Trickling filter

TRICKLING FILTER

• Trickling filter mainly consists of a rotating arm that sprays wastewater on filter medium.
• The filter medium is made-up of rocks, plastic or other medium
• An under-drainage mechanism is set-up at the bottom to collect the liquid for further treatment
• The filter media should have
• high specific surface area
• high percent of void space
• resistance to abrasion during placement
• insoluble in sewage
• resistant to flaking
• round or cubical in shape with a size around 25 to 75 mm

•  Crushed rock is durable and insoluble, locally available, has less void space for passage of air and more surface area per volume for biological growth serves as a suitable filter media for trickling filters
• Sometimes plastic media from random packing media or modular packing media may be used as a filter media
• Concentration of organics in a sewage decreases as waste passes media depth
• Effectiveness of filter decreases as depth increases. Hence, depth of the filter should be limited
• Efficiency decreases as organic loading increases
• Trickling filters are also called percolating filters or sprinkling filters
• Trickling filters are used to give secondary treatment to sewage
• They consist of coarse filtering media over which sewage is sprayed using spray nozzles or rotary distributors
• Percolating sewage is collected at the bottom using an under-drainage system

• Microorganisms present in the sewage get attached to the filter media
• Organic matter in sewage influent is adsorbed on the biological film formed by microorganisms around the filter media
• On the outer surface of the film, a biological mass or slime layer is degraded by aerobic bacteria
• Thickness of the slime layer increases due to growth of microorganisms
• Oxygen is consumed by aerobic microorganisms in the slime layer thereby creating an anaerobic environment near the surface of media particles
• As the slime layer increases in thickness the microorganisms near the media face enter an endogenous growth phase and lose their ability to stick to media surface
• The liquid sewage exerts a shearing action and breaks the slime from the media and a new slime layer begins to grow in its place
• The continuous growth and break-up of slime creates a balance in the thickness of biofilm formed
• The break-up of biomass from slime layer is called sloughing
• This causes turbidity in filter effluent and can be separated in secondary settling tan
• Extent of sloughing is a function of organic and hydraulic loading in filter
• Hydraulic loading is responsible shear velocity and organic loading accounts for rate of metabolism in slime layer

 

The advantages of trickling filters are listed below

• Effluent obtained is highly nitrified and stabilised
• Desired quality of effluent is produced under varying weather conditions
• Removal of 80% solids and 75% to 80% BOD can be achieved
• It is simple in operation
• It is cheap to operate and requires little or no supervision
• It is self-cleaning
• Less mechanical equipment implies less wear and tear
• Trickling filters require less electrical power

The disadvantages of trickling filters are listed below

• High head loss implying increased pumping costs
• High cost of construction
• Large requirement of land
• Primary sedimentation tank is essential before the effluent can be fed to the trickling filters
• Odour and fly nuisance