Zero discharge desalination system by direct contact membrane distillation (DCMC) integrated with solar pond
Creators: name: Bawahab, Mohammed
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401205
subject: Experimental methods in fluid flow, heat and mass transfer
classificationCode: 401703
subject: Energy generation, conversion and storage (excl. chemical and electrical)

description: This research work focuses on developing a desalination system with near zero-liquid discharge (ZLD) by integrating Direct Contact Membrane Distillation (DCMD) with a salinity gradient solar pond (SGSP). By developing the proposed system, this work aims to find an alternative desalination system motivated by the continuous increase in freshwater demand globally. The system must provide freshwater driven by a sustainable energy source without brine discharge, in comparison to conventional desalination technologies that require a high amount of energy (mostly supplied by fossil fuel) and are associated with brine rejection which is seriously impacting our environment.  

A review of the available literature in the fields of water desalination with near zero-liquid discharge shows there has been limited success in achieving an efficient system, especially when coupled with a low-grade temperature heat source such as solar energy or waste heat. 

The system explored for this research study couples a direct contact membrane distillation unit with a salinity gradient solar pond as a source of both thermal energy and feed water. The overall picture of the system is that there are two inputs (heat and saline water) and two output products (freshwater and salt) without waste or brine discharge. The saline water that enters the system could be seawater, aquifer water or any other source of saline water. The heat input to the system is solar radiation collected and stored in the solar pond. The proposed implementation of the system is to utilise solar energy to desalinate water and produce salt and minerals as both a product for industry use, as well as for maintaining the salinity gradient solar pond. No further thermal energy is required for the system, but electrical energy is needed for the pumps to circulate the feed and fresh water through the DCMD unit. 

The novelty of this system is to couple membrane distillation technology, using a DCMD configuration, with a salinity gradient solar pond (SGSP) by directly extracting feedwater from the Non-Convective Zone (NCZ) of the SGSP for freshwater and salt production with near ZLD. The synergy of coupling the two systems (DCMD-SGSP) is that each system would supply the ongoing requirements of the other, thermal energy for DCMD to produce freshwater and salt for the SGSP to maintain its stability. Solar energy is an abundant, freely available, and sustainable energy source, and membrane distillation technology can distil water from high-salinity brine, allowing the system to achieve near ZLD.

A numerical model of the system has been developed using MATLAB script and has been validated with experimental outcomes and used to perform a further investigation in terms of freshwater production and energy consumption. The proposed system was designed and constructed in Melbourne, so the average ambient conditions (monthly average solar energy and monthly average ambient temperatures) for Melbourne are used as inputs to the system where necessary, along with the assumption of 20% SGSP efficiency and 30% of DCMD efficiency. The predicted performance of the proposed system based on these inputs is that it can produce around 125 kg of freshwater daily from high feed salinity by using 1 m2 of DCMD coupled with 250 m2 to 300 m2 of SGSP surface area. The system’s performance can be improved by using an auxiliary source of thermal energy, such as Evacuated tube solar collectors (ETSCs) or any waste heat source, to boost the temperature of the feedwater from the SGSP. 

A case study is presented to predict the economic viability and estimate the performance of the developed system. The outcome of the economic study shows that the proposed desalination system using DCMD coupled with a SGSP and boosted by ETSCs would result in a cost of freshwater approximately equals to AU$10/m3 to AU$11/m3, for a system designed to produce 10 m3/day of freshwater and with an assumed life span of 25 years.