When formulating the model constraints, special attention must be given to the aggregation of processes to sectors. The aggregation level, however, should not go too far because the model should reflect practical requirements from an engineering point of view. The capital stocks for the water infrastructure will be divided into those for civil work and those for mechanical and electrical equipment facilities. Because water losses in the water distribution system, wastewater generation rates, and leakages change over time, the water and wastewater amounts differ sector by sector within the actual and future time intervals. These structural changes depend on the past and future activities.