May 2009, Vol. 21, No.5

Plant Profile

The City of Westminster Wastewater Treatment Plant


Location: Westminster, Md.
Startup date: 1972
Service population: 35,500
Number of employees: 13
Design flow: 5 mgd (18,900 m³/d)
Average daily flow: 4.3 mgd (16,300 m³/d)
Peak flow: 24 mgd (90,800 m³/d)
Annual operating cost: $4.2 million

The City of Westminster (Md.) Wastewater Treatment Plant takes a great deal of pride in being able to push the limits of its design technology and achieve superior effluent quality. Operating in an A²/O™ mode — for single-sludge nitrogen and phosphorus removal — on an almost continuous basis, the plant produces an effluent with total nitrogen effluent concentrations averaging 5 mg/L on an annual basis and never exceeding 6.5 mg/L. That’s not bad for a plant designed for a seasonal permit limit of 8 mg/L. On the phosphorus side of the equation, total phosphorus averages 0.6 mg/L on an annual basis. On top of that, biochemical oxygen demand and total suspended solids concentrations average less than 5 mg/L, while the plant is designed to meet 30 mg/L limits for each parameter.

A History of Growth
The treatment plant was built along Little Pipe Creek in 1972. An older plant had been operating about 3 mi [5 km] upstream since 1935. In the 1980s, a dechlorination facility was added.


A major expansion and upgrade followed in 1991 that increased the capacity from 3 mgd (11,400 m³/d) to the current design capacity of 5 mgd (18,900 m³/d). The upgraded design included an activated sludge process, as well as chemical phosphorus removal. Specific improvements included solids dewatering, flow equalization, additional digesters, improved clarifiers and disinfection tanks, construction of a solids-handling equipment building (including a garage and storage space), expansion of the existing laboratory and grease and grit facilities, and other operational modifications. Also included in the 1991 expansion was construction of a separate septage receiving facility, which incorporates an activated sludge pretreatment of the septage. The septage facility receives both septage from Carroll County residents and businesses and leachate from the local landfill.

In 2000, another upgrade added biological nutrient removal with the ability to operate in either a modified Ludzack–Ettinger (MLE) or A²/O™ mode. In 2005, the plant switched away from gaseous chlorine disinfection when a new liquid chlorination and dechlorination facility was built.

Today’s Plant
Today, influent enters the treatment plant through the headworks, which consist of a 15-mm mechanical bar screen, screenings wash and compaction, an aerated grit-removal chamber using a traveling bridge with a vortex pump and grit classifier, and influent flow measurement and composite sampling.

From the headworks, the flow enters two splitter boxes, with each performing a separate task. If flows exceed 7 mgd (26,500 m³/d), the first box automatically puts the plant into contact stabilization mode by diverting a portion of the flow beyond the biological nutrient removal process. The second box combines the influent flow with return activated sludge from the secondary clarifiers and distributes the flow among the plant’s four aeration trains.


Each aeration train is designed with four distinct zones that can be changed according to plant operating conditions. In the MLE mode, the first two zones become anoxic for biological nitrogen removal, the third can be operated as either anoxic or aerobic, and the fourth is the aeration zone. In the A²/O™ mode, the first zone become the anaerobic zone, the second and third zones become anoxic, and the fourth zone remains aerobic. Nitrate recycles can be fed  from the effluent end of the aerobic zone to either the first or second zone, depending on mode of operation.

Upon exiting the aeration trains, the flow recombines and receives a dose of aluminum sulfate to precipitate out phosphorus, supplementing biological removal. Then, another splitter box distributes the water among the plant’s three secondary clarifiers. Each clarifier is 80 ft (24 m) in diameter, has an 11.5-ft (3.5-m) sidewall, and is equipped with a full-radius skimmer.

Return activated sludge from the clarifiers is sent back to the second influent splitter box to be mixed with new influent and redistributed among the four aeration trains. A scum station returns all skimmed material directly back to the headworks.

Clarifier effluent is combined and sent to the disinfection contact tank. In the tank, sodium hypochlorite disinfects the wastewater, and sodium bisulfite neutralizes excess chlorine. Blowers provide mixing, as well as ensure that effluent dissolved-oxygen parameters are met. At this stage, the effluent flow measurement and composite sampling occurs. Final effluent is discharged directly into Little Pipe Creek.

Solids handling at the City of Westminster Wastewater Treatment Plant, by design, consists of pumping the waste activated sludge into aerated digesters, decanting to reach a solids concentration between 1% and 1.5%, aerobic digestion to reduce pathogens, and belt filter pressing to reach 16% solids before landfilling or applying to agricultural land.

However, all waste activated sludge currently is pumped directly to the belt filter presses, skipping the aerobic digesters. Due to the high mean cell residence time the plant normally maintains, the plant is able to satisfy the 40 CFR 503 biosolids regulations for pathogen reduction with a log mean average below 2 million for bacteria, without additional digestion.

This cost-saving feature eliminates the need for solids storage facilities, eliminates the electrical cost and maintenance associated with digester blowers, and eliminates the need for the plant-dewatering-building odor-control system and its associated chemical costs. The result is a combined savings of more than 17% of the annual solids disposal budget.

Tomorrow’s Expansion
The Westminster plant is still growing and evolving. An upgrade to enhanced nutrient removal is in the later stages of planning and was scheduled to enter the design phase in early 2009. The upgrade is mandated by the State of Maryland and will be fully funded by the Chesapeake Bay Restoration Act — better known as the Flush Tax. It will provide Westminster residents with state-of-the-art wastewater treatment. Enhanced nutrient removal will be designed to achieve total nitrogen and total phosphorus limits of 3 mg/L and 0.3 mg/L, respectively.

©2009 Water Environment Federation. All rights reserved.