In 2021 and 2022, more than half a million gallons of raw sewage spilled in Wake County – with more than 413,000 gallons spilling in Raleigh, alone. As of May 2024, there have already been 30 reported sewer overflows in Wake County, totaling more than 336,000 gallons.
The city said there are about 50 sanitary sewer overflows (SSOs) every year in Raleigh. But SSOs aren't exclusive to Raleigh – they're happening across the state.
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In December 2023, more than 450,000 gallons of sewage spilled into a Durham creek.
So, what are SSOs and why do they matter?
SSOs, or sewer spills, happen when untreated or partially treated sewage is released from a municipal sanitary sewer.
Dr. Lee Fergusonsaid sanitary sewer systems weren't built with any sort of overflow valve.
"You're not supposed to have overflows," said Ferguson, associate professor of civil and environmental engineering at Duke University. "You overwhelm the flow and you see wastewater bubbling up through manholes, or sometimes it'll back up into people's houses, that kind of stuff."
Just this past week, WRAL 5 On Your Side investigated several instances of sewage overflowing into Raleigh residents' homes.
SSOs are not supposed to happen frequently. When they are happening frequently, the Environmental Protection Agency (EPA) said it's an indication that something may be wrong with the sewer system.
It's important to note that in Raleigh the SSO rate per 100 miles of sewer line is between 1.0 and 2.0 – compared to the average for the Southeast, which is around 7.0.
Ferguson said aging infrastructure, growing population and extreme storm events are contributing causes of SSOs in North Carolina.
'A storm that you would think would be a 50-year frequency storm is happening every few years now'
Ferguson said storm water can infiltrate the sanitary sewer systems during a storm, and this can overwhelm the wastewater treatment plant.
"So I think that it's pretty clear that with climate change happening, we are absolutely seeing, you know, larger and more intense storm events that are happening much more frequently," he said.
"So, a storm that you would think would be a 50-year frequency storm is happening every few years now."
Ferguson said these sewer systems weren't built to withstand the kind of extreme storm events we're currently seeing.
"So what typically happens is, during a big rain event, the infiltration will sometimes be enough that the volume is too much for the wastewater treatment plant to handle, and that's when you get overflows," he said.
In 2018, more than 7 million gallons of sewage overflowed into local creeks in Raleigh after a storm.
Ferguson said when you have large amounts of water being dumped in a very quick amount of time, the precipitation doesn't have time to absorb into the ground.
"Instead, it runs into either the storm sewers, and then the storm sewers may have leakage or inadvertent cross connection to the sanitary sewers," he said. "And then you get overwhelmed."
The City of Raleigh said, overall, the trend for SSOs is going down, and it should be understood that weather events have a substantial influence on the number of spills in a given year – but even weather related SSOs are going down, according to the city. However, the bar graph at the top shows that the quantity (per gallons) of these spills increased in Wake and Durham counties from 2021 to 2022.
Ferguson said aging infrastructure can also contribute to SSOs.
"Aging infrastructure is clearly a big issue," Ferguson said. "As the cities and sort of the utilities kind of grow out, you know, you add more sewer lines."
Once the sewer system starts to approach its design limits, Ferguson said it gets closer to reaching capacity.
"And as you get closer to the capacity of the system, then it's not going to take as much to cause an overflow," he said.
"So that's one reason, right... increasing population, increasing density, aging infrastructure. Those sewers aren't getting any younger."
Although extreme weather events and aging infrastructure contribute to SSOs, the City of Raleigh said most are caused by things like grease, wipes and contractor damage.
The City of Raleigh said it has spent roughly $350,000,000 in the past five years to upgrade city sewers. The city has budgeted in the Capital Improvement Program funding for the replacement and rehabilitation of its aging sanitary sewer infrastructure.
Sewer overflows bring water quality back to the 19th century
Another concern with SSOs is the impact they have on the environment and human health.
The EPA said SSOs contain raw sewage, which means they carry bacteria and other potentially dangerous substances. Unfortunately, there is very little information about the environmental or health impacts of SSOs. This makes it difficult to understand how serious of an impact SSOs have.
"The unpredictable nature of most SSO events makes it difficult to monitor and collect the data needed to measure the occurrence and severity of environmental impacts," the EPA said.
Ferguson said he addressed this issue during Hurricane Florence.
"I did some sampling of the Neuse River in the period after Hurricane Florence... there were major flooding events," he said. "And so the flooding caused overflows and basically wash out of raw wastewater down around Goldsboro into the Neuse River.
"And we could pick the signature up chemically when we looked at the sample, so it's pretty clear what happened. I mean, it was easy to see."
Ferguson said the implications of overflows like that are significant.
"Basically you're bringing the water quality kind of back to the late 19th century, right?" he said. "Because, really, what you're doing there is kind of putting us in a scenario where human waste is no longer treated to remove pathogens or remove contaminants, and it just goes directly out into the rivers and then downstream [where] you have drinking water intakes and that sort of thing."
The City of Raleigh said SSOs during heavy rainfall events have minimal impacts to water quality given the massive dilution factor.
"By far and away, it is more important to prevent dry weather SSOs as they do not have a dilution factor to mitigate their impacts to water quality," the city said.
The EPA said underreporting and incomplete tracking make it hard to understand the human health impacts of SSOs.
"Although it is clear that CSOs and SSOs contain disease-causing pathogens and other pollutants, EPA found limited quantitative evidence of actual human health impacts attributed to specific CSO and SSO events," the EPA report said.
You can monitor and track SSOs across North Carolina at deq.nc.gov.