Cleaning can be of both chemical and mechanical type

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Cleaning can be of both chemical and mechanical type. Chemical means involve acid treatment where acid is poured along the scale to loosen it from the surface of the tube. A scale cutter is a device mounted on a fl exible shaft which is inserted from the top of the pipe. This device is moved up and down repeatedly to remove the scale in the entire length of tube. Once all the stages have been cleaned, the evaporator is closed and tested to check for any leaks. Proper seals are made to close these leaks. 6.5.2 Mechanical Vapour Compression The mechanical vapour compression technique is used to concentrate the wastewater ef fl uent. A heat exchanger is used which also acts as an evaporator or condenser. The heat is provided by the simultaneous condensation of the distillate. Latent heat is exchanged between hot and cool water in the evaporation conden- sation process. A compressor which is present provides the energy required for separating the solution by heat transfer. 6.6 Crystallization In crystallization, solid crystals are obtained from a homogeneous solution. It is a solid liquid separation technique. Crystallization occurs only when a solution is supersaturated. Supersaturation of a solution is its state in which the solvent has higher concentration of solute than what can be dissolved in it at that temperature. By using this method, salts can be recovered from the mother liquor. Crystallizers are of two types, namely single-stage and multi-stage. The common chemicals removed from the ef fl uent by crystallization include calcium sulphate, sodium chloride, calcium chloride and sodium sulphate. Therefore, the scale causing chemicals can be removed from wastewater and this can be used a pre-treatment method for wastewater being used for membrane puri fi cation and evaporation. 6.7 Zero Liquid Discharge (ZLD) Zero liquid discharge is a modern separation technique which leads to a solid (or near solid) waste stream and puri fi ed water stream which can be treated further or used for domestic applications (Dalan 2000 ). Reverse osmosis and thermal methods can be used for concentrating the waste stream and formation of pure water. However, since reverse osmosis is more cost-effective, it is more widely used 78 P. Senthil Kumar et al.
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(Heijman et al. 2009 ). ZLD consists of a series of wastewater treatment techniques which include micro fi ltration (MF) for pre-treatment, reverse osmosis (RO) for pre-concentration of waste solutions followed by the application of a multiple-effect evaporator. The major objective is to produce a solid concentrated stream which can be sent for land-based disposal and a relatively clean water stream for domestic use or sewer discharge. This leaves a clean ef fl uent stream without any dissolved solids for further treatment while the fi ltered or concentrated solids can be sent for suitable disposal.
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  • Fall '19
  • N. Gokarneshan

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