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The role of specific substrates in excess biological phosphorus removal Rabinowitz, Barry


The principle objectives of this research were to investigate the role of specific carbonaceous substrates in the excess biological phosphorus (P) removal mechanism and to optimize the design of the nutrient removal activated sludge process so as to maximize the availability of these substrates to the micro-organisms involved in the P removal mechanism. The experimental work reported in this thesis was divided into the following four parts: 1) A series of laboratory-scale anaerobic batch tests which were designed to simulate the conditions present in the anaerobic zone of the process. The objectives of the batch tests were to determine which substrates were most effective in inducing anaerobic P release, the fate of the substrate and its role in the P release mechanism, and to quantify the negative effect of the presence of nitrate in the anaerobic zone on the P release mechanism. 2) A series of pilot-scale experiments in which sodium acetate was added to a simplified nutrient removal activated sludge process with the view to determining what concentration of added substrate is required to reliably induce excess biological P removal in the process. 3) A pilot-scale primary sludge fermentation study in which the objective was to determine the nature of, and the guantity of simple carbonaceous substrates that can be produced on-site at an activated sludge treatment plant, and the optimal fermenter operating conditions for such production. 4) A series of pilot-scale experiments in which primary sludge fermentation was incorporated into the design of a simplified nutrient removal activated sludge process and the UCT process in order to gauge to what extent the P removal characteristics of these processes can be enhanced by such modification. Results of the batch tests show that the simpler short-chain volatile fatty acids (VFA's) acetate and propionate are the most effective in inducing anaerobic P release in activated sludge. Batch tests in which a range of sodium acetate concentrations was fed into the flasks showed that P release and substrate utilization are integral parts of the same exchange phenomenon with a molar exchange ratio of 1.76 moles of acetate (as HAc) utilized per mole of P released by the micro-organisms. With regard to the detrimental effect of nitrate on the P release mechanism, it appears as if the available substrate required for the excess biological P removal mechanism is utilized in the denitrification reaction at a rate of 3.6 mg COD per mg NO₃-N and is thus rendered unavailable for the anaerobic P release mechanism. The addition of 86 mg/L (as COD) of sodium acetate to a simplified nutrient removal process treating raw sewage resulted in excess biological P removal. However, such removal was achieved by the addition of only 39 mg/L (as COD) when the substrate was added to an unaerated zone that received zero influent nitrate, confirming that the substrates required by the excess biological P removal mechanism are utilized in the denitrification reaction, and the importance of adding any additional substrate into a nitrate-free zone. Operation of the pilot-scale primary sludge fermenter showed that acetate and propionate, the two most important substrates in the excess biological P removal mechanism, are also the principle products of primary sludge fermentation, making up more than 95% of the total short-chain VFA production. Optimum VFA yields of 0.09 mg of VFA (as HAc) per mg of primary sludge (as COD) were achieved at fermenter sludge ages in the 3.5-5.0 day range. Incorporation of primary sludge fermentation into the design of the simplified nutrient removal process resulted in an improvement of more than 100% in the P removal characteristics of the process. The same modification to a UCT process that was previously exhibiting some degree of excess biological P removal resulted in a further 5 0% improvement in the P removal characteristics. A proposal for the future design and operation of a primary sludge fermenter for the enhanced P removal activated sludge process that facilitates independent fermenter and process hydraulic and solids detention time control is also outlined.

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