The GATE Grind

GATE Water and Wastewater Treatment and Management: Kinetics and Reactor Design – Previous Year Questions

12 GATE previous year questions on Kinetics and Reactor Design (Water and Wastewater Treatment and Management, Environmental Science and Engineering) with answers and explanations, from every paper.

  1. GATE 2021 ES Q40 (2 marks, Multiple choice) – Ideal PFR and CFSTR treat a pollutant by first-order kinetics at the same inlet concentration, flow and target outlet concentration. Which comparison…
  2. GATE 2021 ES Q65 (2 marks, Numerical answer) – Two completely mixed facultative sewage ponds are in series, each HRT6 days. Overall BOD₅ reduction90%. Both have the same first-order rate constant.…
  3. GATE 2022 ES Q39 (2 marks, Multiple choice) – A continuous culture is at steady state with HRT24 h and growth rate μ₁. HRT is reduced to 12 h, and a new non-washout steady state has rate μ₂. Which…
  4. GATE 2022 ES Q40 (2 marks, Multiple choice) – Which statement is NOT correct for a batch reactor?
  5. GATE 2022 ES Q46 (2 marks, Multiple select) – Which CSTR/PFR comparison statements are NOT correct?
  6. GATE 2023 ES Q65 (2 marks, Numerical answer) – An aeration tank treats180000 m³/day water, lowering VOC from 33 to 12 µg/L. Equilibrium saturation concentration5 µg/L and gas-transfer constant.40/s…
  7. GATE 2024 ES Q39 (2 marks, Multiple choice) – For steady second-order decay r C=-kC2,find CSTR/PFR volume ratio for 90% concentration removal, with equal inlet concentration and flow.
  8. GATE 2025 ES Q18 (1 mark, Multiple choice) – Match reactor types 1 Plug flow,2 Continuously stirred tank,3 Batch with diagrams x, y, z. The stirrer symbol denotes mixing; diagrams are not to…
  9. GATE 2025 ES Q50 (2 marks, Numerical answer) – A stirred tank initially has 500 L brine at 200 g/L. Feed at 100 g/L enters at 15 L/min and an equal fully mixed flow leaves. The equation is…
  10. GATE 2025 ES Q57 (2 marks, Numerical answer) – A batch test reduces a first-order toxic compound from 210-3 M to 210-4 M in 10 min. At the same temperature, a field plug-flow reactor treats1…
  11. GATE 2026 ES Q50 (2 marks, Numerical answer) – The elementary reaction A B follows first-order kinetics with k=.01\,min-1. Find the batch time to reduce A from 100 M to 10 M, to one decimal place…
  12. GATE 2026 ES Q59 (2 marks, Numerical answer) – A steady CSTR receives 300 L/hour with substrate concentration 150 mg/L. Required treatment efficiency is 76%. Substrate decay is half-order with…