Anaerobic digestion of organic fraction of municipal solid wastes. Co-digestion with fat, oil and grease wastes from sewage treatment plants

Resumo

Although a significant number of full-scale anaerobic digestion facilities are nowadays under operation in Spain, in general terms it can be stated that certain issues related with the anaerobic digestion process and digesters performance are hindering its successful application. Some of these factors are directly related with the lack of knowledge of each particular incoming waste, which includes its degradation characteristics and biogas potential. In this context, the performance of biochemical methane potential (BMP) tests of the wastes to be treated would lead to more accurate predictions of biogas production. In Chapter 4 specific values for key parameters were suggested in order to perform biochemical methane potential (BMP) tests of solid organic wastes. The methane potential of different wastes and the suitability of certain organic wastes as hypothetical substrates and co-substrates in further anaerobic digestion processes were assessed in this chapter. Results obtained from four different types of wastes are presented: MSW received from several MSW treatment plants, two different industrial wastes obtained from a processing plant, SC-OFMSW and STP-FOGW. In order to perform the BMP tests two different methodologies, adapted from previously reported protocols, were applied and evaluated: the Field-adapted methodology and the GB21 adapted methodology. Besides, since recent European legislation is promoting the use of biogas from organic wastes as a renewable energy, full-scale facilities must start putting more efforts towards increasing biogas production, avoiding process imbalances and maintaining long-term stable reactor performances. Therefore, a better knowledge and control of the anaerobic digestion processes can contribute to achieve these objectives in full-scale plants. In Chapter 5 the mesophilic anaerobic digestion of SCOFMSW was monitored in order to study the response of different process parameters. Biogas production, methane content, VFAs, pH, alkalinity and alkalinity ratios were monitored, in order to evaluate its suitability as indicators of process imbalances and state simple and comprehensive recommendations for its application in full-scale MBT plants treating OFMSW. Two simultaneously operated mesophilic 5 L reactors were used for this purpose. On the one hand, one of the reactors was established as a reference reactor and was operated at low organic loading rates and high hydraulic retention times during 165 days. On the other hand, severe disturbances were applied to the second reactor during 281 days by means of increasing both hydraulic retention time and organic loading rate. Moreover, the addition of co-substrates appears to be a feasible way to increase biogas production and even the methane yield of the whole process. Again, BMP tests would be necessary to test the suitability of potential co-substrates and to establish the appropriate co-digestion ratios. Finally, it must be also considered that the application of molecular techniques to study the complex microbial populations involved in the anaerobic degradation will provide a deeper knowledge of the whole process. In Chapter 6 the co-digestion SCOFMSW with STP-FOGW was evaluated in continuous lab-reactors. The tested SC-OFMSW:STP-FOGW ratio was established according to the BMP tests results presented on Chapter 4. Two different experiments, at mesophilic (37°C) and thermophilic conditions (55°C) were carried out using the same experimental system. In both experiments, an initial single-substrate stage when only SC-OFMSW was fed to the reactor was followed by the co-digestion stage, when STPFOGW was added. Both experiments involved the study of the reactor performance, in terms of different process parameters (biogas production, methane yield, VS reduction, VFAs, LCFAs). Furthermore, the microbial structure was monitored by using PCR-DGGE and cloning techniques throughout the thermophilic experiment.