Life after filtration
15 Jan 2000
Sterilistation of sludge is vital not only for the dewatered fraction, but for the remaining solids. Sludge used in agricultural fertilisers will runoff and enter lowland surface waters. Organisms present in the runoff contaminate potable water sources and until treatment works guarantee complete pathogen destruction, futher protection of water is required by municipal water services.
Conventional methods of detecting waterborne organisms, such as cryptosporidium, entail concentrating large volumes of water, recovering the oocysts by centrifuge, then asessing possible risk. But this technique does not detect if the oocysts are capable of inducing infection.
Limitations of conventional filtration techniques are causing water companies to optimise processes such as coagulation and granular media filtration. Oocysts are also resistant to chlorine but ozone has proved effective. Unfortunately ozone also breaks down naturally occuring organics producing nutrients for other unwanted bacterium. Ultra-violet irradiation destroys oocysts but can be too expensive on a large scale. Oocysts can however, be inactivated by heating filter elements to 60 degrees C for five minutes.
Cartridge filters and microfilters provide a high degree of security, but complete removal of oocysts can only be guaranteed by reverse osmosis (RO) or ultrafiltration.
RO not only provides protection against cryptosporidial contamination, but can replace processes such as dealkalisation, degassing and base-exchange water softening.
