Project Information

Location: Marlborough, Massachusetts

Project Type: Municipal Wastewater Treatment - Phosphorus Removal

Completion Date: January 2012

Treatment Objectives

Design Flow:
Average: 11,000 m3/day (2.89 MGD)
Peak: 43,990 m3/day (11.62 MGD)

Effluent Total Phosphorus:
May-October: 0.7 mg/L
November-April: 1.0 mg/L


Project Background & Challenges

The city of Malborough, Massachusetts consists of approximately 40,000 people located in Middlesex County, about 32 miles from Boston. Initially settled as a town in 1660, it became a favored stop on the Post Road as populations, business and travel grew in the colonies. By 1890, Marlborough was a major shoe manufacturing center, producing boots for Union soldiers. Marlborough became so well known for this that its official seal is decorated with a factory, a shoe box, and a pair of boots when it was incorporated as a city that year.

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With more than a quarter of its territory as water, Massachusetts is one of the smallest states in the United States but also the third most densely populated, primarily around these waterways. Today, with phosphorus-fed algae blooms plaguing many North American bodies of water, and in response to environmental concerns, regulators in cities like Malborough are pressuring wastewater treatment plants (WWTP) to meet ever decreasing phosphorus limits in order to protect waterways.

These limits can vary, but are usually tied to Total Maximum Daily Loads (TMDL) established for bodies of water. In regions like Malborough where permits have addressed phosphorus pollution, discharge limits traditionally range from 1 mg/L down to 0.1 mg/L total phosphorus (TP). Historically, a limit of 0.1 mg/L has been considered an ultra-low target. But in Malborough, they faced an tough challenge from regulators with an ultra-low phosphorus TMDL limit of < 0.07 mg/L, one of the lowest requirements in North America.

Engineers across North America who are conducting facility planning are now also having to accommodate design for discharge permits ranging from 0.07 down to 0.02 mg/L total phosphorus (70 and 20 µg/L respectively). The permit issued to Malborough, Massachusetts, is currently among the most stringent requirements being met by reactive filtration to date, a TMDL of < 0.07 mg/L. There, the WWTP discharges to the Assabet River. The challenge in Marlborough became not only from the need to achieve a low phosphorus limit efficiently, but the system also needed to be easy to operate and affordable.

The Nexom Answer

The Blue PRO system was chosen for the Westerly WWTP in Marlborough after a competitive pilot project with two other technologies. In collaboration with Carlin Contracting and CDM Smith Engineering, Nexom provided an upgraded system for Malborough by adding the Blue PRO® reactive filtration system for enhanced treatment efficiency and cost minimization. The reactive filtration phosphorus compliance upgrade was sized for 4.15 MGD average daily flow, with a 11.62 MGD peak flow. With a design deliverable of 0.07 mg/L total phosphorus on a rolling 60-day average, its implementation has allowed the WWTP to meet its discharge permit cost effectively.

There are two key features that the Blue Pro reactive filtration offers, that allow it to be so successful for phosphorus removal.

First, it overcomes a critical obstacle to achieving efficient phosphorus removal from high quantities of wastewater, by providing a large reactive surface area within the media bed. This results in guaranteed contact between phosphorus and the highly absorptive HFO. The reactive filter is not simply trapping particles, but instead is actively pulling dissolved phosphorus out of the wastewater.

The second feature is that the adsorptive surface in reactive filtration is a continuously regenerated coating forming on the surface of the sand media. The end result of these two features is an efficient, phosphorus-scrubbing technology that is not limited by surface site exhaustion, unlike more conventional adsorbants. The project was contracted in early 2010, with construction started in December 2011 and commissioned before year end in preparation of the performance test.

Site Construction

Upgraded System Performance

An extensive 30-day system performance test was completed in February 2012. During this test period, with the plant operating within the Nexom design, residual total phosphorus in 24-hour composite samples averaged 0.044 mg/L. Single samples measured as low as 0.026 mg/L. During that time, current TMDL requirements for phosphorus were seasonal with a 1 mg/L winter limit and a 0.1 mg/L summer permit. However, the change in limits meant the Malborough facility needed to meet a mass-based permit of 0.07 mg/L total phosphorus, with the possibility of needing to achieve even lower concentrations in the future.

Ortho phosphorus was never measured above 0.007 mg/L for the entire performance test. Since commission, the only reason the effluent has not flat-lined is that the site sets higher limits in the winter to allow for lower dosing levels.

The Marlborough Westerly plant operates without phosphorus feedback to an online controller, which could allow cost trimming further and would allow the plant to hit even lower targets if needed. Despite only having manual setpoint dose control as process control, the effectiveness of reactive filtration has allowed site operators to maintain the 60-day rolling average target since the reactive filter commissioning.

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