On-site sewage disposal is a necessity in areas that are not provided with city sewers. Even when a site has access to a sewer, it may be desirable to treat sewage on site in order to return water to the local soil, or to provide irrigation for non-food plants. Cesspools, which combine aerobic and anaerobic treatment in the same chamber and leach directly into the soil, may still be allowed in some rural areas, and can give decades of good service. However, in the vast majority of cases, a concrete or plastic septic tank is required in which anaerobic bacteria provides primary treatment. The effluent from the septic tank discharges either directly to a leach field, or indirectly through an aerobic treatment unit and then to a leach field.
Understanding the reason for using a secondary treatment unit or leach field will help avoid accruing unnecessary expense, joining incompatible components, or using the wrong leaching technique for the particular site conditions.
Biochemical oxygen demand, or BOD is a measure of organic material in wastewater. Oxygen is required to break down large organic molecules into smaller molecules and eventually into carbon dioxide and water. Sewage high in BOD can deplete oxygen in receiving waters, causing fish kills and ecosystem changes.
Primary sewage treatment in a septic tank is anaerobic (without oxygen) because any oxygen in the tank is rapidly consumed. While some BOD is removed in the septic tank by anaerobic digestion and settlement, most flows to the leach field. This BOD supports the growth of the microbial biomat that forms at the floor of the leach field trench or bed (or, as claimed by some manufacturers, on the surface of media). A "healthy" (aerobic) biomat contains a variety of micro-organisms that remove many of the undesirable bacteria and viruses in the sewage and that digest most of the remaining BOD. If the effluent BOD is too high (or if the leach field is poorly aerated), desirable aerobic bacteria and protozoans in the biomat die, replaced by anaerobic bacteria producing a mucilaginous coating that clogs the leach field.
Secondary sewage treatment is designed to remove BOD. This is important where sewage effluent flows to a leach field in tight soils (silts and clays). Reducing BOD reduces the biomat and enables the effluent better to infiltrate tight soils. Most enhanced treatment units actively oxygenate the sewage to reduce BOD.
Nitrates are an undesirable byproduct of sewage. Organic nitrogen excreted by occupants is converted to ammonia in the septic tank. Aerobic bacteria in the leach field then convert ammonia to nitrates. Nitrates can have serious health effects if consumed in drinking water, and has deleterious effects on the environment, especially in coastal areas where an excess of nitrates can eutrophy bodies of water. To eliminate nitrates, they must be created aerobically within the secondary treatment unit, then processed anaerobically within the unit into harmless gaseous nitrogen. The anaerobic bacteria use wastewater BOD as a food source, and will die unless sufficient BOD is present. Thus, de-nitrifying systems do not work well in seasonal homes.
Total Suspended Solids or TSS (both organic and inorganic) clog the small pore spaces between soil grains in the leach field if not removed from the effluent. Filters are available to prevent floating matter and fine particles from leaving the septic tank. Many secondary treatement systems also reduce TSS. Fine suspended solids can also clog up the media used in some innovative systems.
Phosphorus in wastewater, unless phosphate-containing detergents are restricted, are typically half from detergents and half from human waste. An unknown amount of phosphorus may be removed in a conventional septic systems by chemical precipitation, especially in the presence of abundant iron. No alternative technologies remove significant amounts of phosphorus, although research is proceeding. One study of pressurized dosing showed a significant phosphorus reduction. Good aeration and good height above groundwater help reduce phosphorus discharge.
All septic systems should be designed by a licensed, experienced designer who is familiar with the various alternative systems and components available in the local area. There are a number of requirements in the design of septic systems, and they experience a number of problems. The secondary treatment equipment and innovative drain field designs described in the Technology Inventory should be used to address these requirements or to overcome one or more of the problems. The following table outlines concerns with conventional systems that are addressed by various alternative technologies:
Reduce Size: There are many reasons why a smaller system might be desirable:
- A conventional leach field may prove inadequate to handle the increased sewage from an addition that increases the number of bedrooms, requiring a leach field with greater capacity for a unit of area.
- It may not be possible to fit a conventional leach field on the site.
- Many jurisdictions require that there be a second area where a (typically, conventional) replacement field can be built if the first one fails. This increases the area needed, leading to designs that reduce field size.
Avoid early discharge:
- Conventional fields fail because water preferentially exits from the earliest holes in the system, causing a buildup of suspended solids that precipitate out, and the accumulation of bacterial growth near the entrance ("bio-mat"), both of which can clog up the piping and field (although cleanouts allow some amount of clearing of obstructions within the distribution piping).
Avoid Stone Aggregate: Stone aggregate may cause several problems:
- Equipment needed to move stone requires clearance to operate and may interfere with existing plantings.
- Heavy equipment and the gravel itself can compact the soil.
- Stone can be coated with fines, leading to clogging the leaching surface.
- Some researchers claim that the stones effectively "mask" a significant portion of the floor of the leach field from access by leachate.
- Conventional systems do not remove much nitrogen from the effluent. Reducing nitrogen loading is required in environmentally sensitive areas.
- From a sustainable viewpoint, a large amount of potentially useful irrigation water is wasted when water flows through an underground system solely by gravity. Sprayed effluent, sub-surface drip and evapotranspiration systems address this problem.
- Three feet of suitable soil must occur between the bottom of the leach pipes and groundwater or an impervious layer of soil (clay) or rock. Another two feet of soil are needed to create a gravel bed for the pipes and provide 18" of cover. An alternative design is needed for the many sites where these criteria cannot be met.
- There are strict limits to the amount of slope in the field and to how close terraced beds can be to each other, enlarging the size of leach fields located in sloping ground. Some innovative systems are better suited for sloping sites
- In extremely pervious soils, effluent may not be treated sufficiently before it joins the underground water table, potentially contaminating nearby wells.
- Some leach field alternative systems make up for the higher unit cost of the product by reducing the size of the field.
- Secondary treatment systems along with, pressurized, dosing, drip, and sprayed effluent distribution all involve added maintenance because of pumps and filters.
Leach field longevity is related both to its size, the larger the better, and to the quality of effluent, the cleaner the better. Over time, surfaces within the field pipes and media may become clogged. Some innovative leach field systems have not been in service long enough to determine whether they can, with little or no risk, be downsized without reducing longevity. On the other hand, conventional gravity-fed fields are not terribly efficient, and new technologies offer the potential for extending the life of the field.
Primary and Secondary Treatment
In nearly all septic systems, sewage flows by gravity through a watertight septic tank, which performs primary treatment. In this phase, inorganic solids settle out into the bottom of the tank, floating solids (oil and grease) rise to the top, and organic material is partially consumed by anaerobic bacteria - those that live on a sulfur-based energy economy, contrasting with oxygen-based aerobic bacteria. If the tank is higher than the house sewer outlet, the house sewage must be pumped upward to flow by gravity into the septic tank. The septic tank requires periodic pumping to remove accumulated solids at the bottom of the tank (sludge) and accumulated floating material at the top (scum). When these become thick enough to interfere with water flow through baffles, the tank will not operate properly. Tanks can be one or two-compartment. A backup generator may be advisable for systems incorporating one or more pumps at sites that experience frequent or prolonged power outages. A filter can be installed to remove suspended particles from the leachate.
In secondary treatment, the effluent from the septic tank is treated before it is discharged into a leach field (some secondary treatment units also perform primary treatment). There are three types described elsewhere in the Technology Inventory
- Recirculating Media Filters
- Constructed Wetlands
- Aerobic Wastewater Units
Depending upon the type chosen, secondary treatment components can accomplish one or more of the following:
- Reduce the size of the leach field (or eliminate the need for one)
- Reduce the depth of soil needed in the leach field
- Allow leaching into formerly unsuitable soils such as clay
- Increase the amount of nitrogen removed from the effluent
- Rehabilitate an existing leach field that is clogged with bio-mat
Secondary treatment units add cost and maintenance, and should be selected based on the particular problem or limitation that needs to be overcome. For example, on a tight site, it would not make sense to opt for a constructed wetland, which requires a considerable area and still needs a leach field. Secondary treatment systems typically require a continued source of waste water to maintain a community of aerobic bacteria, and may not be applicable to vacation homes. In some cases, the leachate is pure enough that a sand leach field, called a "polishing field," can be located under the secondary treatment unit.
Conventional Leach Field
In a conventional system, effluent flows by gravity (or is pumped) to the high point of a leach field. Typically 4" diameter perforated PVC pipe is laid level over 6" of uniform round stones (variously called "gravel", "aggregate", or "drainrock") which also surround the sides of the pipe. Filter fabric is placed over the pipe and gravel to prevent silting, and the array is covered with soil. The stone has four functions:
- Storage: stores effluent that builds up during a surge in load, and stores effluent when the soil is saturated from rain, until it can be released;
- Structural: holds the sides of the trench from collapsing, supports the soil above, and supports the drain piping off the floor of the trench or bed;
- Distributive: distributes effluent along the trench, mitigating its tendency to exit from the first holes in the distribution piping;
- Aerating: the spaces between the stones provide needed air to the aerobic organisms on the floor of the field.
While stone aggregate is not ideal, it is relatively inexpensive, readily available, and familiar.
The field removes most pathogens, organic matter and suspended solids by a combination of physical filtration, biological reduction of organic material by aerobic bacteria, and ion bonding to clay particles, all of which takes place in the soil under the stones.
A conventional leach field consists of a series of properly separated parallel trenches, each typically 2 to 3 feet wide; or a bed not more than 15' wide of more closely spaced pipes. A "contour system" is a single long trench that runs with the line of natural contour. Trenches over 150 feet long need to be pressurized with pumped effluent. The size of the field is determined by the slope of the land, the type of soil, the number of bedrooms, the highest level of the water table, and the distance to an impervious soil layer. Some jurisdictions allow a reduction in field size if the home is equipped with water-saving fixtures. This reduction typically may not be combined with any further reduction through the use of an innovative leach field. Typically, 3 feet of undisturbed, dry soil is necessary under the leach field. For more information, see the Wisconsin Department of Commerce publication at www.wra.org/pdf/government/landuse/Onsite_System_Descriptions.pdf.
Slowly permeable soils, shallow permeable soils over a limiting layer (clay or rock), or permeable soils with high water tables, can accept leachate by the use of an elevated soil absorption bed called a mound. Mounds require more care than conventional systems in site selection, design, and construction. This is partly because the soil and site characteristics are marginal, special sand is required, and contractors are apt to be less experienced with mound construction techniques. Proper location and soil preparation are essential for a properly functioning mound. Excellent detailed instructions for the construction of a mound field are found the following website: www.agcom.purdue.edu/AgCom/Pubs/ID/ID-163.html, which is the source of the diagram; from the City of Austin at www.ci.austin.tx.us/wri/dis7.htm; the Texas Cooperative Extension document L-5414, available at tcebookstore.org/pubbrowse.cfm?catid=115; and the Wisconsin Department of Commerce at www.wra.org/pdf/government/landuse/Onsite_System_Descriptions.pdf
Mounds should be as long and narrow as the site permits, on either flat or sloping topography. A long, narrow mound will minimize the "mounding" of the groundwater table under the absorption bed. Treatment is further enhanced by using a dosing pressure distribution system (see below). 7 to 8 inches of soil under the bed is plowed or roughened to enhance absorption, and the calculated depth of sand is added. Monitoring wells are installed to check conditions within the bed, at its edges and below the field. Distribution piping is laid on 6" of aggregate and covered with 2" of the same material, covered with filter fabric and soil, laid at a maximum slope of 1 in 4. Because of their higher cost, mounds are installed only on sites where conventional absorption systems are not suitable.
This technology is fully discussed in the Technology Inventory: Pressurized Leach Field Dosing. In this technique, effluent is delivered to the leach field in small or large doses (typically but not necessarily distributed through small piping under pressure). Pressurized dosing has shown two benefits. When distributed under pressure, leachate exits evenly from all the distribution piping within the field rather than in a short length of piping near the distribution box itself, reducing clogging and bio-mat overgrowth, and eliminating soil saturation near the entries. Also, the "rest period" between doses enhances soil aeration and hence the effectiveness of pathogen and nutrient removal. This technique requires a pump, which is typically located in a separate chamber, where effluent can accumulate until a standard dose is collected. A holding tank varying in size from 30 to 60 gallons is pumped out whenever it fills, emptying 6 to 12 times a day.
Substitute Aggregate Leach Field
These technologies are fully described in the Technology Inventory: Substitute Aggregate Leach Field. All of these drain field technologies replace gravel by some other material that provides the same (or additional) functions as gravel. Proprietary systems include one in which the distribution pipe is surrounded by plastic beads held in place by a netting; and one with a module made of filter fabric separated by plastic spacers that the manufacturer claims provides additional surface for bacteria to grow on. On the horizon are the use of crushed glass (cullet) or rubber tire fragments in place of stone in conventional or mound systems.
Plastic Chamber Leach Field
This technology is fully described in the Technology Inventory: Plastic Chamber Leach Fields. Plastic chamber systems use formed plastic arches of various sizes and shapes to form voids through which effluent flows, replacing the voids within the stone aggregate. In some jurisdictions, some of these systems required smaller areas relative to a conventional field.
Gravel-Less Pipe Leach Field
This technology is fully described in the Technology Inventory: Gravel-Less Pipe Leach Fields. These systems employ corrugated HDPE plastic piping to distribute the effluent. Either single large (8" to 12" diameter) pipes covered with filter fabric, or arrays (9 to 13 pipes) of approximately 4" diameter are used. Depending on the specific design, the field may be more or less efficient than a conventional field.
Drip Irrigation Leach Field
This technology is fully described in the Technology Inventory: Drip Irrigation Leach Field. Most drip systems require secondary treatment of the wastewater. A high-head pump delivers doses of effluent through a filtering device that removes suspended solids particles to prevent clogging the drip tube emitters; A return flush line is required to periodically back-flush any solids collecting within the drip tubes. A drip system has several advantages: it can be used in clay soils, shallow soils, and moderately saturated soils since it requires only about one foot of unsaturated soils beneath the drip tubing; it re-uses the effluent to water a lawn or non-edible plantings; the plants remove nitrogen from the effluent; and it can be used with proper pressure compensation on relatively steep slopes. The primary disadvantage is its tendency to clog. A drip effluent distribution system typically adds $2,000 to $3,000 to the total cost of the on site wastewater system, with maintenance of about $300 to $600 per year, according to Texas A &M University fact sheet on drip irrigation.
Evapotranspiration Effluent Disposal
In these systems, effluent moves upward from perforated pipes laid in tent-shaped sand beds that maximize contact between the sand and the soil above. The system can either be laid on a waterproof liner, when the soil is too absorptive or when groundwater contamination is to be avoided; or it can be open at the bottom for very slow absorption in highly impervious soil. Among the constraints of these systems is that they must process 24 inches of water in excess of site rainfall. Because the bed can be saturated by rain, evapotranspiration systems are typically used in arid climates. More detailed information is available from the National Small Flows Clearinghouse at Evapotranspiration systems fact sheet and from the City of Austin at City of Austin - Onsite Disposal Systems Fact Sheets- Evapotranspiration systems. A typical system might cost in the range of $25,000, according to the City of Austin fact sheet.
Texas a & M University fact sheet on evapotrasnpiration leaching field
This technology, sometimes known as Individual Residential Spray Irrigation System (IRSIS) is not commonly used for individual systems. Because the effluent is sprayed into the air, pathogens would be a health hazard, and must be removed. Effluent must undergo secondary treatment, then be disinfected with chlorine and held for 30 minutes before spraying. IRSIS provides the opportunity for on-lot sewage disposal using soils that might otherwise not meet the requirements for any other on-lot system type. However, IRSIS is a complex and somewhat expensive alternative. One estimate from the City of Austin, Texas, is around $10,000. The cost is higher because of the filtration system, chlorinators and one to two dose tank/pumping stations, and because all require periodic maintenance. During periods of poor weather, a decision must be made as to whether or not to apply the treated wastewater to the land that day, requiring the homeowner to deal with their sewage disposal system daily. For this reason, an IRSIS may not be allowed in many locations. For more detailed information about IRSIS, see: Pennsylvania State University Agricultural Extension fact sheet on Spray Irrigation; and City of Austin - Onsite Disposal Systems Fact Sheets - Spray Irrigation Systems.
Given the damage that untreated sewage can pose to the environment, and the cost of running sewage lines to rural areas, on site treatment and disposal is a cost effective, practical, and sometimes beneficial way of dealing with sewage. Some methods allow for treatment and then dispersal to vegetation through underground means to supplement their nourishment.
Approval usually comes from the state or local health department. NSF/ANSI 40 - 2004 Standard for Residential Wastewater Treatment Systems details requirements for approval of the system. The NSF/ANSI 40-2004 standard applies to on site wastewater recycling systems with capacities of up to 1,500 gallons per day. The standards require two years of manufacturer maintenance service and renewal options, and alarms to alert the homeowner of malfunctions.