Microbiological characterization and specific methanogenic activity of anaerobe sludges used in urban solid waste treatment

This study presents the microbiological characterization of the anaerobic sludge used in a two-stage anaerobic reactor for the treatment of organic fraction of urban solid waste (OFUSW). This treatment is one alternative for reducing solid waste in landfills at the same time producing a biogas (CH4 and CO2) and an effluent that can be used as biofertilizer. The system was inoculated with sludge from a wastewater treatment plant (WWTP) (Río Frío Plant in Bucaramanga-Colombia) and a methanogenic anaerobic digester for the treatment of pig manure (Mesa de los Santos in Santander). Bacterial populations were evaluated by counting groups related to oxygen sensitivity, while metabolic groups were determined by most probable number (MPN) technique. Specific methanogenic activity (SMA) for acetate, formate, methanol and ethanol substrates was also determined. In the acidogenic reactor (R1), volatile fatty acids (VFA) reached values of 25,000 mg L−1 and a concentration of CO2 of 90%. In this reactor, the fermentative population was predominant (105–106 MPN mL−1). The acetogenic population was (105 MPN mL−1) and the sulphate-reducing population was (104–105 MPN mL−1). In the methanogenic reactor (R2), levels of CH4 (70%) were higher than CO2 (25%), whereas the VFA values were lower than 4000 mg L−1. Substrate competition between sulphate-reducing (104–105 MPN mL−1) and methanogenic bacteria (105 MPN mL−1) was not detected. From the SMA results obtained, acetoclastic (2.39 g COD-CH4 g−1 VSS−1 day−1) and hydrogenophilic (0.94 g COD-CH4 g−1 VSS−1 day−1) transformations as possible metabolic pathways used by methanogenic bacteria is suggested from the SMA results obtained. Methanotrix sp., Methanosarcina sp., Methanoccocus sp. and Methanobacterium sp. were identified.