Wastewater Treatment Plant Halves Screening Costs

Alec Mackie with additional reporting by Clifford Seth

As wastewater pours into the Cobourg Water Pollution Control Facility (WPCF) located on the outskirts of Cobourg, Ontario, it comes loaded with organics, trash, debris and chemicals. A large perforated plate screen system, called the Monster Separation System , is the plant’s first line of defense and removes all unwanted solids to protect downstream processes.

Managers at the facility report significant operational, maintenance and disposal benefits after replacing the old 1-in. bar screen with the new, multifaceted separation system. Funding for a similar installation at Cobourg’s second treatment plant is being sought.

The new system consists of a Finescreen Monster , which features a continuous band of specially shaped, perforated stainless steel panels. The finescreen directs screened solids into an integrated Screenings Washer Monster , a self-contained, hopper-fed system that grinds, washes, compacts and dewaters discharged screens. Both units were designed and built by JWC Environmental of Costa Mesa, Calif.

“It’s unbelievable the difference it has made in cleaning up our facility,” said Bill Peeples, manager of the two facilities. “We had been burdened for years by plastic, rubber and other inorganic and organic solids collecting in various structures all the way through our process. Since the installation of the new system in February 2005, those problems have been eliminated, and it has proved to be very low maintenance.”

Originally built in 1945 as a trickling filter type, Cobourg’s WPCF was upgraded in 1969 to a conventional activated sludge process. The Ontario Ministry of Environment (MOE) now designates it as a Class 4 facility.

The facility, which averages 2 million gal per day (mgd) and can handle a peak flow of 3.5 mgd, serves about 60% of the town’s 18,000 customers, with about 75% of that demand derived from food processing operations, about 20% from residential and the rest from business locations.

The new separation system has saved the facility time and resources in a number of ways:

1. Removal of trash, plastics and latex before they can damage the facility or reach the natural environment;
2. Trash, rags and debris collected by the screen do not end up in the anaerobic digesters;
3. 50% reduction in spending on trash disposal;
4. Improved grit tank operation and prolonged pump life;
5. Fewer dumpster trips to the landfill; and
6. Extended dumpster life since less water pools in the bottom.

Removed solids typically contain up to 50% dry solids, are 80% compacted and are significantly lighter than typical screened solids.

In the Finescreen Monster part of the system, wastewater in the channel flows to a screening zone—a continuous band of perforated panels with 6-mm openings. During the operation cycle, a drive moves the panels from the screening zone to a cleaning mechanism at the discharge point. Debris is removed from the panels by a 2-stage brush/wash water system, and screenings are then conveyed for further processing in the second part of the system.

“There were other screening options with the same 6-mm size, but these were bar screens. Bar screens, regardless of how narrow they are, cannot properly handle slender objects. For example, a Q-Tip can still pass through length-wise, without much difficulty,” Peeples said.

In the Screenings Washer Monster part of the system, solids are removed from the finescreen, flushed through a grinder for size reduction, washed and conveyed by an auger. Soft organics and the wash water are passed through the auger’s perforated trough and are returned to the plant’s waste stream. The captured solids are compacted, dewatered and discharged as a cake into a receptacle for disposal.

“With a much finer screen now in place, by washing through organics instead of removal by raking and with a new compacting operation that squeezes the water out, we’re saving about 50% on costs for hauling the waste to a landfill,” Peeples said. “We’re now emptying our 2-cu-yard dumpster once a month, instead of once a week.”

The MSS installation eliminated several trouble spots and burdens for Peeples and his crew. The grit chamber is one area experiencing a tremendous improvement in operation, removing more sand from the wastewater and producing bone-dry discharge. Plant operators used to empty two wheelbarrows everyday; now, they empty them once a week. In addition, Peeples estimates a 20% to 30% increase in pump service life because of the reduction of abrasive material in the flow.

In planning for the installation, Peeples increased his trash disposal budget, expecting to pay more as the fine screen captured and removed more solids than the old bar screens. Instead, the opposite occurred: The plant discharges only 25% of the volume it used to. “We’re capturing more but sending less to the landfill. It was all that water we were paying for,” he said.

Since the washer monster efficiently breaks up large clumps to wash and dewater them, the screenings discharge is drier and more compact. This has saved the facility money by reducing standing water inside the dumpster, helping to prevent the bottom from rusting out. In addition, the trash truck driver would occasionally get drenched in a shower of nasty water as the dumpster was tipped up and over the cab to empty.

Additionally, the washer monster has eliminated odors inside the headworks building, Peeples said.

“The smell in the headworks would become overpowering at times, especially during the warm summer months,” he said. “We had some health concerns and were looking at an air filtration system or exhaust fan. Once the new screening system was installed, our odor problems disappeared, saving us the $10,000 it would have cost for the air system.”

The benefits of the screening system even extend to the plant’s discharge point. Prior to installation of the fine screen, trash was collecting on top of the chlorine contact tanks, requiring an operator to skim off rags, trash and latex each week. It was an embarrassing problem to have when tour groups and city officials passed through, and an inefficient use of a trained, licensed operator who was acting as a pool skimmer. The finescreen removes all that debris.

Removing trash from the wastewater also keeps it from getting into the biosolids, which, after digestion, Cobourg offers to local farmers to help fertilize their fields.

“Spreading biosolids on land has become a very political issue lately. If someone sees plastic and latex lying on a field being used to grow crops, even though they are biologically inert, it is not viewed favorably,” Peeples said. “If we can install a piece of equipment that will remove these items, it can only serve to help our efforts in making the product more widely accepted.”