Effect of solids retention time and wastewater characteristics on biological phosphorus removal M. Henze*, H. Aspegren**, J. la Cour Jansen***, P.H. Nielsen**** and N. Lee***** * Environment and Resources DTU, Bygningstorvet, Building 115, Technical University of Denmark, DK- 2800 Lyngby, Denmark. (E-mail: moh@er.dtu.dk), corresponding author ** Malmö Water, S-205 80 Malmö, Sweden *** Department of Water and Environmental Engineering, Lund University, S-221 00 Lund, Sweden **** Environmental Engineering Laboratory, Aalborg University, DK-9000, Aalborg, Denmark ***** Lehrstuhl für Mikrobiologie, TU München, Am Hochanger 4, D-85350 Freising, Germany Abstract The paper deals with the effect of wastewater, plant design and operation in relation to biological nitrogen and phosphorus removal and the possibilities to model the processes. Two Bio-P pilot plants were operated for 2.5 years in parallel receiving identical wastewater. The plants had SRT of 4 and 21 days, the latter had nitrification and denitrification. The plant with 4 days SRT had much more variable biomass characteristics, than the one with the high SRT. The internal storage compounds, PHA, were affected significantly by the concentration of fatty acids or other easily degradable organics in the wastewater, and less by the plant lay-out. The phosphorus removal is mainly dependent on availability in the wastewater of fatty acids but also by the suspended solids in the effluent, which is higher in the plant with nitrification- denitrification, probably due to a higher SVI or denitrification in the settler. The addition of glucose to the influent seems to have an effect on the performance of the plants similar to that of acetic acid. In spite of great load variations over time to the pilot plants and the different operational modes, the study of population dynamics showed less significant variations with time which has importance in relation to modelling. The overall conclusion of the comparison between the two plants is that the biological phosphorus removal efficiency under practical operating conditions is affected by the SRT in the plant and the wastewater composition. Thus great care should be taken when extrapolating results from one type of plant to another. Indirectly the experiments confirm that results from lab-experiments with artificial wastewater are difficult to extrapolate through modelling to real life wastewater and conditions. The 2.5 years time series can be valuable in verification of models for Nitrogen and Enhanced Biological Phosphorus Removal. Keywords Activated sludge; design; modelling; nutrient removal; operation; wastewater Introduction Activated sludge processes designed for biological phosphorus removal generally include biological nitrogen removal. This means that nitrate can affect part of the phosphate- release phase and lead to reduced efficiency of the process. Of interest, consequently, is an understanding of the effects of nitrogen removal on BPR performance. Nitrate is usually considered to be an inhibitor to BPR activity, since nitrate introduced to the anaerobic zone via return flows can be denitrified in this zone, thereby reducing the supply of organic sub- strates available for uptake and later utilization by the phosphate accumulating organisms (PAO) responsible for BPR activity. The nitrate does not affect the non-denitrifying PAOs except for the competition for substrate. If there is abundant substrate (acetate), the effect will be a reduced phosphorus release rate. The biomass is a function of influent, design and operation of the activated sludge process. The effect of these factors on the performance of Bio-P processes is not known in detail, because studies have been limited to one process layout at a time, e.g. Østgaard et al., 1997. The microbial diversity of the activated sludge biomass in full-scale plants is most likely Water Science and Technology Vol 45 No 6 pp 137–144 © IWA Publishing 2002 137 Downloaded from http://iwaponline.com/wst/article-pdf/45/6/137/425355/137.pdf by guest