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Article

Shear Enhanced Flotation Separation Technology in Winery Wastewater Treatment

by
David Vlotman
*,
David Key
,
Bradley Cerff
and
Bernard Jan Bladergroen
Department of Chemistry, South African Institute for Advanced Materials Chemistry (SAIAMC), Faculty of Natural Sciences, University of the Western Cape, Cape Town 7535, South Africa
*
Author to whom correspondence should be addressed.
Water 2023, 15(13), 2409; https://doi.org/10.3390/w15132409
Submission received: 3 May 2023 / Revised: 23 June 2023 / Accepted: 26 June 2023 / Published: 29 June 2023
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Abstract

:
The process of wine making is well known to produce large amounts of wastewater with highly variable characteristics. The disposal of untreated winery wastewater is strictly prohibited since it adversely affects the recipient environment. Due to the variability in characteristics of winery wastewater, developing a treatment system which can handle high organic and inorganic loads, especially during the vintage season, is a complex challenge. This study investigated the theory, methodology and implementation of a wastewater treatment technology called shear enhanced flotation separation (SEFS) as a potential primary treatment stage towards the treatment of winery wastewater. Winery effluent was subjected to a coagulation process in a high shear environment, with and without the introduction of air, followed by flocculation. Upon successful optimization of operating parameters, a polymeric-based coagulant AB121 and polyelectrolyte flocculant AB796 yielded the highest reduction in turbidity (95%) with typical values of 630 NTU for the raw wastewater and 25 NTU for the SEFS-treated effluent. A substantial reduction in total suspended solids (97%) was achieved with average raw winery wastewater values of 2275 mg/L compared to the 50 mg/L obtained for the SEFS-treated effluent. Furthermore, a notable reduction (54%) in COD (from 11,250 mg/L to 5220 mg/L) using SEFS technology was achieved.

1. Introduction

Water is a vital resource used throughout the wine-making process ranging from vineyard operations to bottled wine. It is used as a cooling medium, for cleaning, sanitation, sterilization and rinsing [1]. Water scarcity continues to be a serious concern for all types of wineries and while water costs continue to rise, it can undermine a winery’s efforts to remain economically sustainable. During wine making, large volumes of fresh water are used, significant amounts of which become wastewater (50% for small wineries and 80% for medium/large wineries) [2].
Winery wastewater results from the matrix of freshwater usages during wine production and is typically generated from washing operations during grape harvesting, pressing, fermentation as well cleaning of processing equipment and bottles. The wastewater contaminants generated by wineries include unused grape residues and juice, winemaking products such as alcohol and sugars and chemicals such as caustic cleaning agents [3]. According to a study conducted by Van Schoor et al. (2005), the major component of wastewater from wineries is attributed to water used for cleaning processes, which makes up approximately 78% of the wastewater generated [4]. Cleaning and rinse waters are classified as either caustic or acidic. Caustic cleaning agents are intended to dissolve solid deposits of tartrate, pigments, tannins and proteins in processing tanks, whereas acidic cleaning agents (dilute citric and tartaric acid) are typically used to remove caustic residues. Following the use of caustic and acidic chemicals, fresh water is then used to rinse away any traces of cleaning agents [5].
Winery wastewaters are usually characterized by their low pH (pH 4–5) which is primarily ascribed to the presence of organic acids, namely tartaric, lactic, citric, succinic and malic acid [6,7]. The electrical conductivity (EC) of winery wastewater is reported to range between 1.62 and 6.15 mS/cm [8]. Total phosphorous and total nitrogen nutrient levels in winery wastewater range from 240–657 mg/L and 100–640 mg/L, respectively [9,10]. The chemical oxygen demand (COD) of winery wastewaters is used to quantify organic pollution in wastewater and mostly results from highly soluble sugars, alcohols, polyphenols, lignins, tannins and organic acids [11]. The COD values commonly found in winery wastewater typically range between 300 mg/L and 300,000 mg/L with an average concentration of 12,000 mg/L [11,12,13]. The disposal of acidic winery wastewater showing high electrical conductivity and COD levels leads to eutrophication of the recipient environment [9]. Thus, winery wastewater management has become paramount to the sustainability of the wine industry.
Wastewater management is directly linked to efficient winery operations and it is as much a business matter as it is an environmental or technical issue. By efficiently managing winery wastewater, operational and water supply costs may be reduced, wastewater disposal fees/surcharge levels controlled and an enhancement in the relationship and reputation between a winery, their consumers, local community and regulators may be established. The market necessity for sustainable wine production and effective management of water resources including the prevention of pollution is shaping the modern wine industry [14].
There are various winery wastewater management strategies in place with some wineries treating their wastewater onsite, while others direct their semi-treated (typically pH adjusted and clarified) wastewater to municipal wastewater treatment plants. The treatment options generally depend on the initial characteristics of the winery wastewater which vary from winery to winery and are subsequently influenced by climate conditions and the type of wine produced. There is also a large seasonal variation in winery wastewater loads with the highest organic loads (more than 70%) being produced during the vintage season (August–October in the Northern Hemisphere; February–April in the Southern Hemisphere) [15,16].
To date, various technologies have been investigated to treat winery effluent to standards set out by discharge regulations [17,18,19,20], which generally stipulate that all wineries are required to treat their wastewater before being discharged [21]. Since the principal income of a winery is related to the production and sale of wine and grape juice, effluent treatment may be considered as a financial burden. As such, wineries would typically consider the most cost effective, robust and efficient primary treatment technologies capable of reducing the turbidity, suspended particulate matter and COD of the effluent, in line with the requirements imposed by local authorities.
A comprehensive review was conducted by Vlotman et al. (2022) which focused on different winery wastewater treatment options, their basic principles of operation and pros and cons of each treatment technology [22]. In their review, preliminary, primary, secondary and tertiary winery wastewater treatment options were discussed. Primary wastewater treatment techniques based on chemical coagulation, electrocoagulation and flocculation have been adopted as being effective to reduce the colour, suspended particles and insoluble substances in wastewater [23,24,25].
Numerous primary treatment methods based on coagulation/flocculation and sedimentation have been investigated, with many of these techniques capable of reducing turbidity, suspended solids and to a smaller extent, chemical oxygen demand (COD) [26,27,28]. Conventionally, alum (aluminium sulphate), ferric chloride and ferrous sulphate have been used as metal coagulants for the treatment of wastewater. Although these chemicals are relatively inexpensive, widely available and simple to use, they are limited to specific pH ranges (pH 6.5–7.5) and are also prone to producing significant amounts of sludge containing metal hydroxides [29]. These metal salts used as coagulation aids are also known to reduce alkalinity which decreases the pH of the treated effluent water [30]. Researchers have more recently focused on finding alternative polymeric-based coagulant chemicals which can enhance the treatment efficiency of wastewater treatment systems [31].
Pre-polymerized coagulants such as polyaluminium sulfate, (PAS), polyaluminium chloride (PAC) and polyaluminium chloro-sulfate (PACS) have been shown to function efficiently over a wider pH range (pH 6–9.5), temperature and colloid concentration ranges as compared to conventional metal-based coagulants in wastewater treatment [26,32]. The function of a coagulant is to neutralize the negatively charged colloids present in wastewater. Once these colloids have been neutralized/destabilized, van der Waals forces come into play where these charge-neutral species are weakly attracted to one another, causing them to clump together into a group. When enough of these particles have coalesced, they form a floc which either settles or floats to the surface of the water [33]. The advantages of using polymeric coagulants in wastewater treatment include lower coagulant quantity and concentration being required to destabilize colloids as well as less sludge being generated compared to their monomeric metal salt-based coagulant counterparts. Less sludge production correlates to less solid waste being generated, subsequently decreasing solid waste disposal costs for wineries opting to use these types of coagulants in their primary treatment processes.
The efficiency of coagulation can be increased by the use of hydrodynamic shear forces. Hydrodynamic shear forces can alter the stability of the colloids in suspension which assists in particle aggregation. The effect of shear is brought about by bringing particles in close proximity to one another, causing them to collide. Once collided, the particles will aggregate if the net interparticle force is attractive and strong enough to overcome thermal agitation and hydrodynamic drag [34]. The general forces acting on a particle in suspension include van der Waals forces, electrostatic forces and hydrodynamic forces. The attractive forces which favour particle aggregation are known as van der Waals forces, whereas repulsion between particles is caused by electrostatic forces in the electric double-layer around the particles. These electrostatic forces oppose aggregation, which subsequently provides stability to the particles in suspension. The electric double-layer can be suppressed via hydrodynamic shear and the addition of ions of opposing charges which aid in the destabilization of the particles in solution [35]. This occurs via a mechanism called shear flocculation where particles suspended in an aqueous environment are aggregated by applying a shear field of sufficient magnitude [36]. Shear flocculation also aids in producing particles capable of attaching to air bubbles, and as will be shown in this study, shear flocculation flotation enhances solid/liquid separation efficiencies as compared to standard flotation. This process may further be enhanced with the addition of flocculants such as polyacrylamide in a low shear environment (in order for the flocs to remain agglomerated) and eventually floated to the surface of the water [37]. The synergistic effects of hydrodynamic shear, coagulation, flocculation and flotation form the basis of the principal operation of shear enhanced flotation separation (SEFS) used in this study.
The shear coagulation/flocculation process involves: (a) high-speed mixing of the coagulant into the liquid matrix via violent agitation and (b) aggregation of small particles into well-defined flocs by flocculation with gentle agitation. Finally, flocs are driven to the surface of the water (via the flotation process) and removed as froth while the treated water may either be discharged or transferred to an upstream treatment process [38]. The parameters of interest for SEFS treatment used in this study were turbidity, total suspended solids (TSS), total dissolved solids (TDS) and particulate (insoluble) COD. The “optimized” reaction condition used throughout this study relates to the individual parameters (pH, chemical dosage, shearing speed and induced air) that displayed the best results in the treatment efficiency of the overall treatment process.

2. Materials and Methods

2.1. Raw Winery Effluent Collection

Raw winery wastewater was obtained from a winery located on the West Coast of South Africa during the vintage period (February–April 2021/2022) and was used as feed water for batch laboratory studies. Briefly, the inherent set-up at the winery involves the effluent being gravitationally fed through a 2 mm screen grid where solid materials (grape pomace) consisting of skin, stem, residual pulp, seed, small pieces of stalks and yeast cells from the wine fermentation process are removed [39]. The raw effluent for laboratory batch studies was collected immediately after the 2 mm screen grid as shown in Figure 1 and pumped into an intermediate bulk (IBC) container using a Pedrollo RX2/20 submersible pump. Downstream of the grid, the effluent flows into a lime alkalization pit where pre-mixed lime slurry is added to keep the pH of the effluent above 9. The moment the pH in the lime pit drops below 9, the lime water dosing pump is activated. The effluent from the lime pit flows over into three interconnected settling dams (Dams A–C in Figure 1) where solid/liquid separation takes place gravitationally. Thereafter, the effluent is pumped to three biological treatment dams (Dams 1–3 in Figure 1) and finally discharged onto adjacent fields via irrigation pumps in undiluted or diluted form depending on the characteristics of the treated effluent. Since the SEFS process is a primary treatment stage technology, the focus was to treat the raw effluent before it reached the settling dams (the effluent in the upstream secondary biological dams was not investigated).

2.2. Alkalization Study

Batch studies were conducted in 5 L beakers (sample volume = 4 L) at atmospheric pressure and temperature. A lime slurry solution was made by adding 100 g of hydrated lime ((CaOH)2), obtained from Cape Lime (Pty) Ltd., Vredendal, South Africa. To 900 mL of distilled water in a volumetric flask. pH adjustments from raw effluent (pH 4) to a pH of 7, 8, 9 and 10 were carried out using 725 mg/L, 788 mg/L and 825 mg/L and 900 mg/L of lime slurry, respectively. Following pH adjustments in the 5 L beakers, 8 × 500 mL samples were used as a batch for each pH value.

2.3. Coagulation and Flocculation

Based on the treatment efficiency in different types of wastewater, aluminium-based polymeric coagulants and polyacrylamide flocculants were selected as treatment chemicals during this study [34,37,40,41]. The coagulant used in this study was AB121 which is a blend of aluminium chlorohydrol (ACH) with a specific gravity of 1.3 (available as a 1% vol. solution). The flocculant used was AB796, which is a blend of polyacrylamides (granules) and made up to a 1% (w/v) solution with ultrapure deionized–water obtained from a Milli-Q (Millipore Co., Billerica, MA, USA) system. The coagulant and flocculant chemicals were supplied by Abrimix (Pty) Ltd., Johannesburg, South Africa. The coagulant was used as received while a stock flocculant solution of 10,000 ppm was made using 1000 mL of distilled water on a weekly basis. The effect of the coagulant dosage was investigated using the optimum pH values obtained during the alkalization studies. The optimal dosage of the coagulant was evaluated using a zeta potential analysis.

Coagulation and Flocculation Dosages

In a typical coagulation experiment, aliquots of coagulant with various concentrations (1, 2, 3, 4, 5 or 10 mg/L) were added directly to the 500 mL sample while stirring with the high shear mixer for 2 min at 250, 2000, 4000 and 6000 rpm, respectively. At the point where the coagulant dosage reached a zeta potential value close to 0 mV (i.e., the iso-electric point), the addition of flocculant commenced where the sample was stirred using low shear at 250 rpm for 10 min.

2.4. Shear Mixing

The hydrodynamic shearing of the effluent was conducted using a PUXIL JRJ300-1 High Speed Shearing Emulsifier. This unit consists of a gas inlet, rotor, stator, air flow meter, speed control and two pressurized air nozzles (Figure 2). This mixer has a processing capacity of 40 L, working head configuration of 70 mm, output power of 300 W and speed range of 100–11,000 rpm. The lab scale shearing mixing setup was equipped with two pressurized air nozzles which were set to deliver an air flow rate of 450 ln/h (normal litre per hour, defined as a unit of volume for gases equal to the volume of 1 L at a pressure of 1 atmosphere and a standard temperature of 20 °C) [42].

2.5. Analytical Methods

Determination of pH and electrical conductivity were carried out using a portable multi-meter (Hach, HQ40d) with an equipped conductivity probe. The meter calculates an estimate of total dissolved solids (TDS) by multiplying the electrical conductivity (EC) reading with a conversion factor. The factor (from EC in mS/m at 25 °C to TDS in mg/L) is reported to have an average value of 6.5 [43]. The turbidity, measured in NTU (Nephelometric Turbidity Units) of the wastewater was measured using a (Hach 2100Q) spectrophotometer (Hach, Johannesburg, South Africa). The chemical oxygen demand (COD) of the samples was obtained using a Thermo Fisher Scientific (Waltham, MA, USA) Orion Aquafast 3140 colorimeter and an Orion COD165 Thermo-reactor. The reagent vials used were the Aquafast COD HR (High Range), 0–15,000 ppm Thermo Fisher Scientific (Waltham, MA, USA).

Zeta Potential

The zeta potential (ZP) analysis was conducted with a Malvern Zetasizer NanoZS (Malvern Instruments Ltd., Worcestershire, UK) series which uses the electrophoretic light scattering measuring technique [44]. The zeta potential measurements were studied in terms of variations in shear speed and coagulant dosage. The laser used in the Malvern Zetasizer Nano was a 4 mW He–Ne laser of 633 nm wavelength [44]. A dip cell (Malvern Instruments Ltd., Worcestershire, UK) with palladium electrodes (ZEN1002) was used for the determination of zeta potential during this study.

2.6. Determination of Total Suspended Solids

Total suspended solids were determined as per ASTM D5907–18. Briefly, a 0.45 µm filter paper was weighed on a watch glass before filtration; thereafter, samples were filtered in a Buchner filter filtration set-up. The post-filtered solid samples were placed in an oven at 105 °C for 1 h. Thereafter, the sample was removed from the oven and placed in a desiccator for 30 min to allow the sample to cool in a moisture-free environment. Once cooled, the sample was weighed, and the total suspended solids calculated.

3. Results

3.1. Raw Winery Wastewater Composition

The winery crushed a total of 43,500 tons of grapes (38,000 white and 5500 red) during the vintage period (March–April 2021/2022) and the average raw winery wastewater composition during this period is illustrated in Table 1.
The values obtained for the chemical composition of the effluent used in this study corresponds with other studies relating to winery effluent composition in South Africa [13,45,46]. Based on the data displayed in Table 1, the characteristics of the raw effluent was acidic and showed high levels of total suspended solids (TSS), total dissolved solids (TDS) and chemical oxygen demand (COD). Previous detailed studies on winery wastewater composition have concluded that 90% of the total soluble organic load is represented by ethanol and sugars (glucose and fructose) [47].
In order for winery wastewater to be disposed of via irrigation, certain legislation limits must be adhered to [48]. These limits are shown in Table 2. Comparing Table 1 and Table 2, it is evident that the untreated winery wastewater does not comply with the general authorization limits for irrigation water quality in South Africa.

3.2. Alkalization

Wastewater discharge permits generally require that acidic waste be alkalized to the range of a pH of 6.0–9.0 [48]. Winery wastewater is known to have an abundance of negatively charged colloids. These colloids remain in suspension due to their net surface repulsive forces. pH adjustment is commonly used to partially destabilize these repulsive forces. To achieve these goals, lime alkalization is a commonly used pH alkalization option [2,49,50].
In general, acidic conditions present in winery wastewater favours the production of hydrogen sulphide, which is the dominant malodorous gas associated with wastewater originating from wineries [51]. Raising the pH of stored wastewater can greatly reduce the production of hydrogen sulphide and decrease the impact of malodours on neighbouring properties. This step is important, especially in the context of this case study, since the selected winery has had frequent complaints from surrounding neighbours about foul odours being generated by their effluent plant.
The relationship between the zeta potential and turbidity with pH is presented in Figure 3.
An observation that can be made from Figure 3 is that acidic/raw winery wastewater has a larger zeta potential (indicating a greater degree of colloid stability) than the alkalized effluent. This is accompanied by a higher turbidity value since the colloids in suspension of the raw effluent have a lower tendency to agglomerate (and subsequently precipitate) due to higher electrostatic charges (repulsive forces). The optimum pH as determined by zeta potential and turbidity was a pH of 8 which showed optimal conditions for the particulates to coagulate and precipitate. It is well known that lime does not only act as an alkalizing agent but also as a coagulant, which is why a general decrease in zeta potential is observed upon lime addition [52]. There was a decrease in the turbidity as the pH rises from 4 to 8, this observation was also confirmed in a study conducted by Luz et al. (2021) Their findings indicated that alkalization with lime decreases the turbidity of the effluent from 159 NTU to 2 NTU [50]. At pH values above 8, the zeta potential of the solution decreased, which we believe may be due to an oversaturation of lime dosing. This also resulted in resuspension of particles which is evident with the increase in turbidity values. A similar trend was reported by Awodiji et al. (2020) [53].
Figure 4 illustrates the visual effect of alkalization on turbidity during alkalization studies. Herein, hydrated lime not only increases the pH of the solution, but also aids in removing suspended matter. The observations made in Figure 3 and Figure 4 allowed for the selection of the optimum pH (pH 8–pH 9) for hydrodynamic shear and coagulation studies.

3.3. Application of Shear

Following the pH adjustment studies, the effect of applying hydrodynamic shear to the wastewater was investigated. The alkalized effluent (pH 8) was exposed to different shearing speeds of the mixer for various time periods. One batch of such effluent was exposed to high shear speeds (2000, 4000 and 6000 rpm) using a rotor-stator mixer, while another batch was exposed to low shear (250 rpm) using a magnetic stirrer. The samples were sheared for different time intervals, namely 0 min, 1, 5, 10, 20 and 30 min to determine the effect of hydrodynamic shear and shearing speed with increasing time. After the allocated shearing time had elapsed, samples were extracted, 2 min after ceasing agitation, from the center of the container used during the shear experiments (5 cm below surface of liquid) and immediately taken for zeta potential analysis.
Figure 5 illustrates different shearing speeds as a function of time. Here it is shown that aqueous samples exposed to a certain shear environment produced effluent with the zeta potential values closest to 0 mV, which relates to an increased probability for colloid destabilization and subsequent solid/liquid separation to occur. This fast-stirring step destabilizes the colloids in suspension and promotes colloid aggregation via the Van der Waal’s forces acting upon the particles. The effect of hydrodynamic shear based on zeta potential analysis shows that shear plays an important role in the colloid destabilization. The basic principle of shear flocculation in this study is based on the creation of colloid collisions of sufficient momentum, where the kinetic energy of the particles is sufficient to overcome the surface repulsive force, hence allowing the occurrence of particle agglomeration [36].
It should be noted that the purpose of testing zeta potential is to measure the stability of a colloidal suspension. Taking into account that the sample water can be described as a colloidal suspension, the results indicate that at a lower mixing speed there is evidence for aggregation of the particles at lower zeta potentials. The optimum shearing speed was observed to be at 4000 rpm, where the lowest zeta potential was obtained. The results indicate that increasing the shearing speed above 4000 rpm caused moderate re-stabilization of the colloids in suspension. In comparison, batch tests conducted at low shearing speeds (250 rpm) displayed a minimal decrease in zeta potential values. The zeta potential results related to the different shearing speed and times illustrate that after 5 min of agitation, a plateau is reached. Thus, maximum particle destabilization using shear (both low and high shearing speeds) is obtained after 5 min of shearing. At higher mixing speeds, above 4000 rpm there is an increase in the zeta potential inferring a decrease in the colloidal particles size leading to more overall stability.
It can be hypothesized based on the zeta potential analyses, that the surface charge of the particles has been altered during the application of a hydrodynamic shear force. The introduction of hydrodynamic shear in wastewater treatment increases the collision probability between particle/particle and bubble/particle. Based on the increase in number of collisions during the application of hydrodynamic shear, the surface chemistry of the particles may be altered to induce aggregation of the suspended particles to form large clusters which can be removed from the medium via the attachment of bubbles (i.e., flotation) [54,55]. According to the work by Elizaveta Forbes (2011), hydrodynamic shear assists in the recovery of fine particles and produces particle aggregates that are capable of attachment to air bubbles and being recovered by flotation [36].

3.4. Zeta Potential Studies on Coagulation

Real-time determination of appropriate coagulant dosage of varying raw wastewater quality in a treatment plant is a challenging task due to the nonlinear relationship between coagulant dosage and raw wastewater characteristics [56]. In this work, zeta potential results gave insight to the appropriate dosage of chemicals required to achieve maximum destabilization and agglomeration of particles in suspension. The zeta potential after each incremental addition of coagulant was determined where a change towards zero zeta potential indicates that the added coagulant is destabilizing the colloids in the wastewater i.e., shifting towards colloidal aggregation.
During coagulation experiments, an iso-electric point was observed for the AB121 coagulant (1% w/w solution) under the studied pH ranges which is indicated by the dotted line at 0 mV in Figure 6. At the iso-electric point, achieved at a dosage of 5 mg/L (250 µL in 500 mL sample), colloidal particles exist in an optimum destabilization zone where the particles no longer repel one another, making these conditions ideal for agglomeration and flocculation to occur. Adding a flocculant at this point would improve the floc size and floc stability.
The coagulant AB121 was thus shown to have effectively neutralized the counterions in the solution possibly via the adsorption mechanism as proposed by Lee et al., 2014 [57]. Adsorption is possible since this coagulant possesses a specific macromolecular structure containing a variety of functional groups (e.g., carboxyl and hydroxyl) which can interact with the contaminants in the wastewater [58].
As previously stated, colloids in wastewater are generally negatively charged, but after the addition of a coagulant, these negatively charged colloidal particles are neutralized by the cationic coagulant, resulting in an overall surface charge reduction and formation of micro-flocs. However, the process of coagulation may often result in small and fragile flocs which may break up when subjected to physical forces. To overcome this, flocculant chemicals are added to increase the density and solidity of the flocs formed [57]. The following section will investigate the effect of flocculant dosage required to assist the coagulation process. The quality parameters used to evaluate treatment efficiency were total suspended solids and turbidity.

3.5. Flocculation

During flocculation studies, the effluent was alkalized to a pH of 8 using lime, subjected to high shear (4000 rpm) and dosed with 5 mg/L of coagulant (AB121). The pre-conditioned samples were subsequently dosed with a flocculant (polyacrylamide polymer, AB796, Abrimix). It is important to note that during the flocculation process, gentle mixing was used (250 rpm using a magnetic stirrer) since a high shear environment used during coagulation experiments would break the formed flocs. Various flocculant dosages (0–14 mg/L) were investigated. Five minutes after stirring the mixture of flocculant and preconditioned effluent, the stirring was stopped. Two minutes later, the samples were extracted from the center of the holding container using a micropipette. Both turbidity and the value of total suspended solids (TSS) were measured, and results are shown in Figure 7. The results illustrate a reduction of 85% in both turbidity and suspended solids when adding 10 mg/l of flocculant (500 µL to a 500 mL sample) to the pre-conditioned solution. Figure 7 also shows the close correlation between turbidity and suspended solids. Further addition of the flocculant (>10 mg/L) appeared to be unnecessary since both the turbidity and TSS remained unchanged.
In a study led by Iakovides et al. (2014) the combined use of polydadmac and hydrated lime (CaOH)2) to treat olive mill wastewater was investigated. Their results indicated that with a lime dose of 20,000 mg/L and between 750–2000 mg/L of polydadmac flocculant, a reduction of 27% in total suspended solids and 43% in total solids was achieved using a mixing speed of 250 rpm [59]. Although the wastewater types were different, the general characteristics of the raw effluent were comparable to that of winery wastewater. Treatment efficiencies of coagulation/flocculation technologies is well documented in the literature [3,38,60,61]. A comparison can be made where many authors report on using high dosages of chemicals, whilst achieving treatment efficacies generally less than 50% for COD reduction. We therefore postulate that the difference in treatment efficiency may be ascribed to the addition of hydrodynamic shear used in our study.
The optimally destabilized particles readily form agglomerates which are “caught” by the polymer chains of the flocculant, resulting in large settling flocs [62]. The flocculant binds the different flocs together and acts as a backbone to the growing floc, showing improved mechanical integrity. Most of these flocs settle within the time span of a minute, leaving a clear solution behind with significantly reduced turbidity and TSS values. The process of shear coagulation and flocculation causes the effluent contaminant particles to aggregate sufficiently to allow treatment with dispersed air bubbles to float the aggregates to the surface of the liquid (which can be removed as froth), leaving a clear treated effluent as the subnatant.

3.6. Introduction of Air

During the investigation of the final process parameter (the introduction of air in combination with shear/coagulation and flocculation), the effluent was alkalized to a pH of 8 with lime, subjected to high shear (4000 rpm), dosed with 5 mg/L of coagulant (AB121), and finally dosed with 10 mg/L of flocculant (AB796). During the shear process air was bubbled into the region of the rotor-stator cavity. Following the addition of coagulation/shear/air, the shear mixer was turned off and the solution was gently mixed at 250 rpm via a magnetic stirrer, in the presence of added air. Figure 8 illustrates the TSS and the turbidity values of raw effluent (untreated), alkalized (pH 8) and SEFS-treated effluents (shear, coagulated and flocculated) with air (i.e., flotation) and without (i.e., sedimentation) the introduction of air (450 L/h) near the rotor stator cavity.
The small bubbles created by the SEFS rotor were advantageous because smaller bubbles provide a larger surface area which more effectively promotes the bubble particle attachment and buoyancy of the suspended solids. Studies have shown that he need to reduce the bubble size is directly related to an increase in the possibility of collision between bubbles and particles [63].
It is clear that the introduction of air has a positive effect on the quality of treated effluent and confirms that flotation is more effective than sedimentation under the specified conditions (samples were extracted 2 min after the respective procedures, giving very little time for re-sedimentation to occur but sufficient time for flotation). An impressive reduction of 97% and 95% was achieved in TSS and turbidity, respectively, when air was introduced during application of shear. Even for the alkalized solution, introduction of (un-sheared) air bubbles lead to a substantial reduction in TSS and turbidity (compare orange blocks in Figure 8), possibly facilitated by the well-known coagulating property of lime [53].
Table 3 shows the values associated with the bar chart shown in the raw/untreated water, which is effluent sampled from the winery prior to lime, shear treatment, coagulant, flocculant and air addition. The alkalized sample represents the effluent after lime has been added to increase the pH to 8. The SEFS-treated sample represents the alkalized sample which has been exposed to shear (4000 rpm), dosed with 5 mg/L of coagulant: AB121 and 10 mg/L of flocculant: AB796 with and without the introduction of 450 ln/min of air.
The addition of air increased the treatment efficiency of the SEFS reactor. This is shown where a decrease in the suspended solids by 97% (from 2275 mg/L to 70 mg/L) was achieved with the introduction of air compared to the 70% reduction in TSS without air. Similarly, a reduction in turbidity of 95% was achieved with air, whereas only 67% turbidity was reduced without the use of air.

3.7. Influence of SEFS on COD

The performance of the SEFS treatment was tested according to its ability to reduce the chemical oxygen demand pre- and post-treatment with and without the addition of air as shown in Figure 9. The COD for the SEFS treated effluent yielded a 54% reduction (from 11,250 to 5220 mg/L).
The optimized particulate destabilization process, combined with optimized flocculant dosing, may explain these promising results, especially in relation to suspended solids and turbidity, but not so much for COD. COD value measurements include those originating from both insoluble and soluble organic compounds. The insoluble COD fraction is substantially removed by the synergistic effects of the SEFS process as displayed in Table 4. Thus, the smaller reduction in COD compared to the >95% TSS and turbidity is likely to originate from the continued presence of dissolved organic species after SEFS treatment.
With a COD reduction of 54%, we have shown that SEFS may be considered as an effective primary treatment stage for winery wastewater. Based on both COD reduction and the high reduction values of turbidity and total suspended solids, SEFS technology has the potential to be upscaled where a secondary treatment stage such as reverse osmosis can be employed. Through lowering the COD by half and removing most suspended solids using SEFS, upstream biological treatment processes may operate more efficiently.

4. Conclusions

This study focused on the implementation of shear enhanced flotation separation (SEFS) to treat real winery wastewater. We have demonstrated the applicability of this treatment technology with actual wastewater. The individual processing parameters (pH, chemical dosage, shearing speed and induced air) have been optimized based on the overall treatment efficiency of the SEFS treatment process. A substantial (97%) reduction in total suspended solids (TSS) was achieved using SEFS with initial raw wastewater TSS values of 2275 mg/L compared to 50 mg/L for the SEFS-treated samples. The reduction in turbidity was (95%), with initial values of 630 NTU for the raw wastewater and 25 NTU after SEFS treatment. The results indicate a near-linear relationship between turbidity and TSS reductions resulting from SEFS treatment. SEFS technology was also shown to be a suitable technology for the reduction in chemical oxygen demand (COD) of real winery wastewater with reduction values obtained in the study being 54% (raw wastewater initial COD: 11,250 mg/L versus SEFS-treated wastewater COD: 5220 mg/L).
This study highlighted what is achievable with SEFS and shows that this technology may potentially be used as a substitute for conventional primary winery wastewater techniques. Incorporation of an SEFS treatment plant on a winery wastewater treatment site has the potential to alleviate many difficulties in operation turn-around time. SEFS treatment technology may also potentially be used in conjunction with other techniques such as biological treatments and reverse osmosis which will further lower the total COD by reducing the soluble organic content of the treated effluent SEFS.
The potential of this technology has been established on agricultural wastewater but may also be applied to treat other industries’ wastewater (e.g., olive mill and textile industry). There is still additional work to be done where the energy use and total treatment costs will be investigated.

Author Contributions

Conceptualization, D.V. and B.J.B.; methodology, D.V.; formal analysis, D.V.; writing—original draft preparation, D.V.; writing—review and editing, D.V., B.J.B., B.C. and D.K.; supervision, B.J.B. and D.K.; funding acquisition, B.J.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Winetech, project number UWC BB-20-01.

Data Availability Statement

Data is contained within the article.

Acknowledgments

This work was undertaken as part of a Ph.D. research thesis at the South African Institute of Advanced Materials Chemistry, University of Western Cape, South Africa. The authors express gratitude to all those who helped with the performance of this study. A special thank you to B. R. Cerff, for his assistance and knowledge during the finalization of this project. The authors would like to sincerely thank the technical staff at the winery for their cooperation and assistance during this study. Lastly, to Pieter Jansen and Hannes Tolmay, from Abrimix Pty Ltd. for their unselfish knowledge transfer and patience whilst conducting this study.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Water 15 02409 g001 550
Figure 1. Effluent treatment plant at selected winery.
Figure 1. Effluent treatment plant at selected winery.
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Figure 2. Shear enhanced flotation separation lab scale unit.
Figure 2. Shear enhanced flotation separation lab scale unit.
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Figure 3. Zeta potential and turbidity as a function of pH.
Figure 3. Zeta potential and turbidity as a function of pH.
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Figure 4. Lime alkalization studies as a function of pH.
Figure 4. Lime alkalization studies as a function of pH.
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Figure 5. Effect of shearing speed and time (pH 8) on colloidal stability based on zeta potential.
Figure 5. Effect of shearing speed and time (pH 8) on colloidal stability based on zeta potential.
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Figure 6. Effect of AB121 coagulant dosage on pH and zeta potential.
Figure 6. Effect of AB121 coagulant dosage on pH and zeta potential.
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Figure 7. Turbidity and total suspended solids as a function of flocculant dosage.
Figure 7. Turbidity and total suspended solids as a function of flocculant dosage.
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Figure 8. Influence of induced air on turbidity (black arrow/circle) and total suspended solids (blue arrow/blocks).
Figure 8. Influence of induced air on turbidity (black arrow/circle) and total suspended solids (blue arrow/blocks).
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Figure 9. Influence of air on COD reduction.
Figure 9. Influence of air on COD reduction.
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Table
Table 1. Average raw winery wastewater characterization.
Table 1. Average raw winery wastewater characterization.
Sampling DatepHTurbidityECCaTSSTDSCOD
(25 °C)(NTU)(mS/m)(mg/L)(mg/L)(mg/L)(mg/L)
23–28 March 20214.06301751302275354622,620
9–14 April 20224.55702302221840147612,400
Table
Table 2. General Authorizations for legislated limits for irrigation water quality in South Africa.
Table 2. General Authorizations for legislated limits for irrigation water quality in South Africa.
ParameterMaximum Irrigation Volume Allowed (m3/Day)
<50<500<2000
pH6–96–95.5–9.5
COD (mg/L)500040075
EC (mS/m)20020070–150
TSS (mg/L)1000-<25
TDS (mg/L) *13001300488–975
Notes: Chemical Oxygen Demand (COD); Electrical Conductivity (EC); Suspended Solids (TSS), Total Dissolved Solids. * TDS based on EC using a conversion factor of 6.5.
Table
Table 3. Total suspended solids (TSS) and Turbidity values of effluent at different stages of treatment.
Table 3. Total suspended solids (TSS) and Turbidity values of effluent at different stages of treatment.
Sample TypeTSS (mg/L)Turbidity (NTU)
Without AirWith AirWithout AirWith Air
Raw/Untreated22752260660635
Alkalized1750800470280
SEFS treated6807022030
% Reduction (SEFS vs. Raw)70976795
Table
Table 4. COD values of effluent at different stages of treatment.
Table 4. COD values of effluent at different stages of treatment.
Sample TypeCOD (mg/L)
Without AirWith Air
Raw/Untreated11,31011,250
Alkalized98208750
SEFS treated89205220
% Reduction (SEFS vs. Raw)2154
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Vlotman, D.; Key, D.; Cerff, B.; Bladergroen, B.J. Shear Enhanced Flotation Separation Technology in Winery Wastewater Treatment. Water 2023, 15, 2409. https://doi.org/10.3390/w15132409

AMA Style

Vlotman D, Key D, Cerff B, Bladergroen BJ. Shear Enhanced Flotation Separation Technology in Winery Wastewater Treatment. Water. 2023; 15(13):2409. https://doi.org/10.3390/w15132409

Chicago/Turabian Style

Vlotman, David, David Key, Bradley Cerff, and Bernard Jan Bladergroen. 2023. "Shear Enhanced Flotation Separation Technology in Winery Wastewater Treatment" Water 15, no. 13: 2409. https://doi.org/10.3390/w15132409

APA Style

Vlotman, D., Key, D., Cerff, B., & Bladergroen, B. J. (2023). Shear Enhanced Flotation Separation Technology in Winery Wastewater Treatment. Water, 15(13), 2409. https://doi.org/10.3390/w15132409

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