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Greywater
What is Greywater?

       The North Carolina Plumbing Code defines greywater as “waste discharged from lavatories, bathtubs, showers, clothes washers and laundry sinks.” However, in creating a greywater system, certain forms of “blackwater” are separated from the reusable greywater.

Greywater v. Blackwater
  • About 40% of the average daily household wastewater consists of blackwater, which is the term used for water from toilets and garbage grinders, while 60% is greywater.
  • Blackwater and greywater each contain 50% of the daily household wastewater phosphorous output. Blackwater, however, contains 90% of the Nitrogen output, leaving greywater with 10% Nitrogen output.
  • Greywater contains far fewer pathogens than blackwater
  • Greywater decomposes much faster than blackwater.
  • Nitrogen is one of the most serious and difficult to remove pollutants affecting drinking water quality, but nine-tenths of nitrogen is contained in blackwater.
  • Feces is widely viewed as the most significant source of human pathogens, which is also contained in blackwater.
  • Greywater stabilizes quicker, decreasing water pollution, due to more rapid decomposition.

Combined Wastewater

Using Greywater

     Greywater can be used in the lawn for irrigation or rerouted to a holding tank and stored for flushing toilets. The amount of greywater water available for irrigation and toilet flushing is directly proportional to the amount of water used by a household. Water conserving fixtures and appliances produce a lower amount of wastewater as do smaller households.

The irrigation benefits of greywater should be integrated with the landscape design. Low water use landscape is recommended if greywater is the primary source of irrigating water. Year-round outputs of greywater through sub-surface systems make greywater irrigation ideal for maintaining evergreen trees and shrubs. When greywater is used for irrigation, it is put through pretreatment, which may include a septic system, sand filter, and then pump.

Plumbing Codes

       According to the 2006 NC Plumbing Code, treated household greywater may be permitted for use for specific purposes, such as flushing toilets that are located in the same building as the greywater recycling systems and for irrigation purposes, as long as it is treated according to Code Standards and approved by the authority having jurisdiction.

        In North Carolina all wastewater must be disposed of through a permitted system, either a public owned treatment plant or a permitted septic tank system. The North Carolina State Plumbing Code 301.3 requires all plumbing fixtures, drains and appliances that receive or discharge liquid wastes or sewage to discharge to the sanitary drainage system of the structure. There are, however, exceptions for bathtubs, showers, sinks, washing machines and laundry sinks as long as they discharge to an approved greywater recycling system. The Code allows for limited uses of treated or recycled greywater

Some Factors to Consider in Creating a Greywater Systems
  • the size of the lot
  • the topography
  • subsoil texture
  • subsoil structure
  •  soil depth
  • restrictive horizon factors
  • soil drainage – internal characteristics and external factors such as flooding, soil permeability, and flooding characteristics.
  • Sandy and loamy soils are best and the horizon must be over 36 inches from the ground surface.
Greywater Systems

     There are a range of different options for sub-surface systems, but they all need to be approved by appropriate authorities.  Types of systems include:

  • Evapotranspiration
  • shallow trench
  • shallow mound
  • pressure effluent dosing and drip irrigation

         The evapotranspiration system combines the process of evaporation and transpiration (the process whereby plants take in water through the roots, and convert it to vapor which is given off through the leaves) to utilize and dispose of wastewater. A typical evapotranspiration system consits of a septic tank for pre-treatment followed by distribution into a shallow sand bed covered with vegetation. The greywater flows from the house through the septic tank and into the evapotranspiration bed where the water is distributed through perforated pipes.

 

        The shallow trench system consists of water flowing through pipes into shallow trenches after it goes through pre-treatment. The pipes in this system are placed 8 inches deep and close enough to the surface to feed the plant roots.

 

       The shallow mound system uses an elevated absorption field for disposal wastewater. A shallow layer of sand fill and top soil is placed over the existing soil for irrigation. This technique is recommended for when the existing soil is unsuitable for wastewater disposal. Pumping is required for this system and the pipes are placed near the root zone to provide irrigation.

 

       Pressure effluent dosing and drip irrigation systems require more maintenance, but are the most common.  This system can overcome limitations such as shallow soils, high ground water, excessive slopes, and uphill drainfields. The greywater flows through the house, through pre-treatment, and is pumped through perforated pipes into the absorption bed. Low pressure dosing is used for houses of no larger than 2700 square feet or with four bedrooms, and water saving fixtures are required in conjunction with this system.

 

A three step filtration process is advised for treating greywater in the home:

  • Step One: a strainer (pre-filter) in the laundry trough, shower or bath outlet to remove large-sized materials.
  • Step two: A mesh filter installed in the collection tanks to collect hair, soap particles, lint and some entrapped body fats.
  • Step three: a fine filter on the supply line to the irrigation pipes or toilet cistern for precipitates and settled materials.
Filtration with Lignocellulosic Fiber Mats

         Filtration with lignocellulosic fiber mats has the potential to clarify water contaminated by various pollutants. It should be emphasized that this filtration media applies to both greywater and rainwater, which is discussed in a later section. The systems consist of an initial settling tank and a separation tank containing the filter media made of recycled wood fiber. The fiber filter can also be modified chemically using aluminum oxide in order to enhance the removal of phosphate and heavy metals. It has been found that fiber filter media removes not only particles, but also nutrients such as nitrogen and phosphate.

 

 

        

 

 

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