Mega cities with rapid growth are challenged by two main problems concerning water supply and sanitation. One is water scarcity because local demand exceeds local supply. The other is that the infrastructure for water supply and the collection and treatment of wastewater cannot keep up with the rapid growth of the mega cities. The transfer of conventional centralised water and wastewater systems from industrialised countries to mega cities does not seem appropriate, because of the rapid and almost unpredictable growth in mega cities on the one hand and the regional shortage of water which requires an economical use and reuse wherever possible on the other hand. The transition from centralised to semi-centralised supply and treatment systems (SESATS) may be one method of resolution to the grave discrepancy between the rapid growth of cities and the provision of supply and treatment infrastructure. One important aspect of planning semi-centralised wastewater collection and treatment infrastructure including intra-urban water reuse is the assessment of the optimal size. Therefore, factors and indicators, which have an effect on the scale of semi-centralised sanitation systems, have to be developed. Beside the introduction in SESATS some of these factors, criteria and indicators and their effects on the system's scale will be introduced in this paper.