Grey-water harvesting (also called grey-water, grey water and gray water), is differentiated from other harvested water sources like rainwater or condensate in that it has already been “gently used”– usually as water from showers and sinks, but it could also be sourced from ponds, pool backwash or process rinse water. It is most often harvested to flush toilets in a building, but the cleaned water can also be used for irrigation and other applications. Toilet waste water and kitchen sink waste water is termed “black water” and is not normally considered for harvested water systems.
The most important advantage of grey-water over rainwater for harvesting is that in a residential building or school/colleges – where there are lots of uses of showers and sinks – the grey-water provides a constant supply of harvested water for flushing toilets. Our experience is that the grey-water supply can usually meet 100% of toilet flushing requirements. And because the supply is steady and predictable, (unlike rainwater!), the storage requirements are dramatically less, saving storage space and costs.
Harvesting grey-water is a relatively new practice in commercial and institutional buildings, and carries many system and regulatory implications not associated with rainwater or condensate harvesting. Unlike other renewable water sources, grey-water normally contains biological and chemical contaminants that can quickly turn the water to septic “black water”, resulting in unpleasant odors, colors and health hazards if not treated correctly.
How is grey-water reused?
Grey-water from baths, showers, washbasins and washing machines has to be collected separately from black-water, treated and eventually disinfected for reuse as a non-potable water source.
Reusing grey-water can be as cheap and easy (but labor intensive) as bucketing water outside, or as complex and costly (but convenient to use) as installing automatic grey-water diversion, treatment, distribution and/or irrigation systems.
Garden irrigation is most commonly applied, whereby grey-water can be bucketed or diverted to the garden for immediate use. Advanced systems are also available that collect, filter and treat grey-water for indoor use such as toilet flushing or laundry washing.
Laundry washing accounts for 10-30% of the average household water use. Grey-water from laundry is easy to capture and, with the right choice of laundry products, the treated grey-water can be reused for garden watering or irrigation.
Grey water composition and characteristics
Grey water is a reflection of the household activities and its characteristics are strongly dependent on living standards, social and cultural habits, number of household members and the use of household chemicals. Grey water from bathtubs, showers and hand washbasins is considered as the least polluted grey-water source. The average grey water contribution to the total organic load (BOD5) amounts to about 40 – 50%. Grey water also contributes to one fourth of the total suspended solids and up to two thirds of the total phosphorous load. Dish-washing and laundry detergents are the main sources of phosphorous in grey-water. In countries where phosphorous-free detergents are used, these loads are minimal. Kitchen grey-water is the main source of nitrogen in domestic grey-water, while the lowest levels are generally observed in bathroom and laundry grey-water.
Why grey-water recycling?
The main purpose of grey-water recycling is to substitute the precious drinking water in applications which do not require drinking water quality. Non-potable reuse applications include industrial, irrigation, toilet flushing and laundry washing dependent on the technologies utilized in the treatment process.