Actinomycete scum production in activated sludge processes

Abstract

Some activated sludge processes occasionally develop a persistent, viscous, brown which is extremely difficult to deal with. It is different from the usual foams on aeration tanks in that it cannot be collapsed by water sprays. The can completely cover the aeration tanks, sometimes to a depth of 0.6 to 1 m (2 or 3 ft). When it spills over on the walkways between tanks, it will dry leaving them sticky and slick. Some of the car ries over into the secondary settling tanks where it floats on the surface. If the tanks are not skimmed vigorously, the is car ried out in the effluent where it causes ex cessively high suspended solids (ss) and bio chemical oxygen demand (bod) concen trations. This problem was reported at the Jones Island Wastewater Treatment Plant, Mil waukee, Wisconsin, in 1969,1? 2 where it was associated with the growth of large numbers of actinomycetes in the mixed liquor. The was eliminated by reducing the mixed liquor suspended solids (mlss). Scum was reported occurring in the Try on Creek plant, Portland, Oregon, and although the ap parently was not examined for actinomycetes, it was eliminated by decreasing the mlss to increase the loading (F:M ratio).3 It also occurred at the Chicago Heights plant of the Bloom Township Sanitary District in Illinois in the summer of 1972. This occurrence has not previously been reported in the literature, but is included because the author verified the presence of large numbers of actinomycetes in the mixed liquor and in the by micro scope examination and the problem was also eliminated by reducing the mlss. Lechevalier 4 investigated the occurrence of actinomycetes in activated sludge and their association with the problem. (He used the term foam but scum is used in this paper to emphasize the difference from col lapsable foams ). He found the actinomycete problem in 9 out of 49 New Jersey acti vated sludge plants sampled and also was able to isolate actinomycetes from nine sam ples sent in from out-of-state. He identified all of his isolates as members of the genus Nocardia and described the most common isolate as a new species, Norcardia amarae.5 He did not demonstrate why the actinomycete problem occurs in some activated sludge processes and not in others, but he did show that anaerobically digested sludge contains a material which inhibits Nocardia growth. In addition to a description of the problem, this paper presents data which may shed some light on the cause of production and better differentiate it from other activated sludge operating problems. The data were collected for other purposes and it is merely fortuitous that actinomycete counts were made on the samples.