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Types of WwTw

Process design is a highly skilled discipline. Design variations, internal arrangements and layout may be specified in order to address specific effluent types, loadings in conjunction with available space, discharge considerations and the availability of funds. Though they may be similar and true to type in theory, no two Works are exactly the same.

There are two types of WwTW:

  1. Activated sludge works
    These normally service large towns and cities. A typical activated sludge works comprises...
    • Inlet works with screens to remove 'rags' (the term used to refer to typical detritus – plastics, rags, sticks etc.) with or without a macerator (to break up floaters and soft solids).
    • There may be an anaerobic digestor (a large septic tank) where 'soft' COD will be degraded.
    • One or more primary settling tanks where inorganic impurities are removed.
    • A reactor, also known as an aeration 'basin' or 'ditch', where atmospheric air is mixed into the effluent to catalyze the activity of the latent or seeded bacteria.
    • A clarifier, which is a very deep (usually over four metres deep), circular in-ground tank in which the partially treated effluent comes to a virtual rest and stops, allowing any fats or un-degraded COD to float to the surface forming a scum. This is then swept into small hoppers by a crawling centre pivot skimmer arm – the centre of the pivot post forming the in-feed. The scum is then pumped back to the reactor to be circulated again for further processing to break it down.
    • A waste sludge processor (this sludge is mainly composed of 'dead' bacteria), which is normally a belt press – or storage ponds in older plants that have to be cleaned out periodically (at considerable expense).
    • Maturation ponds – there are usually three or four (but there well be more, depending on the layout; it's the surface area of the effluent that's important). Otherwise or as well as in some instances, a maturation river. In either arrangement the idea is for the effluent to be finally 'polished' by sunlight to achieve the desired discharge quality.
    • Disinfectant or biocidical contact (usually a chlorine gas compound) that kills all bacteria prior to discharge into the environment. Chlorine gas bubbled into the treated effluent degrades within a few metres of the discharge so that water downstream (in the natural watercourse) is not contaminated. Usually the 'canal' is 100m or more long to permit degradation even under maximum flow conditions to accommodate hydraulic overload during storm surges.
  2. Oxidation ponds
    An oxidation pond system (often found in smaller towns or as metropolitan satellites to outlying suburbs) is much simpler and comprises...
    • Inlet works with fairly rudimentary screens tended on weekday mornings by hand.
    • An anaerobic digestor (a large septic tank) or a clari-gestor, which is a small primary settling tank-cum-anaerobic digestor.
    • Five or six shallow oxidation ponds in which COD is reduced as the effluent progresses slowly through them.

      Oxidation pond Oxidation ponds
    Although not as 'glamorous', a well-balanced oxidation pond system can produce final effluent as good as a complex activated sludge works. It all depends on the organic load (quality of sewage) and volume of what is received.


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