Waste water treatment can involve physical, chemical or biological processes or combinations of these processes depending on the required outflow standards.
The first stage of waste water treatment takes place in the preliminary treatment plant where material such as oils, fats, grease, grit, rags and large solids are removed. These processes are described in greater detail in the preliminary treatment manual (EPA, 1995). Primary settlement is sometimes used prior to biological treatment. Radial or horizontal flow tanks are normally employed to reduce the velocity of flow of the waste water such that a proportion of suspended matter settles out. Biological treatment of waste waters takes place in fixed media or suspended growth reactors using activated sludge, biofiltration, rotating
biological contactors, constructed wetlands or variants of these processes. Nitrification/denitrification and biological phosphorus removal can be incorporated at this stage and will reduce nutrient concentrations in the outflow.
Chemical treatment is used to improve the settling abilities of suspended solids prior to a solids removal stage or to adjust the properties or components of waste water prior to biological treatment (e.g. pH adjustment, reduction of heavy metals or nutrient adjustment). It may also be used for precipitating phosphorus in conjunction with biological phosphorus treatment. Secondary settlement separates the sludge solids from the outflow of the biological stage
Tertiary treatment refers to processes which are used to further reduce parameter values below the standards set out in national regulations. The term is often used in reference to nutrient removal.
Sludge treatment can be a significant part of a waste water treatment plant and involves the stabilisation and/or thickening and dewatering of sludge prior to reuse or disposal.
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