You Can Expand a Wastewater Plant Without Shutting It Down
Meeting Today’s Demands While Preparing for Tomorrow
Authors: Abraham King, PE & Aaron Dewald, EI
You can’t simply pause wastewater treatment. Wastewater flows occur around the clock, and your NPDES permit holds you to the same limits even when construction is happening. That reality stops many authorities from expanding their operations even when growth and tightening regulations leave no other choice. And so the decision waits, because a violation or an interruption feels like a larger threat, even as capacity quietly runs short.
This dilemma is avoidable. A wastewater plant can keep treating every gallon straight through an expansion, and doing so is not a matter of luck or accepting risk you don’t want to carry. It comes down to a handful of deliberate choices made early in the design stage that pay off during construction. Recent work at the Paradise Township Sewer Authority (PTSA) in Lancaster County, PA, proves the point. Its plant along the busy Route 30 corridor, built in 1989 and rated for 120,000 gallons per day, had reached its limits. Years of steady regional growth added more than seventy new connections and pushed daily flows higher. The aging extended aeration process could no longer keep pace with organic loading, and stricter nutrient limits tied to the Chesapeake Bay watershed raised the bar. PTSA needed to double its capacity, and it needed to do so without ever going dark.
Design for Continuous Operation From Day One
Maintaining continuous operation begins as a design decision, not a construction afterthought. When staying in service shapes the concept from the first sketch, the whole project comes together around that goal. For PTSA, that meant expanding within the existing site and reusing existing structures wherever the process allowed, rather than building an entirely new plant. RETTEW converted the extended aeration system into a modified Bardenpho biological nutrient removal process paired with membrane bioreactor (MBR) technology. The project expanded an existing building to house the new MBR system controls and the installation of an additional 33,000-gallon equalization (EQ) tank to provide increased flow equalization capacity. This combination doubled the plant’s rated capacity to 243,000 gallons per day, strengthened nutrient removal to meet Chesapeake Bay requirements, and accommodated the improvements within the existing treatment plant footprint. Choosing the MBR method also removed the need for a separate final clarifier and delivered the lowest twenty-year present worth among the alternatives our team evaluated. The philosophy holds beyond this one plant; a design that reuses what you already own is far easier to keep running while you build.
Sequence So Nothing Goes Offline
From there, staying online comes down to sequencing. The principle is simple. Each new piece comes online before the old piece it replaces goes out of service, so the plant never loses the ability to treat continuous influent. At the wastewater treatment plant (WWTP), crews installed new rotary drum screens at the headworks first, then built a new EQ tank that added storage and gave operators more room to maneuver during later phases. The MBR building addition was constructed alongside the running plant, with process piping, controls, and instrumentation installed while the existing treatment train kept handling the influent flow. Only after the new headworks and EQ tank were installed did flows transition over and startup begin.
Managing the Riskiest Moments: Process Handoffs
But sequencing alone is not enough. The riskiest moments come when treatment shifts from one process to another. Planning for those handoffs, and bridging them with temporary process strategies, is what keeps a plant compliant from start to finish. At the PTSA WWTP, the added EQ storage absorbed swings in flow while equipment changed over, and careful coordination kept treatment steady as each process gave way to the next. The community never noticed. Behind the scenes of a major expansion, the Authority stayed in compliance and kept service running without interruption.
Procurement as Strategy, Not an Afterthought
A continuous operation plan relies on the equipment showing up on time, which makes procurement a critical piece of the strategy, rather than a detail left to chance. PTSA’s WWTP upgrade moved forward through a difficult construction market. Long lead times for critical equipment, including pumps, blowers, membrane systems, and electrical components, extended well beyond typical procurement schedules, while tariffs and ongoing supply chain disruptions increased equipment costs throughout the project. The most significant challenge came from the substantial delay in delivering the motor control center (MCC), which threatened the overall construction schedule and complicated the sequencing of the electrical and process equipment installation. To mitigate these impacts, RETTEW worked closely with both the general contractor and electrical contractor to develop and implement a temporary electrical service plan that allowed critical systems to be energized and construction activities to continue. This collaborative effort helped maintain construction progress and kept the project close to its planned completion date despite the prolonged MCC delay. Throughout the project, RETTEW also coordinated proactively with manufacturers to monitor production schedules and manage procurement of other long lead-time equipment. This kept materials arriving in step with the construction sequence while supporting uninterrupted plant operations.
The Results
The results of these strategic decisions speak for themselves. PTSA doubled its treatment capacity for a total construction cost of $6.232 million, made full use of the infrastructure the Authority already owned, and gained treatment performance, operational flexibility, and reliability the community will draw on for decades. Throughout the multi-year phased construction process, the WWTP remained fully operational, continuously treated wastewater, and maintained compliance with permit requirements without interruption.
You Don’t Have to Choose
If your plant faces the same pressure from growth and tightening limits, you do not have to choose between expanding and staying in service. With the right design and a construction sequence built around continuous operation, you can meet today’s demands while you prepare for tomorrow.
RETTEW helps authorities plan expansions that keep treating every gallon from the first day of construction to the last.
Abraham King, PE, CCCA & Aaron Dewald, EI
Abraham King and Aaron Dewald are Environmental Engineers on RETTEW's Municipal team, each with more than five years of experience in the planning, design, and construction of water and wastewater infrastructure. Mr. King serves as engineer of record for several mid-size authorities across Pennsylvania, with experience spanning project manager, project engineer, and resident project representative roles. Mr. Dewald's work spans municipal water and wastewater planning, permitting, and construction, including feasibility studies and infrastructure rehabilitation, in roles including assistant project manager, client manager, and resident project representative.
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