Sequencing Batch Reactor (SBR)

A Sequencing Batch Reactor (SBR) is one of the types of biological processes of suspended biomass used for the treatment of sanitary or more frequently industrial liquid effluents.

Reactores Secuenciales Bach 2It is a modification of the activated sludge system and can be described simply as reactors that receive effluent batch. When this batch is treated, a fraction of it is discharged and the remainder is accumulated for the next cycle. There are two types of SBR: intermittent or “true” SBR flow or continuous flow.

Those with intermittent flow accept effluents only at specific intervals and use sequence or phase times within a treatment cycle to achieve the same objective as an activated sludge, but in cycles and in several reactors simultaneously. There are usually at least two reactors in parallel, because one of them will be receiving effluent while the other will be operating in the phase cycle.

The cycle of an SBR can be designed and / or modified in order to: (1) generate an operational strategy to provide conditions: aerobic, anoxic or anaerobic for the development of specific bacteria, (2) biological removal of nutrients (N) And p). These modifications or changes in the phases allow the SBR to treat effluents in quantity and fluctuating composition, maintaining a high quality of treated effluent.

SBRs of intermittent flow can be filled once to their operational level and then the batch is processed through the process phases, or it can also be filled several times in smaller batch until reaching the same condition. However, in the latter case, commonly each charge is treated in the reactor. This type of configuration is used for effluents with a high concentration of organic load and is known as multiple feed SBR.

In continuous flow SBR, as its name indicates, the effluent enters uninterrupted during all phases of the cycle. To reduce short circuits of contaminants, screens or partitions are usually incorporated to separate the turbulent aeration zone from the laminar lapping zone.

The “real” SBR was developed in early 1900 in England and has been used successfully in the US, Canada and Europe since 1920. After the decade of the ’50s, great changes and improvements have been implemented in this type of systems; however, due to the high degree of automation and attention by the operator, the activated sludge system has prevailed over the SBR during these years.

While a system of activated sludge can be operated in various ways, one SBR combines all the treatment of the former within the same reactor, while the activated sludge configuration employs several reactors or enclosures.

The key to the process in an SBR is the control system. Which allows the precision and flexibility for the correct operation of the system. By varying the times of the different phases (oxic / anoxic / anaerobic) of a cycle, the reactions of nitrification, denitrification and biological phosphorus removal can be controlled.

Comparison with an activated sludge

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On the other hand, this type of systems, due to its main characteristic are very used in industries with variable flows.

Entering the process itself, the number of cycles and times of each phase within the SBR cycle are determined by:

  • The flow of effluent to be treated.
  • The composition of the effluent to be treated.
  • The output requirements of the treated effluent.
  • Parallel number of reactors available.

An increase of the flow will result in a shorter treatment time and therefore in the increase of the necessary daily treatment cycles with less time per cycle. An increase in income concentration will require more time and a decrease in the number of daily cycles with an increase in time per cycle. For its part, to achieve nutrient removal objectives, it will require an increase in the treatment time and a decrease in the number of cycles per day.

Having more reactors allows to increase the treatment time per cycle or shorter cycles for each reactor. The usual is between four and six daily cycles for domestic effluents. In any case, the  operative factors that will define the number of cycles per day will be the flow and the minimum time of treatment required.

Phases of an SBR

Although the process of activated sludge of continuous feeding was partly developed to eliminate the inconvenient of fouling of the diffusers in an SBR, separating the solids in phase of aeration and decantation; Many effluents have characteristics that make them unsuitable for these continuous processes and are better treated in batch processes such as SBR. These examples can be: batch treatments of effluents to achieve removal of nutrients such as P and N; industrial effluents with high priority organic load compounds; Batch treatments of effluents in regions with cold climates where conventional decantation presents problems. But the biggest advantage of an SBR compared to continuous flow process is its flexibility to modify reactor  conditions.
However, in order to successfully use an SBR process, two requirements must be fulfilled: control of the automation and adequate training of the personnel and at least one or more reactors in parallel or equalization for correct operation of the  system.

The phases of a SBR of simple feeding per cycle commonly follow the following sequence:

  • Fill
  • Reaction
  • Decantation
  • Emptying

Filling phase:

During the filling phase, the effluent to be treated (substrate or BOD) is admitted to the SBR reactor, from the minimum level to the maximum design level that coincides with the total volume per batch. The time of this phase depends on the incoming flow, the design load conditions, the detention time and the sedimentation characteristics of the biomass.

The filling phase can operate in any of the following modes or combinations of modes:

  • Static filling (anaerobic – no aeration or mixing): used to produce AGVs such as acetate and formic acid necessary to promote the biological removal of phosphorus. It is also used in the start-up of an SBR, as a selector for controlling the growth of filamentous microorganisms or in periods of low load for energy saving.
  • Filling with mixture (anoxic – without aeration but with mixing): used to promote biological denitrification and control of filamentous microorganisms.
  • Llenado aireado (óxico – con aireación y mezcla): utilizado para reducir la carga orgánica antes de la fase de reacción y promover la remoción biológica de fósforo si esta fase es precedida por una fase de llenado estático. La aireación puede lograrse mediante la mayoría de los sistemas de aireación tradicionales incluyendo: difusores, mecánicos, jet, etc.

Reaction phase:

This phase begins when the reactor is completely filled in the previous stage (filled phase). Once this phase is completed, the effluent is transferred to another reactor to continue with the cycle.

The reaction phase continues until complete degradation of the organic load (BOD), if necessary also could be completed the  nitrification processes and / or biological removal of phosphorus.

During this phase, there is no effluent entering the reactor and the aeration and mixing can be continuous or intermittent as needed. The purpose of this aeration and mixing is to complete the degradation of the BOD, promote the nitrification and biological retention of phosphorus (if there is a static filling phase). The objective of not aerating in this phase is related to favor denitrification.

In this phase, as mentioned, no substrate enters, because the effluent entering the SBR was diverted to another reactor, for this reason the OD increases as the time of this phase elapses.

Decanting phase:

During this phase, both aeration and mixing are completed. Flocs formed by biomass (sludge) that degrades organic matter are separated by the effect of gravity and thus generate two streams: one of sludge settled in the bottom and one of supernatant  liquid. If the solids do not decant and are compacted properly, some of them can escape from the reactor during the emptying phase.

Generally this type of systems compared with a conventional activated sludge, generate a supernatant of better quality since the conditions for the clarification are ideal, since there is no continuous income of flow to the reactor. The traditional sweeping  systems of conventional decanters are also avoided to accumulate sludge and then recirculate them.

Drain / purge phase:

In this phase, the clarified and treated liquid is emptied from the SBR reactor. This can be carried out by fixed pipes at a  certain level inside the reactor or by adjustable dumps.

When the liquid level reaches the minimum level, the emptying / purging phase ends.

Adjustable or floating dumps, keep the exit holes slightly below the liquid level to avoid leakage of foams or floatables during this stage.

Commonly, 25% of the volume of an SBR is decanted in this phase. This allows maintaining the biomass inside the reactor for the next cycle.

Eventually, the excess mud purges are made from the bottom of the reactor.

In many cases, there is a fifth phase of “standby” or “standby” where a time passes that allows the SBR to be incorporated  within a cycle. That is, it is a waiting time to complete the batch cycle while another reactor completes its filling phase or completes its emptying / purging phase.

In particular cases where the system is not a traditional SBR and there is continuous flow, there are only three phases of those  described above: reaction phase / decanting phase and emptying / purging phase.

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