Feasibility of Rainwater Extraction from Roof Insulation Surfaces in Ahwaz Weather Conditions

Water demands will increase in the next 50 years due to 40 to 50 percent population growth and the expansion of industries and cities, According to the World Water Association report. This demand will be significant for countries with a water deficits. Water demand for Iran by 2025 will increase by 110% of extractable water resources. With the current climate of the country, access to this volume of water seems impossible. One way to reduce reliance on water resources is to collect rainwater. Rainwater harvesting systems have been adopted in many parts of the world, especially in arid and semi-arid regions, as a practical way to minimize the risk of drought. Because rainwater can be easily collected without special tools and can be used for non-drinking demands. In this study rainfalls that cause to creation of runoff from autumn to late spring in Ahvaz city were investigated.

Two sites were considered for this research. The first was located in the department of parks and green space organization in the south of Ahvaz with the roof area of 115.6 m2, and the second was in the north of Ahvaz town, with the roof area of 35 m2. The roof of both sites had Waterproof insulation levels. The roof runoff of these sites was transferred by gravity from the outlet point to the water storage tanks through a pipe.The tank volume was designed for the first point with a capacity of 2100 liters of polyethylene and the second point with a capacity of 220 liters of plastic (polypropylene). Reservoir volume design was calculated based on 3 components: the average maximum rainfall of the region (December), roof level and runoff coefficient of 0.7. Tanks were installed and calibrated under the outlet pipes of the respective roofs. The volume of water collected at midnight of every day during the test period (in case of rain) was read and the tank was emptied for sampling the next day. On days when the rainfall was more than the volume of the tank design; To prevent the tank from overflowing, the tank water was drained and recorded during the rain. Sampling was started from October 2018 to June 2019. Rainfall data was received from Ahvaz Synoptic Station. The calculation of the roof area of the residential units was based on the urban area houses calculated and was compared with the area of the roofs obtained from Google Earth.

The rainfall statistics reported in Ahvaz by the Meteorological Organization of Khuzestan Province showed that from October 2017 to June 2018, there were 61 rainy days. There were 18 rain events in autumn, 28 events in the winter, and 15 ones in spring. The average runoff coefficient in Ahvaz city was obtained respectively 0.759, 0.711, and 0.797, for autumn, winter, and spring seasons. The runoff coefficient for autumn, winter, and spring seasons were reported 0.66, 0.69, and 0.62 in Mashhad, and 0.75, 0.76, and 0.69, in Noor city, respectively. This difference can be due to the difference in rainfall regime, the type of roof insulation, the slope of the roof, and so on. Also, the results showed that with the extraction of rainwater from the roof surfaces of Ahvaz in the months of October to May, respectively, 2.24, 8.4, 21.2, 14.85, 13.65, 9.17, 7.44, and 4.03% of the domestic needs (except for drinking and cooking), 16.85, 63, 100, 100, 100, 68.84, 55.77 and 30.2% of the public needs, 8.43, 31.5, 79.61, 54.68, 51.19, 34.42, 27.88 and 15.10% of the commercial and industrial needs and 1.06, 4.48, 13.17, 8.88, 7.66, 5.05, 4.3 and 1.88% of the greenhouse needs can be supplied.

The results showed that in the city of Ahvaz, 75% of the rainfall has led to runoff production. The runoffs that can be extracted from the roofs were the most share in December and winter months and the last one in June, July August, and September in Ahvaz. The highest amount of rainfall in Ahvaz, which did not lead to runoff, was 0.2 mm. In other words, rainfall that is less than this amount does not lead to runoff. Also, the lowest amount of rainfall in Ahvaz, which has led to runoff, is 0.3 mm. Therefore, the probability of the threshold of runoff in Ahvaz is 0.3 mm. In other words, rainfall that is more than this leads to runoff production. Also, the results obtained from this study showed that in December, January, and February, water that can be extracted from the roof surface, in addition to providing 100% of the city's general consumption needs in these 3 months, can save 416.14 million liters, Which will be usable in other months. Due to the lack of measurement of quality parameters, the collected water is not recommended for drinking.