What are sequencing batch reactors?
Sequential batch reactors (SBR) or sequencing batch reactors are industrial wastewater treatment tanks. SBR reactors batch-treat wastewater from sewage treatment plants, anaerobic digestion plants, and mechanical biological treatment facilities.
Let’s look at the working and the operating principles of sequencing batch reactors.
Working of SBR
Although, SBRs come in a variety of forms, the fundamental procedure is the same.
The system comprises of a minimum of two similarly furnished tanks, connected by a switchable common inlet. Raw sewage (influent) enters the tanks through one end, and cleaned water (effluent) exits the other end, forming a "flow through" system.
The other tank is aerating and filling while the first is in the settle/decant mode. The bio-selector, a portion of the tank, is located at the inlet. This is made up of a number of walls or baffles that either direct the flow under and over one baffle after another, or from side to side of the tank. This aids in blending the returning activated sludge with the incoming influent, starting the biological digestion process before the liquor enters the tank's main chamber.
What are the Operating principles of sequencing batch reactor?
Basic method ?of treatment
The SBR system is essentially a collection of tanks that run on a fill-and-draw principle. Each tank in the SBR system is topped off for a specific amount of time, before being used as a batch reactor. The cleared supernatant is then removed from the tank, after the combined liquor has undergone the desired treatment and been allowed to settle.
The cycle for each tank in a typical SBR is split into five distinct periods:
Fill, React, Settle, Draw, and Idle. There are several Fill and React times that change depending on the aeration and mixing techniques. Near the end of React, or during Settle, Draw, or Idle, sludge wasting may occur.
The use of a single tank for many stages of wastewater treatment is essential to SBR design.
Stage 1-Fill: Either primary effluent or untreated, unscreened raw wastewater may be used as the tank's influent. It can either be pumped in or left to fall into place naturally. The targeted loading and detention times, as well as the anticipated settling properties of the organisms are some of the criteria, which are used to calculate the feed volume.
The amount of each tank, the number of parallel tanks in use, and how much the wastewater flow rate varies, during the day all affect the fill time. You can use almost any aeration system, including diffused, floating mechanical, and jet.
However, the ideal aeration system should be flexible enough to allow for mixing without aeration, as well as a range of mixing intensities from zero to full agitation. The aerators or mixers can be turned on and off as needed, using level sensing devices, timers, or in-tank probes for the detection of either dissolved oxygen or ammonia nitrogen.
Stage 2-React: Biological processes that were started during Fill are finished during this phase. Similar to Fill, it may be necessary to alternate between low dissolved oxygen concentrations (such as in Mixed React), and high dissolved oxygen concentrations (such as in Aerated React).
Although, the liquid level stays at its maximum throughout the reaction, sludge wasting can happen during this time as a straightforward way, to manage the sludge age. Sludge is removed from the reactor by wasting during react, as a way to maintain or reduce the volume of sludge in the reactor, and decreases the solids volume. Reacting time can take up 50% or more of the overall cycle duration.
Stage 3-Settle: In the SBR, solids separation occurs in a tank that may be more than 10 times larger, than the secondary clarifier used in a typical continuous-flow activated sludge plant, under quiescent conditions (i.e., without inflow or outflow).
The fact that the entire aeration tank functions as the clarifier during the time, when no flow is entering the tank, gives rise to this significant benefit in the clearing process. There is no requirement for the underflow gear often seen in conventional clarifiers, because all of the biomass stays in the tank until some portion needs to be wasted.
In contrast, mixed liquor continuously removes activated sludge from a continuous-flow aeration tank and passes through clarifiers, only to have the majority of the sludge returned to the tank.
Stage 4-Draw (Decant): The withdrawal mechanism may be in the form of a floating or adjustable weir at or just below the liquid's surface, a pipe fixed at a predetermined level with a pump or automatic valve controlling the flow, or any combination of these. In any case, the discharge of floating materials should be avoided, by the design and operation of the withdrawal mechanism.
Draw time can take anywhere between 5 and 30% of the overall cycle time. However, because to the potential issues with growing sludge, the time in draw shouldn't be excessively increased.
Stage 5-Idle: This refers to the time between Draw and Fill. This "idle" time can be utilized to dispose of settled sludge. More regular sludge wasting programmes are advised to preserve process efficiency and sludge settling, even if sludge wasting might occur as infrequently as once every two to three months.
Adjustments in the SBR process: A versatile treatment process
In comparison to traditional activated sludge processes, SBR technology has the benefit of being far more versatile in terms of adjusting reaction times, to the concentration and level of treatment needed for a specific effluent.
The following adjustments can be made in the SBR process in addition to those that can be made in an equivalent conventional process, such as sludge age and operating mixed liquor solids concentration, total cycle duration of each phase within the process cycle pattern of inflow dissolved oxygen profile, during aeration operating top water level operating bottom water level.
For SBRs, numerous different effluent removal techniques have been created;
· Fixed decanters with air-locked multiple pipe layouts and submerged outlet pipes, with automatic syphon control valves.
· Moving equipment such as weir troughs, floating weirs, flexible coupling-connected pipes, tilting weirs, and floating submersible pumps.
· Some decanters experience solids loss because mixed liquid suspended solids are trapped, in the submerged pipework or during the aeration process.
· The effluent must be drained from the tank evenly, which is the main goal of the decanter design.
· Point discharges experience scouring of suspended particles from the settled sludge blanket, and lack process flexibility such as a higher sludge blanket, through a higher tank solids concentration.
· Guard devices are typically used in decanter designs to stop scum and other floating particles, from lowering effluent quality.
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