An earlier Overseas Building Note (No 168, June 1976) reviewed low cost sanitation systems that are not dependent upon sewers for disposal of the wastes. Brief descriptions were given of both the 'Septic Tank' and the' Aqua-Privy', together with an outline of the principles to be adopted in their design and construction. The present note gives a more complete description of the processes involved in these systems, and sets down the calculations needed to ensure an efficient and economical size of tank for particular circumstances.