Septic systems
Septic systems are on-site wastewater treatment systems commonly used in rural areas of the United States to manage domestic sewage and wastewater. They typically consist of a septic tank where wastewater undergoes initial treatment through anaerobic digestion, allowing solids to settle and form sludge. The partially treated effluent then flows into a distribution box and is distributed through gravel-filled ditches known as lateral fields, where it percolates through the soil, further treating the effluent via natural biological processes. While effective in isolated settings, septic systems can pose environmental risks, particularly when not properly sited or maintained. Concerns arise from the potential inadequacy in pathogen reduction, especially from black water, which contains high levels of nutrients and pathogens. As rural development increases, the concentration of septic systems can overwhelm the soil’s capacity to filter wastewater, risking contamination of groundwater and surface water. To mitigate these issues, many states now promote alternative on-site wastewater treatment systems, such as mound systems and constructed wetlands, particularly in areas unsuitable for conventional septic systems. Properly locating and maintaining septic systems is crucial to ensuring public health and environmental safety.
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Septic systems
DEFINITION: Systems for the on-site treatment of sewage and wastewater
Although septic systems work well in isolation and in the appropriate soil environments, increased development in rural areas can cause environmental problems from improperly sited or inadequately separated septic systems.
Septic systems are the most common form of on-site treatment used in the United States. The conventional is a model of simplicity and economy. It consists of a large chamber—the septic tank—into which domestic wastewater flows and where of the occurs. Nondegradable or slowly degradable compounds fall to the bottom of the septic tank, where they accumulate as sludge. The effluent then flows by gravity to a distribution box, where it is distributed by pipes through one or more gravel-filled ditches buried in the soil. These ditches constitute a lateral field where wastes slowly percolate through the gravel and underlying soil. During leaching, pathogenic organisms and residual matter are consumed. The percolating water eventually finds its way to underlying groundwater. The number of American households that use septic system has been declining for decades. According to some studies, about 26 million households—or about 25 percent of US households—used septic systems in the early 2020s.
One of the assumptions underlying the use of septic systems is that there will be some reduction as the wastes are anaerobically digested and as they pass through the lateral field. This is normally the case. Anaerobic digestion reduces the pathogens in waste by about 10 percent, and up to a 99 percent reduction occurs as the pathogens pass through the biological mat that forms at the bottom of the lateral field. The biological mat is a rich collection of bacteria, fungi, and protozoa that consume harmful and viruses in domestic waste.
A second key assumption underlying the use of septic systems is that such systems will be properly sited and maintained. The soil is generally an effective biological filter, so most septic systems are designed to have 45 to 60 centimeters (18 to 24 inches) of permeable soil beneath the lateral field before wastewater encounters and is diluted. Likewise, the nondegradable materials in must periodically be removed and either disposed of in a landfill or further processed in a municipal wastewater treatment system. If the sludge is not removed from the septic tank, it will eventually accumulate to such an extent that it will reduce the wastewater residence time in the tank. This causes inadequately treated waste and some solids to flow into the lateral field, which is subsequently unable to ensure proper wastewater treatment. The solids may also clog the leach field and cause wastewater to pond.
The dominant environmental problems associated with septic systems are the failure of some systems to reduce pathogens adequately and the systems’ potential delivery of nutrients into groundwater. Domestic wastewater consists of gray water—water from showers, sinks, dishwashers, and clothes washers—and black water, which comes from toilets. Gray water is relatively free of nutrients and pathogens. Black water, in contrast, is the major source of environmental concerns regarding septic systems because it is both nutrient-rich and laden with pathogens that are potentially dangerous to humans. Because black water is commingled with gray water in most septic systems, all of the water leaving a house must be treated thoroughly.
In isolation, septic systems work well if properly sited and maintained because there is usually adequate distance between the domestic wastewater source and receiving waters. However, as development finds its way to rural environments that are not served by either treatment or municipal water facilities, the congregation of septic systems in limited areas can overwhelm the environment’s capacity to filter and dilute wastewater effectively before it reaches drinking-water wells or recreational water supplies.
Properly locating septic systems is critical, and home sites may not always be located on soils that are suitable for conventional septic systems. The soils may be slowly permeable, the topography of the land may be too steep to permit an adequate lateral field, or there may be too little soil beneath the lateral field to allow proper before impermeable layers and groundwater are reached. The consequence of these limitations to conventional septic systems is that soluble nutrients such as ammonium and phosphate, which come from the digestion of domestic wastes, may increasingly appear in underlying groundwater or shallow and lead to eutrophication. Protozoan pathogens such as Cryptosporidium and Giardia may be inadequately treated or filtered. The potential for contamination by bacterial pathogens such as salmonella and viral pathogens such as hepatitis A, B, and C is a greater concern because their escape from septic systems into drinking-water or recreational water supplies represents a serious public health crisis.
As a result of these concerns regarding conventional septic systems, many U.S. states now require alternative forms of on-site wastewater treatment if either the soil or the of habitation does not allow for the proper use of conventional septic systems. These alternative treatments include wetlands, in which domestic waste from a first passes through a bed of vegetation before entering a lateral field. Aerobic digestion occurs in the wetland, and soluble nutrients are taken up by the wetland plants. Mound systems are often used where shallow soils exist. In a mound system, a raised bed of soil is constructed to which domestic waste effluent is pumped and then allowed to percolate. A variety of other on-site systems use through sand or peat filters to help and reduce the impact of the nutrients and pathogens in domestic waste.
Bibliography
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