In the late 1990s, one western Pennsylvania sewage treatment
plant (STP) sought to process Class B biosolids on-site, eliminating the
expense of hauling liquid off-site and significantly decreasing associated labor
costs. At the same time, the plant needed to achieve higher cake solids and
provide strict odor control. In addition, the selected equipment also had to be
able to greatly enhance process efficiencies at not just the one plant, but its
sister facility as well. Fortunately,
USFilter offered a fairly simple solution to meet the STP's seemingly complex
The borough of West Mifflin, Penn., is a community located
about nine miles southeast of downtown Pittsburgh. For years, the West Mifflin
Sanitary Sewer Municipal Authority (WMSSMA) hauled liquid from its New England
STP (a 1.2 mgd plant) to the Thompson Run facility (a 4.5 mgd plant), where it
was processed on a belt press for dewatering. However, the hauling and
associated labor proved very costly, and it was not always easy to coordinate
dewatering schedules for both plants. Additionally in the late 1990s, the
WMSSMA faced a requirement to achieve higher cake solids and provide odor
control that would respect nearby homes and businesses. Moreover, space
constraints and staffing reductions necessitated a simple-to-operate
Liquid Clarifying and Solids Dewatering: A Continuous Process
A team which included members of the WMSSMA and West
Mifflin's engineering firm, Chester Engineers of Corapolis, Penn., decided a
liquid clarifying and solids dewatering system would address all of the
borough's biosolids needs.
The team decided to use three USFilter technologies in
combination at West Mifflin's New England STP: a Stranco Products Polyblend®
M Series polymer feed system, a USFilter J-Spin™ centrifuge, and a screw
conveyor from Asdor Products. This would mark the first J-Spin centrifuge
installation in North America.
technological configuration involves first feeding polymer inline to the J-Spin
solid bowl decanter centrifuge via the polymer feed system. The feed slurry
passes into the feed distribution chamber of the centrifuge, where it quickly
accelerates to the speed of the bowl. G-forces drive the solids to the bowl
wall, while the clarified liquid flows to the adjustable overflow weir.
The internal conveyor turns at a slightly higher speed than
the bowl, carrying the solids to the end of the conical section where they are
discharged from the centrifuge. The slower the conveyor moves in relation to
the bowl and the longer the sediment remains in the centrifuge, the greater the
liquid/solid separation efficiency. The differential speed between the bowl and
conveyor is controlled automatically, providing the maximum liquid/solids separation
efficiency and highest cake solids.
At the New England STP, the centrifuge discharges into a
22-in. diameter Asdor screw conveyor, which carries the solids to a roll-off
bin that is hauled to local landfills once a week. Currently the liquid, or
centrate, is passed back through the system for additional processing, although
eventually the STP would like to release it into a nearby tributary.
Less Odor, Less Maintenance, Less Expense
Fully operational since July of 2001, the enclosed J-Spin
design has helped to reduce odor emissions, attests the WMSSMA's general
manager, Ken Frick. Moreover, it is easy to clean and does not allow hydrogen
sulfide to waft throughout the building. And with its small footprint and few
external moving parts, the J-Spin centrifuge takes up minimal floor space and
is safe to operate.
The 50-gallon-per-minute (gpm) centrifuge enables the New
England STP to achieve a cake solid of 22 percent to 24 percent, which is
greater than what its Thompson Run counterpart is producing with its belt
filter press. Now much easier to handle, this denser cake also saves the
facility approximately $5,000 per month, as it is hauled from the facility
directly to the landfill.
"We pay by the ton when hauling biosolids,"
explains Wyatt Sainato, assistant operations manager of the WMSSMA. "The
biosolids that are hauled from the New England STP to the landfill are higher
in solids, so we can pack more into a truck. As a result, we have really
reduced our hauling cost."
The unit itself
is saving the plant additional money, with a sole operator spending only an
average of 15 minutes each on startup and shutdown. In addition, the centrifuge
is low maintenance, requiring minimal hands-on upkeep.
"The integrated system from USFilter has allowed both
facilities to streamline their biosolids processing," reflects Frick.
"We've been able to process more sludge in less time and for lower hauling
costs. And these are just some of the short-term benefits. The long-term ones
will come in time."
For further information, phone USFilter Dewatering Systems
Robert W. Mau and Dean Clemons