It goes without saying, boats are not connected to the sewer or water system like we experience at home. Instead, boats store, collect, and take in water for important onboard amenities. Boats have three types or “water systems”, fresh water, grey water, and black water. Let’s discuss how all three systems work.
Fresh Water- Fresh water is stored in plastic tanks built into the boat, commonly these tanks are under a bed or settee and can be anywhere from 10-100 gallons. On a modern sailboat, tanks are +/- 50 gallons and boats above 40’ have two or sometimes three tanks. Since the water tanks are at or below sea level, a 12 volt freshwater pump is used to pressurize the system and deliver water to the faucets as needed. This pump will come on when it feels the pressure drop below a certain level and turn off once the appropriate pressure is achieved. Without an onboard water maker or access to dock city water, these tanks are the only source of freshwater for extended trip.
Grey Water- Grey water refers to water after it has been used. The most common grey waters are from the sink and shower but any water source that is used in some fashion onboard is now considered grey water. For now, grey water is allowed to be drained directly overboard via the boats plumbing. As an example, sink water drains through a P trap and then exists via a thru-hull located below the drain. Sometimes two drains, like the sink and refrigerator, lead to a single thru-hull for reduced build cost and ongoing maintenance.
The other example of common grey water comes from the shower, but this water typically takes a slightly different path. Considering most shower drains are at or below sea level, it would be impossible for the water to use gravity to exit the thru-hull like the example above. Instead, most boats will have a dedicated shower pump the turns on and evacuates the water collecting at the bottom of the shower. Some boats will have an automatic pump with a sensor while others may have a manual switch.
It’s good to keep in mind that everything you put down the drain leads directly to the ocean.
Black Water- Black water comes from the head and simply put… is sewage. Unlike grey water, black water can only be discharged from the boat in certain situations. When the head is flushed, it goes through a hose and eventually makes its way to the black water tank or holding tank. Traditionally, the waste would first pass through a Y-value or diverter value that would send the waste to the holding tank or to the direct overboard thru-hull. That said, it’s common to now see gravity holding tanks that do not require a Y-valve. Explanation of both coming below.
In the US, it is legal to flush black water into the sea given you are more than three miles from any point of land. In this case, one could simple direct the Y-valve to the overboard position, open the head overboard thru-hull, and use the head. If your boat has a gravity holding tank, you simple open the tank outlet thru-hull and the waste will go from the head, straight thru the holding tank and overboard.
Since you are allowed to pump directly overboard while three miles offshore, you are also allowed to empty your holding tank of all waste. If equipped with a gravity tank, you simple open the valve at the bottom of the tank and gravity will take the waste and empty it overboard. If you are equipped with a traditional Y-valve and macerator, it works a little different. In this set up, the holding tank will have a hose that leads to a macerator pump and then to an overboard thru-hull. One would open the overboard thru-hull and engage the macerator pump to empty the tank.
FAQs
Maritime regulations governing grey water
Per Annex IV of the International Convention for the Prevention of Pollution from Ships (MARPOL), grey water can be directly discharged into most marine environments without further treatment.
Can you pump grey water overboard? ›
Maritime regulations governing grey water
Per Annex IV of the International Convention for the Prevention of Pollution from Ships (MARPOL), grey water can be directly discharged into most marine environments without further treatment.
Where does sewage go on a sailboat? ›
Treated sewage passes through a Type I or Type II Marine Sanitation Devise, or MSD. Depending on the boat set up, the treated waste is then stored in a Type III MSD (holding tank) or discharged directly overboard. Untreated boat sewage may be stored in a Type III MSD or discharge directly overboard.
Where does the waste go in a marine toilet? ›
Marine toilets with dual function pumps draw their own flush-water from a seaco*ck or a water tank and pump the waste to either another seaco*ck or a holding tank. Their capacity to pump waste generally far exceeds their flush water pumping capacity, which means that the bowl is completely emptied.
What happens to waste on a sailboat? ›
There are a handful of ways boaters get rid of their wastewater. Most boats have holding tanks, which can be pumped into an on-shore facility, and some have a Y-shaped valve with the ability to either store waste or directly release the contents of the holding tank into surrounding water.
What is black water on a ship? ›
Blackwater describes wastewater containing fecal matter and urine. Greywater and blackwater are contained on ships in separate holding tanks. On large vessels, whether or not the effluent is treated will depend on the ship's location and destination.
Is it OK to leave your gray water tank open? ›
The Benefits and Drawbacks of Leaving Your Gray Tank Open:
They highlight the convenience of continuous drainage and the prevention of odor buildup as advantages. However, they also caution about potential issues with clogs and the need for regular tank maintenance when leaving the gray tank open.
What can never be dumped in inland waters? ›
Proper Waste Disposal:
If one is within 3 miles of shore or in any lakes, rivers, or inland waters one may not dump any garbage overboard. In some areas, one may dump grey water or fresh fish parts, but never plastics or other forms of trash. Finally, one may never dump untreated human waste in any inland waters.
What is the difference between GREY water and black water on a boat? ›
Gray Water and Black Water
While we explored that gray water is water from showers, baths, and sinks, its toxic counterpart is black water — a.k.a. human sewage. Both gray water and black water can be released untreated into the oceans in many places as long as cruise ships are far enough offshore.
Is urine considered black water? ›
Blackwater can contain feces, urine, water and toilet paper from flush toilets. Blackwater is distinguished from greywater, which comes from sinks, baths, washing machines, and other household appliances apart from toilets. Greywater results from washing food, clothing, dishes, as well as from showering or bathing.
In sailing vessels, the head is the ship's toilet. The name derives from sailing ships in which the toilet area for the regular sailors was placed at the head or bow of the vessel.
Can you put toilet paper in a marine toilet? ›
It's crucial to choose a toilet paper that is designed specifically for marine and RV use, as these papers are specifically engineered to break down quickly and easily in water. Second, it's important to note that not all paper products should be flushed down the toilet.
Do marine toilets smell? ›
However, this is broken down by anaerobic bacteria. These are the very bacteria that cause the odour during the breakdown of the impurities in the water. During this process, these bacteria secrete hydrogen sulphide (or: H2S) and this creates that characteristic rotten-egg smell.
Can a sailboat go underwater and come back up? ›
If a capsized vessel has enough flotation to prevent sinking, it may recover on its own in changing conditions or through mechanical work if it is not stable while inverted. Vessels of this design are called self-righting.
Why do sailboats not tip? ›
How do they stay balanced with so much weight way up high? They do so with by using a ballast, a heavy weight held under the boat that helps lower the center of mass. In sailboats, the ballast is usually part of the keel, a large fin-like structure under the boat that serves two purposes (Figure 1).
How long can you leave a sailboat in the water? ›
Fresh water is less corrosive than salt water, so you can leave your boat stationary longer. Still, avoid leaving your boat in the water for longer than a month to help prevent damage.
What happens if your GREY water tank overflows? ›
Yes - you should be concerned. The only thing overfilling a gray tank should do is to back up into the shower. It should not be leaking into the basem*nt. This would indicate a leak in the shower P-trap or other plumbing associated with that gray tank.
Does a gray water tank need to be pumped? ›
In simple terms, gray water is just water that does not have any sewage inside of it. Some examples of gray water are laundry, a sink located in the basem*nt, and a shower located in the basem*nt. The only time you would need to pump gray water would be if you do not have the ability to gravity drain the water.
What do you do with GREY water on a boat? ›
When at sea, retain your graywater for a pumpout facility. If this is not possible, treat graywater as if it were sewage, and only discharge if you're at least 3 miles offshore. Try water-only wash downs. Often times they can do the trick when cleaning your boat.
Can I use a sump pump for GREY water? ›
A greywater sump can be used to reduce the loading on a septic system that has inadequate leaching capacity. The diagram above shows a typical sump sized for two wash loads a day and is the minimum size for a grey- water sump (96 cubic feet) - see figure below.
