Telephone: (414) 354-9902
Fax: (414) 354-9903
Email: info@ESCOR.org

 
Sole North American distributor for

Aeration Control Systems

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Grafton WWTP Pioneers New Control System

The Grafton, Wisconsin WWTP is a conventional diffused aeration activated sludge wastewater treatment plant. The facility has an average flow rate of 1.1 mgd. A significant portion of the total influent BOD is the result of septage hauling, which creates high slug loads on the aeration system. The treatment processes include primary clarification, BOD removal in two stage aeration basins with fine pore diffusers, and secondary clarifiers. Inlet throttled multistage centrifugal blowers provide air for the aeration basins and post-aeration basins. 
 

Grafton WWTP during side by side offgass testing by Redmon Engineering to confirm A2C system accuracy.

Since 1989 the Grafton plant has controlled their aeration process using a Dissolved Oxygen (DO) control system manufactured by ESCOR – Energy Strategies Corporation – of Milwaukee, Wisconsin. This system integrates the DO control of the aeration basins with automatic control of the blowers. Air flow control to the individual basins is controlled with electrically operated butterfly valves (BFVs). The total system air flow from the blowers is adjusted by electrically operated inlet BFVs and by controlling blower starting and stopping.

Because of Grafton’s successful operating history with the ESCOR DO control system, the Grafton WWTP agreed to assist in field testing a radically new aeration control system being offered by ESCOR. This new (patent applied for) control system, manufactured under license from Advanced Aeration Control, LLC (A2C), abandons conventional feedback control strategies and replaces them with measurement of actual process loads and diffuser performance. This information is used to provide feedforward control of the aeration process. 

The A2C system was designed by an international team of process and control system experts. The heart of the system is real-time offgas analysis of the aeration basins. This technique uses a hood at the influent end of the aeration basin to capture the air escaping from the mixed liquor. The composition of this offgas is analyzed to determine the amount of oxygen absorbed by the mixed liquor. The system calculates the Oxygen Uptake Rate (OUR) and Actual Oxygen Transfer Rate (AOTR) of the aeration system using techniques established by ASCE standards for offgas testing of diffuser efficiency. The calculations establish the current system oxygen requirements and transfer capability. 

Maintaining effluent DO in the aeration basins is an accepted technique for verifying that air flow rates are meeting process requirements. The A2C system employs a second DO transmitter located near the effluent end of the basin for this purpose. This second DO probe is also used to establish non-steady-state oxygen demand of the mixed liquor. 

Once the oxygen demand is known, the actual diffuser configuration and performance characteristics are used to establish the air flow rate required to maintain the set DO level and meet process requirements. Any increase or decrease in the necessary air flow rate is integrated with blower control to provide the correct total air flow rate. 

The result of the feedforward control algorithm is incredibly precise and stable control of the aeration system. Conventional feedback control essentially reacts to errors that have already occurred. Tuning of the feedback control loops is critical. Re-tuning is necessary as wastewater and diffuser characteristics change. In contrast, the A2C controls uses real time OUR of the process and the efficiency of the oxygen transfer system to determine required air flow rates. The chart of a typical six day period illustrates the accuracy of the system. The six day average effluent DO is identical with the DO setpoint of 1.75 ppm, with a standard deviation of only 0.12 ppm. Over 90% of the DO measurements (at a sampling rate of once per minute) were within ±0.15 ppm of setpoint over the entire six day period. 

The A2C system meets the needs of real world operation. Low maintenance was a design priority. Unlike conventional respirometry systems, there are no sample pumps to plug or expensive chemicals to replenish. Tuning is virtually eliminated. Once the basic system geometry and standard diffuser performance characteristics are entered, the control logic does its job 24/7 without operator intervention. Trouble free operation through the rigors of a Wisconsin winter demonstrated the robustness of the system. Based on this field experience minor maintenance on a semi-annual basis is sufficient to sustain performance. 
 

Simplified schematic of the offgas analysis system

Long term accuracy of the system is obviously a concern. This was addressed in many ways. Moisture separation is used to provide dry air to the gas analysis instruments. The CO2 content of the offgas is measured and used to compensate AOTR calculations. The offgas analysis transmitters are self-calibrating: periodic samples of ambient air are used to automatically compensate the output of both O2 and CO2 sensors.

The A2C system provides many significant operational benefits. Obviously, precise control of the aeration system DO provides stable and efficient process performance. Because DO excursions are virtually eliminated, the DO setpoint can be lowered to match actual process requirements. Conventional DO controls require the target DO be set high enough to keep inevitable low DO levels within acceptable limits. This results in significantly improved energy efficiency - as much as 20% compared with conventional DO controls and up to 50% compared to manual aeration control! 

Response of the A2C feedforward system to slug loads is superior to any feedback system. This is demonstrated by the nearly constant DO level despite the slug loading from septage hauling. Recovery of the A2C system from power outages was twice as fast as feedback systems, and DO overshoot was eliminated. 
 

The A2C control provides a wealth of process information that is unattainable by other means. OUR is monitored real time and can track plant loading changes and toxicity. Oxygen Transfer Efficiency (OTE) and F are continuously calculated and can be used to optimize diffuser maintenance. ( = ratio of process water to clean water diffuser performance, F = Fouling factor = ratio of actual to new diffuser performance.)  The A2C system represents a significant advance in aeration controls. The precise process control and operational data produces optimum energy and treatment performance for diffused aeration systems coupled with robust and low maintenance operation.  For additional information about ESCOR contact Tom Jenkins, Vice President, at 414-354-9902 or tom@escor.org. For additional information on Advanced Aeration Control, LLC contact Dave Redmon, Managing Partner, at 262-681-0100 or redmoneng@aol.com.

Typical data displays of the A2C system click here to view samples of individual screens