As the phosphorus mass fraction in volatile sludge is about 2.5% of the VSS concentration in a conventional activated sludge proces, the discharge of excess sludge will also result in the partial removal of phosphorus from the wastewater. However, it will in general be required to lower the effluent phosphorus concentration to a value ≤ 1 mg P/l and when discharge of organic phosphorus with the excess sludge is the only mechanism of phosphorus removal, this is only possible under favourable conditions: i.e. a low P/COD ratio combined with a short sludge age. In waste waters with a higher level of nutrients and/or activated sludge systems operating at a higher sludge age, additional methods of phosphorus removal will be necessary.
Apart from phosphorus removal by chemical precipitation, the other main method applied is biological excess phosphorus removal (or bio-P removal). Under appropriate operational conditions a sludge mass will develop that contains a significantly increased phosphorus content. Using artificial substrate (acetate), phosphorus mass fractions up to 38% weight have been attained. In systems designed for bio-P removal, a mixed population will develop with a mixture of “normal” sludge mass with 2.5% phosphorus content and “enriched” bio-P sludge mass containing 38% phosphorus. In this section the following aspects of bio-P removal are discussed:
(1) Theoric principles of bio-P removal
The conditions required for and the behaviour observed in biological excess phosphorus removal are discussed. These conditions are explained by a biochemical model of the metabolism of phosphate accumulating organisms (PAO). This model is presented in Figure 5.1. Click here to download this section.
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| Figure 5.1 | Metabolism of PAO under anaerobic and oxic conditions, Smolders et al (1994) |
(2) Configurations for bio-P removal
Various system configurations have been developed for bio-P removal, all of which have been extensively applied in practice. The main difference between these configurations is the way in which an anaerobic zone is maintained and how this zone is protected against the introduction of nitrate. In this section several common system configurations are discussed, such as modified Bardenpho, UCT and modified UCT. A general system layout is presented that allows a single wastewater treatment plant to be operated in different bio-P removal configurations. Click here to download this section.
(3) Model of bio-P removal
Based on the concepts presented in the previous sections, a model was developed at the University of Cape Town (UCT) to quantatively describe the processes involved in biological phosphorus removal, including the release of phosphorus in the anaerobic zone and the absorption of excess phosphorus in the subsequent aerobic zone. The model was derived from the results of an extensive study by Wentzel et al (1986), who operated activated sludge systems using acetate as the only source of COD in the influent, resulting in a culture enhanced with PAO. Refer to Example 5.1.
In later research it was confirmed that the model could also be applied to a mixed culture of PAO and "normal heterotrophs, as will be encountered in systems designed for nutrient removal. The steady state activated sludge model has been extended to include the PAO biomass fraction. A design procedure for bio-P removal systems is presented and demonstrated in Example 5.2. The issue of denitrifying PAO is discussed in Example 5.3. To download this section, click here.