Inlet arrangements are substantially different in conventional and "enhanced flocculation" clarifiers.

Conventional clarifiers

Conventional clarifiers are typically center-fed, with vertical inlet pipe discharging mixed liquor horizontally into a cylindrical bottomless well from which is the outlet distributed by ports or other structures into the clarifier. Though some arrangements are claimed to be superior to others, in real life the difference in performance is not dramatic. Principle difference, however, is between shallow and deep inlets.

Shallow inlets frequently cause  currents and produce higher effluent TSS. On the other hand, clarifiers with shallow inlets are rather resistant to short peak flows.

A sketch of a clarifier with flocculation well

During high SVI episodes, sludge blanket rises and the height of the inlet zone (clearance) diminishes. The clarifiers behavior is similar to the deep inlet conventional clarifiers with their positive and negative features (see the right column).

Deep inlets introduce the mixed liquor into the sludge blanket which is in consequence fluidized. Incoming suspended solids are "filtered" (floc-filter) in the sludge blanket and effluent quality can be excellent. Resistance to short term peak flows is poor and the upper layer of fluidized sludge blanket can be lost from the clarifier and produce effluent TSS in the order of hundreds mg/l.

There are many old shallow final clarifiers in Europe and elsewhere. During plant extensions they have been in many cases supplemented by new, deeper clarifiers. A sensible use of the old ones is to load them moderately and by approx. constant flow and convert them to deep inlet. It is completely wrong to admit peak flows to shallow clarifiers with deep inlet. New deeper clarifier can process the variable additional flow.

Enhanced flocculation clarifiers


Flocculation inlet into a large industrial clarifier (winter 1999)



The inlet well has typically a bottom and the influent is discharged into a bottomless flocculation cylinder of a relatively large diameter, 20-30% of the clarifier diameter.


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