Towards Predictive Reactive Transport Modeling

Abstract

Conventional seawater desalination processes like the multi-stage flash (MSF) and multi-effect distillation (MED) are environmentally unsustainable. They consume large amounts of fossil fuels which are a major cause of climate change. Further, desalination plants discharge highly concentrated brine which can cause eutrophication and damage the marine life. Qatar, being a country that faces freshwater scarcity, is highly dependent on desalination for municipal water consumption. On average the daily production capacity of all desalination plants in Qatar is 1.5 million cubic meters per day. This incurs heavy costs on both the economy and the environment. It is expected that by 2020, desalination fuel costs will reach $2.55 billion. Conventional desalination can be made more sustainable by integrating it with solar energy. However, assessing the environmental competitiveness of this solution should be done in a systematic way and reflect the overall system performance. Simplistic models like merely calculating CO2 emissions are not enough and only allow for modest conclusions. Based on a previous literature review by the authors, it was found that the MED process with thermal vapor compression (TVC) is an excellent choice to couple with solar thermal energy that is provided from a concentrating solar collector. The authors also developed a configuration for solar-driven MED with TVC that is simpler in component choices and relies 100% on solar energy to provide the superheated steam required for the MED-TVC process. A model was developed for a 7-effect MED-TVC pilot plant and was validated with actual plant data. Current literature on desalination mainly focuses on membrane technologies and almost completely neglects thermal desalination. In the Arabian Gulf region, thermal desalination is predominant and hence it is required to assess its sustainability from a view point and further investigate how coupling renewable energy can reduce the environmental impacts. This work quantifies the environmental impacts of solar desalination in Qatar using life cycle assessment (LCA). Our work is based on the proposed MED-TVC solar-driven plant. The objective of this study is to assist decision making by providing information about the potential environmental impacts of solar desalination, propose system improvements and suggest references for comparison between different renewable energy-driven desalination processes in general. We identified five impact categories: global warming, freshwater eutrophication, water use, mineral resource scarcity and fossil resource scarcity. GaBi tool was used to carry the LCA. Ecoinvent database, GaBi databases, academic literature and expert opinions were used to construct a comprehensive life cycle inventory for the plant. ReCiPe method was used to assess potential impacts in the five categories. This method was used because it includes characterization factors unique to Qatar and also because it was widely used in the literature hence comparisons can be made. The functional unit was 1 m3 of freshwater at the plant. The results of the LCA are then computed, grouped and weighted. Comparisons with similar desalination systems are also made. The findings of this work are highly relevant to Qatar National Vision 2030 as they provide detailed findings on the environmental impacts of solar-desalination which is a promising solution for the problem of water scarcity in Qatar.