Enhanced Biogas Production in Pilot Digesters Treating a Mixture of

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Biofuels and Biomass

Enhanced biogas production in pilot digesters treating a mixture of sewage sludge, glycerol, and food waste Janaina dos Santos Ferreira, Isaac Volschan Jr., and Magali Christe Cammarota Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00742 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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Enhanced biogas production in pilot digesters treating a mixture of sewage sludge, glycerol, and food waste

Janaína S. Ferreiraa, Isaac Volschan Jr.b, Magali C. Cammarotaa*

a

Department of Biochemical Engineering, School of Chemistry, Federal University of

Rio de Janeiro, Cidade Universitária, Av. Athos da Silveira Ramos, 149, Bl. E, Sl. 203, Ilha do Fundão, 21941-909, Rio de Janeiro, Brazil, [email protected], [email protected]. b

Water Resources and Environmental Engineering Department, Polytechnic School,

Federal University of Rio de Janeiro, Cidade Universitária, Av. Athos da Silveira Ramos, 149, Bl. D, Sl. 202, Ilha do Fundão, 21941-909, Rio de Janeiro, Brazil, [email protected].

*Corresponding author: Phone: +55 21 3938-7568; Fax: +55 21 3938-7567; E-mail address: [email protected]

ORCID: 0000-0002-0637-9707 (Magali C. Cammarota), 0000-0001-8634-8857 (Isaac Volschan Jr.), 0000-0003-0248-7898 (Janaina S. Ferreira).

Abstract Anaerobic co-digestion of a ternary mixture of sewage sludge with food waste and residual glycerol was evaluated in pilot scale units, aiming to increase methane production in sludge anaerobic digesters at sewage treatment plants. Two pilot scale anaerobic digesters (useful volume of 320 L) operated simultaneously for 60 days at room temperature (24 °C on average). One digester received only sludge in the feed and the other received the ternary mixture. The feeding routine of the digesters respected a 1 ACS Paragon Plus Environment

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semi-continuous mode. The units continuously promoted the mixing of the sludge through a bottom-up recirculation system and according to a hydraulic retention time of 30 d. The digester fed with just sewage sludge had greater instability during the whole period of operation, producing about 3 L d-1 of biogas with 23% methane, volatile solids (VS) removal of 65.3%, and methane yield (MY) of 98.5 LN CH4 kg-1VS add. On the other hand, the digester fed with ternary mixture produced 21 L d-1 of biogas with 43% methane, VS removal of 73.4%, and MY of 174.5 LN CH4 kg-1VS add. The mass and energy balances showed that through methane recovery, the ternary mixture substrate contributed for the production of 12 times more energy than digesters fed with pure sewage sludge.

Key words: sewage sludge, glycerol, food waste, anaerobic co-digestion, biogas, pilot digesters

Introduction

Although the sludge generated in sewage treatment plants represents only 1% to 2% of the treated sewage volume, the increased rates of population growth has led to increased sludge production in these plants. Sludge management is very complex (densification, stabilization, conditioning, dehydration, and final disposal) and expensive1. Although the trend is towards the application of technologies that produce less sludge2, the use of so-called conventional systems, which are known to generate appreciable amounts of sludge, such as activated sludge1, cannot be ruled out. The treatment and final disposal of the sludge generated in the sewage treatment plants is a major environmental problem for public or private sanitation companies1,3. To

2 ACS Paragon Plus Environment

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produce a quality effluent and reduce costs, sewage treatment plants have sought to solve this problem. Among the several options currently available, anaerobic digestion is probably the most widely used sludge treatment technology. The growing concern about energy security, environmental impacts, and increased energy costs for wastewater treatment has restored the anaerobic digestion process as a major renewable energy production technology4-6. The biodegradability of organic compounds depends on the composition of carbohydrate, protein, and lipid fractions. The generation of methane as the final product of anaerobic digestion is improved if initial hydrolysis of organic compounds becomes the slowest step in comparison to acidogenesis, acetogenesis, and methanogenesis steps. The general term of hydrolysis usually expresses the concepts of disintegration, solubilization, and enzymatic hydrolysis. Although the first-order rate hydrolysis coefficient of sewage sludge varies from 0.17 up to 0.60 d-1, values of 0.34 and 0.55 d-1 are observed with kitchen waste and food waste, respectively7. Co-digestion of sludge with one or more substrates may improve the yield of sludge anaerobic digesters. The co-substrate can provide nutrients that are deficient in the sludge and at the same time provide a positive synergetic effect in the mixture, leading to stable digestion and improved biogas yield8-10. Sewage sludge is the second major substrate in quantity for anaerobic co-digestion, behind only the organic fraction of municipal solid waste – OFMSW9. Food wastes are part of the OFMSW discarded from various sources, including food processing plants, commercial and domestic kitchens, and restaurants. Anaerobic digestion is a promising approach for its treatment11-13. Anaerobic co-digestion of sewage sludge and OFMSW has been the most reported in the literature, with satisfactory results being obtained with this technology14-16.


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Another possible co-substrate for anaerobic co-digestion with sludge is glycerol, which is the main byproduct of biodiesel production. The mass generation of glycerol (with purity between 55 and 90%) is approximately 10% of biodiesel production17. The final disposal of this organic residue is a serious concern, and anaerobic digestion can be employed for its treatment and stabilization. However, the efficiency of the anaerobic treatment of glycerol can be compromised by the presence of residual concentrations of the alkaline catalyst and alcohol (usually methanol), both used in the transesterification reaction of biodiesel production, as well as the presence of free fatty acids. The absence of nitrogen in the residual glycerol is also a factor that compromises anaerobic digestion18,19. On the other hand, its high theoretical potential for methane production, around 0.43 Nm3 CH4 kg-1 glycerol20, makes the use of glycerol as co-substrate in the co-digestion of organic waste quite attractive. The literature contains several studies about co-digestion of sewage sludge with crude glycerol21-26. This study investigated the co-digestion of a ternary mixture of sewage sludge, food waste, and crude glycerol to demonstrate the potential that substrate diversity offers anaerobic digestion process efficiency. Organic content stabilization and methane production were evaluated in experiments in pilot scale anaerobic digesters. The application of co-digestion technology for this specific ternary mixture contributes to the adequate final destination of important organic wastes such as food waste and glycerol from the biofuel industry.

Experimental Section

Biological excess sludge from an activated sludge system was used during the experiment and characterized in terms of humidity, total solids (volatile and fixed),


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carbon, and nitrogen concentrations. The activated sludge plant is part of the Environmental Sanitation Experimental Center of the Federal University of Rio de Janeiro, located in Rio de Janeiro, Brazil. Weekly, food waste from the university restaurant was crushed and diluted with distilled water (food waste: distilled water ratio equal to 1:3 v v-1) in an industrial blender to reduce size and homogenize the food waste. The addition of distilled water was needed to correct the humidity of the food waste (about 73%) to a value close to the humidity of the waste activated sludge used (93% on average) without addition of interfering chemical elements (e.g. iron and chlorine present in the local water supply). An aliquot of water and food waste mixture was submitted to physicochemical characterization, and the remaining part was stored in a freezer (-20 °C) for later use during the experiments. According to a semi-continuous feeding regime, a specific amount of frozen mixture was removed from the freezer and acclimated at room temperature. Mixture characterization consisted of determining the pH, humidity, total solids (fixed and volatile), carbohydrate, protein, oil and grease, carbon, and nitrogen concentrations. The residual glycerol came from a biodiesel industry that used bovine fat and soybean oil as the basic sources for the biodiesel production process. Residual glycerol was collected after particle separation and alcohol (methanol) recovery phases. Samples of the residual glycerol were characterized for glycerol, total carbon, chemical oxygen demand (COD), chloride, methanol, and humidity. Then the samples were stored under refrigeration until the moment of use. Due to its high viscosity, the residual glycerol was diluted 10 times with tap water before mixing it with food waste and sludge. According to the experimental design, preliminary assays were conducted at the bench scale to define the best ternary mixture composition to use during the following


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experimental phase in pilot scale digesters. The results of these previous assays indicated the ratio of 89.6% of sludge, 10.0% of food waste, and 0.4% residual glycerol (v v-1) as the best condition27. Two digesters operated simultaneously for 60 days at room temperature. Digester D1 received only sludge in the feed, while digester D2 was fed with the ternary mixture. The acrylic digesters were 1.7 m height and 0.55 m diameter, with a total volume of 400 L and a useful volume of 320 L. To maintain mixed conditions in the reactors, an external pump device provided a bottom-up recirculation of the entire volume under digestion per day. Every 30 minutes, the external mechanical device with a pump capacity close to 27 L h-1 was automatically turned on for 30 min, extracted part of the volume content from the bottom of the digesters, and pumped to near the top of the digesters. The digesters were inoculated with sludge from an operating anaerobic digester (10% of the useful volume) and the remainder volume (90% of the useful volume) completed with sewage collected at Environmental Sanitation Experimental Center28. Both digesters remained in batch for 90 days for system adaptation and then were fed in a semi-continuous mode (twice a week), and the monitoring of the main parameters was started. The mode of feeding was established to provide consistent feed in the digesters, due to the small and variable daily volume of waste activated sludge generated at the Environmental Sanitation Experimental Center. Before each feeding operation, the recirculation pump device was turned off for supernatant separation and digested sludge discard to maintain the hydraulic retention time (HRT) equal to 30 d. Then an amount of ternary mixture was replaced according to the required feeding load. Figure 1 shows a schematic drawing of feeding and discarding operations as well the dimensions of the digesters.


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For operational conditions and behavior of the digesters, the following parameters were monitored: biogas volume and composition, total suspended solids (fixed and volatile), pH, temperature, volatile fatty acids, and total alkalinity. The volume of biogas was measured using Ritter TG5 meter, coupled to a Rigamo software, which recorded the biogas volume at ambient temperature (average of 23.6 ± 1.6 °C) and atmospheric pressure every 30 min. The biogas composition was measured in GEM 2000 portable equipment (Landtec). Temperature and pH and were monitored by automated Etatronds controllers (eControl), the data were stored every 30 min on an SD card coupled to the system. Carbohydrates and proteins were determined according to Dubois et al. (1956)29 and Lowry et al. (1951)30. Volatile Fatty Acids (VFA) and Total Alkalinity (TA) were determined according to DiLallo and Albertson (1961)31 and Ripley et al. (1986)32. Glycerol content was measured according to Bondioli and Bella (2005)33. Oil and grease, COD, and VS concentrations were measured according to standard procedures34.

Results and Discussion Table 1 presents characteristics of the substrates used in the co-digestion experiments. Excess sludge was poorly concentrated due to the atypical characteristics of the university campus sewage, which presented low biochemical oxygen demand (BOD) values (161±83 mg L-1). The C: N ratio of 9.5 of the sludge, close to the range suitable for anaerobic digestion (15 to 30)35, indicates sufficient nitrogen. The concentration of nitrogen in the food waste was higher than required, in relation to carbon (C: N 6.4). Values below 6 indicate low carbon availability combined with high concentrations of ammonia, which may cause toxicity to anaerobic microorganisms36. The food waste had 69.8, 26.2, and 22.3 mg g-1 of oil and grease, proteins, and carbohydrates, respectively. 7 ACS Paragon Plus Environment

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Lipid molecules are more complex to biodegrade than carbohydrates and proteins, and this should influence methane production. For potential methane production, expressed in standard pressure (1 atm) and temperature (0 °C) conditions (STP conditions) per g volatile solids (VS), lipids have higher potential (1.014 L CH4 g-1 VS) than carbohydrates and proteins (0.415 and 0.496 L CH4 g-1 VS, respectively)37. The low methanol values found in the residual glycerol are not detrimental to anaerobic digestion. Previous studies have found good methane production and COD removal in anaerobic biodegradability tests of effluent containing methanol concentrations up to 4,000 mg L-1 (date not shown). The residual glycerol is basically carbon (glycerol) and salinity. The high salinity is due to the use of NaOH in the esterification reaction via basic catalysis and HCl in the neutralization step. The chloride concentration would be extremely detrimental to anaerobic digestion, because concentrations between 4 and 9 g Cl- L-1 strongly inhibit methanogenesis38. However, the dilution of the residual glycerol and the small percentage in the ternary mixture contributed to acceptable levels of salinity. In general, sewage sludge is characterized by a relatively low C: N ratio and high buffer capacity and combines well with substrates containing high amounts of easily biodegradable organic matter and low alkalinity9. The optimum proportion of substrates in the evaluated ternary mixture greatly increased the organic load to be introduced into the digester (increase of 3.3-fold in VS concentration compared to pure sludge) without changing the C: N ratio, which remained below 10, indicating even more nitrogen than necessary. Figure 2a shows the organic loading rate added (OLRadd) values for the digesters D1 (fed with sludge only) and D2 (fed with the ternary mixture). An increase in OLRadd was observed after 20 days operation of digester D2, which can be explained by the


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great VS variability of the food waste, because it was collected weekly (Figure 2b). Even with a higher concentration of VS in the ternary mixture, OLRadd values during the operation of digester D2 were still low, due to the characteristics of raw sewage, as well as the biological excess sludge, with a mean VS value of 1.3 g L-1. A typical low-load sewage has an average COD of 250 mg L-1; while the thickened secondary sludge shows VS of 12 g L-1 on average39. Due to this variability, the parameters that were measured during the 60 days that the digesters were operated are divided in Table 2 into two periods of operation lasting 30 days each. The two operating periods were delineated based on the behavior of the pH and the VFA / TA relation in digester D2, as described below. During the first 30 days of operation, NaHCO3 was added to the digester to adjust pH and increase alkalinity, while in the next 30 days the digester stabilized and the operation was continued without addition of alkalis. The mean OLRadd for D1 was 0.03-0.04 and for D2 0.14-0.23 kgVS add m-3 d-1, characteristic values of low rate digesters. The pH, as well as the OLRadd, remained stable during the entire operation of digester D1 (6.9 ± 0.3). In the first days of operation of digester D2, the pH of the ternary mixture (7.2) was reduced to values of 5.6 ± 0.5 (Table 2, Figure 3). The addition of food waste and glycerol to the sludge resulted in a higher production and accumulation of volatile fatty acids (VFA) in the digester, and pH correction with sodium bicarbonate was required in the first month of operation. When pH had been stabilized at 6.1 ± 0.5, adjustment procedures were discontinued. The acidification of the medium is reported in studies of co-digestion of sludge with food waste40 or glycerol21, requiring correction of the pH with sodium bicarbonate until the stabilization of the system. The concentration of VFA in D2 reached 2200 mgHAc L-1 in the first month of operation, while the total alkalinity (TA) reached 2120 mgCaCO3 L-1,


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which corresponds to a VFA / TA relation of 1.04 and complete instability of the system. On average, the VFA / TA relation was 1.31±0.85 (Table 2, Figure 3). Low values of the VFA / TA relation are indicative of a good buffering of the medium and efficient anaerobic digestion. In the second month of operation, after the pH adjusting and stabilization period, the average TA values increased to 2500 mgCaCO3 L-1 and the average VFA concentration reduced to 500 mgHAc L-1, recovering the stability of the system with VFA / TA relation 0.32±0.06 (Figure 3). Generally, the higher the temperature, within the favorable limit to methanogenesis, the greater the rate of anaerobic decomposition35. However, the operation of the pilot digesters on a pilot scale, at a mean temperature of 23.6 ± 1.6 °C, showed a higher biogas rate than the operation at 30 °C on the bench scale (data not shown). One explanation would be the beneficial effect of the agitation promoted by the recirculation pump on the pilot scale. Mixing conditions in the reactors provided by the bottom-up recirculation of sludge must have contributed to a greater homogeneity of the digester content and a better transfer of substrates to the microorganisms41. The VS reduction suffered a great variation in the first 30 days of operation in both digesters and stabilized in the second month of operation (Table 2 and Figure 3). Nevertheless, higher values were observed in digester D2. During this period, VS reduction achieved by digester D2 (73.4±6.0 %) was 12% higher than the one achieved by digester D1 (65.3±7.3 %). There are no data published in the literature related to the anaerobic co-digestion of a ternary mixture of sewage sludge, residual glycerol, and food waste in pilot digesters, which makes an analysis of the results obtained difficult. Co-digestion of different binary mixtures with sewage sludge and crude glycerol or food waste in mesophilic pilot digesters operated under HRT around 20 d resulted in VS reductions of 40 to 70%42-44. In a CSTR bench reactor, operated under mesophilic


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conditions and with HRT of 33 d, Rodríguez-Abalde (2017)45 obtained SV reduction 83% higher for the co-digestion of a ternary mixture composed of pasteurized slaughterhouse waste, pig slurry, and glycerin than the digestion of just pig slurry. Figure 4a shows the cumulative volume of biogas produced in each of the digesters as a function of time; the daily production of biogas can be calculated from this figure. The digester that was fed just with sludge (D1) remained unstable throughout the operation, and the daily production of biogas varied between 0.5 and 7.3 L d-1 (Figure 4a). The average biogas rate values, presented in Table 2, confirm this instability, which probably occurred due to failures in the feed pump of the digesters. Considering only the periods of regular operation, the average biogas rate in digester D1 was 5.0 ± 2.4 L d-1. On the other hand, digester D2 presented less instability and a lower biogas rate in the first month of operation due to acidification caused by the addition of crude glycerol and food waste to the sludge, as previously reported. In the second month of operation, biogas rate in digester D2 increased and stabilized at around 21.3 L d-1. Compared to the values obtained in the second month under regular operational conditions, there was a considerable increase in biogas rate of 5.0 L d-1 (in digester D1) to 21.3 L d-1 (in digester D2), with the addition of food waste and glycerol to the sewage sludge. After microorganism inhibition due to acidification, the continuous higher organic load introduced in digester D2 allowed a greater availability of biodegradable organic matter and, consequently, a larger production of biogas. Figure 4b shows that the methane content in biogas also varied considerably up to 45 days of operation (from 11 to 36% in D1 and from 4 to 45% in D2) and then stabilized at the mean values shown in Table 2. Again, values obtained during the most stable operation period indicate that digester D2 produced biogas with almost twice the methane content in the biogas. Because the volume and composition of the biogas


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produced in the digesters were very instable, the daily production of methane (shown in Table 2) was calculated from graphs of cumulative methane volume as a function of time (Figure 4c). Production of methane varied in both digesters during the first month of operation, and then production stabilized according to the values presented in Table 2. Daily production of methane achieved mean values of 0.1 and 14.5 L in digesters D1 and D2, respectively, proving the increased methane production in the anaerobic codigestion of the ternary mixture. The biogas yield (BY) was also calculated by the angular coefficient of linear correlations obtained for the period before and after system stabilization (Figures 4d and 4e). Values of BY and methane percentages were used to estimate methane yield (MY). The BY and MY values presented in Table 2 indicate that digester D1 performed better only in the first month of operation, while digester D2 had improved performance in the second month. MY values obtained in digester D2 were 1.8 up to 2.0 times higher than in digester D1. Table 2 also shows the biogas and methane productiveness, considering the mean biogas and methane yields as well as the useful volume of digesters. Low values, compared to literature data, are due to the atypical characteristics of the university campus sewage, as mentioned earlier, resulting in low values of VSadd and OLRadd. Anaerobic co-digestion of mixtures of sewage sludge and glycerol (OLR 1.0 kgVS add m3

d-1, HRT 23-25 d)21 or food waste (OLR 6.0 kgVS add m-3 d-1, HRT 20 d)42 resulted in

MP of 0.78 and 2.33 LCH4 L-1 d-1, respectively. While co-digestion of a ternary mix of pasteurized slaughterhouse waste, pig slurry, and glycerin (OLR 2.5 kgCOD add m-3 d-1 and HRT 33 d)45 resulted in MP of 0.48 LCH4 L-1 d-1. These studies also achieved increased MP in the co-digestion of mixtures compared to feeding with a single substrate. The same occurred with the ternary mixture of sewage sludge, residual


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glycerol, and food waste evaluated that, despite the low values obtained, increased the MP by 3.8 times, compared to the digestor fed with only sludge. The higher values of MY and MP in the co-digestion of the ternary mixture in relation to the control is due to the good VFA / TA relation (0.3 after stabilization), to a good synergy between the three substrates, and to the greater availability of carbon from glycerol and food waste24. In a reactor operated under mesophilic conditions and HRT of 33 d, evaluating the co-digestion of a ternary mix of pasteurized slaughterhouse waste, pig slurry, and glycerin, Rodríguez-Abalde et al. (2017) obtained a 2.5-fold increase in MY over the digestion of only one of these co-substrates45. Considering the results obtained in the second month of operation of the pilot digesters (VS removal, BY, and % CH4), complementary data from the literature, and a hypothetical inflow of 10.7 L d-1 (equivalent to HRT 30 d), mass and energy balances were carried out. Table 3 presents the results obtained for the mass and energy balances of digesters D1 and D2. In digester D1, daily production of biogas from just sludge digestion may achieve 10.5 MJ, while the digested sludge maintained a potential energy of 84.5 MJ. The sum of the produced energy (95 MJ) is lower than the one obtained from typical raw sludge (203.3 MJ). In digester D2, the values of energy produced are 69.9 (from the biogas) and 67.6 MJ (from the digested sludge), totaling 137.5 MJ, lower than the sum of the energy fraction of each constituent of the raw ternary mixture (202 MJ), but 40% higher than in D1, making the co-digestion of the three substrates more energy efficient. Considering the burning of the purified biogas (biomethane), digester D2 presents even more promising results, as it would lead to an energy production 12 times larger (43.2 MJ) than digester D1 (3.5 MJ). Therefore, one may say that the anaerobic co-digestion of the ternary mixture of sewage sludge, food waste, and residual glycerol is technically feasible and may contribute to the increase of methane


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production in the sludge digesters of sewage treatment plants, allowing its energy utilization. In addition, the same strategy may contribute to volume reduction and treatment of three potential pollutant wastes.

Conclusions The anaerobic co-digestion of the ternary mixture consisting of sludge: food waste: glycerol in the proportion of 89.6: 10.0: 0.4 (in % v v-1) in pilot semi-continuous digesters favored methane production, with a MY of 189 LCH4 kg-1VS add against 107 LCH4 kg-1VS add obtained in a digester operated under the same conditions but fed with only sewage sludge. The 1.8-fold increase in MY occurred without prejudice to the stabilization of the ternary mixture, which showed a VS removal of 73% under 30 d of HRT. In the mass and energy balances, there was a 12-fold increase in energy production for the ternary mixture compared to the control (pure sludge). The acidification caused by the addition of crude glycerol and food waste to the sludge must be avoided through a balanced ratio of residues and pH adjustment at the beginning of digester operation. Once the initial inhibition of the microorganisms by the volatile acids is overcome, the higher organic load introduced into the digester fed with the ternary mixture provided greater availability of biodegradable organic matter and, consequently, larger production of biogas/methane.

Acknowledgements This work was supported by project funds from the Brazilian National Council for Research and Development (CNPq), and Carlos Chagas Filho Foundation for Research Support in the State of Rio de Janeiro (FAPERJ).


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(14) Nielfa, R., Cano, R.; Fdz-Polanco, M. Biotechnol. Reports 2015, 5, 14-21. (15) Silvestre, G.; Fernández, B.; Bonmatí, A. Bioresour Technol. 2015, 193, 377-385. (16) Grosser, A. Energy 2018, 143, 488-499. (17) Yang, F.; Hanna, M. A.; Sun, R. Biotechnol. Biofuels 2012, 5, 13. htpp://dx.doi.org/10.1186/1754-6834-5-13. (18) Thompson, J. C.; He, B. B. Appl. Eng. Agric. 2006, 22, 261-265. (19) López, J. A. S.; Santos, M. M.; Pérez, A. F. C. Bioresour. Technol. 2009, 100, 5609-5615. (20) Astals, S.; Ariso, M.; Galí, A. J. Environ. Manage. 2011, 92, 1091-1096. (21) Fountoulakis, M. S.; Petousi, I.; Manios, T. Waste Manage. 2010, 30, 1849-1853. (22) Jensen, P. D.; Astals, S.; Lu, Y.; Devadas, M.; Batstone, D. J. Water Res. 2014, 67, 355-366. (23) Nartker, S.; Ammerman, M.; Aurandt, J.; Stogsdil, M.; Hayden, O.; Antle, C. Waste Manage. 2014, 34, 2567-2571. (24) Athanasoulia, E.; Melidis, P.; Aivasidis, A. Renew. Energy 2014, 62, 73-78. (25) Nghiem, L. D.; Nguyen, T. T.; Manassa, P.; Fitzgerald, S. K.; Dawson, M.; Vierboom, S. Int. Biodeter. Biodegr. 2014, 95, 160-166. (26) Zahedi, S.; Rivero, M.; Solera, R.; Perez, M. Fuel 2018, 215, 285-289. (27) Ferreira, J. S.; Cammarota, M. C.; Volschan Junior, I. J. Energy Power Eng. 2017, 11, 569-583. (28) Niazi, S.; Brown, J. L. Fundamentals of Modern Bioprocessing; CRC Press: Boca Raton, 2016. (29) Dubois, M.; Gilles, A.; Hamilton, J. K.; Rebers, P. A.; Smith, F. Anal. Chem. 1956, 28, 350-355. (30) Lowry, O. H.; Rosebrough, N. J.; Lewis F. A.; Randall, R. J. J. Biol. Chem. 1951,


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(45) Rodríguez-Abalde, Á.; Flotats, X.; Fernández, B. Waste Manage. 2017, 61, 521528. (46) Kim, Y.; Parker, W. Bioresour. Technol. 2008, 99, 1409-1416. (47) Giudicianni, P.; Bozza, P.; Sorrentino, G.; Ragucci, R. Waste Manage. 2015, 44, 125-134. (48) Bohon, M. D.; Metzger, B. A.; Linak, W. P.; King, C. J.; Roberts, W. L. Proc. Combust. Inst. 2011, 33, 2717-2724.


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Figure captions

Figure 1. Experimental scheme and details of pilot digesters used in the study.

Figure 2. Organic loading rate added (OLRadd) applied in digesters D1 (Control – fed with sludge) and D2 (fed with ternary mixture) over the operating time (A) and variability of the VS concentration in the food waste each preparation week (B).

Figure 3. Variation of VS input and output, and VS reduction during the 60 days of operation for digesters D1 (Control – fed with sludge) and D2 (fed with ternary mixture).

Figure 4. Volume of biogas accumulated (A), methane percentage in biogas (B), and volume of methane accumulated (C) as a function of the operating time, and volume of biogas accumulated versus VS applied or removed (D and E) in digesters D1 (Control – fed with sludge) and D2 (fed with ternary mixture).


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Table 1. Characteristics of the waste activated sludge (WAS), food waste (FW), residual glycerol (RG), and ternary mixture used in the study.

Parameter

RG

WAS

FW

Ternary mixture

8.0

6.9±0.9

5.5±0.6

6.6±0.2

92.2±1.5

93.5±1.6

92.5±2.0

92.0±0.4

1,119±247

nd

nd

nd

Chloride (g Cl- L-1)

37.1±1.1

nd

nd

nd

Methanol (mg L-1)

19.0±1.3

nd

nd

nd

Glycerol (%)

74.0±3,5

nd

nd

nd

1.26

nd

nd

nd

TS (mg g-1)a

nd

12.2±0.4

236.6

34.6

TVS (mg g-1)a

nd

8.2±0.4

212.5

28.6

32.8

36.2

53.5

37.9

Nitrogen (%)

0

3.8

8.3

4.3

C: N

-

9.5

6.4

8.9

pH Humidity (%) COD (g L-1)

Density at 20 ºC (g mL-1)

Carbon (%)

a

wet content. COD = chemical oxygen demand, TS = total solids, TVS = total volatile

solids, nd = not determined.


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Table 2. Results obtained during 60 days of operation of the semi-continuous pilot digesters, operated with HRT 30 d. Operation period

Operation period

1 – 30 days

31 – 60 days

Parameter D1

D2

D1

D2

T (°C)

24.5±1.9

24.5±1.9

22.5±1.2

23.2±0.6

OLRadd (kgVS add m-3 d-1)

0.04±0.02

0.14±0.10

0.03±0.00

0.23±0.05

pHinput

7.1±0.1

7.2±0.0

7.1±0.1

7.2±0.0

pHoutput

6.9±0.3

5.6±0.5

6.9±0.1

6.1±0.5

VFA (mgHAc L-1)

382±328

1626±380

163±92

936±356

TA (mgCaCO3 L-1)

1027±733

1523±603

750±517

2873±595

VFA/TA relation

0.43±0.18

1.31±0.85

0.25±0.11

0.32±0.06

VS reduction (%)

35.4±20.3

44.7±21.9

65.3±7.3

73.4±6.0

Biogas rate (L d-1)

3.8±3.1

8.1±0.2

2.8±3.3

21.3

% CH4 (LCH4 100 L-1biogas)

23.3±7.5

14.0±7.1

23.0±1.6a

43.0±4.2a

Methane rate (LCH4 d-1)

1.0±0.9

3.8±3.6

0.1

14.5a

BY (Lbiogas kg-1VS add)b

665.9

369.2

464.4

440.1

BY (Lbiogas kg-1VS rem)b

952.3

479.1

106.8

551.7

MY (LCH4 kg-1VS add)c

155.2

51.7

106.8

189.2

142.1

47.3

98.5

174.5

221.9

67.1

24.6

237.2

203.3

61.4

22.7

218.8

BP (Lbiogas L-1reactor d-1)d

0.012

0.025

0.009

0.067

MP (LCH4 L-1reactor d-1)d

0.003

0.012

0.000

0.045

0.003

0.011

0.000

0.042

-1

c

(LN CH4 kg

VS add)

-1

c

MY (LCH4 kg

VS rem)

-1

(LN CH4 kg

c

VS rem)

(LN CH4 L-1reactor d-1)d a

value obtained in a stable period (last 15 days of operation); b determined in graphs of volume of

accumulated biogas × VS applied or removed accumulated; c determined from mean values of BY and % CH4. d determined from mean values of biogas and methane rates. OLRadd = organic loading rate added; VFA = volatile fatty acids; TA = total alkalinity; VS = volatile solids; BY and MY = biogas and methane yield; BP and MP = biogas and methane productiveness. LN = volume in standard temperature and pressure.


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Table 3. Mass and energy balances of digesters at a flow rate of 10.7 L d-1 of sludge (D1) or ternary mixture (D2). Mass balance Digester

Substrate Sludge (ρ =1 kg L-1)

% in feed (v v-1) 100.0

Volumein (L) 10.7

Massin (kg TS) 10.7

Massin (kg VS) 6.4

Removed solids (kg VS) 4.2

Digested Biogas sludge (kg TS) (m3) 6.5 0.45

Methane (m3) 0.104

D1 D2

Ternary mixture:

100.0

10.7

10.7

7.5

5.5

5.2

1.290

Sludge (ρ =1 kg L-1)

89.6

9.59

FW (ρ =1.26 1 kg L-1) 10.0

1.07

RG (ρ =1 1 kg L-1)

0.054

0.4

3.0

Energy balance Digester

Substrate

Fed in digester

D1

Massin (kg TS) -1 10.7 Sludge (ρ =1 1 kg L )

Calorific value (MJ) 203.3

Massout (kg TS) 6.5


Volume (m3) 0.45


Volume (m3) 0.104


D2

Ternary mixture:

10.7

202.0

5.2

67.6

3.0

69.9

1.290

43.2

Sludge (ρ =1 1 kg L-1) 9.59

182.2

FW (ρ =1.26 1 kg L-1) 1.07

18.9

RG (ρ =1 1 kg L-1)

0.86

0.054

Digested sludge

Biogas

Methane

FW = food waste; RG = residual glycerol; TS = total solids; VS = volatile solids; VS/TS = 0.6 (D1) and 0.7 (D2); Calorific value (MJ kg-1 TS) of raw sludge (19)46, digested sludge (13)1, food waste (17.7)47, residual glycerol (16)48; Calorific value (MJ m-3) of biogas (23.3)1.

22

ACS Paragon Plus Environment

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Figure 1


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Figure 2


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Figure 3


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Figure 4


Enhanced Biogas Production in Pilot Digesters Treating a Mixture of

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