The atypical design of the Cohasset (Mass.) Wastewater Treatment Plant confuses visiting operators and challenges those who run it. Mark Cataldo, project plant manager with United Water, inherited the 46-year-old facility in January 2011.

Although the treatment process had been upgraded to tertiary membrane bioreactors in 2001, many necessary improvements confronted him. Tim Swimm, operations and maintenance technician, joined Cataldo in July 2012. Together, they often worked with very little while significantly revitalizing the plant.

David Burns, environmental engineer with the state Department of Environmental Protection, recognized their efforts and was instrumental in the plant receiving the 2013 Best Small Wastewater Treatment Facility award from the Massachusetts Water Pollution Control Association.

“We don’t fool around,” says Cataldo, a Grade 7 wastewater operator with 26 years of experience. “We get to the root of problems and fix them. Tim and I have become adept at doing things quicker and better.”

Tertiary process

The plant may be small (450,000 gpd design flow), but its process is sophisticated and its effluent quality is high. Wastewater enters the pump building through a Channel Monster grinder (JWC Environmental) before spilling into a wet well with two pumps (Aurora/Pentair) that send it 300 feet to the rear of the plant.

After passing through a SpiraGrit vortex grit removal system and Hydronic T screening system (both from Lakeside Equipment Corp.), the water flows to an anoxic selector with a mixer (Flygt – a Xylem Brand), then enters one of two parallel treatment trains. Each ZeeWeed ultrafiltration system (GE Water & Process Technologies) has seven immersed cassettes holding hollow-fiber membranes with 0.1-micron pores.

Valves control the treatment cycle. Upon activation, the first valve opens, enabling the permeate pump to backflush the membranes. That valve closes and another opens to draw liquid through the fibers to fill the backpulse tank. After it closes, a third valve opens, sending permeate from the tank through the UV disinfection system (TrojanUV), and then to a chamber with three alternating submersible pumps (Goulds) that discharge to dedicated duckbill diffusers in the Cohasset Inner Harbor.

The plant averages 225,000 gpd from 4,500 customers. It treats 5,000- to 6,000-gallon batches every 15 minutes (the time it takes to fill an aeration tank) during peak flows, and every 60 minutes during low flows. About 30 percent of customers tie into 12 miles of 8-inch gravity sewer. The rest of the collection system has 22 miles of 2-inch low-pressure sewer with grinder pumps, eight lift stations, 62 watertight manholes and 44 air-release manholes.

Call the experts

Cataldo and Swimm have become adept at diagnosing issues in the treatment process and either devising economical solutions or getting appropriate help. As they learned more about the plant, they wondered why the pumps for the treatment trains’ manifold air-vacuum system — which keeps permeate in the piping and air out — were shutting off sporadically. Because the SCADA system could not activate the second pump, “One of us had to drive to the plant and hit the on button,” says Cataldo.

Of more concern was why the pumps, rated to last five years, were burning out in seven months. The $5,000 replacements were eroding the annual $600,000 operations budget. “If we can’t solve a problem, we ask engineers and manufacturers to evaluate it,” says Cataldo.

Cataldo called the pump manufacturer, SIHI Pumps Americas, and sent a photograph of the piping. It showed the discharge line rising above the pump, then traveling horizontally to the aeration tank and down to it. The representative called immediately to ask the height of the discharge pipe, which the operators reported as 32 inches. “He said it should be no more than 12 inches above the pump,” says Cataldo.

The additional height allowed permeate to drop back into the pump when it shut off, causing the 3,500 rpm impeller to reverse, slam and loosen. Cataldo and Swimm cut out the piping on both pumps, mounted hose barb connections on the discharge pipes, and ran hoses to the floor drain that feed back to the headworks. The pumps have run smoothly since then.

Tough surprises

In one case, the air-vacuum system caused a sanitary sewer overflow on a Sunday afternoon in 2012 when it shut down the plant for 24 hours. Responding to the alarm, Swimm started the second pump, while Cataldo shut off the influent pumps and let the wastewater fill up the sewer main. A contractor cleaned up the 5,000 gallons of wastewater that escaped.

The following morning, an electrician put a magmeter on the pump wires and found a short. When he pulled out the wiring, the casing was missing in some areas. Replacement wiring fixed the problem. “The damage happened during the plant upgrade,” says Cataldo. “We also inspected a 3-inch cast-iron drainline to the headworks and found it clogged with debris and small hand tools.” They jetted and cleaned the pipe to restore its capacity.

Another puzzle was why the pneumatic actuators on the treatment trains broke every seven months. “We were told the supplies on hand were new, but they failed, too,” says Cataldo. “A closer look revealed that parts were used.” Replacements at $800 apiece broke just as fast. Cataldo eventually ran out of actuators for one train and had to shut it down for a day.

“I asked the town, which has always been cooperative, for $25,000 to buy replacements for every component in a train,” says Cataldo. “That’s eight valves, eight actuators, eight solenoids and 20 pressure snubber valves.” The operators also filled a toolbox with everything needed to repair or replace valves and actuators. The idea reduced a two-hour task to 30 minutes.

Harsh environment

The reason for the actuator failures remained a mystery until Cataldo brought in Energy Machinery, a compressed air specialist. The technician discovered that the filters on the air dryer had never been changed and were clogged, and that the air line to the air compressors had no oil/water separator. Without them, the actuators gummed up and broke.

“For $2,000, we piped in a separator and two air dryers,” says Cataldo. “We also added filter changes to our maintenance schedule.” At his request, the Energy Machinery technician installed a connection for running the system with a trailer-mounted compressor during emergencies.

As for the membranes, the maintenance schedule calls for cleaning every six months. First, the operators hoist out a cassette, hose it down, and remove rags and debris trapped between the vertical fibers — that takes two to 2 1/2 hours. Then they transfer the cassette to the 5,000-gallon dip tank at the rear of the building.

“Our instructions were to add 15 gallons of sodium hypochlorite to 1,000 gallons of water and aerate the solution,” says Cataldo. “In June 2013, a GE representative said the correct procedure was to maintain 1,000 parts per million of hypochlorite for 12 hours, then leave the cassette in the solution overnight.” Because there was no ventilation system, aeration of the heavy-dose solution set off chlorine alarms in the plant. The town and United Water are discussing covering the dip tank and installing a scrubber to remove the chlorine from the air.

The cassettes also need an annual citric acid cleaning after the hypochlorite cleaning. “We hypo-clean one train, put it back online, and clean the other train,” says Cataldo. “Then we pull and hose off the first cassettes before submerging them in an aerated citric acid solution [pH 2.4] in the dip tank for 24 hours.”

Grinding away

At the customer level, Cataldo and Swimm inherited maintenance of 400 grinder pumps. “They’re old and clog more readily when homeowners flush disposable wipes and other forbidden objects,” says Cataldo. As the original centrifugal pumps fail, they replace them with new semi-positive displacement E/One pumps (Environment One Corp.) maintained by distributor F.R. Mahony & Associates.

“I’m amazed at the hours the operators put in,” says Cohasset Sewer Commission administrator Diane Hindley. “They’re working on grinder pumps on weekends, holidays and nights.” Their efforts have produced positive compliments from residents and a substantial drop in service calls.

Cataldo observes, “What we do at the plant and in the field is called time, energy and money. We’re thorough in our approach and concerned about what we do. As a result, our neighbors greet us warmly. Such a response tells us we’re doing our job. I couldn’t ask for a better partner than Tim. He is a self-starter — ambitious, reliable and mechanically gifted. We work well together and both of us enjoy troubleshooting.”

More Information

Pentair - Aurora Pump - 630/859-7000 - www.aurorapump.com

Environment One Corporation - 518/579-3068 - www.eone.com

Flygt - a Xylem Brand - 855/995-4261 - www.flygtus.com

GE Water & Process Technologies - 866/439-2837 - www.gewater.com

Godwin, a Xylem brand - 800/247-8674 - www.godwinpumps.com

Goulds Water Technology - a xylem brand - 866/325-4210 - www.goulds.com

JWC Environmental - 800/331-2277 - www.jwce.com

Lakeside Equipment Corporation - 630/837-5640 - www.lakeside-equipment.com

SIHI Pumps Americas - 716/773-6450 - www.sihi-pumps.com

TrojanUV - 888/220-6118 - www.trojanuv.com