Responsible Investment in Advanced Treatment at One of New England’s Largest Wastewater Treatment Facilities

Along the headwaters of the Blackstone River in Millbury, Mass­a­chu­setts sits one of New England’s largest wastewater treatment plants. It’s the home of Upper Blackstone Clean Water (UBCW), which serves about 250,000 people in the greater Worcester area. Managing wastewater flows for seven member communities and ten additional communities, the plant provides a high level of advanced biological wastewater treatment. Due to the strict regulations implemented by the National Pollutant Discharge Elimination System (NPDES) permit, the facility was required to meet limits on both nitrogen and phosphorous in its effluent that stretch the limits of available technology. Working collab­o­ra­tively, Upper Blackstone and CDM Smith approached these upgrades in a fiscally responsible manner that maximizes the use of existing assets and links investment at the plant with water quality benefits in the Blackstone River.  

To rise to this challenge, UBCW and CDM Smith partnered to achieve a seasonal total phosphorus limit of 0.45 mg/L and a seasonal total nitrogen limit of 6 mg/L through biological nutrient removal (BNR) methods. BNR has enabled the facility to achieve much lower nutrient effluent standards than the plant was designed for initially. Plant operations, control, and performance operations were streamlined to promote stable activity with upgraded effluent quality. The team developed a three-phase, multi-year capital improvement plan to make the recommended facility improve­ments while remaining in the plant’s existing footprint. Completing these upgrades with a phased approach allowed it to manage the cost to ratepayers while improving water quality in the Blackstone River. 

The facility’s existing activated sludge system was converted to an A2O system for nitrogen and phosphorous BNR to treat peak flows up to 120-mgd using fine-bubble diffused aeration with new 800-hp blowers. A new aeration basin and two new secondary clarifiers were installed with internal recycle pumping along with the flexibility for system operation in A/O and MLE process modes. The team also replaced all RAS, RAS, grit, and drain pumps with provisions to allow for hydraulically wasting sludge directly from aeration tanks. 

 To evaluate potential benefits of improvement work and assess the effluent’s impact, CDM Smith and the University of Mass­a­chu­setts teamed up to create a long-term water quality monitoring program for the Blackstone River. Since 2006, water quality samples have been gathered from the Blackstone River to develop a database of river water quality and demonstrate the positive impacts of plant upgrades on the river water quality. This data has been shared with regulators and the general public to both demonstrate the benefits of past investments and guide decisions regarding the need for future investments in additional nutrient removal at the plant. 

Addi­tion­ally, in response to ongoing discussions with regulators regarding future nutrient discharge limits, CDM Smith planned and implemented a pilot-testing program that compared the performance of three phosphorus removal tech­nolo­gies under actual field conditions. Based on the results of the pilot program, Upper Blackstone and CDM Smith proceeded with the design of a new 120 MGD tertiary phosphorous removal facility which includes an ACTIFLO system with ferric chloride chemical addition, an inter­me­di­ate pump station to route secondary effluent to the ACTIFLO process, and a new laboratory/admin­is­tra­tion building. The design of these upgrades is complete, with the construc­tion schedule for this $150 million project to be determined based on the results of ongoing nego­ti­a­tions with regulators. This upgrade will allow the facility to meet a proposed seasonal total phosphorus limit of 0.1 mg/L.

Aside from nutrient removal improve­ments, CDM Smith aided in bidding assistance, construc­tion services, SCADA engineering, startup services and O&M assistance, and facilitated Construc­tion Manager at Risk delivery. The team completed design and construc­tion oversight, including expansion of the facility from 56 mgd to a peak hour flow of 160mgd, with an average flow of 45 mgd. These upgrades strength­ened sludge handling facilities, disin­fec­tion, blowers, odor control, and more.

Several of these upgrades allowed for operational flexibility; for example, our design of the new phosphorous removal facility allowed for the possible use of this system to treat wet weather flows under certain conditions, thereby maximizing the treatment benefit of this major investment. In these and other ways, the facility upgrades have provided flexibility for UBCW’s operators to make process adjustments to reflect both seasonal and short-term changes in influent conditions, all while meeting stringent nitrogen and phosphorous limits.

Through BNR and an array of plant improve­ments, UBCW is equipped to achieve its mission of providing high quality, clean water services to the surrounding communities while preserving water quality of the Blackstone River. The team continues to conduct river monitoring to assess the Blackstone River and guide discussions with regulators regarding investment priorities for Upper Blackstone and its member communities. The cost-effective and innovative improve­ments are a tribute to the long-term collab­o­ra­tion between Upper Blackstone and CDM Smith and showcase the potential of great minds working together to maintain remarkably low levels of nutrients in wastewater effluent.