Anaerobic digestion of municipal solid waste: Development of a thermophilic inoculum. L. Ortega1,2, S. Barrington1 and S.R. Guiot2. 1 McGill University ...
Proc. 10th World Congress on Anaerobic Digestion (29 August - 2 September 2004), Montreal, Canada (Editor S.R. Guiot), Vol. 3, pp. 1682-1686
Anaerobic digestion of municipal solid waste: Development of a thermophilic inoculum L. Ortega1,2, S. Barrington1 and S.R. Guiot2 1
McGill University, Bioresource Eng. Department. 2111 Lakeshore Road, Ste-Anne-de-Bellevue (Quebec), H9X 3V9 Canada. 2 National Research Council of Canada, Biotechnology Research Institute. 6100 Royalmount Av. Montreal (Quebec), H4P 2R2 Canada.
Abstract A thermophilic anaerobic inoculum was developed for the treatment of the organic fraction of a separately collected municipal solid waste (OFMSW) by an abrupt change from mesophilic to thermophilic conditions. Such an instantaneous increase in temperature from 35 to 55°C did not result in the pasteurization of the biomass. The substrate consumption rate (SA) indicated the presence of thermophilic fermentative activity as well as methanogenic activity related to both acetoclastic and hydrogenophilic microorganisms. The inoculum generated was used to treat OFMSW under thermophilic conditions. During 4 batch-cycles (21 days-each), a shredded, 17.9%-solid OFMSW obtained from a cafeteria was fed at a substrate/biomass (F/M) ratio between 0.12 and 4.43 kgVS/kgVSS. The potential for gas and methane production varied between 0.2 to 1.4 m3/kgVSfed and between 0.1 to 0.84 m3CH4/kgVS, respectively. Keywords anaerobic digestion, thermophilic, inoculum, organic municipal solid waste.
Introduction Notwithstanding that anaerobic digestion represents the best and more energy-efficient way to treat the organic fraction of a separately collected municipal solid waste (OFMSW) (Baldasano and Soriano, 2000 and De Baere, 2000), this subject still needs further investigation in key topics such as process starting-up and improvement and active biomass generation among others. The present research shows the results of the first stage of an ongoing investigation regarding anaeobic treatment of municipal solid waste. This stage concerns the development of a thermophilic anaerobic inoculum for the treatment of a separately collected organic fraction of municipal solid waste (OFMSW). In particular, this paper discusses biomass activity and performance when a particular anaerobic seed-biomass is exposed to an instantaneous increase in temperature, from mesophilic (35°C) to thermophilic (55°C) conditions. The performance and microbial evolution of such an inoculum for the treatment of OFMSW under thermophilic conditions are discussed as well. Materials and methods To develop a thermophilic inoculum, a seed biomass consisting of a mixture of different available granular and non-granular anaerobic sludge was used. After the thermophilic inoculum development stage was completed, a shredded and manually sorted OFMSW obtained from the cafeteria at the Biotechnology Research Institute of the National Research Council of Canada (Montreal, Quebec) was used as substrate (SC-OFMSWBRI) and treated anaerobically under 55°Cthermophilic conditions. The substrate’s solids content was 17.9% (95% volatile solids). Its organic content was 1.2 g COD/gVSS and included food scraps (e.g. meat, egg, fruit and vegetables, bread, pastries, etc.), paper napkins and packaging, plastic packaging, some wood sticks, and plastic tablesettings.
Batch experiments were carried out with a complete mixed and temperature-controlled laboratory scale 5 L-glass reactor. The pH was daily monitored by an epoxy-pH probe (ColeParmer Instrument Co. Vernon Hills, Il.) and a Fisher Accumet pH-meter model 825 MP. As well, after condensate recovery, the biogas generated was measured daily by a Wet Tip-like water displacement system. The reactor’s influent and effluent were analyzed in for total and suspended solids (TS and SS), volatile solids and volatile suspended solids (VS and VSS) (all in g/L) and total and soluble chemical oxygen demand (COD in g/L). Additionally, the concentration of volatile fatty acids (VFA in g/L) in the reactor’s effluent were measured. All analysis were carried out using the techniques described in Standard Methods (1995). The composition of biogas (hydrogen, nitrogen, oxygen, methane, and carbon dioxide) was analyzed by means of a HP gas chromatograph 68900 Series (Hewlett Packard, Wilmington, DE). Daily determinations of ultimate gas potential (Go in m3/kgVSrem) and ultimate biodegradation potential (Bo in m3CH4/kgVSrem) were calculated in accordance with Mata-Alvarez (2002). Finally, in order to evaluate the microbial diversity of the anaerobic biomass, measurements of the substrate activity (SA) were done (Guiot et al., 1995). Results and discussion In order to generate an inoculum adapted to thermophilic conditions (55°C), the reactor was seeded with 4 L of an anaerobic sludge without extraneous substrate supply and the system was started-up under mesophilic conditions (35°C). Soon after the reactor was started, the rate of gas production reached its maximum value (1.9 LNPT/day). By the beginning of the 13th day of operation, the gas production dropped to 0.32 LNPT/day indicating that mesophilic microorganisms had consumed all the available substrate. After 13 days, once the sludge was organically stabilized the system temperature was instantaneously increased form 35 to 55°C; constant 55°C-thermophilic condition was reached after ten minutes. After 4 hours of 55°C-operation, the volume of gas produced was 0.75 LNPT. After 22 hours of thermophilic operation the total volume of gas produced during this stage was 1.4 LNPT of which only 47% was related to microbial activity; the remaining 53% was related to an abrupt gas release from the liquid phase, which was caused by the change of methane and carbon dioxide solubilities due to the change in temperature. The effect of increasing the temperature not only affected the gas production rate, but the gas composition as well. After one day of operating at 55°C (day 14), the composition of methane dropped from 65 to 38%. A small recovery of methane production (38% to 41% from day 14 to day 20) showed that methanogenic microorganisms survived the increment in temperature. To evaluate the shift in microbial content SA tests were carried out at the beginning and at the end of the seeding stage for both temperatures 35 and 55°C (Table 1). The first obvious observation was that increasing the temperature did not have a pasteurization effect on neither bacterial nor methanogenic populations.
Table 1 Initial and final biomass substrate activity measured at 35 and 55°C (seeding stage) Activity (g substrate/gVSS•d) Substrate Test temperature Initial (Day 0) Final (Day 20) Glucose 35°C 5.09 ±1.82 2.12 ±1.09 55°C 1.85 ±0.49 3.58 ±2.06 Propionate 35°C 0.09 ±0.02 0.06 ±0.006 55°C 0.03 ±0.02 0.02 ±0.01 Acetate 35°C 0.62 ±0.4 1.18 ±0.39 55°C 0.6 ±0.11 0.47 ±0.51 Hydrogen 35°C 1.04 ±0.39 0.01 ±0.001 55°C 2.52 ±1.12 0.26 ±0.02
The results in Table 1 also indicated that the abrupt change in temperature from 35 to 55°C on day 13 produced a positive selection pressure for thermophilic fermentative bacteria. Thus, thermophilic glucose consuming bacteria improved their activity almost two fold from 1.85 to 3.58g glucose/gVSS.day. On the contrary, mesophilic glucose consuming bacteria reduced its SA by 58.3%. For propionate consuming bacteria, mesophilic and thermophilic microorganisms reduced their activity from 0.09 to 0.06 and from 0.03 to 0.02g propionate/gVSS.day, respectively. By the end of the inoculum development stage, thermophilic methanogenic activity was reduced 22% in the case of acetoclastic methanogenesis (0.47g Substrate/gVSS.day) and 89.6% for the case of hydrogenophilic methanogenesis (0.26g Substrate/gVSS.day). Interestingly, mesophilic acetoclastic methanogens were not affected by the abrupt change of temperature, in fact their SA improved since such a trophic group was under starvation conditions. The SA increased two fold; from 0.62 to 1.18g acetate/gVSS.day. Once the thermophilic inoculum development stage was completed, the SC-OFMSWBRIfeeding stage was initiated and included four batch cycles (C1 to C4). The retention time for each cycle was 21 days and the average process temperature was 55 ±0.3 °C (Table 2). Table 2 Reactor performance during the 55°C-Inoculum development feeding stage Bo % % F/M Average pH Go m3CH4/kgVS CH4 H2 Cycle kgVS/kgVSS units m3/kgVS C1 0.12 1.40 0.84 60 0 C2 1.15 7.3 0.89 0.61 64 0 C3 4.43 5.17 0.2 0.01 4 14 C4 2.78 7.03 1.26 0.80 64 0
The results in Table 2 showed that the thermophilic inoculum developed in the previous stage was able to accommodate low and intermediate F/M ratios. In C1, a low F/M ratio of 0.12 kgVS/gVSS was applied. For C2, the F/M ratio was increased ten fold (1.15 kgVS/gVSS). This increment did not cause a negative effect on the reactor, since the production of gas increased significantly from 11 LNPT. during C1 to 56 LNPT during C2. The concentration of methane also increased from 60% during C1 to 64% during C2. A F/M ratio of 4.43 kgVS/kgVSS applied in C3 resulted in acidification of the medium from day two with an average pH of 5.17. This acidification was a result of volatile fatty acids accumulation. At the beginning of C3, the concentrations of acetate, propionate and butyrate were 4,580, 65, and 7,000 mg/L, respectively. By the end of C3, only 45 and 40% of acetate and butyrate respectively were consumed, while the concentration of propionate remained unchanged throughout C3. The F/M ratio applied during C3 also increased the production of gas and the proportion of hydrogen within the gas (up to an average of 14%). Such a production of hydrogen was related to both the consumption of rapidly available carbohydrates by fermentative bacteria (Clostridium thermopalmarium) and to the degradation of butyrate by syntrophic bacteria (Coprothermobacter proteolyticus and C. platensis).
At the next cycle (C4), a mixture consisting of 1.6 L of sludge produced at the end of C2, 0.3 L of the original seed biomass, and 0.25 L of a 0.2M NaHCO3 solution (pH 8), was added to the reactor, in order to restore methanogenic conditions. At the end of C4, values of 1.26, 0.8 m3/kgVSREM and 64% were obtained for Go, Bo and %CH4, respectively. Conclusions This experiment showed that the usual, time-consuming acclimatization stage can be avoided. In this case, favorable conditions were quickly achieved (20 days) and the substrate feeding stage could be initiated immediately afterwards. An instantaneous upgrading procedure (IUP) from mesophilic to thermophilic conditions did not have a pasteurization effect on anaerobic microorganisms. The IUP provided a positive selection pressure on mesophilic acetoclastic methanogens and thermophilic fermentative bacteria. However the substrate activities of both mesophilic and thermophilic propionate and hydrogen methanogens were reduced. The thermophilic biomass developed was able to accommodate the substrate supplied (SCOMSW) and could sustain F/M ratios of 0.12 and 1.15 kgVS/kgVSS, however a F/M ratio of 4.43 displaced methanogenesis towards hydrogen production. Acknowledgements: The authors acknowledge both the Mexican Consejo Nacional de Ciencia y Tecnología (CONACyT) and the National Research Council of Canada (NRCC) for respectively supporting and funding this research. A. Corriveau and S. Deschamps are acknowledged for analytical chemistry support.
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