by Sharon Beder
first published by Allen & Unwin, Sydney, 1989
SYDNEY'S SEWAGE is hardly treated at all. Conventionalsewage treatment is identified by stages. Primary treatment, which comes after the sewage is screened to remove large solid objects, is simply sedimentation – the sewage is put into tanks and the heavier solids settle to the bottom. These solids are scraped of the bottom and are called sludge. The scum that rises to the surface is also scraped off. Then comes secondary treatment, a biological process that utilises naturally occurring micro-organisms to break down the organic material in the sewage. Tertiary treatments usually provide further biological breakdown.