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
· 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.
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,
Therefore, BOD5 removal efficiency: or, 96.64%