Three Arkansas Wastewater Plants Find Adjustable Speed Drives Improve Process Performance

Pine Bluff Upgrades Pump Stations
Pine Bluff, population 58,000, is located about 515 miles southeast of Little Rock. The city has an average dry weather wastewater flow of 10 million gallons per day (mgd) into the treatment plant. During extremely wet weather, the flow has reached 30 mgd.

With the city's industrial development and growth over the years, the wastewater collection system had outgrown its ability to keep pace with the demand for effective wastewater pumping capacity in certain areas of the municipality. As a result of this need, the Pine Bluff Wastewater Utility, after investigating possible solutions, settled on the installation of adjustable speed drives for two of the wastewater pumping stations.

Installing the drives, which were acquired from the Graham Company in Milwaukee, Wisconsin, solved two problems at these stations. Jim Ferguson, chief engineer for the Pine Bluff Wastewater Utility, discussed the conditions that prompted the selection of the drives for Pump Stations 5 and 9. He said, "Our concern began with Station 5, which was built around 1945. This station is one of the oldest structures in our system. Today it serves a large residential area, the University of Arkansas Pine Bluff Campus, and a large Tyson chicken processing plant. The wet well in the pump station is rather small. At the time of construction there were no large daily flows, or the stormwater inflow problems we have today."

The utility's facilities were small relative to the load they had to handle. Large quantities of inflow and infiltration can tax older systems like this. When it rains, stormwater enters through the sanitary sewerage. In this case, before the adjustable speed drives went in, this could overwhelm the station,. And the pumps cycled on and off frequently. The only realistic solution to this problem was to install larger pumps. However, if only float switches were used for on/off control of larger pumps, on and off, there could be even more cycling of the motors, and motor life would be significantly diminished.

A decision was made to install two 60 hp pumps units with adjustable frequency drives. These would control the pump speeds and maintain a steady flow out of the station. They would also reduce electrical consumption during low flow conditions. A bubbler-type, proportional-output, wet well level sensor and computerized pump controller were installed to operate the pumping units, as were two Graham 1703AFC60 drives. In operation, the pump controller activates the drives at given liquid levels in the wet well, attempts to maintain a certain level in the well, and commands the drives to run accordingly.

In the modified system effluent wastewater from Station 5 now is pumped to Station 9 through a new 15,000-ft 20-in. force main, before proceeding on to the treatment plant. Station 9 serves approximately one third of the city. In order for Station 9 to handle its original flows and the additional flows from Station 5, it also had to be upgraded. Station 9 had the same problems as Station 5; large flow fluctuations between dry and wet weather, an undersized wet well, and heavy electrical consumption rates.

Ferguson continued, "At Station 9, we installed two Graham 1576AFC150 drives, three 150-hp, 1800-rpm pumps, and the same control setup as we installed at Station 5. In addition to motor speed control as an efficient means to handle the influent, we wanted to install the adjustable speed drives for the electrical savings as well. We have a very long 35,000-ft, 30-in. force main from Station 9 to the influent lagoon at the treatment plant. Now that we can control the pump motor speed, we've been able to realize a sizable power savings in that system."

Most of the time only one of the drives and motors is needed to operate, according to Ferguson. However, as the inflow increases, pumps two and three will come on-line. Each of the rebuilt stations two drives and a third stand-by pump that is constant speed. The pumps controlled by the drives originally had 1,200 rpm motors. Capacity was boosted by installing 1,800 rpm motors. They can run up to 78 percent of full speed, or 1,400 rpm before pump cavitation begins. When all three pumping units are on-line, the speed of the drive units is reduced to 1,200 rpm to maintain a load sharing condition.

Ferguson concluded by saying that in addition to reducing energy consumption and providing good influent control, the new drives also offer a means to reduce wear and tear on the equipment when the units are run at reduced speed. In the past, with float switches and constant speed motors, the station discharge was either zero or 100 percent-and nothing between. The upgraded system has evened out the flow, with the result that the treatment plant sees a much more consistent influent flow, and no more periodic large infusions of wastewater.

Russellville Food Plant Gets Wastewater Pre-Treatment System Improvements
A large ConAgra food processing plant is located in Russellville, Arkansas. The facility's primary products are TV dinners, but other frozen food products under various private labels are produced there. As a result of a somewhat novel arrangement, ConAgra and the municipality have joined forces to develop and operate a wastewater pre-treatment plant. It handles only ConAgra's wastewater, removing grease and solids from the waste stream before it enters the city's sewer system. This joint operation of the Russellville Water and Sewer System and the food processor is called City Corporation. The corporation owns the pre-treatment facility, while ConAgra purchases all the equipment, pays all the bills, and makes any necessary capital improvements.

In describing the pre-treatment plant, Danny Teeter, plant operator, explained that previously all the ConAgra wastewater discharge entered two large concrete basins within the municipal facility. However, the inflow was particularly high on one shift, and it was clear that the process could be improved if the feed was more uniform. Also, a more consistent rate would lower the cost of pre-treatment chemicals.

Engineers from ConAgra and City Corporation joined forces to find ways to enhance performance. Their design work resulted in the addition of a 300,000 gallon concrete equalization tank at the head of the treatment process. It was determined that this would help even the flow rates to the plant's 450-sq ft dissolved air flotation (DAF) units.

Teeter explained that the wastewater from ConAgra contains grease, vegetable oils and starches. He went on: "The grease and oils do not readily come to the surface where they can be skimmed off like motor oil. What we do is force the oil out of the water by using a compressed air system which creates bubbles that the oil clings to and then is brought to the top of the tank. It's then mechanically skimmed from the surface."

After the equalization tank was completed, three Gorman-Rupp self-priming, solids-handling centrifugal pumps were installed to transfer the ConAgra waste stream from the tank to the DAF units. Only one pump is used at a time, and they are alternated on a weekly basis. But then a new process problem arose. As the level in the equalization tank was reduced through pump-down, the discharge head was reduced and the flow rate would fall to less than 200 gpm. This is below the desirable rate for the DAF process, and the possibility of odor development became a new consideration as a result.

To solve the problems the joint engineering team decided to install an adjustable speed drive with a motor selection transfer switch. It was connected to two of the pump motors through the transfer switch. The pumped volume is controlled using a flowmeter with an analog output device installed between the pump and the DAF process section. Pump sheaves were changed to provide a speed corresponding to a 600-gpm flow when the level in the tank is just above the high-speed switching point. The flowmeter senses the flow and signals the drive to change pump speed to maintain a constant flow as the level in the tank changes. Flow to the DAF is maintained at ± one percent of the set rate.

According to Teeter, "Because the pump is able to run at reduced speed and head, improvements include significant energy savings. Also, motor life is extended by reducing the number of starts and stops that used to occur. The drive unit, which as in the case of Pine Bluff was supplied by the Graham Company, automatically gives a controlled soft start as well as the controlled speed of the motor and pump." Other improvements that have been observed are a reduction in chemical consumption because of less waste, faster pump-down of the equalization tank, and minimization of unpleasant odors around the facility.

Batesville Wastewater Flow is Stabilized, Process Improved
Batesville is a community of approximately 9,200 located on the White River in north central Arkansas. However, according to Eugene Townsley, superintendent of the Batesville Wastewater Treatment Plant, as a result of the city's industrial load on the local wastewater system, the treatment facility processes wastewater volumes that could equal those from a population of 40,000 to 50,000.

Townsley explained the disparity. The major industrial contributors to the treatment plant are two large poultry processors that each discharge close to one million gallons of wastewater per day. Also, there are a bakery, a cheese manufacturer, two other industrial sites, a municipal incinerator and two groundwater remediation projects all discharging to the sewerage system. Adding the industrial contribution to the effluent already entering the system from the general population, it is not uncommon for this plant to treat over 3 mgd. In fact, the records showed the daily average in 1993 was 3.95 mgd.

To handle this flow rate the plant has an underground lift station, 20 ft below grade, with one 75 hp and three 100 hp pumps. This facility is the main lift station, which is located about 4,000 ft from the treatment plant. All wastewater from the collection system goes through this pump station, which lifts it an average of 16 ft to the treatment plant.

According to Eugene Townsley, under normal dry conditions, a single 100 hp pump is adequate for transferring all wastewater from the pump station to the plant. When the city experiences a light rainfall, the adjustable drive in the pump station may reach 100 percent speed. If the flow continues to rise, as in the case of a heavy rain event, the 75 hp unit comes on line, followed by the second 100 hp pump. The third 100 hp unit will start up if the flow continues to rise.

In the late 1970's the pump station was equipped with an adjustable AC drive that ran one of the four pump motors at variable speed, and the relay logic and bubbler controls that staged all four of the motors was dependent upon the depth of the water in the wet well. In 1982 the adjacent White River flooded the pump station, and the existing drive was destroyed and never repaired. Discussing the system in place prior to the upgrade, Townsley said that the waste stream to the plant used to be quite intermittent. Mercury switches, controlled by a bubbler system, were used to turn the pumps on and off. With this arrangement the plant was either receiving 4,000 gallons per minute (gpm) of influent or nothing.

Now, the system installed more recently, with its Graham adjustable frequency drive, can operate at a low speed of 45 percent, a 1,800 gpm flow rate. There is a continuous flow to the treatment plant, and the process is more stable and operating better. In addition to the adjustable frequency drive, a new Allen Bradley PLC control system, and new relay circuits to achieve better control, was incorporated in the equipment improvements.

Townsley summed up his comments this way: "At this point I haven't seen any electrical failures or pumping problems. Also, since we installed the new system mechanical problems have been greatly reduced. Previously, we had experienced situations where pumps would cycle four to six times an hour. This in turn would cause mechanical stress on the equipment-the motors, starters, breakers and seals. It also appears to have improved the treatment process at the plant. With a constant flow rate, we have eliminated the septic influent shocks that would occur regularly, especially during the summer months."

Editor's Note: More information about these three projects may be obtained from the Graham Company: Tel. 414-355-8800; Fax. 414-355-6117. This article first appeared in a Graham Company newsletter, Water & Waste Control Update. It has been edited to fit 's journalistic style.

Edited by Ian Lisk