Abteilung Umweltchemie

Since the prohibition of phosphates in detergents, a considerable shift of the nutrient ratio P/TOC (phosphorus/organic carbon) in untreated sewage could be observed. The decrease of phosphate concentrations led to an increase of the removal rate in conventional activated sludge plants from 15 % up to 40%, and allows to consider additional biological P-elimination by luxury uptake as feasible treatment process in Switzerland. A literature survey reveals insight into recent research of the microbial processes of phosphate release and uptake and the consequent design considerations for the necessary anaerobic/aerobic activated sludge systems. Experimental results of pilot and full scale plants show in general satisfactory results. The influence of incontrollable nutrient conditions, concerning above all readily biodegradable substances and recycled nitrates, causes, however, instabilities of the process performance. It is therefore doubtful, if the stringent Swiss effluent standards of 0.8 mg P/l could be reached. Additional support by chemical precipitation would be necessary. Although biological phosphate removal offers many advantages compared to chemical precipitation, high investments and additional modifications concerning the water and the sludge handling flow schemes may not favor a wide application.

Solids separation, adsorption onto particulates and volatilization are the main physico-chemical mechanisms responsible for the removal of organic matter in wastewater treatment plants. In addition, solids separation as well as adsorption performance may be enhanced by chemical flocculation/precipitation with hydrolyzing metal salts. With the help of particle size distributions and other characteristics of the particulates at different treatment stages, more insight can be gained into the interactions between particulate properties and solids separation performance. Significant removal of trace organics is achieved by adsorption onto particulate surfaces and stripping in aeration tanks. Examples illustrate the adsorption of the relatively large quantities of substances contained in detergents and reveal the effect on water and sludge quality. Octanol/water partition coefficients of trace organics may be used together with oxygen transfer characteristics to determine the potential of volatilization into the air. The effect of flocculants on particulate and dissolved organics removal in wastewater treatment is illustrated when Fe-salts are dosed to raw sewage, to the biological stage or to tertiary filters.