The 17-year-old polymer feed system for biosolids at the Northside Wastewater Treatment Plant in Woodstock, Ill., was outdated and parts were no longer available because the manufacturer had gone out of business.

“The old unit was working flawlessly, but we worried that one day it would break down,” says Anne George, plant superintendent. She budgeted for a new polymer feeder system, and EnPro Technologies won the bid. Two operators installed the company’s ParaDyne polymer feed system in one day.

“It reduced our polymer usage by 13 percent,” says George. “Because we’ve had the unit for less than a year, we’re estimating an annual savings of $1,350.”

Teardown and installation

The 10.5 mgd (design) activated sludge plant treats 2.5 mgd from 18,000 residents. Before the polymer feed system was replaced, operators spent seven hours once a week pumping biosolids with 2 percent solids to a 2-meter belt press (Komline-Sanderson) at 175 gpm. Each ton was mixed with 16 pounds of Praestol K290 FLX flocculant (Ashland Chemical). Cake at 12 percent solids was stored on drying beds.

Before changing out the system, operators dewatered the biosolids. Then they cut the PVC piping to the polymer/biosolids mixer, unbolted it from the floor, and removed it with a forklift. The new system with a smaller footprint enabled the operators to line it up with the existing piping. “The swap went very smoothly,” says George. “Everything is pre-piped and pre-wired.”

As an electrician connected the 230-volt power to the belt press, George asked him to wire a signal that would shut down the press if it ran out of polymer. “If we missed changing containers with the original system, the press kept running and we had a mess,” she says. “The operators really appreciate that shut-off feature.”

The next day, service personnel from Energenecs, an EnPro representative in Cedarburg, Wis., inspected the installation and programmed the parameters using settings from the original feeder. “Their visit was part of our purchase arrangement,” says George. “They also trained our staff to program the control panel, which is simple and user friendly.”

Getting going

On startup, the service technicians reduced the polymer feed rate from 1.9 to 1.4 gph and set the water flow rate at 480 gph, producing a polymer solution concentration of 0.3 percent. The combination reduced polymer usage by 13 percent and increased cake solids content from 12 percent to 14 percent.

When activated, a non-impinging rotor controlled by a variable-frequency drive pulls polymer into the mixing zone at the bottom of a high-shear mixing chamber. The rotor creates high fluid shear between itself and stator plates to invert the polymer as it is introduced into the dilution water. The hydrated solution then flows through baffles in the chamber without recirculation to avoid fracturing polymer molecules. A stay-clean check valve spring beyond the flow path prevents it from gumming or clogging.

“The non-impinging rotor definitely produces a better mix, and the operators say the system is simpler to use,” says George. “They like seeing the polymer and water mixing in the chamber. The old system hid the process under a stainless steel cover.”

The operators made an important discovery at a local operators association meeting: Adrian Pino from the Village of Huntley told them he used a polymer Drumstik to discharge liquid from 55-gallon containers of polymer.

“It works like a straw and is much cleaner and easier than tilting the drum on a rock-it drum stand and allowing the polymer to gravity feed to the system,” says George. “The Drumstik goes to the bottom of the container, so the operators don’t have to lift it to pour out remaining polymer or move the drum onto the stand. That was hard on their backs.

They all like the unit. It was a good investment.”