Guide to COST-BENEFIT ANALYSIS of investment projects - Ramiri

An alternative option considered to be technically feasible, taking into account the hydrological and geomorphologicalconditions of the project area, is the construction of a dam and a long aqueduct (more than100 km long), that supplies water to the irrigation and industrial networks. This option has been rejectedfor economic reasons. No other alternative was considered feasible from a technical point of view.Figure 4.9 Diagram of the overall scheme for the project infrastructuresCity’s sewerage networkEffluent mainWaste water treatment plantT ti t t t l tIndustrial areaWater body of thewater final dischargePumpingstationsIrrigatedagricultural landFOCUS: THE WATER DEMANDThe volume of water to be treated has been estimated on the basis of an average daily water actual supply of 190 litres per inhabitant, taking intoaccount a reduction factor of 0,8 for the wastewater collected by the urban sewerage system.The size of the daily water supply was determined on the basis of a study of the needs of the civil population of areas similar to those of theproject (similar social customs, similar consumption levels, same geographic area, etc.) and corrected in the light of data on historic consumptionavailable from the Municipality 81 .For the industrial area the water demand was estimated on the basis of the specific consumption of industrial plants, taking into account aperiod of activity of about 11 months per year.The water demand for irrigation, has been calculated on the basis of the expected water consumption of various specific kind of agriculturalcultures, taking into account a period of activity of about 6 months per year (the dry season). For the cultivation in greenhouses the full year ofactivity is assumed.The total supply is considered gross of leakages in the water network. The real consumption is calculated as follow:real consumption = total supply – leakagesThe water demand quantification- Drinking water supplied to the urban users: 725,000 inhabitants x 190 l/inh.d. x 365 / 1,000 = 50.3 Mm 3 /y- Wastewater to be treated in the plant: 50.3 Mm 3 /y x 0.80 = 40.3 Mm 3 /y- Water supplied to the urban users: 12.1 Mm 3 /y- Agricultural water demand for irrigation: 3,500 he x 4,500 m 3 /ha y = 15.75 Mm 3 /yThe overall recycled water is 27,9 Mm 3 /y, that is about 70% of the total amount of the treated wastewater.The dynamics of demandThe dynamics of Household demand were determined by the forecast of the population resident in the city, which has two components:- a demographic rate of growth (the average for the region) of 0.06% per year;- a migratory flow with a positive balance, that gives an average rate of growth of 0.09%;- as result, an average yearly rate of growth of 0.15% has been set in the CBA model.The industrial demand is assumed to decrease (average yearly rate = -0.3%), because of the reduction of the network leakage and because of therecovery of efficiency for the water usage inside the factories.For similar reasons the irrigation water demand is assumed to decrease as well, with an average yearly rate = -0,5%For simplicity, no other dynamic of demand is considered in the present case study.81As it has been mentioned, the volumes of wastewater take into account a reduction coefficient of 0.80. The contamination level (BODBiochemical Oxygen Demand, COD, Chemical Oxygen Demand) was estimated using standard environmental engineering methods.169