Development of an improved synthetic sludge: a possible surrogate for studying activated sludge dewatering characteristics

Activated sludge is a complex mixture of inert suspended solids, various microorganisms, and extracellular material. Research on the physical characteristics of activated sludge is complicated because the living microorganisms continuously change sludge characteristics, making it almost impossible to carry out controlled experiments or reproduce test results. Therefore, in dewatering, settling and conditioning studies of activated sludge, there is a clear need for an activated sludge surrogate that is chemically well defined, stable, and successfully represents activated sludge properties. One such chemical surrogate, named “synthetic sludge”, suggested by Sanin and Vesilind [Sanin, F. D. and Vesilind, P. A. (1996) Synthetic sludge: a physical/chemical model in understanding bioflocculation. Water Environ. Res. 68 (2), 927–933] is composed of polystyrene latex particles, alginate, and calcium ions. In this sludge the polystyrene latex particles simulate individual bacteria, and alginate simulates extracellular material tied into a matrix with the calcium. However, one important component, something to simulate the filamentous microorganisms, is missing. The lack of filaments in the synthetic sludge causes the flocs to be weaker and smaller than activated sludge flocs. In this study synthetic sludge is improved by adding cellulose fibers to simulate the filamentous microorganisms found in activated sludge. The addition of cellulose results in the formation of strong flocs, improving the dewaterability and settleability of synthetic sludge.