In order to treat domestic wastewater, a sequencing batch reactor (SBR) system operated in the sequence of anaerobic (3 hr) - aerobic (3 hr) - anoxic (2 hr) phases was used. Because of the exhaust of organic materials at the anaerobic phase, external carbon addition was indispensable for denitrification. Although methanol has been used for denitrification due to its inexpensive cost, it is urgent to find a new alternative carbon compound for the methanol, as an effort to reduce operation cost in the wastewater treatment plants. Acetate is known to be the most efficient electron donor for nitrogen removal, and other volatile fatty acids (VFAs) such as propionate and butyrate are reported to be more favorable than methanol. Furthermore, short-chain fatty acids, especially acetate, are recognized as the most suitable carbon substrates for the biological phosphorus removal process. Therefore, Korean food wastes which contain a higher percentage of fermentable components with less N- and P- than any other wastes were anaerobically fermented in batch reactor, and the VFA solution obtained in the fermentation was added in the biological nutrient removal (BNR) of municipal wastewater using the SBR process in order to promote the denitrification and P removal processes. The removal rates of nitrogen and phosphorus improved from 44 and 37% to 92 and 73%, respectively when the VFA solution was added. The concentrations of nitrogen and phosphorus were maintained below 3 and 1mg/L, respectively. The N- and P- content of the VFA solution was low enough not to influence the final N- and P- concentrations of the wastewater.
Each VFAs showed different efficiencies in denitrification and phosphate removal. In the case of denitrification, acetate was the most effective source, followed by butyrate, propionate and valerate. Mean specific denitrification rate of acetate was ca. 2 times higher than those of other VFAs. Other VFAs showed similar efficiency. The best P-release and uptake ratio was also obtained with acetate. Therefore, it is favorable that the organic acid solution obtained from acidogenesis of food wastes, which will be used as the carbon source for BNR, contains more acetate and butyrate. In order to determine the appropriate conditions for production of VFA containing larger percentage of acetate and butyrate from food wastes, the influence of various operational parameters such as HRT, OLR, pH, and temperature on acidogenesis in semi-continuous mode reactor was investigated. Among the investigated operations, Run 9 (35 ℃, pH 6.0, HRT 8 days, OLR 9 g/L·d) was found to be the most suitable operation. In this condition, concentrations of TVFA, SCOD, acetate, and butyrate were 25, 39.5, 12, and 5.25 g/L, respectively.
To optimize the operation strategy of SBR process, modeling and track studies were performed. The models were set up with material balances on SBR operation and Monod kinetics. The model parameters were obtained to best fit the experimental results in a small scale SBR. The results of simulation and track studies were useful in optimizing HRT. HRT of 5.5 hr consisting of the sequence of anaerobic (1.5 hr) - aerobic (2.5 hr) - anoxic (1.5 hr) phases was found to be enough for an appropriate level of removal rate, at least in the wastewater composition investigated. So as to evaluate the applicability of acidogenic digestion product of food wastes to BNR, fermentation broths prepared in various acidogenic conditions in semi-continuous reactor were added in anaerobic and anoxic phases. The best result was obtained when the VFA solution of Run 9 was used as a carbon source, and the shortened HRT (5.5 hr) was applicable to this system while the removal efficiency is not affected by it.
In order to stably operate the BNR system, separation of denitrification stage from SBR system was attempted. Packed bed reactor (PBR) was used for high rate denitrification. The volumetric denitrification rate of PBR observed in this study was 45.8 mg N/L·h. Due to high denitrifying performance achieved by the PBR described in this study, the volume of such a reactor will be at least 2.5 times smaller than that of the anoxic zone of a conventional denitrifying activated sludge process facility (including SBR system in this study). The separation of a denitrifying biomass in a specific bioreactor led to a development of a very active biofilm and a stable nitrogen and phosphorus removal regardless of the presence of suitable carbon source for P-release in anoxic phase.
