• Energy Research

The influence of total solid contents during anaerobic mesophilic treatment of the organic fraction of municipal solid waste (MSW) has been studied in this work. The work was performed in batch reactors of 1.7L capacity, during a period of 85-95 days. Two different organic substrate concentrations were studied: 931.1 mgDOC/L (20% TS) and 1423.4 mgDOC/L (30% TS). Experimental results showed that the reactor with 20% total solids content had significantly higher performance. Thus, the startup phase ended at 14 days and the total DOC removal was 67.53%. The startup in reactor R30 ended at 28 days obtaining 49.18% DOC removal. Also, the initial substrate concentration contributed substantially to the amount of methane in the biogas. Hence, the total methane production in the methanogenic phase was 7.01 L and 5.53 L at the end of the experiments for R20 and R30, respectively.

Abstract Anaerobic digestion of the Organic Fraction of Municipal Solid Wastes from non-selective collection systems is widely implemented at industrial scale. Classically, single-temperature anaerobic digestion systems (mesophilic or thermophilic) have been used although some limitations were found in the processes performance. However, the specific advantages of the two processes could be exploited by using a temperature-phased anaerobic digestion (TPAD). Thus, in this paper, the application of a TPAD process without microbiological separation (thermophilic 55 °C–mesophilic 35 °C) to the biomethanation of OFMSW has been studied. The TPAD assays were carried out in batch reactors. The processing times for the thermophilic (55 °C) stage have been located in the range from 12 to 3 days. After the first stage, the waste treatment was continued in a second mesophilic reactor (35 °C) until completing the degradation process. The higher values for the organic matter removal (VS removal 82–85%) and the maximum specific growth rates of microorganisms (0.31–0.43 days-1), were obtained for thermophilic phase operation at 5 and 4 days, respectively. Lower operation times, namely 3-days, for thermophilic first-phase are viable and the methane productivity obtained was comparable with respect to that obtained in the above mentioned conditions (0.6 L/gVSremoved). However, in this conditions, a decrease in the maximum specific growth rate of microorganisms (0.26 days−1), a lower SV removal (75%), and a higher final amount of non-biodegradable substrate (1231 mg/L) were obtained, working at 3 days, despite of using higher operational times (29 days). According to these results, it can be concluded than the optimum time for the thermophilic first-phase in TPAD would be between 5-days and 4-days.

Abstract Dry mesophilic anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) is a widespread technology. However, OFMSW is a very heterogeneous waste and therefore specific studies are needed to optimise the AD process with each type of OFMSW that will be used. The main operational variables to optimise are the solids retention time (SRT) and the organic loading rate (OLR), which are closely related to each other when the concentration of organic matter of OFMSW remains relatively constant. In this work three different experimental conditions (corresponding to the SRT of 30, 20 and 15 days) were tested in a semi-continuous stirred tank reactor, operating at mesophilic range (35 °C) and high solids concentration (20% TS). The OLR corresponding to the above mentioned SRT were 22.8, 27.3 and 35.9 mg DOC/L reactor /d, respectively. The results obtained in this study indicate that 20 days is the optimum SRT for the dry mesophilic anaerobic digestion of the OFMSW used. Thus, in general, all the parameters analysed show better performance for 20-day SRT with regard to 30-day and 15-day SRT. More specifically, it can be pointed out than at SRT of 20 days (27.3 mg DOC/L reactor /d), both the highest productivity of methane (0.11 L CH 4 /g waste-fed) and the highest organic matter removal rate (66.3% DOC removal) were reached.

Abstract Temperature is a significant variable in anaerobic digestion (AD) since it determines the values of the main kinetic parameters for the process and, hence, the rate of the microbiological process. Thus, in this study, batch AD experiments were carried out, at mesophilic (35 °C) and thermophilic (55 °C) conditions, with the aim to compare the kinetic of both processes. Tests were performed in dry conditions (concentration of Total Solids (TSs) of 20%) using the Organic Fraction of Municipal Solid Waste (OFMSW) as feedstock. Romero model [1] was used to fit the experimental results from both, the organic matter consumption and the biogas production. This model has been used extensively in the analysis of results of AD process in a wide range of experimental conditions [2–4] . The values of the maximum specific growth rate of microorganisms ( μ MAX ) are 27–60% higher for thermophilic process than for mesophilic ones and, therefore, the same level of organic matter degradation and methane production can be achieved in a shorter operating time, 20 days in thermophilic instead of 40 days in mesophilic. Moreover, the yield coefficient for methane production ( α P/S ) and the initial amount of active microorganisms ( X V0 / Y X/S ) show values 107% and 129% higher, respectively, in thermophilic processes.

The anaerobic mesophilic degradation of municipal waste has been studied in discontinuous lab-reactors with two different initial concentrations of OFMSW: R20, with 931.1mg DOC/L (20% TS), and R30, with 1423.4 mg DOC/L (30% TS). The anaerobic digestion process was favoured when it was carried on material with a total solids content of 20% in comparison to a similar process with 30% TS. A higher level of organic matter, in terms of DOC and VFA, 18.18% and 8.09% respect, was removed in the system with the lower amount of solids. The kinetics parameters showed higher active biomass and a higher coefficient for the production of methane at the lower loading. The highest maximum specific growth rate for the microorganisms (mu(MAX)) in the reactors with 20% TS, a value of 0.192 d(-1), was achieved in comparison to 0.131 d(-1) in the reactors with 30% TS. The inverse of the F:M ratio was higher in the R20 system with a value of 0.0104 (cf. 0.0006 in R30) and, as a result, the R30 system required longer hydrolytic and acidogenic stages. X(V0)/Y(X/S) and the yield coefficient for product generation (alpha(P/S)) were higher, by around 53%, for the R20 digester.