C. M. Hooijmans

IMPACT OF EXCESSIVE AERATION ON BIOLOGICAL PHOSPHORUS REMOVAL FROM WASTEWATER

Abstract It has been reported that deterioration of biological phosphorus removal (BPR) efficiency at some wastewater treatment plants (WWTPs) regularly occurred after heavy rainfall or weekends. The deterioration has been attributed to low plant loading that took place during such events. However, it is hypothesized in this study that the cause of such deterioration may have been the excessive aeration that took place at some of those plants due to inadequate control of aeration system during weekends and rainfall periods. In order to prove this hypothesis, the influence of excessive aeration (aeration during starvation conditions) on BPR processes was studied using a laboratory anaerobic-aerobic-settling sequencing batch reactor (SBR). It was clearly demonstrated that the phosphorus uptake stops due to a gradual depletion of poly-hydroxy-butyrate (PHB) in an over-aerated process. If organic substrate is introduced to the system, phosphorus release is immediately at its maximal rate. However, the released phosphorus cannot be taken-up fully again because the PHB content limits the uptake rate. Consequently, incomplete phosphorus uptake leads to temporary reduction of BPR efficiency. This causal effect can explain the deterioration of BPR efficiency after heavy rainfall or weekends. Since excessive aeration clearly negatively affects the BPR processes, the aeration should be properly controlled at sewage treatment plants. Some other findings of this study deserve to be mentioned. • • It was confirmed that the presence of acetate under aerobic conditions provokes phosphorus release. This may also contribute to deterioration of the BPR efficiency. • • The aerobic phosphate uptake was found to depend not only on the PHB but also on polyphosphate (poly-P) content of the cells. • • A maximal poly-P (0.18 g-P/g-VSS) and minimal PHB content of the cells (2.11 mg-COD/g-VSS) were observed in the enriched sludge during excessive aeration experiments. • • It was shown that, under aerobic starvation conditions, glycogen can not replace PHB for phosphate uptake and is only used for maintenance. During this period, no oxygen consumption due to decay processes has been observed.

Biological phosphate removal processes

Abstract Biological phosphate removal has become a reliable and well-understood process for wastewater treatment. This review describes the historical development of the process and the most important microbiological and process-engineering aspects. From a microbiological point of view, the role of␣poly(hydroxyalkanoates) as storage material in a dynamic process and the use of polyphosphate as an energy reserve are the most important findings. From a process-engineering point of view, the study of biological phosphate removal has shown that highly complex biological processes can be designed and controlled, provided that the importance of the prevailing microbiological ecological processes is recognised.

Modeling COD, N and P removal in a full-scale wwtp Haarlem Waarderpolder

Abstract In this study the following was evaluated: (a) application of a complex activated sludge model on a full-scale plant (Phostrip ® -like process), (b) influent and sludge characterization procedures for bio-P modeling, (c) the use of batch tests for model evaluation and (d) different alternative BPR process configurations (A/O, UCT and BCFS ® ). An integrated model for aerobic and denitrifying biological phosphorus removal (Delft bio-P model) replaced the module for P removal of ASM No. 2 and was combined with retained equations for COD and N conversion of the ASM No 2. This combined model proved well capable of describing the wastewater treatment plant (wwtp) Haarlem Waarderpolder with adjustment of only three (out of the sixty) default parameters. Some of batch tests were satisfactorily described by the model too. Batch tests proved as a useful tool for sludge characterization and model validation. The standard Dutch procedure for influent and sludge characterization proved satisfactory for the model construction. Modeling of alternative BPR process configuration showed that good P removal was achieved by all three alternative process configurations, by the latest one with comparatively lowest construction and operational costs (lower building costs and no acetate addition). Valuable experience from practical application of the model was obtained; study indicated where the model should be improved; the plant operation and treatment processes were better understood and plant performance was further optimized.

Influence of temperature on biological phosphorus removal: process and molecular ecological studies

Abstract This paper describes the impact of long-term (weeks) temperature changes on stoichiometry and kinetics of the anaerobic and aerobic phases of the biological phosphorus removal process. Steady state conversion of relevant compounds for biological phosphorus removal was studied at 20, 30, 20, 10 and 5°C, following chronological order. Integrated in the process study, two methods (electron-microscopy and dry denaturing gradient gel electrophoresis) were applied to investigate the complexity of the bacterial community of biological phosphorus removing sludge cultivated at different temperatures. The coefficient for metabolic conversions obtained from long-term temperature tests was similar to the temperature coefficient observed in short-term (hours) tests ( θ =1.085 versus θ =1.078, respectively).Temperature had a moderate impact on the aerobic P-uptake process rate ( θ =1.031) during long-term tests. However, a strong temperature effect on other metabolic processes of the aerobic phase, such as polyhydroxyalkanoate consumption ( θ =1.163), oxygen uptake ( θ =1.090) and growth ( θ >1.110), was observed. Different temperature coefficients were obtained for the aerobic phase from long-term and short-term tests, probably due to a change in population structure. This change was also visible from molecular ecological studies. The different temperature coefficient found for P-uptake compared to the other metabolic processes of the aerobic phase underlines that, in complex processes such as BPR, it is dangerous to draw conclusions from easily observable parameters (like phosphate) only. Such consideration can easily lead to underestimation of the temperature dependency of other metabolic processes of the aerobic phase of BPR.

The dynamic effects of potassium limitation on biological phosphorus removal

Abstract A full-scale sewage treatment plant designed for biological phosphorus removal may experience short- or long-term shortage in potassium of the influent. In this study, using an anaerobic-aerobic sequenced batch reactor system, inoculation sludge from laboratory-, pilot- and full-scale phosphorus removal plants was exposed to different potassium-phosphorus ratios in the influent. By simulating the conditions which may occur in practice, it was shown that potassium is an essential factor in biological phosphorus removal processes. When the system was exposed to severe shortage of potassium in the influent: (a) phosphorus removal was absent, (b) polyphosphate concentration in the biomass decreased exponentially due to sludge wasting and (c) the anaerobic phosphorus release and the related acetate uptake was only affected after several days of potassium absence, likely due to insufficient content of polyphosphate in the biomass to allow full acetate uptake under anaerobic conditions. In contrast, the system achieved complete phosphorus removal when potassium was present in excess amounts.

Effect of polyphosphate limitation on the anaerobic metabolism of phosphorus-accumulating microorganisms

Abstract There are two types of microbial populations described in the literature as being capable of anaerobic storage of acetic acid in activated-sludge processes: the polyphosphate-accumulating organisms (PAO) and the glycogen-accumulating non-polyphosphate organisms (GAO). Both groups use the conversion of glycogen to poly-hydroxyalkanoate to produce ATP and NADH; however, the first group can also produce ATP from polyphosphate (poly-P). No representative pure cultures are available from either group. The question arises: is the observed activity of GAO due to PAO that are depleted in poly-Pu2009? In this study, using a laboratory sequencing batch reactor containing an enriched culture, the ability of the enriched PAO to utilize organic substrate (acetate) anaerobically was investigated under conditions of poly-P limitation and surplus glycogen content of the biomass. This study showed clearly that, under these conditions, almost no acetate was taken up. Furthermore, this strongly suggests that PAO can not use glycogen conversion to poly-hydroxyalkanoate as the sole energy source under anaerobic conditions, which seems to be the restricted to a separate group of GAO. On the basis of the results and literature data, an improved scheme for the anaerobic acetate accumulation is presented.

Minimal aerobic sludge retention time in biological phosphorus removal systems

The methodology for determination of the minimally required aerobic sludge retention time (SRTminaer) in biological phosphorus removal (BPR) systems is presented in this article. Contrary to normal biological conversions, the BPR process is not limited by a SRTmin resulting from the maximum growth rate of the organisms. This is because the aerobic SRT should be long enough to oxidize the amount of poly-hydroxy-alkanoates (PHA) stored in the anaerobic phase. This means that the SRTminaer will primarily depend on the PHA conversion kinetics and the maximal achievable PHA content in the cell (storage capacity). The model for the prediction of the minimally required aerobic SRT as a function of kinetic and process parameters was developed and compared with experimental data used to evaluate several operational aspects of BPR in a sequencing batch reactor (SBR) system. The model was proved as capable of describing them satisfactorily.Copyright 1998 John Wiley & Sons, Inc.

Nitrification activities in full-scale treatment plants with varying salt loads

The effect of salt on the nitrification activity in full-scale wastewater treatment plants (WWTP) was investigated. Not only the activity of ammonia and nitrite oxidisers was measured, but also the nitrifying population was assessed (by Fluorescent In Situ Hybridisation) - in full-scale domestic and industrial WWTPs, operated under various salt levels. The results demonstrate a decline in the activity of ammonia and nitrite oxidisers with an increase in salt content: the domestic WWTP with the lowest salt level (0.13g Cl−l−1) had the highest specific activity of ammonia and nitrite oxidisers (4.3 and 2.4mg N (gVSS)−1 h−1, respectively), while the lowest specific activities of ammonia and nitrite oxidisers (1.1 and 0.5mg N (gVSS)−1 h−1) were measured at the highest NaCl concentration (16g Cl− l−1). However, comparing the nitrification activity of different types of sludge developed under different operational conditions with the reported values was not directly possible. So we have used the activated sludge model (ASM) to translate the routine operational data into parameters to enable the calculation of the actual fraction of nitrifiers and consequently the actual specific activity of ammonia and nitrite oxidisers. Expressing the activity of ammonia oxidisers in terms of actual specific activity makes the results from pure cultures, enriched cultures, pilot scale and full scale WWTPs comparable. Moreover, these results confirm the behaviour of nitrifiers under salt stress and validate the results obtained from pure and enriched cultures to be extrapolated to full scale.