Solving Combined Sewer Overflow (CSO) Without Breaking the Bank
Reduce the environmental, financial, and health & safety risks of CSO with data-driven decisions
By Tom Garrity, Director of Sales, Ti-SALES
Combined sewer overflow, or CSO, is every wastewater or stormwater professional’s nightmare. CSO flooding is a public health risk, can damage local infrastructure, and leads to complaints from unhappy residents with flooded basements.
I’m going to briefly cover what CSO is and what causes it – and why it’s such a serious problem – and then share some of the solutions and recommendations we’re seeing work for our customers.
What is combined sewer overflow (CSO)?
While newer water systems have separate sewer infrastructure for stormwater runoff and sanitary waste, combined sewer systems (CSSs) still serve many towns and cities across the country. (Per the EPA’s latest CSO data, it’s about 860 communities serving around 40 million people.) Unlike a separate sewer system, which sends sanitary waste to wastewater treatment plants and surface runoff to various outfall sites, a CSS sends everything down the same pipes.
Combined sewer overflow, or CSO, happens when a CSS exceeds its capacity. Populations often grown faster than the infrastructure can keep up with; combined with an increase in severe weather events including heavy rain and snow falls, these systems are simply overmatched. That combined sewage either ends up being discharged, untreated, into a local water body; or in severe cases, backing up into streets, showers, and other input sources.
Why is CSO such a problem?
The thought of raw sewage flooding your basement is enough to make anyone’s stomach turn, not to mention the property damage. It’s not just unpleasant – raw sewage is a major health and safety concern. Industrial and human waste, toxic materials, and debris pollute local waterways and work their way back into our drinking water supplies.
Unfortunately, even municipalities with separate sanitary and stormwater sewer systems aren’t immune to these concerns. Blockages, malfunctioning equipment, and infiltration/inflow (water entering from breakages or inappropriate connections) can all contribute to sanitary sewer overflow (SSO) events. SSOs can create even greater health and environmental risks than CSOs, as the contaminants are more concentrated.
CSO reporting mandates
In 2019, only 14 states had mandatory notification laws for CSO events (more details in this 2019 WBUR article on CSOs). However, the number is rising; public awareness has led to pressure from environmental and public health activists alike to prioritize combined sewer overflow; as of January 2021, Massachusetts now requires public notifications within two hours. (In Vermont, it’s just one hour.)
While CSO reporting is undoubtedly important, these requirements can be a heavy burden for wastewater utilities, many of whom are operating on outdated technology and an overtaxed infrastructure. Suffice to say, it’s a complex issue with no one right answer.
Options for reducing combined sewer overflow
There are two main categories of techniques for addressing combined sewer overflow:
- Gray infrastructure initiatives include improvements to and reconstruction of the sewer system itself.
- Green infrastructure initiatives focus on reducing the amount of stormwater that enters the system through low-impact development, or environmental changes.
Gray infrastructure: high impact, high cost
Most gray infrastructure options face the same hurdle: they’re costly to implement. Sewer separation, or building a second sewer system, is a massive undertaking; in many circumstances it isn’t even feasible due to the amount of disruption it would cause or other physical limitations.
Another tactic involves increasing wastewater treatment capacity, either through expanding the facilities for treatment or by creating a CSO storage facility: basins or tanks that temporarily store CSO runoff during peak events, then returning it to the system once the event is over and the sewer has capacity again. In addition to the logistical challenges of managing those flows back and forth, CSO storage basins and tanks have limited capacity and must be maintained (accumulated solids & debris removed) between uses. Furthermore, sewage that remains in a covered environment for an extended period carries the risk of turning septic (a condition where anaerobic bacteria have the opportunity to multiply, leading to a buildup of acids and organic gases). The resulting septic material is highly corrosive as well as dangerous to handle, as the gases are both toxic and flammable.
Yet another tactic involves treating the overflow, either in a retention basin to treat and store or a screening and disinfection, or “flow-through” facility, which treats sewage on its way to an outfall. While either solution is more economical than building a new wastewater treatment plan, they come with drawbacks. Retention basins face many of the same challenges as CSO storage facilities; and while flow-through facilities are effective at removing large solids and other debris and providing basic disinfection, they require a high level of maintenance, and lack the advanced capabilities of full wastewater treatment plants (clarification, aeration, pH balancing, testing, etc.).
Green infrastructure: easy to implement, hard to target
A wide variety of green infrastructure techniques can reduce the amount of stormwater entering a combined sewer system via low-impact development, or environmentally friendly land planning and engineering design. With new green technology being developed every year, the number of options can be overwhelming, but a few techniques have risen to the top.
Rainwater harvesting involves collecting runoff during wet weather to use for irrigation or other uses (with proper treatment). Graywater collection is similar, but targets the water used in sinks, showers, dishwashers, etc – any source that does not collect industrial or human waste or other pollutants – for on-site use.
Permeable paving materials divert stormwater runoff from sewer systems & help recharge groundwater tables.
Other tactics center around letting stormwater directly return to the ground. Using permeable paving materials (right) for walkways, parking lots, and even (some) roads lets water flow through the surface material and any aggregate below it and reenter the groundwater table, while also performing basic filtration. Rain gardens, also called bioretention cells, perform a similar function – absorbing stormwater and filtering out common pollutants – while also providing visual interest and environmental benefits. (The EPA maintains a great list of resources for creating rain gardens, including regionally specific tips and information!
While green infrastructure options for reducing the amount of stormwater runoff entering a CSS tend to be cost-effective and relatively easy to implement, the real challenge is in the details. For any of these techniques to succeed, a utility needs detailed information about stormwater levels and other environmental conditions, all of which influence the capacity needed and overall effectiveness of the green initiative.
Remote monitoring: real-time results and data-driven decisions
With so many options available to tackle the challenge of CSO, the number one recommendation we make to our customers is to implement a remote monitoring system. We’ve written before about how our customers are using remote monitoring all throughout their water distribution and treatment systems, and CSO prevention and reporting are at the top of that list.
At its most basic, remote monitoring at an outfall point can simplify or even completely automate CSO reporting, eliminating a heavy burden from overtaxed wastewater departments (as well as potential fines!).
Beyond reporting, remote monitoring can give utility staff the real-time data that can let them reduce CSO in the first place: an abnormally high water level in one location could indicate a blockage that can be quickly cleared to prevent flooding. (Check out this short video from Ayyeka about stormwater management via their remote monitoring solution!)
At an even larger scale, remote monitoring gives a utility a wealth of data to inform their infrastructure and investment decisions – knowing exactly where to strategically locate a few bioretention cells or implement permeable paving could help avoid the expense of building a new CSO storage facility or retention basin.
Even more benefits: remote monitoring enables dynamic technology
With the growth of cloud computing and Internet of Things (IoT) technology, remote monitoring is poised to help utilities in even more impactful ways. Imagine a water system that can dynamically adapt to peak usage events, in real time: an automated movable gate or pump station diverting water from an overtaxed to one with capacity. Or a rainwater storage tank automatically releasing water in advance of a heavy rainfall to minimize runoff overflow without losing a drop. It’s all possible, and as IoT technology continues to improve even more possibilities for automation and CSO mitigation are sure to become available.
Summing Up
Combined sewer overflow, or CSO, is a thorny issue with environmental, financial, and health & safety risks – and if that’s not enough, reporting mandates can be a heavy burden for wastewater utilities. The challenge of CSO is likely to grow in coming years as populations continue to grow and severe weather events increase.
Fortunately, utilities have plenty of options for tackling these challenges, including both gray infrastructure and green infrastructure initiatives. While no two CSO situations are the same, we enthusiastically recommend starting with a remote monitoring solution. (We also fully endorse Ayyeka’s Wavelet device – it’s rugged, powerful, compatible with virtually any device or sensor, and connects seamlessly and securely to a best-in-class cloud-based software platform and/or third-party software systems such as SCADA.)
Not only is a remote monitoring solution fast and easy to implement, it provides data that can be acted on immediately to streamline CSO reporting or even avoid CSO events (and the fines that come with them!). That same data can help a utility target their infrastructure investments – gray and green alike – to achieve the best and most cost-effective solutions for combined sewer overflow.
If you’re currently wrestling with CSO, don’t hesitate to reach out. We’d love to learn more about your challenges and help you find a solution that meets your goals.
Tom Garrity is the Director of Sales for Ti-SALES, a leading distributor of water and wastewater supplies and supplier for Ayyeka’s Wavelet remote monitoring device. The Wavelet is a full-stack hardware, software, and service solution enabling data collection from any sensor, over any communication network, using any server, and can be integrated into any SCADA or software analytic platform.