Grey Water Tanks are typically small tanks of about 1 gallon which collect the waste from showers or sinks and then pump the waste overboard automatically using a bilge type pump with an inbuilt float switch

Grey water tanks are designed to collect waste water from showers, sinks, air conditioners and washing machines. Some boats have a grey water tank for the shower only and is known as sump tank. Quite often the shower outlet is below the waterline and needs to be pumped overboard above the waterline. By installing a simple grey water sump tank the waste water can be automatically discharged. These tanks have either a pump mounted inside the tank with a level switch or an external pump with an internal level switch. There is no facility to store the water in these tanks which can be a problem when visiting environmentally sensitive areas such as Greece and Turkey where pumping soapy water into the harbour is not allowed.

Grey water tanks


Grey water systems on larger boats are designed to take far more grey waste water and store it until ready for discharge similar to a black waste holding tank. These larger tanks can also be run on automatic which will allow the waste to be discharged in a similar way to the smaller sump tanks. Larger systems will also have a manual override switch which enables the tank to be emptied at will.

Grey water tanks
These larger tanks tend to use 1 ½” diaphragm pumps which can handle foreign bodies such as hair from the shower, peas from the sink and money from the washing machine!  The pumps are controlled by float switch type senders that energise a relay when full and then shut down the pump when empty. The switch position is critical on these systems to avoid the pump running continuously when empty.


Smaller off the shelf units have an array of ports ranging from ½” to 1 ½” to accept hoses from the shower, sinks etc. These ports are supplied blanked off and are “opened up” to suit the number of hoses required. Larger custom built tanks will have the same sizes but situated where the customer specifies.


The tank size will depend on how many appliances are going to be plumbed in, the output from these appliances and how often the tank can be emptied. It would be a good idea to check with the manufacturer of the appliance how much water it uses, for example a shower using a pump that can potentially deliver 4 litres a minute being run for 5 minutes twice a day will give 40 litres a day for the shower alone.
There are not many places in the world that forbids the discharge of grey water but they are becoming more common so grey tank sizes should be reviewed carefully when designing your system.
1. Fuel tanks
2. Water tanks
3. Grey water tanks
4. Waste tanks