Maintaining the proper concentration of dissolved oxygen (D.O.) in an aeration basin is necessary to keep microorganisms alive to break down organic waste
When the measured D.O. decreases below a desired concentration, the best instrument systems add air automatically to the aeration basin to provide life-sustaining oxygen for the microorganisms and to facilitate thorough mixing of the organic waste.
Without enough D.O. concentration, the beneficial microorganisms will die while troublesome filamentous microbes proliferate, causing some sludge settling problems. If the D.O. content becomes too high, energy is then wasted, and the expensive aeration equipment endures a great deal of unneeded wear.
Recently, a small mid-Western city decided to investigate ways to improve the efficiency of their waste treatment plant. The facility operator was interested in pursuing further aeration system improvements at the plant. He had learned from an operator of a nearby treatment plant that their use of a continuous measurement system for monitoring process D.O. helped them respond to rapid increases in plant load. The operator wanted to apply this strategy to achieve automatic control of blower speed and further reduce energy costs.
The city consulting engineer and plant operator joined forces to complete a simple study that evaluated the cost effectiveness of process D.O. measurement and automatic aeration control. Over the course of a five-month period, the following data was collected:
- D.O. concentration in the contact tank
- D.O. concentration in the re-aeration tank
- Time of day that D.O. measurement was made
- Energy consumption (in kWh) for the preceding day
- Percent speed at which the variable frequency drive (VFD) had been operating during the preceding day.
A portable D.O. meter was used to obtain D.O. measurements in the process tanks. The VFD unit was manually controlled to operate the blower motor at 50%, 60%, 70%, 80%, and 90% of full speed (1750 rpm). A second motor blower motor was also operating at 1750 rpm. The operator collected 91 sets of data. Seventy-six percent of the data sets involved readings at about 8 a.m. when D.O. concentrations tended to be high due to the lower nighttime loading. Twenty-four percent of the data sets involved afternoon D.O. readings taken during higher loading conditions. With high blower motor speeds, the afternoon D.O. values were never unacceptably low.
Analysis of the data disclosed that each 10% reduction of blower speed decreased the daily energy use by about 40 kWh. These results confirmed that using an automatic controller to regulate blower speed would significantly reduce energy use. Subsequently, the plant operator installed an automatic D.O. monitoring /control system for a six-month trial. A control set point of 2 mg/l D.O. concentration was established for the trial.
During part of this trial a single blower was used. When two blowers were used, they were operated simultaneously under the control of the automatic system. Both VFDs were set to operate at a minimum speed of 50%. Over the course of this trial period, total plant energy consumption was reduced by about 8%.
The U.S. Environmental Protection Agency Design Manual on Fine Pore Aeration Systems (EPA/625/1-89-023) states: "energy saving achievable by automatic aeration on D.O. control is 25% to 40%, but can be as high as 50%." Realistically, a waste treatment plant can expect energy savings of anywhere from 0% up to 50%. Many factors influence the magnitude of energy savings, including plant size, mixing limitations, type of aeration equipment used, and plant loading. It is recommended to conduct a feasibility study to determine the potential cost savings.