Two general methods are considered for wastewater treatment planning and design — centralized and decentralized models. Centralized wastewater treatment involves collection of wastewater from a large area to one centralized location where it is most commonly treated and discharged to local surface waters. Decentralized wastewater treatment provides treatment and discharge either directly on site or within close proximity. Advancements in technology allow either method to provide equivalent levels of treatment and protection of public health.
However, centralized and decentralized wastewater treatment systems vary greatly, as do the impact they have on environmental health and potable water supplies. Centralized systems obtain potable water from one location, that water is utilized by the public, then collected as wastewater and transported to the treatment plant following which it is discharged to surface waters. This process short-circuits or bypasses the local water cycle and can cause aquifer depletion.
The decentralized model can collect, treat, and then discharge to the subsurface all within a local area. This has the benefit of replacing the original water resource back to the local aquifer. When adequately designed and installed, decentralized wastewater systems have the capacity to process large quantities of wastewater into the underlying soils, making it one of the most passive, sustainable forms of aquifer recharge.
Rather than partially to fully treating wastewater effluent then discharging it to a surface pond or injection well before recharging the aquifer, decentralized systems can provide both wastewater treatment and groundwater recharge in one step. Via this sustainable practice, the replenished aquifer can then supply wells, recharge wetlands for wildlife, maintain base flow, and in the case of coastal cities and towns, counteract saltwater intrusion.
Restoring aquifers and reducing costs
Decentralized systems are being designed and implemented for large-scale municipal and commercial wastewater projects across the United States and Canada. While decentralized systems have and will continue to serve rural areas outside city limits, the notion that the decentralized system is only there to serve small, single-family homes has been transformed with large decentralized systems handling flow rates in excess of 1 million gallons per day (mgd).
Given that the technologies and the operations and maintenance infrastructure are available, many engineering firms are offering their services in this new niche. Large businesses and communities no longer have to wait or pay exorbitant tap fees to tie-in to existing centralized services. Consultants will perform feasibility studies reviewing options for their clients, and the decentralized solution may yield the most beneficial cost position.
As technology continues to advance, an increasing number of sites have become viable candidates for decentralized systems. For example, designing a small-scale, advanced treatment train prior to dispersal of the effluent into the native soil can decrease both the absorption area and depth to limiting layers that are required for adequate treatment. This has allowed decentralized systems to be placed in areas previously inaccessible, in turn increasing the volume of groundwater recharge associated with decentralized systems.
Community decentralized systems are being designed and installed on an increasing basis where sewers have become prohibitively expensive, or where lot sizes or site conditions limit the use of individual onsite wastewater treatment systems. Large systems can also be developed for large-flow commercial and industrial sites, decreasing the hydraulic and nutrient stress placed on centralized wastewater treatment plants.
Centrally managed, decentralized wastewater treatment systems, such as publicly and privately owned community systems, are being staffed with trained and educated personnel in the same manner as centralized systems.
Community wastewater treatment system
The community of Gold Beach, Ore., has approximately 2,250 residents. Wastewater is treated by a Sequencing Batch Reactor Plant, followed with ultraviolet disinfection, and then discharged to a series of subsurface drainfields. Under existing permit conditions, treated effluent discharge was to subsurface absorption fields located at an airport; however, the existing drainfields did not have the capacity for the peak wet weather flows and were experiencing effluent surfacing along with overflows discharged to a creek.
The Port of Gold Beach, which leases the property to the city, expressed concerns about construction of additional drainfields and requested that drainfield operations be confined to the limited, existing easement. Aaron Speakman of The Dyer Partnership Engineers designed a new system based on the maximum allowable area within the existing easement. It is strategically placed adjacent to an airport runway and is designated as a Runway Safety Area (RSA), defined as an area “prepared or suitable for reducing the risk of damage to airplanes in the event of an excursion from the runway.” As the RSA must be clear of obstacles and capable of supporting an occasional aircraft, the 2.2 mgd Infiltrator chamber drainfield design was modified to include stone, increasing the structural capacity of the drainfield in case of an airport emergency condition. The system is covered with a geotextile to prevent sand intrusion into the stone.
The innovative design with 21 drainfields rather than the original nine offers flexibility and redundancy to the city. Flushing ports and isolation valves also allowed the city to flush the lines during new plant construction, which was critical when the existing plant failed prior to startup of the new plant.
The Town of Hopkinton, Mass., had a municipal sewer system but did not have a wastewater treatment plant. Wastewater was being sent to other towns but they were at their discharge limits. A study was conducted to investigate town-wide alternatives and to perform a cost analysis, which led to construction of a new 350,000-gallon-per-day treatment plant.
An innovative sewer-mining solution was designed that partially diverts flows from an existing sewer main to the new treatment plant. A new surface discharge was impossible; therefore, a subsurface outfall consisting of a large infiltration basin was proposed. Due to the proximity to natural wetlands, the space available for the system was constrained. A highly efficient 80,223-square-foot Infiltrator chamber infiltration system solved this problem. Another challenge solved by the infiltration system was that it could fit on the constrained and irregular site as well as recharge the local aquifer and adjoining wetlands.
Centralized and community decentralized treatment
Loudon County, Va. has adopted a combined approach to wastewater treatment, utilizing both centralized and community decentralized treatment facilities. Community decentralized treatment facilities are publicly owned and maintained by Loudon Water in the same manner as the county’s centralized treatment plant. As the county continues to grow, developers integrate new community decentralized facilities then transfer ownership and operation to Loudon Water.
All homes connected to the community systems pay taxes and fees matching those paid by homes connected to the centralized system, which allows for sustainable operation and maintenance. One of the subdivisions serviced by decentralized wastewater treatment is the Elysian Heights wastewater treatment system. This system was designed to serve a 324-home subdivision and five acres of commercial land.
Decentralized wastewater treatment can provide equal protection of public health while outshining centralized wastewater treatment in environmental protection. Where individual lots are not suitable for decentralized treatment onsite, community decentralized systems can provide sustainable and responsible wastewater treatment. With aquifers becoming depleted at rapid rates and water tables dropping, minimizing the impacts to the water cycle that are under our control is critical for sustainability.
Decentralized systems provide a low-cost, environmentally friendly, passive form of both wastewater treatment and aquifer recharge, combatting the declining groundwater table and protecting public health.
DENNIS F. HALLAHAN, P.E., technical director at Infiltrator Water Technologies (www.infiltratorwater.com), has more than 25 years of experience with onsite wastewater treatment system design and construction. He is responsible for technology transfer between Infiltrator and the regulatory and design communities and consults on product research and testing for universities and private consultants. He can be contacted at email@example.com.