Black water systemes: problems and solutions!

20 March 2019 | | Reading time 4 minutes

There are various water systems on a boat, including black water systems (wastewater from the heads). It is not a subject that people talk about that much, but dealing with black water can be a real headache. It is a delicate but essential matter. If you have a weak stomach, read no further!



The first thing to understand is that the heads on a boat are systems that grind up and eliminate wastewater. As everyone will tell you, when it comes to toilets, prevention is better than a cure! With this in mind, here are a few basic rules for correct use.

First of all, be extremely careful about what you dispose of in the system. You should only use specially-adapted toilet paper. Next, always clean the heads with special eco-friendly products. Do not use any products containing acetone, trichloroethylene, or any other product that can damage rubber parts.



To keep the system in good working order, it is essential to understand the mechanisms involved.

There are two types of toilet:

  • Manual toilets, which you have to pump by hand to create suction to flush.
  • Electric toilets: simply push a button to activate the inlet of freshwater into the bowl and flush away the black water.

In some navigable waters, you are advised not to discharge black water into the sea, and indeed prohibited from so doing in some regions. There are two ways to empty out your toilets: either have them emptied by a company specialized in doing this in port, or discharge black water into the sea (provided that this not prohibited). Even so, out of respect for everyone, you MUST discharge black water in offshore waters, at least six nautical miles from the coast.

In practice, when you flush the toilet, the black water is pumped into a black water tank. If the outlet valves are closed, then the black water remains in the holding tank.


You can prevent disaster by following a few simple rules:

  • To keep the hull outlet valves, connected to the heads, in good working order, you should open and close these valves (concealed under the sole) at least once a month.
  • If you are offshore, in a zone where discharging black water is allowed: rinse out the tank as often as possible, by opening the valves, filling the tank (using the deck filler) with freshwater and pumping it out via the through- hull outlets. It is advisable to take the boat out of the water once a year and clean all the through-hull fittings.
  • If you are in waters where the temperature is below zero, you should also drain the black water system to prevent water in the pipes from freezing. Use of antifreeze is not recommended, since it is not compatible with the system.
  • Lastly and most importantly, drain your black water system before leaving your boat. Also remember to flush the system through using freshwater (never seawater) so that your system will contain only clear water. If you fail to do this, the black water will dry out, block the pipes and damage the tanks, and your boat will reek of bad smells.


Should your toilets ever become blocked, it could be for one of several reasons: there may be something blocking the grinder, or blocking the manual pump or your through-hull may be blocked. If you want to repair the system yourself, then certain precautions should be taken, including closing all valves connected with the heads, and cutting off the water, to carry out the repairs safety. You can dismantle the grinder, and its motor if necessary, to remove whatever is causing the blockage. If you have manual toilets, you can dismantle the pump.