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Watercare Services Ltd, Mangere Wastewater Treatment Plant (WTP), situated on the Manukau Harbour, Auckland, New Zealand has recently completed a major upgrade to change the treatment process from a combined trickling filter/oxidation pond system to a step-feed activated sludge process to achieve nitrification and denitrification. This upgrade, at a cost of approximately NZ$450 million, includes, as well as the activated sludge process, sludge handling upgrades and reclamation of the foreshore area previously dedicated to the oxidation ponds. Correspondingly, power consumption has increased on-site with the added facilities and increased treatment requirements. Some of this cost has been offset by the maximization of biogas generation, but Watercare continues to seek opportunities to further lower operational costs of the plant. This evaluation has been undertaken to investigate reduction of the power requirements for aeration of the activated sludge process under an alternative airflow control system.

The Mangere WTP has a maximum day inflow to the plant of 1,209,600m3/day (319 mgd) with a mean daily flow of 390,000m3/day (103 mgd). The newly completed plant is designed to achieve a 15mg/L TBOD, 15mg/L TSS maximum monthly mean effluent standard with a seasonally adjusted Total Nitrogen target of 9.5mg/L (December to March inclusive) or 35mg/L (April to November inclusive). To achieve these effluent criteria requires biological nitrogen removal. Nine discrete reactor/clarifier activated sludge units have been designed for nitrification and denitrification as well as organics removal. Central to this function is the need for substantial quantities of aeration air at a fairly high discharge pressure, which results in high operation and maintenance costs with electrical power being the dominant operation cost. Five highly efficient variable-vane single-stage HV Turbo blowers deliver between 40,000 (23,600 scfm) and 120,000m3/hr (70,800 scfm) of air at 85kPa (12.3 psi) to the aeration distribution system. The cost of operation of the aeration system is in the order of 40% of the site's total electricity demand, costing about NZ$ 1.7 million per year. Experience with the existing aeration control approach is that the DO concentrations in the four aerated zones of each R/C may vary substantially, and there may be periods of over-aeration. This in turn implies higher operating costs.

The DO control philosophy has been reviewed in order to quantify the potential for power savings in aeration. The existing approach creates periods of highly variable DO concentrations in the aerobic zones in each reactor. An alternative DO control system based on local airflow control valves for each aerobic zone coupled to feedback from DO sensors in each zone is evaluated. The Bio Win™ process simulation model has been used to quantify the potential power savings by simulating this alternative DO control philosophy at the Mangere WTP. Using operational data over several one-week periods, the total airflow of the existing versus proposed control regime has been assessed in terms of power demand. This evaluation indicates that the power consumption over a week's simulation period is reduced by about 28% when local DO control is implemented. This paper presents the modeling approach taken to assess this potential power saving and the final results obtained from field testing of this method of DO control on a single reactor clarifier unit.

Document Type: Research Article


Publication date: 2005-01-01

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