What is Grey water?
Grey water can be defined as any domestic wastewater produced, excluding sewage. The main difference between grey water and sewage (or black water) is the organic loading. Sewage has a much larger organic loading compared to grey water.
Some people also categorise kitchen wastewater as black water because it has quite a high organic loading relative to other sources of wastewater such as bath water.
People are now waking up to the benefits of grey water re-use, and the term “Wastewater” is in many respects a misnomer. Maybe a more appropriate term for this water would be “Used Water”.
What Can Grey water Be Used For?
With proper treatment grey water can be put to good use. These uses include water for laundry and toilet flushing, and also irrigation of plants. Treated grey water can be used to irrigate both food and non food producing plants. The nutrients in the grey water (such as phosphorus and nitrogen) provide an excellent food source for these plants.
What Are the Benefits of Grey water Re-use?
Re-using water provides benefits on many levels.
Two major benefits of grey water use are:
- Reducing the need for fresh water. Saving on fresh water use can significantly reduce household water bills, but also has a broader community benefit in reducing demands on public water supply.
- Reducing the amount of wastewater entering sewers or on-site treatment systems. Again, this can benefit the individual household, but also the broader community.
How the Grey water is treated For Re-use?
There are many ways by which to treat grey water so that it can be re-used. The various methods used must be safe from a health point of view and not harmful to the environment.
All systems that store grey water have to incorporate some level of treatment, as untreated grey water deteriorates rapidly in storage.
This rapid deterioration occurs because grey water is often warm and rich in organic matter such as skin particles, hair, soap and detergents. This warm, nutrient-rich water provides ideal conditions for bacteria to multiply, resulting in odour problems and poor water quality. Grey water may also contain harmful bacteria, which could present a health risk without adequate water treatment or with inappropriate use. The risk of inappropriate use is higher where children have access to the water.
Direct reuse systems (no treatment)
It is possible to reuse grey water without any treatment provided that the water is not stored for long before use. For example, once bath water has cooled, it can be used directly to water the garden.
Using grey water in this way may not suit everyone, but it does provide an inexpensive and easy way of saving water and avoids grey water storage issues. It is particularly useful for keen gardeners when water is scarce. The grey water is not treated, so must not be stored for too long because the water quality will deteriorate rapidly.
Short retention systems
These systems take wastewater from the bath or shower and apply a very basic treatment such as skimming debris off the surface and allowing particles to settle to the bottom of the tank. If this water thus cleaned is not used within a certain time, the stored treated water is released and the system is topped up with mains water. Potential water savings are dependent on usage patterns. Major benefit of short retention systems is that they can be located in the same room as the source of grey water, reducing the need for expensive, dual-network plumbing
Basic physical and chemical systems
Some systems use a filter to remove debris from grey water before storing and use chemical disinfectants (e.g. chlorine) to stop bacterial growth during storage.
Pros & cons can be described as follows
• Water savings range from less than 20 to over 32 per cent of total water use;
• Reliability varied;
• Filters required regular cleaning to avoid blockages;
• Occurrence of odour problems due to either poor water quality or high levels of disinfectant;
Biological systems vary in their complexity and form, but the concept is the same: bacteria are used to remove organic material (contamination) from wastewater. The process uses the principles employed at sewage treatment works. Oxygen is introduced to wastewater to allow the bacteria to ‘digest’ the organic contamination. Different systems supply oxygen in different ways; some use pumps to draw air through the water in storage tanks while others use plants (Reed Beds) to aerate the water.
The most advanced domestic grey water treatment systems use a combination of biological and physical treatment. Such systems are best installed during construction and are not suitable for retrofitting into existing buildings due to cost and other practical difficulties.
Organic matter is removed by microbial cultures formed on media. Solid material is allowed to settle to the bottom of the tank and is removed automatically. The system encourages bacterial activity by bubbling oxygen through the water. The final stage of the system is UV disinfection to remove any remaining bacteria.
Combining physical and biological treatment generally produces the highest quality water, but need careful operations & maintenance.
This high level of water quality may not be required if the use of treated greywater is restricted in an individual property for toilet flushing. But where stored grey water is treated to a high standard, there is potential for its use in other applications such as vehicle washing. A high standard of water quality may also be required in communal systems to overcome both real and perceived risks associated with using the treated water.