Gray water, grey water, or greywater from the shower, lavatory and kitchen sink can be routed to a leach field, a waterway with hyacinths or other biodegrading plants, a dry well/French drain application, or even a decorative planting bed. Leach fields, unlike septic fields, do not handle black water. Some municipalities do not differentiate between gray water and conventional septic systems and as a result require an organized design procedure to avoid problems. The recommended steps in designing a leach field for gray water management are as follows:
1. Investigate the permit process.
2. Prepare the plan.
3. Design the gray water system.
4. Submit the plan for review and approval.
5. Install the system.
6. Arrange for inspection and approval.
7. Use, monitor, and maintain the system.
More often than not, the authority having jurisdiction will assign the same methodology to leach field design as to a septic field. This means that the resultant leach field will be assumed to handle black water and therefore will be sized according to the number of bedrooms in a house or the total number of fixture units in a commercial building. Some jurisdictions are more open to engineered systems than others. Those who balk at engineered systems are concerned that failure of an installation would reflect badly on public health agencies by relinquishing control to a new technology. You can help address their concerns by showing competence and thoroughness in your methodology.
Three concepts interrelate to handle gray water: evaporation, absorption, and transpiration. The local rate of evaporation from the soil to atmosphere is a function of relative humidity and wind velocity. Such information typically can be found at local university agronomy departments. A soils engineer can determine the rate of absorption of gray water into the soil, usually after analysis of a soil sample. Horticulture or agriculture sources can relay the transpiration of water into plants as part of their growing process. You will hear these terms in combination (e.g., evaporation, evapotranspiration, and evapoabsorption), each referring to the primary method of handling gray water. Absorption into the soil, which is how a septic field works, may not be possible due to soil conditions, water table, or proximity to a nearby body of water. In this case, a water barrier is used, and gray water is processed by evapotranspiration.