Both conventional and alternative OSDS usually include a soil absorption field. These absorption fields require a certain minimum area of soil surrounding the system to effectively remove pathogens and other pollutants. Setbacks from wells, surface waters, building foundations, and property boundaries are necessary to minimize the threat to public health and the environment. The setback should be based on soil type, slope, presence and character of the water table (as defined on a map developed by the implementing agency), and the type of OSDS. Setback guidelines should be set for both traditional and alternative OSDS. The Design Manual for Onsite Wastewater Treatment and Disposal Systems (USEPA, 1980) recommends the following setbacks for soil absorption systems, although other increased setbacks may be necessary to protect ground water and surface waters from viral and bacteria transport to account for tidal influences and accommodate sea level rise. (NOTE: Setback distance requirements may vary considerably based on local soil conditions and aquifer properties):

 Water supply wells 50 to 100 feet
 Surface waters, springs 50 to 100 feet
 Escarpments 10 to 20 feet
 Boundary of property 5 to 10 feet
 Building foundations 10 to 20 feet
 (30 feet when located up-slope from a
 building in slowly permeable soils)

Studies have shown that at least 4 feet of unsaturated soil below the ponded liquid in a soil absorption field is necessary to (1) remove bacteria and viruses to an acceptable level, (2) remove most organics and phosphorus, and (3) nitrify a large portion of the ammonia (University of Wisconsin, 1978). The majority of coastal States already require a minimum separation distance of at least 2 feet (Woodward-Clyde, 1992). Massachusetts requires a minimum separation of 4 feet; 5 feet is required by towns with sensitive surface waters. Several towns on Cape Cod have adopted 5 feet as the minimum. A prescribed minimum distance is necessary to prevent contaminants from directly entering ground water and surface waters. Areas with rapid soil permeabilities (e.g., a percolation rate of less than 5 minutes/inch) may require a greater separation distance. However, because of local variation, these numbers are provided only as guidance.

A study on a barrier island of North Carolina (Carlile et al., 1981) found high concentrations of nitrogen, phosphorus, and pathogens in shallow ground-water wells located beneath septic system soil absorption fields. These high concentrations were suspected to be the result of inadequate separation distance to the water table. Further analysis revealed that, at the design loading rate, a greater separation distance reduced the ground-water concentration of indicator organisms from 4.6 to 2.3 logs, and phosphorus by 93 percent. Nitrogen levels were also reduced, but this improvement (10 percent) was not as dramatic as that observed for bacteria and phosphorus.

Site assessments should be performed to determine the soil infiltration rate, soil pollutant removal capacity, acceptable hydraulic loading rate, and depth to the water table prior to issuing permits for OSDS. Percolation tests are usually performed to determine the soil infiltration rate. However, Hill and Frink (1974) stated that percolation tests are often performed improperly and system failures have resulted from improper siting and inadequate percolation rates. In addition, regulatory values based on acceptable percolation rates vary considerably (e.g., Delaware - 6 to 60 min/in; Georgia - 50 to 90 min/in; Michigan - 3 to 60 min/in; and Virginia - 5 to 120 min/in (Woodward-Clyde, 1992). States such as Florida and Mississippi require soil evaluations to determine the suitability of an absorption field. A soil evaluation should also be used in conjunction with percolation test results to determine whether a site is acceptable, and soil percolation requirements should be phased out, if appropriate. These evaluations should examine the organic content of the soil, the grain size distribution, and the structure of the soil. In addition, hydraulic loading should be evaluated to determine the suitability of a site for septic tank use.

A system such as DRASTIC methodology (USEPA, 1987) can also be used to map areas where aquifers may be vulnerable to pollution from OSDS. DRASTIC considers soil permeability, depth to ground water, and aquifer characteristics.

In areas where nitrogen is a problem pollutant, it is important to consider the density of OSDS. As the density of residences increases, lot sizes decrease and impacts (especially from nitrogen) on underlying ground water may intensify. One-half to 5-acre lots are generally the minimal requirement for siting OSDS, but the lot size may need to be larger if nitrogen is a problem pollutant. Limits on the density of absorption fields should also reflect variations in climate (Rutledge et al., undated). In Buzzards Bay, Massachusetts, a minimum lot size of 70,000 square feet was recommended as necessary to avoid nitrogen-induced degradation (Horsely Witten Hegeman, 1991). However, this practice should not preclude implementation of the use of cluster development to retain open areas necessary for controlling NPS pollution.

A number of treatment systems are known to remove nitrogen using denitrification. Such systems include sand and anaerobic upflow filters, and constructed wetlands. These systems are described in practice "f." Most of these systems require nitrification of septic tank effluent as an initial stage of the treatment process. When properly operated, these systems have been shown to have the potential to remove over 50 percent of the total nitrogen from septic tank effluent.

As stated previously, the majority of OSDS soil absorption field failures are attributed to hydraulic overload. Solids loads from garbage disposals can also lead to clogging and failure of an absorption field. To address these problems, plumbing codes that minimize the potential for soil absorption field failure should be implemented.

Plumbing codes that require the use of high-efficiency plumbing fixtures in new development can reduce these water loads considerably. Such high-efficiency fixtures include toilets of 1.5 gallons or less per flush, shower heads of 2.0 gallons per minute (gpm), faucets of 1.5 gpm or less, and front-loading washing machines of up to 27 gallons per 10- to 12-pound load. Implementing these fixtures can reduce total in-house water use by 30 percent to 70 percent (Consumer Reports July 1990, February 1991).

Selection of an OSDS should consider site soil and ground-water characteristics and the sensitivity of the receiving water(s) to OSDS effluent. Descriptions and design considerations for systems have been provided below. Table 4-21 (31k) contains available cost and effectiveness data for some of these systems. Design and operation and maintenance information on these devices can be found in Design Manual for Onsite Wastewater Treatment and Disposal Systems (USEPA, 1980).

Conventional Septic System. A conventional septic system consists of a settling or septic tank and a soil absorption field. The traditional system accepts both greywater (wastewater from showers, sinks, and laundry) and blackwater (wastewater from toilets). These systems are typically restricted in that the bottom invert of the absorption field must be at least 2 feet above the seasonally high water table or impermeable layer (separation distance) and the percolation rate of the soil must be between 1 and 60 minutes per inch. Also, to ensure proper operation, the tank should be pumped every 3 to 5 years. Nitrogen removal of these systems is minimal and somewhat dependent on temperature. The most common type of failure of these systems is from clogging of the absorption field, insufficient separation distance to the water table, insufficient percolation capacity of the soil, and overloading of water.

Mound Systems. Mound systems are an alternative to conventional OSDS and are used on sites where insufficient separation distance or percolation conditions exist. Mound systems are typically designed so the effluent from the septic tank is routed to a dosing tank and then pumped to a soil absorption field that is located in elevated sand fill above the natural soil surface. There is evidence suggesting that pressure dosing provides more uniform distribution of effluent throughout the absorption field and may result in marginally better performance. A major limitation to the use of mounds is slope. In Pennsylvania, elevated sand mound beds are permitted only in areas with slopes less than 8 percent (Mancl, 1985).

Where adequate area is available for subsurface effluent discharge, and permanent or seasonal high ground water is at least 2 feet below the surface, the elevated sand mound may be used in coastal areas. This system can treat septic tank effluent to a level that usually approaches primary drinking water standards for BOD5, suspended solids, and pathogens by the time the effluent plume passes the property line for single-family dwellings. A mound system will not normally produce significant reductions in levels of total nitrogen discharged, but should achieve high levels of nitrification.

Intermittent Sand Filter. Intermittent sand filters are used in conjunction with pretreatment methods such as septic tanks and soil absorption fields. An intermittent sand filter receives and treats effluent from the septic tank before it is distributed to the leaching field. The sand filter consists of a bed (either open or buried) of granular material from 24 to 36 inches deep. The material is usually from 0.35 to 1.0 mm in diameter. The bed of granular material is underlain with graded gravel and collector drains. These systems have been shown to be effective for nitrogen removal; however, this process is dependent on temperature. Water loading recommendations for intermittent sand filters are typically between 1 and 5 gallons per day/square foot (gpd/ft2) but can be higher depending on wastewater characteristics. Primary failure of sand filters is from clogging, and the following maintenance is recommended to keep the system performing properly: resting the bed, raking the surface layer, or removing the top surface medium and replacing it with clean medium. In general, the filters should be inspected every 3 to 4 months to ensure that they are operating properly (Otis, undated).

Intermittent sand filters are used for small commercial and institutional developments and individual homes. The size of the facility is limited by land availability. The filters should be buried in the ground, but may be constructed above ground in areas of shallow bedrock or high water tables. Covered filters are required in areas with extended periods of subfreezing weather. Excessive long-term rainfall and runoff may be detrimental to filter performance, requiring measures to divert water away from the system (USEPA, 1980).

Recirculating Sand Filter. A recirculating sand filter is a modified intermittent sand filter in which effluent from the filter is recirculated through the septic tank and/or the sand filter before it is discharged to the soil absorption field. The addition of the recirculation loop in the system may enhance removal effectiveness and allows media size to be increased to as much as 1.5 mm in diameter and allows water loading rates in the range of 3 to 10 gpd/ft2 to be used. Recirculation rates of 3:1 to 5:1 are generally recommended.

Buried or recirculating sand filters can achieve a very high level of treatment of septic tank effluent before discharge to surface water or soil. This usually means single-digit figures for BOD5 and suspended solids and secondary body contact standards for pathogens (in practice, 100-900 per 100 ml). Dosed recycling between sand filter and septic tank or similar devices can result in significant levels of nitrification/denitrification, equivalent to between 50 and 75 percent overall nitrogen removal, depending on the recycling ratio. Regular buried or recirculating sand filters may require as much as 1 square foot of filter per gallon of septic tank effluent.

Anaerobic Upflow Filter. An anaerobic upflow filter (AUF) resembles a septic tank filled with 3/8-inch gravel with a deep inlet tee and a shallow outlet tee. An AUF system includes a septic tank, an AUF, a sand filter, and a soil absorption field. As with the sand filter, dose recycling can be used to enhance this system's performance. Hydraulic loading for an AUF is generally in the range of 3 to 15 gpd. An AUF resembles a septic tank or the second chamber of a dual-chambered tank. It should be sized to allow retention times between 16 and 24 hours. There is a high degree of removal of suspended solids and insoluble BOD. Dosed recycling between sand filter and AUF can result in 60 to 75 percent overall nitrogen removal.

A growing body of data at the University of Arkansas and elsewhere suggests that an AUF can provide further treatment of septic tank effluent before discharge to a sand filter. This treatment allows a drastic reduction (by a factor of 8 to 20) in the size of sand filter needed to attain the performance described above, with major reductions in cost (Krause, 1991).

Trenches and Beds. Trenches are typically 1 to 3 feet wide and can be greater than 100 feet long. Infiltration occurs through the bottom and sides of the trench. Each trench contains one distribution pipe, and there may be multiple trenches in a single system. Like conventional septic systems, they require 2 to 4 feet between the bottom of the system and the seasonally high water table or bedrock, and are best suited in sandy to loamy soils where the infiltration rate is 1 to 60 minutes per inch. Gravelly soils or poor-permeability soils (60 to 90 minutes per inch) are not suitable for trench systems. However, where the infiltration rate is greater than 1 minute per inch, 6 inches of loamy soil can be added around the system to create the proper infiltration rate (Otis, undated).

Beds are similar to trenches except that infiltration occurs only through the bottom of the bed. Beds are usually greater than 3 feet wide and contain one distribution pipe per bed. Single beds are commonly used; however, dual beds may be installed and used alternately. The same soil suitability conditions that apply to trenches apply to bed systems.

Trenches are often preferred to beds for a few reasons. First, with equal bottom areas, trenches have five times the sidewall area for effluent absorption; second, there is less soil damage during the construction of trenches; and third, trenches are more easily used on sloped sites.

The effluent from trenches or beds can be distributed by gravity, dosing, or uniform application. Dosing refers to periodically releasing the effluent using a siphon or pump after a small quantity of effluent has accumulated. Uniform application similarly stores the effluent for a short time, after which it is released through a pressurized system to achieve uniform distribution over the bed or trench. Uniform application results in the least amount of clogging.

Maintenance of trenches and beds is minimal. Dual trench or bed systems are especially effective because they allow the use of one system while the other rests for 6 months to a year to restore its effectiveness (Otis, undated).

Water Separation System. A water separation system separates greywater and blackwater. The greywater is treated using a conventional septic system, and the blackwater is contained in a vault/holding tank. The blackwater is later hauled off site for disposal.

For extreme situations or for seasonal residents, some form of separation of toilet wastes from bath and kitchen wastes may be helpful. Most nitrogen discharges in residential wastewater come from human urine. A very efficient toilet (0.8 gallon per flush), if routed to a separate holding tank, would need pumping only three or four times per year even for a family of four permanent residents.

Constructed Wetlands. Constructed wetlands are usually used for polishing of septage effluent that has already had some degree of treatment (processing through a septic tank or other aggregated system). The performance of constructed wetlands will be degraded in colder climates during winter months because of plant die-off and reduction in the metabolic rate of aquatic organisms.

Cluster Systems. For the purposes of this guidance, a cluster system can be defined as a collection of individual septic systems where primary treatment of septage occurs on each site and the resulting effluent is collected and treated to further reduce pollutants. Additional treatment may involve the use of sand filters or AUF, constructed wetlands, chemical treatment, or aerobic treatment. The use of cluster systems may provide advantages due to increased treatment capability and economy of scale.

Evapotranspiration (ET) and Evapotranspiration/Absorption (ETA) Systems. ET and ETA systems combine the process of evaporation from the surface of a bed and transpiration from plants to dispose of wastewater. The wastewater would require some form of pretreatment such as a septic tank. An ET bed usually consists of a liner, drainfield tile, and gravel and sand layers. ET and ETA systems are useful where soils are unsuitable for subsurface disposal, where the climate is favorable to evaporation, and where ground-water protection is essential. In both types of systems, distribution piping is laid in gravel, overlain by sand, and planted with suitable vegetation. Plants can transpire up to 10 times the amount of water evaporated during the daytime. For an ET system to be effective, evaporation must be equal to or greater than the total water input to the system because it requires an impermeable seal around the system. In the United States, this limits use of ET systems to the Southwest. The size of the system depends on the quantity of effluent inflow, precipitation, the local evapotranspiration rate, and soil permeability (Otis, undated). Data were unavailable on this BMP, so its cost and effectiveness were not evaluated.

Vaults or Holding Tanks. Vaults or holding tanks are used to containerize wastewater in emergency situations or other temporary functions. This technology should be discouraged because of high anticipated overloads due to difficult pumping logistics. Such systems require frequent pumping, which can be expensive.

Fixed Film Systems. A fixed film system employs media to which microorganisms may become attached. Fixed film systems include trickling filters, upflow filters, and rotating biological filters. These systems require pretreatment of sewage in a septic tank; final effluent can be discharged to a soil absorption field. Cost and effectiveness data for this BMP were not available.

Aerobic Treatment Units. Aerobic treatment units can be employed on site. A few systems are available commercially that employ various types of aerobic technology. However, these systems require regular supervision and maintenance to be effective. They require pretreatment by a septic tank, and effluent can be discharged to a soil absorption field. Power requirements can be significant for certain types of these packages. Cost and effectiveness data for this BMP were not available.

Sequencing Batch Reactor. A sequencing batch reactor is a modified conventional continuous-flow activated sludge treatment system. Conventional activated sludge systems treat wastewater in a series of separate tanks. Sequencing batch reactors carry out aeration and sedimentation/clarification simultaneously in the same tank. They are designed for the removal of biochemical oxygen demand (BOD) and total suspended solids (TSS) from typical municipal and industrial wastewater at flow rates of less than 5 MGD. Modification to the design of the basic system allows for nitrification and denitrification and for the removal of biological phosphorus to occur.

The sequencing batch reactor is particularly suitable for small flows and for nutrient removal. Sequencing batch reactors can be either used for new developments or connected to existing septic systems. Small reactors can be sited in areas of only a few hundred square feet. While sequencing batch reactor cost and operation and maintenance requirements are greater than those for conventional OSDS, sequencing batch reactors may be suitable alternatives for sites where high-density development and/or unsuitable soils may preclude adequate treatment of effluent.

Sequencing batch reactors can also be used where municipal and industrial wastes require conventional or extended aeration activated sludge treatment. They are most applicable at flow rates of 3000 gpd to 5 MGD but lose their cost-effectiveness at design rates exceeding 10 MGD (USEPA, 1992). Sequencing batch reactors are very useful for the pretreatment of industrial waste and for small flow applications. They are also optimally useful where wastewater is generated for less than 12 hours per day.

Disinfection Devices. In some areas, pathogen contamination from OSDS is a major concern. Disinfection devices may be used in conjunction with the above systems to treat effluent for pathogens before it is discharged to a soil absorption field. Disinfection devices include halogen applicators (for chlorine and iodine), ozonators, and UV applicators. Of these three types, halogen applicators are usually the most practical (USEPA, 1980). Installation of these devices in an OSDS increases the system's cost and adds to the system's operation and maintenance requirements. However, it may be necessary in some areas to install these devices to control pathogen contamination of surface waters and ground water.

(NOTE: The use of disinfection systems should be evaluated to determine the potential impacts of chlorine or iodine loadings. Some States, such as Maryland, have additional requirements or prohibit the use of these processes.)

Massachusetts has adopted a provision of its State Environmental Code that allows for "approval of innovative disposal systems if it can be demonstrated that their impact on the environment and hazard to public health is not greater than that of other approved systems" (310 CMR 15.18). Commonly referred to as Title 5, this legislation requires evaluation of pollutant loadings as well as management requirements prior to approval of alternative systems (Venhuizen, 1992).

In preparation of site plans and designs for OSDS, it is recommended that a suitable area be identified and reserved for construction of a second or replacement soil absorption field, in the event that the first fails or expansion is necessary. Oliveri and others (1981) determined that continuously loaded soil absorption fields have a finite life span and that 50 percent of all fields fail within 25 years. Consequently, dual systems or a plan for a backup system is necessary. The area for the backup soil absorption field should be located to facilitate simultaneous or alternate loading of the old and new systems. With trench systems, the area between the original trenches can serve as the replacement area as long as sufficient vertical spacing exists between the trenches.

Care must be taken during the construction of OSDS so that the soil in the absorption field area is not compacted. Compaction could severely decrease the infiltration capacity of the soil and lead to failure of the absorption field.

A postconstruction inspection program should be implemented to ensure that OSDS were installed properly. The inspection should ensure that design specifications were followed and that soil absorption field areas were not compacted during construction. Many local governments in Massachusetts require postconstruction inspection for OSDS (Myers, 1991).

5. Effectiveness Information and Cost Information

The availability of high-quality, water-efficient plumbing fixtures (1.6-gallon toilets, 1.5-gpm showerheads, etc.) can provide a reduction of 50 percent in residential water use and wastewater volume, at an incremental cost of only about $20 to $100 for new homes. For on-site treatment, the higher influent concentrations are counterbalanced by longer septic tank retention time. This water conservation can allow further reductions in the size of sand filters or other forms of treatment (Krause, 1991).

The elimination of garbage disposals will reduce hydraulic loadings to OSDS and decrease the potential for solids to clog the absorption field, as shown in Table 4-22.

Performance data on sequencing batch reactors show that typical designs can achieve BOD and TSS concentrations of less than 10 mg/L and that modified systems can denitrify to limits of 1 to 2 mg/L NH3-N (EPA, 1992). Some modified sequencing batch reactors have been shown to exhibit denitrification. Biological phosphorus removal to less than 1.0 mg/L has also been achieved (EPA, 1992).

The costs for sequencing batch reactors, adjusted to 1991 dollars, for constructing and operating sequencing batch reactors were determined for several existing systems. The capital costs for six treatment systems were found to range from $1.93 to $30.69/gpd of design flow (USEPA, 1992). The operating costs for three existing systems, based on 1990 average flow rates, ranged from $0.17/gpd to $2.88/gpd (USEPA, 1992).

Costs for a complete mound system, including a septic tank, in the rural Midwest are typically $7,000 installed (Krause, 1991). The cost for a residential septic tank/AUF/sand filter combination in the rural Midwest normally ranges from $3,000 to $4,000 (Krause, 1991). Costs for buried or recirculatng sand filters depend on the filter size and the availability of sand of the proper texture. Costs for a complete system in the rural Midwest may range between $5,000 and $10,000 (Krause, 1991).