Temperature Phased Anaerobic Digestion and Bioaugmentation of Single-Stage Thermophilic Reactor

Temperature phased anaerobic digestion (TPAD) systems are composed of two single-stage anaerobic reactors operated at different temperatures in which typically the thermophillc reactor (55°C) precedes the mesophilic reactor (35°C). Among the advantages of the system are operational stability, good volatile solids destruction, pathogen inactivation and effective solids stabilization. In this document, the performance of a TPAD system operated for about 1.5 years is reported. The digesters were fed daily with a mixture of 70% municipal primary sludge and 30% waste activated sludge from three different municipal treatment plants, each fed at different periods. The pH, volatile solids destruction, biogas and methane production of the system were monitored over time. Being consistent with other research, the overall volatile solids destruction of the system depended mainly on the thermophilic digester since most of the solids destruction was performed in this reactor (greater than 70% of the overall solids destruction). The mesophllic reactor was a complement digester that improved the quality of the effluent. The system reached quasi-steady state three times during the operation period. It was observed that during these periods the highest volatile solids destruction and pH stability were achieved. The maximum stability and solids destruction (64%) were achieved during the last 40 days of operation. A typical problem in the operation of anaerobic digesters is the potential drop of the pH, making the reactors go sour , which leads to a decrease in the methane production. It has been studied previously that the cause of this pH drop is often the accumulation of propionic acid. In this report a method to avoid the build-up of acetic and propionic acids in the anaerobic digestion process is described. It has been previously studied by Pipyn et al. (1981) that the fermentation of substrates to lactic acid which is easily converted to acetic acid and then methane is an alternative to avoid the propionic acid problem. However, the organisms that produce lactic acid in the anaerobic systems may be very few and propionic acid bacteria can predominate. The method studied herein was to increase the activity of lactic acid producers in the system by bioaugmentatlon..