Context

Context and Motivation for Decentralized Water and Sanitation

Water and sanitation can underpin a healthy society when solutions are effective in protecting public health and preserving environmental quality while being affordable, socially acceptable, and sustainable. In the United States water and sanitation infrastructure evolved during the 20th Century in response to a growing recognition that providing safe drinking water and adequate treatment of wastewaters were needed to protect public health and preserve water quality. During this evolution there was always a mix of onsite systems serving individual homes and businesses in rural and peri-urban areas, decentralized systems serving suburban residential and mixed-use developments, and larger centralized systems serving densely populated urban areas. However, the relative proportion of the population and development served by different types of infrastructure has varied and evolved over time.

During much of the 20th Century some viewed onsite and decentralized wastewater systems as a means of providing temporary service until sewers and a centralized treatment plant became available to provide permanent service. Early versions of onsite systems (e.g., pit privy, cesspool) were often designed with simple and short-term goals of human waste disposal to prevent human exposure to infectious waste materials and to achieve basic public health and environmental protection. As water-using fixtures and appliances became commonplace, onsite system designs evolved to include raw wastewater treatment through solids separation and anaerobic digestion in a tank-based unit (e.g., a septic tank) followed by effluent disposal to the land (e.g., a soil drainfield). Continuing to evolve, onsite and decentralized systems were increasingly designed and implemented to achieve purification as well as disposal, and even considered for beneficial water reuse. But many designers and regulatory officials continued to view onsite and decentralized systems as inherently deficient compared to centralized systems. As a result, during the latter half of the 20th Century there were major Federal and State programs that provided funding for construction of wastewater collection systems and centralized treatment plants. But the push to expand service areas and increase the percentage of the population connected to centralized wastewater systems eventually faded for a number of reasons. The construction grants program that provided funding for centralization ended plus there was a growing realization that large centralized systems were not appropriate for all rural and many suburban and peri-urban areas and there were growing concerns about the sustainability of large infrastructure. By the end of the 20th Century, about 25% of the United States population was served by onsite and decentralized wastewater systems and approximately one-third of new development was being supported by such systems. This amounted to roughly 25 million existing systems with about 200,000 new systems being installed each year.

Near the end of the 20th Century and into the 21st Century a series of activities and events in the United States helped catalyze a reevaluation of how water and wastewater infrastructure could be made more sustainable. There was growing interest in how onsite and decentralized systems could help provide more sustainable infrastructure by:

Reducing the use of drinking water to flush toilets and transport waste to remote wastewater treatment plants.
Preventing pollutant discharges from large centralized systems including sanitary sewer overflows, combined sewer overflows, and leaking sewers.
Recharging water near the point of water extraction and avoiding water export and depletion of local water resources.
Enabling recovery and reuse of wastewater resources including water, organic matter, nutrients and energy.
Lowering consumption of energy and chemicals, and reducing greenhouse gas emissions.
Providing infrastructure that is more robust and resilient to natural disasters and climate change.

During this period that was also a growing recognition that the capabilities of 21st Century onsite and decentralized systems should not be judged based on the performance of older 20th Century systems. The early versions of onsite and decentralized systems (e.g., cesspools, seepage pits, leachfields, septic systems) were designed to be simple and cheap but not to achieve long-term treatment or reuse goals. During the latter decades of the 20th Century, increased water use and wastewater generation and more widespread use of disposal-based systems in a growing suburban America, led to hydraulic malfunctions, groundwater contamination, and surface water quality deterioration. As a result, these older disposal-based systems became known as “legacy systems”. In contrast to the legacy systems of years past, modern systems can be designed for effective treatment as well as water reclamation and resource recovery.

Based on major research and development efforts over the past two decades or more, 21st Century onsite and decentralized systems (hereafter referred to as decentralized systems) have evolved to include a growing array of approaches, devices and technologies that can achieve wastewater treatment and enable resource conservation and reuse (Figs. 1 and 2). Ultra efficient fixtures and source separation plumbing can enhance water infrastructure by minimizing water and energy demands and enabling reuse. Wastewater treatment can be achieved using engineered reactor-based unit operations (e.g., aerobic bioreactors, porous media biofilters, membrane bioreactors) or engineered natural system unit operations (e.g., constructed wetlands, subsurface soil infiltration, landscape dispersal). Nutrient reduction strategies and technologies can remove and, in some cases, recover nitrogen and phosphorus. Reuse of reclaimed water can occur through garden and landscape irrigation, toilet flushing, and other functions. Sensors and monitoring devices can be used to verify performance and enable remote process control to monitor and automatically correct a system malfunction. Systems can mimic natural processes to achieve performance objectives while minimizing water, energy and chemical use, and enabling beneficial reuse.

Today, decentralized infrastructure involving wastewater treatment and water reclamation can be used to serve buildings and developments with design flows of less than 100 to 100,000 gal/d or more. Common and emerging applications within the United States and similar industrialized countries include approaches, technologies and systems that are deployed for one or more of the following purposes:

To provide effective wastewater treatment for homes and businesses in rural and peri-urban areas and residential, commercial and mixed-use developments in suburban areas.
To provide effective wastewater treatment for buildings and developments while also producing a reclaimed water for nonpotable reuse purposes such as toilet flushing, cooling, or irrigation.
To recover valuable wastewater resources including nutrients, organic matter, and energy.
To earn points for a green building or sustainability rating through the low impact water and wastewater management options enabled by decentralized systems.
To provide appropriate treatment and recovery of stormwater runoff in suburban and urbanized areas.

Decentralized systems are also critical to providing safe drinking water and adequate sanitation in developing regions of the world. In developing regions worldwide concerns about sustainability of large water and wastewater infrastructure are not yet paramount. Rather, concerns are still focused on how best to provide solutions for safe drinking water and effective sanitation – solutions that are effective, affordable and socially acceptable.