This is necessary to ensure that these suspended

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separation methods and if necessary, ions such as iron and magnesium. This is necessary to ensure that these suspended solids and ions do not foul the semiper- meable membrane. After pre-treatment the wastewater is pressurized and sent to the reverse osmosis chamber. When the applied pressure is greater than the osmotic pressure across the membrane, clear water passes from wastewater to the clear water chamber, emerging at atmospheric pressure. Reverse osmosis can be used to treat ef fl uent from primary, secondary and tertiary stages of wastewater treatment. The reverse osmosis models include spiral wound system, tubular systems, Hollow fi bre membranes and the disc module. The main downfall of this process is fouling of the membranes in use due to organic materials, colloids and micro-organisms. Also scale formation can take place due to hardness-causing constituents (carbonates) in the ef fl uent stream. Chlorine and ozone can oxidize the membrane and hence should not be present in wastewater to be treated. 6.3.2 Ultra fi ltration Ultra fi ltration and reverse osmosis follow similar techniques. The difference between the two processes is primarily the retention properties of the membranes. Fig. 3 Reverse osmosis (Wayman 2015 ) Nanochemicals and Ef fl uent Treatment in Textile Industries 75
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Ultra fi ltration membranes have higher pore size and can retain only macro-molecules and suspended solids. Reverse osmosis membranes on the other hand have smaller pore size and hence can be used for removal of all solutes including salts. The osmotic pressure difference across an ultra fi ltration membrane is negligible as the salts pass through along with the permeate water. This makes it necessary for a much lesser pressure to be applied for the process to take place (Babu et al. 2007 ). Ultra fi ltration membranes materials include cellulose derivatives such as cellulose acetate, polymers of nylon and other inert polymers. This makes it possible to treat acidic or caustic streams and not limiting the process unlike that for reverse osmosis due to chemical attack of the membranes. 6.3.3 Nano fi ltration When the salt concentration in the permeate water is not of prime importance as compared to the reverse osmosis operation, nano fi ltration can be used for wastewater treatment (Nguyen et al. 2012 ). Hardness-causing elements in water are salts of calcium and magnesium, and nano fi ltration can be used to remove these salts. It can also be used for the removal of micro-organisms such as bacteria and viruses. Nano fi ltration is suitable to use when permeate with TDS, but without colour, COD and hardness is acceptable. It is a more cost-effective method. For higher ef fi ciency, turbidity and the amount of colloidal content in wastewater should be low. Fouling is a major problem whenever membrane separation techniques are used for purifying wastewater (Jiraratananon et al. 2000 ). Wastewater has dissolved elements such as silica, calcium, barium, iron and strontium which may precipitate on the surface of the membrane and in turn clog the pores of the membrane. This phenomenon is termed as fouling of the membrane. Bacteria present in wastewater
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  • Fall '19
  • N. Gokarneshan

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