walls offer greater rigidity and permeation resistance, and are generally well worth the price difference. Advantages: Readily available in a wide range of shapes and sizes. Generally the least expensive option. Provided visual indication of tank content level. Disadvantages: None Flexible tanksAdvantages: Can often be used when other types of tanks will not fit in the space available. Disadvantages: Susceptible to permeation. Susceptible to leakage due to chafing and to flexing at piping connections. Fiberglass reinforced plastic (FRP) tanksAdvantages: Highly resistant to permeation. Relatively inexpensive as a "do-it-yourself" project. Can be custom fit to the space available. Disadvantages: Typically cost more than a plastic or flexible tank. Metal tanksAdvantages: Highly resistant to permeation. Easy to fabricate to space available. Disadvantages: Susceptible to corrosion. Sewage, additives and cleaning agents may aggravate corrosion rates. Tank level monitors and warning systemsFederal regulations require that holding tanks have a means of indicating when the tank is more than 3/4 full by volume. A translucent plastic tank can meet this requirement if its location permits easy regular access for viewing contents level. Tank level monitoring systems are fairly simple to install. These typically include a sensor installed at the top of the tank and a small remote panel with a warning light that indicates when the tank is more than 3/4 full. Seacocks and through-hullsA seacock is a valve in the hull of a boat or ship that may be opened to let water in so as to flood a ballast tank, for example. Toilet intake and overboard discharge lines should be equipped with seacocks to stop the inflow of seawater in the event of a hose failure. Seacocks are operated by a lever-type handle that gives clear indication whether the...
