The effect of microorganisms on pulp mill effluent induced flocculation in receiving waters

Recent studies have shown that organic matter discharged from a pulp mill flocculates and accumulates on the river bottom more rapidly than predicted by previous transport models, a phenomenon termed pulp mill effluent induced coagulation and flocculation (PMEICF). Pulp mill effluent induced coagulation and flocculation can cause accumulation of material on the river bottom that may induce anoxic or toxic conditions, subsequently harming benthic organisms and the entire food chain. Pulp mill effluent and river water were mixed in a standard jar test apparatus. Heterotrophic plate counts revealed that a variety of microorganisms was present in the floc material and the liquid portion. Individual colonies were isolated, identified, and tested for their role in flocculation. Seven isolates were found capable of enhanced flocculation: Comamonas testosteroni, species belonging to the Pseudomonas, Enterobacter, and Aeromonas genuses and an unidentified isolate. These isolates did induce flocculation, though not consistently. Results varied with the environment available for the microorganisms. Further tests are required that consider the changing effluent and river water characteristics. Key words: pulp mill effluent, coagulation, flocculation, bacteria, identification, sediment transport model.