Activated sludge sewage treatment plants that were affected by biological foams attributed to the presence of actinomycetes were surveyed in Queensland, Australia. Nocardia amarae and Nocardia pinensis were the predominant organisms found to accumulate selectively in the foam phase. Foaming was shown to be a significant problem in Queensland with 92% of surveyed plants being affected. A total of 60% of these plants suffered from bulking, however, no correlation between these two problems was noted. No operational, design, influent or weather feature was found to be consistent in all foaming plants. Apart from the fact that the plants were all variations of the activated sludge process, the only other aspect shared by them was that large numbers of filamentous bacteria were present in the foam. Field scale trials that evaluated control strategies against actinomycete foaming or scumming in activated sludge plants are described. The effects were assessed of: altering the rate of return activated sludge recycle, dosing of the aeration tank with anaerobic digester products, the addition of a commercial product specifically designed to control foaming, altering sludge age, varying levels of mixed liquor suspended solids (MLSS) and altering other operational features. These strategies did not efficaciously control scumming in activated sludge, however, some operational features were highlighted as exacerbating the problem.
