WWTP Cuts Screening Costs in Half with Solids Separation System
As wastewater pours into the Cobourg, Ontario, Water Pollution Control Facility, it comes loaded with organics, trash, debris and chemicals. A large perforated plate screen system, called a Monster Separation System, is the plant’s first line of defense, and it removes all unwanted solids in order to protect downstream processes.
Managers at the facility report significant operational, maintenance and disposal benefits through replacement of the old 1-ft bar screen with the new, multi-faceted Monster Separation System. Funding for a similar installation at Cobourg’s second treatment plant is being sought.
The new system consists of a Finescreen Monster that features a continuous band of specially shaped, perforated stainless steel panels, which directs screened solids into an integrated Screenings Washer Monster (SWM), a self-contained, hopper-fed system that grinds, washes, compacts and dewaters discharged screenings. Both units, and their integrated operation, were designed and built by JWC Environmental.
“It’s unbelievable the difference it has made in cleaning up our facility,” said Bill Peeples, manager of the Water Pollution Control Facility. “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 proven to be very low maintenance.”
Originally built in 1945 as a trickling filter type, Cobourg’s Water Pollution Control Facility was upgraded in 1969 to a conventional activated sludge process, and is now designated as a Class 4 Facility by the Ontario Ministry of Environment.
The facility, which averages 2 mgd and can handle a peak flow of 3.5 mgd, serves about 60% of the town’s 18,000 residents, with about 75% of that demand derived from food processing operations, about 20% from residences and the balance from business locations.
The new Monster Separation System has saved the facility time and resources in a number of ways:
- Removal of trash, plastics and latex before they damage the facility or reach the natural environment;
- A 50% reduction in spending on trash disposal;
- Improved grit tank operation and prolonged pump life;
- Fewer dumpster trips to the landfill; and
- Extended dumpster life since less water pools in the bottom.
The system is designed to provide screenings of unparalleled dryness and cleanliness, ready for disposal. Removed solids typically contain up to 50% dry solids, are 80% compacted and are significantly lighter than typical screened solids.
A fine screen
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 two-stage brush/wash water system, and screenings are 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,” Peeples said. “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.”
In the SWM part of the system, solids are removed from the fine screen, flushed through a grinder for size reduction, and washed and conveyed by an auger. Soft organics and the wash water are passed through the auger’s perforated trough and 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 noted. “We’re now emptying our 2-cu.-yard dumpster once a month, instead of once a week.”
The Monster Separation System 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 every day, but now have not emptied them for three months. In addition, Peeples estimates a 20 to 30% increase in pump service life due to 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.
Because the SWM efficiently breaks up large clumps to wash and dewater them, 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.
Peeples has discovered another benefit of the SWM—it eliminates odors inside the headworks building.
“The smell in the headworks would become overpowering at times, especially during the warm summer months,” he recalled. “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 JWC screening system even extend all the way 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 and every week. According to Peeples, it was an embarrassing problem to have when tour groups and city officials passed through, and a licensed operator was acting as a pool skimmer. All of that debris is now removed by the Finescreen Monster at the headworks.
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.”
In just over a year, Peeples and his crew have seen labor and resource savings in dozens of different treatment plant processes, and the only change was a new screening system at the front. The Monster Separation System provides better screening and more efficient washing and compaction. It is a new system that will be very beneficial to the operation of many wastewater treatment plants.