Water Miles

The following was first printed in The Last Straw Journal, an international Straw Bale building journal, in their special feature on Water Conservation and Management:

Greywater, Greywater Everywhere – But not a drop to drink?

by Craig Brown – Waiheke Island, New Zealand

To answer one of the questions I get asked a lot, it is possible to treat greywater to potable standard. However, that would be excessively expensive and energy and resource intensive, especially when there are uses for it which do not require the water to be of such a high standard. If lower quality water, produced cheaply, can be used for purposes like flushing toilets and watering gardens, then more high quality water is available for drinking. It is possible to save 70% or even more of your water use by recycling your greywater. At least 50% is very achievable for people who normally water their garden.

There is another benefit: your wastewater system (whether it is an on-site septic tank type system, or a council treatment plant) will have less load on it if you take out the greywater. This can make the difference between a well-mannered septic and a smelly bog or, on a large scale, could help municipalities avoid significant costs in upgrades and operation. There are energy benefits too. In New Zealand 24% of councils’ greenhouse gas emissions result from water and wastewater services because of the large amount of power used to pump and treat it[1].

What is greywater?

Greywater is wash-water, or sullage. It is the water produced by your home that is not ‘blackwater’, or sewage. Of course, once mixed, all water leaving your home is sewage, but if you plumb greywater separately it can be reused. It makes sense to consider only baths, showers and washing machines as a greywater source for reuse since they are the cleanest and provide the most water (approximately 70% of the water produced by an average household). Everything else (wash tub, kitchen and bathroom sinks, dishwasher and of course toilets) should be plumbed as waste, since these sources have a higher chemical and bacterial load, and some water is required to ensure that the pipes don’t clog.

How can it be used?

Note: Actual uses of greywater may be restricted by law. Laws vary from place to place and often contradict each other. The following is my opinion of what should be allowed.

There are a variety of ways to reuse greywater – these are just some of them. The longer you wish to store it and the more contact you wish to have with it, the higher the level of treatment that is required.

One simple option is to let your washing machine discharge into a pipe which runs downhill to a dry part of your garden. It would be advisable to either have some holes along the length of the pipe so that some water can escape across a wide area, or have mulched pits for it to flow into. It is important not to allow greywater to collect in pools where bacteria can breed, enter waterways or leave your property. Be aware that you will have to be careful to use only eco-friendly, low salt washing products.

You will want to avoid watering acid-loving plants as greywater is alkaline. Some plants thrive on this water (e.g. bananas and citrus), others are indifferent (e.g. oleander) and some may not like it at all (e.g. camellias and azelias). Do not water vegetables with greywater. It may be a good idea to have several areas in the garden which you can switch between to rest them. It may be possible to plumb your bath and shower water into this system too, but this increases the possibility of overloading an area.

Another option is to store the water for later use. This water will go ’septic’ if you leave it without treatment and become a health hazard but it is difficult to design a system for yourself that is able to treat the greywater to the right level (that is sufficient treatment to kill the bacteria, but avoiding heavily dosing the water with damaging chemicals). It is still important to use eco washing powders and to avoid high bacterial loads like nappies (otherwise the treatment level required is high). Also, unless the use is manual (e.g. with a bucket) you can encounter the problem of filtration. Often homemade systems fail because the filter (or lack of one) doesn’t protect the pump, or clogs the irrigation line. Alternatively, the filter protects the pump but collects everything, so that every time you run the washing machine or take a shower you need to empty it of hair, lint and synthetic fibres.

Cleaning filters and adding treatment chemicals can be a chore and systems often get abandoned due to high maintenance requirements. Commercial systems are available now that can filter, treat, store and pump the water through drip irrigation pipes (an efficient irrigation method) with minimal maintenance requirements. This means that irrigation can be distributed more evenly (avoiding issues of ponding and overloading) and can be uphill from the source. These systems also enable other uses, such as using the greywater to flush toilets and can be fully automated. Make sure you ask the manufacturer what maintenance is required for the system and if possible ask an existing user to share their experiences with you.

A final option is to treat the water to an even higher standard, so that it can be used for spray irrigation, car washing, or even clothes washing. If these uses are required then there are rather expensive systems available to achieve this, but maintenance and operational costs, as well as purchase costs, should be considered, since it is likely that the water produced will be more expensive than fresh water and some systems may use a lot of electricity. If rainfall in your area is reasonable then it may make more sense to capture it from your roof than to treat the water to this extent, but these systems can make sense in very water-poor areas, if you are required by law to meet very stringent standards of treatment, or if you need to relieve the load on your wastewater system. They may also be more appropriate for a commercial or multi-unit building.

Greywater heat recapture

Greywater is usually warm water and it is possible to plumb the greywater through a heat exchanger so that fresh water entering the house for the hot water supply can be pre-warmed. This works very well for showers where water is being supplied at the same time as it drains. Otherwise systems that store the greywater may need to be used since the hot water input may not correspond to the greywater output.

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Craig Brown is the Business Development Manager of ECOplus, which manufactures a New Zealand designed greywater recycling unit <craig@greywater.co.nz>, www.ecoplus.co.nz. He is moving into sustainability teaching and consulting from his background in Ergonomics and has plans to build a straw-bale house soon.

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[1] International Council for Local Environmental Initiatives – Australia/New Zealand (2006) Communities for Climate Protection – New Zealand: Inventory Report 2006. [Electronic Version]. Retrieved 14 March, 2007, from ICLEA website: http://www.iclei.org/fileadmin/user_upload/documents/ANZ/CCP/CCP-NZ/Measures/CCP-NZ_InventoryReport2006.pdf