Plumbing the waste. Tank sizes etc

We spent a couple of hours yesterday trying to make detailed plans for the grey water waste system.

It is very difficult to find much information on this, obviously our goals are not very common yet. I was quite shocked to find lots of people in discussions still advocating discharging raw sewage into the sea (and even in some cases into rivers and harbours). A really good reminder of why we won’t be using salt water to do washing up.

Anyway, our needs are unusual because we won’t have any black sewage to store or dispose of. Again, I want to push for how awesome composting toilets are, we both agree that if we had our own home rather than one provided by my job we would be switching to composting toilets there too.

So we don’t have any black sewage to worry about. Fantastic, makes life really easy. Except we also don’t want to put grey water out into the sea either. Our grey water is going to be as safe as we can make it in terms of the lack of chemicals and plastics in our soaps, shampoos, washing up liquids etc. However, it is still not what we or anyone wants to go into the water in our harbours, rivers and coastline where we swim and where there should be lots of marine life.

Therefore we want to collect the grey water to give us the option of pumping it out where there are facilities, or pumping it into a portable tank to empty into a proper drain or as a last resort pump out well out to sea. But what tank capacity do we need and and how do we connect it up? There are lots of articles on the capacity of black sewage tanks, I’m struggling to find stuff on grey water tanks (many people mix the two anyway).

Starting with the immediate tanks

If we begin with where the waste water comes from. On Vida there are essentially 4 sources of grey water (basin in aft heads, sinks in the galley, basin in the forward heads and shower in the forward heads).

Beyond this there are a couple of other sources. The hand powered washing machine also has a hose so that can be directed into a sink or basin depending on where you use it. We will also have a hose on the spin dryer which will go into the nearest sink (galley if used in the cockpit, basin if used in the forward toilet compartment).

So each basin/sink will drain straight into a waste tank directly below it. That gives us storage at each place the waste comes from, except the shower, that runs into a the sealed bilge area under it as a temporary waste water catchment but it can’t stay there.

We have now found tanks to use in the galley (31 Litre tank from C.A.K.) and for both wash basins (32 litre tank from Tek-Tanks) based on what will fit best.

Where does it go next?

Until they are full the grey water can stay in the 3 intermediate tanks. But we do not want them all to have a deck pump fitting. So all three plus the shower drain will be connected to a pump to bring them to a central larger tank.

So the next step is to calculate how much extra storage is needed. In an ideal world you would have huge tanks to give the greatest freedom from needing to empty them. We don’t have that much space.

So far we have found the largest tank we can fit under the saloon U-seating (the least accessible storage that can fit a tank), this is a 51 litre tank again from Tek-Tanks. We will have a single pump that can bring grey water from any of the 3 other tanks, plus the shower bilge into this central tank. Then this tank will have a hose to a standard pump out deck fitting.

I’m planning to use 22mm pushfit plastic pipe and fittings for the connections between the tanks. The standard for marine fittings seems to be 19mm so I’ll need to figure out a way of connecting them. The inlet to the pump will have T connectors and a tap for each tank (and the shower). I figure that when you start the pump you just open the taps in sequence (the pump can run dry for 30 minutes so there is no desperate rush).

So when you have a shower you will turn the pump on for the whole time. The other tanks will only be pumped out when they are close to full or when the whole system is being pumped out.

We will have to see how long these tanks last before they fill up. I suspect that if we have a lot of showers the big tank will fill up first. If that happens we will have to look at interconnecting some more tanks with that main one to add capacity. We can probably add one more 30 litre under the saloon bench, beyond that we would be looking at pumping it somewhere else in the boat eg somewhere in the forecabin depending on how we configure that.

Pumping out

The main central tank will have a deck pump out point which meets the international standard. That will be the only way to get grey water out of the boat, so when we empty it ourselves we will use a portable pump. That is simpler than having extra plumbing, valves and a seacock to drain it out to sea. It also discourages us from taking short cuts (just opening the seacock and pretending nothing is happening). If we are in a really restricted area without pump out facilities we can always pump it out into a jerrycan and carry it ashore.

Cleaning, maintenance and monitoring

All the tanks will have inspection hatches and we will make sure they are accessible. So if the main pump or tank has a problem the portable pump could be used to empty any tank (we just need to make sure we have a long enough hose). They can also be used to clean the tank (hopefully several smaller tanks won’t mean lots more cleaning needed.

I’m also going to fit as many as possible of the tank connections into the top of the tanks (using an internal pipe to draw water up from the bottom where needed). That should mean I don’t need to empty tanks to replace hoses, reseal fittings etc.

Adding the intermediate tanks does appear to make things more complicated. However, it adds capacity and makes plumbing easier as I don’t need to worry about relative heights (and how these change when the boat heels). It also means the pump can be controlled manually as needed.

I’m guessing that we will need to add gauges to measure tank levels, hopefully we can manage for a while and that can be a job for another year.

Still to plan

We have not started looking at taps and basins for the heads yet. Nor anything for the shower.


Hopefully this is useful for anyone else wanting to be serious about not releasing grey water into the sea (and without a solution you could be looking at big fines in Turkey, we expect others to follow their example.

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