The Need

Onsite wastewater treatment systems (OWTS) serve approximately 25% of households in the United States, corresponding to over 25 million households (Amador and Loomis, 2018), and are the technology of choice in rural and suburban areas where population density and cost preclude the use of centralized sewer collection and treatment systems. In unsewered watersheds they are the sole means of wastewater treatment, even for non-residential wastewater applications. Onsite wastewater treatment systems are an integral part of the water infrastructure throughout the country and are expected to protect ground and surface waters from inputs of carbon, nutrients, pathogens, and pharmaceutical and personal care compounds. These systems are expected to function under a wide range of environmental conditions with little intervention. Properly-functioning OWTS help protect public health without which ground and surface waters used as drinking water supplies would become contaminated with pathogens, nutrients and other compounds, making them unsuitable for human consumption (Ahmed et al., 2005; O’Reilly et al., 2007; Borchardt et al., 2011; Wallender et al., 2014).

To design OWTS that function effectively under a wide variety of conditions, and meet our performance expectations, we must have a thorough understanding of the processes on which these systems rely to treat wastewater. This is particularly challenging for OWTS, which rely on complex interactions of hydraulic, hydrologic, physical, chemical and biological processes to treat wastewater. Despite their ubiquity and importance as part of the nation’s water infrastructure, our understanding of these processes at work in OWTS lags behind that for centralized sewage treatment systems.

The systematic study of OWTS has evolved considerably over the past half century, leading to improvements in understanding of how contaminant removal takes place within components in the treatment train and the receiving soil. This has led to more effective contaminant removal from changes in system design, improved understanding of the biogeochemical processes that remove contaminants, biomimicry of natural ecological systems, improved selection of soils receiving wastewater, and better placement of systems both within the soil profile and within watersheds to maximize treatment and minimize impact. These improvements have come about, in large measure, from the efforts of scientists, engineers and outreach professionals in Land Grant and private universities across the U.S., funded by federal, state and local agencies, and through collaborations with regulators and private industry. However, few options still exist for regions with challenging soil conditions (e.g. shallow soils on steeply sloping landscapes, soils with shallow depth to a limiting layer or soils with unpredictable water movement, such as regions with high clay content or karst topography), where conventional OWTS designs are difficult to accommodate. More work is needed to identify low-cost solutions for effective decentralized wastewater treatment in rural communities with challenging soil conditions, to improve residents’ quality of life and protect human and environmental health in these regions.

In addition, several new challenges have developed in different parts of the country, such as more stringent nutrient and pathogen reduction regulations, removal of chemicals of emerging concern (CECs; such as pharmaceuticals, personal care products, nanoparticles, flame retardants, etc.) present in wastewater, and high strength commercial wastewater. In addition, a changing climate presents a continental-scale challenge to OWTS. Soil-based wastewater treatment systems are regulated, designed, and built based on assumptions about the volume of wastewater applied, the magnitude and distribution of past precipitation events, the historical range of variations in depth to water table, and soil temperature over the long-term (decades). These assumptions are no longer valid in many parts of the country because of climate change related variability in precipitation, temperature, and weather patterns.

As climate change continues to alter the temporal and spatial patterns of precipitation and temperature, we must expect attendant consequences as sea levels rise and changes in groundwater levels develop. These changes will affect treatment dynamics in OWTS, through changes in soil moisture dynamics, surface and groundwater hydrology, water use patterns and associated changes in wastewater composition and volume within soil-based treatment technologies (Mihaly, 2017). Changes in precipitation (e.g. more frequent and intense events) and long-term gradual sea level and/or groundwater table rise represent poorly understood threats to OWTS. Short-term catastrophic flooding brought on by intense storm or precipitation events may present challenges to proper OWTS function and longevity. The short-term excess water infiltrating from above during floods, and the long-term upward creeping of the water table (from sea level rise or changes in groundwater table elevation) from below the drainfield reduces the unsaturated soil required for adequate wastewater renovation. The treatment performance of different types of OWTS in response to these different types of flood events has been poorly characterized, despite the substantial risks improperly treated wastewater presents to human and environmental health.

In contrast to wet conditions in some regions, climate change may produce even drier conditions in arid regions.  This is likely to produce a more concentrated wastewater as more stringent water conservation measures are used to save scarce potable water, placing a greater burden on soil treatment areas to effectively renovate wastewater.  New innovative OWTS designs will need to be developed in these regions to reach treatment standards that enable wastewater reuse and recycling to save precious potable supplies and still be protective of public and environmental health.

Regulatory decision-makers that set codes and policies and other stakeholders need to understand the consequences of these changes and the options available to mitigate, adapt, and plan for climate change and its effects on OWTS. As in the past, they rely on scientists, engineers and outreach professionals to carry out the research and provide them the necessary information in an effective and timely manner.

Importance of the Work

Scientists and engineers have developed a reasonable understanding of several of the processes that underlie the functioning of soil-based wastewater treatment over the past five decades, with work conducted recently by NE 1045 and NE 1545 members helping to provide a deeper understanding. This understanding continues to develop, as does our understanding of the challenges that climate change and its attendant consequences (e.g. flooding) poses to OWTS function and longevity. We must also gain a better understanding of how to help rural communities with challenging soil conditions to get access to cost-effective and reliable wastewater treatment to safeguard public health and environmental quality.

As a “green technology” (Lindbo, 2015), OWTS are relied upon by a large majority of rural and suburban populations to help protect public health and sensitive ecosystems. Onsite wastewater treatment systems rely on an intricate set of hydrologic, biogeochemical and physical processes to renovate wastewater. These are controlled, directly and indirectly, by soil type, precipitation, temperature, and depth to groundwater. Climate change will affect the treatment capacity of OWTS as a result of changes in precipitation patterns as well as soil moisture dynamics and depth to saturation, during both short and long-term fluctuations in water table levels, compounded by the effects of sea level rise in coastal areas. Major and minor flooding events are occurring both in inland (e.g. Mississippi River and tributaries watersheds) and in coastal regions with increasing frequency, with potentially serious consequences for OWTS function during and after these events. Although these factors are likely to vary in magnitude geographically, climate change is expected to affect most areas of the nation, making the proposed research efforts applicable to a large proportion of the US population.

We are already familiar with the effects of failed or poorly functioning OWTS from previous experiences, contamination of ground and surface waters with human pathogens leading to the spread of enteric diseases (McKenna et al., 2017), as well as increased inputs of N and P to aquatic ecosystems (Chen, 1988; Fisher et al., 2016; Lapointe et al., 2017), resulting in eutrophication, anoxia and ecosystem collapse. In most cases, incidents of malfunctioning OWTS have had a modest impact on public health and ecosystem functioning, because they are generally limited in geographic scope to a relatively few systems. As climate change affects larger populations at regional and continental scales, the number of malfunctioning systems, and the magnitude of their impact, is expected to be more widespread in scope, affecting a much larger portion of the population, as well as regional aquifers and surface drinking water systems. Ecosystem effects are expected to linger for decades, particularly for nutrients like P, for which removal pathways are physical, and for N, which relies on microbial processes, with severe constraints, for removal from ecosystems.

Traditional OWTS designs that rely on deeply placed soil treatment areas are unsuitable in certain landscapes, including near-shore coastal regions with shallow water tables (e.g. the eastern seaboard of the US) or low relief coastal plain areas with shallow water tables, soils with shallow bedrock, as well as in regions where soil parent material, depth, texture and structure prevent water from moving through the soil in a timely manner (e.g. the Piedmont regions of North Carolina and Georgia in the US). Some economically disadvantaged regions of the US still lack adequate onsite wastewater treatment systems (e.g. Appalachia (Hughes et al., 2005; Cook et al., 2015; Arcipowski et al., 2017)), resulting in the rise of preventable enteric diseases (e.g. rural Alabama (McKenna et al., 2017)). Rural areas with challenging soil conditions would benefit from cost-effective innovative ecological wastewater treatment designs that mimic natural systems, maximize wastewater treatment, and protect public and environmental health.

In Coastal areas of the United States there are various additional changes to soil systems as mean sea level is projected to rise at an accelerated rate through time and greater than 30 cm by 2100 (Pierfelice et al., 2017). Rises in mean sea level coupled with increased drought will likely allow for extensive saltwater intrusion from the ocean up estuaries and into current upland areas. Changes in soil salinity alter fundamental soil biogeochemical processes, which will in turn impact the functionality of OWTS. For example, salinity is harsh on many terrestrial microorganisms and biogeochemical treatment of wastewater effluent may slow as more salt-tolerant organisms replace less tolerant ones (Pierfelice, 2013). Cycling of nutrients will likely change due to saltwater stress and reduction of porewater sulfate into sulfides will likely become an alternate pathway for anaerobic microbial respiration. Research in coastal soils undergoing salinization has shown greater P mineralization and turnover, short-term desorption of exchangeable ammonium, and reduction in long-term net nitrification rates (Noe et al., 2013). Thus, OWTS in coastal areas are the most likely systems to be adversely impacted by climate change and sea level rise in the next 100 years.

Because drinking water sources for large urban areas are often found in rural landscapes where OWTS are common, the impact will not be limited to rural areas alone. This is particularly true with respect to increased inputs of organic C and nutrients to surface and groundwater reservoirs from OWTS which can interfere with water disinfection processes and result in the production of trihalomethanes, known human carcinogens. In addition, as CECs become pervasive in drinking water resources, more research is needed to determine what soil conditions within the soil treatment area help remove these contaminants and help mitigate their impacts.

The Technical Feasibility of the Research

The proposed research is technically feasible. The tools and techniques necessary to carry out the research have been developed, and include traditional water quality measurements (e.g. 5-day Biochemical Oxygen Demand (BOD₅), coliform enumeration, nitrogen and phosphorus concentrations), and measurements of hydropedological properties of soils, which can be combined with cutting-edge approaches from a variety of science and engineering fields to understand and model the fate of contaminants from wastewater. Combining traditional approaches to assessing water quality and soil properties with 21^st century techniques, such as advanced computer modeling of hydrological and biogeochemical processes, identifying nutrient transformations, and applying molecular genetic tools to identify the microorganisms responsible for both water quality degradation and improvement can lead to important insights and implications for improving wastewater treatment. The research efforts of biological and environmental engineers, pedologists, hydrologists, soil physicists, soil microbiologist, computer modelers, environmental and public health scientists, and extension and outreach professionals at state and private universities have led to a better understanding of the challenges presented by climate change and the potential solutions.

Advantages for Doing the Work as a Multistate Effort

Addressing the research and outreach challenges presented by a changing climate requires the expertise of a broad spectrum of scientists, engineers and outreach professionals. Developing novel cost-effective approaches to wastewater treatment in areas with challenging soil conditions also necessitates collaboration among these researchers and outreach personnel from several different disciplines. The necessary breadth of expertise and perspectives is already found in select Land Grant and private universities across the US. Researchers are currently working on various aspects of the science and engineering of OWTS and the wastewater-related challenges many communities face today. Collaboration among professionals with different areas of expertise leads to better insights, experimental designs, analyses and overall better outcomes. In some instances, the results of this research are broadly applicable, as is the case with studies focusing on fundamental processes. In other instances, the research focuses on addressing problems that are local or regional in nature as a result of unique geological, geographical or regulatory issues (e.g. coastal zones subject to sea level rise or areas with challenging soil conditions). Similarly, outreach professionals develop materials that are tailored to national, regional and local issues, depending on the circumstances. In order to carry out and disseminate research that is responsive to the broad range of problems and constituencies in the US, the proposed work needs to be a comprehensive and collaborative effort done at a multistate level.

The Likely Impacts from Successfully Completing the Work

We anticipate that completion of the proposed work will lead to evidence-based solutions to OWTS problems associated with ecological design of OWTS that addresses the challenges of climate change and challenging soil conditions. Specifically, we expect that results from examination of fundamental physical, chemical and biological processes – and their response to changes in hydrologic regime – will, in combination with studies focusing on local and regional aspects of OWTS function and design, provide solutions to the challenges of wastewater treatment in light of a changing climate and regional soil constraints at appropriate spatial and temporal scales. Communities served by OWTS across the US and abroad impacted by flooding will benefit from the proposed research efforts to understand the impacts to OWTS during and after flood events – from both short-term precipitation-induced floods and chronic changes in sea level and groundwater tables. Developing innovative, cost-effective wastewater treatment options for areas with challenging soils will improve the quality of life of rural residents by protecting public health, as well as the health of water resources in their communities and farther downstream. Understanding the removal mechanisms for CECs in STAs located in different soil types and landscape positions will help local, state, and national decision makers make informed policy and regulation revisions to protect at risk water resources.  This will be come even more important as the areas become more impacted by climate change and sea level rise.  Furthermore, we will share these solutions among OWTS outreach professionals and make them available to clientele and stakeholders so that policy and management decisions can be developed to optimize the use of decentralized wastewater treatment to remediate or avert large-scale public health and ecosystem crises.