Alternatives of hydraulic design and implementation of the second line of treated wastewater transfer to the Varamin plain

The use of treated wastewater has been common in various countries around the world for a long time. With the increase in world population and the need for more water resources, the use of treated municipal wastewater for irrigation is expanding. One of the limitations of using treated wastewater for irrigation is the cost of transporting it to agricultural areas. In general, water conveyance line costs are estimated using pricing for constructed projects that may be similar in size, material, and depth. The water supply system for irrigation in the plains of Varamin and Pakdasht in the south of the capital has always been considered due to its size and socio-economic importance. The presence of a large treatment plant in the south of Tehran can be a reliable source of recycled water. This treatment plant with a capacity of 700,000 m3/day produces effluent of suitable quality for agriculture from the municipal wastewater treatment process. Part of this wastewater has been transferred to the irrigation network of Varamin through a 36 km long open canal for the last 30 years. The effluent extracted from the Tehran water treatment plant provides the conditions for the implementation of an efficient plan to solve the existing problems and provide safe water. Based on this, a 36 km transmission line with a water delivery structure to the downstream irrigation network was considered. After the implementation of the project, the transmission line with a discharge capacity of 9 cubic meters per second and the existing canal with a transfer capacity of 4 cubic meters per second, will perform the effluent transfer work with an integrated system. The existing canal will normally act as a substitute and complement, and in critical times and in the presence of rain and floods, it will enter the transmission circuit and operate with the main system.

The case study is the second line of wastewater transfer near the Tehran canal in the southeast of Tehran province. The Tehran canal, approximately 36 km long, starts from the southern Tehran treatment plant located in the south of the Shahre’Rey and joins the main canal of the Varamin irrigation network. Then it feeds the main canal (AB) of the Varamin irrigation network and secondary canals by a water dividing structure. In this section, different options of material and type of pipe for the second line of wastewater transmission, such as closed pipes (pipes and concrete boxes) or open canals are examined. The following is a brief description and economic estimate of the proposed options. In this study, by examining six different options and based on the material and type of transmission line and implementation feasibility, a suitable transmission line is selected technically, economically and environmentally. The considered options were as follows: building a new canal parallel to the existing canal, constructing two concrete boxes parallel to the existing canal, constructing a concrete twin box parallel to the existing canal (in one side of the Tehran canal) ), Compound cross-section option (correction of Tehran canal), re-lining the Tehran canal and using a pipeline parallel to it, and finally, implementing a transmission line parallel to the Tehran canal and maintaining the current operating conditions of the Tehran canal. To estimate the costs of executive operations, the proposed options are examined in detail. After selecting the appropriate option, which was to be implemented with thick polyethylene pipes, the production line of this type of pipes was imported to Iran by Mohammadian Oil and Gas Engineering and Development Company to produce pipe with a diameter of 3000 mm.

In this section, the transmission line flow was analyzed at different hours of the day and the design flow was obtained. Different materials were investigated for the transmission duct, the roughness coefficient proportional to the diameter and the material of the transmission pipe for the sewage was proposed, a hydraulic model was implemented to simulate the flow behavior in the duct and to determine the location of manholes. Finally, the economic analysis of the project was performed for different options and the sixth option was selected for implementation.

Open canal option in terms of environmental issues and sedimentation problems and maintenance and operation problems (especially because due to sewage, is prone to algae growth and needs regular and continuous attention, and given the short life cycle of the project) was not recommended. The cost of execution of concrete box in comparison with SRPE pipes does not show a significant difference and in terms of technical and operational issues, execution time, design safety (in terms of leakage), and difficulty of execution are not comparable to transfer using the pipe. Also, reinforced polyethylene pipe has a significant advantage over the concrete box. Therefore, the sixth option for the design was selected and implemented.