Cause and control of activated sludge foaming

Activated sludge foaming refers to the formation of stable foam, which is a widespread problem in biological wastewater treatment plants (WWTPs) all over the world. This foam is highly enriched with floating biomass and covers activated sludge tanks, secondary clarifiers and sometimes also occurs in anaerobic digesters. It causes severe operating problems, increases maintenance efforts and may lead to poor effluent quality. Decades of research could not fully clarify the key processes behind this phenomenon; and there is still no reliable control strategy. The goal of this thesis is to systematically identify and investigate factors that play a crucial role in activated sludge foaming. Therefore, sub-processes of activated sludge foaming were identified and separately studied: 1) growth of activated sludge of a certain foaming potential, 2) the actual foam formation and stabilization, 3) distribution and accumulation of foam in the WWTP.

The studies took place in several full- and pilot-scale WWTPs, conventional activated sludge plants and membrane bioreactors. Therefore, methods to quantify bacteria in activated sludge and to measure the foaming potential under laboratory conditions were developed.

The results show distinct seasonal variations of the abundance of bacteria that are often suspected to play an important role in foaming, Microthrix parvicella and nocardioform actinomycetes. A mathematical models was developed to describe and further study the growth of Microthrix parvicella. The foaming potential was demonstrated to be controlled by the solids concentration and properties. It was not correlated with the foam coverage of the reactors, the floc structure and surprisingly not with the abundance of the suspected foam producing bacteria Microthrix. Furthermore, it is particularly interesting that foam samples and sludge from membrane bioreactors exhibited a considerably higher foaming potential than mixed liquor from conventional plants.

Future research should clearly distinguish the sub-processes of foaming and consider the distinct dynamics of the processes, e.g. by analyzing time-series of samples.To control foaming we recommend to selectively remove floating sludge and to add poly-electrolyte as an emergency measure. As it seems that we have to live with foam in modern nutrient removing WWTPs and particularly in membrane bioreactors, the foam tolerance of existing and future plants should be increased.

Summary PhD thesis [pdf file, 28 KB]