In non-sewered urban and rural residential developments, domestic wastewaters are treated and disposed of on-site. Surveys indicate that the performance of domestic systems needs to be significantly improved to overcome potential public health and nuisance problems caused by failing systems. Many current effluent disposal practices are clearly not sustainable according to the accepted definitions of ecological sustainability. It is essential that the on-site disposal of domestic effluent is considered as an integral part of the land development process. Individual soil and site assessment needs to be undertaken in the design and sizing of effluent disposal systems. An important implication from the consideration of land capability criteria for on-site effluent disposal is that individual lot sizes will need to be larger in future developments of this nature.