The City of Mt. Pleasant constructed a primary
treatment plant in 1957. The original plant
consisted of raw wastewater metering,
comminution and pumping; grit removal; primary
clarification; chlorination; one anaerobic
sludge digester; and sludge drying beds. The
treatment facility underwent a major upgrade in
1978 -1981 to meet new secondary treatment
standards and to provide additional capacity.
The major improvements in this upgrade included
new raw wastewater comminution, pumping and
metering; a new aerated grit chamber; additional
primary settling tanks; new secondary treatment
facilities, including rotating biological
contactors; final clarifiers, and chemical
storage and metering facilities for phosphorus
removal; a new chlorine contact tank and
effluent aeration; addition of a new secondary
anaerobic digester; modifications to the
existing primary digester; and addition of a
centrifuge for sludge dewatering. An inflow
containment basin was constructed in 1984. Two
sludge storage tanks were added to facilitate
liquid sludge disposal on a year-round basis
subsequent to 1984. In 2000 the plant added a
pump station, two BioTowers and a SCADA system
to the system. The 2000 – 2003 upgrade also
included a new in-line grinder, relocating the
chemical feed systems, and remodeling the
interior of the plant to utilize the useable
space. The plant replaced equipment that was
worn or obsolete and repaired other units to
bring the plant up to specifications for
treating wastewater.
Influent wastewater passes through the influent
diversion chamber prior to entering the
screening facilities and wet well in the
basement of the administration building. The
diversion chamber is equipped with an adjustable
weir that allows excessive storm event peak
flows to be diverted to the inflow storage
basin. The elevation of this weir dictates the
maximum flow rate to the treatment facility, and
any flow in excess is diverted to the storage
basin. A pair of return pumps within the
diversion chamber allow for the diverted flow to
be brought back into the treatment facility once
the peak flow event has subsided.
The wastewater enters the treatment plant on the
west side of the administration building where
it empties into a wet well after passing through
either an inline grinder, or a bar screen. The
bar screen acts as an automatic diversion device
to allow the continuous flow of wastewater into
the treatment plant in the event of power
failure, or inline grinder equipment failure.
The normal operation of the inline grinder is to
grind any large debris from the influent sewer
to a smaller size, which will not create
equipment malfunctions or pipeline plugging in
the later treatment processes. The bar screen
has openings of 1.5 inches to protect downstream
equipment from large debris, and to allow
adequate flow into the plant should the inline
grinder fail. The current facility has one
additional influent channel, which is closed
off, and was intended for a future inline
grinder or screening device.
After the incoming wastewater passes through the
screening facilities, the flow is diverted into
two wet wells. The raw wastewater is then pumped
and metered to the aerated grit chamber. All of
the four raw wastewater pumps are variable
speed, which allows the flow rate to the
downstream unit processes to be matched to that
of the incoming flow.
The aerated grit chamber is designed to remove
larger, heavier particles, sand, stones, etc.
that would cause excessive equipment wear in the
other plant treatment processes. The airflow to
the grit chamber is to prevent the relatively
lighter organic solids from settling out with
the grit, and to prevent septic odors in the
wastewater. The grit that settles out in this
chamber is pumped through a grit washer and
dewatering screw. Grit washing is intended to
remove any organic solids bound in with the
grit, and return them to the treatment plant. A
phosphorus removal chemical is added to the grit
chamber, oxidized iron, which enhances the
reaction and settling of phosphorus-generated
solids.
The discharge from the grit chamber enters the
inlet channel of the five primary clarifiers by
gravity. The flow is split into each of the
individual tanks through the use of influent
gates. Each primary settling tank is equipped
with a longitudinal sludge collector, which
moves the settled sludge to a sludge hopper. The
sludge hopper contains a screw type cross
collector to move the primary sludge to the
inlet of the draw off piping. Each primary
settling tank is also equipped with scum removal
pipes that drain to a common scum pit. The scum
pit is connected to the suction of the primary
sludge pumps for transferring the scum to the
digesters. Discharge from the primary clarifiers
is collected in a series of weir troughs, which
maintain a nearly constant level in the tanks
over the entire flow range. Recycle flows are
discharged to the influent of the primary
settling tanks to allow these higher strength
flow streams to receive treatment for a portion
of the solids, carbonaceous biochemical oxygen
demand (CBOD), ammonia, and phosphorus.
The chemical storage and feed area for
phosphorus removal is located in the basement of
the administration building. The system was
designed for use of either ferric chloride or
alum. The treatment plant currently uses ferrous
chloride because of cost considerations. The
ferrous chloride is oxidized in the aerated grit
chamber to ferric chloride. The resulting
phosphorus sludge is settled with the primary
sludge within the primary clarifiers.
The effluent from the primary clarifiers flows
by gravity to the secondary treatment process
pump station. From there it is pumped up to the
BioTowers, which are designed to remove the
soluble CBOD from the primary settling tank
effluent. The biotowers are about 50 feet high
including the covers. There is 22 feet of media
inside the towers and the distributor arms
evenly discharge water over the surface of the
media. The media looks like corrugated cardboard
standing on edge. This allows the bacteria a
place to attach for growth. Also the media is
placed in the biotowers in such a way as to
inhibit water from free falling to the bottom
and not being treated. A drop of water has to
travel diagonally though the BioTower to reach
the bottom. The bacteria are the same type that
grows on rocks in a river. These bacteria use
the soluble material in the water as their food
supply. The CBOD and dissolved solids are
consumed by the bacteria (carbonaceous biomass)
which grow on the media surface, and is
converted to sludge as a portion of the biomass
dies (sloughs off). This material becomes a food
source for the bacteria in the next stage of
treatment. The growth of new biomass is
beneficial to the treatment process and
sloughing is a desired phenomenon that actually
increases the effective surface area for
treatment by new growth.
The discharge from the biotowers flows by
gravity to the advanced portion of the secondary
treatment process, a series of Rotating
Biological Contactors (RBCs). Each RBC consists
of plastic media discs supported on a 20-foot
long shaft and rotated by a mechanical drive.
Bacteria grow on the plastic discs inside the
RBC. The RBCs do the same type of work as the
biotowers but in this plant they are the primary
unit for the removal of Ammonia Nitrogen. The
bacteria are the same type of bacteria that grow
in the biotowers. Different types of bacteria on
the discs are different colors. The bacteria
that utilize the CBOD portion of the water are a
gray/brown color and those that use the ammonia
portion are a red color. Approximately 40% of
the disc assembly is submerged, and each unit is
equipped with a fiberglass housing to reduce
heat loss. The Ammonia Nitrogen is consumed by
the bacteria which grow on the disc surface, and
is converted to sludge as a portion of the
biomass dies (sloughs off) and is settled out in
the final clarifiers. As with the BioTowers,
sloughing of the old biomass and the regrowth of
new is necessary for effective treatment.
Sloughing is also required to prevent an
excessive weight buildup on the RBC shaft, which
can result in mechanical failure.
Another unit process within the secondary
treatment train are the final clarifiers. The
biomass that has sloughed off of the RBCs must
be settled out and removed to meet the permit
limits for suspended solids and total CBOD. The
sludge generated in these final clarifiers is
generally less dense than that of the primary
clarifiers. The final clarifiers are designed
with a lower flow rate for this reason. The
sludge removed from the final settling tanks is
also thinner (lower percent solids) than the
primary sludge. The treatment plant currently
recycles this sludge for co-settling in the
primary tanks to obtain a higher solid
concentration of the combined sludge removed
from the system.
Disinfecting of the treated wastewater is
currently accomplished by sodium hypochlorite
(bleach). The two chlorine contact tanks are
underground surrounding the final clarifiers.
The chlorine contact tanks give the chlorine and
water time to mix and kill harmful bacteria.
After the chlorine contact tanks, any residual
chlorine is reduced by the addition of sodium
sulfite prior to discharge. The flow leaving the
plant is metered through a Parshall flume before
it cascades down a series of steps to increase
the dissolved oxygen content before it enters
the Chippewa River.
The primary and secondary sludge generated from
the clarifiers is pumped to the first of two
anaerobic digesters. The first stage digester is
designed for heating at 100 to 105 degrees
Fahrenheit and is gas mixed. The second stage
digester is also heated to 100 to 105 degrees
Fahrenheit and is mixed only by the
re-circulation of the sludge pumped through the
heat exchanger. The first stage (high rate)
digester reduces the sludge volume by converting
a portion of the solids to methane gas, carbon
dioxide, and water. The second stage digester
(moderate rate) is used primarily to store the
methane generated to allow it to be used in
heating the system and to allow further
digestion of the sludge to reduce the overall
volume. The sludge from the secondary digester
is pumped to either of two holding tanks where
it is stored until it can be land applied. The
liquid biosolid holding tanks contain a series
of valves, which allow the lighter solids on the
top of the liquid to be decanted and recycled
back to the plant. Decanting reduces the overall
volume that must be hauled to the application
sites.
