Web Exclusive Column: Dilution is Not the Solution

March 26, 2020

Decentralized wastewater treatment solves challenges for rural & growing communities

About the author:

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


One in every three homes in the U.S. is served by an individual onsite septic system. Community treatment needs, design challenges and regulations can vary greatly whether searching for the best solution for individual septic systems or larger community systems. In both cases, long-term performance at an affordable cost is most often the desired outcome.

In the past, the solution to pollution was dilution. Everything that was considered waste or garbage went into the water. While most of that waste was organic, such as human and animal wastes, as villages grew and continued to dump that waste into rivers and bodies of water they were transformed into open sewers. During the Industrial Revolution, with the ongoing belief that the dilution strategy worked, non-organic chemicals and pharmaceuticals that did not break down naturally were added to the mix. It was much later that health officials realized that dilution of these bio-accumulating chemicals and pharmaceuticals was not working and that these pollutants were actually mimicking hormones and interfering with bird and fish reproduction.

Today, extensive cleanups are still in progress to reverse the faulty pollution to dilution solution and innovation regarding effective methods of wastewater treatment of varying strengths is top of mind for communities.

Decentralized Strategies for Communities Expand

From small flow systems in rural areas to decentralized treatment facilities that serve entire communities, advances in products, regulations, and treatment and disposal strategies have expanded. A decentralized wastewater treatment approach is increasingly at the forefront of the options available; even at design capacities of more than 1 million gallons per day.

In many communities plagued with nitrogen loading issues, contaminated waterways, or proximity to sensitive environments, advanced decentralized wastewater treatment strategies enable exemplary treatment often in a lesser footprint and with aquifer recharge close to the point of origin. In the case of coastal communities, these systems also combat saltwater intrusion, a growing environmental and water quality challenge.

The best solution for any individual or community wastewater treatment system results from assessing the environmental, financial, and community development challenges, evaluating all the design/product options, and considering the plan for operation and maintenance.

What to Consider When Choosing a Community Wastewater Treatment System:

Communities should ask the following questions prior to making any decision on the best approach for them:

What Wastewater Treatment Challenges Currently Exist?

Communities should assess the shortcomings of the existing wastewater treatment system and future community plans. An over-capacity wastewater treatment plant, leaking sewers, sewage overflows, watershed proximity issues, groundwater pollution including saltwater intrusion for coastal areas, and regulatory noncompliance of individual residential systems are all factors that impact the decision on the best solution for the future. Funding available for existing system rehabilitation or a new community approach can be the most critical determining factor when evaluating choices. Educating residents about the options and costs of proposed solutions is key to moving forward.

What Are the Projections for Community Growth?

And how do they impact anticipated wastewater treatment needs? Each community has a unique set of circumstances and needs. While centralized sewers offer the possibility for large-scale commercial and residential growth, many communities are more focused on retaining historic and community character. Others simply do not have the funds for large infrastructure improvements. A decentralized treatment approach can enable a community to focus on treating the areas of most critical need or risk currently, thereby taking a phased approach to upgrading treatment community-wide. In this scenario, smaller design flows, smaller disposal areas, and lower costs are possible. Additionally, the financial burden is placed on those at-risk properties.

What Treatment Options are Available?

The most common choices are decentralized, centralized, or a combination of the two. An important step toward making the best choice without being bias-directed toward only one of the options is to hire a consultant specialized in community treatment solutions.

What are the True Costs?

Decentralized treatment can offer many cost advantages. Short-term costs to consider for any new approach include design, land acquisition, permitting, legal and construction. Long-term costs include operations and management, licensed operators, billing structure, district vehicles (specialized trucks) and specialized equipment.

Advanced Decentralized Treatment Options

Advanced system designs, modular products such as chambers and geosynthetic aggregate bundles, and lightweight, high-strength plastic septic tanks enable individuals and communities to effectively protect resources and public health. Additionally, Advanced Wastewater Treatment Units (ATUs) such as Fixed Film Bioreactors (FFBR) can be effective where poor soils, shallow vertical separation distances to limiting conditions, horizontal setback restrictions, and tight lots are a challenge. ATUs also provide effective treatment when high strength wastewater and locations in environmentally-sensitive areas are a challenge. Additionally, ATUs provide higher levels of treatment where nitrogen removal is critical to the treatment plan.

Communities without public sewer systems or adequate onsite septic systems can also pursue community systems to serve a neighborhood. These systems can often connect with a neighboring community wastewater treatment plant by employing a Septic Tank Effluent Pump (STEP) system approach.

Rural Areas Benefit From Decentralized Applications

In rural locations where a centralized wastewater treatment plant is not available, decentralized treatment is often the only solution to protect public health. With a host of options to choose from, most environmental, system sizing, depth to water table, or difficult access challenges can be addressed by decentralized treatment. The shift to a decentralized approach is well underway even in areas where decentralized treatment has not previously been the choice of area regulators.

Aquifer Recharge is Key

Because decentralized wastewater treatment systems return water to the aquifer thereby replenishing groundwater supplies, they are preferred in remote desert or conservation areas. In these situations, they provide exceptional treatment and preserve the sensitive environmental balance.