Pennsylvania plant cuts $20K annual O&M costs
The Mon Valley Sewage Authority Wastewater Treatment Plant (WWTP) serves the Borough of Donora in Washington County, Pennsylvania and the City of Monessen in Westmoreland County, Pennsylvania. Both communities are in the historic ‘Mon Valley’ steel and coal industrial area close to the Appalachian Mountains. All domestic and industrial water supplies are extracted from and returned to the Monongahela River.
“The presence of water for steel and coal extraction and processing was a key factor in establishing the area in the late 19th century,” says Thomas A. Salak, General Manager at the WWTP, “In the early days, water was being re-used without much regard for sanitation or health, but then a series of initiatives for treating industrial and domestic sewage was undertaken during the 20th century.”
A legacy of heavy industry
Now part of the so-called ‘Rust Belt’ region of the United States, Donora and Monessen have seen the demise of heavy steel, wire-making and coal industries. Still, relics of the region's proud industrial heritage are evident all around and this legacy is also evident in the sewage and drainage collection network serving the treatment plant.
“As a result of the long history of incremental development of the sewer network in the area, the sewage entering the plant is fed by combined sewers that carry surface water runoff as well as domestic sewage,” says Salak. “Because we often have heavy snowfall in the winters, roads are treated extensively with grit for traction control. After the snow melts, or especially after heavy rainfall, a lot of grit ends up in the treatment plant.”
Maintenance cost problems
Before the new system was installed, headworks grit removal consisted of grit sedimentation channels with scrapers and chains to regularly pull the piles of settled grit aside for disposal. A twin channel arrangement allowed the WWTP to divert all flow through one channel at low flow periods in the summer, to allow for the other to be cleaned out.
“The grit build up was approximately 3 cu ft per day,” says Salak. “It was removed once a year, as part of annual downtime for maintenance. I estimate it cost us approximately $20,000 in terms of maintenance every year, in labor and hiring vacuum trucks, to extract the settled grit for disposal. The chains and scrapers also needed frequent replacement as they wore out; new parts cost around $40,000.”
Downstream wear costs
In addition to this costly maintenance regime, the inefficient grit removal system allowed a large quantity of grit to pass into downstream processes, particularly during times of high flow.
Efficient sludge collection in particular was negatively affected. Grit accumulated in the settled sludge before being pumped for dewatering and disposal. Despite using twin lobe rotary pumps with anti-abrasion rubber linings, the pumps suffered extensive wear and had to be replaced every two to three years, adding to operating or maintenance costs. In other areas, equipment also had to be replaced frequently as a result of excessive wear, for example in the aeration system.
Return on investment
To reduce the excessive cost burden at a time when budgets were tight, the Authority recognized the payback it would gain from a new grit management system as part of headworks upgrade and the return on investment from savings in maintenance and operating costs.
Completed in 2013, the new headworks includes a 15 mgd Grit King® grit removal system from Hydro International. The Authority’s Consulting Engineer, Mr. Jason McBride, P.E., worked with Mr. Salak to develop a system which would fit neatly into the headworks space, incorporating a ½ inch screen to remove trash and floatables before the grit system.
“Hydro International’s engineers worked very closely with us to ensure the Grit King® was configured to our requirements, which were to remove 95% of grit above 150 micron and larger during peak flow, and wash and classify it for landfill disposal,” says Salak. “The Grit King® process operates on a regular basis throughout the year and we have retained a grit channel as a bypass for when the Grit King® is being serviced for annual maintenance. Due to service requirements now being much quicker and easier, with less need for outside contractors with vacuum trucks and labor, our annual maintenance costs are negligible. We haven’t yet quantified any reduction in other costs such as wear on other downstream facilities, as we haven’t had to replace much yet. But even based on annual maintenance, we are looking at a favorable payback period.”
Advanced grit separation
The Grit King® is an advanced hydrodynamic separator combined with a grit washing / classification unit. Flow is introduced into the Grit King® via a tangentially positioned inlet causing a rotational flow path around a dip plate. The flow spirals down the wall of the chamber as solids settle out by gravitational and rotational forces.
The grit collects in the grit pot as the center cone directs flow away from the base, up and around the center shaft into the inside of the dip plate. The upward flow rotates at a slower velocity than the outer downward flow. The resulting "shear" zone scrubs out the finer particles. The concentrated grit underflow is pumped or gravity fed to a grit classifier for dewatering from the bottom of the Grit King®.
"We are totally content with the Grit King® set up and we are 100% satisfied after two years of operation," says Salak. "It does everything it claimed, so it exceeds my expectations - normally it’s good if you are within 80% of claimed performance!"