GATE — Environmental Science and Engineering (ES) · India · IITs and IISc on rotation (2026: IIT Guwahati)
Original practice questions written to the published syllabus · not past papersMaking water safe to drink and wastewater safe to release or reuse: unit operations, biological treatment, reactor kinetics, sludge, industrial effluents and zero liquid discharge. Farm-scale lagoons and manure systems are in the Agricultural Engineering subject waste-management-environmental-engg; this subject covers municipal and agro-industrial treatment to GATE ES depth.
In the exams: GATE ES Section 5 (whole section, numerical); ICAR AICE subject 71 Unit 4; UGC NET Unit 6; NCEES FE/PE Environmental
GATE -- Environmental Science and Engineering (Section 5)
A whole numerical section: BOD kinetics and temperature correction, hardness, population forecasting and design flows, settling (overflow rate, removal fraction), G and Gt, filter area, chlorine dose and CT, F/M, SRT, SVI, Monod effluent, NRC trickling filter, CSTR/PFR/series reactors, sludge volume, methane yield, CETP and ZLD concepts.
How our questions for GATE spread across the units of Water & Wastewater Treatment. Difficulty: 21 easy, 16 medium, 12 hard.
| Screening, sedimentation, coagulation and flocculation | 5 | |
| Activated sludge, trickling filters and their design parameters | 5 | |
| Water and wastewater quality parameters: BOD, COD, solids, nutrients | 4 | |
| Population forecasting and design flows | 4 | |
| Filtration, desalination and disinfection | 4 | |
| Anaerobic treatment and UASB reactors | 4 | |
| Reactor kinetics: batch, CSTR and plug flow | 4 | |
| Sludge treatment and disposal | 4 | |
| BOD kinetics | 3 | |
| Water distribution and storage | 3 | |
| Sewers, storm sewers and appurtenances | 3 | |
| Primary, secondary and tertiary treatment | 3 | |
| Agro-industrial effluents: dairy, sugar, distillery, food processing | 2 | |
| Common effluent treatment plants, recycling and zero liquid discharge | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
Water contains 40 mg/L calcium and 12 mg/L magnesium. What is its total hardness as CaCO3?
B. 149 mg/L
Hardness = 40 x 50/20.04 + 12 x 50/12.15 = 99.8 + 49.4 = 149 mg/L as CaCO3. Adding masses ignores equivalents; the third ignores Mg.
The BOD rate constant is 0.23 per day at 20 degrees C. Using theta = 1.047, what is it at 30 degrees C?
C. 0.364 per day
k30 = 0.23 x 1.047^10 = 0.364. Dividing gives the 10 degrees C value.
A wastewater has BOD5 = 200 mg/L with k = 0.23 per day (base e). What is its ultimate BOD?
C. 293 mg/L
L0 = BOD5/(1 - e^-1.15) = 200/0.683 = 293. Multiplying by 0.683 gives BOD5 from L0, the reverse calculation.
A town of 100,000 grows geometrically at 2% a year. What population is expected after 30 years?
D. 181,136
Pn = 100,000 x 1.02^30 = 181,136. 160,000 is arithmetic growth.
In an ideal settling tank with overflow rate 30 m3/m2/d, what fraction of particles with settling velocity 0.2 mm/s is removed?
B. 58%
vs = 0.2 mm/s = 17.28 m/d < vo, so removal = vs/vo = 0.58. Forgetting to convert units gives nonsense.
A plant treats 10,000 m3/d at a chlorine dose of 2.0 mg/L. How much chlorine does it use per day?
C. 20 kg
10,000 m3 x 2.0 g/m3 = 20,000.0 g = 20 kg. Forgetting to convert grams to kilograms overstates the amount a thousandfold.
The minimum velocity that prevents deposition in sanitary sewers (CPHEEO) is about:
C. 0.6 m/s
Storm sewers need higher velocities because of grit.
The CSTR volume is split into two equal tanks in series (total theta = 4 days, k = 0.5/day). What fraction remains?
A. 0.250
C/C0 = 1/(1 + k theta/n)^n = 1/(1 + 1)^2 = 0.250. Two tanks in series perform between a single CSTR and a PFR.
A sludge cake must be reduced from 80% to 60% moisture. What fraction of its mass remains?
A. 0.5
Solids mass constant: M2/M1 = (100 - 80)/(100 - 60) = 20/40 = 0.5.
In a ZLD train, which unit recovers most of the water as permeate?
B. Reverse osmosis
RO permeate is reused; reject goes to evaporators.
Ten here, 49 in the app. The Laisha app has every practice question we have written to GATE in this subject, marked as you go, with timed papers, a jump-to-unanswered list and a certificate when you pass at 70 per cent.
Free for ten days, then one subscription of US$10 a month. We would rather say so here than have you find it later.
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