When a design engineer is developing a new clarifier or reviewing existing units, side water depth (SWD) is crucial. The side water depth/sidewall depth is the depth of the water at the basin's wall.
How does a clarifier or settling tank influence side water depth?
The application of the clarifier/settling tank is heavily influenced by the side water depth. The side water depth of a grit collector that operates according to Stokes Law, might be quite shallow.
It is established by holding solids inventory in waste treatment primary units. In chemical clarifiers, the side water depth may be determined by the solids inventory and detention time, for flocculation and separation. The same criteria are utilized for water treatment units, as for chemical clarifiers/settling tanks. All of these characteristics are also location specific.
The material that follows is based on waste activated sludge final clarifiers. With regard to SWD, the design looks to be more of an art than just following a set of scientific guidelines.
Final Clarifiers for Waste Activated Sludge/Settling Basins
The surface overflow rate and mass loading are commonly used to construct and evaluate clarifiers. Previously, detention time was utilized to determine the depth of the clarifier.
The side water depth is chosen independently, of the detention time now-a-days. However, there has been little or no empirical evidence gathered on the effects of side water depth, on the performance of final clarifiers. This is most likely owing to the speculative character of the objects to be tested.
Depth for Efficient Operation
A clarifier should be deep enough to accommodate the conveyance and solids inventory, required for optimal operation. If the solids-holding depth is insufficient, gross carry-over of solids into the effluent might occur, lowering effluent quality.
Thus, the following question arises:
Will the clarifier require more depth to handle more mixed liquor suspended solids (MLSS)?
With this in mind, it appears that the side water depth should be chosen based on both the required sludge detention duration, and the necessity to handle the aforementioned additional mixed liquor suspended solids.
Examining the multiple settling zones (with varying settling characteristics) formed by waste-activated mixed liquor, demonstrates the importance of the correct clarifier design depth.
What are the several zones in a clarifier?
The several zones (beginning at the top of the basin) are as follows:
• Zone of clarified water
• Particle zone
• Restricted zone
• Zone of transition
• Zone of compression
• A different concentration zone
Which zones are important for final clarifiers?
For final clarifiers, the top and bottom zones are the most crucial. The removal device and mass loading have an impact on the bottom concentration zone. The type of input and overflow rate have an impact on the top clarified zone.
Because, the major goal of a final clarifier is to provide ultimate clarification of the treated sewage, these zones should be given special attention.
How can we assist?
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