leather wastewater.

Parameter	Value
Rapid Mixing
Quantity	1
Flow Rate	0.05 m3/day
Coagulant	FeCl3
Concentration of coagulant	500 mg/L of wastewater
Velocity gradient	800 s−1
Detection time	120 s
Rotational speed	0.75 rps
Power	0.56 kW
Mixer type	Turbine with six flat blades
Volume of the tank	0.88m3
Impeller diameter	0.41 m
Tank diameter	1.33 m
Surface Area	1.40 m2
Depth	0.63 m
Flocculation Tank
Type	Paddle wheel
Type of flow	Cross–flow horizontal- shaft
Retention time	3600 s
Speed gradient	10,000 s−1
Number of tapered flocculation	3
width	0.27 m
Paddle wheels type	Blades with a 6-in
Length	0.01 m
Volume	0.42 m3
Height	5.76 m
Separation of the wheels	0.04 m
Power	17.64 W

Table A10. Technical specifications for the construction of the coagulation/flocculation tank to treat the leather wastewater.

Parameter	Value
Rapid Mixing
Quantity	1
Flow Rate	0.05 m3/day
Coagulant	FeCl3
Concentration of coagulant	500 mg/L of wastewater
Velocity gradient	800 s−1
Detection time	120 s
Rotational speed	0.75 rps
Power	0.56 kW
Mixer type	Turbine with six flat blades
Volume of the tank	0.88m3
Impeller diameter	0.41 m
Tank diameter	1.33 m
Surface Area	1.40 m2
Depth	0.63 m
Flocculation Tank
Type	Paddle wheel
Type of flow	Cross–flow horizontal- shaft
Retention time	3600 s
Speed gradient	10,000 s−1
Number of tapered flocculation	3
width	0.27 m
Paddle wheels type	Blades with a 6-in
Length	0.01 m
Volume	0.42 m3
Height	5.76 m
Separation of the wheels	0.04 m
Power	17.64 W

7. 

Sedimentation tank design

The wastewater flow is not considerably high, therefore, only two sedimentation tanks are going to be used. Moreover, some data for the tanks are going to be assumed according to the specifications reported by ref. [48]. These assumptions are as follows: type = horizontal flow—rectangular tank, depth = 3.5 m, length-to-depth ratio = 6:1, surface loading rate = 250 m³/m²·day, horizontal mean velocity = 0.05 m/s, detection time = 5 min, density of the particle (${\rho }_p$) = 1500 kg/m³, minimum size particle to be removed = 60 mm, drag coefficient (C_d) = 0.44, and bottom slope = 1:100 m/m. So, the volume of the tank (V) is equal to

$$V_T=Q\times t=0.05\times 300=15m^3$$

Surface Area, A_s

$$A_s={\displaystyle \frac{V}{H}}={\displaystyle \frac{15}{3.5}}=4.28m^2$$

Length, L

$$L={\displaystyle \frac{A_S}{H}}={\displaystyle \frac{4.28}{3.5}}=1.22m$$

Width, W

$$W={\displaystyle \frac{V}{L\times H}}={\displaystyle \frac{15}{1.22\times 3.5}}=3.51m$$

Surface overflow rate, SOR

$$SOR={\displaystyle \frac{Q}{L\times W}}={\displaystyle \frac{0.05}{3.51\times 1.22}}=0.01{\displaystyle \frac{m}{s}}$$

Settling velocity, V_s

$$V_s=\sqrt{{\displaystyle \frac{4\left({\rho }_p-{\rho }_W\right)g{d}_p}{3{\rho }_W{C}_d}}}=\sqrt{{\displaystyle \frac{4\left(1500-1000\right)\times 9.8\times 0.06}{3\times 1000\times 0.44}}}=0.95{\displaystyle \frac{m}{s}}$$

Percentage of removal

$$\%Removal={\displaystyle \frac{V_s}{SOR}}={\displaystyle \frac{0.95}{0.01}}=95\%$$

Sludge generated, S_g

$$\begin{array}{l}{S}_g={COD}_i\times \%Removal=3150.30\ast 0.95\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}=2992{\displaystyle \frac{\mathrm{m}\mathrm{g}}{L}}\times {\displaystyle \frac{1kg}{1000mg}}\times {\displaystyle \frac{1000L}{1m^3}}\times {\displaystyle \frac{0.05m^3}{s}}\times 3600s=538kg\end{array}$$

The principal features about the design of the sedimentation tank are summarized in Table A11.

Table A11. Technical specifications for the construction of the sedimentation tank to treat the leather wastewater.

Parameter	Value
Quantity	2
Type	horizontal flow—rectangular tank
Flow Rate	0.05 m3/day
Volume	15 m3
Depth	3.5 m
Surface loading rate	250 m3/m2·day
Horizontal mean velocity	0.05 m/s
Velocity gradient	800 s−1
Detection time	300 s
Surface area	4.28 m2
Length	1.22 m
Width	3.51 m
Surface overflow rate	0.01 m/s
Settling velocity	0.95 m/s
Percentage of removal	95%
Sludge generated	538 kg/day
Separation of the wheels	0.04 m
Power	17.64 W

Table A11. Technical specifications for the construction of the sedimentation tank to treat the leather wastewater.

Parameter	Value
Quantity	2
Type	horizontal flow—rectangular tank
Flow Rate	0.05 m3/day
Volume	15 m3
Depth	3.5 m
Surface loading rate	250 m3/m2·day
Horizontal mean velocity	0.05 m/s
Velocity gradient	800 s−1
Detection time	300 s
Surface area	4.28 m2
Length	1.22 m
Width	3.51 m
Surface overflow rate	0.01 m/s
Settling velocity	0.95 m/s
Percentage of removal	95%
Sludge generated	538 kg/day
Separation of the wheels	0.04 m
Power	17.64 W

8. 

Treatment of the generated sludge

8.1. Gravity thickener

The sludge generated during the sedimentation process is going to be considered as the parameter of the design for the gravity thickener. There is going to be only one tank considered. Moreover, some data for the tanks are going to be assumed according to the specifications reported by [48]. These assumptions are as follows: type = rectangular tank, detention time: 3 h, total solids (S_TSS) = 200 kg/day, solids loading rate for mixed sludge (SLR) = 50 kg/ TS/m²·day, removal efficiency = 90%, sludge influent (Q_s) = 538 kg/day, hydraulic loading rate, (HLR) = 20 m³/m²·day, Side Wall height, (H) = 1.5 m, bottom slope factor, S_F = 0.083. So, the area of the tank (A) is equal to

$$A={\displaystyle \frac{S_{TSS}}{SLR}}={\displaystyle \frac{200}{50}}=4m^2$$

Dilution water, (W)

$$W=\left(HLR\times A\right)+Q_S=\left(20\times 4\right)+538=618m^3$$

Total flow, (Q_t)

$$Q_t=Q_s+W=618+538=1156m^3$$

Diameter, (D)

$$D=\sqrt{{\displaystyle \frac{4A}{\pi }}}=\sqrt{{\displaystyle \frac{4\times 4}{\pi }}}=2.25m$$

Bottom slope, S_L

$$S_L=H\times S_F=1.5\times 0.083=0.13m$$

Center height, H_C

$$H_C=H+S_L=1.50+0.13=1.63m$$

Volume, V

$$V=A\times H=4\times 1.5=6m^3$$

The principal features about the design of the thickener tank are summarized in Table A12.

Table A12. Technical specifications for the construction of the thickener to treat the sludge.

Parameter	Value
Quantity	2
Type	rectangular tank
Flow Rate	538 kg/day
Retention time	3 h
Surface area	4 m2
Diameter	2.25 m
Horizontal mean velocity	0.05 m/s
Side Wall Height	1.5 m
Center Height	1.63 m
Efficiency	90%

Table A12. Technical specifications for the construction of the thickener to treat the sludge.

Parameter	Value
Quantity	2
Type	rectangular tank
Flow Rate	538 kg/day
Retention time	3 h
Surface area	4 m2
Diameter	2.25 m
Horizontal mean velocity	0.05 m/s
Side Wall Height	1.5 m
Center Height	1.63 m
Efficiency	90%

Table A13. Heat balances for the tanning process by using 10% Tara powder.

Conversion of Power Units (hp to kw)	Time Conversion Units (Hours to Seconds)	Formulation $\mathit{W}=\dot{\mathit{W}} \mathbf{\ast } \mathit{t}$	Engine Work (kJ)
$0.5 \sout{\mathrm{hp}}\ast {\displaystyle \frac{1.341 \mathrm{k}\mathrm{w}}{1\sout{\mathrm{hp}}}}=0.6705 \mathrm{k}\mathrm{w}$	$13.3 \sout{\mathrm{h}}\ast {\displaystyle \frac{3600 \mathrm{s}}{1 \sout{\mathrm{h}}}}=47880 \mathrm{s}$	${\mathrm{W}}_{\mathrm{m}}=1.341 {\displaystyle \raisebox{1ex}{$\mathrm{k}\mathrm{J}$}\!\left/ \!\raisebox{-1ex}{$\sout{\mathrm{s}}$}\right.}\ast 47880 \sout{\mathrm{s}}$	${\mathrm{W}}_{\mathrm{m}}=64207.08 \mathrm{k}\mathrm{J}$
Observations: 1 hp = 746 W
Formulation	Variable Substitution	Results
$∆{\dot{\mathrm{H}}}_{1-2}={\displaystyle \frac{64207.08 \mathrm{k}\mathrm{J}}{13.33 \mathrm{h}}}$	$∆{\dot{\mathrm{H}}}_{1-2}={\displaystyle \frac{∆{\mathrm{H}}_{1-2}}{13.33 \mathrm{h}}}$	$∆{\dot{\mathrm{H}}}_{1-2}=4816.74 {\displaystyle \frac{\mathrm{k}\mathrm{J}}{\mathrm{h}}}$
$∆{\dot{\mathrm{H}}}_{\left(1-2\right)\mathrm{e}}={\displaystyle \frac{∆{\dot{\mathrm{H}}}_{1-2}}{{\mathrm{f}}_{\mathrm{m}}}}$	$∆{\dot{\mathrm{H}}}_{\left(1-2\right)\mathrm{e}}={\displaystyle \frac{4816.74 \raisebox{1ex}{$\mathrm{k}\mathrm{J}$}\!\left/ \!\raisebox{-1ex}{$\sout{\mathrm{h}}$}\right.}{11.304 \raisebox{1ex}{$\mathrm{k}\mathrm{g}$}\!\left/ \!\raisebox{-1ex}{$\sout{\mathrm{h}}$}\right.}}$	$∆{\dot{\mathrm{H}}}_{\left(1-2\right)\mathrm{e}}=426.109{\displaystyle \frac{\mathrm{k}\mathrm{J}}{\mathrm{h}}}$
Observations: ${\mathrm{f}}_{\mathrm{m}}$ = Mass flow

Table A13. Heat balances for the tanning process by using 10% Tara powder.

Conversion of Power Units (hp to kw)	Time Conversion Units (Hours to Seconds)	Formulation $\mathit{W}=\dot{\mathit{W}} \mathbf{\ast } \mathit{t}$	Engine Work (kJ)
$0.5 \sout{\mathrm{hp}}\ast {\displaystyle \frac{1.341 \mathrm{k}\mathrm{w}}{1\sout{\mathrm{hp}}}}=0.6705 \mathrm{k}\mathrm{w}$	$13.3 \sout{\mathrm{h}}\ast {\displaystyle \frac{3600 \mathrm{s}}{1 \sout{\mathrm{h}}}}=47880 \mathrm{s}$	${\mathrm{W}}_{\mathrm{m}}=1.341 {\displaystyle \raisebox{1ex}{$\mathrm{k}\mathrm{J}$}\!\left/ \!\raisebox{-1ex}{$\sout{\mathrm{s}}$}\right.}\ast 47880 \sout{\mathrm{s}}$	${\mathrm{W}}_{\mathrm{m}}=64207.08 \mathrm{k}\mathrm{J}$
Observations: 1 hp = 746 W
Formulation	Variable Substitution	Results
$∆{\dot{\mathrm{H}}}_{1-2}={\displaystyle \frac{64207.08 \mathrm{k}\mathrm{J}}{13.33 \mathrm{h}}}$	$∆{\dot{\mathrm{H}}}_{1-2}={\displaystyle \frac{∆{\mathrm{H}}_{1-2}}{13.33 \mathrm{h}}}$	$∆{\dot{\mathrm{H}}}_{1-2}=4816.74 {\displaystyle \frac{\mathrm{k}\mathrm{J}}{\mathrm{h}}}$
$∆{\dot{\mathrm{H}}}_{\left(1-2\right)\mathrm{e}}={\displaystyle \frac{∆{\dot{\mathrm{H}}}_{1-2}}{{\mathrm{f}}_{\mathrm{m}}}}$	$∆{\dot{\mathrm{H}}}_{\left(1-2\right)\mathrm{e}}={\displaystyle \frac{4816.74 \raisebox{1ex}{$\mathrm{k}\mathrm{J}$}\!\left/ \!\raisebox{-1ex}{$\sout{\mathrm{h}}$}\right.}{11.304 \raisebox{1ex}{$\mathrm{k}\mathrm{g}$}\!\left/ \!\raisebox{-1ex}{$\sout{\mathrm{h}}$}\right.}}$	$∆{\dot{\mathrm{H}}}_{\left(1-2\right)\mathrm{e}}=426.109{\displaystyle \frac{\mathrm{k}\mathrm{J}}{\mathrm{h}}}$
Observations: ${\mathrm{f}}_{\mathrm{m}}$ = Mass flow

Table A14. CAPEX analysis for the construction of the proposed wastewater treatment plant.

Item	Volume	Unit	Price Unit, USD	Total Price, USD
Warehouse	10	m2	29.89	298.90
Pipes	30	m	15.00	450
Pipes accessories	10	unit	12.00	120
Equalization tank	2	unit	1500	3000
Grit chamber	4	unit	150	4600
Clarifier	2	unit	1000	2000
Aeration tank	2	unit	2000	4000
Coagulation/flocculation tank	1	unit	3000	3000
Sedimentation tank	2	unit	1500	3000
Gravity thickener	1	unit	2500	2500
Pumps and control panel	2	unit	850	1700
Electrical installations	1	unit	590	590
Total				25,258.50

Table A14. CAPEX analysis for the construction of the proposed wastewater treatment plant.

Item	Volume	Unit	Price Unit, USD	Total Price, USD
Warehouse	10	m2	29.89	298.90
Pipes	30	m	15.00	450
Pipes accessories	10	unit	12.00	120
Equalization tank	2	unit	1500	3000
Grit chamber	4	unit	150	4600
Clarifier	2	unit	1000	2000
Aeration tank	2	unit	2000	4000
Coagulation/flocculation tank	1	unit	3000	3000
Sedimentation tank	2	unit	1500	3000
Gravity thickener	1	unit	2500	2500
Pumps and control panel	2	unit	850	1700
Electrical installations	1	unit	590	590
Total				25,258.50

Table A15. OPEX analysis for the construction of the proposed wastewater treatment plant per month.

Item	Volume	Unit	Price Unit, USD	Total Price, USD
NaOH 50%	70	L	1.50	105
FeCl3	970	kg	0.30	291
Electricity	20	kW	4.65	93
Maintenance	1	unit	250	250
Total				739

Table A15. OPEX analysis for the construction of the proposed wastewater treatment plant per month.

Item	Volume	Unit	Price Unit, USD	Total Price, USD
NaOH 50%	70	L	1.50	105
FeCl3	970	kg	0.30	291
Electricity	20	kW	4.65	93
Maintenance	1	unit	250	250
Total				739

Table A16. Sensibility analysis to determinate the maximum cost/benefit in the production of chromium-free leather.

Leather Price, dm2	Cost/Benefit, USD	Tara Cost, USD/kg	Cost/Benefit, USD	Wastewater Treatment Cost, USD/lt	Cost/Benefit, USD
0.3	1	0.8	1.1287	0.4	1.077
0.35	1.03	1.2	1.03	0.5	1.03
0.4	1.08	1.45	0.97	0.6	1.01
0.5	1.1	1.5	0.82	0.7	0.99

Table A16. Sensibility analysis to determinate the maximum cost/benefit in the production of chromium-free leather.

Leather Price, dm2	Cost/Benefit, USD	Tara Cost, USD/kg	Cost/Benefit, USD	Wastewater Treatment Cost, USD/lt	Cost/Benefit, USD
0.3	1	0.8	1.1287	0.4	1.077
0.35	1.03	1.2	1.03	0.5	1.03
0.4	1.08	1.45	0.97	0.6	1.01
0.5	1.1	1.5	0.82	0.7	0.99