By FLOW Board member Bob Otwell and Traverse City Commissioner Tim Werner
High Lake Michigan water levels are are forcing more water to flow into Traverse City’s wastewater treatment system. This forces city residents to pay more in energy, maintenance and other operational costs.
Wastewater collection systems are designed to transport wastewater (sewage) from homes and businesses to a wastewater treatment plant. Old sewers constructed with clay pipes often have cracks that allow clean groundwater to infiltrate into the system through gaps in both the private sewer leads and the public sewer mains. Some improper building construction also has allowed basement drains to be connected directly to the sewer system. In addition, stormwater inflow can happen during rain events through uncapped cleanouts and faulty manhole covers, and through improper roof-drain connections. See the figure below.
In Michigan, with our glacial soils and humid climate, which means more rainfall than evaporation, groundwater flows downhill to the nearest surface water body. Groundwater behaves similarly to streams that also drain into lakes and rivers. But groundwater is invisible, below the ground surface. If you dig a hole at the beach, the water you encounter is groundwater.
As Great Lakes levels rise, groundwater levels adjacent to the lake rise as well. Last year’s historic high Lake Michigan water levels have increased groundwater levels, and hence increased the volume of unwanted clean groundwater entering the wastewater sewer systems in lakefront communities.
The figure above shows the level of Lake Michigan over the past 100 years. Record high levels were set in 1986, only to be broken in 2020. Record-low levels occurred in the early 1960s and were matched in 2012/2013. The average level has been about 579 feet above sea level, and the difference between high and low lake levels is about 6 feet.
To quantify how much high groundwater levels have impacted wastewater flows in Traverse City, the daily flow into the municipal wastewater plant was compared with Lake Michigan water levels over the past eight years. See the comparison below.
Lake Michigan levels below 579 feet showed relatively constant inflows into the plant (blue dots above), with an average of 2.3 million gallons a day (MGD). As Lake Michigan rose above an elevation of 579 in 2016, daily base flow into the plant increased (red dots). In the first 10 months of 2020, high Lake Michigan water levels created 450 million gallons of extra wastewater flow. This clean groundwater inflow all had to be treated at the wastewater plant.
Where did all of the water come from? Although the city has already completed some rehabilitation of old sewers, more work must be done. In addition, the Union Street dam provides a separation on the Boardman River between the river section affected by Lake Michigan, and the unaffected upper reaches of the river. Wastewater sewers in the near-bay neighborhoods (Slabtown, Downtown, Boardman, Oak Park and the neighborhoods at the base of Old Mission peninsula) are affected by high Lake Michigan levels and are where these excess flows originate. Sewers in neighborhoods farther south (Traverse Heights, Old Town, and much of Central neighborhood) are unaffected by Lake Michigan levels.
Solutions
Different techniques are available to reduce the impact of infiltration and inflow on a community’s wastewater treatment system. Within the public system, old mainline sewers can be either replaced, or improved with trenchless rehabilitation that re-lines the sewer. Manholes can be inspected, and leaks can be sealed, and leaky covers replaced.
On private property, stormwater downpipes and basement drains need to be disconnected from the wastewater system. Broken laterals need to be replaced or re-lined.
To determine where the majority of inflow and infiltration originates, a proactive, systematic analysis of a community’s wastewater system needs to be completed. Different techniques are available to assess the problem.
- TV inspection—Inspect the inside of sewers by using a small camera that travels down the length of a pipe to produce a visual representation of the pipe’s condition. The camera can also spot excessive lateral inflow.
- Manhole inspections—These inspections can identify leaks from joints due to tree root intrusion, settling, or design issues. Manholes can contribute to significant inflow through leaky covers as well.
- Smoke testing—This testing locates inflow and infiltration sources by identifying stormwater drain cross connections, broken pipes and laterals, and unsealed manholes.
- Private property inspections—The inspection of private properties consists of visual assessments of the stormwater and wastewater networks within a property.
A planned course of action is important to conserve and protect our resources. Water—and the energy to move it—is too precious to waste.
