What is Mixing regime and Flow Scheme in ASP?
Activated sludge (AS) is a treatment method for sewage and industrial wastewaters that was invented between 1912 and 1914. Although there are many different designs, all AS have three main components: an aeration tank that serves as a bio reactor; a settling tank (final clarifier) for separating AS solids from treated waste water; and return activated sludge (RAS) equipment to transfer settled AS from the clarifier to the aeration tank's influent.
To generate a biological floc, atmospheric air or, in rare situations, pure oxygen is supplied to a combination of primary processed or filtered sewage (or industrial effluent) coupled with organisms.
The concentration of biodegradable components in the influent is lowered in all activated sludge plants owing to biological (and occasionally chemical) activities in the aeration tank. Different boundary conditions, such as the hydraulic residence time (HRT) in the aeration tank, which is determined by the aeration tank capacity divided by the flow rate, affect the removal efficiency. Other considerations include the influence load (BOD-5, COD, Nitrogen, etc.) in proportion to the AS solids in the aeration tank (Food: Microorganism Ratio or F:M Ratio), oxygen supply, temperature, and so on.
Mixing Regime in Activated Sludge Processes
In the activated sludge process, two types of mixing regimes are of particular interest:
Plug flow and total mixing.
In the first, the flow of mixed liquor through the aeration tank is orderly, with no element of mixed liquor overpowering or combining with any other element. There may be lateral mixing of mixed liquor, but no mixing along the flow channel is permitted.
The contents of the aeration tank are thoroughly mixed and consistent during total mixing. As a result, the effluent from the aeration tank has the same composition as the contents of the aeration tank in steady state.
The kind of mixing regime is critical because it influences:
(1) Oxygen transfer needs in the aeration tank,
(2) Biomass sensitivity to shock loads,
(3) Local environmental conditions in the aeration tank, and
(4) The treatment kinetics.
Flow Scheme in Activated Sludge Processes
The flow s?cheme involves:
1: The pattern of sewage addition;
2: The pattern of sludge return to the aeration tank, and
3: The pattern of aeration.
Sewage can be added at a single location at the input end or at several spots throughout the aeration tank. Sludge can either be returned directly from the settling tank to the aeration tank or via a sludge reaeration tank. Aeration can be done at a constant pace or at different rates depending on where you are in the aeration tank.
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