Sewage Treatment in Graywater Systems

An additional treatment system is necessary if graywater is separated from other wastewater

Sewage Treatment in Graywater Systems

Most of the time, systems treat the combined wastewater from all sources in a structure. Occasionally, the decision is made to split the system so one or more components treat one source of wastewater while another component treats other sources. This will require separate plumbing networks in the residence or other building.

Graywater is the wastewater captured from nonfood preparation sinks, showers, baths, spa baths, clothes washing machines and laundry tubs. Toilet wastes from the residence or other establishment have to be treated in some other system, or the residence has to have a privy. To prevent hooking up a flush toilet onto a graywater system, it is best if the plumbing of the system has a 2-inch diameter pipe, rather than 4-inch (a requirement in many states). Graywater systems cannot accept garbage disposal waste.

The primary reason a homeowner may want to separate this waste is to conserve space on their lot, as graywater soil treatment systems are typically allowed to be sized at a reduced footprint. Other reasons include using graywater systems to reduce nitrogen loading in sensitive areas and reusing nontoilet water for other applications. Other separation of flows may also benefit other establishments with high fat, oil or grease content.

The primary reasons for splitting flows include:

  • Have separate systems treat blackwater and graywater:
    • Nitrogen being discharged to ground or surface water in nitrogen-sensitive areas by using nondischarging blackwater toilets, such as composting or incinerating toilets. These toilets retain approximately 70% of the nitrogen in residential wastewater flows since most of the nitrogen is in the blackwater.
    • Reuse wastewater where wastewater reuse is a priority, by treating and reusing graywater for landscape irrigation, toilet flush water or other uses.
  • Keep wastes containing high concentrations of fats, oils and greases (like from commercial kitchens) from fouling components handling wastewater from other sources until the FOG concentrations have been significantly reduced.

Sometimes, even though the flows are initially split, they are combined somewhere downstream. For example, when a grease trap or interceptor is used to handle wastewater from the kitchen only, the resulting effluent may be combined with the rest of the wastewater in a downstream septic tank. Other times this separation continues throughout the entire system, so there are two complete wastewater systems. Examples of this are when nondischarging toilets (composting toilets and holding tanks) are used to handle the blackwater and another pretreatment and dispersal system is designed and constructed to handle the graywater.

Graywater may go through one or more treatment processes so that the graywater can be used for one or more nonpotable uses such as irrigation, toilet flushing and greenhouses. This allows the graywater to be reused as a resource. Graywater may be collected in a holding tank, where permitted, and periodically pumped and hauled away to a site that can treat and dispose of it properly. Data indicate that graywater contains significant concentrations of organic and inorganic material (whatever is poured down a sink or drain). Graywater also can contain fecal coliform concentrations as high as those found in blackwater. According to the National Academies of Sciences (2016), graywater has the range of characteristics shown in Table 4-1.   

Graywater characteristics can vary from house to house. Graywater may contain elevated levels of chemicals and disease-causing microorganisms (pathogens), but the quality of graywater can vary greatly from location to location based on the contributing sources (e.g., laundry, showers, baths), the amounts and types of chemicals used or disposed there (e.g., detergents, bleach, solvents, cleansers, personal care products), and the health of the residents in the source area. Because pathogens are only excreted by infected individuals, the greater the number of people contributing to graywater, the greater the likelihood of the presence of a range of pathogens. 

However, even in waste streams to which a small number of people contribute, when an infected individual is excreting pathogens, the concentration can be very high because of the relative lack of dilution. Graywater contains all of the concerns present in normal sewage, just at slightly lower levels. It is interesting that they do not combine the kitchen sink waste in these graywater levels as the organic loading, bacteria and virus levels are much higher from this source.

Thus, graywater must be treated as sewage. If a typical septic system is used, it may be reduced in size since just the graywater is being treated. When graywater is being treated for a later nonpotable use (toilet flush water, landscape irrigation), there must be assurances that the treatment is being reliably provided. Ongoing monitoring and maintenance is critical. Effects of not meeting treatment standards include clogging of pipes, valves and orifices by nutrients, algae and solids; and exposure of humans to pathogens in inadequately treated reuse water.

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation, and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to


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