Guidelines

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Evapotranspiration (ET) System
Shallow Trench
Shallow Mound
Pressure Effluent Dosing and Drip Irrigation

Wastewater systems that incorporate greywater irrigation should permit flexibility in response to occupant's wastewater patterns, seasonal variations in demand for irrigation, and weather. This system would entail a separate sewer or septic connected blackwater system that is linked to the greywater system for overflow and other backup purposes. However, the fact that blackwater can occasionally be generated by greywater sources is less of a concern in sub-surface systems than in above-ground systems.

1.0 Factors Affecting Approval of Greywater Systems

• Size of lot and topography (As a general rule of thumb, lot sizes well under one half an acre will need professional engineering. Steeper slopes - beyond 15% - may need more design and engineering work as well.)

• Subsoil texture (sandy and loamy soils are best)

• Subsoil structure (again, sandy and loamy soils are best)

• Soil depth

• Restrictive horizon factors (none within 36 inches of the ground surface)

• Soil drainage - internal characteristics and external factors such as flooding

• Soil permeability

• Flooding characteristics

• There is a range of options in sub-surface systems that can respond to the different factors listed above. The following are not the only options, as qualified engineers can design systems specific to unique circumstances. The systems mentioned herein, again, do not assure automatic approval by virtue of being listed. Case-by-case, site-specific approval is always needed.

• It is important to note that all these systems should have vegetative cover specified on the plan to cover the drainfield area. All plans should specify erosion control procedures to prevent loss of top soil during the vegetation establishment period.

• Innovative or experimental systems will require property owners to submit a letter stating they know the system is experimental. In the event of property sale, the new owner must be informed about the experimental nature of the system. Each plan must have an Operation and Maintenance Manual, one to attach to the permit submittal and one for the owner.

  2.1 Evapotranspiration (ET) Systems

  • This system combines the process of evaporation and transpiration to utilize and dispose of wastewater. (Transpiration is the process whereby plants take in water through the roots, and convert it to vapor which is given off through the leaves.) A typical evapotranspiration system consists of a septic tank for pre-treatment (removal of solids) followed by distribution into a shallow sand bed covered with vegetation.

  • The greywater (wastewater from baths, showers, laundry and lavatory sinks) flows from the house through the septic tank and into the evapotranspiration bed. The greywater is distributed through perforated pipes. Once in the sand, greywater is taken into the plant root system. Underneath the bed is either a plastic lining or very impermeable soil which prevents the grey water from seeping into the ground. Blackwater (wastewater from toilets and kitchen sinks) flows into a sewer line or an alternative treatment.

  • There are special variations on this approach such as rock/plant systems that offer distinct advantages where the greywater movement and use needs to be highly controlled. These alternative systems can be used as a pre-filtering device. These filters can be thought of as constructed wetlands, that use living beds of marsh plants combined with gravel to break down wastewater pollutants that become food for the plants. Canna lilies, iris, ginger lily, elephant ears, and cattails have been used with these types of systems. Trenches are lined with PVC liner and filled with 1 to 1 1/2 inches of river gravel and topped with 6 inches of pea gravel. Plants are planted directly in the gravel, so no soil is used. An average two bedroom house requires about 210 square feet of trench, or a 3 foot x 70 foot trench. Maintenance involves cutting away dead leaves and plant stems seasonally.

  2.2 Shallow Trench
  • In this system, greywater flows from the house through pre-treatment and is piped into shallow trenches (pipe placed 8 inches deep). These pipes are placed close enough to the surface to feed the plant roots.

  • The distinction between a conventional septic tank system and a shallow trench subsurface landscape irrigation system occurs in the absorption field design.

  • Conventional septic tank systems are designed for disposal only; therefore, the distribution pipes are usually placed too deep for efficient irrigation and the spacing of the trenches is sometimes too wide. Consequently, irrigation gaps may occur which will need additional watering to prevent a lawn from having a striped effect. Mulched plant beds would pose less of a problem as the wicking effect of the mulch will even out the water concentration.

  

  Figure 2
  Shallow Trench Section View

  • The shallow trench greywater system can provide irrigation via shallow placement of distribution pipes and the optimum spacing of trenches.

  Figure 3
  Shallow mound Section View

  2.3 Shallow Mound

  • The shallow mound uses an elevated absorption field for disposal of wastewater. For irrigation, a shallow layer of sand fill and top soil is placed over existing soil. This technique is usually done when existing soil is unsuitable for wastewater disposal. Pipes are placed near the root zone to provide irrigation.

  • The Shallow Mound system will require pumping of the greywater to function properly.

  2.4 Pressure Effluent Dosing and Drip Irrigation
  • Low pressure effluent dosing is an option to gravity distribution. Although more maintenance is required, this type of greywater system is the most common. It can overcome many site limitations such as shallow soils, high ground water, excessive slopes, and uphill drainfields. Low pressure dosing uses a pump to distribute greywater through perforated pipes in the absorption bed. This modification is applicable to ET, shallow trench, shallow mound, and sub-surface drip irrigation systems. The greywater flows from the house through pre-treatment and is pumped into absorption alternatives.

  • Low pressure dosing is used for houses of no larger than 2700 square feet or with four bedrooms. Water saving fixtures (see Indoor Water Conservation) are required in conjunction with this system.

  • With the addition of a pump to a greywater system, distribution is more uniform than gravity flow, and greywater dosage into the bed can be controlled. Also, there is less soil clogging (which lengthens the life of the system) with properly designed and installed pressure dosing. Pump tank size is site specific; pump tanks must be a minimum of 500 gallon capacity and large enough to avoid overflow in the event of pump failure.

  • Trench locations and lengths are determined by the contours and the site slope. Trenches are narrow and 18-24 inches deep, and must be cut with a trenching machine or a rock saw. Trench cutting must be done to exact specifications (level within 2 inches) and may not be backfilled in order to achieve the desired level.

  • Trenches are covered with 6 inches of gravel before the distribution pipe is installed. Geotextile filter fabric must cover the gravel layer. The trenches are finally backfilled with sandy loam, and covered with a 6 inch mound of sandy loam so that rainwater will be diverted away from the area.

  • An initial distribution test must be done before the trenches are backfilled, including a head distribution check and leak check. This test must be repeated with a Health Department inspector present. The final landscape inspection will require adequate cover by the mound, good surface drainage, and a cover of seeded or sodded grass.

  • A drip system will require a filter in addition to the settling tank (septic tank) to prevent clogging the emitters. Specific application rates will be required depending on slope and soil types. Required separation distances of 50 feet for sharp slopes and breaks in slope can be reduced 10 feet.

  • Drip irrigation system emitters should be the pressure compensating type that will prevent soil from clogging the emitters. Greywater in the drip system is released at the emitters just below the surface in strategic locations in the landscape.

• The amount of greywater available for irrigation is obviously directly proportional to the amount of water used by a household. Water conserving fixtures and appliances produce a lower amount of wastewater as do smaller households. In a highly conserving home, 30 gallons per person per day of greywater is likely to be produced. This amount can vary according to individual usage patterns.

• If greywater is the primary source of irrigating water, a low water use landscape should be used. Year-round outputs of greywater through sub-surface systems make greywater irrigation ideal for maintaining evergreen trees and shrubs. The irrigation benefits of greywater should be integrated with the landscape design.

• The availability of greywater for irrigation on a square foot basis will vary according to the greywater system's area requirements. The system's area requirement is determined by the site's soil and slope characteristics discussed earlier. It is best to assume that some additional water from other sources than greywater will probably be needed on a seasonal basis.