Design of a high-recovery zero-liquid discharge solar desalination pilot: aiming a beyond state-of-the-art concept

Abstract

Whereas the need for additional water resources becomes ever more pressing in a framework of combined population and specific water consumption increase with climatological impacts of climate change, the increased reliability and efficiency of desalination technologies renders marine or brackish water resources a viable solution, especially in coastal areas where, in turn, the largest population conglomerates occur. In view of the coincidence of water scarcity regions with high solar irradiation potentials, the use of solar energy as energy input for the higher specific energy consumption pertained by desalination technologies emerges as a technological solution for the reduction of its energy related environmental impacts, ever closer to economic competitiveness in a wider range of applications and contexts. A thorough approach to the potential impacts of desalination, though, implies due consideration of its effluent - brine - which has led to the development, along the most recent years, of so-called “zero liquid discharge” concepts, aiming at an increased recovery ratio and/or at the economic valorization of this effluent - some of its components extracted over consecutive processes and/or, ideally, fully eliminating liquid discharges from the desalination/effluent valorization plant. In the scope of project Sol2H2O - European Twinning for research in solar energy to (2) water (H2O) production and treatment technologies, funded by the European Commission through Horizon Europe GA Nr. 101079305, a pilot of a zero-liquid discharge solar desalination plant is being designed and engineered, for ensuing experimental testing and demonstration: an innovative hybrid solar-driven water production and water treatment pilot powered by solar energy is proposed to ensure the sustainability of the overall process, leading to the maximization of freshwater production, and approaching the ZLD. The pilot aims at demonstrating a “beyond state of the art” technological combination, surpassing the existing produced water recovery rate in current systems situated at around 50%, to levels close to 80%. The treatment chain system will be able to treat 1 m3 of water per day and consists of the following equipment: photovoltaics panels with reverse osmosis (PV/RO) system, a multiple feed plug flow reactor (MF-PFR), a membrane distillation (MD) plant and at the final treatment stage two solar evaporation ponds. Seawater is sent to the RO unit powered by the electrical energy produced by PV panels with a capacity to operate with water salinity up to 45 g/l and providing a brine recovery rate of 30%: It produces two streams: a permeate with high-grade quality water for industrial and drinkable purpose and a brine retentate, enriched in all ions present in seawater. The brine is sent to the MF-PFR unit for selective recovery of magnesium and for removal of calcium in the form of hydroxides. The recovery/removal is carried out by a fractionated reactive crystallization adding an alkaline reactant, such as sodium hydroxide water solution. After solids separation, the clarified effluent will be sent to solar powered vacuum-enhanced air-gap membrane distillation unit, with a capacity to treat water with high salinity. Operating in batch conditions, it can produce two different streams, one is a pure distillate and the other one is a retentate, whose concentration can reach about 250,000 ppm. The high concentrate brine is sent to a first solar evaporation pond (Solar Pond 1) to produce NaCl with high purity to be used at food grade and a remaining brine depleted in NaCl is sent to solar evaporation pond 2, for complete evaporation and precipitation of a mixture of chloride and sulfate salts, which can be used for de-icing purposes. At this point the whole treatment chain meets the zero liquid discharge concept because no effluents are produced and then released into the environment. The pilot installation a