Optimisation of primary sludge acidogenic fermentation

Laboratory batch experiments were conducted, at room temperature, to investigate ways of optimising the performance of primary sludge acidogenic fermentation systems, used at four major Johannesburg waste-water treatment plants. Retention time, seeding, solids concentration and mixing were found to be important variables governing volatile fatty acid (VFA) production. At least 75% of the potential VFA yield was reached within 6 d fermentation time. The addition of partially fermented sludge to fresh primary sludge improved the VFA yields after 6 d from between 48 to 170%, using a one-to-three-day-old fermented seed sludge, of 10 to 20% (v/v) the total volume of sludge in the fermenter reactor. A threefold dilution of the primary sludge with primary settled tank effluent, and the absence of mixing in the batch reactor, each showed at least a 70% improvement in the VFA yields. Furthermore, dewatering of fermented sludges with a cationic polyelectrolyte can be applied as a final solid-liquid separation technique with elutriation of the desired adsorbed organics, generated during the digestion process, to the supernatant liquor.