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Holding Tanks

Over the past 30 years, sewage handling aboard boats has become an emotionally charged topic. While it has been unlawful to discharge untreated sewage in US territorial waters for decades, these laws have been unevenly enforced. Now there is a growing movement to establish no-discharge areas where even treated sewage will be banned. These laws will affect those sailors having expensive onboard sewage treatment systems installed on their boats.

Boats confined to areas with uniform no-discharge laws such as the Great Lakes have no option other than the installation of a holding tank. Long-distance sailors transiting many cruising grounds have more decisions to make. Ideally, they would have a direct overboard discharge for non-US waters, a treatment system for coastal cruising and a holding tank for no-discharge areas. And while this is possible, it is expensive, complicated and space consuming-a sailor's triple nightmare.

Flex holding tankMuch of the opposition to holding tanks has come from two myths: first that they must take up valuable stowage space. Except on very small pocket cruisers, most boats have hard-to-reach places in the bilge, beneath the sole or under settees. Two obstacles keep many sailors from filling these spaces with solid holding tank capacity: firstly, some portion of the boat's furniture and cabinetry will probably have to be removed and put back after installation. Secondly, the tank may have to be patterned and custom built to fit an oddly shaped area.

An often-overlooked option is to install a flexible tank. These can be folded into a petite package and slipped into place through a small opening. They automatically conform to odd shapes, rising and falling with the level of contents. Flexible tanks are robust and will last for many years, even if mounted vertically. Models up to 50 gallons are readily available through most marine stores.

Some tips on installing flexible tanks
  • Tanks with fittings pre-installed at the factory are designed to lie horizontally. For mounting vertically or on a severe angle against the hull side, purchase a tank without fittings and install the inlet and vent near the top of the tank, placing the outlet at the bottom.

  • Strongly secure a flexible tank by its lashing points. A 50-gallon tank weighs more than 350 pounds when full.

  • Ensure that there are no stray fiberglass needles, sharp corners or rough spots that will chafe the tank. A rubber mat under it would not hurt.

  • The second myth about holding tanks is that they smell. An odoriferous sewage system is the sign of a poor installation. Whether installing new equipment or refurbishing older hardware, the entire system must be vapor tight. Here are some things to look for.

  • Polyethylene Holding TankThe tank itself must be one piece. Whether metal, polyethylene or other plastic, it must not have any open seams. Tank fittings must be bedded and inspection plates must have O-rings or gaskets. Flexible tanks must be designed for sewage and not water or fuel.

  • Only white seamless plastic sanitation hose should be used. Impregnated cloth hose, especially if it is wire reinforced, is permeable to most sewage gases and will allow odor to pass through.

  • Check all connections between hose and hose barbs making sure that they are airtight. Make as few connections and use double hose clamps where possible.

  • Make all hose bends as sweeping as possible to help prevent waste from settling. Avoid kinks and very tight radius bends in hoses, as well as 90-degree elbows.

  • Sanitation PumpIf a diaphragm pump is used for overboard discharge, guarantee that the valves and diaphragm are made of nitrile. Most diaphragm pumps are supplied with standard neoprene parts that will deteriorate rapidly in the presence of sewage. Nitrile will not allow odors to pass as readily.

  • Use Y-valves intended for waste systems rather than stock ball valves or gate valves from the local hardware store. The latter will not seal well enough.

  • Make sure that the tank vent is in a location where odors are not blown back into the boat. Venting out the transom is generally best if the distance isn't too great. Check to see that the vent line has no low spots forming a liquid trap that prevents free passage of gasses through the vent hose.

  • Flush and clean the tank frequently. After a pump out, fill the tank again with a hose and pump it a second time. This will stir up solids and prevent them from adhering to the tank and hoses.

  • Use holding tank additives with caution. Most contain nothing more than formaldehyde, chlorides or other powerful chemicals to kill all bacteria with added deodorants to hide any remaining problems. These are particularly harmful to the environment and very difficult to get out of the tank once applied. Others contain enzymes, live bacteria or nitrates that actually promote bacterial growth in the tank. If you decide to experiment with holding tank treatments, follow the manufacturer's instructions assiduously.

Photographs by Kathy Barron

Tom Wood is offline  
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