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Gravity types are simple but have to be placed higher then the holding tank. Plumbing has to be fairly straight and downhill. The best location for a gravity head is on the main deck more or less above the holding tank.
The push type uses gravity, compressed air [4], water-hydraulics [10], or a manual or electrical pump to evacuate the toilet and push the contents through the pipes to the holding tank.
The pull type uses a vacuum to suck the contents from the toilet down to the holding tank. Of these, only three types are suited to a yacht:
A manual-pump head typically uses a foot pedal or handle to activate a manual pump to send the effluent to the holding tank. Manual pump heads are not dependent on electrical power, thus reducing the demand on the electrical system. They are very dependable and easier to install, and have relatively low maintenance costs. They can use seawater for flushing. Chief disadvantages are that users must learn how to use the pump (open the intake valve, select a dry or a wet position); users may not know if the flush is complete to the holding tank; and pipes can get clogged.
Typical vendors are Groco [6http://www.groco.net/ Groco], [http://www.jabsco.com/ Jabsco ] (includes Par) [7], Lavac [8http://www.lavac.com/ Blake Lavac], Raritan [3http://www.raritaneng.com/ Raritan] and [http://www.wilcoxcrittenden.com/ Wilcox Crittenden [9].
=== Electric Head ===