Aug 05, 2019

Thinking Off Center

Mitigating combined sewer overflows with decentralized sewer systems

Mitigating combined sewer overflows with decentralized sewer systems

Nearly 50 years since the Clean Water ACT (CWA) passage in the early 1970s, many communities have not met CWA goals and continue to pollute. CWA aimed to solve wastewater treatment challenges, and it offered a rush of funding aimed at tackling those wastewater problems. Supporting a centralized model for wastewater treatment, CWA funding helped communities construct collection systems and centralized wastewater treatment plants (WWTP), but in numerous cases, the cost to operate and maintain those systems as they aged burdened the communities that built them. As communities grew, treatment standards increased, and as the CWA funds ran out, the funds to expand those WWTPs were unavailable. 

The centralized treatment system approach has the problem of infiltration and inflow (I&I), which leads to combined sewer overflows (CSO) and sanitary sewer overflows (SSO). Due to costly expenses to repair pipe, community violations continue for nearly 50 years in some areas. The U.S. EPA has resorted to enforcement actions—typically Consent Decrees—against numerous cities. 


Centralized & Decentralized Wastewater Treatment

Today, communities are faced with environmental and community challenges including CSOs from outdated centralized systems. Most do not have the budgets for wastewater treatment infrastructure projects. 

Finding less capital-extensive solutions to extend the life and expand the capacity of existing centralized systems also is a high priority. Many communities are open to smart sustainable development, but have aging or undersized wastewater treatment plants. These communities are turning to a combination approach of centralized and decentralized wastewater treatments to handle capacity needs and to sustain development. New regulations are resulting in an expanded number of choices for communities, as well. 

This new approach to combine centralized and decentralized wastewater treatment, includes expanding a wastewater district service area without extending the collection system to new or distant parcels. The district can incorporate decentralized strategies by installing a satellite plant with subsurface disposal. 

Septic tank effluent pumping systems, in which individual or groups of homes have tanks for primary treatment and wastewater that are pumped to a centralized treatment facility, are an example of the intersection of decentralized and centralized approaches. Likewise, large commercial systems with multiple wastewater generating sources are combining technologies that traditionally were considered either centralized or decentralized.

In some cases, constructed wetlands using green infrastructure are effective resources for storm water runoff. These can benefit the local community with a visually appealing treatment and runoff control system that also mitigates residential flooding.
In some cases, constructed wetlands using green infrastructure are effective resources for storm water runoff. These can benefit the local community with a visually appealing treatment and runoff control system that also mitigates residential flooding.

Capture Runoff, Meet Objectives

Capturing runoff where it falls in urban settings remains one of the primary goals in reducing CSOs. However, given the urban setting, it can be difficult to find eligible open land. Decentralized strategies have the capability to meet multiple CSO objectives including flow rate attenuation, volume reduction and water quality improvement. 

The Water Environment Research Foundation (WERF) published a report in 2006 titled, “Decentralized Stormwater Controls for Urban Retrofit and Combined Sewer Overflow Reduction.” This report supported the use of decentralized source controls in conjunction with redeveloping land in urban regions. Over time, this approach creates opportunities to develop greener communities that achieve higher levels of ecological and receiving water protection.

Wet-weather water management can be difficult for communities that face the challenge of discharging large volumes of minimally treated wastewater. The budgets of these communities often are strained, compelling engineers to look for innovative, low cost solutions. Many designers have discovered the decentralized model meets community needs at a cost communities can afford. In lieu of installing more pipe and building larger wastewater treatment plants, designers are using decentralized methodologies, such as engineered wetlands, to treat CSOs. 


Natural Processes for Runoff Reduction  

When reintroduced into the design of semi-urbanized environments, green infrastructure that uses vegetation and soil to reduce storm water runoff volume may also reduce air pollution and air temperature through evapotranspiration. This can minimize the urban heat island effect, while also providing ground cover that serves a habitat function. Designing with nature also can be seen in a larger sense, as land development that is more sustainable—economically, environmentally and socially.

Large decentralized systems with flow rates of more than 1 mgd are being designed and implemented for large-scale municipal and commercial wastewater projects across the U.S. and Canada. While decentralized systems have, and will continue to serve the rural areas outside city limits, the notion that the decentralized system is only there to serve small, single family homes has been transformed by these large decentralized system designs. 

Large businesses and communities no longer need to wait or pay exorbitant tap fees to tie into existing centralized services. Consultants can perform feasibility studies to review options for their clients. In some cases, the decentralized solution may yield the most beneficial cost position.


Environmentally Vulnerable Sites & Watershed Areas

Increasingly restrictive regulations with the goal of protecting vulnerable environments have caused municipalities to push engineers and developers to present wastewater treatment solutions that can perform long term and contribute to sustainable development. Health codes that regulate onsite wastewater system design and installation have grown increasingly stringent with elevated awareness of nutrient damage to the environment from nitrogen and phosphorus. The value of preserving the water is recognized worldwide as one of the greatest challenges of our time.


Decentralized System Reduced Waterway Pollution

The community of Washington, Ind., had problems with CSOs. Like many communities, Washington’s centralized infrastructure was old and decaying. Most of its water infrastructure was constructed in the 1930s, and its storage capacity was minimal. Things had gotten so poor that as little as 0.1 in. of rain produced CSOs. Even worse, the water pooled and then dried up, concentrating pollutants between rain events. The city struggled with this problem for decades. Early attempts to abate the pollution included enclosing drainage ditches and creeks in large pipes, but these did not address the water quality problems. 

Facing federal mandates to clean up its water, the city was desperate. It employed various firms to study the problem and devise a solution. The resultant studies proposed conventional, centralized solutions with the most cost effective solution estimated to be in the $53 million range. Given the average income of the city’s 12,000 residents, this expense was not a viable solution. The town needed to find an alternative its constituents could afford. 

Discussions involving city officials, project engineer Bernardin, Lochmuller & Associates, and the Indiana Department of Environmental Management resulted in the proposal of a decentralized solution featuring a constructed engineered wetland. The city chose that decentralized approach rather than a chemically enhanced high rate clarification system due to the difference in capital costs. 

The constructed engineered wetland saved Washington residents approximately $26 million. Also, the city was projected to save an additional $1.6 million or more annually in operations and maintenance expenses by taking this decentralized approach. Another key benefit was the associated diversion of storm water to reduce nearby residence and roadway flooding during rain events.

Engineered wetlands are different from other treatment processes because they employ vegetation as part of the treatment process, which requires minimal energy input. In the Washington wetland, wastewater is directed into a 4-million gal storage tank and then discharges to a treatment plant. Excess flow is released via two 84-in. culverts to the wetland, which encompasses 27 acres. A SCADA system determines how long the overflow stays in the wetland. Following treatment, the effluent passes through an ultraviolet disinfection system before discharging to the local creek.

The new system includes tools for fine particle removal, an underdrain system to allow dewatering for maintenance purposes, and a recirculation system to control soil moisture and help with purification. A landscape plan includes specific plant species known for adaptability to changes in water level and soil moisture. The wetland remains saturated even in times of drought thanks to its depth and the natural geology of the area, benefiting the plants and enhancing nutrient uptake.

In Washington, Ind., only 1/10th of an inch of rainfall would set off a series of CSOs in the community. To combat this, Washington built a wetland, which not only treated the runoff effluent, but also reduced flooding of nearby residences and occurrences of water running across a nearby road.
In Washington, Ind., only 1/10th of an inch of rainfall would set off a series of CSOs in the community. To combat this, Washington built a wetland, which not only treated the runoff effluent, but also reduced flooding of nearby residences and occurrences of water running across a nearby road.


For many communities facing consent decrees to address wet-weather water management problems, the decentralized model has emerged as a viable treatment option. An objective analysis can compare quantifiable parameters, such as environmental treatment, affordability and sustainability to determine the best decentralized approach. 

Large system designs, such as infiltration basins, subsurface infiltration chambers or constructed engineered wetlands, have strong treatment capabilities that also eliminate CSOs and can expand to keep pace with community growth. The subsurface system approach allows for land use above the system, such as parks, ball fields, agriculture or parking lots.

Every situation is different. Each application has variable wastewater volumes, treatment needs, design challenges and local regulations. The right solution is one that serves the water quality, treatment and financial needs of the community. When communities and engineers choose a sustainable development and wastewater treatment path, they base the choice on these factors, in addition to community planning, anticipated growth, economics and environmental sensitivity.

About the author

Dennis F. Hallahan, P.E., is technical director for Infiltrator Water Technologies. Hallahan can be reached at [email protected]