Log in
Article search:

Q & A

Grey Water recycling

I am in the process of designing a 60' x 11.5' liveaboard with the build starting in July this year and want it to be as environmentally friendly as possible. I want to use the grey water on board to flush the toilet but can find no information regarding this on a boat. I have emailed companies who advertise their services for commercial buildings and houses but had no response. Can anyone tell me if it is possible and point me in the right direction

Asked by: David Nei  | 7.33pm, Friday 26 April

WW says:

Boat toilets are usually supplied with water from the boat's domestic water tank via a pump with a pressure switch. This is the same pump that supplies water to the basins, sinks, and showers. To do as you describe, you would need a dedicated pipe from the grey water tank to the toilet(s) with an additional pump somewhere along the route to move the water.
I can't think of any boat that has used this method but I can't see why it should not work. My only slight reservation is that you might generate more grey water than you need to flush the loo. This would depend on which type of loo you choose as some use much more flushing water than others. If you opted for a vacuum loo which uses very little water, you would need to have some means of emptying the grey water tank to prevent it overflowing.

Graham Booth  | 1.51PM, Saturday 27 April

Boat toilets are optimised to use as little water as possible, as unlike their domestic counterparts they discharge into a tank; which does not want to be filled up with unnecessary amounts of fresh water, since it costs money and/or effort to empty.
As Graham says, it is certainly possible to arrange a pump to supply the grey water to the toilet; but a good filtration system would be also required to ensure that the flushing valves do not get clogged, letting grey water past into your black water tank.
A typical boat toilet uses between 1 and 2 litres of water to flush, so two people could use around 15 to 20 litres of water a day just flushing the loo; whether the extra range between filling up the water tank achieved by recycling grey water, is worth the extra infrastructure cost would be your decision. An interesting idea, do please let us know if you decide to implement it.

Rupert Smedley  | 2.44PM, Saturday 27 April

Dutch boats tend to have grey water holding tanks and some of these have been adapted to use for toilet flushing systems: there are moves afoot on some busy continental waterways to prevent discharge of grey water overboard in harbours, for instance.
Ideally, the water entering the grey water tank from washbasins, showers, etc should pass through a fine filter, as you need to remove hair, good residue, fluff, etc. the filter would need to be reasonably accessible for (hopefully infrequent) cleaning.
The tank will need a lower drain point to enable the inevitable sediment to be removed (though most tanks would have this anyway) as sand etx find their way through.
As graham points out, you need to work out some form of overflow control to the tank.
You may also need to consider some form of
Treatment for the tank on a continual basis, as the stored water would become a potential pathogen breeding ground. Although low-level chlorine dosing may be appropriate (to achieve around 2 to 5ppm available chlorine) silver ion treatment and others might be more sensible. Regular additions of a low level disinfectant will prevent slime mould and other biofilms developing within the tank. Without some form of microbial control, there is a risk of pathogens being released as an aerosol when the toilet is flushed.
A grey water recycling system sounds excellent- although there is another alternative- just using canal/river water as the flushing fluid. This is frequently done on river cruisers and reduces the drinking quality water demand for toilet flushing. However, toilet flushing on boats is only a small proportion of the water taken onboard by boats- a toilet uses around 0.5 to 1.5 litres a flush onboard a boat, which is only less than 5% of domestics loos. So whether it is cost-effective to fit a grey water recycling system may Br a moot point... Also you would need to check that the toilet manufacturer will approve the system. It is however very feasible to fit such a system.

Mark Langley  | 3.10PM, Saturday 27 April

Readers say:

Thanks very much for the rep]ies they were helpful, it seems from what you say that no system is available "off the shelf".I will contact boat toilet manufactures and take their advice and attempt design a bespoke system. Thanks again

David Nei  | 7.00PM, Saturday 27 April

You must log in to post an answer.