Process flow - step 2

Primary treatment

The next stage of treatment takes place in the primary sedimentation tanks.

The chemical treatment precipitates phosphates, organics and heavy metals dissolved in the sewage. It is at this stage of the process that polymer is added to cause flocculation (or binding together) of precipitated particles as suspended solids. This assists in settling of the solids to the bottom of the tanks, where they are removed as settled sludge.

The sludge is held in the primary sedimentation tanks for two hours.

Primary treatment also involves skimming off surface scum and fats. Scum and fats are the soaps used in the bathroom and the oils and fats that are put down the drain in the kitchen. Fats will stick to almost everything. Fats should never be poured down the drain as they can cause blockages in household sinks and clog sewerage pipes.

Sludge collection

Mechanical scrapers in the primary sedimentation tanks are used on the bottom of the tank to collect the sludge, and also on the surface to collect the floating scum. The sludge and the scum are collected separately in troughs which are emptied regularly.

The sludge and scum removed in this process are treated in Step 7.

Next: Step 3