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Waste Treatment
Increasing the efficiency of ZLD Separating all of the water out of the product requires large amounts of energy . It takes roughly six times more energy to evaporate water ( latent heat ) at its boiling point than the energy needed to actually bring it to that boiling point ( sensible heat ). For that reason , ZLD processes often start with a separation process based on ( reverse osmosis ) membranes . Membrane separation does not require phase change / boiling . Electrical energy ( pumping ) is used to push water through the pores of the membrane and separate it from the dissolved solids . Membrane can only work to bring the product up to a certain concentration . To achieve complete separation , evaporation / crystallization processes are needed for completing the process . As explained before , evaporation ( due to the latent heat ) is highly energy consuming . Therefore , it is wise to choose an evaporation process that involves ways of energy optimization , the most popular being :
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Multistage evaporation : using the latent heat of the evaporated water as energy source in a next evaporation stage reduces the overall consumption of the boiler to the evaporation plant .
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Thermal Vapour Recompression ( TVR ): evaporated steam is mixed with boiler steam . The reuse of the evaporate steam reduces the energy demand .
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Mechanical Vapour Recompression ( MVR ): An MVR compressor ( driven by an electrical motor ) can be used to compress the evaporated steam , thus increasing its pressure , and use this steam as the energy input for the process . MVR compression is very efficient in terms of energy consumption .
Due to the factors outlined above , ( multistage ) vapour compression plants remain the main method employed for ZLD processing globally , with evaporation typically recovering around 95 per cent of wastewater as distillate . Any remaining concentrate is then further treated physically or chemically to produce solid residues ( such as crystals ) and water . Evaporators used in ZLD systems are often run at lower pressures in order to reduce the boiling point of the liquid being treated .
The HRS ZLD solution Depending on the product to be concentrated , HRS can select from a series of technologies for designing the most optimal ZLD process . Energy optimization methods ( multistage , TVR , MVR ) can be combined with several types of heat transfer technologies ( plate evaporators , corrugated tube evaporators , scraped surface evaporators ). Whatever the technology applied , the overall process can be separated into three steps : 1 . Evaporation / concentration : The product is concentrated to just below its maximum concentration ( saturation ). The evaporation plant is usually a multistage evaporator setup .
2 . Cooling : if the maximum solubility curve is steep ( large concentration at high temperature , low concentration at low temperature ), the product obtained in step 1 is cooled , provoking immediate precipitation of dissolved solids .
3 . Crystallisation : Crystallisation / sedimentation of the solids produced in step 2 occurs in specially designed crystallisation tanks . A supernatant layer of
HRS Unicus Series scraped-surface evaporators are used to maintain thermal efficiency and remove fouling during evaporation in ZLD installations . Image courtesy of HRS Heat Exchangers .
concentrated solution remains after this stage and is returned to step 1 for reprocessing .
For products without a steep solubility curve , it is necessary to concentrate inside the evaporator to above the maximum solubility . This means that the step 1 process is equipped with a final evaporator stage ( finisher ) that is specially designed to work with suspended solids . The fluid with suspended solids is then transferred directly to the crystallization tanks in step 3 . The brine cooler and evaporator finisher work with solids in suspension and often this means dealing with fouling products . A typical HRS evaporator / finisher will use Unicus scraped surface evaporators that are self-cleaning and maintain optimal evaporation rates . Typically , our R series scraped surface coolers are used for cooling the saturated brines that are sent to the crystallization tanks . The result is an efficient process which can work continuously without requiring scheduled downtime . Whatever kind of evaporator is employed , heat exchangers have a crucial role to play in ZLD systems in reducing running costs by utilising heat from process water and other existing sources , and also recapturing heat at the end of the process and reusing it to boost the energy efficiency of the overall ZLD system .
References
[ 1 ] “ Zero Liquid Discharge Market is Estimated to Rise at a CAGR of 11.95 % during the Forecast Period , notes TMR Study ”, https :// www . prnewswire . co . uk / newsreleases / zero-liquid-discharge-market-is-estimatedto-rise-at-a-cagr-of-11-95-during-the-forecastperiod-notes-tmr-study-866877417 . html
[ 2 ] Tong , T . & Elimelech , M . ( 2016 ) The Global Rise of Zero Liquid Discharge for Wastewater Management : Drivers , Technologies , and Future Directions in Environmental Science & Technology , https :// pubs . acs . org / doi / 10.1021 / acs . est . 6b01000
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