It shouldn't be surprising that a boat's head often becomes a source of foul odors. Waste is periodically flushed into it down to the holding tank, where bacteria—some of which produce very foul gases—move in and begin to dine on the organic matter inside. Using seawater to flush your head adds a whole new dimension of living organisms, some of which die and begin to rot away inside your sanitation system.
Thankfully, there are a few ways that you can eliminate or at least severely reduce the odors emanating from your boat's head, allowing you and your guests to use it without clearing out the bottom deck.
1. Remove Odors at the Source by Eliminating Anaerobic Bacteria in the Holding Tank
The absolute best way to eliminate head odors on your boat is to stop them at the source—your holding tank. The culprit behind the odors are anaerobic bacteria, which metabolize the organic waste in the tank into foul-smelling compounds such as hydrogen sulfide. This quickly leads to a pervasive rotten egg smell coming from the head.
Aerobic bacteria, on the other hand, transform the organic waste in the holding tank into carbon dioxide and water, which are both odorless. The problem is that they need oxygen in order to accomplish this, and it's used up as part of their metabolic process. Once the oxygen is gone, the aerobic bacteria either die off or switch to anaerobic metabolism.
In order to cut down on odors and keep the aerobic bacteria in your holding tank alive and working, you'll need to ventilate it. One way to accomplish this is to attach two holding tank vents to it, plumbing one to the port side of your boat and the other to the starboard side. This creates a cross-breeze that keeps oxygen circulating around your holding tank.
However, this can be difficult to accomplish on many boats and requires some extensive plumbing work. Another way to add oxygen to a holding tank is to purchase a holding tank air pump from a marine plumbing supply store and attach it to the tank. It continually brings fresh air into the tank, keeping the oxygen levels high.
2. Check Your Sanitation Hoses to See if Odor Is Permeating Them
Another way to eliminate head odors is to make sure odor isn't permeating through your sanitation hoses. Sanitation hoses degrade over time, and the smell of waste will eventually begin to break through. If your hoses are a few years old, it's probably time to simply replace them all.
Additionally, odor permeation is more likely to occur in any bends in your hoses where sewage is allowed to collect—the increased pressure causes the hose to break down faster. If you decide to replace your sanitation hoses, make sure that they're all sloping straight downwards towards the holding tank and don't have any bends that allow sewage to collect.
3. If You Use Seawater for Your Head, Either Switch to Freshwater or Periodically Flush Your Sanitation System
Heads that are flushed with seawater rather than freshwater have an additional odor issue to contend with along with human waste—plankton and other tiny forms of sea life. These collect in your boat's sanitation system, die off, and become food for bacteria. Over time, it results in a persistent rotting odor.
The best way to deal with this is to flush your sanitation system. Take your boat to a pump-out station, run fresh water through the head until the holding tank is full, and then pump it out. You'll need to do this about ten times to make sure all of the seawater residue is fully flushed out of the system.
Another option is to purchase an electric head from a marine plumbing supply store that uses water from your freshwater tank to flush. These don't require much energy or water to operate, and they will eliminate any odors that are caused by the organisms floating around in seawater.
4. Add Enzymatic Odor Remover to the Holding Tank
Finally, if you're looking for a cheap, fast solution to cut down on odors, find a marine plumbing supply store and purchase an enzymatic odor remover. The enzymes in these products convert some of the foul-smelling waste produced by anaerobic bacteria into odorless compounds. They're simple to use—you simply flush them down the head to add them to your holding tank.
Overall, the best solution is to keep anaerobic bacteria out of your holding tank in the first place by ventilating it. If the cost is prohibitive or if it's not an option for your boat, try switching to flushing with freshwater, make sure your sanitation hoses are in good condition, and add enzymatic odor removers to your holding tank after every pump out.
With a bit of work and some parts purchased at a marine plumbing supply store, neither you nor your boating guests will have to fear foul smells emanating from your boat's head.