In 2004, operators at the Ford Road Wastewater Treatment Plant (Xenia, Ohio) converted their conventional activated sludge system to an enhanced biological phosphorus removal (EBPR) system solely through the ingenuity, knowledge, efforts, and utilization of in-house talents for design, installation, and startup. The plant made the change to comply with a 1-mg/L summer phosphorus discharge limitation that began in 2006. The limit stemmed from the implementation of a phosphorus total maximum daily load for the plant’s receiving stream, the Upper Little Miami River. The total maximum daily load has the potential for a further mass-based reduction down to 0.45 mg/L in 2013.
|What makes completing the EBPR project in-house even more impressive is that the City of Xenia Wastewater Treatment Division operates two wastewater treatment facilities — the Ford Road and the Glady Run wastewater treatment plants — with only 10 employees. To operate with so few people, the plants use a comprehensive monitoring system that consists of several ethernet I/O units located at each facility that communicate with a central server through the Xenia fiber-optic ethernet loop. The server system operates with the supervisory control and data acquisition software to access, display, graph, track, and report all configured process parameters.
Xenia’s two facilities are staffed only one shift throughout the normal work week. Therefore, the monitoring system is used to analyze and report any significant failures or problems to plant staff. Tags, such as pump flows, sump levels, and pressures, are monitored within the system to maintain mechanical and regulatory stability. Critical parameters are logged, alarmed, and periodically sent to wireless devices for monitoring by the wireless device recipients.
Operators at the Ford Road Wastewater Treatment Plant (Xenia, Ohio) converted their conventional activated sludge system to an enhanced biological phosphorus removal system. (Photo: Jason Tincu)
The division functions as one single unit with staff trained to operate and maintain either facility. Individual staff members are urged and expected to make educated decisions on their own while being held accountable for their results.
In fact, demonstrated staff expertise in operating a biological phosphorus removal process at the Glady Run plant helped the division decide to pursue EBPR instead of chemical precipitation as the phosphorus removal option for Ford Road. Increased solids generation associated with chemical precipitation further tipped the scales toward EBPR.
The EBPR Process
At the Ford Road plant, influent passes through coarse bar screens (0.25-in. [6.5-mm] openings), fine step screens (0.24-in. [6-mm] openings), and a grit-removal tank before entering the five-tank EBPR process. Plant staff reconfigured the original conventional activated sludge tanks to provide EBPR by converting the middle tank (Tank 3) to an anaerobic zone, as shown in the figure below.
Enhanced Biological Phosphorus Removal Process
The screened, degritted wastewater combines with return activated sludge in the influent box upstream of a 4-ft-wide (1.2-m-wide) influent channel. Influent wastewater and return activated sludge are directed to the anaerobic basin (Tank 3). Mixed liquor from Tank 3 can be diverted to two parallel aerobic zones, one flowing through tanks 2 and then 1, and the other flowing through tanks 4 and then 5. Slide gates control flow into each of the 90-ft-long (27.4-m-long) × 21-ft-wide (6.4-m-wide) × 15-ft-deep (4.6-m-deep) concrete tanks. Each tank has a volume of 212,000 gal (802,000 L), and total bioreactor volume is about 1 million gal (3.8 million L).
In the EBPR process, Tank 3 operates as a biological selector for phosphorus-storing organisms. No free oxygen and minimal-to-no nitrates are to be present in this zone. Free oxygen and nitrates would force this zone aerobic or anoxic, inhibiting the release of soluble phosphorus. If anaerobic conditions are met, flow leaving Tank 3 will contain more soluble phosphorus than the flow entering. This signifies the initial phosphorus release required to promote biological phosphorus removal.
Liquid waste activated sludge at the Ford Road Wastewater Treatment Plant (Xenia, Ohio) is stored in one of two aerated sludge holding tanks prior to dewatering. (Photo: Jason Tincu)
To divert flow and create an anaerobic zone in Tank 3, Xenia’s operators were forced to drop operating elevations in the system. They accomplished this by installing two knife gates in the influent channel and lowering the weir elevations in tanks 1 and 5 by 1.5 ft (0.5 m).
There are two submersible mixers in the anaerobic tank — one located 30 ft (9.1 m) inside the tank and facing the influent opening, and one located 60 ft (18.3 m) inside the tank and facing the effluent openings.
Tank 3 also contains a 24-in. (610-mm) gate valve on each side. These gate valves allow flow into the adjacent tanks.
As the biomass enters either Tank 2 or Tank 4, it is introduced into an oxygen-rich environment that promotes the aerobic breakdown of biochemical oxygen demand and begins nitrification. Individual air-header valves allow operators the flexibility to adjust airflow rates for each tank. Tanks 1 and 5 are also oxygen-rich environments that allow for the completion of the nitrification process and for the luxury uptake of phosphorus.
The aeration system, consisting of centrifugal blowers (one multistage and two single-stage) and fine-bubble membrane diffusers, provides air for mixing and oxygen to support aerobic reactions in the two aeration passes. Blower output is controlled automatically by a programmable logic controller that adjusts blower inlet and outlet valves based on dissolved oxygen at the tail end of the aerobic tanks as measured by an online dissolved-oxygen meter.
In the aerobic environment, bacteria produce energy by oxidizing stored biochemical oxygen demand and uptaking phosphorus. As the population of phosphorus-storing bacteria increases, so does the system’s capability to remove increased phosphorus loadings. Removal of phosphorus from the system occurs only via sludge wasting. Plant staff can utilize one or both aerobic zones, depending on the total aerobic volume needed to accomplish treatment objectives, providing operational flexibility to accommodate short-term or seasonal variations in hydraulic and organic loading conditions.
Following the EBPR process, solids are allowed to settle in one of two secondary clarifiers. Clarifier effluent is disinfected using ultraviolet (UV) light and aerated to increase dissolved oxygen to 7.0 mg/L. The UV-disinfection system consists of four banks of 40 vertical low-pressure–high-output lamps in series — a total of 160 lamps. A 2.6-hp (1.9-kW), 110-ft³/min (3.1-m³/min) high-pressure regenerative blower pumps air into treated effluent through six 9-in. (229-mm) fine-bubble diffusers located downstream of the UV-disinfection system. Finally, the disinfected and aerated effluent is discharged to the Little Miami River through a 30-in. (750-mm) outfall.
Liquid waste activated sludge (WAS), which is generated in the secondary clarifiers, is held in one of two aerated sludge holding tanks prior to dewatering. Each tank is 60 ft (18.3 m) in diameter with a 13.5-ft (4.1-m) side-wall depth and has a capacity of 280,000 gal (1 million L). The tanks also receive liquid WAS trucked from the Glady Run plant. The tanks’ aeration system consists of a dedicated 650-ft³/min (18.4-m³/min) positive-displacement blower and 300 9-in.-diameter (229-mm-diameter) membrane diffusers.
WAS pumps move the aerated WAS to a 6.6-ft (2-m), eight-roller belt filter press. The press feed rate varies between 100 and 185 gal/min (380 and 700 L/min), depending on sludge conditions. Dewatered cake with a total solids concentration between 17% and 23% is discharged onto a belt conveyor and transported to alkaline stabilization, where concentrations rise to between 20% and 24% due to alkaline addition.
Dewatered solids are mixed with dry lime to increase pH and inactivate pathogenic microorganisms. The product is classified in Ohio as Class B material and is used by local farmers as a soil stabilizer and growth supplement for feed crops.
Since completion, the EBPR project has had no phosphorus violations and has created a sense of ownership, confidence, and unity throughout the Xenia Wastewater Treatment Division.