Softening

Softening is commonly used for GW by Lime-soda ash process and Ion-exchange process.

In lime-soda ash process, hydrated lime is added to water raising pH to 10.3 meaning increasing OH- plus adding Na2CO3 precipitating CaCO3. Removal includes CO2, Ca-CH, Mg-CH, Ca-NCH and Mg-NCH. After softening, CO2 is bubbled into water to decrease the pH and precipitating calcium carbonate.

In Ion Exchange process, replacement of unwanted ions (Ca2+ and Mg2+) with ions of different species (Na+) attached to insoluble resin (Zeolites) through a column. The removal is 100% effective yet Na+ causes heart problem to consumers. Thus, backwashing the bed with solution NaCl forming Na2R: Ca(HCO_3 )_2+Na_2 R→CaR+2NaHCO_3

In membrane processes, waste water (feed) passes through micro-filtration, ultra-filtration, nano-filtration and reverse osmosis completing permeate.