What Is Gray Water?
Gray water is wastewater from homes that comes from bathing and washing clothes. Gray water does not contain food waste, human waste, toxic or hazardous substances, or water softener backwash water.
Gray water can be reused and recycled in Ohio with a permit from the local health department (Ohio Administrative Code 3701-29-17). Recycling is allowed in both rural areas and in cities, even if a sanitary sewer system is available.
All pipes and fixtures conveying gray water must be purple or marked with a purple stripe and be labeled “Nonpotable water, do not drink” as shown in Figure 1. Because gray water does contain some dissolved organic matter, if held too long, it becomes anaerobic and starts to smell bad. Gray water should be dispersed within a few hours to prevent odors.
Figure 1. Purple pipes used for recycled wastewater.
The Four Types of Gray Water Systems Permitted in Ohio
A Type 1 gray water system recycles less than 60 gallons per day. It is used for subsurface irrigation of gardens, lawns and landscape plants during the growing season.
Food crops can be irrigated with gray water as long as they are not root crops or as long as the edible portion of the crop is not in contact with the gray water. An example would be a fruit tree irrigated with gray water at the roots as shown in Figure 2. This is permitted because the water is not sprayed on the fruit. Keep gray water application at least 25 feet away from the exposed, edible portion of plants.
Figure 2. An example of a Type 1 gray water reuse system.
Source: Santa Cruz County Environmental Health Services,
A Type 2 gray water system recycles more than 60 gallons per day but less than 1,000 gallons per day. It is used for subsurface irrigation of gardens, lawns and landscape plants during the growing season (Figure 3).
Food crops can be irrigated with gray water as long as they are not root crops or as long as the edible portion of the crop is not in contact with the gray water. Keep gray water application at least 25 feet away from the exposed, edible portion of plants.
Figure 3. An example of a Type 2 gray water reuse system.
Source: Building Green Futures, buildinggreenfutures.com/services/graywater.
A Type 3 gray water system recycles hand-carried gray water that is poured through a screen into a disposal sump tank before disposal as shown in Figure 4. The gray water is discharged into leach field trenches. Siting and designing these systems requires a soil assessment and an estimation of the wastewater loading rate to determine the number and length of the leaching trenches.
This is a common system in campgrounds, Amish homes and remote vacation homes. The tank that serves as a sump is equipped with a screen to keep debris from clogging the pipes.
Figure 4. A diagram of gray water reuse system for manual reuse.
Source: Karen Mancl, The Ohio State University.
A Type 4 gray water system recycles treated gray water that is reused year-round either outside or inside the building. The gray water is treated, disinfected and stored for no longer than 24 hours before reuse. Siting and designing these systems requires a soil assessment and an estimation of the wastewater loading rate.
Outside the building, gray water can be used for green roofs (Figure 5), living walls, surface and subsurface irrigation of lawns, gardens, and landscape plants.
Inside the building, gray water may be used to irrigate houseplants or living walls.
Figure 5. The green roof on Howlett Hall at The Ohio State University.
Source: Chadwick Arboretum and Learning Gardens, The Ohio State University,
Part of a Total Wastewater Reuse System
Gray water reuse systems can be combined with composting toilet systems to achieve reduction in water use. Composting toilets can treat both human and food waste (Figures 6 and 7). Wastewater from the kitchen sink or dishwasher must still be discharged to a sewer system, or it must be treated and dispersed into a septic system. The rest of the wastewater from the home is gray water and can be handled through an onsite reuse system.
Figure 6. A composting toilet used in a LEED platinum home.
Source: BPC Green Builders, bpcgreenbuilders.com/green-building/tour-a-leed-house.
Figure 7. An illustration of a composting toilet.
Source: WaterNSW, waternsw.com.au/water-quality/catchment/living/wastewater/systems/composting-toilets.