By Marcia Sherony
The city of Peru, Ind., is a typical, small Midwestern city, where
manufacturing and industry drive the local economy. At the end of the
20th century, this industrial city 75 miles north of Indianapolis
experienced a period of economic growth. However, this welcome growth
did not come without consequences. The addition of a bacon processing
plant and large recreational vehicle manufacturer brought jobs to the
area, but it also strained the resources of the city's existing
wastewater treatment plant.
The plant struggled to keep up with the increased flows. During heavy
rains, the level of suspended solids in the plant effluent exceeded
state safety limits. To solve this and other water treatment problems,
the city opted to revamp the entire treatment plant, adding significant
capacity and improving its processes.
Another serious problem facing the city of Peru was grit. There was an
excessive amount of grit being transported to the plant from the aging
collection system, especially during rainstorms. The facility, state of
the art in the 1930s, had a Dorr Oliver grit rake and Cyclone degritter
to remove grit. However, despite numerous rebuilds, the original
equipment was no longer up to the task. The biosolids, which were land
applied, were loaded with grit. At one point, a 3- to 4-foot buildup of
grit was cleaned from the primary clarifier.
An effective grit-removal system was an important criterion to the new
design. City officials were strongly considering a solids-reduction
process to eliminate the need for land application of the biosolids.
Advanced processes such as solids reduction must be protected from even
fine grit. Another important consideration was the ability of the
system to perform effectively at all flow levels, especially
considering the wide swings in flows, with an average flow of only 3.5
million gallons per day (mgd) and a peak design flow of 26 mgd.
The city needed to find a highly efficient grit-removal system to
alleviate these concerns. Failure to effectively remove grit and other
nuisance abrasives results in downstream plugging, loss of process
space, and excessive wear on mechanical equipment.
The engineering firm Advanced Technology Services of Westchester, Ohio,
proposed installation of a Headcell grit-removal system, a technology
capable of removing grit particles as small as 100 microns. "This was
very attractive to us because grit had been such a problem in the
plant," said Mike Dalquist, superintendent of Water and Sewer
Management for Peru Utilities. "There wasn't another system out there
that offered that kind of efficiency in the same small footprint."
|Settled solids are continuously pumped to a SlurryCup grit washer and classifier and Grit Snail dewatering escalator.|
The Headcell, supplied by Hydro International (at the time, operating
as Eutek Systems, an Oregon-based maker of grit-removal, dewatering,
and transport systems), is a modular, multi-tray settleable solids
concentrator that removes grit with minimal headloss. The Headcell Its
stacked tray design provides a large surface area and short settling
distances capture fine grit particles. The Headcell evenly splits flows
and eliminates the short circuiting that often degrades the performance
of conventional grit basins.
The small footprint of the Headcell made it easy to incorporate into
the new wastewater treatment plant headworks. The high-efficiency flow
distribution header evenly distributes influent over multiple conical
trays. Tangential feed establishes a vortex flow pattern where solids
settle into a boundary layer on each tray, and are swept down to the
center underflow collection chamber. These settled solids are
continuously pumped to a SlurryCup grit washer and classifier and Grit
Snail dewatering escalator.
The SlurryCup uses a combination of an open free-vortex-type flow
regime and boundary layer effects to capture, classify, and remove fine
grit and other high-density fixed solids from grit slurries, and both
primary and secondary sludge.
The grit underflow from the SlurryCup passes through a hydraulic valve,
which provides secondary grit washing before being discharged into the
large clarifier pool of the Grit Snail which is designed to retain fine
particles. A cleated belt moving 1 to 2 feet per minute gently lifts
captured solids out of the clarifier, allowing the grit to be dewatered
without re-suspending it and reintroducing grit to the plant. Dewatered
grit is carried to the top of the Grit Snail, where it is discharged
into a disposal container for landfilling. The cleaned grit typically
contains 60 percent total solids with less than 20 percent volatile or
organic solids. This approach minimizes the volume and weight of
material hauled to solid waste disposal sites, and reduces odors and
"The volume of grit being removed has increased substantially,"
Dalquist said. "The original plant typically was able to remove just
half a cubic yard of grit, and allowed particles the size of coffee
grounds to get past the headworks building. With the new grit-removal
technologies in place, the upgraded plant is removing 1.5 to 2 cubic
yards of grit during average flows, and turning this into clean, dry,
odorless material to be landfilled. We conducted inspections in the lab
and found the grit-removal system performing exactly as advertised.
cleated belt moving 1 to 2 feet per minute gently lifts captured solids
out of the clarifier, allowing the grit to be dewatered without
re-suspending it and reintroducing grit|
"Initially we had planned on installing a third component to remove
inert material, but the new biosolids-reduction process and Headcell
systems are working so well we decided against the further technology
installation," Dalquist said. "This alone saved us about $1 million."
By February 2006, the new plant was online. At that time, Peru was able
to retire its original treatment facility, which could only handle 4 to
6 mgd. The city is now able to treat 8 mgd, up to a peak of 26 mgd. The
increased capacity ensures that the system runs optimally, even during
heavy rains. The quality of the effluent that flows into the Wabash
River has improved significantly—now measuring less than 10 mg per
liter in total suspended solids, down from an average in "the high
teens" and periodic spikes over 45, Dalquist said. Before, plant
sensors "almost always" detected ammonia in the effluent; now they
hardly ever do, he said.
Moreover, the wastewater plant has a sustainable means to remove grit
and keep the treatment facility running smoothly long into the future.
It was so successful that this model treatment plant has now been
emulated by other facilities across the country.
Marcia Sherony is a national solutions manager for Hydro
International's wastewater division.. She has worked with market leaders
in liquid/solid separation for more than 20 years.