Practical Engineering Combined with Sound Operations Optimizes Phosphorus Removal

March 28, 2002

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.


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.

Image courtesy Institute of Chemical Research of Catalonia (ICIQ).
All images courtesy of Ecosorb.