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Gray Water and Black Water

Gray water (also spelled grey water) is untreated wash water from bathtubs, showers, lavatory fixtures, and clothes washing machines. Although no longer potable, it’s water that is relatively free of toilet wastes. Kitchen waste is usually not considered gray water because it contains excessive levels of grease, oils, and fats and waste disposal residue. Although gray water will contain bacterial contamination (e.g., total and fecal coliforms and heterotrophic plate count [HPC] bacteria) from normal bathing and washing of underwear and diapers, it does not contain bacterial contamination at the same level as does water with toilet wastes.

About 50 to 80% of domestic wastewater is gray water.


Although gray water may contain grease, food particles, hair, and other impurities, it may still be suitable for irrigation and/or sewage processing (flushing toilets). An obvious use of gray water is that it may potentially replace treated tap water used for landscape irrigation. It may benefit plants because it often contains nutrients such as nitrogen or phosphorus. Gray water use also helps overtaxed municipal wastewater treatment plants be cause gray water use diminishes sewer flows, thereby lessening the need to overwork or expand such treatment facilities.

Black water contains toilet wastes (e.g., feces, cellulose from toilet paper, and nitrogen compounds) from water closets (toilets), urinals, and bidets. Black water may also carry hazardous chemicals from activities such as cleaning car parts, washing greasy/oily rags, or disposing of waste solutions from home photo labs or similar hobbyist activities. Compared to gray water, black water has a large amount of organic and pathogenic pollutants.

A significant difference between black water and gray water is in the rate of decay of the pollutants in each. The pollutants in gray water decompose rapidly. For example, fecal matter from underwear and diapers washed in a clothes washing machine breaks down, exposing potential pathogens to detergent. Conversely, black water contains a substantial quantity of organic compounds that have already been exposed to the digestive tract of the human body, so they don’t rapidly further decompose when placed in water. Cellulose from toilet paper in black water also decays slowly.

It’s estimated that about 35 gal of gray water is generated each day per person in new construction and approximately 46 gals each day per person in existing homes. Provided that it can be treated and reclaimed, gray water represents a large water conservation resource in buildings. It can result in a significant reduction in the need for potable water and a reduction in the amount of wastewater entering on-site sewage treatment systems or a community wastewater treatment system. For ex ample, in a residence, reuse of gray water can amount to an annual savings of 30 000 to 50 000 gal per year. As a result, innovative systems that safely recycle gray water for landscape irrigation and/or water for toilet flushing are being installed.

Gray Water Reuse

Depending on health regulations, technology for gray water reuse can be as simple as saving rinse water from bathing and the clothes washer to rather complex treatment systems, such as one in which the gray water flows to an aerobic treatment unit, then to a recirculating filter. Although the level of contamination of gray water is low in comparison to black water, concerns about health and public safety need to be managed, especially those related to the potential for transmission of disease. As a general rule, gray water originating at a building is required to be contained and used within the property boundary. See Fgrs 10 and 11.

Gray water needs to be filtered to remove large waste particles before being used or treated. This primary treatment will reduce the solids in the wastewater. Once coarse filtered, gray water may be treated using a sand filter. The basic structure is a waterproof box filled with coarse sand laid over a gravel bed. Gray water flows in at the top and out the bottom. A number of commercial sand filters are available. Reed beds and sand filters treat the wastewater through filtration and some biological nutrient uptake. Gray water needs to be pretreated to allow removal of large particles, or else clogging will occur and the lifetime of the system will be reduced. Secondary treatment removes pollutants from the remaining liquid. Treated gray water has had most nutrients like nitrogen and phosphorus removed, so it’s safer to use in large quantities. Disinfection is required for general reuse of gray water. All disinfection requires regular maintenance. Chlorine is most commonly used for disinfection, but it has been found to have adverse environ mental impacts. Ultraviolet (UV) or ozone disinfection can be used in place of chlorination.

The amount and quality of gray water will to a certain extent determine how it can be reused. Gray water is commonly used for irrigation. Untreated gray water can only be reused for subsurface irrigation. Treated gray water can be reused for either subsurface or above-ground irrigation. Only treated and disinfected gray water should be used for above-ground irrigation because of the potential presence of pathogens. The health effect of use of untreated gray water directly on edible vegetable plants has not been fully researched. The use of gray water for irrigation of vegetable gardens should be avoided if vegetables will be eaten raw or lightly cooked.

Healthy soil is a complex system that requires moisture, organic material, microorganisms, and other life forms (e.g., worms, and so forth). By introducing large amounts of deter gent into the soil, the microorganisms can be destroyed, which adversely affects the quality of soil. Gray water from the wash cycle of a clothes washer contains higher chemical concentrations from soap powders and soiled clothes, and is high in suspended solids, lint, turbidity, and oxygen demand; if applied untreated, it can damage the soil and lead to environmental damage. Water from the first and second rinse cycles contains pollutants, although these are greatly reduced.

FGR.11 A gray water treatment (GTS) system that collects, stores, and treats gray water to a quality water standard for irrigation.


Gray Water Diversion Devices

Gray water diversion devices don’t treat gray water but use it directly as irrigation water. Gray water is piped to outdoor vegetation through subsurface irrigation lines. Human contact with gray water must be avoided. To prevent human exposure, a soil barrier of at least 4 in (100 mm) must separate the surface from the subsurface irrigation lines. There are two types of diversion devices: gravity diversion devices and pump diversion devices.

A gravity diversion device is a fitting configuration that diverts gray water from a plumbing fixture directly to an irrigation line. A Suldi Gray Water Diversion Valve is designed to fit under a laundry sink. A hose extends from the valve out through the wall of the building and is linked to a garden hose, which is then connected to a slow release soaker hose. It can then be easily turned off to allow soapy wash water to go down a conventional drain line and on to use the rinse water for irrigation.

A pump diversion device has a similar irrigation piping arrangement to the gravity diversion device, but includes a surge tank to temporarily store and limit the flow of gray water during sudden surges. The surge tank should be capable of momentarily storing a volume of water equivalent to the emptying discharge of fixtures connected to it. However, it should not serve as a storage tank. The surge tank should be fitted with a high water level alarm to warn of pump failure or system blockage.

In both systems an overflow connection is joined to the conventional drain line. The flow of gray water in a gray water diversion device is usually activated through a hand operated valve, tap, or switch. During wet or cold (below freezing) weather, the valve, tap, or switch directs gray water to a conventional drain line, where it’s discharged to the sewer. A diversion device does not treat gray water but it’s good practice to install a course screen to remove material that may clog pumps, block pipes, or pollute the soil.

Gray Water Treatment Systems

Gray water treatment systems (GTS) are more complex systems that collect, store, and treat gray water to a quality standard for irrigation and/or sewage processing (flushing toilets). A GTS includes components such as wetlands, intermittent sand filters, soil filters, gray water septic tanks, and aerated wastewater treatment systems.

A GTS requires separate black water and gray water waste lines. Separation of lines is achieved through the use of two independent plumbing arrangements. Plumbing configurations should not allow contamination by wastes from the toilets, cause undesirable odors, and result in aesthetic degradation of yards and gardens. Wastewater separation is relatively easy to accomplish in new construction, but can be difficult in retrofitted systems in existing dwellings. The treatment process varies according to how the gray water is used and includes settling of solids, floatation of lighter materials, anaerobic digestion in a septic tank, aeration, clarification, and finally disinfection.

Commercially available gray water treatment systems include the following:

• BioSeptic Pty Limited Aerated wastewater treatment systems (AWTS)

• Wattworks

• Graywater Saver

• Rain Reviva

• ECO Wastewater Recycling System

• Clivus Multrum Graywater Prefilter

• Gough Plastics Gray Water Reuse Unit

Gray Water Reuse Regulations

Regulations affecting reuse of gray water vary from state to state and from municipality to municipality within the same state. In many locations gray water reuse is not approved be cause it’s assumed to carry pathogenic (disease-causing) bacteria and other hazardous pollutants. In jurisdictions where gray water systems are permitted, they are strictly regulated. In many cases, they must meet all design and construction standards for OSST (septic) systems, so disadvantages include cost for additional equipment and the requirement of gaining approval of a septic system.

As codes are revised, a septic tank may not be required for disposal of gray water only. An approved filter system may be used in place of the septic tank as long as no garbage disposal waste or waste from a toilet enters the gray water disposal system.

If garbage disposal waste or liquid waste from a com posting toilet enters the gray water disposal system, the gray water must be treated before discharge. The conventional treatment method, a septic tank large enough to provide at least a two-day retention time, allows grease to cool, solidify, and float to the top. This is particularly important when kitchen wastes are part of the gray water flow. Garbage disposals are strongly discouraged as they stress the conventional septic system and provide an extremely high amount of solids to a system designed to handle only gray water. If a garbage disposal is installed, it should be plumbed as part of the black water system.

The State of Arizona has a progressive law on offering permits for gray water systems. It’s a performance-based code that uses a three-tiered method of classifying systems:

Tier 1 Private residential systems that process less than 400 gal per day don’t need a special permit if all the following conditions are met:

1. Human contact with gray water and soil irrigated by gray water is avoided.

2. Gray water originating from the residence is used and contained within the property boundary for house hold gardening, composting, lawn watering, or landscape irrigation.

3. Surface application of gray water is not used for irrigation of food plants, except for citrus and nut trees.

4. The gray water does not contain hazardous chemicals derived from activities such as cleaning car parts, washing greasy or oily rags, or disposing of waste solutions from home photo labs or similar hobbyist or home occupational activities.

5. The application of gray water is managed to minimize standing water on the surface.

6. The gray water system is constructed so that if block age, plugging, or backup of the system occurs, gray water can be directed into the sewage collection system or on-site wastewater treatment and disposal system, as applicable. The gray water system may include a means of filtration to reduce plugging and extend system lifetime.

7. Any gray water storage tank is covered to restrict access and to eliminate habitat for mosquitoes or other vectors.

8. The gray water system is sited outside of a floodway.

9. The gray water system is operated to maintain a minimum vertical separation distance of at least 5 ft from the point of gray water application to the top of the seasonally high groundwater table.

10. For residences using an on-site wastewater treatment facility for black water treatment and disposal, the use of a gray water system does not change the de sign, capacity, or reserve area requirements for the on-site wastewater treatment facility at the residence, and ensures that the facility can handle the combined black water and gray water flow if the gray water system fails or is not fully used.

11. Any pressure piping used in a gray water system that may be susceptible to cross-connection with a potable water system clearly indicates that the piping does not carry potable water.

12. Gray water applied by surface irrigation does not contain water used to wash diapers or similarly soiled or infectious garments unless the gray water is disinfected before irrigation.

13. Surface irrigation by gray water is only by flood or drip irrigation.

Tier 2 Systems that process over 400 but less than 3000 gal a day or don’t meet the list of conditions (e.g., commercial, multifamily, and institutional systems) require a standard permit.

Tier 3 Systems that process over 3000 gallons a day re quire a special permit and are considered on an individual basis.

In many states and local communities, regulators apply oversight to gray water systems based on their possible impacts.



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