Forty years after passage of the Clean Water Act, sewers still routinely overflow and pollute waterways and collection systems leak to deplete groundwater resources.
Sustainable development that includes natural and resilient designs for water and wastewater infrastructure has moved away from the marketing hype and has become real, with installed projects worldwide. These performance-engineered solutions for decentralized wastewater treatment designs preserve groundwater supplies as they serve large and small individual and community wastewater needs. The shift from centralized wastewater treatment plants and systems is partially due to high installation and operations costs to homeowners and municipalities and a scarcity of government grants and subsidized loans to fund these methods.
What is also at issue but not often talked about is the groundwater depletion that is a detrimental by-product of centralized treatment. Groundwater serves the needs of people everywhere, from drinking water supplies to agricultural water use. Long-term water level declines or groundwater depletion occurs when water is removed from the ground faster than it is replenished. It has many negative effects, including a deterioration of water quality and a reduction of water available in lakes and streams. In many areas of the U.S. and worldwide, the situation has become extreme, exacerbated by chronic drought and by centralized wastewater treatment systems that discharge water often miles away from its source.
With a decentralized wastewater treatment approach, groundwater is extracted, utilized, and treated onsite; then it is returned close to its point of origin to recharge the aquifer. From small residential systems to large-scale facility or community discharges of more than 1 million gallons per day (mgd), these natural approaches provide suitable long-term treatment solutions. Natural wastewater treatment designs also offer engineers and developers improved environmental options for best development practices and can be more cost-effective than centralized systems. Due to the compact nature of the decentralized model, there is less energy consumption.
The Los Osos, Calif., recharge system will protect the community against saltwater intrusion. Instead of valuable water lost to an ocean outfall, 1.6 mgd will be utilized for recharge.
Decentralized benefits include the following:
- Treatment — Decentralized systems can treat to the same level as centralized systems. The technologies available for large-scale systems such as membrane treatment systems are now available for smaller flows.
- Increased flow — Through advances in treatment and disposal technologies and management structures, decentralized systems can now treat flows in the 1-mgd flow range; several facilities are operating at a capacity of more than 1 mgd.
- Green and sustainable — Decentralized systems typically are installed at or near the point where the wastewater is generated, thus requiring less energy input.
- Decentralized wastewater treatment can provide a long-term and cost-effective solution for communities by avoiding large capital costs, reducing operation and maintenance costs, and promoting business and job opportunities.
- Decentralized wastewater treatment provides aquifer recharge, replenishing local groundwater supplies.
In the recharge arena, wastewater is viewed more as a resource than a nuisance. Recharging the aquifer allows replenishment of overused aquifers and water storage underground for reclaiming during dry periods.
Disposal of treated wastewater to the subsurface prevents the evaporation loss that can be an issue with natural and artificial storage reservoirs. Centralized system reservoirs can have water losses as great as 50 percent in arid areas. Recharge maintains the local water table, thereby providing essential base flow to wetlands, streams, and rivers to maintain ecology. Recharge can also offset saltwater intrusion in coastal regions.
Current technology makes it possible for treated wastewater to meet drinking water standards at a cost less than desalinization or importing water. Indirect potable reuse (IPR) is the process by which treated wastewater is utilized to recharge a groundwater aquifer that is used as a potable water source.
Decentralized systems can meet the same treatment levels as centralized systems. This large-capacity system has a membrane treatment plant being constructed in the background, which will discharge to a subsurface recharge basin in the foreground.
Southern California’s large population, living in a semi-arid region, has long relied upon outside sources for much of its drinking water. Importing water has become more expensive because of the demand from larger populations, multi-year droughts, and increased environmental regulations. During the 1950s, saltwater intrusion was measured as much as five miles inland. With Southern California losing more than 1 billion gallons of water per day due to ocean outfall discharges, the increased water demand to support the population and agriculture, and the need to curb saltwater intrusion, innovation was a necessity. Here, wastewater is now viewed as reliable and constant resource water.
Systems in action
Orange County, Calif. — Due to Southern California’s water challenges, 70-mgd Orange County Groundwater Replenishment System (GWRS) incorporated decentralized methodologies at a very large scale. The GWRS purifies resource water and recharges the groundwater aquifer. The process utilizes methods that consume up to two-thirds less energy than importing water and three times less energy than desalinization.
Village of Omemee, Ontario — Located within the City of Kawartha Lakes in Ontario, the Village of Omemee had choices to make. Flow to the existing wastewater system of lagoons and spray irrigation was exceeding the system’s capacity. The village was placed under regulatory orders, halting community growth until a solution was provided. Initial proposals recommended a “big pipe” solution, however the high capital and operating costs of such a system was not financially feasible to residents. Engineers then proposed a decentralized solution. A large benefit of this system is the resulting recharge to the vital aquifer supplying water to the wetland and river in contrast to a centralized solution where it would have been piped out of town. The modular recharge system design, sized for 350,000 gpd, handles the existing flow and allows for community growth. Even better, the cost was less than $3 million; a dramatic savings compared with the $14.9 million cost of a centralized solution.
Los Osos, Calif. — The community of Los Osos had a long struggle in choosing the direction for wastewater treatment. The coastal community had for generations met its wastewater needs with onsite wastewater treatment systems that were now outdated. As urban core density increased, code-sized systems could not fit the small lots. With existing systems far exceeding their design life and now contributing to nitrogen pollution, the Regional Water Quality Control Board declared a prohibition zone that included a building moratorium. To make matters worse, the agricultural community was experiencing saltwater intrusion further inland as groundwater was pumped to meet demand.
Initial studies that recommended an ocean outfall for a centralized wastewater treatment system raised concerns that saltwater intrusion would be exacerbated. A decentralized solution that included final discharge to a community drain field, recharged groundwater supplies, minimized saltwater intrusion, and served the community for the future was ultimately selected. The completed large community treatment system has a capacity of 1.6 mgd and includes collection, conveyance, treatment, and recycled water reuse.
Communities have options when facing wastewater challenges. In spite of the perceived benefit of centralized sewers, they continue to contribute to surface water pollution 40 years after the CWA passage and they deplete, not replenish, groundwater supplies. While the centralized model is applicable to highly urbanized areas, the notion to continue with this model needs to be questioned in outlying areas and regions where drought and water supply issues are present.
Dennis F. Hallahan, P.E., technical director, Infiltrator Systems Inc., has more than 25 years of experience with onsite wastewater treatment system design and construction. He has authored articles for industry magazines and has presented nationally on the science and fundamentals of onsite wastewater treatment systems. He can be reached at firstname.lastname@example.org.