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Reactors discontinuous cycle (SBR)

Sequencing batch reactors (SBR) are discontinuous flow treatment units where the different treatment steps are finalized within a single vessel. No development in space as in the case of traditional activated sludge units, where the flow is pumped to the following vessel, but they are time-oriented units, leading to flow and volume changes in the vessel in compliance with the different operating strategies.

The reactor consistent flexibility complies with different requirements. In fact the same system can be intensively operated, with consistent inlet loads or at idling speed, thus leading to a consistent shock resistance depending on the abrupt increase of the organic load during feeding.
Sequencing batch reactors are suitable for the small and medium size community waste treatments, mainly industrial wastes, as less sensitive to the hydraulic and organic load.
On a SBR, the treatment cycle essential consists of 4 different time steps: Fill, React, Decant/settle and Discharge.
Reactors discontinuous cycle (SBR)
During filling (1 step), the liquor is pumped to the reactor, where it is mixed with the residual biomass from the previous cycle. The initial volume assessment, that is the liquor volume present in the vessel at the beginning of filling, depends on different factors, among which sludge settling sedimentation, biomass concentration and hydraulic retention times. The initial liquor filled volume can vary up to 40-50% of the total volume. Thus the liquor is diluted or consists of an initial mixed liquor volume accounting for at least 50-60% of the total volume.
During filling, it is possible to forecast different operations, alternating or in series. The filling can be forecast under normal conditions or from simple mixing to aeration at different rates.

When the scheduled maximum level is reached, the mixing-aeration started during filled comes to an end during the Reaction step (step 2). As during filling, during reaction as well, there are different anaerobic, anorexic and aerated steps, at the aeration system level: two different degradation steps are therefore possible, affecting microorganisms and different enzymes, mutually interfering thus leading to a more effective treatment, with no side effects such as bulking, which generally often appear under specific conditions in the traditional treatment units.
The batch process leads to a constituent concentration within the reactor, which on average is higher than in the continuous stirred tank reactors (CSTR) and as a consequence the average speed of the constituent removal is higher.
In other words, the higher the constituent (pollutant) concentration in the reactor, the higher the constituent degradation speeds.

The decanting/settling starts (step 3), when suspended solids are settled disabling the aeration-mixing units, therefore the newly recovered calm inside the oxidation vessel leads to the activated sludge build-up on the vessel bottom, leaving the surface cleaned.
After settling, the clarified liquor is removed (step 4).
The vessels are emptied through one of the installation dedicated outlets, as it is necessary to provide for the effluent good quality, thus avoiding removing the biomass or the suspended solid dragging.
The excess sludge is removed during settling, even though sometimes the entire process is delayed during the waiting period.

The system is entirely automated, through an automation and control panel managing the different treatment step durations according to the tank level and the electro-mechanical unit operations (compressors, mixers, feeding pumps, bleeding pumps of the cleaned effluent).


  • Small and medium-size domestic waste water treatment;
  • Agriculture and food waste water treatment, as poorly sensitive to the hydraulic and organic load of the industrial waste, connected to the operating activity of the manufacturing departments:
    • Dairies;
    • Meat processing;
    • Wine processing;
    • Canning food industry.


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