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 papersDesigning waste out of the system: circular-economy principles, the biological cycle, industrial symbiosis, material flow analysis and the bioeconomy. Composting, biogas and briquetting technology are in the Agricultural Engineering subjects bioenergy-systems and waste-management-environmental-engg; this subject covers the system design, metrics and policy.
In the exams: GATE ES Sections 5 and 7 (recycling, zero liquid discharge, waste hierarchy); ICAR AICE subject 71 Unit 5 (5R concepts); SEA (GBCI) core concepts
GATE — Environmental Science and Engineering (ES)
Waste hierarchy and 3R/resource recovery in Section 7, zero liquid discharge and recycling of water in Section 5, sustainable development and resource efficiency in Section 9. Expect definitions, ordering of the hierarchy, and short numericals: mass balances, recovery fractions across ZLD or recycling chains, stoichiometric by-product yields.
How our questions for GATE spread across the units of Circular Economy & Bio-based Resources. Difficulty: 9 easy, 11 medium, 4 hard.
| Material flow analysis and resource-efficiency metrics | 6 | |
| Bio-based and biodegradable materials: claims and evidence | 6 | |
| Industrial ecology and industrial symbiosis | 5 | |
| Linear versus circular economy; the butterfly diagram | 3 | |
| Cascading use of biomass | 1 | |
| Circular agriculture: nutrient and carbon recycling | 1 | |
| Food waste valorisation and biorefineries | 1 | |
| Extended producer responsibility and eco-design | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
What is the order of priority in the EU waste hierarchy, from most to least preferred?
D. Prevention, preparing for reuse, recycling, other recovery, disposal
Directive 2008/98/EC sets prevention, preparing for reuse, recycling, other recovery (including energy recovery) and disposal. The other orders misplace prevention, reuse or disposal.
Further reading: Directive 2008/98/EC (Waste Framework Directive)
Which town is the classic example of industrial symbiosis, with a power station, refinery, plasterboard factory and pharmaceutical plant exchanging by-products?
C. Kalundborg, Denmark
Kalundborg's symbiosis grew from bilateral exchanges from the 1970s: steam, heat, gypsum, fly ash, water and fermentation biomass. The other cities are major ports and industrial centres but not the textbook case.
Further reading: Chertow 2000
A country has domestic extraction of 500 Mt, imports of 200 Mt and exports of 150 Mt. What is its domestic material consumption (DMC)?
C. 550 Mt
DMC = DE + imports − exports = 500 + 200 − 150 = 550 Mt. 700 Mt is DMI (DE + imports); 850 Mt adds exports; 450 Mt subtracts imports and adds exports.
Further reading: Eurostat EW-MFA handbook
One tonne of dry straw contains 38 % cellulose. Hydrolysis adds water (glucose = 1.111 × cellulose mass) and the theoretical ethanol yield is 0.511 g per g glucose. What is the maximum ethanol from the cellulose?
D. 216 kg
Glucose = 380 × 1.111 = 422 kg; ethanol = 422 × 0.511 = 216 kg. 194 skips the hydration factor; 422 is the glucose; 511 applies the yield to the whole straw mass.
Which polymer is largely fossil-based yet biodegradable?
D. PBAT (polybutylene adipate terephthalate)
PBAT is made from petrochemical monomers but is biodegradable and often blended with PLA or starch. Bio-PE is bio-based but not biodegradable; PET and PVC are fossil-based and not biodegradable.
Further reading: Bioplastics textbooks
Why did the EU ban oxo-degradable plastics under the Single-Use Plastics Directive?
A. They fragment into microplastics without fully biodegrading
Oxo-degradable plastics are conventional polymers with additives that accelerate fragmentation; the fragments persist as microplastics. They are fossil-based, not food-crop-based, and are chemically distinct from PLA.
Further reading: Directive (EU) 2019/904
PLA is made by polymerising lactic acid (C3H6O3, 90.08 g/mol), with each unit losing one water molecule to give a repeat unit of 72.06 g/mol. What is the maximum PLA from 1 kg of lactic acid?
D. 0.80 kg
PLA = 1 × 72.06/90.08 = 0.80 kg; the rest leaves as water. 1.00 kg ignores the water lost; 1.25 inverts the ratio; 0.20 kg is the water removed.
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