Planning of Crop Irrigation with Treated Wastewater Using Geographic Information System - The Case of Sana’a Wastewater Treatment Plant

There is water scarcity problem in arid and semiarid areas like the Sana’a basin, eventhough the cities produce a large amount of wastewater. Therefore, reuse of treated wastewater in irrigation is warranted. However, the sewerage system covers only 20 percent of the city of Sana’a. The flow to the Sana’a Wastewater Treatment Plant (WWTP) is estimated to be 50,000 m3/day by the year 2005. This is the available amount for irrigation use. The main objective of this study was to determine the possible reuse of treated wastewater for irrigation and the extent of the agricultural area that could be irrigated with the treated wastewater. A Geographical Information System facility was then used to select suitable crops and determine the irrigation water requirement. In spite of the low treatment efficiency of the Sana’a WWTP, especially the microbiological test result, reuse of this water in irrigation has less environmental problems compared to other means of wastewater disposal. Moreover, wastewater is a renewable source of water and provides nutrients for crops and organic matter for soil conditioning. The soil map was prepared by a consultant within the Sana’a Basin Water Management Project. The soil map was used as the base map layer of this work and the crops to be grown were grouped into three types of crops: deep rooted; shallow rooted; and tree. The soil characteristics such as salinity, texture and the crop suitability were keyed into text tables and joined to the attribute table of the soil map to produce the crop blocks map. From soil units data and command percolation factors of the soils, the map of texture percolation factor was created. The soil texture map was overlaid with the crop blocks map to calculate the percolation losses of every block. The crop water requirement for every block was calculated per month while considering the crop coefficient, kc, for different growth stages throughout the year. The peak water requirement was in May. The total irrigation water requirement was calculated by aggregating the percolation water losses, the peak crop water requirement and the leaching requirement. The total water requirement was found to be 13,058,855 m3/month, which was equal to 5,595 m3/month/ha. The minimum possible irrigated area using wastewater of this project was 268 ha, depending on the irrigation schedule and operation system of the lagoon storage/balance. An irrigation system was designed for a 30 ha area. The nearest area to the water source was chosen, and a sprinkler irrigation system was suggested to avoid losses due to the scarcity of water in the study area. AutoCAD was used to draw the layout of the irrigated blocks and distribute the sprinklers. This layout was exported to ArcView GIS to show the irrigated area by making a buffering of the sprinklers wetted diameter. An irrigation schedule was suggested as an economic system design with appropriate quantity of water use. This study recommends connecting the rest (80 percent) of the city of Sana’a to the trunk sewerage system to convey the wastewater to the WWTP for treatment and reuse for irrigation purposes. And the treatment process in the WWTP should give more emphasis to the chlorination and disinfection process and control of the trace element inlet to the WWTP or its removal as a primary treatment of the WWTP. Other wastewater treatment plants should be established at appropriate locations around the city for a satisfactory effluent quality.

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