ReviewAdvances in enhanced biological phosphorus removal: From micro to macro scale
Introduction
Phosphorus (P) is a key nutrient that stimulates the growth of algae and other photosynthetic microorganisms such as toxic cyanobacteria (blue-green algae), and must be removed from wastewater to avoid eutrophication in aquatic water systems. The risk of adverse effects to the plant and animal communities in waterways declines as P concentrations approach background levels (Mainstone and Parr, 2002). Around the world, a growing awareness of the need to control P emissions, which is reflected in increasingly stringent regulations, has made P removal more widely employed in wastewater treatment. Enhanced biological phosphorus removal (EBPR) promotes the removal of P from wastewater without the need for chemical precipitants. EBPR can be achieved through the activated sludge process by recirculating sludge through anaerobic and aerobic conditions (Barnard, 1975). Usually, biological nutrient removal (BNR) refers to the combination of biological nitrogen removal and the EBPR process.
The group of microorganisms that are largely responsible for P removal are known as the polyphosphate accumulating organisms (PAOs). These organisms are able to store phosphate as intracellular polyphosphate, leading to P removal from the bulk liquid phase via PAO cell removal in the waste activated sludge. Unlike most other microorganisms, PAOs can take up carbon sources such as volatile fatty acids (VFAs) under anaerobic conditions, and store them intracellularly as carbon polymers, namely poly-β-hydroxyalkanoates (PHAs). The energy for these biotransformations is mainly generated by the cleavage of polyphosphate and release of phosphate from the cell. Reducing power is also required for PHA formation, which is produced largely through the glycolysis of internally stored glycogen (Mino et al., 1998).
Aerobically, PAOs are able to use their stored PHA as the energy source for biomass growth, glycogen replenishment, P uptake and polyphosphate storage. Net P removal from the wastewater is achieved through the removal of waste activated sludge containing a high polyphosphate content. While the majority of P removal from the EBPR process is often achieved through anaerobic–aerobic cycling, anaerobic–anoxic operation also allows P removal to occur, due to the ability of at least some PAOs (i.e. denitrifying PAOs or DPAOs) to use nitrate or nitrite instead of oxygen as electron acceptors and, therefore, perform P uptake and denitrification simultaneously. Maximising the fraction of P removal achieved anoxically can reduce process operational costs, due to savings in aeration as well as in the amount of carbon sources needed for denitrification. Currently, many different process configurations exist where both P and nitrogen removal are combined (Henze et al., 1997; Tchobanoglous et al., 2002).
When operated successfully, the EBPR process is a relatively inexpensive and environmentally sustainable option for P removal; however, the stability and reliability of EBPR can be a problem. It is widely known that EBPR plants may experience process upsets, deterioration in performance and even failures, causing violations to discharge regulations (Hartley and Sickerdick, 1994; Blackall et al., 2002; Thomas et al., 2003; Stephens et al., 2004). In some cases, external disturbances such as high rainfall, excessive nitrate loading to the anaerobic reactor, or nutrient limitation explains these process upsets. In other cases, microbial competition between PAOs and another group of organisms, known as the glycogen (non-polyphosphate) accumulating organisms (GAOs), has been hypothesised to be the cause of the degradation in P removal. Like PAOs, GAOs are able to proliferate under alternating anaerobic and aerobic conditions without performing anaerobic P release or aerobic P uptake, thus they do not contribute to P removal from EBPR systems. GAOs are believed to use glycogen as their primary energy source for anaerobic VFA uptake and PHA formation, while PHA is oxidised aerobically, leading to biomass growth and glycogen replenishment (Liu et al., 1994; Satoh et al., 1994; Mino et al., 1995). Since GAOs consume VFAs without contributing to P removal, they are highly undesirable organisms in EBPR systems. GAOs have indeed been found in numerous full-scale EBPR plants (Crocetti et al., 2002; Kong et al., 2002b; Saunders et al., 2003; Thomas et al., 2003; Gu et al., 2005; Wong et al., 2005; Kong et al., 2006; Burow et al., 2007), and studies have suggested that they increase the anaerobic VFA requirements of these plants (Saunders et al., 2003; Thomas et al., 2003; Gu et al., 2005). Minimising the growth of GAOs in EBPR systems has been a widely researched topic recently, due to the opportunities that exist for increasing the cost-effectiveness of this process.
The aim of this review is to discuss recent advances in EBPR research, related to the microbiology and biochemistry of the microorganisms involved as well as process modelling and optimisation. The main focus of the review is to summarise the many new findings that have been made since previous reviews of this subject (e.g. van Loosdrecht et al., 1997; Mino et al., 1998; Kortstee et al., 2000; Blackall et al., 2002; Seviour et al., 2003). Future developments for this process are discussed as well.
Section snippets
Isolated organisms proposed as PAOs
The first attempts to identify the microorganisms involved in EBPR, over 30 years ago, were based on culture-dependent techniques. Postulation of which bacteria were thought to be responsible for P removal were made based on the number of viable bacterial colonies that grew on defined media (Barker and Dold, 1996). Through these techniques, Acinetobacter was first proposed to be the primary organism responsible for P removal in EBPR (Fuhs and Chen, 1975), and was long believed to be the sole
Limitations of biochemical studies
One limitation with many of the biochemical studies discussed below is the lack of characterization of the microbial population present in the culture that was used. In particular, it is likely that GAOs were present and active in a number of previous studies that have focussed on the biochemistry of PAOs (as suggested by low anaerobic P release to VFA uptake ratios), making the results more difficult to interpret. In studies focussed on determining the metabolic pathways of GAOs, the microbial
The competition between PAOs and GAOs
Successful operation of the EBPR process depends on numerous process operational factors. Process upsets and the deterioration of P removal in EBPR plants can be explained by such disturbances as the presence of nitrate in the anaerobic zone (Kuba et al., 1994), potassium and/or magnesium limitation (Brdjanovic et al., 1996; Pattarkine and Randall, 1999), over-aeration due to e.g. excessive rainfall (Brdjanovic et al., 1998b) and the microbial competition of GAOs with PAOs (Thomas et al., 2003
Process operation
Recent advances have been made in relation to the development of novel processes incorporating both P and nitrogen removal. The ability of PAOs to denitrify is a key factor in many EBPR process designs, due mainly to the reduced levels of oxygen and carbon sources that are needed for simultaneous denitrification and P removal. This can lead to savings in plant operational costs.
Process modelling
Two different types of models, namely activated sludge models (ASM) and metabolic models, have been used to describe the EBPR process. Both types of models consist of sets of stoichiometric and kinetic expressions that describe the biochemical transformations of the process. One key difference between the two types of models is that the yield coefficients in metabolic models are derived theoretically through substrate, energy and reducing power balances, minimising the need for site-to-site
Conclusions and future directions
Over the years, our knowledge of the EBPR process has steadily improved, largely due to the high number of valuable studies from various disciplines. However, many unanswered questions about the process still remain in relation to numerous issues. Discussed below are a list of the primary conclusions from this review and some of the main issues that still remain to be resolved.
Acknowledgements
The Fundação para a Ciência e Tecnologia (FCT) in Portugal is gratefully acknowledged through the project POCI/AMB/56075/2004. Adrian Oehmen would like to acknowledge the FCT for Grant SFRH/BPD/20862/2004. Gilda Carvalho acknowledges the FCT for fellowship SFRH/BPD/6963/2001. The Environmental Biotechnology Cooperative Research Centre (EB CRC) of Australia is also gratefully acknowledged for their continued support.
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