Development of a two phase anaerobic digester for the treatment of mixed abattoir wastes

Abstract

A two phase anaerobic digestion system was developed for the treatment of mixed abattoir wastes composed of mixtures of cattle blood and cattle gut fill (rumen paunch contents). The reactor system, and its mode of operation, overcame the problems associated with a single pass anaerobic digestion process by alleviating toxicity problems associated with the accumulation of volatile fermentation intermediates and high ammonia concentrations. The principle of operation of the two phase system was to separate the hydrolysis reactions from those of methanogenesis and, by introducing a hydraulic flush regime, to prevent accumulation of intermediate products in the first phase of the process. The hydraulic flush operates in such a manner whereby the liquid retention time in the first reactor was significantly shorter than the solids retention of the fibrous components of the feedstock. The first phase reactor was run in this mode using solids retention times of 5, 10, 15, 20 and 30 days with a liquid retention time of 2 days. Up to 87% solids reductions were achieved compared to a maximum 50% when control reactors were operated in a single pass mode with solids and liquid retention times of equal duration. The performance of the first phase hydrolysis reactor was also monitored in terms of volatile fatty acid production, COD removal efficiency and ammonia accumulation potential. The liquefied effluent from the hydraulic flush reactor was found to be a suitable substrate for a second phase high rate methanogenic reactor operated over a range of retention times of between 2 - 10 days; this gave equivalent process loadings of 1.4 - 7.0 kg COD/m3/day. Methane conversion efficiencies of around 0.3 m3 CH4/kg COD removed were achieved. By use of the two phase system it was possible to operate at a loading to the first phase of 7.22 kgTS/m3/day with a resultant effluent from the second phase with a COD of 4270 mgl−1. The overall performance of the system showed a process loading of 3.6 kgTS/m3/day was achievable with a methane production rate of 0.27 m3CH4/kgTS added and 63% TS destruction. The results suggest that further optimisation of the two phases might further improve this overall performance.