Operation 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 flow^1,2. 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.

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.

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 (NO_x) or sulfur compounds (SO_x), 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.

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.

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.

 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.

1. AguaSigma, SEQUENTIAL BIOLOGICAL REACTOR (SBR), https://aguasigma.com/en/procesos/sbr/
2. 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)
3. Metcalf& Eddy, Wastewater Engineering Treatment and Reuse, 2003
4. 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)
5. Suez, Help in analysing malfunctions, https://www.suezwaterhandbook.com/processes-and-technologies/biological-processes/suspended-growth-cultures/help-in-analysing-malfunctions