The City of Nashua added secondary treatment to its wastewater treatment facility (NWTF) in the late 1980s. Like many facilities of its age, the NWTF had several significant assets that required substantial repair and upgrades. In addition, the 20-year-old blowers at the NWTF were oversized for the plant’s treatment needs and accounted for 30 percent of the facility’s total energy use. Replacing the blowers represented a significant opportunity to reduce operating costs. The City and Woodard & Curran worked together to identify and design energy-saving upgrades to the aeration system as well as complete a structural condition assessment, implement recommended repairs, and carry out several equipment upgrades to improve and sustain plant operations for many years to come.
NEW BLOWER UPGRADES IMPROVE PERFORMANCE AND SAVE ENERGY
A detailed analysis of the entire treatment system in Nashua indicated that the City’s 20-year old blowers were too large for the facility’s needs. As a result, the blowers were running at a lower speed, reducing their efficiency. Woodard & Curran concluded that three new high-efficiency blowers (sized to actual aeration demands based on biological modeling and statistical analysis of historic airflow rates) totaling one-half the existing horsepower of the older system would be sufficient to meet the facility’s treatment process needs. Woodard & Curran and the City evaluated several direct-drive turbo blowers, but eventually selected a gear-driven blower. In addition to proven efficiency and reliability, another major advantage of this technology was that it could be powered by the existing medium-voltage electrical system, which saved an estimated $200,000 in construction costs. Modern instrumentation and controls were also included in the design to precisely monitor dissolved oxygen and maximize efficiency. Woodard & Curran estimated that Nashua’s new aeration system would use 50% less energy than the previous system and would reduce the plant’s overall electricity use.
STRUCTURAL CONDITION ASSESSMENT AND REPAIRS
Woodard & Curran appraised the structural integrity of the facility’s 20-year old secondary treatment tanks: four 150-foot-long aeration basins, three 120-foot-diameter secondary clarifiers, and steel sludge collection mechanism in each clarifier. The inspection revealed leaking cracks and control joints, spalled concrete, and corroded steel components. The major assets were determined to be structurally sound and their useful lives could be significantly extended through repairs. Control joint sealant was replaced, and cracks were injected with polyurethane. Spalled areas were repaired, and the vertical concrete surfaces in the aeration basins and secondary clarifiers were coated with an industrial epoxy-based coating system. Different systems were specified for common walls where adjacent tanks would need to be kept in service and for walkways subject to wear.