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September 25, 2025The Superpower of Wetlands
July 17, 2025Defoamer Series – How foams are generated and the additives that break them down.
September 25, 2025Operation of an SBR: Which Parameters Influence Performance?
Over the decades, the sequencing batch reactor (SBR) has established itself as a benchmark technology in wastewater treatment. The main difference between this process and others is that the treatment stages are separated in time rather than space. In fact, it’s not a question of different zones, but rather of different periods. This configuration reduces construction and operating costs, while also offering greater flexibility when faced with variations in inlet flow1,2. The treatment sequence generally consists of five phases: filling, aerobic reaction, anoxic settling, supernatant removal and sludge removal3. The time allotted to each of these phases varies according to their role. For the aerobic reaction, the average duration ranges from 1.5 to 3 hours, while settling generally lasts from 30 minutes to 1 hour. The removal phases, meanwhile, last around an hour4. Operating this system can be complex if the factors influencing it are not well understood. Therefore, the remainder of this article will be devoted to explaining these different phases, as well as the main operational parameters that can be acted upon.
PHASE I – FILLING
During the filling phase, wastewater is introduced into the reactor. It is at this point that the new fresh feed comes into contact with the biomass that is already in the reactor. Therefore, it is necessary to maintain a balance between the flow of incoming raw water and the amount of material extracted during the removal phase, not only to maintain a relatively stable volume, but also to sustain a very active young biomass. The ratio of filling volume to total volume in the bioreactor is generally around 25-30%. This design feature ensures that an adequate volume of sludge with active bacteria is maintained in the tank throughout the cycles3. In terms of the incoming raw water feed, measuring the concentration of suspended solids (SS) in the mixed liquor is an effective method of ensuring consistency over time. The system is highly dependent on this value, since the overall balance of the process depends on the ability of the bacteria to abate the load.
|
Operational Parameter |
Modification |
Influence of the Parameter |
|
Volume of Wastewater Added to Bioreactor |
↑ |
An increase in the volume of wastewater corresponds to an increase of the contaminant load of the total volume. As a result, the active biomass must be sufficiently competent to remove the additional load. This addition also translates into an increase in the amount of sludge that will be generated in the treatment process. |
|
↓ |
A decrease in the volume of added wastewater corresponds to a decrease of the contaminant load of the total volume. As a result, the process is not operating at its optimum capacity, and there is no gain in overall treatment performance. |
PHASE II – AEROBIC REACTION
The aerobic phase is when air is injected into the system. The introduction of oxygen into the environment enables the biological elimination of organic matter by oxidation, as well as the nitrification of ammonium (NH4+) into nitrite (NO2–) and nitrate (NO3–)1. The duration of the aeration phase depends on the composition of the wastewater and the desired level of removal. A longer aeration period promotes microbiological activity, enabling more complete degradation of the targeted contaminants. This phase also produces the bulk of the biological sludge. It is therefore essential to adequately assess the volume generated in order to maintain an operational balance between sludge production and removal. Consequently, the amount of SS in the mixed liquor, the reaction time and the level of injected oxygen are key parameters whose optimization is essential to the performance and stability of the treatment.
|
Operational Parameter |
Modification |
Influence of the Parameter |
|
Quantity of SS in the Mixed Liquor |
↑ |
An increase in SS concentration due to a reduction in sludge removal increases the available active biomass. By retaining more sludge, a higher concentration of active biomass is available to treat environmental contaminants. However, it’s important to understand that a balance needs to be maintained, as an excessive SS concentration in the bioreactor can also impair the performance of the process. |
|
↓ |
A decrease in SS by greater sludge removal reduces the concentration of active biomass. This can reduce biological treatment capacity and lead to the incomplete treatment of contaminants. | |
|
Reaction Time |
↑ |
An increase in reaction time gives microorganisms more time to consume contaminants, promoting more complete oxidation and nitrification. |
|
↓ |
A reduced reaction time limits the time available for biological reactions, which, in turn, decreases pollutant degradation and compromises the achievement of targeted discharge standards. | |
|
Oxygen Level |
↑ |
An increase in oxygen supply improves aerobic conditions, favouring the oxidation of organic matter (COD) and the nitrification reactions needed to eliminate ammonia. |
|
↓ |
A reduced oxygen level limits essential aerobic reactions. This can slow down COD oxidation and inhibit nitrification, thus compromising treatment efficiency. |
PHASE III – ANOXIC SETTLING
The settling phase is quantified as anoxic due to the complete interruption of the oxygen supply. Only bound oxygen, present in certain nitrogen compounds (NOx) or sulfur compounds (SOx), remains in the environment. These conditions favour the reduction of nitrate (NO3–) to nitrogen gas (N2), which, once in this form, can be removed from the water. By putting the brakes on aeration and, therefore, agitation, the settling phenomenon can begin. This is when biological flocs, made up of biomass and residual waste, settle to the bottom of the bioreactor. The efficiency of settling will dictate the SS removal performance from the clarified water. When the sedimentation of biological flocs is not sufficiently effective, the addition of a coagulant, alone or combined with a polymer, can help correct the situation. Moreover, in cases where the phosphorus discharge standard is a critical parameter, chemical treatment often becomes indispensable, since the chemical removal of phosphate is the only way to meet regulatory requirements. Although a chemical treatment can be added directly to the biological reactor, it is generally preferable to apply it downstream so as not to interfere with microbial activity.
|
Operational Parameter |
Modification |
Influence of the Parameter |
|
Settling Time |
↑ |
Increasing the settling time allows more flocs to settle, resulting in a clearer supernatant. |
|
↓ |
A shorter settling time reduces the time available for settling flocs that are less dense, increasing the risk of them being carried over in the clarified water and consequently reducing the quality of the supernatant. | |
|
The Presence of a Coagulant |
With |
The addition of a coagulant promotes the agglomeration of biological flocs, thus improving their settling speed. In addition, when a phosphorus discharge standard must be met, coagulant-induced chemical removal of phosphate helps to achieve this objective. |
|
Without |
Flocs can naturally settle to the bottom of the tank without the addition of coagulant. However, the process is slower, and smaller flocs may remain in suspension, adversely affecting the quality of the clarified water. What’s more, in the absence of the chemical removal of phosphate, compliance with phosphorus discharge standards may prove more difficult. |
PHASE IV – SUPERNATANT REMOVAL
The supernatant removal phase corresponds to the stage when the water, now clarified, is extracted from the bioreactor. It is essential to check that this water complies with current discharge standards. This is usually done before the sludge is removed to avoid remixing solids with the clarified water. At this stage, a good knowledge of sludge production, and consequently of the height of the sludge blanket in the reactor, is necessary to prevent any risk of an accidental carryover of sludge towards the clarified water outlet.
|
Operational Parameter |
Modification |
Influence of the Parameter |
|
Sludge Level in Reactor |
↑ |
If the sludge level is too high, there is a significant risk of sludge being carried over when the clarified water is removed. A reassessment of the amount of sludge to be removed per cycle is therefore necessary to avoid this problem. |
|
↓ |
A lower sludge blanket height has no immediate impact on the quality of the clarified water extracted. However, it is essential to understand that a minimum volume of sludge is necessary to maintain the equilibrium of the biological treatment, since the active biomass is found in the sludge. |
PHASE V – SLUDGE REMOVAL
Sludge removal is a critical phase, essential for maintaining the overall balance of the treatment. As mentioned above, most of the sludge is generated during the aerobic phase and consists mainly of biomass (dead and living) and waste. The removal of this sludge must be matched to its rate of production to maintain an adequate volume of active biomass in the bioreactor throughout the cycles. The aim is to keep the sludge relatively young, but sufficiently active to ensure effective biological treatment. To achieve this, the sludge retention time (SRT) is generally maintained between 10 and 40 days, depending on the treatment objectives3.
On the other hand, biomass that is too old can lead to an overall loss of system performance. In addition, rising sludge can occur in systems where aged sludge, accumulated at the bottom, remains too long in anoxic conditions. This leads to the production of gas, which forms micro-bubbles that adhere to the biological flocs, causing them to float back up into the clarified water5.
|
Operational Parameter |
Modification |
Influence of the Parameter |
|
Age of Mud |
↑ |
Overall aging of the sludge in the reactor leads to a reduction in biological activity, thus reducing treatment efficiency. In addition, excessive aging at the bottom of the tank can cause degasification, which causes flotation of biological flocs to the surface and, consequently, towards the clarified water outlet. |
|
↓ |
Sludge that is too young results in insufficient retention time in the reactor, preventing optimal biomass development. This limits treatment capacity and reduces contaminant removal efficiency. |
|
|
Amount of Sludge Removed |
↑ |
Excessive sludge removal is detrimental to the proper renewal of the biomass present in the environment. The amount of sludge removed must respect the sludge retention time of 10-40 days. |
|
↓ |
Insufficient sludge removal can lead to a build-up of old sludge, which, in turn, leads to a loss of treatment performance and intensifies the degasification phenomenon. All this increases the risk of flocs being carried over in the clarified water. |
The interdependence between the phases of the process and the operating parameters demonstrates the complexity of the optimization of an SBR. Controlling the sludge retention time, the aeration phase and decantation are essential to guarantee an effective and stable treatment. In addition, daily analysis of the main contaminants (SS, phosphorus, nitrogen, COD and BOD) throughout the cycles has proven to be a key indicator for diagnosing any loss of performance and targeting the origin of an imbalance more quickly. At Aquasan, we can support you in this optimization process by sharing with you the knowledge that we have developed over many years of experience in the wastewater treatment sector.
- AguaSigma, SEQUENTIAL BIOLOGICAL REACTOR (SBR), https://aguasigma.com/en/procesos/sbr/
- Lamarre, Optimisation de l’opération d’un réacteur biologique séquentiel pour l’élimination des nutriments d’un effluent industriel-agro-alimentaire fortement chargé en carbone, azote et phosphore, 1996, https://publications.polymtl.ca/8701/ (in French only)
- Metcalf& Eddy, Wastewater Engineering Treatment and Reuse, 2003
- Gouvernement du Québec, Guide pour l’étude des technologies conventionnelles de traitement des eaux usées d’origine domestique, 2023, https://www.environnement.gouv.qc.ca/eau/eaux-usees/domestique/1-introduction.pdf (in French only)
- Suez, Help in analysing malfunctions, https://www.suezwaterhandbook.com/processes-and-technologies/biological-processes/suspended-growth-cultures/help-in-analysing-malfunctions
