Investigating the best method for saline wastewater discharge: A case study of one of Assaluyeh desalination plants

Introduction
In today's world, fresh water is known as a limited resource that all economic and social activities of human beings and more importantly human life and other organisms depend on this limited resource and this limited resource is decreasing day by day. At present, in most countries, desalination of the seas and oceans is the most important source of water supply near the coast. One of the products of desalination plants is saline effluent that is discharged into the sea environment. Improper discharge of this effluent causes damage to the environment and can cause irreparable damage to human life and other organisms. In this research, in a case study, the most optimal method of discharging the effluent of one of the Assaluyeh desalination plants is investigated in order to achieve the maximum amount of effluent dilution in the near and far fields. Also, the effect of increasing the number of dischargers on the dilution rate of effluent discharged from multi-port dischargers in the far field is investigated.
Methodology
The most important environmental problem of desalination plants is the production of brine (containing high concentration of salt) that is discharged directly into the sea. In this research, using the VISJET integral model, the dilution of effluent from an Assaluyeh desalination plant is investigated. VISJET is an integral model that uses the Lagrangian method to solve equations and predict the amount of effluent dilution in the Water environment. VISJET has the ability to simulate a multilayer environment and simulates effluent with positive, neutral and negative buoyancy. VISJET does not consider effluent dilution in the remote field. Therefore, in this research, the CORMIX model is used to dilute the effluent in the far field. The CORMIX model has been developed to analyze and predict the discharge of effluents with positive, neutral and negative buoyancy into the water environment. Discharge of various types of industrial and toxic effluents in the form of single- port and multi- port submerged and surface discharge, as well as considering environmental conditions such as wind speed, speed and direction of ambient flow and land slope are among the features of this model. In addition to near-field effluent mixing, this model also predicts effluent mixing in the far field. CORMIX consists of three sub-models: CORMIX1 (discharge using single-channel discharge), CORMIX2 (discharge using multi-duct discharge) and CORMIX3 (surface discharge).
Results and Discussion
In this section, using the CORMIX model, the dilution of the effluent of Assaluyeh desalination plant is investigated. Then, using CORMIX and VISJET models, the submerged discharge scenario of this plant at different depths is investigated, with the aim of achieving the maximum amount of effluent dilution in near and far fields. At a distance of 200 meters from the discharge site of Assaluyeh desalination plant, the salinity of the ambient fluid increases by 16%. Therefore, Iranian environmental standards are not observed. This type of discharge (surface discharge) has the least dilution in the nearby field due to minimizing the contact of the effluent with the sea environment. In a submerged discharge, the height of the effluent in the plume condition increases with increasing discharge height from the ground. In this case, due to more contact of the effluent with the water environment and having more time for mixing, the dilution of the effluent increases. Using a mile discharge with a froude number of 27.4 at a depth of 9 meters is the best method for discharging the effluent of Assaluyeh desalination plant as a single port. In this case, the concentration of ambient fluid at a distance of 200 meters from the discharge site will increase by 1.5 percent, which is fully in accordance with Iranian environmental laws. Effluent dilution in a dynamic environment, in addition to the Froude number, discharge angle and ambient flow, also depends on the mass flux of the effluent. According to the results, increasing the number of outlets in multi-port dischargers (by keeping the froude number constant, ambient flow velocity and discharge angle for all dischargers) increases the dilution rate of effluent in near and far fields.
Conclusion
Surface discharge of Assaluyeh desalination plant effluent increases the concentration of the receiving fluid by 16% at a distance of 200 m from the discharge site. The use of a 60-degree multi-port discharger with 10 outlets and a Froude number of 27.4 at a depth of 3.8 meters to discharge the effluent of the Assaluyeh desalination plant increases the concentration of ambient fluid by 0.8% at a distance of 200 meters from the discharge site. This scenario is recommended for optimal discharge of effluent according to environmental standards.