If you are going to install a holding tank on your boat, don't just choose arbitrarily. Choosing the wrong holding tank or getting one just because it was a good buy can spell disaster down the road. The following are considerations to be made when choosing a holding tank to install on your boat:

1. To determine the capacity of the type of toilet you will need, you must realistically calculate how many people will be using it, how many daily flushes, and how often you will be pumping out your holding tank.

2. Determine the shape needed to fit your boat, the location and the dimensions of the tank. The holding tank should be as close as possible to the head.

3. Look closely at the position of the fittings on the tank: where the intake, outlet clean out and vents are to be located.

4. Draw up a plumbing scheme: from the head where will the venting be, the pump out fitting and the y-valves.

5. Determine what type of tank material will be used: Polyethylene is recommended, 3/8" minimum wall thickness.

6. You must consider the weight distribution of the water in the tank. As a guideline, 1 gallon of water = approximately 8 pounds.  This weight distribution when full can affect your boats performance and displacement.

7. Odor control is a big issue. Use the right additives, the best hose (PVC if possible) and provide ample venting (the bigger the better, around 1" minimum recommended).

8. If you want to install a tank level indicator system, what type will it be? Visual, probe/float, or electronic sensor type.

9. What kind of tank discharge pump will you install: types to consider are electric (macerator/diaphragm) or manual diaphragm pump.

10. Lastly, what type of tank do you need, portable or permanent?

    The number one problem that people complain about are odors which emanate from holding tanks. Odors are caused by an anaerobic bacteria in the waste that emits a gas as it multiplies. Anaerobic bacteria does not need oxygen to live, therefore one cause of this problem is a lack of oxygen in the tank. If there is little oxygen, then aerobic bacteria, which eats the anaerobic bacteria, cannot multiply in the tank and you have an overgrowth of anaerobic bacteria and a nasty odor as a result. Aerobic bacteria does not emit a foul smelling gas as it multiplies, which is why in this case it is considered more beneficial than aerobic bacteria.

     One way to correct this problem is by making sure there is adequate venting of the tank. Holding tank manufacturers suggest a vent of 1 inch, or even better yet, 1.5 inches in diameter. If your tank has an adequate vent, but there is still an odor, one possibility could be that the vent is clogged or stopped up. Always check this first.

     Secondly, odor can come from the hoses connected to the tank, especially if residue has collected in them. To determine if your hose is causing an odor problem, take a moist towel and wipe the hose. If the odor is transmitted to the towel, then the hose is contaminated. The only way to fix this is by replacing the hose.

     If the hose is not causing the offending odor, then check the holding tank and its fittings for leaks and repair as needed.

     If there is an obnoxious odor outside the boat, then you might have to have the contents of the tank neutralized with a chemical additive. In order to use this solution you must have an adequate vent line. One recommended product is "KO" but do  not use a disinfectant with it because it could kill the active ingredient in KO or a similar product.

     Another problem that boaters have with holding tanks is when the toilet will not flush or becomes hard to pump. This may indicate that the tank is full, so first get it pumped out to rule out this possibility. If the problem persists, check the tank's vent. It may be clogged, and you may have to clean out the vent and the vent line. If the line is clogged due to mineral build up or paper product clog, then remove the hose and clean it out. If the vented loop vent fitting is clogged then you must clean out the valve and the fitting. In order to avoid this situation, install a tank level monitor system. 

     Some boaters experience their tanks filling up when the boat is not in use. A common cause of this problem is a tank that is partially or completely below the water line and the tank discharge thru-hull sea cock was left open. When the boat is not in use you must close the valve and install a vented loop between the pump and sea cock approximately 12" or more above any angle of heel.

     What about a tank that won't empty when being pumped out? Check the vent line or vent fitting to see if it is clogged and clean it out as needed. It could be that the pump out equipment is not functioning properly, so check with the operator. If your tank has sludge build up it can cause the tank to not empty completely. In this case you must use an emulsifier, a chemical that will break up the sludge and, along with water, dilute it to a state where it can be pumped out. Add the combination of chemical (i.e. KO) and water, let it stand for a few days then pump it out. Another possible cause of a tank not pumping out is that the Y-valve to the discharge pump is not closed or its leaking. In this case either close the valve or replace it.

     Using biodegradable paper which breaks down up to five times faster than regular paper will prevent this. Also use KO or similar additives with frequent pump outs. 

A 1" or larger diameter vent line is recommended or two smaller lines (5/8" minimum).

Mechanical means of aeration is helpful.

Run the vent line horizontal to the tank whenever possible.

Locate the holding tank forward on sailboats in most cases.

From the toilet to the tank there should be a maximum run of 6 to 8 ft. and no 90? fittings, dips or valleys. One 90? turn is equal to 15 ft. of line that a pump needs to clear. 

Do not use flexible bag type tanks.

Use only the very best sanitation hose or PVC.

Always flush toilet with enough water to rinse out the lines.

Rinse a bowlful of fresh water through the system each day.

Do not use chemicals that disinfect to clean toilet or add to holding tanks when using "KO" or similar products. 

Example: if you have a small manual toilet and only one person is making 4 flushes per day, you need 5 gallons of capacity. But since the tank should only be filled up to 90% of its total capacity, you need to get a tank that will accommodate more than 5 gallons, therefore a 6 to 8 gallon tank is recommended, if you are going to pump out every 5 days.

The information for this article was provided by Dale Weatherstone, Managing Director, Raritan Engineering Company, Inc. 3101 SW 2nd Ave., Ft. Lauderdale, Fl. 33315. (954) 525-0378. You can find more useful information on holding tanks and toilets at their website: www.raritaneng.com .