Blower energy usage is typically one of the largest energy consumers in an aerobic wastewater treatment process. Operators at the main Lockport (Ill.) Wastewater Treatment Plant (WWTP) are using a blower control system that reduces the amount of energy used for the blowers.
By utilizing dissolved oxygen (DO) sensors, modulating valves, multiple PID control loops, and variable frequency drives on the blowers, plant operators are able to maintain an efficient, precise, optimal DO concentration in each of the six new aeration chambers. By precisely controlling pressure in the air header and modulating airflow to each aeration zone to maintain set point DO levels, the energy usage is minimized through efficient operation.
The city of Lockport, which is located southwest of Chicago in Will County, is undergoing tremendous growth. With construction beginning on Interstate 355, the unprecedented population growth is sure to continue. The city of Lockport contracted Robinson Engineering to design a treatment plant expansion that would keep operation and maintenance costs low.
The WWTP was expanded from 2.28 mgd to 3.40 mgd during this initial phase, and was built with 5.0 mgd in mind for the future. Early in the design, Robinson Engineering determined that fine bubble aeration would provide the necessary oxygen transfer efficiencies.
Metropolitan Industries, brought in early during the design process, found additional ways to save energy. Typically, the biggest users of energy in a DO are the aeration blowers (generally between 50 and 80%). Metropolitan had utilized sophisticated blower control methodologies on previous projects, demonstrating substantial energy savings.
“Based on previous wastewater treatment projects, we felt the inherent energy-savings would save Lockport thousands of dollars in energy costs over the years,” said John Kochan, Jr., president of Metropolitan Industries.
Executing phases
The first phase of the WWTP involved construction of aeration tanks, the blower building and necessary blowers. The second phase involved adding additional blowers and aeration tanks. A three-pass, plug flow, conventional aeration system was chosen due to its ability to reach Lockport’s lower effluent limits.
The mixed liquor resides in the aeration basin being treated by aerobic bacteria for about 14 hours.
Blower airflow to diffusers is traditionally controlled by manually throttling the inlet 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.
“Typically, what you see in older treatment plants are constant running blowers controlled with throttling valves that drive up energy costs,” said Kochan, Jr. “As energy prices continue to increase nationally, municipalities like all of us are looking for ways to cut energy consumption.”
A method that substantially automates the aeration process to the changing organic loads realized at municipal WWTP’s is controlling the concentration of DO with a PID loop that automatically adjusts blower speed. To balance the flow of air between each aeration basin and zone within each basin, the zone header pipe has a motor-operated valve. The valve is automatically modulated to maintain the proper balance of air to each treatment zone.
The local RTU/SCADA-RTU (Remote Terminal Unit) is programmed to hold the adjustable DO set-points for several zones, using multi-level, cascaded, PID loop strategy that automatically compensates for BOD (Biological Oxygen Demand), air density, blower efficiency, plant flow, and provides blower surge mitigation.
“Minimizing blower speed based on demand is the single most efficient method of dissolved oxygen control,” said Metropolitan Controls Division Manager Rich Potter.
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 DO readings in strategic zones to control the blower speed and valve positions.
Controlling DO levels
DO 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. DO sensors are located in strategic locations to transmit signals to the controlling SCADA-RTU.
The operator can set the DO levels on each zone’s air supply pipe. Zones that are not furnished with DO 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 DO sensor controls a zone on the SCADA-RTU control screen.
“I can see what is happening in real-time from my desktop and have comfort in the fact of knowing that my processes are running properly at all times,” said Plant Operator Joe Findley.
Controlling the number of blowers operating and their operating speeds regulates air pressure delivered to the zone air valves. The zone having the greatest air demand determines the air pressure set point. DO levels were maintained at 2.0 mg/L. Because a DO concentration above 4.0 mg/L does not improve operation of the system, but does increase aeration costs, DO is monitored and blower speeds are adjusted to accommodate varying loads to the plant at night and during storms.
A pilot study done by Metropolitan Industries showed an average energy savings around 16% and concluded, “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.”
