The Wallace R. Knox Wastewater Treatment Plant in Texas City, Texas, operates one of two belt presses seven days a week during normal work hours, dewatering an average of 72,000 gpd of feed sludge.

 

Every three to four weeks, however, the spring-loaded check valve in the aging emulsion blending system became caked with polymer. It took staff one day to dismantle the chemical feed pump and clean the mixing chamber with polymer solvent.

 

“We had capacity to store sludge in the digester for a day or two, but interrupting the dewatering process wasn’t very efficient,” says wastewater superintendent Calvin Bremer. “Neither was the polymer mixing chamber. It was breaking the polymer chains and not giving us a homogenized mix.”

 

Bremer consulted Matt Davis at Hartwell Environmental Corp., a distributor in Tomball, Texas. Davis brought in George N. Argiris from Pulsafeeder Inc. in Houston, who suggested two systems.

 

Bremer chose the Polyfeeder emulsion polymer blending system from Pulsafeeder Inc. in Rochester, N.Y. Since it went online in June 2009, the unit has worked without interruption, increasing dewatering production, and saving the city thousands of dollars in polymer expenses.

 

Vertical loop reactor

The Knox plant flow averages 4 to 6 mgd. Wastewater is treated in a vertical loop reactor, basically an oxidation ditch system flipped on its side. A horizontal baffle divides each rectangular tank into upper and lower compartments. Disc aerators are on top and coarse bubble diffusers are on the bottom. The under-over flow pattern allows reactors to be installed in deeper tanks, reducing land requirements.

 

The aerobically digested sludge is dewatered in a 2.2-meter Klampress filter press from Ashbrook Simon-Hartley and a 2.5-meter Series 518 Sludgepress from Ovivo, that the staff is rehabilitating. The waste activated sludge runs 8,000 to 10,000 mg/l mixed liquor suspended solids (MLSS) with 2 to 3 percent solids.

 

“With the old injection system, we ran the presses at 150 gpm and got 16 to 18 percent solids cake,” says Bremer. “Today, we process at 200 to 250 gpm and get 18 to 21 percent solids cake.”

 

The real eye-popper came when Bremer looked at the data for polymer used. The old system required about 13 gpd, and Bremer spent $2,139 on a 275-gallon tote every three weeks. “The Polyfeeder uses about 5.4 gpd,” he says. “I was astounded when I saw the numbers. Now we go a month and a half before ordering the next tote.”

 

Making it fit

The staff installed the modular polymer system in one day. To make room for it on the catwalk, they unbolted and moved the hydraulic pump. John Condame and another associate from Pulsafeeder then programmed the unit’s straightforward digital control and trained the entire staff to operate it in one session.

 

“There was a bit of a learning curve, but they caught on quickly,” says Bremer. “What’s really cool is that the operators don’t have to make a bunch of adjustments. They just set the water to where they want it, adjust the polymer percentage, and the computer does the rest. For the most part, the system runs the same day in and day out.”

 

In two months, operators learned to get the most from the system. The machine has a clear, high-shear mixing chamber and an anti-clog injector lance that discharges into the process stream. The unit has run without major maintenance. Operators change a hose occasionally, but they do it in the morning before starting the press.

 

“The new system is far more efficient,” says Bremer. “It mixes the polymer through the sludge better so it dewaters faster with a much drier cake. It has improved our dewatering process quite dramatically.”

 

The Knox plant lime stabilizes and heat processes some cake into Class A biosolids. The city’s Solid Waste Management Department mixes the material with wood chips to make mulch used in ditches and parks. “The pH is too high for flowerbeds,” says Bremer. “We’re looking at doing some Class B composting, but that’s down the road.”

 

Within seven months, the Polyfeeder paid for itself in polymer saved. The Knox treatment plant staff is considering upgrading the emulsion blending system for the Sludgepress when it is rehabilitated.