Moving towards Valorization of Biowastes Issued from Biotrickling Filtration of Contaminated Gaseous Streams: A Thermochemical Analysis-Based Perspective

Abstract

This paper investigates the valorization potential of two biowaste types resulting from biotrickling filtration of volatile organic compounds (i.e., ethanol) and carbon dioxide from air by co-immobilized microalgae and compost heterotrophs, which were either attached on polypropylene spheres or entrapped within the alginate beads. In this regard, biomass samples from the surface of the packing spheres (S1) and the waste alginate beads (S2) underwent thermal and energy characterization via thermogravimetry and calorimetry techniques as a screening step for establishing some possible biomass valorization pathways. The heat release capacity (HRC) values for the samples S1 and S2 were 95.67 J/(g·K) and 44.11 J/(g·K), respectively, while the total heat release (THR) values were 11.03 kJ/g and 3.64 kJ/g, respectively. The results of this study indicate that the S1 biomass could be suitable for undergoing thermal decomposition processes-based applications, while the S2 biomass could have a potential application for improving flame retardancy of some materials. These findings show that the biowaste issued from such air biotreatment can become a valuable resource for different applications instead of being disposed of. Further research referring to the implementation of these solutions for the development of the final applications is needed.

1. Introduction

Microalgae-based systems are attracting increased attention among air purification options, taking into consideration the photosynthesis capacity in the presence of light, the ecofriendly attribute, and the versatility of the involved microalgae that are able to adapt to various environments. For example, adding microalgae to a biotrickling filter treating volatile organic compounds (VOCs) in air by heterotrophs can contribute to the uptake of the carbon dioxide emissions emerging from the VOCs’ biodegradation in the same unit under appropriate illumination [1,2]. In any case, valorization of the waste biomass resulting from the air biotreatment process is a collateral technological advance that can be further addressed, taking into consideration the research need for sustainable solutions in any industrial sector. In this regard, the microalgae content can also increase the biomass potential to undergo different valorization routes. For instance, harvested microalgae can be involved in the production of biofuels (biodiesel, bioethanol, biohydrogen, etc.) [3,4]. Moreover, during their development, microalgae accumulate lipids, pigments, proteins, polysaccharides, carbohydrates, etc., which confer them other potential applications: animal feed [5], biosorbents, polymer fillers, metal nanoparticles, etc. [6].

As far as biochemical conversion of microalgae waste and other biowastes is concerned, there is a number of disadvantages related to the long processing time, high production costs, and low efficiency. Therefore, thermochemical conversion is preferred as it has a much higher yield [7]. Venkatachalam et al. [8] conducted a comparative assessment of the thermochemical conversion of the lignocellulosic and algal biomass. Hydrothermal liquefaction is a thermochemical conversion technique used to produce biomass oil with very good yields. This method also has the advantage that no chemical, physical, or mechanical pretreatment of biomass is required and energy recovery is better than in the case of other dry conversion processes [8]. Chen et al. analyzed the effects of using various amounts of microalgae along with coal in the pyrolysis process. The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves for mixtures were similar to those obtained for algae and different from those of coal. They revealed the interactions between the solid phases of coal mixtures, which actually inhibited its thermal decomposition. It was also found that the amount of waste increases with the increase in the microalgae amount in the mixtures [9]. On the other hand, Baloyi and Dugmore recorded thermogravimetric curves for microalgae, coal waste, and mixtures thereof in an inert atmosphere and noted higher decomposition rates for mixtures within the 200–400 °C temperature range and a smaller amount of waste [10]. Other studies have suggested mixing different types of waste with microalgae in order to streamline their combustion processes [11,12]. Thermogravimetric analysis in inert atmosphere was used by Azizi et al. for microalgae, wood, and used tires in binary and ternary mixtures. The interaction between these materials was assessed and found to be inhibitory rather than synergic. An increase in total weight loss was detected in binary mixtures microalgae–tires and wood–tires, but not in the microalgae–wood mixture [11]. The same technique was used by Sukarni to reveal the improvement of the combustion process of solid waste to which different percentages of microalgae were added. The onset temperature of the decomposition process for the mixtures was found to decrease and the percentage mass loss was found to increase up to 480 °C, compared to the findings recorded under the same conditions for solid waste [12]. Dessi et al. have recently revealed the influence of the work environment on the thermal decomposition processes of two types of microalgae [13]. By means of thermogravimetric analysis in an oxidizing atmosphere, López-González et al. concluded that microalgae samples may be considered a biomass with potential combustion applications. Differential scanning calorimetry (DSC) results showed that the amount of heat released during microalgae combustion is of the same order of magnitude as in the case of the lignocellulosic biomass [14]. Microscale combustion calorimetry (MCC) is an analysis method that simulates the pyrolysis and combustion stages of a combustion process in a combustion-free test. This technique has a unique and accurate way of measuring the heat release capacity. Moreover, it requires a short analysis time and a small amount of material. There is no literature research that uses this technique to assess the energy characteristics of particular microalgae or mixtures thereof with other materials.

In other order of idea, valorization of the wastes resulting from air biotreatment processes is currently little addressed. This paper aimed to determine the thermal and energy characteristics of some biowaste samples resulting from air treatment by photobiotrickling filtration in order to reveal some possible pathways for the valorization of the biowaste issued from such process. Results of a thermogravimetric analysis in an inert atmosphere (nitrogen) and air as well as of microscale combustion calorimetry (MCC) are presented and discussed for this purpose.

2. Materials and Methods

Biomass samples were collected from the two (photo)biotrickling filters (BTFs) treating ethanol in air which exhibited the best environmental performance (complete VOC removal and the lowest CO₂ emissions) in the previous studies [1,2]: one packed with polypropylene spheres and one with alginate beads, as a microorganisms’ support. Both BTFs involved co-immobilized microalgae and compost heterotrophs, which were attached on the polypropylene spheres and entrapped within the alginate beads, respectively (Figure 1).

Microalgae Arthrospira platensis PCC 8005 (Pasteur Culture Collection, Paris, France), commercial compost (peat-based and rich in active humus), and sodium alginate were the main ingredients used for the different biological substrate preparations as described in [1,2]. Thus, the biomass samples collected (scraped) from the surface of the spheres (S1) and the waste alginate beads (S2) were the biowastes investigated in the actual study. The physical appearance of these biowastes is provided in Figure 2.

Some microscopy images of the abovementioned microorganisms are provided in [1], where small rod-shaped bacteria are the dominant compost microorganisms, while large spiral filaments belong to the microalgae Arthrospira (Spirulina) platensis. Overall, the following generic molecular formulas describe the bacterial biomass and the microalgae biomass [15,16]: C₅H_8.3NO_1.35 and CH_1.83O_0.48N_0.11P_0.01, respectively. In particular, carbon and nitrogen are the major biomass components (about 60% of the biomass’ molecular composition) [16,17].

The two abovementioned sample types were subjected to thermogravimetric analysis in a Mettler Toledo 851^e device (Mettler Toledo, Greifensee, Switzerland). The thermogravimetric (TG), derivative thermogravimetric (DTG), and differential thermal (DTA) curves were recorded in both air and inert atmosphere (nitrogen) at a 20 mL/min flow rate within the 25–700 °C temperature range. Sample amounts of 3.2–3.8 mg were used, and the heating rate was 10 °C/min. The Mettler Toledo 851^e equipment ensures a mass measurement error of less than 1 microgram and a temperature adjustment accuracy of 0.01 °C. For each sample, three TG/DTG/DTA tests were performed under the same conditions to verify reproducibility. The differences between the recorded thermogravimetric curves were less than 1%.

The thermogravimetric curves were processed using the STAR^e software (Version 9.10 (Giessen, Germany)) developed by Mettler Toledo. The samples S1 and S2 were dried in laboratory conditions (temperature of 23 ± 2 °C and humidity of 40 ± 5%) for a month, and then their thermal behavior in a nitrogen atmosphere was analyzed. After another two months of drying in laboratory conditions (temperature of 22 ± 1 °C and humidity of 42 ± 7%), they were tested for thermal behavior in air and subjected to microscale combustion calorimetry (MCC) tests. The MCC tests were carried out according to the following experimental protocol: progressive temperature increase in a pyrolizer at a 2 °C/s heating rate up to 750 °C in a nitrogen environment at a 80 cm³/min flow rate and maintaining a 900 °C constant temperature of the resulting gas in the combustor in a nitrogen (80%) and oxygen (20%) environment at a 20 cm³/min flow rate. Three tests were performed on each sample under the same conditions to determine the reproducibility of the experimental measurements. Approximately 20 mg of a sample were used for each experimental determination.

3. Results and Discussion

3.1. Assessment of Thermal Decomposition in Air and Nitrogen

A comparison was drawn between the thermogravimetric curves (TG), derivative thermogravimetric curves (DTG), and differential thermal curves (DTA) of the two tested samples in air (Figure 3a, Figure 4a and Figure 5a) and in an inert atmosphere (nitrogen) (Figure 3b, Figure 4b and Figure 5b).

The processing of the TG, DTG, and DTA curves recorded in the two working atmospheres has revealed the main thermogravimetric characteristics: onset mass loss temperature (T_onset), peak mass loss temperature (T_peak), end mass loss temperature (T_endset), percentage mass loss (W) in each stage, and the amount of waste at 700 °C, respectively, which are shown in

Table 1. Main thermogravimetric characteristics.

Sample	Working Atmosphere	Stage	Tonset, °C	Tpeak, °C	Tendset, °C	W, %	Residue, %
S1	air	I	62	88	138	9.14	22.55
		II	236	283	344	38.14
		III	454	495	553	30.17
	nitrogen	I	49	84	143	14.65	13.74
		II	252	285	344	42.11
		III	473	515	559	29.50
S2	air	I	48	62	114	6.55	58.03
		II	114	142	226	6.39
		III	226	242	332	15.85
		IV	332	448	558	6.60
		V	558	566	570	6.58
		VI	656	–	–	5.54
	nitrogen	I	44	61	81	17.88	39.60
		II	217	237	332	20.81
		III	332	454	564	9.22
		IV	564	–		12.49

.

The findings reveal the existence of three decomposition stages for sample S1 regardless of the working atmosphere. The first stage is related to moisture removal. As expected, the water loss percentage was higher for the inert atmosphere test after one month of drying the material in laboratory conditions, i.e., 14.65%. In the case of the test carried out in air, after three months of laboratory drying, the percentage decreased to 9.14%. The second stage in which the T_peak is approximately 284 °C corresponds to the decomposition of glycoproteins and glycolipids [18,19,20,21,22,23]. The last stage in which the T_peak is 495 °C when the working atmosphere is air and 515 °C when working in nitrogen corresponds mainly to cellulose and lignin degradation [18,19,20,21,22,23].

According to the TG, DTG, and DTA curves shown in Figure 3, Figure 4 and Figure 5 for sample S1, the rate of thermal decomposition processes was similar regardless of the atmosphere in which the tests were performed. The data shown in Table 1 reveal slightly lower T_onset temperatures in the second and third stages when air was the working atmosphere. Most literature studies dealing with thermal decomposition of microalgae were performed in an inert atmosphere at various heating rates [18,19,20,21,23]. Greque de Morais et al. [22] reported the 232–329 °C range for the main stage of spirulina decomposition in air at a 10 °C/min rate. Their findings are very close to those reported in this research, in similar working conditions. When sample moisture is not considered, regardless of the working environment, the percentage mass loss within the 236–559 °C temperature range for sample S1 is about 70%.

The TG, DTG, and DTA curves recorded for sample S2 are also shown in Figure 3a,b, Figure 4a,b and Figure 5a,b. The STAR^e software was also used to process the thermogravimetric curves and determine the main characteristics shown in Table 1.

According to our findings, decomposition of sample S2 involved six stages when working in air and four stages when working in an inert atmosphere, e.g., nitrogen. Sample moisture removal, which is influenced by the duration of the drying process of the material under laboratory conditions, occurred in the early stages and was accompanied by mass losses of 6.55% in air and 17.88% in nitrogen.

When the working atmosphere is air, the mass loss is 6.39% within the 114–226 °C temperature range, which may be associated with the soluble polysaccharide and carbohydrate decomposition onset [13]. The rate of the TG, DTG, and DTA curves in the two working atmospheres was similar within the 220–560 °C temperature range for sample S2. These stages mainly show the decomposition of proteins, lipids, and other insoluble polysaccharides [13,14]. Thermogravimetric analysis in air revealed the existence of a strong exothermic peak (Figure 5a) within the 558–570 °C temperature range. This peak can be associated with the decomposition of carbonate, which is an intermediary product of alginate thermal degradation [13,14,24,25]. Some carbonate can also accumulate during the VOC bioconversion to CO₂ under alkaline conditions. According to our findings, at temperatures higher than 550 °C, the decomposition mechanism of sample S2 was influenced by the working environment. In an inert atmosphere, i.e., nitrogen, the decomposition process of sample S2 at temperatures higher than 560 °C occurred slowly over time. There was also a larger amount of waste if the working environment was air. Soares et al. also reported working atmosphere-dependent differences between the alginate degradation mechanisms [26].

The comparative analysis of the decomposition processes of the two biowastes reveals that, at 700 °C, the amount of waste is much more significant in sample S2 than in sample S1 irrespective of the working atmosphere.

3.2. Assessment of Energy Characteristics by Microscale Combustion Calorimetry (MCC)

The amount of heat released when a material is exposed to combustion provides information about its flammability and energy characteristics [27]. The most important data collected from the MCC tests on the samples S1 and S2 are shown in

Table 2. Data collected by MCC analysis.

Sample	Mass (mg)	Waste (mg)	Char Yield (wt%)	Decomposition Rate (%)	THR (kJ/g)	HRC (J/(g·K))
S1	20.19 ± 0.06	6.44 ± 0.04	31.89 ± 0.20	68.11 ± 0.20	11.03 ± 0.39	95.67 ± 2.09
S2	20.17 ± 0.03	10.92 ± 0.21	54.15 ± 1.01	45.85 ± 1.01	3.64 ± 0.20	44.11 ± 4.05

and

Table 3. Characteristics of the peaks in the time- and temperature-dependent HRR curves.

Sample	Peak Number	PHRR (W/g)	TPHRR (°C)	Time (s)
S1	1	158.84 ± 5.03	314.72 ± 1.63	63.50 ± 8.41
	2	51.16 ± 2.37	450.30 ± 4.86	127.00 ± 11.27
S2	1	21.41 ± 2.88	261.66 ± 3.35	33.00 ± 1.32
	2	22.75 ± 1.89	303.32 ± 8.72	54.83 ± 5.48
	3	25.24 ± 1.77	401.60 ± 5.44	104.33 ± 3.69
	4	36.46 ± 1.82	472.76 ± 0.88	139.83 ± 2.89

. These parameters are: HRC—heat release capacity, THR—total heat release, PHRR—peak heat release rate, T_PHRR—peak heat release rate temperature, time—the time when PHRR occurs, char yield—char percentage; decomposition rate—percentage of the mass consumed during combustion.

Figure 6 and Figure 7 are a graphical representation of the time- and temperature-dependent HRR curves of the microalgae waste samples.

According to Figure 7, biowaste sample S1 had an HRR curve with a narrower peak (1) and PHRR of 158.84 W/g at 314.72 °C, while (2) showed a PHRR of 51.16 W/g at 450.30 °C. The S2 biowaste sample showed four wide peaks and the following PHRR readings: 21.41 W/g, 22.75 W/g, 25.24 W/g, and 36.48 W/g at 261.66 °C, 303.32 °C, 401.60 °C, and 472.76 °C, respectively. The PHRR readings in sample S2 were undeniably significantly lower than the PHRR readings in sample S1. Figure 7 shows the complex decomposition mechanism of sample S2 as compared to sample S1. These findings agree with those previously revealed by the thermal decomposition assessment within the thermogravimetric analysis of the two samples. A higher number of decomposition stages and a higher amount of residue were found from thermogravimetric analysis for S2 compared to S1. A higher amount of char residue led to a reduction in the amount of flammable gases and lower values for the HRR [28].

HRC is the most important parameter at the microlevel when analyzing the energy performance of materials using the MCC technique. Low HRC values indicate low flammability in the MCC test and low real-scale fire hazard, while THR may be an indicator of the total amount of the generated fuel [29]. A considerable HRC and THR decrease was noted for the microalgae waste sample S2 compared to the microalgae waste sample S1. Thus, the HRC readings for the microalgae waste samples S1 and S2 (Table 2) were 95.67 J/(g·K) and 44.11 J/(g·K), respectively, HRC suffered a twofold decrease, and THR (Table 2) was 11.03 kJ/g and 3.64 kJ/g, respectively, suffering a threefold decrease. This behavior was due to the presence of alginates in the biowaste sample S2. The fireproof nature of alginates is well-known in literature. They are used in various composite materials to increase flame-retardant properties [30,31].

Since the percentage of carbonized waste is an indicator of the amount of unburned fuel in a material, the material with a higher percentage of carbonized waste is expected to produce a lower amount of heat during combustion [32]. The amount of waste remaining after the combustion of each sample was recorded and the char yield ratio between the mass of the waste and the initial mass of the sample was calculated. The char yield value for sample S1 was lower than that for sample S2, i.e., 32.12% compared to 55.14%, which means that the char yield of the biowaste sample S2 was 71.67% higher than that of the biowaste sample S1. These findings are consistent with those previously revealed by thermogravimetric analysis.

3.3. Perspectives

Assessing the thermal behavior of the biowastes resulting from the treatment of contaminated air by biotrickling filtration is important for their energy recovery. As mentioned earlier, there are literature studies suggesting the potential of the microalgae biomass to be used as biofuels [33]. It has been shown that the generation of pollutants during microalgae pyrolysis can be avoided by controlling the temperature at about 300 °C. The first PHRR value found for sample S1 in our study was similar to that reported for wood by other researchers [34]. It is to be noted, however, that the T_PHRR was 315 °C for sample S1 and 380 °C for wood [34].

As shown by the analysis of the energy characteristics of the S1 and S2 biowastes, the presence of alginates induces a considerable decrease in the amount of heat released. Thus, these aspects suggest the potential use of the S2 waste towards the flame retardancy area for some materials. Gady et al. [28] used alginates and the algae biomass to obtain composite materials with improved flame-retardant properties subject to further investigations with standardized procedures.

These findings show that the biowaste issued from air biotreatment can become a valuable resource for different applications instead of being disposed of. Depending on the particularities of the air biotreatment processes and the type of generated biowastes, other characterization techniques could be envisaged. Moreover, a LCA (life cycle analysis) could be useful for the identification of the most suitable technical solutions for biowaste valorization.

4. Conclusions

The thermogravimetric and microscale combustion calorimetry studies applied to two types of biowastes (S1 and S2) resulting from biotrickling filtration of ethanol in air by a mixture of microalgae and compost heterotrophs co-immobilized on different supports revealed their thermal and energy properties. The purpose was to determine how such wastes could be better valorized if one or the other version of the biotrickling filter is approached as this aspect could be important for the practitioners. The rate of the DTG and DTA curves in the two working atmospheres was similar for the S1 sample (biomass collected from the surface of the polypropylene spheres used as a microorganisms’ support) when thermal decomposition comprised three stages with various percentage mass loss values. The percentage mass loss was about 70% for sample S1 within the 236–559 °C temperature range irrespective of the working atmosphere. As far as sample S2 is concerned (waste alginate beads with microbiological content), the rate of the DTG and DTA curves within the 220–560 °C temperature range was similar in both working atmospheres, while the percentage mass loss was about 30%. At temperatures higher than 560 °C, the decomposition mechanism of sample S2 was influenced by the working atmosphere, which means that a strong exothermic peak occurred within the 558–570 °C temperature range in the air, while in nitrogen, it continued slowly until the end of the test. When the thermogravimetric analysis was performed in air, mass losses in the temperature range of 558–570 °C could be associated with decomposition of the carbonate. At 700 °C, the amount of waste was found to be much higher in sample S2 than in sample S1 regardless of the working atmosphere.

For the first time, assessment of the biowaste used in a biotrickling filter via microscale combustion calorimetry was performed, revealing the possibility of energy recovery for sample S1. The PHRR value (158.84 W/g) of this biomass was comparable to that of wood but occurred at a lower temperature. The HRC values for the samples S1 and S2 were 95.67 J/(g·K) and 44.11 J/(g·K), respectively. As far as sample S2 is concerned, the presence of alginates contributed to a significant decrease in the amount of heat released. Therefore, these wastes could have a potential application for improving the flame retardancy of some materials.