Assessment of slaughterhouse wastewater treatment by using intermittently-aerated sequencing batch reactors (IASBRs)
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A large amount of slaughterhouse wastewater is generated during meat product manufacturing due to the cleaning process. It contains high concentrations of organic matter, oil and grease, and nitrogenous compounds (proteins and amino acids). Hence, its treatment before discharge is necessary for protection of the water environment. Given the high biodegradability of slaughterhouse wastewater, biological processes are considered to be suitable for organic matter removal. Traditional aerobic and anaerobic processes are limited by a number of factors, like high cost, and temperature. The annual average temperature in Ireland is 11 C, which is considered to inhibit nitrification and as a consequence, nitrogen removal would be deteriorated. Thus, the overall aim of this PhD research was to explore new techniques for high strength slaughterhouse wastewater treatment at 11 C, in particular nitrogen removal. During this research, after comparing the effects of intermittent aeration and continuous aeration patterns on nutrient removals, an IASBR technology was chosen to treat high strength slaughterhouse wastewater. Three IASBR reactors were constructed in the Environmental Engineering Laboratories at the National University of Ireland, Galway and operated in a temperature-controlled laboratory where the temperature was 11 C. During the course of laboratory-scale research, three operational parameters were examined: (1) cycle durations (8 hours and 12 hours); (2) organic loading rates (OLRs; 0.61, 0.82, 0.93, and 1.02 g COD/(L.d)); and (3) aeration rates (0.4, 0.6, 0.8, 1.0, 1.2, and 1.4 L air/min). The specific objectives of this PhD research included: (1) to investigate the intermittent aeration strategy treating slaughterhouse wastewater at 11 C; (2) to obtain the optimal operation condition based on the studies of different OLRs and aeration rates; (3) to investigate the influence of the operation conditions on the populations of nitrifers and phosphorus accumulating organisms (PAOs) in activated sludge biomass; and (4) to study the characteristics of N2O emissions in IASBRs treating slaughterhouse wastewater. Comparing two cycle durations on slaughterhouse wastewater treatment at 11 C, a longer IASBR cycle duration (12 hours) was required to efficiently treat slaughterhouse wastewater to meet wastewater emission standards. With a 12 h cycle duration, the optimum aeration rates were 0.6, 0.8, and 1.2 L air/min at the average OLRs of 0.61, 0.82, and 1.02g COD/(L.d), respectively. The aeration rates adversely affected the PND efficiencies. High AOB/NOB ratios in activated sludge biomass benefited nitrogen removal via PND. Both the NLR and the aeration rate influenced AOB and NOB activity and population sizes. The ratio of AOB/NOB (R sub(AOB/NOB) ) varied with the NLR (N sub(LR) . mg TN/(L.d)) and the aeration rate (A , L air/min): (R sub(AOB/NOB) =0.07N sub(LR) - 4.65A+0.27 R2 = 0.9, P= 0.01). Because N2O has a high global warming potential (approximately 300-fold greater global warming potential than CO2), it is necessary to understand N2O generation and emission from wastewater treatment systems, especially from enhanced nitrogen removal processes. In this study, the mean N2O emission in IASBRs in pseudo-steady state was up to 3.6% - 12.0% of the incoming TN. Higher N2O was generated at lower aeration rates and N2O generation increased when the NLR was increased. The amount of N2O generation in the aeration period was reduced with an increase in the aeration rate; however, the highest N2O generation in the non-aeration period occurred at the optimum aeration rates. In this study, the correlation of the N2O-N generation (G sub(N20) . mg) and the incoming TN (N . mg) at the optimum aeration rate was established as: ( G (sub(N20) = 0.13N - 14.83 R2 = 0.98). This study shows that control of air supply with the occurrence of "DO elbow" at the end of the last aeration period would achieve efficient TN removal with high PND efficiency in the IASBR technology.