INTEGRATION OF DESALINATION AND PURIFICATION PROCESSES FOR THE TREATMENT AND VALORISATION OF INDUSTRIAL BRINES

Abstract

The industrial sector should shift more towards sustainability. The industrial production is continuously growing driven by the increasing demand, which leads to heavy consumption of raw materials and to the release of significant amounts of highly-concentrated wastewater streams into the environment. To achieve a more sustainable development, it is fundamental to decouple these phenomena by introducing circular economy models. These would allow for reducing the environmental impact of the industrial process by recovering energy and materials and recycling pre-treated effluents. However, so far, very few studies have dealt with the technical implementation of circular economy at the industrial scale and have performed economic analysis of large-scale plants. To fill this gap, the activities performed during my Ph.D. project were based on three research questions, which concerned (i) the selection of treatment processes to purify industrial effluents and to recover raw materials; (ii) the development of economically feasible treatment chains; (iii) the estimation of the energy demand of the treatment chains and the possibility to couple the chains with more environmentally friendly energy supply systems. Such questions may be applied to any industrial sector producing industrial wastewater and to answer them, I developed a novel multi-step method able to simulate and analyse integrated processes (chains) for the treatment of industrial effluents. The proposed method is given by four steps: (a) implementation of techno-economic models for pre-treatment and concentration technologies; (b) definition of suitable inputs and parameters and of representative outputs for each model; (c) development of integrated platforms simulating the treatment chains by interconnecting the models of single technologies; (d) definition of global assessment criteria informative about the performances of the entire chain. Concerning the last point, the technical performances are assessed by estimating the total electric and thermal energy demand; the economic feasibility is based on the calculation of the levelised cost of the target product of the chain and the environmental impact is evaluated via the specific CO2 emissions connected to the energy requirements. In this regard, I included different thermal and electric energy supply systems in the scenarios analysed in the thesis by giving suitable costs and CO2 emission factors. The developed method is flexible and able to simulate treatment chains for different wastewater effluents. The thesis presents the results obtained by applying the novel method to two case studies: water softening industry and coal mines. In the first case, I developed treatment strategies to purify the wastewater and recover a target NaCl-water solution reusable as a reactant. Conversely, for the coal mine effluent, the treatment chains were designed to produce NaCl crystals competitive with the market. For each case, I identified the most economically feasible and the least energy intensive chain among various alternatives. The environmental impact of the industrial process decreased because the treatment strategies allowed for minimising the discharge of polluted effluent into the environment and for reducing the demand for raw materials. In addition, for the softening industry case, the specific CO2 emissions connected to the energy requirements of the treatment systems resulted to be lower than those due to the production of the fresh regenerant, both with grid supply and with a photovoltaic-battery system. The chains turned out to be beneficial also from the economic point of view. In fact, the levelised cost of the7 brine produced by treating the wastewater of the softening industry was 40 to 50% lower than the current cost of the regenerant. In the case of the coal mine effluent, I found that the most feasible levelised cost of salt was comparable with the lower bound of the market price range of high purity sodium chloride. Overall, the method developed and presented in this thesis is a powerful tool that can be used for decision support by the industries to minimise the environmental impact of the processes, by introducing economically feasible treatment and recycling strategies.