By Dennis F. Hallahan, PE and Donald Prince
Two general methods are considered for wastewater treatment planning and design: centralized and decentralized models. As environmental and community challenges become more complex and budgets for infrastructure projects continue to be diminished, large commercial projects and communities that do not have access to a centralized wastewater treatment plant or where the plant is at capacity are utilizing decentralized approach to wastewater treatment. This approach can serve the community and sustain development without huge infrastructure costs.
The growing awareness of nutrient damage to the environment from nitrogen and phosphorus is the latest in a push for increasingly stringent health codes to protect vulnerable environments. These codes, enacted by states and municipalities, are challenging engineers and developers to propose wastewater treatment solutions that can perform long term and protect the environment without a huge community financial burden. The value of preserving the water resource is recognized worldwide as one of the greatest challenges of our time.
Centralized and decentralized wastewater treatment systems vary greatly as does 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. This can be particularly challenging in coastal areas where salt water intrusion is a concern.
The decentralized model collects, treats, and then discharges to the subsurface all within a local area. This has the benefit of replacing the original water resource back to the local aquifer at a much lower energy cost. When adequately designed, installed, and maintained decentralized wastewater systems have the capacity to process large quantities of wastewater into the underlying soils, making this option one of the most passive and 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.
Decentralized Approaches for Large Commercial and Community Applications
While decentralized systems 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 with large decentralized systems handling flow rates more than 1 MGD. Large businesses and communities no longer need wait or pay exorbitant tap fees to tie-in to existing centralized services. Often these large commercial systems utilize a combination of technologies that could be traditionally considered on one or the other side of the centralized/decentralized fence.
In many communities, where centralized wastewater treatment facilities (WWTF) are overburdened and the addition of new sewer lines is prohibited, or where individual septic systems are frowned upon, cluster systems are being recommended to developers by local health departments and planning agencies.
Large commercial and industrial systems
In some cases, towns and municipalities have a centralized WWTF but it is overburdened and cannot support community growth. In this case, large decentralized systems can be designed to handle new, large flow commercial and industrial development, thereby decreasing the hydraulic and nutrient stress placed on centralized wastewater treatment plants and augmenting the capacity to sustain community growth. In the case of community wastewater treatment facilities that are reaching or over capacity, adding an exfiltration bed utilizing subsurface infiltration, such as an engineered chamber system, can extend the life and community investment in the WWTF and have the added benefit of reducing phosphorus and eliminating outfall discharges to bodies of water.
Recognizing the need to advocate advanced wastewater treatment systems of a scale that will support positive development, health officials also recognize and often require these systems to be professionally managed. Professional management provides more control on the quality of the waste treatment process. If competent management is available, some utilities are even favoring this approach as the most cost effective long-term solution.
Applications in Action
Paradise, California FEMA Workforce Housing Camp Utilizes a Combined Treatment and Dispersal System to Speed Construction Timeline and Overcome Site Limitations
The 2018 “Camp Fire” devastated the community of Paradise, California, killing 85 people, destroying 11,000 homes, and displacing nearly 50,000 people. The Federal Emergency Management Agency (FEMA) needed to quickly stabilize the situation and support rehabilitation of the community. This required a 1500-person workforce housing camp, including 400 temporary housing units, a laundromat, and food preparation and dining facilities.
Design and installation complications included accelerated deadlines and shallow lava formations, which precluded subsurface dispersal, impeded construction, and required specialized excavation equipment. A General Order Permit was acquired to speed development.
NexGen Engineering designed a 100,000 gallon-per-day (GPD) Advanced Enviro-Septic (AES) combined treatment and dispersal system that receives gravity-flow influent to four, 40,000-gallon septic tanks configured in series. The effluent is then segregated into four treatment paths to facilitate isolation during maintenance. The flow is split to four, lined, AES beds for passive, secondary treatment. Each 25,000 GPD bed contains 8,400 feet of AES pipe surrounded by specified sand for a total 33,600 feet of AES pipe. Treated effluent is collected and gravity-distributed to four, UV disinfection units, each followed by a pump tank. These pumps distribute purified effluent to two evapotranspiration ponds, which allow for possible reuse.
The low maintenance, high flow, AES system includes disinfection and allows for full occupancy of the FEMA work housing camp. This provides needed resources close to the devastated community. New building codes and improved street planning are being put in place to prevent similar, future devastation.
Expansion of New Hampshire Town Community Treatment Plant Accommodates Growth and Preserves Natural Beauty
As the population in the town of Newbury, New Hampshire grew, facility operators began detecting elevated levels of nitrogen in the effluent and groundwater. This catalyzed an update to the Blodgett Landing Community Treatment Plant, a 50,000 gallon per day (GPD) facility. 0riginally designed in 1959 as a seasonal wastewater treatment facility, the upgrade utilizes passive treatment, denitrification, and dispersal to purify the town’s wastewater. A limited budget necessitated finding an effective treatment plant upgrade solution that was affordable both in upfront cost and ongoing operations and maintenance expense. Additionally, Newbury’s economic dependence on recreational tourism required a solution that would preserve the area’s natural beauty.
Several options were explored before the engineers employed by town officials contacted Presby Environmental, Inc. (PEI) for help. PEI recommended incorporating the Enviro-Septic technology into the existing sand filter configuration, which enhances aerobic nitrification and allows for increased denitrification in the recirculation process. The passive treatment performance of the Enviro-Septic technology utilizing naturally-occurring bacteria eliminates the use of harmful chemicals, biological additives, or additional electrical energy. The subsurface location and low maintenance requirements of the system design make it ideal for Newbury and the approach allowed for the continued use of an existing Imhoff tank and significant other existing infrastructure. Ultimately, the Enviro-Septic technology allows the Blodgett Landing WWTF to significantly reduce certain wastewater contaminates, including Fecal Coliform, Total Nitrogen (TN), 5-day Biochemical Oxygen Demand (BOD5), and Total Suspended Solids (TSS).
This wastewater treatment solution enables the Town of Newbury to protect its natural resources while allowing continued development within the community unfettered by wastewater capacity restrictions. Moreover, because the system performs well in varying climates, including freezing temperatures, it is ideal for the location of this project where cold winter months are the norm.
Decentralized systems offer many benefits for large commercial or community applications and the best approach varies from case to case. Local codes, proximity to water bodies, and anticipated daily flows are additional key factors for individual, commercial, or community systems. New products and applications approaches has broadened the options available to engineers, developers, and communities. In many cases, the decentralized approach may offer a lower cost, high performance, and versatility when compared to a centralized option.
Dennis F. Hallahan, PE has more than 30 years of experience with onsite wastewater treatment systems’ design and construction. Currently Technical Director at Infiltrator Water Technologies, 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. Hallahan received his MS in civil engineering from the University of Connecticut and his BS in civil engineering from the University of Vermont. He is a registered professional engineer in Connecticut and holds several patents for on-site wastewater products. He can be reached at email@example.com.
Donald Prince holds a Civil Engineering Degree from Vermont Technical College and is a licensed septic system designer and evaluator. He has provided technical support for AES and other Presby Environmental products since 2012. Presby Environmental is an Infiltrator Water Technologies company. Don lives with his family in northern Vermont. He can be reached at firstname.lastname@example.org.