A fill-and-draw reactor is a single reactor in cyclic operation :

1. phase 1 : wastewater is fed and the reactor is aerated (accumulation process).
2. phase 2 : regeneration phase, during which no wastewater is fed.
3. phase 3 : the sludge is allowed to settle, so that the clean effluent can be discharged. A part of the sludge is wasted.

It is clear that the water level in the reactor cannot be constant and that continuous feeding is not possible.

On the other hand, the fill-and-draw reactor allows wastewater feeding into a reactor with a relatively high substrate concentration. In these conditions, the so-called floc forming bacteria grow much faster than the filament forming bacteria. These floc forming bacteria tend to settle much more easily than the filaments (better sedimentation).

In a conventional system, the substrate concentration is lower and this means that the filaments tend to dominate. This leads to less easily settling sludge. To solve this problem, a conventional system sometimes uses a separate so-called selector tank in which the influent is mixed with the highly concentrated recycled sludge.

Advantages

Easy and compact construction

Control in time

Selection of well settling sludge (= 'integrated selector effect')

Quiescent settling conditions.

Disadvantages

Discontinuous influent feeding and effluent discharge

The variable level and volume

Complex moving equipment

SEQUENCING BATCH REACTOR (SBR)

There is a fairly simple way to make a fill-and-draw system continuous: place several fill-and-draw reactors (each operating in batch mode) in parallel, in such a way that always at least one reactor is in 'fill' mode and one in 'draw' mode. This way, some of the advantages of the fill-and-draw system are maintained, on the other hand this method retains some of the disadvantages of fill-and-draw (variable level) and of the conventional system (several reactors).

Waterleau Lucas® system merges the fill-and-draw (time control) and conventional system (place control), combining the advantages of both systems and eliminating the disadvantages.