Fermentation of raw sludge on an industrial scale and applications for elutriating its dissolved products and non-sedimentable solids

Raw sludge was pre-fermented in a pilot plant to examine the potential for short-chain fatty acids production in Swiss municipal waste-water treatment plants. The significance of both batch and continuous fermentation in sludge thickeners depends strongly on the efficiency of elutriating the fermentation products. Elutriating the pre-fermented sludge with additional water means diluting the dissolved products. To describe the efficiency of elutriation, the settling property of pre-fermented sludge was therefore examined. Next to dissolved products such as short-chain fatty acids, NH4 and PO4 the elutriant contains suspended solids. Elutriation of non-sedimentable solids rapidly increases the thickening of the fermented sludge. This improved thickening of the sludge is thus an important advantage in addition to the first of all wished short-chain fatty acid production in a sludge thickener operated as a pre-fermenter for enhanced nutrient removal in the activated sludge system. The optimized operation of the sludge thickeners in the Zurich-Werdhoelzli WWTP is described as an example. The dissolved fermentation products in common thickeners have the potential to denitrify an additional 2 g NO3-N m−3, which represents 10% of the total inflow of the ammonium load.