Greywater is defined as the wastewater produced from baths and showers, clothes washers, and lavatories. The wastewater generated by toilets, kitchen sinks, and dishwashers is called blackwater. The primary method of greywater irrigation that will be discussed is through sub-surface distribution. This is the method that is readily approved in Austin when conditions are suitable.
The use of greywater for irrigation requires separate blackwater and greywater waste lines in the house. This is not a difficult task in new construction but can be problematic in existing buildings.
In certain parts of Austin, difficult conditions such as steep slopes, poor soil percolation qualities, close proximity to lakes, or other problems may require the services of a licensed Professional Engineer. However, in areas without unusual conditions, a sub-surface greywater system can be approved.
Sub-surface distribution systems are required by the local Health Department for greywater. Sub-surface systems are not as effective as above-ground spray systems for turf areas but are highly conserving and effective for providing root zone irrigation of plant beds, shrubbery, and trees. The best applications for greywater will be in conjunction with low water demanding landscapes (Xeriscape).
Regulation of site wastewater disposal systems is provided by the Austin-Travis County Health Department and LCRA (where appropriate) under rules established by the State Legislature, the Texas Water Commission, and the City of Austin. The rules that govern greywater systems are currently based on modifications of septic system guidelines. Some variances are permitted for greywater since less volume is created than in a septic system. Low pressure dosing systems allow for uphill and smaller drainfields. Above-ground greywater spray systems are not permitted in Travis County.
|Greywater Irrigation System|
|Satisfactory in most conditions|
|Satisfactory in Limited Conditions|
|Unsatisfactory or Difficult|
Greywater systems have been used informally for a long time. Greywater systems are modifications of septic system technology and, thereby, use components standard to septic systems. The primary modification is the location of the drainfield in the root zone of plants making the greywater most useful as irrigation. An important feature of a greywater system is the isolation of blackwater to a separate system, leaving the greywater available for reuse – most often irrigation. Additional research by some companies is focusing on greywater treatment processes prior to its release for irrigation or other uses.
Septic system components are readily available. Consultation for greywater system engineering is similarly available in our area.
A sub-surface greywater irrigation system is similar to the cost of a downsized septic system for a home. Greywater irrigation systems cost more than conventional spray and drip irrigation systems to install due to the tank used in greywater systems (a filter and special emitters are also needed for greywater drip irrigation systems). When a sewer line is available, the added cost of a greywater system is significant.
Greywater systems are a popular alternative water system in Austin. Public presentations regarding greywater systems have been well attended over the past several years. Illegal greywater use has been noted to be occurring currently in Austin, primarily from clothes washers.
A greywater system must be approved by the Austin-Travis County Health Department. Current regulations deal with sub-surface greywater systems similarly to septic systems, with the following differences:
- A smaller area on a lot can be used for a greywater system.
- The lot size can be less.
Ordinance #880310-H & I establishes regulations of individual septic tank systems and septic tank system use in subdivisions. These regulations are found in Chapter 12-4 of the 1992 Code of the City of Austin and govern the construction, inspection, and approval of all septic systems, greywater systems, and composting toilets within the jurisdiction of the City of Austin.
A section to this ordinance allows the Austin – Travis County Health Department to approve “Innovative Systems”. Innovative Systems are defined as those systems not specifically described in any technical reference found in Chapter 12 of the 1992 Code of the City of Austin, or issued by the Texas Water Commission, or in the LCRA’s Supplemental Standards to the Texas Department of Health Construction Standards for Private Sewage Facilities.
The Innovative Systems section to ordinance #880310-H & I does not grant categorical approval to non sub-surface greywater irrigation systems. The regulation allows a case by case review of innovative approaches. The possibility of system failure causing a public health threat, liability and maintenance issues, and potential negative environmental effects are central concerns in considering approval of innovative systems. Currently, the Health Department will not approve above-surface greywater irrigation systems.
The Health Department has a “cookbook” of acceptable septic system and greywater designs. Standard designs may be allowed in accordance with lot size and conditions such as slope. Any system outside of the “cookbook” typically requires submission by a registered Professional Engineer.
The following is the step by step process to gain a license to operate a greywater irrigation system (sub-surface only):
- Apply for a permit from the Health Department. The permit fee is $200 for a non-engineered system and $300 for an engineered system and is paid with the application.
- The Health Department will conduct a percolation test and a preliminary site inspection. This can be done prior to applying for a permit if you are not certain if a greywater system can be used in that location. The cost for this earlier evaluation is $50.
- A permit to construct the system is issued and periodic inspections by the Health Department will occur during the installation.
- A license is granted to operate the system providing the installation is done properly.
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)
Restrictive horizon factors (none within 36 inches of the ground surface)
Soil drainage – internal characteristics and external factors such as flooding
2.0 Types of Systems
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.
The shallow trench greywater system can provide irrigation via shallow placement of distribution pipes and the optimum spacing of trenches.
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.
3.0 System Capacity
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.