12 Questions
What is the main action of a trickling filter?
Metabolization
What is the purpose of passing settled effluent through a trickling filter?
To reduce BOD
What forms on the surface of the support material in a filter bed?
Slime layer of biologically active material
What is the most common packing material used in a trickling filter?
Granite
Which organisms primarily remove the majority of the organic loading in the biological film?
Bacteria
Why is it important for a trickling filter's packing material to have a voidage of 45-55%?
To reduce the likelihood of blockages
What are nitrogenous matter and ammonia oxidized to in the filter bed?
Nitrates
What role does the secondary sedimentation tank play after a trickling filter?
Remove suspended matter
How does a rotary system in some trickling filters improve performance?
Allows more uniform hydraulic loading
What role do higher organisms like protozoa play in the filter bed?
Prevents the filter from blocking
What can poison the active slime in a filter bed?
Toxic chemicals
What is a primary drawback of operating a simple filter at abnormally high organic loading rates?
Inefficiency
Study Notes
Trickling Filter
- A trickling filter is not a true filtration process, but rather a fixed film bio-reactor.
- Settled effluent to be treated is passed down through a packed bed counter-current to a flow of air.
- Micro-organisms adhering to the packing matrix adsorb oxygen from upflowing air and organic matter from the downflowing effluent.
- The organic matter is then metabolized, and the stream's BOD (Biochemical Oxygen Demand) is reduced.
Design and Operation of a Trickling Filter
- A conventional trickling filter typically consists of a cylindrical concrete tank, 2 to 3 m in depth and 8 to 16 m in diameter.
- The tank is packed with a bed of stone (usually granite) or special plastic packings, underlaid with drains.
- The packing material should have a diameter of 50 to 100 mm to provide a specific surface area of around 100 m2/m3.
- The packing material should be packed to give a voidage of 45 to 55% to minimize the risk of blockages.
- Synthetic packing material has a higher surface area and voidage, allowing higher treatment rates per unit volume of bed and reducing the likelihood of blockages.
Importance of Secondary Sedimentation
- A trickling filter is always followed by a secondary sedimentation tank or humus tank to remove suspended matter (e.g. biofilm sloughed off the packing) from the treated effluent.
Effluent Treatment and Biological Film
- In conventionally loaded or low-rate filters, the effluent from which the suspended solids have been removed is fed onto the upper surface of the bed by spray nozzles or mechanical distributor arms.
- A slime layer of biologically active material (bacteria, fungi, algae, protozoa, and nematodes) forms on the surface of the support material.
- The bacteria in the biological film remove the majority of the organic loading, breaking down complex organic materials, and oxidizing nitrogenous matter, ammonia, and other compounds.
- Higher organisms (protozoa, etc.) control the accumulation of the biological film and improve the settling characteristics of the solids (humus) discharged with the filter effluent.
Learn about the concept of a trickling filter in the context of unit operations, where settled effluent is treated by passing down through a packed bed counter-current to a flow of air. Discover how micro-organisms adhere to the packing matrix, absorb oxygen, and metabolize organic matter to reduce BOD in the effluent.
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