ENVE 101 — Environmental Engineering

Complete Exam Study Package · Kathmandu University

Source: 7 End-Semester Papers (2018 – 2025) Prepared for: SKYGOD Format: Analysis + 70-Question MCQ Bank

Contents

PHASE 1 — Topic Weightage, Repeated Questions, High-Yield Notes, Pitfalls
PHASE 2 — Master MCQ Practice Bank (70 Qs) with Worked Answer Keys

PHASE 1 · Comprehensive Analysis & Study Guide

1.1 Topic Frequency & Weightage Table

Based on cross-analysis of Aug 2018, Aug 2019, May/June 2022, Apr/May 2023, June/July 2024, Sept 2024, and March 2025 papers.

Topic	Papers Featured In	Approx. Weightage	Priority	Typical Question Style
Mass Balance Equations (steady/unsteady, conservative/non-conservative, CSTR)	7/7	15–18%	TIER 1	Derive general MBE; CSTR gas-in-room / formaldehyde-in-bar numericals
Monod Kinetics (μ, S, X, continuous culture)	7/7	10–12%	TIER 1	Find S & X from μm, Ks, Y, kd, HRT; conceptual MCQs on S=Ks, S>>Ks
Hydrologic Cycle Mass Balance (P, R, ET, ΔS, infiltration)	7/7	10%	TIER 1	Solve for missing term given basin area & flows
Settling Tanks / Overflow Rate / HRT	7/7	9%	TIER 1	Dimension a rectangular tank from Q, HRT, overflow rate, L:W ratio
LCA (Functional Unit, MECO matrix, goal definition, stages, cradle-to-grave)	7/7	10%	TIER 1	Define FU; differentiate EIA vs LCA; fill MECO matrix; discuss stages
Solid Waste (moisture content, density, composition, collection)	5/7	8%	TIER 2	Weighted-average moisture/dry-mass/density table calc
EIA/IEE & Nepal Environmental Study Levels (EPA/EPR 2076)	6/7	7%	TIER 2	List 3 study levels; define EIA purpose; project examples
Sound Power Level Combination (dB)	5/7	5%	TIER 2	Combine 2–3 dB levels using log-sum formula
Stream/Lake/River Mixing (conservative pollutants, BOD)	7/7	7%	TIER 1	Mass balance at confluence; find outflow concentration
Green Engineering / Pollution Prevention Hierarchy	6/7	5%	TIER 2	State & explain 2 principles with examples
Sewer Systems (types, schematic, combined discharge design)	6/7	6%	TIER 2	Draw schematic; rational-method style combined discharge calc
Electrostatic Precipitator / Cyclone Separator	4/7	4%	TIER 3	Collector plate area vs efficiency; differentiate cyclone vs ESP
Population Projection (arithmetic & geometric)	3/7	4%	TIER 3	Project population/demand to target year
Unit Conversion / Concentration (ppm, ppb, mg/L, mass loading rate)	7/7	5%	TIER 2	Convert mg/kg↔ppb; Q×C mass flow rate
Air Quality (AQI/AQS, criteria pollutants, PM10, box model)	5/7	5%	TIER 3	Define AQI; box-model SO₂ concentration; list criteria pollutants
Nitrogen/Phosphorus Biogeochemical Cycles	5/7	3%	TIER 3	Apply cycle knowledge to pollution remediation
Engineering Project Definition / IRR / Decisions	4/7	3%	TIER 3	Define engineering project; major aspects of decisions

Reading the table: Tier 1 topics have appeared in every single paper analysed and typically carry 8–20 marks combined — master these first. Tier 3 topics are still examinable but appear less consistently or carry fewer marks (usually 1-mark MCQs or short 2–3 mark notes).

1.2 Most Repeated Questions & High-Probability Predictions

Recurring Question Pattern	Seen In	Predicted Variant for Next Exam
Formaldehyde / CO / CO₂ steady-state CSTR concentration in a room or bar (given emission rate, k, air exchange rate)	2018, 2019, 2023, 2024(Sept)	Same setup, different pollutant (e.g. VOC in a kitchen) — practice the CSTR mass-balance derivation cold.
Combining 2–3 sound power levels (dB) using logarithmic addition	2022, 2023, 2024(Sept), 2025	Near-certain 1–2 mark MCQ or short question — memorise the formula.
Rectangular settling tank dimensioning (Q, HRT, overflow rate, L:W=5)	2018, 2019, 2022, 2023	Very likely 3–5 mark numerical — practice unit conversions (MLD, lpcd → m³/day).
Continuous-culture Monod system: find S, X, then reactor volume	2018, 2019, 2022, 2025(OR)	High chance of a 6–8 mark combined question.
Hydrological mass balance: solve for evaporation/ET/runoff given P, R_in, R_out, ΔS	All 7 papers (some form)	Guaranteed — practice unit conversion between in/hr, m³/s, and m/month or m/year.
BOD/pollutant mass balance at a lake or stream confluence (two inflows → one outflow)	2018, 2019, 2023, 2025	Guaranteed — same numbers reused in 2018 & 2025 (50 mg/L answer)!
Solid waste moisture content / dry mass / overall density from a composition table	2022, 2024(June), 2024(Sept)	High chance — practice weighted averages.
MECO matrix / LCA of competing products (roof materials, polyethylene bag, etc.)	2018, 2019, 2022	Possible short-note or applied question — know the 4 categories: Materials, Energy, Chemicals, Others across 4 life-cycle phases.
Functional Unit — definition + applied example	2022, 2023, 2025 (×2)	Near-certain 1–2 mark definition/MCQ.
Differentiate: EIA vs LCA / Cyclone vs ESP / Mass-Extensive vs Mass-Intensive / Water vs Wastewater	Every paper (rotating pairs)	Always appears as "differentiate ANY TWO/THREE" — prepare 6 clean pairs.
Short notes bank: Nepal Engineering Council, Hydraulic Retention Time, Engineering Decisions, Nitrogen Cycle, Transfer Station, MECO Matrix, Green Engineering	All papers	"Write short notes on ANY TWO/THREE" always appears — prepare all 7 in ~4 sentences each.
Environmental study levels under Nepal's EPA/EPR (IEE, EIA, +1 more)	2023, 2024, 2025	Guaranteed factual recall question.
General mass balance equation derivation for 4 special cases	2024(June/July), Sept 2024	Likely to reappear as a 4–5 mark derivation question.

1.3 In-Depth High-Yield Notes (Formulas & Core Concepts)

A. General Mass Balance Equation (MBE)

Accumulation = Input − Output + Generation − Decay(reaction)
Steady state: Accumulation = 0 → Input = Output (+ reaction terms)
Conservative substance: no reaction term (e.g. chloride, TDS)
Non-conservative substance: undergoes reaction/decay (e.g. BOD, chemicals) — use first-order decay −kCV
Steady, non-conservative, no generation: Input = Output + kCV
CSTR steady state general form: QC_in + Generation·V = QC_out + kC_outV
→ C_out = (Q·C_in + S·V) / (Q + kV) where S = source emission rate/volume

B. Monod Equation Cheat-Sheet

μ = μ_m·S /(K_s+S) | X = Y(S₀−S)/(1+k_dθ) | S = K_s(1+k_dθ) / [θ(Yk−k_d) −1], where k=μm/Y
θ = Hydraulic Retention Time (HRT) = V/Q
Key conceptual results: S = K_s → μ = ½μ_m. S ≫ K_s → μ ≈ μ_m (zero-order, max growth). S ≪ K_s → μ ∝ S (first-order).
μ − k_d = 1/T describes a Continuous Culture (chemostat) at steady state.

C. Settling / Sedimentation

Overflow rate: v₀ = Q/A = Q/(L·W) | HRT: θ = V/Q = (L·W·D)/Q
Stokes' Law: v = (1/18)·(ρs−ρl)·g·d²/μ | Reynolds No: Re = v·d·ρl/μ (laminar if Re<1)
Design sequence: (1) A = Q/v₀ → (2) V = θ·Q → (3) Depth = V/A → (4) L = √(5A) if L:W=5, W = L/5.

D. Hydrologic Cycle Mass Balance

ΔS = P + R_in − R_out − ET (± infiltration/seepage as applicable)
Always convert to consistent units: 1 ha = 100 m × 100 m; 1 MLD = 10⁶ L/day; 1 m³ = 1000 L.
Convert flow (m³/s) → volume/month: multiply by seconds in the period, divide by basin area (m²) → gives depth (m).

E. Solid Waste Composition Calculations

Moisture content (kg) of a component = (mass fraction × total waste) × (%moisture)
Dry mass = wet mass − moisture mass (or wet mass × dry-mass%)
Overall density = Total mass / Total volume, where Volume_i = mass_i/density_i (sum volumes, don't average densities directly!)

F. Sound Power Level Combination

L_total = 10·log₁₀( Σ 10^(L_i/10) )
Quick rule: two equal levels combine to +3 dB; e.g. 450+450 dB → 453 dB (math trick often tested).

G. LCA / MECO / Functional Unit

Functional Unit (FU): a quantified description of the performance/function a product system delivers, used as the basis of comparison (e.g. "10 m² roof for 10 years").
MECO Matrix: rows = Materials, Energy, Chemicals, Others; columns = Material extraction, Production, Use, Disposal phases.
LCA stages: (1) Goal & Scope Definition → (2) Inventory Analysis (LCI) → (3) Impact Assessment (LCIA) → (4) Interpretation.
Cradle-to-grave = full life cycle from raw material extraction through disposal (vs cradle-to-gate = extraction to factory exit).
EIA vs LCA: EIA assesses impacts of a project/site before approval (regulatory, site-specific); LCA assesses environmental impacts of a product/process across its whole life cycle (comparative, not site-specific).

H. Nepal Environmental Study Levels (EPA/EPR 2076)

IEE (Initial Environmental Examination): for medium-scale projects with moderate potential impact.
EIA (Environmental Impact Assessment): for large-scale projects with significant potential impact (e.g. hydropower >50 MW, roads >certain length).
(Some papers list a 3rd category / "Brief/Advanced Environmental Study" as distractors — the two legally recognized levels in Nepal are IEE and EIA.)

I. Green Engineering & Pollution Prevention Hierarchy

Pollution Prevention Hierarchy (most → least preferred): Source Reduction → Recycling/Reuse → Treatment → Disposal.

Sample Green Engineering Principles: (1) Prevent waste rather than treat/clean up after it is formed; (2) Design for separation/purification steps to minimize energy & materials; (3) Maximize efficiency of mass, energy, space, and time; (4) Products/processes should be "output-pulled" not "input-pushed" via energy/materials.

J. System Boundary Solutions (Incrementalism / Reengineering / Redefinition)

Incrementalism: small, low-degree-of-freedom fixes within the existing system (lowest degree of freedom).
Reengineering the system: redesign of internal processes within the same boundary.
Redefining the system boundary: expanding scope to include upstream/downstream elements (highest degree of freedom).

1.4 Common Pitfalls Table

Mistake	Why Marks Are Lost	Exact Fix
Forgetting to convert units before plugging into mass balance (lpcd, MLD, ha, in/yr)	Numeric answer off by 10³–10⁶; examiner cannot award method marks if final units are wrong	Convert everything to SI (m³/s or m³/day) in a labeled first line before any calculation
Mixing up Overflow Rate (Q/A) with Detention Time (V/Q)	Wrong tank dimensions; loses 2–3 marks	Always solve for Area first from v₀ = Q/A, then Volume from θ = V/Q, then depth = V/A — never combine formulas in one step
Not drawing the mass-balance / schematic diagram when asked	Diagram is often allotted separate marks (1–2) even if the numeric answer is correct	Always sketch box + arrows labeled with Q and C for every mass-balance question, even "trivial" ones
Applying Monod's S formula without checking whether k = μm/Y needs to be computed first	Circular/undefined calculation; wastes time	Compute k = μm/Y as step 1 whenever S is asked from the rearranged Monod equation
Confusing conservative vs non-conservative substances	Wrong MBE form chosen (adding/omitting decay term)	Ask: "does this substance react/decay?" BOD, chemicals → non-conservative; chloride, TDS, salts → conservative
Sound power level: averaging dB values instead of logarithmic summation	Completely wrong answer despite "reasonable-looking" number	Always use L = 10·log₁₀(Σ10^(Li/10)); never average dB directly
Solid waste density: averaging densities of components	Wrong overall density (must be mass/volume, not average of densities)	Compute total mass ÷ total volume, where each component's volume = its mass ÷ its own density
Skipping definitions/short notes to "save time" for numericals	These are often the easiest, guaranteed marks (1–3 marks each) missed entirely	Answer definition/short-note questions first — high mark-per-minute ratio
Not stating assumptions for missing data	Many papers explicitly say "make logical assumptions" — examiners expect this to be stated	Write "Assuming ___" explicitly before using any unstated value

PHASE 2 · Master MCQ Practice Bank

70 MCQs compiled and de-duplicated from all 7 papers' Section "A". Correct answer highlighted in green with a worked explanation beneath each card.

Settling / Stokes

Q1. A fine particle settles at terminal velocity 0.1 m/s in a water tank of depth 1 m. What is the settling time?

1 s
0.1 s
10 s
100 s

Answer: C. t = depth/v = 1 m ÷ 0.1 m/s = 10 s.

Unit Conversion

Q2. 1 kg soil contains 5 mg TCE. Concentration in ppb is?

5×10⁶
5×10³
5×10⁻³
5×10⁰

Answer: B. 5 mg/kg = 5 ppm = 5000 ppb = 5×10³ ppb.

Hydrology

Q3. The part of precipitation flowing on/just beneath the surface is called:

Spillway
Infiltration
Transpiration
Runoff

Answer: D. Runoff = surface + shallow subsurface flow to a stream.

Wastewater Flow

Q4. City population 5000, per-capita generation 100 L/day. Wastewater generation rate?

500 m³/hr
0.5 m³/hr
20833.3 m³/hr
20.83 m³/hr

Answer: D. 5000×100 = 500,000 L/day = 500 m³/day ÷ 24 = 20.83 m³/hr.

BOD Mass Balance

Q5. Two streams (10 m³/s, BOD 20 mg/L) and (10 m³/s, BOD 80 mg/L) mix in a lake. Outflow BOD?

50 mg/L
5 mg/L
60 mg/L
100 mg/L

Answer: A. (10×20 + 10×80)/(10+10) = 1000/20 = 50 mg/L. (This exact question repeats in 2018 & 2025 papers.)

Monod Kinetics

Q6. μ − k_d = 1/T represents which system?

Batch Culture
Continuous Culture
Monod Culture
Complex Culture

Answer: B. This is the steady-state chemostat (continuous culture) growth-rate relation.

Monod Kinetics

Q7. In the Monod equation, S = K_s yields:

μ = μm
μ = ½μm
μ = 2μm
No relation

Answer: B. μ = μm·S/(Ks+S); if S=Ks → μ = μm·Ks/2Ks = μm/2.

BOD/DO

Q8. Which statement about BOD and DO is correct?

BOD increases with increase in DO
BOD and DO always equal
BOD increases with decrease in DO
Independent of each other

Answer: C. Higher organic pollution (BOD) consumes more oxygen, lowering DO — inverse relationship.

Water Quality

Q9. Which is the unit of turbidity?

NTU
mg/L
m³/m² day
day⁻¹

Answer: A. NTU = Nephelometric Turbidity Unit.

Hydrology

Q10. ____ replenishes underground aquifers.

Percolation
Infiltration
Sedimentation
Nitrogen-fixing bacteria

Answer: B. Infiltration is the process water uses to enter soil and recharge groundwater (percolation also appears as an option in some papers referring to the same downward movement — infiltration is the KU-preferred term).

Mass Balance

Q11. For steady state with conservative pollutants:

Transformation rate ≠ 0
Accumulation = Input − Output
Input rate = Output rate
Transformation = Input − Output

Answer: C. Steady state → Accumulation = 0; conservative → no reaction term → Input = Output.

Reactors

Q12. Which statement is NOT valid for a CSTR?

Completely mixed
Concentration higher as depth increases
Input flow = output flow
Reactor conc. = effluent conc.

Answer: B. A CSTR is completely/uniformly mixed — concentration does NOT vary with depth/position.

Mass Transfer

Q13. Adsorption of pollutants on activated carbon is an example of:

Solid–liquid mass transfer
Solid–solid
Gas–liquid
Liquid–solid

Answer: A. Pollutant moves from the liquid phase onto the solid carbon surface.

Colloids

Q14. Which is NOT true for colloidal particles?

Electrically charged
Always in motion
Size 1 micron–1 milli-micron
Can be removed by filtration

Answer: D. Colloids are too small to be removed by ordinary filtration — require coagulation/flocculation.

Wastewater

Q15. Which does NOT contain human faeces/urine?

Black water
Grey water
Night soil
Sewage

Answer: B. Grey water = wastewater from sinks/showers/laundry (no faecal matter).

Sewers

Q16. Which sewer is the largest in size?

Main
Lateral
Branch
Outfall

Answer: D. Order (smallest→largest): House > Laterals > Branch/Mains > Outfall. Outfall carries the full collected flow.

Wastewater Treatment

Q17. Skimming tank refers to removal of ____ from wastewater.

Fats, oils, and grease
BOD
Pathogens
Sludge

Answer: A. Skimming removes floatable FOG (fats, oils, grease) at the surface.

Air Quality

Q18. PM10 does NOT refer to:

Dust, dirt, soot
Ultrafine particles
Particles <10 microns
Coarse particles

Answer: D. PM10 = fine/inhalable particulate matter under 10 μm, not the coarse fraction.

Cyclone Separator

Q19. In cyclone separators, ___ forces drive particles to the wall.

Gravitational
Electrostatic
Centrifugal
Magnetic

Answer: C. The vortex creates centrifugal force pushing particles outward to the cyclone wall.

Sound Power Level

Q20. Combining 450 dB and 450 dB gives:

453 dB
450 dB
900 dB
903 dB

Answer: A. L=10log₁₀(2×10^45)=10(45+log2)=450+3.01≈453 dB. Two equal sources always combine to +3 dB.

Sound Power Level

Q21. Combine 65 dB, 60 dB, 70 dB:

≈72 dB
75 dB
70 dB
65 dB

Answer: A. L=10log₁₀(10^6.5+10^6.0+10^7.0)=10log₁₀(1.416×10⁷)≈71.5→72 dB.

Solid Waste Mgmt

Q22. The least preferred method of solid waste management is ___ of waste.

Disposal
Reduction
Recovery
Recycling

Answer: A. Waste hierarchy (best→worst): Reduction > Reuse/Recycle > Recovery > Disposal.

Solid Waste Mgmt

Q23. The dirty liquid extracting solutes as it passes through landfill layers:

Leachate
Faecal sludge
Sewage
Sewerage

Answer: A. Leachate is contaminated liquid percolating through landfill waste.

Population Calc

Q24. Kathmandu Valley: 715 t/day organic waste = 65% of total; per capita 0.4 kg/day. Population?

2.75 million
2.75 million (check)
1.8 million
1.16 million

Answer: A (≈2.75 million). Total waste = 715/0.65 = 1100 t/day = 1,100,000 kg/day ÷ 0.4 kg/capita/day = 2,750,000 ≈ 2.75 million.

LCA

Q25. Which is NOT a goal of LCA?

Identify alternative product
Minimize pollution
Reduce production cost
Maximize recycling

Answer: C. LCA targets environmental impact reduction, not primarily cost — cost is an economic (LCC) tool, not LCA.

Systems Thinking

Q26. Which has the LOWEST degree of freedom?

Incrementalism
Redefining the problem
Re-defining system boundary
Re-engineering the system

Answer: A. Incrementalism = smallest, most constrained changes within an existing system.

Sedimentation Design

Q27. Sedimentation tank 100m×10m×4m treats 8000 m³/day. HRT is:

12 hrs
48 hrs
12 days
0.5 hrs

Answer: A. V=100×10×4=4000 m³; θ=V/Q=4000/8000=0.5 day=12 hrs.

HRT / Lagoon

Q28. Lagoon volume 1512 m³, inflow 3 m³/hr. Retention time?

189 days
21 days
504 days
4536 days

Answer: B. θ=1512/3=504 hr ÷24=21 days.

Wastewater Solids

Q29. Plant discharges 1.5 m³/s at 20 mg/L solids. Solids discharged per day (kg)?

26
2600
260
2.6

Answer: B. Q=1.5×86400=129,600 m³/day; mass=129,600×20 g/m³=2,592,000 g≈2600 kg/day.

Mass Transfer

Q30. Methane release from anaerobic wastewater treatment is an example of:

Liquid–gas mass transfer
Solid–gas
Gas–liquid
Liquid–solid

Answer: A. Dissolved organics in liquid are converted and released as gaseous CH₄ — liquid to gas phase transfer.

Water Supply

Q31. To move water from a lower to higher elevation on a hill, the required additional unit is:

Pumping system
Digital meter
Control valve/sensors
Break pressure tank

Answer: A. Uphill transport needs energy input → pumping. (Break-pressure tanks are instead used going DOWNHILL to dissipate excess pressure.)

Water Treatment

Q32. ___ removes turbidity and non-settleable colloidal matter.

Filtration
Aeration
Screening
Sedimentation (with coagulation)

Answer: D (Coagulation/Sedimentation). Screening removes large solids; filtration is a later polishing step — coagulation-sedimentation targets colloidal turbidity.

Sewers

Q33. Access point for inspection/cleaning of a sewer line is known as:

Manhole
Inlet
Cover
Catch basin

Answer: A. Manholes provide periodic sewer access for maintenance.

Microbial Growth

Q34. The phase when microbes start declining from peak is known as:

Lag
Log
Stationary
Endogenous

Answer: D. Growth curve order: Lag → Log (exponential) → Stationary → Endogenous/Death (decline).

Water Supply

Q35. The process of carrying water from intake to the treatment unit is called:

Collection
Distribution
Transmission
Treatment

Answer: C. Transmission mains carry raw water to the plant; distribution mains carry treated water to consumers.

Monod Kinetics

Q36. If S ≫ Ks in Monod equation:

μ ≈ μm
μ = ½μm
μ = 2μm
No relation

Answer: A. When substrate is abundant, growth rate saturates at its maximum, μm (zero-order kinetics).

Air Quality

Q37. Air Quality Index (AQI) is best described as:

A measure of how clean/polluted the air is
Presence of contaminants only
Devices preventing pollutants
Acceptable threshold level only

Answer: A. AQI is a composite index communicating overall air cleanliness/pollution to the public.

EIA

Q38. Environmental Impact Assessment (EIA) is conducted for:

Products
Policies
Projects
None

Answer: C. EIA is a project/site-specific regulatory tool (contrast with LCA for products).

Solid Waste Density

Q39. 5000 households fill one 100 L bin/week (density 100 kg/m³). What density must a 12 m³ truck compact to, to collect all in 2 trips/week?

30000 kg/m³
2083 kg/m³
100 kg/m³
500 kg/m³

Answer: B. Total waste = 5000×100 L=500,000 L=500 m³ (at 100 kg/m³) = 50,000 kg/week. Per trip (2 trips): 25,000 kg in 12 m³ → density=25,000/12≈2083 kg/m³.

LCA / Functional Unit

Q40. A Functional Unit is:

A quantified description of the performance of the product system
Description of material/energy flows only
Critical balance of supplies
Defines how big a part of the life cycle will be

Answer: A. The FU is the reference unit that allows fair comparison between competing product systems.

LCA

Q41. Which is NOT a product life-cycle phase?

Raw material extraction
Manufacture and use
Inventory
End of life

Answer: C. "Inventory" is an LCA analysis stage (LCI), not a physical life-cycle phase of the product.

Mass Properties

Q42. Which is a Mass-Intensive property?

Density
Volume
Mass
Internal Energy

Answer: A. Intensive properties (density, temperature, concentration) don't depend on system size; extensive ones (mass, volume, energy) do.

Mass Balance

Q43. Mass balance equation for steady-state non-conservative system:

Input = Output
Accumulation = Input + Output
Input = Output + KCV
Accumulation = Input − Output

Answer: C. Steady (Accum.=0) + reacting substance → Input = Output + decay term (kCV).

Process Systems

Q44. A bypass stream does not go through:

Process
Feed
Both a & b
Product

Answer: C. A bypass diverts a fraction of the feed around the process unit entirely, skipping it.

Water Supply

Q45. Wet weather flow is:

Water from rainfall only
Wastewater from kitchen
Sewage flows + runoff (infiltration/inflow during rain)
Treated water

Answer: C. WWF = normal sewage flow plus stormwater-derived inflow/infiltration during rain events.

Water Treatment

Q46. First step of water treatment removing large floating matter via uniform openings:

Filtration
Aeration
Screening
Sedimentation

Answer: C. Screening (bar racks) is the preliminary treatment step.

Wastewater Treatment

Q47. Which is a biological wastewater treatment process?

Coagulation
Sedimentation
Activated Sludge Process
Filtration

Answer: C. ASP uses microorganisms to biologically degrade organics — the others are physical/chemical.

Sewers

Q48. Decreasing order of sewer size (diameter):

House>Laterals>Mains>Outfall
Outfall>Mains>Laterals>House
House>Laterals>Outfall>Mains
Outfall>Laterals>Mains>House

Answer: B. Sewer size increases downstream as more flow accumulates: House < Laterals < Mains < Outfall.

Air Pollutants

Q49. Which is NOT one of the six "criteria" air pollutants?

Carbon monoxide
Lead
Hydrogen peroxide
Sulfur oxides

Answer: C. The 6 US EPA criteria pollutants: CO, Pb, NO₂, O₃, PM, SO₂. H₂O₂ is not one of them.

Air Pollution Control

Q50. Purpose of an Electrostatic Precipitator (ESP):

Filter coarse dust only
Remove fine particulate matter from air using charged plates
Regulate CO emissions
Neutralize SOₓ

Answer: B. ESPs charge particles electrostatically and collect them on oppositely charged plates — effective for fine PM.

Sewers

Q51. The sewer that unloads sewage at the treatment point is called ____ sewer.

Outfall
Branch
Laterals
Sub-mains

Answer: A. Outfall sewer = final conveyance to the treatment plant/disposal point.

Solid Waste Mgmt

Q52. Process of "transfer and transport" in solid waste management involves:

Gathering & storage of waste
Moving waste from collection to larger transport equipment to disposal sites
Recovering usable materials
Final disposal in landfill/incinerator

Answer: B. Transfer stations consolidate small collection loads into larger, more efficient long-haul vehicles.

EIA

Q53. Which project requires only an IEE (not full EIA) in Nepal?

Road construction up to 25 km
Hospital with 10 beds
Hydropower above 50 MW
Water supply project 5 MLD

Answer: A/D (small/medium projects). Large hydropower (>50 MW) requires full EIA; smaller-scale road/water-supply projects typically fall under IEE thresholds per Nepal's EPR schedules.

System Property

Q54. Which property does NOT depend on the amount of matter present?

Extensive properties
Intensive properties
Physical properties
Chemical properties

Answer: B. Intensive properties (e.g. density, concentration, temperature) are independent of system size.

Catchment

Q55. Catchment area is defined as:

The area of land draining into a stream/water course
Small rivers joining the main river
Place where a river flows into another body of water
Water lost from seepage

Answer: A. This is the standard definition of a drainage basin/catchment area.

Hydrology

Q56. Primary process by which water changes from liquid to gas in the hydrologic cycle:

Precipitation
Evaporation
Condensation
Runoff

Answer: B. Evaporation converts liquid surface water to water vapour, driven by solar energy.

Chemical Process

Q57. Main function of a purge stream in a chemical process:

Maintain reaction temperature
Remove unreacted residue/inerts and prevent accumulation
Reduce reaction pressure
Increase reactant concentration

Answer: B. A purge stream bleeds off a small fraction of a recycle loop to prevent build-up of inert/unreacted material.

Water Treatment

Q58. Killing harmful microorganisms before distribution uses:

Filtration
Aeration
Chlorination
Screening

Answer: C. Chlorination (disinfection) inactivates pathogens before treated water is distributed.

Population Projection

Q59. Arithmetic population after 20 yrs: base year 74,100, growth 10,350 (over 20 yrs, avg used per decade in original data). Which is closest?

84,450
95,000 (context-dependent — see note)
138,000
74,100

Answer: Verify with your own arithmetic-increase formula: P_n=P₀+n·(avg. increase). With avg. increase ≈10,350/decade and n=2 decades: P=74,100+2×10,350=94,800≈95,000 (Option B).

Wastewater Solids

Q60. Plant processes 10,000 m³/day at 100 mg/L SS. Total solids/day?

500 kg
10 kg
100 kg
1000 kg

Answer: D. 10,000 m³ × 100 g/m³ = 1,000,000 g = 1000 kg/day.

Solid Waste Density

Q61. City of 25,000 people, 2 kg/capita/day, landfill area 50 m², 4 m depth of daily waste. Density of solid waste?

500 kg/m³
250 kg/m³
2000 kg/m³ (check working)
5000 kg/m³

Worked: Daily mass = 25,000×2 = 50,000 kg. Daily volume = 50 m²×4 m = 200 m³. Density = 50,000/200 = 250 kg/m³ → Correct Answer: B (250 kg/m³).

Nepal EPR

Q62. Which is NOT an environmental study level under Nepal's Environmental Protection Rules?

Brief Environmental Study (BES)
Initial Environmental Examination (IEE)
Environmental Impact Assessment (EIA)
Advanced Environmental Study (AES)

Answer: D (or A — both are distractors). Nepal officially recognizes only IEE and EIA as statutory study levels — "BES" and "AES" are not real categories.

HRT Definition

Q63. Hydraulic Retention Time (HRT) of a tank represents:

Time to fill the tank
Average time a fluid particle spends in the tank
Volume of the tank
Flow rate through the tank

Answer: B. HRT = V/Q, the average residence/detention time of fluid in the reactor/tank.

Groundwater

Q64. Which hydrological process replenishes groundwater reserves?

Evaporation
Infiltration
Runoff
Condensation

Answer: B. Infiltrated water percolates downward to recharge the aquifer.

Dissolved Solids

Q65. Which unit is NOT used to measure dissolved solids concentration?

mg/L
kg/m³
ppm
kg/day

Answer: D. kg/day is a mass FLOW RATE, not a concentration.

Monod / Bio-treatment

Q66. According to Monod kinetics, when substrate concentration (S) is very high:

Growth rate remains constant (low S baseline)
Growth rate decreases
Growth rate reaches maximum
Growth rate stops completely

Answer: C. At high S, μ saturates near μm (zero-order — substrate is no longer limiting).

Engineering Economics

Q67. Internal Rate of Return (IRR) is:

Estimate of lowest total cost
Environmental impact caused by project
An indicator reflecting project profitability
Information of synthesized solution

Answer: C. IRR is the discount rate at which a project's NPV = 0 — a standard profitability/economic-feasibility metric.

Design Discharge

Q68. Sediment mass flow rate for a 25 lps stream with 2000 mg/L load:

432 kg/day
864 kg/day
500 kg/day
0.05 kg/s (equiv. 4320 kg/day)

Worked answer: Mass rate = Q×C = 0.025 m³/s × 2000 g/m³ = 50 g/s = 0.05 kg/s → ×86,400 s/day ≈ 4320 kg/day. (Cross-check the paper's exact given numbers against this method — the technique Q×C is what's tested, regardless of the specific figure.)

Solid Waste Truck

Q69. Landfill site: solid waste compacted to 1000 kg/m³. Annual waste from a slum providing 3 m depth — asked to find landfill area in anna. Which formula step is essential first?

Directly divide mass by depth
Volume = Mass/Density, then Area = Volume/Depth, then convert m² → anna
Multiply mass by depth
Use density × depth directly for area

Answer: B. Always: Annual mass → Volume (mass/density) → Area (volume/depth) → unit convert using 1 anna = 31.79 m².

Green Engineering

Q70. The pollution prevention hierarchy places which option as MOST preferred?

Source reduction / prevention
Recycling
Treatment
Disposal

Answer: A. Preventing waste at the source is always preferred over managing it after generation.