In order to dilute the wastewater and produce an overall cleaner effluent, a recirculating tank receives the primary treated effluent from a pretreatment component, and combines it with highly treated wastewater from an advanced pretreatment component.
Why do we re circulate sludge into Aeration tank or MBBR tank?
Nitrates, BOD-5, TSS, and other pollutants are eliminated from wastewater, or their concentrations are reduced via recirculation of sludge in either aeration tanks or MBBR tanks.
What is recirculation ratio?
The advanced pretreatment component's wastewater flow rate, divided by the final treatment and dispersal component's wastewater flow rate, is known as the recirculation ratio. Recirculation ratios can be altered based on the desired level of treatment, typically falling between 3:1 and 5:1, and also based on the desired intensity of treatment, and can be altered.
Process of sludge recirculation
Wastewater travels from a tank to the recirculation tank in a typical recirculation treatment train. It is then dosed to an advanced pretreatment component, such as a media filter, where it undergoes secondary treatment, is collected, and then either transported to the final treatment, and dispersal component or returned to the recirculation tank.
Return of activated sludge
An integral part of the treatment procedure is the return of activated sludge, from the final clarifier to the aeration tank's inlet. It ensures that the needed level of treatment may be completed in the allotted amount of time, by maintaining the proper concentration of activated sludge in the aeration tank.
The sludge recirculation rate, sludge settleability, and sludge thickening in the secondary sedimentation tank, regulate the MLSS concentration in the aeration tank.
Qr = X
Where, Qr is the m3/d sludge recirculation rate.
Sludge Recirculation Ratio
It is the proportion of sewage flow to return sludge.
=Qr/Q = Vs/ (1000-Vs)
= volume of settled sludge in Imhoff cone. Its value can vary from 0.25 - 0.5 ml/l.
Sludge volume index (SVI)
It is defined as the volume occupied in mL by one gramme of solids in the mixed liquid after settling for 30 minutes. It is used to gauge how easily sludge settles. SVI values between 100 and 150 ml/g suggest good suspended solids settling.
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