Practical Engineering Combined with Sound Operations Optimizes Phosphorus Removal
By: Daniel Bolduc and James Fitch, P.E.
Built in the early 1970s, The Oakland, Maine, Wastewater Treatment Facility (WWTF) treats and discharges approximately 300,000 gallons per day (gpd) of wastewater to the Messalonskee Stream. The facility was designed as a conventional activated sludge secondary treatment system to be used principally for BOD and TSS removals. The secondary effluent enters the Messalonskee Stream upstream of several impoundments. This practice has resulted in a steady decline in the water quality of the stream as evidenced by increased algae blooms and other signs of euthophication in impoundments located downstream of the discharge.
In the early 1990s, the Maine Department of Environmental
Protection (MeDEP) ordered Oakland to reduce the phosphorus discharged. As a
result, the WWTF staff designed and constructed a ferric chloride addition
system and began monitoring phosphorus concentrations in the effluent. However,
the desired water quality improvements were not achieved. The MeDEP began
discussions with the town regarding replacing the facility with effluent
disposal via land application. To avoid this costly option, the town conducted
a detailed evaluation of the WWTF, focusing primarily on implementing capital
and operations improvements necessary for improving and stabilizing phosphorus
removal. The end result has been a dramatic improvement in receiving water
quality and the avoidance of costly permitting, design and construction of a
land application system.
Oakland, founded in 1873, is a small town located in central
Maine. The towns’ location along the Messalonskee Stream provided a
source of hydromechanical power for the early mills. These mills served as a
foundation of growth for the town with the development of a modest downtown
commercial district and outlying residential areas. The population is roughly
6,000 and growing as people move from Central Maine cities into the country to
take advantage of the rural beauty and low taxes.
A sewerage system initially was constructed in the 1930s
under the WPA program to collect the storm drainage and wastewater developed by
the community and the commercial district. The system discharged to the
Messalonskee Stream at a number of locations. The original system was
constructed using clay pipe of various sizes. This system has been modified and
improved many times over the years and now includes AC and PVC pipe, though the
majority of the system is still clay. Due to the water quality impact of these
discharges and the degradation of the waterway in downstream areas, a
wastewater treatment plant was constructed in 1972 using grant funds from the
federal and state government. The sewerage system was modified to send the
collected storm and sanitary sewage to the wastewater treatment facility. Many
of the existing outfalls were maintained as overflows that were active during
high groundwater and storm events.
The wastewater treatment facility constructed under PL92500
is located downstream from the populated center of the community. It included a
headworks, extended aeration activated sludge fitted with mechanical aerators
(two trains), secondary clarifiers and a chlorine contact tank. The solids
train included a small sludge storage tank and vacuum filters designed for use
of ferric chloride and lime. The disposal of the processed biosolids was by
land application on local farmland. The treated wastewater discharges into an
impoundment in the stream located between an old mill dam and a hydroelectric
facility. The treatment facility was upgraded in 1990 to include an
equalization tank located on the dedicated line serving the woolen mill to
dampen the surges from the mill and remove the excessive amount of fibers in
the mill wastewater.
The Messalonskee Stream is a small, slow moving stream that
is impounded for much of its length. It drains a series of central Maine lakes
that are controlled by dams to maintain desirable water levels during the
summer season. These lakes have served as recreational areas for Maine
residents, and several summer camps were built along the shores. Recently, many
of these camps have been converted to year-round dwellings, increasing the
waste load from septic systems to the lakes. This has led to the deterioration
of the water quality in these water bodies and in the Messalonskee Stream. Over
time, this has resulted in algae blooms in the impoundments of the stream
particularly downstream from the Oakland wastewater treatment facility
discharge.
The Problem
In the early 1990s the MeDEP began a statewide program to
reduce the impact of wastewater treatment facilities on the lakes of the state.
One of the initiatives was to remove the discharge of treated wastewater from
the lakes and “Great Ponds of the State.” The goal was to reduce
the nutrient discharges to these water bodies and reduce the likelihood of
water quality degradation and the appearance of algae blooms. At this time, the
town’s wastewater discharge came under scrutiny because the impoundment
in the stream at the discharge point made it qualify as a “Great
Pond” under Maine law. The history of algae blooms downstream of this
discharge and the continued discharge of CSO flows also were of concern to the
MeDEP.
The MeDEP believed that the best approach was to eliminate
this discharge (regardless of how well it was treated) from the stream. Two
alternatives were proposed: pump the treated wastewater more than five miles
through adjacent communities to a discharge point in the Kennebec River (a
large river draining the central portion of the state) or to eliminate the
wastewater treatment facility and pump raw wastewater to a neighboring
community for transport with wastes collected in that community to a regional
treatment authority for management and disposal.
However, the town could not afford to expend significant
capital to install the infrastructure necessary to discharge to the Kennebec,
and an extension of an outfall through neighboring communities was not
politically viable. Town officials were uncomfortable sending the waste to
neighboring communities for transport and treatment by the regional authority
because they would lose control of the cost of managing and disposing of the
wastewater.
The Plan
The MeDEP believed that the discharge of phosphorus was the
primary cause of the algae blooms and that the plant’s discharge could
continue, providing that the phosphorus discharged could be adequately
controlled. Plant operators began field trials to determine whether ferric
chloride could be used to effectively coprecipitate phosphorus along with the
activated sludge in the secondary clarifiers. The plant converted its vacuum
filters to lime and polymer and had the large storage tanks designed with the
original plant available for storage of the ferric chloride. They set up a
temporary system for adding the ferric chloride into the discharge trough from
the aeration tanks and began a full scale field trial of the coprecipitation
system.
The field trial initially showed promise, reducing
phosphorus concentrations in the effluent to between 0.5 mg/L and 1 mg/L. However, the removal reductions and
consistency were not sufficient to eliminate the formation of algae blooms
during the summer. Based on the field trial results, the MeDEP considered
requiring the town to evaluate the two alternate disposal methods. Oakland
officials turned to Portland, Maine-based Woodard & Curran to identify and
evaluate other alternatives. The firm identified two alternatives that could be
implemented in a phased manner. First, evaluate the treatment facility focusing
on improved and more consistent phosphorus removal. Second, if these
improvements did not reduce the phosphorus load on the stream to acceptable
levels, to land apply a portion of the treated effluent to forestland using a
trickle irrigation system.
The MeDEP agreed to this phased approach and began a
modeling effort designed to determine the probable level of phosphorus that
could be discharged to the stream without causing algae blooms. The town agreed
to evaluate its plant, implement recommended improvements, monitor the water
quality of the stream and search for sources of phosphorus in the service area.
The town determined that the Messalonskee Stream had a fairly
high background level of phosphorus due to the poor performance of subsurface
disposal systems around the upstream lakes. In addition, the town uncovered an
unlikely source of phosphorous. The water company serving that region was
adding phosphorus compounds to its supply system in order to control corrosion
in the distribution system. This practice led to high levels of phosphorus in
the raw wastewater received at the treatment plant.
Plant Evaluation and Improvements
While the plant evaluation identified capital improvement
needs related to age and condition, it focused primarily on what improvements
would upgrade the amount and consistency of phosphorus removal achieved. The
evaluation concentrated on better control of ferric chloride addition, better
solids removal in the final clarifiers and improved efficiencies in solids
handling.
During the field trials of ferric chloride addition, the
dosage was manually set with periodic adjustments based on flow. Recommended
improvements to the chemical addition systems included automatic, proportional
flow-paced control of the ferric chloride and sodium bicarbonate feed systems.
The ferric chloride feed system simply automated the manually controlled system
designed and installed by the plant staff. The sodium bicarbonate system
converted an abandoned septage receiving tank into a batch mix tank and
included a small blower for mixing and a flow paced chemical feed pump. The pH
in the aeration basins is monitored continuously and used to manually adjust the
automatically controlled proportionate feed rate.
An evaluation of the performance of the final clarifiers was
a priority because of the likelihood that short circuiting was occurring. This
preliminary observation was based on the vintage and design of the clarifiers,
along with the need to retain and not have the solids flushed out of the
clarifiers during high flow events. Dye testing of the final clarifiers in
summer 1993 determined that there was significant short circuiting in the
clarifiers with velocity currents across the bottom, up the side walls and over
the weir. The location of the velocity currents suggested that Crosby baffles
likely would reduce the short circuiting and improve the settling
characteristics. During the summer of 1993 one of the clarifiers was brought
off line and Crosby baffles were installed. Once this installation was
completed and the clarifier operation was stabilized, the clarifiers were dye
tested again. Retesting the clarifiers verified that the hydraulic short
circuiting had been eliminated and performance improved. These results were
sufficient to warrant improving the other clarifier and the installation was
scheduled for completion as part of the capital improvement program in 1994.
At the time of the plant evaluation in 1993, waste sludge
was “stored” in the final clarifiers from which it was directly
wasted to a vacuum coil filter for dewatering. While the practice of storing
waste sludge in final clarifiers is a typical practice from design and operations perspectives, it
was determined that this technique also was likely resulting in the periodic
inadvertent loss of solids from the clarifiers. A more positive means of
wasting sludge from the clarifiers and control of the sludge blanket depth was
desired to optimize solids retention and removal, thereby improving phosphorus
removal. Recommended improvements to the waste sludge and subsequent biosolids
handling systems included the following.
• Installation
of aerated waste sludge holding tanks where waste sludge could be wasted
automatically on a regular, timed basis. The waste sludge holding tanks were
equipped with telescoping valves to enable decanting of the waste sludge prior
to dewatering.
• New
controls and a flowmeter were provided for the waste sludge pumps to enable
automatic wasting and measurement of waste sludge removed from the final
clarifiers.
• The
vacuum coil filter was replaced with a new belt filter press (BFP)
to improve dewatering efficiency, reduce energy costs, save
chemicals, improve solids capture, reduce operating cost, etc.
• Dewatered
sludge from the BFP is discharged to a new cut flight screw conveyor where lime
is added to stabilize the sludge. Lime stabilized sludge is land applied for
disposal on farm fields in the vicinity of the plant.
Implementation of these improvements enabled the plant
operators to more positively control the amount of solids stored in the final
clarifiers, essentially eliminating periodic washout of solids from the
clarifiers. The time required to dewater sludge has been reduced from
approximately 92 hours per month with the vacuum coil filter to 37 hours per
month with the BFP. With the higher solids concentration achievable with the
BFP, the tons of dewatered sludge requiring disposal has been reduced from
approximately 48 tons per month to 36 tons per month. Lime use with the new
biosolids handling system has been reduced from approximately 4,800 lbs./month
to 4,300 lbs./month. Dewatered sludge cake from the BFP averages approximately
24 percent prior to lime addition.
End Result
Implementation of the recommended plant improvements began
in May 1994 and was completed in October 1998. Since the improvements were
made, the plant has been operating to optimize phosphorus removal during the
summer months by the addition of approximately 54 gallons of ferric chloride
per 250,000 gallons per day of influent flow. During winter months, phosphorus
removal also is completed, although the ferric chloride feed rate is one-half
of the summer’s rate. Effluent concentration data for phosphorus, TSS and
BOD is provided in Figures 1, 2 and 3, respectively. Figure 4 presents TSS
removals on a percent removal basis. Since optimizing phosphorus removal at the
plant, algae blooms in the Messalonskee Stream downstream of the plant have
been eliminated. The last reported algae bloom occurred in summer 1993 prior to
completion of the plant improvements. The renovations both met Maine DEP
requirements and saved the town thousands, if not millions, of dollars. They
also serve as a model for any small- to medium-sized town faced with similar
costly improvements.
About The Author: James Fitch, P.E., is a vice president with Woodard & Curran, Portland, Maine, with special expertise in municipal facilities. Daniel Bolduc is superintendent of the Oakland, Maine, Wastewater Treatment Facility.
