# Disposal of Sewage Effluents

 Question 1
A sewage treatment plant receives sewage at a flow rate of 5000 $m^3/day$. The total suspended solids (TSS) concentration in the sewage at the inlet of primary clarifier is 200 mg/L. After the primary treatment, the TSS concentration in sewage is reduced by 60%. The sludge from the primary clarifier contains 2% solids concentration. Subsequently, the sludge is subjected to gravity thickening process to achieve a solids concentration of 6%. Assume that the density of sludge, before and after thickening, is 1000 $kg/m^3$.
The daily volume of the thickened sludge (in $m^3/day$) will be_________. (round off to the nearest integer)
 A 5 B 10 C 20 D 30
GATE CE 2022 SET-2   Environmental Engineering
Question 1 Explanation:
Wt. of TSS at inlet of PST
$=5000 \times 10^3\frac{L}{d} \times 200 \times 10^{-6}Kg/L=1000Kg/d$
Wt. of solid in sludge from PST $=0.6 \times 1000=600 Kg/d$
Wt. of sludge before thikeninig $=\frac{600}{\frac{2}{100}}=30000 Kg/d$
Wt. of sludge before thikeninig $=\frac{600}{\frac{6}{100}}=10000 Kg/d$
Daily volume of thickened sludge $=frac{10000}{1000}=10m^3$
 Question 2
A city generates $40 \times 10^{6}$ kg of municipal solid waste (MSW) per year, out of which only 10% is recovered/recycled and the rest goes to landfill. The landfill has a single lift of 3 m height and is compacted to a density of $550 kg/m^{3}$. If 80% of the landfill is assumed to be MSW, the landfill area (in $m^{2}$, up to one decimal place) required would be ______
 A 25743.3 B 36548.4 C 27272.7 D 26845.6
GATE CE 2018 SET-1   Environmental Engineering
Question 2 Explanation:
Total weight generated by city
$=40\times 10^{6}\: kg/year$
Weight of MSW going into landfill
$=0.9\times 40\times 10^{6}\: kg/year$
$=36\times 10^{6}\: kg/year$
Compacted density
$=550\: kg/m^{3}$
Compacted volume of MSW
$=\frac{36\times 10^{6}\: kg/year}{550\: kg/m^{3}}$
$=65454.5454\: m^{3}/year$
Total landfill volume = Vol. of MSD + Vol. of cover
Given, Volume of MSW $=0.8\times$ Total landfill volume
$\therefore$ Vol. of cover $=0.2\times$ Total landfill volume
$\therefore$Total landfill volume $=\frac{65454.5454}{0.8}m^{3}/year =81818.18175m^{3}/year$
Height of landfill $=3\: m$
$\therefore$ Area of lanfill$=\frac{81818.18175}{3} =27272.7\: m^{2}/year$
 Question 3
The wastewater form a city, containing a high concentration of biodegradable organics, is beingsteadily discharged into a flowing river at a location S. If the rate of aeration of the river water islower than the rate of degradation of the organics, then the dissolved oxygen of the river water
 A is lowest at the locations S B is lowest at a point upstream of the location S C remains constant all along the length of the river D is lowest at a point downstream of the location S
GATE CE 2017 SET-1   Environmental Engineering
Question 3 Explanation: As given rate of aeration is less than rate of degradation which decreases with time/distance, minimum DO is observed downstream of point of disposal 'S' where both rate of a reaction and degradation becomes equals.
 Question 4
In a certain situation, waste water discharged into a river mixer with the river water instantaneously and completely. Following is the data available :

Waste water:
DO = 2.00 mg/L
Discharge rate = 1.10 $m^{3}$/s

River water:
DO = 8.3 mg/L
Flow rate = 8.70 $m^{3}$/s
Temperature = 20$^{\circ}$C

Initial amount of DO in the mixture of waste and river shall be
 A 5.3 mg/L B 6.5 mg/L C 7.6 mg/L D 8.4 mg/L
GATE CE 2005   Environmental Engineering
Question 4 Explanation:
\begin{aligned} DO_{mix} &=\frac{Q_{W}DO_{W}+Q_{R}DO_{R}}{Q_{W}+Q_{R}} \\ &=\frac{\left ( 1.10\times 2.00 \right )+\left ( 8.70\times 8.3 \right )}{1.10+8.70} \\ &=7.6\: mg/L \end{aligned}
 Question 5
Match Group-I (Characteristics of sewage discharged into inland waters ) with Group-II (Allowable limit, mg/L). A P-2 Q-5 R-4 S-2 B P-4 Q-1 R-6 S-4 C P-3 Q-1 R-4 S-2 D P-2 Q-1 R-6 S-3
GATE CE 2003   Environmental Engineering
Question 5 Explanation: There are 5 questions to complete.