Advanced Regulatory Control of Dissolved Oxygen at the Mangere Wastewater Treatment Plant
Doughnut shaped aeration tank/clarifiers (bioreactors) are an integral part of a recently commissioned secondary wastewater treatment process at Watercare Services Ltd’s Mangere Wastewater Treatment Plant. This biological nutrient removal process reduces the organic (biological oxygen demand and total suspended solids) and nutrient loads (ammonia and total nitrogen) in a series of anoxic and aerobic zones.
Central to the success of the process is the aeration control of these various zones of the bioreactors. Each of the nine bioreactors has four aerobic zones and four anoxic zones that alternate. Air control to each aerobic zone is individually controlled by an air-actuated valve and dissolved oxygen feedback. The air to all thirty-six aerobic zones onsite is fed from five blowers on a separate pressure control system.
This paper describes the successful application of advanced regulatory control (feed-forward plus feedback) at Watercare Services Ltd.’s Mangere Waste Water Treatment Plant. The dissolved oxygen process control system has recently been upgraded to incorporate feed-forward control on primary treatment effluent flow, gain scheduling has been introduced on the feedback controllers and the whole system fine-tuned. This has enabled control of the dissolved oxygen to well within +0.5ppm of set point, resulting in nitrogen levels being well within consent requirements, a 25% reduction in blower energy usage amounting to 13,000 kWh/d and a more environmentally friendly process operation.