We’ve written previously on the blog about how working with a consultant to conduct a Sewer System Evaluation Survey (SSES) contributes to a much more effective asset management program. In the most recent issue of Water Environment & Technology, I published an article on our experience working with the City of Lawrence, Massachusetts to pinpoint the cause of a serious uptick in their wastewater flow using an SSES, with the intent of preventing a significant rate increase for its residents. WEF members can log in at the link to the magazine to read the full article, but for nonmembers and anyone who wants the short version, here are the highlights.
Solving a municipal mystery
Once a thriving industrial community, Lawrence now ranks the lowest among the 351 Massachusetts communities in per capita and median family income. The city also has an aging collection system with structurally impaired portions, putting it in a seemingly impossible position for funding a proactive asset management strategy. However, when the city began noticing that its wastewater flows had nearly doubled in a very short period and remained high, they had no choice but to find a way to address whatever issue was behind this change.
Between December 2013 and April 2014, Lawrence’s flow increased from an average monthly flow of about 10 MGD to 18.31 MGD with no extreme precipitation events that would explain this sudden increase. In May, the city reached out to us, as we were about to kick off an SSES of its collection system, to help investigate the problem. Our first step in finding the inflow source was to narrow down the system infrastructure to pipes with a large enough diameter to accommodate this amount of increased flow, leaving us with Lawrence’s largest diameter combined system infrastructure, located closest to the Merrimack River in the downtown industrial mill section of the city. On each side of the Merrimack River, a power company had installed canals, the north canal and the south canal, to divert river water through private factories to power equipment. The power company retained ownership of the dam and the two canals, while the private factories owned all the land and infrastructure that connected the canals and the river and the Greater Lawrence Sanitary District (GLSD) owned the large diameter sewer interceptors along each side of the Merrimack River. Because of this arrangement, the City of Lawrence had little to no authority to inspect the infrastructure between the canals and the river.
It was clear that the collaboration of the other parties would be necessary to complete the investigation, and the city reached out to the GLSD and the power company. The GLSD installed temporary flow meters on the north and south bank interceptor downstream of the canals, and with these installed along with two weeks of flow data being received, the power company drained the water from each canal at the city’s request so that the GLSD could compare pre- and post-draining flows as well as total inflows at the treatment facility. When the south canal was drained the flow returned to normal, confirming that the inflow source was directly linked to the water elevation in the 3,000 LF south canal.
Further inspections of the drained canal revealed dozens of other connections the city hadn’t been aware of, one of which was external 48-inch slide gate. After injecting dye and hydrant water through a hole in the wooden slide gate, dye emerged at a manhole connection to the GLSD interceptor. On, November 4, 2014, 11 months after dry-weather flows increased abruptly, it was confirmed that the defects in the privately owned slide gate coupled with the water elevation in the canal was contributing millions of gallons of daily dry-weather flow to the system through a 48-inch connection.
Perhaps as remarkable as the persistence and dedication necessary to find the source of this inflow was the ultimate cooperation of all the parties involved. Ultimately, the city authorized the abatement of the source and continued to work closely with the GLSD to meter flows and inspect manholes to identify additional sources of inflow, all in the name of avoiding a rate increase of up to $500,000 spread across the community’s 12,000 customers. While it is unlikely every community will have a 5,550 GPM inflow source, this is a meaningful example of the real value communities can see from implementing an SSES and other proactive public and private infrastructure assessments.
