1.      Describe the differences in the nature of recycling between activated sludge process and trickling filter. (3)




Activated Sludge Process


        The sludge or biomass is recycled in the activated sludge process

        Purpose of recycling is to maintain a high biomass concentration in the aeration tank

Trickling Filter


        Treated wastewater is recycled in the trickling filter

        Purpose of recycling is to maintain adequate hydraulic loading rate, without changing organic loading rate, so that all portions of the filter may be wetted adequately all the time.





2.      A tricking filter with the following dimensions is available. Depth: 2 m, Surface area: 150 m2. The media consists of stones of 7-10 cm diameter. This filter will be used to treat 0.6 MLD wastewater with BOD5 = 300 mg/L. The trickling filter will be operated in the high-rate mode, i.e., OLR: 0.48 0.96 Kg/m3/d, HLR: 10 40 m3/m2/d, re-circulation ratio: 1-2. Based on this information, calculate the expected BOD5 removal efficiency.


Text Box: High-Rate Trickling Filter 

















, ,

Where, So = BOD5 in Raw Wastewater, mg/L

Se = Total BOD5 of settled effluent from the filter, mg/L

Sa = Total BOD5 of wastewater applied to the filter, mg/L

k = Treatability constant, 2.36

D = Depth of the Trickling Filter, m

Q = Total Flow rate applied to the filter without recirculation, m3/d

A = Surface Area of the Trickling Filter, m2

n = 0.5

V = Volume of the Trickling Filter, m3 (7)




Volume of trickling filter = D.A = (2).150 = 300 m3

Organic Loading Rate (OLR) = (within limits)

Without recycle, Hydraulic Loading Rate (HLR) =

(The above value is inadequate and hence must be increased). So, let R = 2

Hence, HLR = . This value is adequate.

Now, to calculate BOD5 removal efficiency.

It is known that, , Where,


or, or,

or, ;


Therefore, BOD5 removal efficiency: or, 96.64%