SBR System (Sequencing Batch Reactor)

Advantages of SBR Systems

SBR systems have found wide application, especially abroad, for treating wastewater in small communities (particularly coastal ones, where nutrient removal is crucial for eutrophication control) or for industrial wastewater treatment.
In many cases, these systems have been implemented by simply retrofitting traditional activated sludge plants or existing septic tanks.
This application has certainly been favored by the system's structural simplicity and flexibility, allowing for rapid and effective adaptation to diverse conditions.
In summary, the distinctive features of SBR reactors can be summarized as follows:
- High resistance to potential shocks caused by sudden increases in organic load, due to the very nature of these reactors and the presence of an equalization tank upstream;
- Structural simplification, as all different phases of a traditional activated sludge plant, including sedimentation, are combined into a single tank;
- Operational simplicity and process reliability, thanks to automatic control of all pumps (feed, sludge wasting, and effluent/sludge discharge) via a microprocessor and timer system connected to level sensors and/or characteristic physical-chemical parameters;
- Process flexibility, with the ability to easily modify phase durations and achieve high purification efficiency even under non-steady-state conditions. This is particularly relevant for industrial wastewater, characterized by continuous variability in composition.
Particularly important for the achievable purification yield is the ability to vary the duration of the sedimentation phase based on the sludge's settleability characteristics. Moreover, sedimentation occurs under conditions of complete stillness with zero upflow velocity, preventing the formation of preferential paths (short-circuiting), as often happens in continuous-flow sedimentation units.
Furthermore, given the sedimentation surface area, the solids loading per unit area is extremely low, allowing for the removal of even the most difficult-to-settle particles.
Operating flexibility should also be highlighted for the reaction phase, which can be modified by simply varying durations and operating modes.
It can be conducted in environments with different characteristics, allowing for the removal of: organic matter alone, nitrogen (nitrification/denitrification), and phosphorus.
This enables optimal management of the "resources" used in purification, following a logic inspired by sustainable pollutant control criteria.
- Absence of recirculation (both for sludge and aerated mixture): this feature allows for highly effective use of organic matter for denitrification, which is decisive for wastewater with low COD/N ratios.
Additionally, the absence of recirculation pumps provides a clear economic advantage.
- Higher oxygen transfer efficiency in the tank during the aeration phase, since dissolved oxygen concentrations decrease to near-zero values during the feed and reaction phases (both anoxic and anaerobic). Numerous authors report transfer efficiencies in the range of 10% to 30%.
- Improved biomass settleability characteristics, due to microbial selection resulting from alternating anoxic, anaerobic, and aerobic phases. While it cannot be stated with absolute certainty that these process conditions always discourage the growth of filamentous bacteria, experimental evidence demonstrates the high settleability and resistance to mechanical shock of activated sludge in SBR systems.
- Greater understanding of ongoing biological processes; indeed, the batch reaction allows for easy determination of pollutant removal kinetics. Furthermore, the batch reaction leverages higher kinetics due to high initial pollutant concentrations (increasing the substrate diffusion rate into the sludge flocs), making SBR systems particularly suitable for treating high-concentration wastewater.