Phosphorus is a constituent of wastewater, averaging around 10 mg/liter in most cases. The principal form in food and agricultural wastewater is organically bound phosphorus. Organically bound phosphorus originates from body and food waste and, upon biological decomposition of these solids, is converted to orthophosphates.
Biological phosphate removal is a relatively new technology dating back to the late 1950s; it wasn't until the 1970s that there were full-scale processes developed for practical use in advanced wastewater treatment. Based on a series of tests and experiments on biological phosphorus removal, Fuhs and Chen (1975) determined that a genus called Acineto-bacter was responsible for biological phosphorus removal and postulated that these bacteria utilized substrates, a type of volatile fatty acid (VFA) produced from an anaerobic phase, for growth and excessive phosphorus uptake under aerobic conditions. However, this explanation of biological phosphorus removal was challenged by a number of researchers using molecular ecology techniques (e.g., Bond et al., 1995; Mino et al., 1998). Nevertheless, it was recognized from a process engineering point of view that the necessary condition for biological phosphorus removal is existence of a true anaerobic phase in the process; this insight has helped develop several process configurations of biological phosphorus removal in the world.
It was later discovered that the main function of the anaerobic phase was not only to provide polyphosphate-accumulating bacteria with VFAs but also to enable this type of bacteria to use phosphate as an energy reserve to pick up substrates (Wentzel et al., 1986; Arun et al., 1987; Smolders et al., 1994; Maurer et al., 1997; Mino et al., 1998). The available VFAs enable bacteria to utilize the VFAs as a carbon source under anaerobic condition and release phosphate into the solution, and a subsequent aerobic phase as well as anoxic conditions take up the phosphate in water. As a result, a greater amount of phosphorus is removed from waste-water as sludge. Glycogen is also utilized under anaerobic conditions and replenished during the aerobic phase of the cycle.
Acinetobacter is only responsible for some portions of biological phos phorus removal. It is clear today that phosphorus-accumulating organisms (PAOs) include many heterotrophic microorganisms. However, not all heterotrophic bacteria are PAOs, and in a wastewater treatment plant with biological phosphorus removal processes, these non-PAOs may compete with those heterotrophic PAOs for the substrate, particularly those low molecular fatty acids, which is needed for the phosphorus storage mechanism. The result of this competition determines the success of the biological phosphorus removal process. As mentioned previously, the anaerobic phase is of great importance in steering the substrate utilization toward the direction of the heterotrophic PAOs.
The biological treatment or removal of phosphorus from wastewater depends on the accumulation of a large amount of bacteria that are capable of storing phosphorus in the form of polyphosphate inside the bacterial cells; polyphosphate as stored energy for bacteria is produced as a result of sequestering volatile fatty acids by aerobic bacteria under anaerobic conditions, resulting in poly(hydroxyalkanoates) (PHAs) under simultaneous use of glycogen. This requires that the influent of wastewater for biological phosphorus removal has to first mix with sludge in order to create a true anaerobic environment free from electron acceptors such as oxygen and nitrate. In the anaerobic environment or zone, volatile fatty acids (may be formed by fermentation) in the incoming wastewater stream can be accumulated by polyphosphate-accumulating bacteria. Thus, a successful design of a biological phosphorus removal process relies on the creation of such a true anaerobic zone; this will also be influenced by the characteristics of incoming wastewater streams. Depending on whether there is a presence of volatile fatty acids produced by fermentation, the size of the anaerobic zone or reactor varies with the predominant anaerobic process. The volatile fatty acids-containing wastewater streams require small reactors; incoming wastewaters from an aerobic process without volatile fatty acids need larger reactors because the anaerobic phase has to be based on a slower fermentation process. Fig. 5.2 shows a basic schematic diagram of a biological phosphorus removal process. In this diagram, substrate is taken up by polyphosphate-accumu-lating bacteria and phosphate is released into the liquid phase in the anaerobic phase. In the aerobic phase, the bacteria grow and accumulate phosphate in the cells, resulting in removal of phosphate from the waste-water. It is generally observed that in order to operate a successful biological phosphorus removal process, it is crucial that the incoming waste-water stream should contain the correct balance of nutrients, carbon
sources, and pH; careful considerations must be given to the food:mi-croorganism ratio, hydraulic retention time, solid retention time, temperature, and DO concentration (Mulkerrins et al., 2004). In addition to this basic biological phosphorus removal process, there are several combined processes of chemical and biological phosphorus removal. Fig. 5.3 gives a schematic diagram of metabolisms of PAOs under aerobic and anaerobic conditions.
It has been shown that some PAOs called denitrifying PAOs can also accomplish denitrification while accumulating phosphorus. The combined processes of biological phosphorus and nitrogen removal by denitrifying phosphate-accumulating bacteria such as UCT (University of Cape Town)-type processes have been demonstrated (Kuba et al., 1996, 1997). In these combined processes, PAOs used nitrate or nitrite as an electron acceptor instead of oxygen. Contrary to the earlier view of different PAOs involved in denitrification and biological phosphorus removal in the combined processes, it now appears that Accumulibacter was the denitrifying PAOs in both anaerobic and anoxic conditions in the combined processes (Ahn et al., 2002; Zeng et al., 2003). Further microbial analysis in the study of Zeng et al. (2003) revealed that Accumuli-bacter was the dominant species in both PAO and denitrifying PAO sludge. The current trend in biological phosphorus removal is development of a simultaneous nitrification, denitrification, and phosphorus removal process that can save capital and operational costs as well as leave a smaller environmental footprint.
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