New Polymer Feed System Improves Pennsylvania Wastewater Plant's Performance

Built in 1977 in a project that cost $6 million, the Pennridge wastewater treatment facility upgraded a polymer feed system there about five years ago with new hydraulic diaphragm metering pumps that have provided new levels of performance and reliability, according to the plant manager.

Located on the East Branch of the Perkiomen Creek north of Philadelphia in Pennsylvania, the Pennridge plant provides advanced treatment and disinfection of wastewater before it is discharged to the creek. It has a capacity of 4 million gallons per day (mgd) and serves 25,000 people in six municipalities in Bucks County.

A plant with a traditional process scheme, the facility has a pretreatment step for debris removal, followed by a raw wastewater pumping station which lifts the stream to the primary treatment stage where solids settle out and are then removed.

Chemicals are added in a flash mixing tank to precipitate phosphorus-bearing compounds. Then gentle agitation in the flocculators promotes coagulation of large, readily settleable flocs. Precipitated solids are removed as the wastewater flows through primary settling tanks. From these a clarified effluent flows by gravity to two trickling filters, where bacteria and other microorganisms attached to the rock media of the filters consume organic material in the stream. The waste stream now passes through aeration tanks and into secondary clarifiers where a final polishing step is carried out to produce a clear effluent for discharge into the creek.

Pumping Polymer for Clean Effluent
The coagulant ferric chloride is injected into the wastewater in both the primary clarifiers and at the end of the aeration stage for total phosphorous removal.

An anionic polymeric chemical is fed into the effluent from the aeration tanks for final clarification before discharge. This polymer acts as a polishing agent to remove trace suspended solids, BOD and insoluble phosphorus from the water. "It's the polymer that helps settle and coagulate the solids and bring them all together so you have a clean effluent, to meet today's stringent permit limits," said Pennridge's manager, Pat Donovan. He added that the polymer plays a critical role in obtaining an effluent that meets the regulatory limits before discharge into the Perkiomen Creek, and it's important that the correct feed be maintained to achieve the proper dose.

Two hydraulic diaphragm metering pumps now are used to feed the polymer to the effluent. But several years ago, according to Donovan, when the plant staff wanted to increase the polymer dosage, they found the pumps being used at the time presented a problem. He said they were never sure of the dosage rates provided by these old belt driven pumps, which were not considered to be very accurate. These pumps did not have flooded suction, so frequently were being starved. This resulted in the tanks and lines having to be cleaned out.

Rebuilding the pumps was attempted but, according to Donovan, they still were not capable of increasing the polymer dosage sufficiently. So, in 1993, Pennridge installed two new Series 560 hydraulic diaphragm metering pumps built by Neptune Chemical Pump Co., (Lansdale, PA) to feed the anionic polymer.

The polymeric material, which is received in dry crystal form in sacks, is mixed with treated wastewater in 1,000-gal tanks in a pump room located in the main control building. The new pumps inject the solution at rates up to 40 gals per hour. "The flow rate is set according to the flow demands of the system at the time," explained Donovan. "This varies with the time of day, rainfall and other factors that impact the demand on the plant." A 4-20 mA signal is fed from the effluent flow meter to the pumps, and automatic controllers are used to set the flow rate.

System Has Proved Reliable
"There is not a whole lot to go wrong with these pumps compared to the ones we used before," Donovan noted. "They have cast iron bodies, and they're easier to adjust and calibrate. You set that knob (on the micrometer dial) to 50%, (of flow capacity, or 50% of 40 gal/hr), do the draw-down, and you know you're going to get 50% (20 gal/hr) right on the button."

Commenting that the simplicity of the Neptune pumps is to his liking, Lloyd Dooley, the maintenance supervisor, said that since the units were acquired in February 1993 the only significant maintenance performed on them has been to rebuild the check valve system. And as a result of their success, the Pennridge staff, which is right now looking into installing a new pump system to replace the three pumps presently feeding the ferric chloride solution, plan to investigate feed pumps for that task from the same manufacturer.