State Dissolved Oxygen Levels lead to Financial Savings
AEA Authority View – February 2006

Two wastewater plants in the Township of Morris have experienced significant savings in electricity costs, sludge removal and chemical usage after installing a new system to control dissolved oxygen (DO) levels in the aerated digestion tanks. Regulated DO levels optimized the rate of tank aeration and reduced pumping requirements. It also provided a stable environment for the microorganisms that processed the wastewater and reduces sludge production and chemical usage.

Sludge removal and electricity were the 2.2 mgd Butterworth plant’s biggest expenditures. The majority of the electric bill was for pumping air into the aeration tanks and for ultraviolet disinfection.

When the microorganisms in the tanks experience swings in the wastewater’s dissolved oxygen, the plant produces more sludge. Seasonal temperature changes also affect dissolved oxygen. Pumping more air into the tanks than necessary wastes a great deal of electrical power.

In 2001, the Butterworth plant installed a dissolved oxygen control system on the aeration tanks to defray costs. The system consists of DO sensors in the final tanks of the three aeration trains, an electronic recorder/controller, variable frequency drive and a positive displacement blower driven by a 150 hp motor.

Using manual DO meters, plant personnel developed the proper air balance for the four tanks in a single train. When the four tanks are in equilibrium, a fixed relationship exists among the DO levels from tank to tank. The analysis determined that holding the DO level to 3 ppm in the last tank would optimize the DO levels in the preceding three, with the first tank assuming about a 1 ppm level.

The Benefits New Jersey Central Power and Light monitored the electrical usage at the Butterworth plant before and after the installation of the DO control system. The DO system significantly reduced peak and routine power requirements. The plant received a one-time $80,000 rebate from JCP&L based on the reduction in energy usage.

The plant is also consistently exceeding all the operational parameter mandated by the state. It seems that consistent, uniform control of dissolved oxygen is the key to sludge reduction. Microorganisms require a stable environment for optimum efficiency.

Another benefit from consistent DO control is a reduction in chemical usage. In the past during the winter, the plant would experience a massive foaming in the aeration tanks caused by the growth of undesirable microorganisms that thrived on excess air. Chlorine was used to control the foam, but it came at a cost. Using chlorine increased operating costs and was a health and safety hazard. The stable DO levels have eliminated the need for chorine and the need for additional chemicals to remove the chlorine.