Development, Modeling, Analysis, and Optimization of a Novel Inland Desalination with Zero Liquid Discharge for Brackish Groundwaters

摘要

Groundwater is considered the major source of domestic water supply in many countries worldwide. In the absence of surface water supplies, the use of groundwater for domestic, agricultural, and even for industrial purposes becomes essential, especially in rural communities. Groundwater supplies typically are of good quality, and the quality is reasonably uniform throughout the year compared to that of surface water, thus making it suitable for direct use, or simple to treat. A disadvantage of groundwater is the content of dissolved salt as many have a moderate-to-high salinity. The high salinity makes water brackish and thus it requires desalination before use. This has led to wide use of groundwater desalination to produce good-quality water in many regions around the world.Nevertheless, a problem of desalination processes is the generation of a concentrate stream, sometimes called brine or reject, which must be properly managed. The management of brine from brackish groundwater desalination is a significant issue if located far from the coast (i.e. inland plants) or far from public channel to discharge such brine. Some options for brine disposal from inland desalination plants are evaporation ponds, deep-well injection, disposal to municipal sewers, and irrigation of plants tolerant to high salinities. Each of these disposal methods may result in many environmental problems such as groundwater contamination, the decline in crop yields from agricultural lands, the formation of eyesores, decreasing the efficiency of biological wastewater treatment, and making treated sewage effluent unsuitable for irrigation. As a result, the brine management from inland desalination of brackish groundwater is very critical, and the need for affordable and environmentally benign inland desalination has become crucial in many regions worldwide.This work aims to develop an efficient and environmentally benign process for inland desalination of brackish groundwater, which approaches zero liquid discharge (ZLD), maximizing the water produced and minimizing the volume of concentrate effluent. The technical approach involves utilization of two-stage reverse osmosis (RO) units with the intermediate chemical treatment of brine stream that is designed to remove most of the scale-forming constituents, which foul membrane surface in RO and limits its water recovery and hence enable further recovery of water in the secondary RO unit.The treatment process proposed in this work is based on advanced lime softening processes, which have the ability to effectively remove scale-forming constituents, in addition to heavy metals and natural organic matters that might be present in the brine. The process has been applied to the brine produced from 1st stage RO i.e. primary brine stream, to minimize the volume of the stream to be treated chemically, which in turn reduces the capacity of the treatment equipment.Analysis of groundwater quality and scale-forming constituents that are present in the brine stream upon desalination of groundwater has been performed. The analysis has revealed that in most cases of brackish groundwater desalination the recovery is limited by scaling due to calcium sulfate i.e. gypsum, and amorphous silica. Thus, the main objective set for the chemical treatment of the brine stream focused on removal of calcium, sulfate, and silica. Advanced lime softening based on high lime doses along with sodium aluminate, as in ultra-high lime with alumina UHLA process, has been proposed for chemical treatment of brine. Bench-scale experiments conducted to evaluate the effectiveness of the proposed chemical treatment for removal of scale-forming constituents, particularly calcium, sulfate, and silica by studying the different factors affecting the removals efficiency from synthetic solutions containing sulfate-only, silica-only, and model brine solution. The results obtained have revealed that the proposed process was very effective and results generally in high and quick removals of calcium, sulfate, and silica of more than 80% within 2 hrs under different experimental conditions. In addition, beneficial uses of different solid byproducts formed are investigated, by analyzing the solids resulted to qualitatively and quantitatively to identify the different solids present. This offers the potential to lower both costs and solid disposal problems of solids formed being considered as added value product rather than solid waste that has to be properly managed. Results have shown that the solid precipitate contains a wide range of solids that generally composed of calcium, magnesium, aluminum along with carbonate, sulfate, and silicate, which have several potential applications as soil sub-grade, and in cement industry.Equilibrium model to simulate the chemical treatment process that is able to predict the required chemical reagents doses, effluent water quality for a given influent water quality and treatment levels has been developed utilizing OLI stream analyzer, the developed model was found to well predict the performance of the chemical treatment at equilibrium conditions. Rigorous membrane separation model has developed in Aspen Custom Modeler to more accurately model RO desalination, which is to be combined with the developed equilibrium model to formulate a complete 1st Stage RO–Chemical Treatment–2nd Stage RO process model. The developed complete and validated model has been then used to fully and accurately simulate the performance of the proposed Zero Liquid Discharge desalination process. The present work results in three novel achievements: first, introducing a very effective intermediate chemical treatment, which efficiently remove sulfate, particularly from brine. Most of the previously proposed intermediate treatment processes remove sulfate as calcium sulfate i.e. gypsum, however in the introduced process, sulfate is removed in calcium-aluminum-sulfate complexes, which has very low solubility, making the brine highly undersaturated with respect to gypsum, and hence lowering the fouling propensity in the secondary RO, leading to maximizing the overall recovery. In addition, the chemical treatment has been successfully modeled for better simulate of its performance for different brine qualities, which are usually encountered in brackish ground desalination due to the high location-specific nature of groundwater quality.Second, the developed membrane model has treated the species present in water as ions, accounting for monovalent and divalent ions separately, and obtaining a different permeability coefficient for their transport through the membrane. This is different from most developed RO models, which simplify the transport through the membranes to only water and salt permeability coefficients. This treatment results in better and more refined modeling and simulation of the RO membrane separation, as the RO membrane interact differently to ions present in water.Third, the complete process model, results from combining the developed equilibrium model of the chemical treatment, and membrane separation model, has revealed very promising results of achieving high recovery desalination of about 93.5% suitable for drinking water purposes, which is higher by about 90% than most of the reported literature, whose result in reducing the brine volume from 25% in conventional desalination to only 6.5% in the proposed process, i.e. brine volume reduction of 74% relative to conventional inland desalination, and 35% relative to other high recovery processes, at reasonable chemical treatment levels.