Table 8.1 The stages of wastewater treatment
| Treatment Stage | Waste constituents treated * | Remaining waste | |||
|---|---|---|---|---|---|
| Organic material (BOD) | Suspended solids (organic) | Bacteria and viruses | Salts:nitrates & phosphates | ||
| Primary (settling) | Up to 35% captured | Up to 65% captured | Not removed | 'Raw' sludge and primary effluent | |
| Secondary(aerobic bacteria growths) | Can be reduced to 20 g/m3 | Can be reduced to 30 g/m3 | Some removed | Not removed | Biological sludge; secondary effluent with some salts, metals, bacteria etc |
| Tertiary (various techniques) | Can be reduced to15 g/m3 | Can be reduced to 10 g m/3 | Can be disinfected to remove | Can be treated to reduce salts | Tertiary effluent and solid residues with metals, etc. |
| Sometimes better tertiary quality than this can be achieved | |||||
| Land (septic tanks and soil soakage) | Will reduce total amounts of organic material, salts and bacteria - levels depending on system design | Remaining scum and sludge (septage) with metals, etc. | |||
| Treatment of sludge | Takes primary and secondary treatment sludges and uses anaerobic digestion to convert them to ‘humus solids’, known as biosolids, plus methane gas | Methane gas; biosolids with metals, etc. | |||
| Treatment to produce reclaimed water | Further treatment for non-potable purposes | ||||
* Other waste constituents (see Section 2.3) that are not removed by standard treatment processes will need to be assessed to determine whether there is a tertiary technique(s) that can be used to treat them. If there is no applicable process available, the method of ecosystem re-entry used will have to address the environmental risks associated with those substances not managed through the treatment technologies.
