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A recirculating toilet has a storage tank of water and chemicals that it uses for all liquid-related purposes, combining and neutralizing waste with these chemicals. The storage tank doubles as both waste containment system and flushing system. This compact toilet is usually used in places where there is neither access to normal plumbing nor room for a larger system, most often small boats or aircraft.
The chemical composition of the toilet's liquid differs based on the supplier, but its main job is to destroy bacteria, such as coliform, that build up in the waste. This makes the toilet safer to use by reducing the possibility of contamination and helps to eliminate odors. More complicated systems may have separate tanks of chemicals that are periodically injected into the main tank to help dissolve and disinfect the waste.
More traditional recirculating toilets use a macerator to help dissolve solid waste into smaller particles. A macerator system is equipped with high-speed blades that grind the waste down into pieces small enough to dissolve into the liquid of the main storage chamber. A macerator is generally added into the system to reduce the possibility of clogging and to allow the liquid from the main chamber to be used for flushing purposes.
While macerators do dissolve solid waste, they do not affect the speed at which the chemicals neutralize coliforms. This creates a balance problem when the chemicals have not yet had time to fully treat the floating particles of waste but the liquid is being reused for flushing purposes already. To solve this problem, some recirculating toilets use a filter system that divides the waste between several different compartments. Solid waste is caught by the filter and siphoned off into a large holding tank designed to neutralize it. Liquids intending to be reused for flushing purposes are sent into a much smaller tank, which is then saturated with bacteria-killing chemicals from a separate, larger storage pump.
Boats and aircraft usually pump the storage tank liquids into a septic tank when they return to land again. The chemicals and water are replenished and cycled through the system so that the toilet is ready to be used again. This process differs from toilet to toilet, as do chemical choices, based on effectiveness and odor removal.