The aqua privy is a simplified variation of the septic tank. It is a simple storage and settling tank immediately under the latrine floor. Excreta drop directly into the tank through a pipe. Te bottom of the pipe is submerged in the liquid in the tank, forming a water seal to prevent escape of flies, mosquitoes and smell. Effluent usually infiltrates into the ground through a soak pit. Accumulated solids (sludge) must be removed frequently and enough water must be added to compensate for evaporation and leakage losses (in order to maintain the water seal).