WASTEWATER TREATMENT-V

SOLUTION

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

Solution:

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.

Hint:

, ,

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

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

S_a = 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)

Solution:

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,

Also,

or, or,

or, ;

Therefore, BOD₅ removal efficiency: or, 96.64%