Index >> Waste Water Microbiology >>Tertiary Treatment

Ammoniacal nitrogen can also be removed by breakpoint chlorination by adding hypochlorous acid in 1:1 ratio. Removal of ammoniacal nitrogen lowers the BOD because nitrification would consume oxygen dissolved in the remaining water. Removal of heavy metals like mercury, lead, chromium and cadmium also occurs during tertiary treatment. The absorbed metal ions are generally converted into either toxic products or residues that remain associated with the micro  biopolymer matrix and are either released during sludge treatment or are remobilised after sludge disposal. The general tendency of bacteria to concentrate heavy metals in their biomass is favourable to effluent quality, but it complicates the disposal of sludge (rectified to some extent by microbial mining. e.g. acid produced by thiobacilli would solubilise heavy metals and leach them from sludge). Heavy metals can be subsequently fed and reprocessed for use or permanently immobilised.

Disinfection is the final step in tertiary treatment. This is to kill escaped bacteria or viruses. This is accomplished by chlorination (chlorine gas, hypochloride a sodium hypochlorite).

Disadvantages
More resistant types of organic molecules including some lipids and hydrocarbons are not oxidised completely but instead become partially chlorinated. Chlorinated hydrocarbons tend to be toxic and are difficult to mineralise. Alternative means of disinfections are more expensive, hence chlorination remains the principle means of sewage disinfectant.