Water recycling - bathing in greywater!

Yes, it is happening already! Greywater, which is non-industrial wastewater generated from domestic processes such as dish washing, laundry and bathing, comprises more than 50% of residential wastewater. It gets its name from its cloudy appearance and from its status as being neither fresh (white/clear potable water), nor heavily polluted (blackwaters). Hence it has the potential for and has been considered as a solution for water scarcity for a while now. It can be used either as it is, for irrigation for example, or it can be cleaned up somewhat and then re-used for various purposes inside the home. In fact, many people have used greywater before to irrigate their garden at least, when there’s been a hose pipe ban. It might be seen as a disgusting thing to do, to reuse spent water inside the home though, but as a water scientist, I find it can be perfectly ok. Read on to see why.

Schematic of a greywater system (Google images)

What happens is: the water used around the house is collected, and then filtered, chemically treated, i.e. chlorinated, to kill pathogens, if necessary, and finally used again to flush the toilets and could even be used to shower and to do the washing, depending on the level of treatment it received. Of course, there are some exceptions, water from toilet flushing, as well as washing machine and dishwasher waters are not used in the greywater process to be re-used in the home or garden yet. Can we expect to be using greywater more widely soon though?

An article in this month’s Water & Wastewater Treatment (1) shows that Premier Travel Inn is reducing its water consumption and meeting stringent new energy efficiency targets through the installation of a greywater recovery system. The system has reduced the hotel’s water consumption by 20% by reusing water from baths and showers to flush the hotel toilets. The technology (ARC60B) used by Premier Inn has recently been added to the government’s water technology list. The programme is managed by Envirowise and Defra and provides practical advice to businesses including details of the enhanced capital allowance (ECA) programme. Waterscan’s technology is one of the many environmental technologies eligible for 100% first year capital allowances. Other technologies include rainwater management systems.

There used to be a problem with particle-associated coliforms remaining in the treated greywater, as they were shielded by the larger particles. However, an article (2) in CAB Abstracts (Winward et.al., 2008) shows this problem can be solved, thanks to advanced membrane technology, as membranes filter out particles greater than a few microns. In fact, the core element of the ARC60B is the ultrafiltration membrane, which is submerged directly into the greywater. A slightly negative pressure of only 0.1 bar draws the filtrate through.  However, the level of greywater treatment is dictated by the final water quality requirement. Greywater for landscape gardening isn’t treated with the same rigour as that to be used in the home for bathing and doing the washing.

Treated greywater has the potential for consideration as a resource, since it can be used as a supplement or replacement for potable water in landscape irrigation and other agricultural activities in rural and urban areas. Greywater is sometimes the only water resource available for crop irrigation in some parts of the world. Anybody wanting to save some water, the simplest greywater system is to simply divert the water directly to the garden. But there are some do’s and don’ts which you might want to consider, as compiled by the environment protection agency (EPA) and listed below:

Greywater dos

• Only use wastewater from baths, showers, hand basins and the final rinse from washing machines.
• Only use greywater on the garden and rotate which areas you water.
• Only apply enough water that the soil can absorb.
• Wash your hands following watering with greywater.
• Stop using greywater during wet periods.
• Stop using greywater if odours are generated and plants do not appear to be healthy.

Greywater don'ts

• Water vegetable gardens if the crop is to be eaten raw or uncooked.
• Use greywater that has faecal contamination, for example wastewater used to launder nappies.
• Use kitchen wastewater (including dishwashers) due to the high concentration of food wastes and chemicals that are not readily broken down by soil organisms.
• Store greywater for more than 24 hours.
• Let children or pets drink or play with greywater.
• Allow greywater to flow from your property or enter stormwater systems.

References

1. Hotel chain chooses greywater system. Water & Wastewater Treatment, 2008, 51(8), Pg 41.
2. Winward, G. P.; Avery, L. M.; Stephenson, T.; Jefferson, B. (2008). Chlorine disinfection of grey water for reuse: effect of organics and particles. Water Research, 42 (1-2), Pg 483-491.

Comments

[Djenk]

Great ideas for saving our most precious resource.

[Vera Barbosa]

Thanks for reading and for the positive comment.

Vera