Polished to Perfection

A treatment process with aerobic and anaerobic cells and a polishing reactor enables a plant in northwest Iowa to meet ammonia permit levels
Polished to Perfection
Workers from Penro Construction unload 6-foot-square, 8-foot-tall polishing modules and place them on pallets.

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The three-cell, 6-acre aerated lagoon at the Remsen (Iowa) Wastewater Treatment Plant could not meet new ammonia permit levels of less than 4 mg/l in winter and 2 mg/l in summer. The lagoon averaged 14 mg/l in summer and 27 mg/l in winter.

Each 12-foot-deep cell held 5 million gallons, but the primary cell had six feet of sludge in areas. “We were losing quite a bit of detention time and had no idea because we didn’t use a Sludge Judge (Nasco),” says chief operator Mike Ruden.

The complete-mix cell had 20 submerged coarse-bubble diffusers and the partial-mix cell had four, but dissolved oxygen still hovered about 6 mg/l in winter. The third cell was the quiet zone.

The city hired DeWild Grant Reckert and Associates Co., consulting engineers in Rock Rapids, to upgrade the plant. They selected the LemTec biological treatment process from Lemna Technologies, a combination of aerobic and anaerobic cells followed by a polishing reactor. Since its installation, the plant has achieved year-round effluent readings of 4 mg/l BOD and TSS, 2 mg/l ammonia, and 6 mg/l DO. The city has the only LemTec system in northwest Iowa.

 

Laying the groundwork

The 317,000 gpd (design) treatment plant handles on average 250,000 gpd from 300 homes within the city’s one square mile. Remsen has 1,700 residents. “To meet new requirements, the plant needed a redundant system,” says Keith Miller of Lemna Technologies. “We redesigned cells 1 and 2 to mirror each other and treat the split flow from the plant after it passed through a 1/4-inch bar screen at the headworks.”

Two baffles divide each cell into complete mix, partial mix and settling. Then the wastewater moves to the polishing reactor and disinfection. The baffles also reduce short-circuiting. To prepare for the system, Ruden’s staff drained the lagoon, removed the aerators, and land-applied the sludge. They squeegeed the bentonite clay liner in the first two cells, checked for leaks and found none.

Workers from Penro Construction of Pender, Neb., then installed four Lemna mixers and 24 fine-bubble submerged diffusers in the complete-mix zones and 28 in the partial-mix zones. Each zone has a 30 hp Gardner Denver blower. A third unit serves as backup. “This is not an economical process,” says Ruden. “Our electric bill went from $800 to $1,700 the first month. Activating the UV system increased it to $4,000 a month, but we’re getting the desired result.”

 

Attached growth

Simultaneously, Penro workers decommissioned the third cell, then used the space to pour the 60-foot-long polishing reactor basin with two 7.5-foot-square by 10-foot-deep treatment zones. Workers also poured the open basin for the UV3000B disinfection system from Trojan Technologies and installed and plumbed the 3-foot-wide by 2-inch-deep stainless steel channel.

Stacks of 6-foot-square, 8-foot-tall treatment modules arrived with hooks welded to the tops for lifting them into the basin. The modules, assembled from honeycomb-like layers alternating 90 degrees to each other, are secured by a threaded rod.

“Both reactor zones have six stacks of 48-density modules to treat BOD and ammonia,” says Rhett Arens of Lemna Technologies. Depending on the performance requirements, zones can be designed with two different media densities for cross-use on BOD or ammonia. Units are attached to an air rack, fed by an airline controlled by a ball valve at the aeration header. Each polishing zone is dedicated to a treatment cell.

To enhance system kinetics, retain heat, control odor, and prevent algae growth in the two cells, Penro workers assembled the LemTec insulated modular cover as soon as the water returned to operating levels. They fastened the 6- by 40-foot sections together on shore, floated them across using winches, and anchored the ends with cables and deadmen.

 

Snug and warm

“The first cell was covered last January during a particularly cold winter,” says Ruden. “When the men finished, they were working in ice and subzero temperatures.”

Within days, Ruden saw a rise in water temperature as the black cover absorbed and transferred heat. With the second cell covered, water temperatures rose to 43 degrees. The lowest temperature Ruden recorded that winter was 41 degrees.

The disinfection system went online in August. Effluent runs from the polishing reactor to a collection box, which slows the flow before releasing it into the channel with 10 banks of four energy-efficient amalgam lamps on either side.

“The agent from the Department of National Resources arrived just after the UV system went online,” says Ruden. “I grabbed a sample and held it next to a beaker of tap water for him. He couldn’t tell the difference and was really impressed.” The sample Ruden pulled on E. coli showed less than 16 colonies per 100 ml.

Sensitivity monitors on the lights indicate when they need cleaning, at which time Ruden pulls them out and squirts them with diluted muriatic acid. “It’s just like washing a window,” he says.

 

Plant maintenance

Besides sampling and testing the effluent, Ruden removes buildup on the treatment modules with occasional blasts of air. “The units are like king-sized trickling filters where bacteria collect on the rocks,” he says. “If air won’t dislodge the scum, we lift out the modules and power wash them or disassemble the layers to clean them.”

It is too soon for Ruden to tell how often he will have to clean the sludge in the polishing reactor. “It leaves a lot of suspended solids, so right now I’m backwashing it twice a year,” he says. “I’m not complaining. The reactor is a key component to us meeting our ammonia permit limits.”



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