Whether undertaking an upgrade or planning for a new facility, assessing risk and building in resiliency measures are an important focus during the design process. This is even more critical for towns like Warren, Rhode Island with a wastewater treatment plant (WWTP) predicted to be highly vulnerable to sea level rise, situated on a small plot of land near the mouth of the Warren River where it flows into the northern reaches of Narragansett Bay.
Facility Upgrades Increase Flexibility and Address Nitrogen Limits
In 2016, residents of Warren approved a $20 million bond to undertake a major upgrade to the existing WWTP, which sits between the Waterfront Business District and residential neighborhoods. Originally built in 1940, the last major renovation to the facility happened in 1981. This latest project improved and expanded wastewater treatment processes with additional reactor capacity, new mechanical and electrical equipment designed to meet more stringent water quality standards set by the Rhode Island Department of Environmental Management (RIDEM).
The Rhode Island Pollutant Discharge Elimination System (RIPDES) permit collaboratively developed by RIDEM, the town, and Woodard & Curran not only allowed the WWTP flexibility in accommodating historically high spring flow conditions, but also provided a high level of nitrogen removal at a lower cost. This informed the upgrade design, leveraging the permit flexibility through selection and implementation of a Variable Operating Mode Process. This approach allows the WWTP to optimally function in either a 4-Stage dual anoxic zone or a Modified Ludzack-Ettinger (MLE) mode of operation. An additional feature allows the WWTP to temporarily switch to operate in Contact-Stabilization Mode during extreme wet weather events. The system was also configured with multiple swing zones to allow operators to modify the sizes of the aerobic and anoxic zones in the process.
These upgrades have provided significant process performance stability, resulting in consistently low effluent BOD, TSS, and total nitrogen concentrations even during high flow periods.
Opportunity for Resiliency Measures
About five years ago, Woodard & Curran worked with RIDEM to evaluate the implications climate change has on the state’s wastewater infrastructure and identify ways to integrate climate change considerations into wastewater planning and operations. This included site-specific adaptive strategies, which were applied in the design of this WWTP upgrade. Specifically, our team evaluated resiliency to future climate change effects based on model projections from the U.S. Army Corps North Atlantic Coast Comprehensive Study, New England Interstate Water Pollution Control Commission (NEIWPCC) 2016 revision of TR-16 Guides for the Design of Wastewater Treatment Works, STORMTOOLS, STWAVE, and NOAA sea level rise. Our experts teamed up with RIDEM and the Rhode Island Coastal Resources Management Council (RI CRMC) to establish flood design criteria and evaluate flood resiliency.
A detailed structural analysis, with cost estimates, was conducted to evaluate several system scenarios. A cost-benefit analysis considered two magnitudes of flood protection – one for the base case that would allow some structures to flood with modifications to improve equipment resiliency and the other to upgrade structures to avoid floodwaters. The team also evaluated measures for operational flexibility to better respond and recover after flood conditions occur.
Ultimately, the resiliency measures taken as part of the WWTP upgrade included raising the elevation of new structures, relocating critical electrical and controls equipment above future flood elevations, and replacing the plant’s main pump station with dry-pit submersible pumps.