Enhanced Biological Phosphorus Removal -Processes, Competing Substances and Tools for Operation of Wastewater Treatment Plants

Abstract: The idea of phosphorus removal at wastewater treatment plants is to transfer dissolved phosphorus into a solid phase. Together with the rest of the produced sludge it is then removed from the water, resulting in a decreased concentration of phosphorus in the effluent and the prevention of the eutrophication in the recipient. Phosphorus can either be stored biologically, with enhanced biological phosphorus removal (EBPR), or with chemical precipitation. The EBPR-process is performed by polyphosphate accumulating organisms (PAOs) which are promoted by alternating anaerobic and aerobic conditions with volatile fatty acids (VFAs) available during anaerobic conditions. Also glycogen accumulating organisms (GAOs) are able to take up VFAs anaerobically and are consequently regarded as competitors to the PAOs. The method of laboratory batch tests is thoroughly described and the evaluation of the results is discussed. It is concluded that the test is a useful tool for the characterisation of the EBPR activity of an activated sludge. However, the batch tests have some disadvantages and therefore a combination with for example microbial methods is suggested to obtain a more reliable evaluation of the process. The results from a study where laboratory batch tests and microbial methods jointly characterised sludge from a full-scale plant correspond well with models found in the literature originating from studies of sludge grown in laboratory reactors. Studies in laboratory reactors, where populations of PAOs and GAOs can be highly enriched, are important in order to increase the knowledge of the EBPR process and also to improve activated sludge models. A laboratory reactor was used to investigate the effect of chemical precipitation. It was found that simultaneous chemical precipitation can compete with the PAOs for available phosphorus, resulting in favourable conditions for the GAOs. The proportion between phosphates and VFAs has been shown to be important for the competition between PAOs and GAOs. A decreased ratio (P/VFA) in the influent wastewater is suggested to promote the growth of GAOs. To be able to continuously reach very low effluent phosphorus concentrations at full-scale plants, chemical precipitation is often used in addition to the EBPR process. The sludge hydrolysis is also a common method at treatment plants for production of the necessary VFA. Both chemical precipitation and sludge hydrolysis can decrease the P/VFA ratio. Even if both methods have been proved to be successful and sometimes also necessary, they are suggested to be used with caution in order to avoid the growth of GAOs. However, neither the significance of GAOs at full-scale plants nor the reason for the competition between PAOs and GAOs is understood yet. Furthermore, it was found that it is possible to operate full-scale plants with EBPR successfully, using chemical precipitation and sludge hydrolysis to improve the operation. The release and uptake of phosphorus are strongly correlated to the release and uptake of magnesium (Mg2+) and potassium (K+). These ions are suggested to be counter-ions to the negatively charged orthophosphates. The proportions in Mg/P and K/P are not found to be constant. The supply of cations in the wastewater can be important for the choice of counter-ions, but it was also discovered that different ratios were found in batch tests when acetate or propionate was used as the carbon source, which suggests that other factors in addition to the concentrations are important for the choice of counter-ions. Microbial studies as well as both laboratory- and full-scale studies are important for the understanding of the EBPR process. However, the results from laboratories have to be verified applicable also at full-scale plants before the knowledge can contribute to improve the phosphorus removal process.