For communities with no access to centralized wastewater treatment, finding cost-effective and sustainable solutions for residential and commercial treatment and dispersal/disposal is a challenge. The need for high-level treatment strategies that address nitrogen reduction, watershed protection, and sensitive environments has catalyzed the design of advanced treatment systems to solve these issues. This is mostly due to the high cost of traditional centralized wastewater collection and treatment and the innovation with small-scale treatment systems.
To sustain the environment and community health, engineers, municipal health officials, and regulators are adding to their wastewater solutions tool chest the use of pre-engineered, decentralized treatment systems as well as incorporating nontraditional collection methods and soil dispersal for final disposal.
Where individual onsite septic systems are the norm, town officials have a host of traditional and advanced decentralized treatment system possibilities to protect local waterways and water supplies, upgrade outdated systems to reduce nitrogen loading, and improve overall wastewater management for the community. Because of the advances in collection, treatment, and dispersal, decentralized systems are no longer limited to small-flow systems and those in remote locations. Some decentralized communities are collecting and treating wastewater in excess of 3.75 million liters per day. It’s a boost to rural and growing communities that the wastewater technologies and solutions available for large-scale decentralized systems are now available for small-scale systems as well. Communities, homeowners, and developers can move ahead with projects without waiting for a sewer extension to reach their site or the treatment plant to be expanded.
Pre-engineered, decentralized treatment systems provide long-term treatment solutions at an affordable cost; however, they do require engineers and regulators to be comfortable with how these Alternative Onsite Wastewater Treatment Systems (AOWTS) have been designed and sized by the manufacturers. If they are accustomed to designing, engineering, and incorporating individual components to make up a centralized treatment system, this approach saves time and money that would normally go into the design/engineering phase of the treatment.
Engineers who are embracing these new options are finding they can easily be adapted to various site conditions and that these space-saving and flexible technologies do work well when applied appropriately, installed correctly, and maintained consistently. This flexibility can lend itself whether designing and installing individual AOWTS, several smaller-scale satellite AOWTS, or a single larger AWOTS to serve the needs of treating the wastewater in a development.
Successful nutrient removal
Although not always attributed to onsite septic systems, nitrogen can be a problem in coastal estuaries throughout the United States including Cape Cod, Long Island Sound, New Jersey, Delaware, Chesapeake Bay, and Puget Sound. While tides, currents, or fresh water naturally flush estuaries, nitrogen can accumulate in highly populated coastal areas or those with extensive agricultural runoff when the estuaries don’t flush adequately.
Nitrogen pollution issues have been catalyzing initiatives to develop new ways of treating wastewater at individual sites as well as community and municipal systems. AOWTS have been used successfully to replace or rehabilitate outdated onsite systems such as cesspools and eliminate surface discharge.
Finding less capital-intensive solutions that can extend the life and expand the capacity of existing centralized systems is also a high priority in many communities that have aging or undersized wastewater treatment plants but are open to smart sustainable development. AOWTS are often paired with a range of updated components and new disposal field products such as plastic chambers, geosynthetic aggregate bundles, and large, plastic septic tanks. This increases the opportunity to protect natural resources and public health at a cost that communities can afford.
Collection: STEP systems
Septic Tank Effluent Pump (STEP) collection systems can be a cost-saving alternative for many communities. In these cases, septic tanks, like the versatile yet tough plastic tanks mentioned above, are placed at each home or every couple of homes to retain solids onsite. The tanks have pumps that pressurize a small-diameter collection line that can be buried at a shallow depth, making them perfect for rocky or rolling terrain. These lines deliver settled wastewater to either an existing centralized facility or to single or multiple satellite treatment and disposal facility locations throughout the development. This eliminates the need for additional primary settling at the treatment locations.
Treatment: Secondary and tertiary
Many packaged treatment systems have proven performance in not only obtaining exceptional secondary effluent quality but also accomplishing nutrient removal and disinfection. Just about any treatment system technology available for large-scale plants is now available to develop small “plug and play” systems to meet even the most stringent effluent requirements. Activated sludge systems, trickling/media filters, Integrated Fixed Activated Sludge (IFAS), Moving Bed Bioreactors (MBBRs), and even Membrane Bioreactors (MBRs) have been scaled down for use in some of the toughest applications. A range of applications from schools with high incoming total nitrogen, to restaurants with very high biological oxygen demand (BOD), to even direct-discharge applications can be addressed with decentralized packaged systems.
Dispersal: Using the soil
One diminishing resource is groundwater. In some areas, sinkholes are swallowing up land and homes, but even worse, the elevation of large areas of land such as the San Joaquin River Valley in California fell in recent years due to groundwater depletion. Using soil dispersal as part of the overall design of a decentralized wastewater collection/treatment/distribution plan, groundwater is extracted, consumed, treated onsite, and returned close to its point of origin to recharge the aquifer. This is in sharp contrast to the centralized model, where treated wastewater is discharged to a river where it gets taken miles away from the origination point and is essentially a wasted resource.
STEP system example
Surgoinsville is a low-income community in northeastern Tennessee with a population of 1,800, no public sewer system, and limited economic development opportunities. Existing homes had failing or compromised subsurface sewage disposal systems and the current package plant was at the end of its useful life. With no previous wastewater infrastructure experience, Surgoinsville faced big challenges, including project funding. Grants or loans from the Rural Development Community Program, the Appalachian Regional Commission, the U.S. Environmental Protection Agency, and USDA Rural Development made the $4.5 million project possible.
Designed to serve 700 properties and installed on lots with existing homes, outbuildings, and landscaping, the low-pressure STEP sewer system currently serves 247 residences, two schools, and nine businesses. It delivers wastewater to the neighboring city’s wastewater plant via nine miles of low-pressure sanitary sewer collection lines, two pump stations, and individual STEP services utilizing Infiltrator IM-1060 plastic tanks. An excellent example of successfully incorporating decentralized design concepts into centralized infrastructure, Surgoinsville benefited from the best possible solution to its wastewater treatment challenges at a cost it could manage with the help of funding sources.
In Suffolk County, N.Y., nitrogen from residential septic systems is a significant factor in the degradation of Long Island estuaries. When New York State listed portions of the Long Island Sound, Peconic Estuary, and South Shore Estuary as impaired water bodies, Suffolk County officials began a review to understand the source of the water supply pollution.
Seventy-four percent of Suffolk County utilizes onsite sewage disposal systems. Currently, the drainfield systems in Suffolk County consist of deep leaching pits, concrete rings, or leaching pools that can be installed as deep as 25 feet. A study revealed numerous pollution sources; however, nitrogen pollution from cesspools and septic systems was identified as the primary cause of water quality issues. Impacts include beach closures, toxic algae blooms, and shell fishing restrictions — all detrimental to resident health and tourism revenues.
Sewering was financially unfeasible so the Suffolk County Health Department began a “Reclaim Our Water Initiative” focused on reducing nitrogen by replacing existing septic systems with individual AOWTS. The initiative includes a demonstration program to pilot these advanced treatment technologies. In this public-private partnership, manufacturers donate the systems, including advanced treatment technologies and shallow dispersal drainfields, to the demonstration program. Tested and monitored by the county for six months, if the system meets the county “pass criteria” standard of a 50 percent reduction in nitrogen, it gains Provisional Use Approval. The county also upgraded its sanitary code to allow for the changes needed to incorporate these advanced technologies.
Decentralized systems offer many benefits for small residential or community applications and the best approach varies from case to case. Community leaders and residents need to define the community’s environmental challenges and anticipated growth in making the best wastewater treatment choice. Local codes, proximity to water bodies, and anticipated daily flows are additional key factors for individual, commercial, or community systems. In most cases, the decentralized approach can offer lower cost, high performance, and versatility when compared with a centralized option.
The National Onsite Wastewater Association (NOWRA), the largest wastewater organization in North America, is dedicated to providing resources to members and professionals wishing to incorporate decentralized treatment. Organization members include regulators, educators, engineers, and manufacturers of decentralized wastewater products and systems. NOWRA also has an extensive training program in place that is accessed via national or state affiliate shows. Find out more at www.nowra.org.
Another resource for wastewater engineers and designers is numerous state and university-based demonstration programs and test centers such as the Massachusetts Alternative Septic System Test Center (www.masstc.org) and the North Carolina Soil and Water Environmental Technology Center (http://soils.ces.ncsu.edu/training).
Allison Blodig, REHS, is a senior regulatory specialist at Infiltrator Water Technologies. She has more than 20 years of experience in designing, inspecting, and troubleshooting many types of decentralized systems. She can be contacted at firstname.lastname@example.org.