1. Working Principle and Basic Operations
The SBR process treats wastewater. Its core treatment equipment is a sequencing batch reactor (SBR). The SBR process eliminates many treatment structures. All reactors operate within a single SBR reactor. Time control is used to achieve the operational objectives of each stage within the SBR reactor. The flow pattern is completely mixed, achieving push flow over time, and organic pollutants are degraded over time.
The entire SBR process cycle consists of five basic steps: water inlet, reaction, sedimentation, water outlet, and idle time. These steps are carried out sequentially within a reactor equipped with aeration or agitation. This cycle repeats itself repeatedly during the treatment process to achieve the desired wastewater treatment results.
1.1 Water Intake Process
Before wastewater is injected, the reactor is in standby mode. The supernatant liquid after sedimentation has been drained, and the reactor still contains a high-concentration activated sludge mixture. The water level in the reactor is at its lowest. Sewage is injected, and the reaction proceeds after completion. In this sense, the reactor also acts as a regulating tank. Therefore, the SBR process is less affected by load fluctuations and is more adaptable to changes in water quality and quantity.
1.2 Reaction Process
When the wastewater reaches a predetermined height, the reaction process begins. The appropriate operation can be selected based on different treatment objectives. For example, controlling the aeration time can achieve different requirements, such as BOD removal, digestion, and phosphorus absorption. Controlling the aeration or agitator intensity can maintain anaerobic or anoxic conditions in the reactor, enabling digestion and denitrification.
1.3 Sedimentation Process
In this process, the SBR reactor acts as a secondary sedimentation tank. Aeration and agitation are stopped, allowing the mixed liquor to remain static, and the activated sludge to settle by gravity and separate from the supernatant. Sludge sedimentation in the SBR reactor occurs in a completely static state, minimizing external interference. Furthermore, static sedimentation avoids the problem of continuous effluent carrying away light, highly active sludge. Consequently, the SBR process offers short settling times and high sedimentation efficiency, maintaining high sludge activity. The settling time is determined by the wastewater type and treatment requirements, typically ranging from 1 to 2 hours.
1.4 Discharge Process
The supernatant after sedimentation is discharged, restoring the water level to the minimum at the start of the cycle. The remaining treated water serves as circulating water and dilution water. Most of the settled activated sludge serves as return sludge for the next cycle, while the remaining sludge is discharged.
1.5 Idle Process
When the SBR tank is idle, microorganisms regenerate through endogenous respiration, reducing the dissolved oxygen concentration and enabling a certain degree of denitrification and nitrogen removal, creating favorable initial conditions for the next cycle. Because the microorganisms are in a state of starvation after the idle period, the activated sludge has a larger surface area. Therefore, during the influent phase of the new cycle, the activated sludge can exert its strong adsorption capacity for the initial removal of organic matter. Furthermore, the standby process further reduces the dissolved oxygen level in the tank, providing favorable conditions for the denitrification process.
2. Performance characteristics of SBR process
(1) The process flow is simple, the operation is flexible, and the capital construction cost is low. The main equipment in the SBR process is an SBR reactor. From the above analysis, it can be seen that an SBR tank plays multiple roles: a mixing tank, a reaction tank (anaerobic, anoxic and aerobic), a sedimentation tank and a partial concentration tank. Basically, all operations are completed in such a reactor, and mud and water mixing, organic matter oxidation, digestion, denitrification, phosphorus absorption and release, and mud and water separation are carried out at different times. It does not require a secondary sedimentation tank and sludge return equipment, and generally does not require a regulating tank and a primary sedimentation tank. Therefore, the sewage treatment system using the SBR process greatly reduces the number of structures, saves capital construction costs, and often has the advantages of compact layout and space saving.
(2) The treatment effect is good and the effluent is reliable. From the perspective of reaction kinetics, the SBR reactor has its unique advantages. According to the activated sludge reaction kinetic model, the current continuous flow biological treatment reactor mainly has two flow states: complete mixing and plug flow. In the continuous flow plug flow reactor, there is only horizontal mixing on each section of the aeration tank, and there is no vertical "back mixing". The substrate concentration gradually degrades from the highest concentration at the inlet to the lowest concentration at the outlet, providing the greatest driving force for the biochemical reaction. During the aeration reaction stage of operation, although the mixed liquid in the reactor is in a completely mixed state, the concentration of its substrate and microorganisms gradually decreases over time, which is equivalent to a plug flow state in the time sense. Therefore, the SBR reactor realizes the characteristics of two reactors in the continuous flow.
(3) Better phosphorus removal and nitrogen removal effect. Phosphorus removal and nitrogen removal is a relatively complex process. It is necessary to provide anaerobic, anoxic and aerobic stages in the treatment process to achieve the purpose of nitrification, denitrification and phosphorus absorption and release. In the SBR method, different purposes can be achieved in a single reactor. Because the SBR method can easily achieve the alternation of anaerobic, anoxic and aerobic states through the time arrangement of the five processes, it can maximize the satisfaction of the theoretical environmental conditions for biological denitrification and phosphorus removal.
(4) Good sludge settling performance. Activated sludge bulking is a common problem in the activated sludge treatment process. More than 90% of sludge bulking problems are caused by filamentous sludge bulking. Due to the excessive growth of filamentous bacteria, the growth and reproduction of bacterial flocs are inhibited, and many filamentous bacteria extend beyond the sludge surface, making the flocs loose and the sedimentation properties worse. The SBR method can effectively control the excessive growth of filamentous bacteria, and the sludge SVI is low. It is a process with relatively good sludge settling performance.
Strong adaptability to changes in water quality and water quantity ratio. The treatment effect will be affected by water quality and water quantity, mainly because it will change the treatment environment, and microorganisms are often more stringent in their living environment conditions. Therefore, from a theoretical analysis, the completely mixed reactor has a stronger ability to withstand shock loads than the plug flow reactor. While the SBR process is an ideal plug-flow treatment process, its reactor structure maintains the typical characteristics of a complete mixing reactor. Therefore, it can withstand large fluctuations in water quality and quantity and has a strong ability to withstand shock loads.