Sump Tank Plumbing

[jammy]

Hi - I posted this on Scuttlebutt but was suggested put here for more responses:

I want to refresh the plumbing for my grey water systems and wanted some advice for installation. Currently my sink and shower drain for fwd port and stb heads drain to a single shared sump tank. If on port tack then this seems to flush back through the starboard shower drain, and vice versa for the other tack. Should I have one way valves in the system or an anti-syphon swan neck?

Cheers,

James

 

[kengill]

Your problem is gravity - not pressure so a swan neck solution is unnecessarily complex.
IMHO you need two one way valves immediately depending from the drains themselves to reduce the amount of swilling back water.

An alternative is to introduce simple ball valves which can be operated each time as standard but that will mean creating a removable access cover in each system so you can turn the handles each time and making sure they're operated correctly.

i reckon the one way valves are a fit and forget solution.
Fair winds
Ken

 

[HeadMistress]

Since you're not likely to shower, or even be using those sinks much if at all, while underway under sail...is there any reason why you can't just pump out the sump after motoring out of the marina or anchorage, before raising sail?

Seems to me that would be much simpler solution than retrofitting check valves (that can very easily become clogged with hair, soap scum, toothpaste etc)...Or am I missing something?

 

[Ships_Cat]

I think there is only one full fix for the problem and that is to replace the tank. If you are replacing the plumbing then this may be possible.

The waste should enter the tank either through the top (usually not possible on a modern yacht due to insufficient headroom under the sole) or on the centreline at the top of the fore or aft end. The tank should be sized and the level switch inside it set at a height so that when the boat is heeled on either tack and the level switch is just about to actuate the pumpout pump, the grey water level is still below the intake from the bathrooms.

The tank need not be very big if the pump is of sufficient capacity. The tank can be sized by using the flow in and the pump capacity to see how frequently the pump will cycle when grey water is continually flowing into it. The cross sectional area of the tank in plan needs to be sufficient such that the tank holds the desired cycle volume between the on and off positions of the float switch. Our own tank is only around 8-10 litres and when showering the pump cycles almost continuously. Obviously, with this arrangement the pump capacity needs to exceed the expected flow from the bathrooms which is normally easy to do as the fresh water flow to the bathrooms is limited in total by the capacity of the freshwater pump.

On our own boat which is arranged as described, the top of the grey water sump tank is only the thickness of the sole (maybe 12mm) lower than the level of the bottom of the shower tray and we have never had a feedback problem.

Other than that I think you are stuck with non return valves (which are likely to cause problems) or, as Peggy suggests, resorting to pumping the tank before sailing. On some large boats I have seen used long flexible duck bill type rubber valves which insert into the waste pipe to prevent "feedback" along the sewage plumbing system when, say, a toilet is flushed. However, as far as I know these are only available in domestic plumbing type pipe sizes eg around 50 mm - it may be possible that smaller ones are available somewhere and if so may do the job as they are not affected by hair, etc.

John