Dear Mary Rose, your question ...

Dear Mary Rose, your question is quite generic and may have multiple responses.  I will take the assumption that you may have a situation of low cost housing in an area with a shallow water table and little space between buildings.  If you are constrained to having an on-site solution (no possibility of conventional sewers) but compelled to a water flush system as in Asia (vs acceptability of dry sanitation as in Africa), you have limited options.  However it is normally possible to install a septic tank even under these circumstances - or at least to share a septic tank between a few households.  The main challenge will be to prevent the tank floating soon after construction and when it is de-sludged.  This will mean the concrete cover must be designed to provide sufficient gravitational downward force to counteract the upward flotation forces when it is empty.  The tank must also be water-tight.  The next challenge though is to deal with the outflow from the septic tanks as having a high water table minimises the opportunity for sub-surface disposal into infiltration drains. Some solutions include simple filtration of the septic tank effluent through a sand filter and then discharging to a reed-bed wetland area, treating it for re-use as suggested by others, or connecting a number of tanks to a small diameter sewer and draining to a safe disposal place (this will be a solids free sewer and hence can be of a smaller diameter and not necessarily always be at a positive slope (usually 75mm HDPE).  The septic tank may need to be modified for the particular space constraints you are faced with, but it is always beneficial to have an initial anaerobic tank under these circumstances.