Trickling Filters

The effluent from primary settling tank which contains unstable organic matter should be converted into stable forms like nitrates & Sulphates by oxidation ( aerobic process)

Filtration: lnfluent should be sprinkled over the open beds of coarse aggregate. Effluent from filters again settled in secondary settling tank. Sludge in secondary clarifiers or sedimentation tank is digested us sludge digestion process, if required.

Types of Filters:              

• Intermittent sand filters
• Contact beds
• Low rate trickling filters 
• l-11gb rate Trickling Filters

Intermittent Sand filters:

• Used in special cases for treating sewage form Tuberculosis hospitals 
• BOD removal is 90 to 95 %.

Contact beds(contact filers):

• Filter media consists of Gravel, broken bricks or stone.
• BOD will be removed to an extent of 6Oto 75%.

Trickling filters : They work on the principle of “Attached growth process”.

• Conventional or low rate trickling filters: The purification of sewage is brought about mainly by aerobic bacteria which forms a bacterial film around the particles of filtering media.Size of filter media 25 min to 75 mm stone aggregate, depth of filter is 2 to 3m Provided with open top and there will be free circulation of air through the medium. BOD removal efficiency is 75 to 80 %.

Suitability: Conventional trickling filter is very useful to medium to and indwtria1 cities requiring full treatment of sewage.

OPERATIONAL TROUBLES:

• Fly nuisance : Psychoda (Fly).
• Odour nuisance : The remedy is chlorination of sewage.
• Ponding troubles : Filters media gets clogged due to growth of algae and fungi and this is called ponding.

Adding chlorine or Copper Sulphate to the sewage kills algae there by ponding can be avoided

Efficiency = BOD removed/BOD Applied

Efficiency = 100/1+0.0044(u)1/2

n= Efficiency of filter in terms of percentage of applied B.O.D. removed.

u = Organic loading in Kg/ha. m/ day applied to the filter. (unit organic loading)

High Rate Trickling Filters:

• Same as low rate Trickling lifters but re- circulation of sewage is essential. In high rate T.F the rate of filtration and efficiency are high compared to the low rate T.F
• Due to re-circulation. it is possible to pass sewage at greater loading thus requiring lesser space and lesser filter media.
• BOD removal efficiency is 80 to 95 %.

Single Stage Re-circulation Process:

• Recirculation helps in seeding the sewage with bacteria and accelerating biological oxidation process.
• Recirculation also helps in reducing odours fly nuisance.

Efficiency of High rate trickling Filters:

• Re- circulation Ratio(R / I): It is the ratio of volume of sewage ( R) to the volume of raw sewage (I).

FILTER LOADING :

• Hydraulic Loading (h): It is the amount of sewage flow that can be applied per unit surface area of the filter per day. 
• Organic Loading (u): It is the amount of B.O.D. in kgs. applied on to the litter per unit volume of the filter