Utilization of membrane separation processes for reclamation and reuse of geothermal water in agricultural irrigation of tomato plants-pilot membrane tests and economic analysis

The quality of irrigation water is critical for enhancing agricultural productivity. As a result, this research was carried out with the aim of treating spent geothermal water before it is used for agricultural irrigation. While doing that, cost analysis of the system was taken into consideration as well. The product water was targeted to suit irrigation water standards for tomato plants. Two commercially available pressure driven membranes (NF8040-70 as NF membrane and TM720D-400 as RO membrane) were employed for this task. A constant applied pressure of 15 bar and 60% of water recovery were kept constant during the product water production while mode of operation for the membrane system was continuous. According to Turkish Ministry of Environment and Urbanization irrigation water standards and the results obtained from this study, it was clearly seen that both NF and RO product waters meet the quality I class irrigation water standards with respect total dissolved substances (TDS), electrical conductivity (EC), concentrations of Na+ and Cl− ions. Quality 1 means that the produced water will not cause any environmental effect when employed for irrigation purpose. Nevertheless, the produced water was found not to obey the irrigation standards with respect to sodium adsorption ratio (SAR) and boron concentration (quality III class). Quality III explains that the water will cause soil infiltration problems when employed for irrigation purpose. Since most of the minerals needed for plant growth were rejected by NF and RO membranes, an appropriate mixing ratio of the product water with well water for remineralization was determined. Mixing 50 and 60% of well water with the product waters of NF (50%) and RO (40%) membranes, respectively was found to be the optimum mixing ratios to produce the requested water quality for tomato irrigation. Quality II class irrigation water which can be applied with caution was targeted in terms of SAR as well as boron concentration (2–4 and 4–6 mg/L) while determining the mixing ratios. The cost of the product water was found as 0.76 and 1.56$/m3 for NF and RO processes, respectively.