Clean Plant, Clean Effluent

Meticulous housekeeping and top plant performance go hand in hand for the staff at the new MBR treatment plant in North Libert­y, Iowa

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In the wastewater profession, a clean plant usually means a well-run plant. So it’s nosurprise that the performance record at the North Liberty (Iowa) Water Pollution Control Plant is essentially perfect. The place is spotless as a result of a rigorous cleaning and maintenance program run by plant manager Dave Ramsey and his staff.

“Dave runs a tight ship,” says project engineer Steve Troyer of Fox Engineering, who partnered with Shive-Hattery Engineers in the design of a recent expansion at North Liberty that includes Iowa’s first membrane bioreactor (MBR). “When you walk into the plant, right away you notice how clean and organized everything is.”

Ramsey explains, “We have a regular cleaning schedule set up, and we allot a large amount of time to cleaning and maintenance. Our operators are cross-trained in all areas of plant operations, and they know exactly what needs to be done and when.” One reason the plant is kept so pristine is that it represents the largest capital investment in city history — $8.4 million.

The money went to expand capacity and improve treatment in this community of 12,000, located in the fast-growing corridor between Cedar Rapids and Iowa City. The community had operated a sequencing batch reactor (SBR) system since 1998, but with new homes and schools lining up along the plant’s receiving stream, city officials wanted better treatment. They opted for the MBR system, and residents agreed to pay the higher cost.

“There was little hesitation on the community’s part,” observes Troyer. “They wanted to go to advanced treatment, and the MBR seemed to fit.”

Growth spurt

As late as the 1960s, North Liberty had fewer than 1,000 residents. Sewers were installed and a trickling filter plant went online in 1967. As the popu-lation grew, the city built a new plant using SBR technology for biological treatment.

But within a few years, that plant was overwhelmed; a 5.5-million-gallon equalization basin built in 2005 was being used to store influent instead of smoothing flow through the plant during rain events. “The SBR facility was running at 125 percent of capacity,” Ramsey says. “The community was simply outpacing the treatment plant.”

The move up to an MBR presented Ramsey and his staff with several challenges. “Before plans were finalized, we visited a number of MBR plants in Georgia,” says Ramsey. “We wanted to see the equipment and get answers to a lot of questions we had.”

During construction of the new treatment plant, he kept the old SBR running, even as the new MBR was coming online. “I needed to know the MBR was going to work,” he says. “It needed to be proven to me 100 percent, because once we shut the SBR system down, there was no going back.”

Sludge hauling increased significantly, too, as one of the aerobic digesters was converted to an aeration tank for the MBR. “The new digesters for the MBR plant were still being used for the SBR, which left us with very little digester capacity for treatment of waste activated sludge,” says Ramsey. The MBR plant (4.3 mgd design capacity) started up in August 2008. It is already piped and prepared for a Phase II expansion to 5.27 mgd in the future.

Integrated process

North Liberty’s wastewater is collected in 55 miles of sewers, helped by eight lift stations that Ramsey and his staff maintain. It enters the treatment plant through a 1/4-inch bar screen (Lakeside Equipment Corporation), and then flows to a grit removal system (Hydro International). Three 2,000 gpm self-priming pumps (Gorman-Rupp Company) deliver it to the MBR.

During times of high inflow and infiltration, wastewater can be diverted through an additional series of pipes and pumps to the equalization basin. In the MBR, two 1-mm rotary-drum fine screens (Baycor Fibre Tech Inc.) remove hair and other inorganic material. The membranes need very clean activated sludge for efficient operation.

The wastewater then enters a biological treatment area consisting of an anoxic zone where denitrification takes place, followed by an aerobic zone for nitrification and BOD removal. Four submersible machines (ABS USA) mix the contents. Three 125 hp blowers (Aerzen USA) provide air through fine-bubble diffusers (ITT Water & Wastewater – Sanitaire).

Mixed liquor from the biological process tanks is transferred to the membrane tanks by three return activated sludge pumps (Flowserve Corporation) and distributed to three 35- by 9- by-12-foot membrane tanks, each containing four ZeeWeed 500 ultrafiltration membrane cassettes (ZENON Environmental Inc., A Division of GE Water & Process Technologies), each containing 48 modules. The hollow-core fibers have a 0.04-micron pore diameter and provide a physical barrier to suspended solids, colloidal material, bacteria and viruses. Total membrane surface area is 195,840 square feet.

Generated by three 40 hp permeate pumps (Boerger LLC), a relatively low-pressure vacuum draws water through the surface of the membranes from the outside in. Festo Corp. Air Cyclic actuators and ABZ valves provide aeration to the bottom of the membrane cassettes.

Biosolids withdrawn from the system pass to a pair of 570,000-gallon aerobic digesters and then on to an 800,000-gallon storage tank. A private contractor, Nutri-Ject Systems Inc., hauls the liquid biosolids sludge to area farm fields for use as fertilizer several times each year.

The effluent, with turbidities that average 0.03 to 0.05 NTU, is near drinking water quality and is well below all the plant’s NPDES permit limits. The extremely clear water is discharged to Muddy Creek which flows to the Iowa River. A UV disinfection system (Trojan Technologies) is available if needed.

Pushing performance

Ramsey and his staff have worked diligently to get the utmost performance from the system. Foaming is an ongoing problem, especially with Iowa’s extreme climate changes and with extremely old sludge age and mixed liquor in the 10,000 to 13,000 mg/l range.

The city asked the plant’s system integrator to change the SCADA system so that operators can adjust the on/off cycles for the aeration tanks. “This gives us operational control over foaming and also provides substantial electricity savings,” says Ramsey.

The staff also altered the membrane backwash system, which originally pumped permeate water in a reverse flow through the membranes every 12 minutes to expand the fibers and remove solids. “Now we run it in relaxed mode,” says Ramsey. “We shut off the suction and let the biomass fall off. This has increased our removal efficiencies of bacteria and viruses from ultrafiltration to the nanofiltration range. It also creates less wear and tear on the pumps without affecting transmembrane pressures.”

Another change added a control matrix to the aerobic digester controls to cycle the air on and off throughout the day. This operational strategy provides more anoxic time, increasing alkalinity and in turn helping the staff control pH in the digesters. The SCADA system supplier is Automatic Systems Company in Ames, and Ramsey and his staff work closely with the company’s programmers to make modifications like these.

Squeaky clean

Still, it’s the clean and well-main-tained condition of the plant that turns heads. “We get compliments whenever a group comes to tour,” says Ramsey. “They say it’s the cleanest plant they have ever seen, especially our laboratory. You could eat off the floor in there.”

Mark Farrier, plant operator II, manages the state-certified laboratory. “He is charged with all operations, such as lab procedures, control testing, and record keeping, and does so with exceptional expertise,” says Ramsey. “He performs at least three times the amount of testing that our NPDES permit says we need to do.”

Assistant superintendent Kevin Stensland explains that every floor in every office gets mopped once a week, and the entire plant is swept and vacuumed regularly. “We have an ongoing painting schedule,

and wipe down every motor, pipe and blower weekly,” he says. “We keep a regular cleaning and painting checklist, and operators get to these items as they have downtime. We probably devote 15 man-hours a week to upkeep.”

The plant maintains a complete supply of spare parts and materials. “An advanced treatment plant is expensive to run and maintain,” he says. “We spent $90,000 on spare parts last year and plan to budget for several hundred thousand dollars more in the future.”

Stensland keeps a detailed computerized inventory of all spare parts and runs the maintenance program for the entire plant and the eight lift stations. He monitors times and hours on all equipment and initiates work orders accordingly. Amp readings are taken monthly on every piece of equipment to monitor motor efficiencies.

Plant maintenance specialist Tom Arey and plant operator I Drew Lammers check every piece of equipment in the plant at least twice a day. “They do an exceptional job of maintaining all equipment,” says Ramsey. The city also maintains a comprehensive safety program, including lock-out/tag-out, confined-space entry, arc flash and more.

Staff members perform monthly safety checks, including fire extinguishers, emergency lighting, eyewash and emergency showers, hoists, gas meters, SCBAs and fall protection. “All inspections are documented, and so are the safety classes the staff attends regularly,” Ramsey says. “The MSDS are kept up regularly, and we do a full yearly inventory of chemicals.”

For its environmental impact and exceptional effluent quality, the North Liberty plant earned the 2009 Iowa Governor’s Environmental Excellence Award in Water Quality. You can bet the plaque on the wall gets polished once a week — maybe more often than that.


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