Green Wastewater Treatment Plant combats excessive blower energy usage

By:  Jon Zabrocki, P.E. & Paul L. Larson, P.E.

Introduction

Blower energy usage is typically one of the largest energy consumers in an aerobic wastewater treatment processes. By utilizing dissolved oxygen sensors, modulating valves, multiple PID control loops, and variable speed drives on the b

lowers, the plant operators are able to maintain an efficient, precise, optimal dissolved oxygen concentration in each of the 6 new aeration chambers.  By precisely controlling pressure in the air header and modulating airflow to each aeration zone to maintain set point dissolved oxygen levels, the energy usage is minimized through efficient operation. This technical paper presents a discussion of the energy saving blower control system utilized at the main City of Lockport, Illinois Wastewater Treatment Plant (WWTP). 

History
The City of Lockport, which is located southwest of Chicago in Will County, is undergoing tremendous growth.  With construction beginning on Interstate 355, which will run through the City of Lockport, the unprecedented population growth is sure to continue.  The City of Lockport contracted Robinson Engineering, Ltd. to design a treatment plant expansion that would keep operation and maintenance costs low.  The Lockport WWTP was expanded from 2.28 Million Gallons a Day to 3.40 MGD during this initial phase, and was built with facility planning to 5.0 MGD in mind.  Early in the design, Robinson Engineering determined that fine bubble aeration would provide the necessary oxygen transfer efficiencies.  Metropolitan Industries, located in Romeoville, Illinois, was brought in early in the design process to find additional ways save energy.  Typically the biggest users of energy in a wastewater treatment plant are the aeration blowers (generally between 50 and 80%).  Metropolitan Industries had utilized sophisticated blower control methodologies on previous projects, demonstrating substantial energy savings.       

 

The Lockport WWTP expansion was designed in 2 phases (Phase 1 to 3.4 MGD, Phase 2 to 5.0 MGD).  The first phase involved construction of aeration tanks, the blower building, and necessary blowers.  The second phase involved adding additional blowers and aeration tankage.  A three-pass, plug flow, conventional aeration system was chosen due to its ability to reach lower effluent limits.  The mixed liquor resides in the aeration basin being treated by aerobic bacteria for about 14 hours.

Traditional Blower Control

Blowers and airflow to diffusers are traditionally controlled by manually throttling the inlet butterfly valve on each blower.  The blower runs at a constant full speed, which is analogous to driving a car with the accelerator to the floor and using the brake to regulate speed

Variable Speed Blower Control based on Dissolved Oxygen

A method that substantially automates the aeration process to the changing organic loads realized at municipal wastewater treatment plants is controlling the concentration of dissolved oxygen with a PID loop that automatically adjusts blower speed.  To balance the flow of air between each aeration basins and zone within each basin, the zone header pipe has a motor operated butterfly valve.  The valve is automatically modulated to maintain the proper balance of air to each treatment zone regardless of possible water elevation differences between aeration tanks.  The local RTU/SCADA-RTU is programmed to hold the adjustable dissolved oxygen set points for several zones, using multi-level, cascaded, PID loop strategy that automatically compensates for BOD, air density, blower efficiency, plant flow, and provides blower surge mitigation.

An airflow meter and an air pressure transducer were included on the header pipe in the blower room.  This information is used along with the dissolved oxygen readings in strategic zones to control the blower speed and valve positions. 

The controller automatically sets blower speed and valve positions.  By receiving inputs from the dissolved oxygen probes and other instrumentation, judgments are made about blower speed and modulating inlet valve position.  The PLC ensures that sufficient air is provided to the aeration basins at all times to prevent settling of solids in the basins.  The PLC also automatically alternates the blower motors.

Dissolved oxygen levels are controlled by modulation of the air delivered to each aeration zone. The system automatically compensates for ambient air density, temperature and process demand, to minimize electric power input to the blower motors.  Dissolved oxygen sensors are located in strategic locations to transmit signals to the controlling SCADA RTU.

The operator can set the dissolved oxygen levels on each zone air supply pipe.  Zones that are not furnished with dissolved oxygen sensors (or out of service probes) follow the air supply of a similar zone.  An offset ratio may be assigned to interpose between any follower zone and the controlling zone.  The operator can select which dissolved oxygen sensor controls a zone on the SCADA RTU control screen.

Air pressure delivered to the zone air valves is regulated by control of the number of blowers operating and their operating speeds.  The zone having the greatest air demand determines the air pressure set point.

In order to control the process for each system, dissolved oxygen levels were maintained at 2.0 mg/L.  Because a dissolved oxygen concentration above 4.0 mg/L does not improve operation of the system, but does increase aeration costs, the D.O. is monitored and blower speeds are adjusted to accommodate varying loads to the plant – at night, and during storms.  Return activated sludge rates (RAS) are maintained around 50% of the incoming flow to reseed the bacteria into the reaction basins.  Waste activated sludge (WAS) is drawn off the RAS to remove excess solids from the process.

A pilot study done by Metropolitan Industries showed an average energy savings around 16%.  The conclusion of the study was, “Because of wide variations in the biological oxygen demand over the course of any given day; real and significant savings can be realized if the oxygen volume delivered to the process matches the requirements at that specific time.”

To minimize energy usage, fine bubble aerators were selected to maximum oxygen transfer efficiency.  A control scheme was also developed with the assistance of Metropolitan Industries.  The plug flow process monitors dissolved oxygen in each pass.  Modulating valves are adjusted within limits to adjust air to that pass.  As valves are opened, the pressure in the blower header drops.  The blower speed is increased to maintain the pressure at a set point.  This causes more air to be supplied to the system by the blowers.  Each zone then has a 2.0 mg/L of dissolved oxygen, which maintains mixing and selects the proper bacteria for efficient biological reduction of TSS and BOD.

The Lockport WWTP has had excellent effluent of approximately 5 BOD and 5 TSS.  One blower operating during the day is currently handling the entire 1 MGD average flow.  Typical operation has dissolved oxygen is maintained at 2.0-mg/L.

Conclusion

The City of Lockport has been extremely pleased with the outcome of this project.  With very little capital expenditure, the treatment process has resulted in excellent effluent, matching of oxygen added to demand, and soft starting of the blowers.  Amperage readings and estimated energy usage have shown that the blower control system supplied by Metropolitan is reducing energy usage over traditional throttling inlet valve blower control.  As a result, upgrades to the existing aeration zones are on order.

About the authors

Jonathon Zabrocki, P.E. is a registered professional engineer in the States of California, Colorado and Illinois.  He works as a senior engineer for Robinson Engineering, Ltd, which is located in South Holland, Illinois.  Jonathon attended Northwestern University where he received a B.S. is Civil Engineering and a M.S. in Environmental Engineering. 

Paul L. Larson, P.E. is a registered professional engineer in the State of Illinois.  He works as the chief mechanical engineer for Metropolitan Industries, Inc., which is located in Romeoville, Illinois.  Paul attended South Dakota School of Mines and Technology where he received a B.S. in Mechanical Engineering.