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TERTIARY TREATMENT
Tertiary treatment may be considered as the further treatment of sewage that has already been treated to a standard achievable by biological treatment. The most common application of tertiary treatment is the reduction of suspended solids (and associated biochemical oxygen demand) but it may also be used to reduce ammoniacal nitrogen, total nitrogen, phosphorous or micro-organisms.
- Upward Flow (Pebble Bed) Clarifier
A simple process that should not be confused with conventional clarification tanks used in primary and secondary treatment. Secondary effluent from a biological oxidation plant passes in an upward direction through a shallow bed of pea gravel supported on a steel/plastic mesh. The suspended solids in the effluent are removed by natural flocculation and settlement Upward flow clarifiers will accommodate wide fluctuations in suspended solids concentration and hydraulic loading and there are no mechanical moving parts.
- Pressure Filters
A closed vessel in which the influent is forced (in a downward direction) by pressure generated by feed pumps through a static filtration bed. The filter is fully automatic in operation with flows controlled by air operated butterfly valves mounted on a frontal pipe manifold. As the solids level in the filter medium increases air scour and backwashing is necessary (backwash liquor is returned to the head of the works for further treatment), thus flow balancing or sufficient duplex filtration capacity is required to ensure continuous operation.
- Rotating Drum/Disc Filters
Drum/Disc filters are packaged units that can be used to replace conventional humus tanks or as tertiary filters. They are able to accommodate a wide range of suspended solids loadings and are designed to receive a gravity flow thus negating the need for high pressure feed pumps.
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