Greywater is defined as waste from laundry, sinks, and showers. It does not include wastewater from toilets.  
Greywater systems take non-fecal water from showers, bathtubs, sinks, washers and dishwashers and reuse that water for irrigation, cooling devices and in some cases toilet water.  
One type of a graywater system is called a pass through system. The filtering process in pass through systems ideally uses only gravity, bacteria, plants and gravel or sand. The graywater may pass through a settling tank or sand filter prior to entering these systems in order to settle out solids. The water can then drain or be pumped through these pass through systems. Once through the system, the water may be recycled, used for irrigation, or for a landscape amenity such as a pond, fountain, or pool.  
To ensure a clean, healthy, and efficient pass through system it is important to keep in mind the following suggestions:

--Keep all organic material out of the system other than the small bit which will come from bathing and dishwashing. This includes any liquid organic matter such as juice, beer, or oils
--Eliminate the use of all toxic chemicals and non-biodegradable soaps in the water that enters the systemDo not use cleaning products containing boron as it is toxic to most plants
--Limit human contact with the graywater by filtering it below the surface of the ground
--Do not drink the graywater or allow the water to come into contact with edible portions of crops
--Do not allow the graywater to pool on the surface

Getting a grasp on the how much graywater may be available for use for a pass through system is fundamental:·    

--The amount of graywater generated per person per day varies from 25 to 45 gallons, plan for about 30 gallons per person in a water conservative home
--42-79% of household graywater comes from the bathtub or shower, 5-23% from the laundry,  10- 17% from the kitchen sink or dishwasher, and 5-6% from the bathroom sink

Two examples of a pass through system:  
Evapotranspiration: a system in which plants absorb the graywater and transpire the moisture into the air in the form of vapor. One example consists of a shallow trench with a waterproof lining (such as clay or plastic), filled with an inch or two of standard gravel, and six inches of pea gravel. It is important to have pea-sized gravel so that there will be a lot of surface area on which bacteria can grow. The bacteria consume some of the waste themselves to reproduce and grow more bacteria, and the rest of the converted waste is used by the plants. No soil is added, the plants are rooted in the gravel. This system also works with a shallow sand bed covered with vegetation. Good plants for this system include canna lilies, iris, cattails, and ginger lily. An average two bedroom house may require a three foot wide and seventy foot wide trench. This system is great for residential properties as it is a smaller system that can easily run the length of a side of the house.  
Subsurface Flow Constructed Wetlands: this system consists of aquatic plants in a wet substrate, usually gravel. Keeping the water below the surface of the gravel medium is ideal for this type of system, especially if used in a residential setting, as it helps to keep any odor down, there is less human contact, faster treatment of the water (more contact with roots and gravel), and is less likely to freeze. The gravel should be uniformly small to medium sized, and from one to three feet deep. Sand, mulch, or topsoil can be added over the gravel medium, though just gravel is fine too. Good plants for this system include cattails, rushes, sedges, and reeds. One cubic foot of artificial wetland is estimated for every gallon of graywater produced. An average one bedroom house may require a 120 square foot wetland that is one foot deep. The water that passes through this system may be recycled, or may help to recharge ground water, or may flow into an open surface wetland for more wildlife habitat. The subsurface wetland is also referred to as vegetated submerged bed, root zone method, rock reed filter, microbial rock filter, soil filter trench, and reed bed treatment.  
Greywater systems originated as a set of ideas to conserve and reduce the amount of water that we use on a daily basis. A simple explanation of greywater is that greywater is wash water excluding wastewater from toilets. Greywater is different than usual wastewater (Blackwater) in that it includes no human or organic waste.  
  Figure 1  
Figure 1 shows with mixing the two types of wastewater together the increase of the biological oxygen demand (BOD) is much greater then leaving them separate. On a more detailed level the difference between grey and black water is that greywater contains less than one-tenth the nitrogen that is present in blackwater (Figure 2).  
  Figure 2.