Anaerobic Fermentation of Sulphite Pulp Mill Effluents

Abstract

Despite internal measures modern sulphite pulp mills are still releasing considerable amounts of effluents loaded with organic pollutants. The main sources are the spent liquor evaporator and the bleach plant. In the laboratory scale two anaerobic systems (stirred fermentation vessel and fixed bed reactor) were tested for the fermentation of effluents from a magnesium sulphite mill. The efficiency of the fixed bed reactor was substantially better than the efficiency of the fermentor. Upon treatment of evaporator condensates, COD removal exceeded 90 % with corresponding BOD removal of 99 % after a residence time of 36 hours, whereas the necessary residence time for the same results was about 16 days in the fermentor. The methane production increased with increasing loading, reaching a maximum at a COD loading rate of 7 kg.m−3 .d−1. BOD elimination from bleaching effluents was better than 95 % with fixed bed reactor, whilst COD removal remained below 50 %. All experiments were run over a period of more than two years, exhibiting stability of the fixed bed filter, low needs for maintenance and negligible biomass generation.