Gray Water Reuse
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.
Designing Gray Water Evaporation Fields
Assuming a calculated approach to determining water volume is accepted by the local jurisdiction, the following is a guide discussing the fundamentals for designing gray water evaporation fields and how to determine the size required to handle a typical gray water application in concert with a composting toilet installation.
I. Determine System Demand
Determining the expected demand is the first step in sizing a leach field. Following is an example of the fundamentals involved in determining demand for a typical single-family house:
- The use of water is partly based on the flow rate of the domestic water supply pump. Most pumps are set at 40 pounds of pressure. At that pressure, the average shower uses about 1¼ to 1½ gallons per minute. Using that rate, you can determine that a 15-minute shower will use 22 gallons of water.
- Flush versions of composting toilets use only about 1 pint per flush. Therefore, this water can be evaporated in the composting system itself, resulting in no water discharge.
- The average person should use only about 1 gallon a day for good hygiene
- Washing dishes should take only 2 gallons per day per person.
- A high-efficiency, front-loading washing machine uses about 15 gallons per load. A normal top-loader uses about 40 gallons and has less capacity. Using the high-efficiency machine and two loads of laundry per person per week equals 30 gallons per week, or about 2 gallons per day per person.
Thus, the total water use per person per day is approximately 27 gallons.
II. Size the Evaporation Field
First must determine the amount of water that will be used in the dwelling per month, which is based on the number of people who can sleep in the house, not the number of bathrooms. If you conclude that each person’s use is a minimum of 27 gallons per day, total usage is 810 gallons per person per month.
Local soil evaporation rates can be obtained from local university agronomy departments. (Agronomy departments gather this information as part of their efforts to monitor soil evaporation rates in conjunction with agricultural crop growth.) For instance, the evaporation rate in Miami averages 11.4 gallons per square foot per month. The actual rate varies based on humidity levels during a particular month.
If you divide the total usage of 810 gallons by the evaporation rate of 11.4 gallons per square foot, you get 71 square feet per person. That means you need an evaporation field that has a 71-square-foot surface area for each person living in our example house. Thus, you can estimate that a cottage with two people living full time would need an evaporation field of about 142 square feet, or 10 feet by 14¼ feet in dimension. Keep in mind that guests will boost system demand, so increasing evaporation field size for added capacity is advisable in some situations.
III. Boost Evaporation Rates
The evaporation rates we have used to calculate the field size are based on straight evaporation of water from the ground. If local climate allows, the rate can be boosted significantly by the use of broadleaf plants. Some studies have shown that plants with large leaves can expel water into the air at a rate five times faster than an open pool of water. Thus, if you design a system to cover just the two people living in the house, by planting a full field of plants, you should be able to boost capacity to evaporate enough water to accommodate any guests. Note: Care must be taken to ensure that excessive rainwater does not get into your field. Evaporation rates take normal rainfall into account. They do not account for runoff draining into the field. Be sure to raise the ground around the field to prevent rainfall runoff from flowing into the field. Also, you can construct a plastic roof over the evaporation field. That will allow light for the plants and keep rainwater completely out of the field. (Do not allow rain runoff from the roof to fall in the field.) For proper operation of an evapotranspiration field, an important concept to remember is that evaporation per square foot must exceed rainfall per square foot where absorption into the soil does not occur.
Bob Boulware is President of Design-Aire Engineering, Inc. Bob is a past president of the American Rainwater Catchment Systems Association (ARCSA) and an Accredited Rainwater Systems Design Professional. He is a 30+-year member of ASHRAE and past president of the Central Indiana Chapter of American Society of Plumbing Engineers (ASPE) and serves on the ASPE National Standards Committee. Mr. Boulware is a member of the International Association of Plumbing and Mechanical Officials (IAPMO) Alternative Water Sources Committee, and helped to develop the Green Plumbing Supplement to the upcoming editions of the Uniform and the International Plumbing Codes. Mr. Boulware has taught Environmental Design for mechanical and electrical systems at Ball State University and plumbing design at IUPUI. Follow us @daengineering on Twitter & www.daengineering.com. Bob can be reached at firstname.lastname@example.org.
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