LESSON 3.2 — Environmental Pollution and Controls

A. Standard Map

Topic	Governing Source	Exam Focus
Air pollution — parameters	CPCB NAAQS (2009); Air (Prevention and Control of Pollution) Act, 1981	PM2.5, PM10, NOx, SO2, CO — annual/24-hr limits
Water pollution — BOD, COD, DO	Water (Prevention and Control of Pollution) Act, 1974; CPCB standards; IS:3306:1990	What each measures; relationship (COD ≥ BOD); threshold values
Noise pollution	Noise Pollution (Regulation and Control) Rules, 2000	Zone category; day/night limits in dB(A)
Land degradation	NDMA; MoEFCC guidelines; standard practice	Causes; reclamation methods
Control hierarchy	Standard environmental management	Source → treatment → buffer — in that order
Sewage treatment — three stages	CPCB; IS:3306:1990; Metcalf & Eddy (2003)	Primary/secondary/tertiary; efficiency per stage; UASB reactor

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B. Why It’s Used

Environmental Pollution and Controls is one of the most consistently tested clusters in GATE AP. CPCB standards, the Solid Waste Management Rules 2016, BOD/COD relationships, and noise zone limits appear as direct factual recall questions almost every year. The topic rewards exact number recall (PM2.5 = 40 µg/m³, BOD₅ at 20°C for 5 days, silence zone = 50/40 dB(A)) over conceptual understanding — making it high-return territory for prepared candidates.

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C. Mechanism in Words

1. Pollution occurs when contaminants enter the environment at a rate exceeding the natural system’s capacity to absorb or neutralise them.
2. Air pollutants are emitted by vehicles, industry, and construction; they cause respiratory disease, reduce visibility, contribute to acid rain, and interact with sunlight to form secondary pollutants like ground-level ozone.
3. Water pollutants consume dissolved oxygen (DO) as they are decomposed by bacteria — a polluted water body has high BOD, high COD, and low DO; aquatic life suffocates at low DO.
4. Noise pollutants cause hearing damage, sleep disturbance, and cardiovascular stress — the effect depends on both level (dB) and frequency weighting (dB(A) weights frequencies to match human hearing).
5. Land is degraded when soil structure, nutrient content, or biological activity is permanently reduced by misuse, contamination, or erosion.
6. The correct response to pollution follows a hierarchy: prevent the source first, treat what cannot be prevented at source, then use spatial planning buffers to protect sensitive receptors from whatever remains.

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D. Core Concept Explanations

C1. Air Pollution — Key Pollutants and CPCB NAAQS

The National Ambient Air Quality Standards (NAAQS) are prescribed by the Central Pollution Control Board (CPCB) under the Air (Prevention and Control of Pollution) Act, 1981 and the Environment (Protection) Act, 1986. The current standards were notified in 2009.

Pollutant	Description	Primary sources	NAAQS Annual average	NAAQS 24-hour average
PM10	Particulate matter ≤ 10 µm diameter; inhalable into upper airways	Road dust, construction, industrial emissions, vehicle exhaust	60 µg/m³	100 µg/m³
PM2.5	Fine particulate matter ≤ 2.5 µm; penetrates deep into lungs and bloodstream	Vehicle exhaust, biomass burning, secondary formation	40 µg/m³	60 µg/m³
NO₂ (NOx)	Nitrogen dioxide; reddish-brown gas; precursor to photochemical smog and ozone	Vehicle exhaust (especially diesel), thermal power plants	40 µg/m³	80 µg/m³
SO₂	Sulphur dioxide; acidic gas; precursor to acid rain	Coal combustion, power plants, industrial smelting	50 µg/m³	80 µg/m³
CO	Carbon monoxide; colourless, odourless; toxic at high concentrations	Incomplete combustion — vehicles, fuel burning	—	2000 µg/m³ (8-hr average)

Health effects by pollutant:

Pollutant	Primary health effect	Secondary environmental effect
PM2.5	Deep lung penetration; cardiovascular and pulmonary disease	Visibility reduction; climate forcing
PM10	Upper respiratory tract irritation; asthma aggravation	Visibility reduction
NO₂	Bronchial irritation; lung damage	Precursor to photochemical smog; contributes to acid rain
SO₂	Respiratory irritation; bronchoconstriction	Precursor to acid rain (H₂SO₄)
CO	Haemoglobin binding; oxygen displacement; toxic at high concentrations	Contributes to ozone formation

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C2. Water Pollution — BOD, COD, and Dissolved Oxygen

Three parameters dominate water quality assessment in examinations:

Parameter	Full name	What it measures	Test method	Relationship
BOD	Biochemical Oxygen Demand	Dissolved oxygen consumed by aerobic microorganisms decomposing biodegradable organic matter in a water sample	5-day test at 20°C (BOD₅)	BOD ≤ COD always
COD	Chemical Oxygen Demand	Oxygen required to chemically oxidise ALL organic matter (biodegradable + non-biodegradable) using a strong chemical oxidant	Chemical test (dichromate method); faster than BOD	COD ≥ BOD always
DO	Dissolved Oxygen	Oxygen dissolved in water and available to aquatic life	Electrochemical probe or Winkler titration	DO ↓ as BOD ↑

Interpreting typical values (mg/L):

Water quality condition	BOD (mg/L)	COD (mg/L)	DO (mg/L)
Clean unpolluted water	< 1	< 10	> 8
Slightly polluted	2–3	10–40	6–8
Moderately polluted	5–10	40–150	4–6
Heavily polluted	> 10	> 150	< 4
Severely polluted / anaerobic	> 100	> 500	~0
Raw sewage (typical)	150–300	300–600	Near 0
Treated effluent (CPCB discharge standard)	≤ 30	≤ 250	≥ 4

DO and aquatic life:

DO level	Effect on aquatic life
> 8 mg/L	Healthy; supports most fish species
5–8 mg/L	Acceptable; most fish and invertebrates can survive
2–5 mg/L	Stressed conditions; sensitive species begin to die
< 2 mg/L	Hypoxic; mass fish kills possible
~ 0 mg/L	Anaerobic; only anaerobic bacteria survive; foul odours

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C3. Noise Pollution — dB(A) Scale and Zone Limits

The dB(A) scale:
Sound is measured in decibels (dB), a logarithmic scale. For environmental noise assessment, the dB(A) scale is used — it applies a frequency weighting that approximates the sensitivity of the human ear, which is more sensitive to mid-range frequencies than to very low or very high frequencies. The A-weighting makes dB(A) the standard unit for noise regulations worldwide.

dB scale characteristics:
– Every 3 dB increase = doubling of sound power
– Every 10 dB increase = perceived as roughly twice as loud (subjective)
– Quiet bedroom: ~30 dB(A); Normal conversation: ~60 dB(A); Road traffic: ~70–80 dB(A); Jet engine: ~120 dB(A)

Noise Pollution (Regulation and Control) Rules, 2000:
Notified under the Environment (Protection) Act, 1986. Define ambient noise standards by area category:

Area category	Definition	Day limit (6 AM – 10 PM)	Night limit (10 PM – 6 AM)
A — Industrial	Areas predominantly with industrial land use	75 dB(A)	70 dB(A)
B — Commercial	Shopping, office, and mixed commercial areas	65 dB(A)	55 dB(A)
C — Residential	Areas used predominantly for housing	55 dB(A)	45 dB(A)
D — Silence zone	Areas around hospitals, educational institutions, courts, religious places	50 dB(A)	40 dB(A)

Health effects of noise:
– > 85 dB(A) sustained → permanent hearing damage
– > 65 dB(A) residential → sleep disturbance, cardiovascular stress
– > 55 dB(A) at night → WHO guideline for outdoor residential areas

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C4. Land Degradation — Causes and Reclamation

Causes of land degradation:

Cause	Mechanism	Typical context
Deforestation	Removal of vegetative cover → soil erosion, nutrient loss, compaction	Watershed areas, hill slopes
Overgrazing	Trampling and overconsumption of vegetation → bare soil exposed	Semi-arid and dryland regions
Waterlogging	Prolonged saturation of soil → anaerobic conditions → root damage, salt accumulation	Over-irrigated agricultural land; low-lying areas
Salinisation	Salt accumulation from evaporation of irrigation water	Arid zone agricultural land
Mining and industrial contamination	Heavy metals, toxic compounds leach into soil	Mine tailings, industrial sites
Urbanisation and impervious cover	Sealing of soil surface; disruption of natural drainage	Urban fringe; conversion of agricultural land

Reclamation approaches:

Approach	Application
Afforestation / reforestation	Restores vegetative cover; reduces erosion; rebuilds soil organic matter
Phytoremediation	Specific plants (hyperaccumulators) absorb heavy metals from contaminated soil (e.g., sunflower for lead)
Bioremediation	Microorganisms (indigenous or introduced) degrade organic pollutants (hydrocarbons, solvents)
Soil amendment — liming	Adding lime (CaO, Ca(OH)₂) raises pH of acid soils; improves nutrient availability
Soil amendment — gypsum	Adding gypsum (CaSO₄) to saline/sodic soils; replaces sodium ions; improves structure
Controlled landfill capping	Impermeable cap over contaminated fill; prevents leachate migration; planted surface above cap

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C5. Pollution Control Hierarchy

The correct response to environmental pollution follows a source–treatment–buffer hierarchy — each level is applied in order, and lower-cost upstream interventions are always preferred over expensive downstream treatment.

1. SOURCE REDUCTION (most preferred)
   ↓ Prevent or minimise the generation of the pollutant at origin
   → Cleaner production technology, fuel switching, low-emission vehicles

2. TREATMENT
   ↓ Capture and neutralise pollutants that cannot be eliminated at source
   → Effluent treatment plants, air scrubbers, noise barriers

3. PLANNING BUFFERS (spatial separation)
   ↓ Where pollution cannot be fully controlled, separate sensitive uses from sources
   → Industrial buffer zones, setback requirements, green belts

4. RECEPTOR PROTECTION (least preferred)
   ↓ Last resort — protect the receptor itself from unavoidable pollution
   → Personal protective equipment, double glazing, insulation

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C6. Sewage Treatment — Three Stages

Stage	Type	Process	Key removal	Typical BOD reduction
Primary (Physical)	Physical/mechanical	Screening and bar screens remove gross solids; grit chambers settle heavy inorganic particles; primary sedimentation tanks allow suspended solids to settle by gravity; scum collected from surface; pH adjusted with lime if needed	Large suspended solids, grit, floating material	30–40% BOD; 50–60% TSS
Secondary (Biological)	Biological	Microorganisms digest dissolved and colloidal organic matter. Two common methods: (a) Activated Sludge Process — aeration tanks where aerobic bacteria degrade organics; (b) UASB Reactor (Up-flow Anaerobic Sludge Blanket) — anaerobic bacteria decompose organics; produces methane as by-product	Dissolved organic matter; foul odour	85–90% BOD
Tertiary (Advanced)	Chemical/physical	Disinfection (chlorination, UV radiation, ozonation); nutrient removal (nitrogen, phosphorus stripping); filtration; activated carbon adsorption for trace organics	Residual organics, pathogens, dissolved nutrients	>95% BOD; water suitable for non-potable reuse

Discharge standards (CPCB, for discharge into surface water bodies):

Parameter	Permissible limit
BOD	≤ 30 mg/L
COD	≤ 250 mg/L
Suspended Solids (SS)	≤ 100 mg/L
pH	6.5–9.0
Dissolved Oxygen	≥ 4 mg/L

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E. Design/Parameter Table

Parameter	Value	Source
PM2.5 annual NAAQS	40 µg/m³	CPCB NAAQS 2009
PM2.5 24-hr NAAQS	60 µg/m³	CPCB NAAQS 2009
PM10 annual NAAQS	60 µg/m³	CPCB NAAQS 2009
PM10 24-hr NAAQS	100 µg/m³	CPCB NAAQS 2009
NO₂ annual NAAQS	40 µg/m³	CPCB NAAQS 2009
SO₂ annual NAAQS	50 µg/m³	CPCB NAAQS 2009
CO 8-hr NAAQS	2000 µg/m³	CPCB NAAQS 2009
Noise — Industrial (day/night)	75 / 70 dB(A)	Noise Rules 2000
Noise — Commercial (day/night)	65 / 55 dB(A)	Noise Rules 2000
Noise — Residential (day/night)	55 / 45 dB(A)	Noise Rules 2000
Noise — Silence zone (day/night)	50 / 40 dB(A)	Noise Rules 2000
BOD — clean water	< 1 mg/L	Standard water quality
BOD — CPCB discharge limit	≤ 30 mg/L	CPCB
COD — CPCB discharge limit	≤ 250 mg/L	CPCB
DO — minimum for aquatic life	≥ 5 mg/L	Standard water quality
UASB BOD reduction	~85%	CPCB; Metcalf & Eddy
Primary treatment BOD removal	30–40%	IS:3306; Metcalf & Eddy
Secondary treatment BOD removal	85–90%	IS:3306; Metcalf & Eddy
Tertiary treatment BOD removal	> 95%	IS:3306; Metcalf & Eddy

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F. Design Criteria

Parameter	Value	Source
PM2.5 annual standard	40 µg/m³	CPCB NAAQS 2009
PM2.5 24-hour standard	60 µg/m³	CPCB NAAQS 2009
PM10 annual standard	60 µg/m³	CPCB NAAQS 2009
PM10 24-hour standard	100 µg/m³	CPCB NAAQS 2009
BOD₅ test conditions	5 days at 20°C	IS:3306:1990
COD vs BOD relationship	COD ≥ BOD always	Standard environmental science
Silence zone — day limit	50 dB(A) (0600–2200 hrs)	Noise Rules 2000
Silence zone — night limit	40 dB(A) (2200–0600 hrs)	Noise Rules 2000
Residential zone — day/night	55 / 45 dB(A)	Noise Rules 2000
Commercial zone — day/night	65 / 55 dB(A)	Noise Rules 2000
Industrial zone — day/night	75 / 70 dB(A)	Noise Rules 2000
BOD removal — primary treatment	30–40%	Standard practice
BOD removal — secondary treatment	80–95%	Standard practice
C:N ratio optimal for composting	~25–30:1 (closest = 30)	Standard composting practice

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Code Comparisons

Code / Rule	Scope	Key Numbers for GATE
CPCB NAAQS 2009	National Ambient Air Quality Standards — limits for 12 pollutants	PM2.5: 40/60 µg/m³; PM10: 60/100 µg/m³ (annual/24-hr)
Water (Prevention & Control) Act, 1974	Regulates water pollution; establishes PCBs	Legal framework for effluent discharge standards
Air (Prevention & Control) Act, 1981	Regulates air pollution; establishes SPCBs	Legal framework for emission standards
Noise Pollution (Regulation & Control) Rules, 2000	Zone-wise ambient noise limits	Silence zone 50/40; Residential 55/45; Commercial 65/55; Industrial 75/70 dB(A)
Solid Waste Management Rules, 2016	Duties of generators; segregation requirements; processing rules	Replaces 2000 rules; co-processing definition; ODF++ criteria
IS:3306:1990	BOD test standard	5 days at 20°C

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G. Application Zones

Pollution Type	Key Controls	Governing Rule/Act
Air pollution	Source controls; NAAQS compliance monitoring; emission standards	Air Act 1981; CPCB NAAQS 2009
Water pollution	Effluent treatment before discharge; UASB for sewage	Water Act 1974; CPCB standards
Noise pollution	Land use separation; noise barriers; limits by zone category	Noise Rules 2000
Solid waste	Segregation at source (wet/dry/hazardous); composting; sanitary landfill	SWM Rules 2016
Sewage treatment	Primary → Secondary → Tertiary sequence; UASB reactors (anaerobic)	Standard practice; MoEFCC

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H. Common Confusions

Confusion	Correct Distinction
COD < BOD for some samples	COD is ALWAYS ≥ BOD for the same sample. COD measures everything BOD measures (biodegradable organics) plus non-biodegradable organics. There is no case where COD < BOD.
High DO = polluted water	High dissolved oxygen indicates CLEAN, healthy water. LOW DO (caused by high BOD) indicates polluted water. Higher DO = better water quality.
PM10 has a stricter limit than PM2.5	PM2.5 has a STRICTER (LOWER) annual limit (40 µg/m³) than PM10 (60 µg/m³). Smaller particles are more hazardous — they penetrate deeper — hence the lower allowable concentration.
dB and dB(A) are the same	dB is the raw sound pressure level. dB(A) applies A-frequency-weighting to match human hearing sensitivity. Environmental regulations use dB(A) — not plain dB.
Silence zone = no noise allowed	Silence zones have a day limit of 50 dB(A) — not zero. The label “silence zone” indicates more restrictive limits near sensitive receptors, not complete prohibition of noise.
UASB is a tertiary treatment process	UASB (Up-flow Anaerobic Sludge Blanket) is a SECONDARY treatment method — it is a biological reactor. Tertiary treatment includes disinfection (chlorination, UV) and nutrient removal.
Primary treatment removes dissolved organics	Primary treatment is PHYSICAL (screening, settling) and removes only SUSPENDED and FLOATING solids. DISSOLVED organics require secondary (biological) treatment.

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I. Compare & Contrast

Pair	Key Difference
BOD vs COD	BOD: oxygen demand for biodegradable organics only (5-day test, 20°C). COD: oxygen demand for ALL organics (chemical test). COD ≥ BOD always; difference represents non-biodegradable fraction.
Primary vs Secondary treatment	Primary: physical removal (screening, sedimentation) — 30–40% BOD removal. Secondary: biological removal (activated sludge, trickling filter, UASB) — up to 90%+ BOD removal.
Aerobic vs Anaerobic treatment	Aerobic: uses oxygen; produces CO₂ + H₂O + biomass; activated sludge process. Anaerobic: no oxygen; produces biogas (CH₄ + CO₂); UASB reactor.
PM2.5 vs PM10	PM2.5: fine particles ≤2.5 µm; penetrate deep into lungs; stricter limits (40/60). PM10: coarse particles ≤10 µm; less harmful; less strict limits (60/100). Lower number = finer particle = more dangerous.
Wet vs Dry composting	Wet: high moisture content; anaerobic conditions if not managed. Dry: lower moisture; aerobic composting; better odour management. C:N ratio ~25–30:1 optimal for both.
Primary vs Secondary air pollutants	Primary: emitted directly from source (CO, SO₂, NOx, PM). Secondary: formed in atmosphere by chemical reaction (ozone O₃, secondary PM2.5).

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J. Memory Hooks

• PM2.5 = 40/60; PM10 = 60/100: “Smaller particle, smaller limit — 40 then 60; 60 then 100.”
• COD ≥ BOD always: “Chemical tests more than biological tests — COD catches everything BOD misses.”
• Silence zone = 50/40: “Silence is FIFTY by day, FORTY by night — the quietest zone.”
• Noise zone order: “Silence → Residential → Commercial → Industrial = 50 → 55 → 65 → 75 (day).”
• BOD₅ at 20°C for 5 days: “Five degrees below boiling, five days of waiting.”
• UASB = anaerobic: “U Are Sans Breathing — UASB has no oxygen.”
• SWM Rules 2016: “Segregate in 2016 — wet, dry, hazardous.”
• Pollution hierarchy: “Source → Treatment → Buffer — fix the factory before building the wall.”

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K. Revision Ladder

Tier 1 — Must Know:
– PM2.5: 40 µg/m³ annual, 60 µg/m³ 24-hr (NAAQS 2009)
– PM10: 60 µg/m³ annual, 100 µg/m³ 24-hr (NAAQS 2009)
– COD ≥ BOD always; BOD₅ test at 20°C for 5 days
– Silence zone: 50 dB(A) day, 40 dB(A) night (Noise Rules 2000)
– SWM Rules 2016 — replaces 2000 rules; segregation at source mandatory

Tier 2 — High Probability:
– Noise limits: Residential 55/45; Commercial 65/55; Industrial 75/70 dB(A)
– Primary treatment: 30–40% BOD removal; secondary: 80–95%
– UASB reactor: anaerobic; produces biogas
– Pollution control hierarchy: source reduction → treatment → buffer
– C:N ratio optimal for composting: ~25–30:1

Tier 3 — Worth Knowing:
– Primary vs secondary air pollutants distinction
– Dissolved oxygen (DO): low in polluted water bodies; fish kill threshold ~4 mg/L
– Activated sludge: aerobic biological secondary treatment
– Water Act 1974; Air Act 1981 — year and scope
– ODF++ criteria under Swachh Bharat

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L. Exam Traps

Trap	Incorrect Assumption	Correct Answer
T07	“BOD measures all organic matter in water”	BOD measures only biodegradable organic matter. COD measures total organic (biodegradable + non-biodegradable). A sample with synthetic detergents will have a high COD but the BOD will not fully capture the non-biodegradable fraction.
T08	“Residential noise limit at night = 55 dB(A)”	Residential NIGHT limit = 45 dB(A). The 55 dB(A) is the residential DAY limit. Day limits are always higher than night limits in all categories.
T09	“PM10 has a lower NAAQS limit than PM2.5 because it is bigger”	NAAQS limits: PM2.5 annual = 40 µg/m³ (lower/stricter); PM10 annual = 60 µg/m³ (higher/less strict). Smaller particles (PM2.5) get the tougher limit despite being physically less visible.
T10	“Source reduction and treatment are equivalent options”	Source reduction is ALWAYS preferred over treatment. Treatment is a secondary response for pollution that cannot be eliminated at source. Treating pollution is always more costly and less effective than preventing it.
T11	“UASB reactor uses aerobic bacteria”	UASB is an anaerobic process. Anaerobic bacteria degrade organic matter in the absence of oxygen, producing methane. Aerobic bacteria are used in the activated sludge process, not UASB.
T12	“Noise Pollution Rules 2000 apply only to industrial areas”	The Rules apply to ALL four area categories — Industrial (A), Commercial (B), Residential (C), and Silence zones (D). Each category has different day and night limits.

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M. Answer-Writing Cues

For BOD/COD distinction:

For sewage treatment stages:

For noise zone limits:

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N. PYQ Integration (2007–2026 verified)

Topic	Exam Appearance	Question Pattern
PM2.5 vs PM10 limits	GATE recent, UPSC-CPWD	MCQ: “NAAQS annual standard for PM2.5 is ___”
BOD vs COD — which is higher	GATE, UPSC-CPWD, ISRO	MCQ: “For the same water sample, COD is always ___ BOD” → ≥
BOD definition — 5-day test	GATE, UPSC-CPWD	MCQ: “BOD₅ is measured over ___ days at ___°C”
DO — relationship with pollution	GATE	MCQ: “In a heavily polluted water body, dissolved oxygen is ___” → Low
Noise zone limits — residential night	GATE, UPSC-CPWD	MCQ: “Residential night ambient noise standard per Noise Rules 2000 is ___”
Silence zone limits	UPSC-CPWD, state PSC	MCQ: day and night limits for silence zones
UASB — aerobic or anaerobic	GATE, UPSC-CPWD	MCQ: “UASB reactor uses ___ process” → anaerobic
Primary treatment efficiency	UPSC-CPWD	MCQ: “BOD removal in primary treatment is approximately ___”
Pollution control hierarchy	UPSC-CPWD	MCQ: correct sequence; “which is most preferred?” → source reduction
As per the Municipal Solid Waste Management Manual by the Central Public Health and…	GATE AR 2026 Q19 — Ans: B,C	MSQ: As per the Municipal Solid Waste Management Manual by the Central…
Which of the following is/are mandatory requirement(s) of an area to be declared as ODF++…	GATE AR 2026 Q21 — Ans: C	MSQ: Which of the following is/are mandatory requirement(s) of an area to be…
Identify the correct relationship with respect to water quality from the following…	GATE AR 2025 Q16 — Ans: C	MCQ: Identify the correct relationship with respect to water quality from…
As per the Solid Waste Management Rules, 2016, co-processing is the use of ___ and…	GATE AR 2025 Q17 — Ans: A	MCQ: As per the Solid Waste Management Rules, 2016, co-processing is the use…
For composting, the optimum Carbon to Nitrogen (C:N) ratio is closest to ___.	GATE AR 2025 Q18 — Ans: B	MCQ: For composting, the optimum Carbon to Nitrogen (C:N) ratio is closest…
Solid Waste Management Rules 2016 — duties of waste generators. (A) Segregate in 4…	GATE AR 2021 Q42 — Ans: B,C	MSQ: Solid Waste Management Rules 2016 — duties of waste generators. (A)…
As per the Central Pollution Control Board’s National Air Quality Index (AQI) of India…	GATE AR 2022 Q44 — Ans: A,C	MSQ: As per the Central Pollution Control Board’s National Air Quality Index…
Which among the following is/are the component(s) of the assimilative carrying capacity…	GATE AR 2022 Q77 — Ans: A,B,D	MSQ: Which among the following is/are the component(s) of the assimilative…
NOT considered as primary air pollutant(s).	GATE AR 2023 Q27 — Ans: D	MSQ: NOT considered as primary air pollutant(s).
As per the Solid Waste Management Rules 2016, which of the following statements are…	GATE AR 2023 Q40 — Ans: A,C	MSQ: As per the Solid Waste Management Rules 2016, which of the following…
Which of the following statements about composting are correct?	GATE AR 2023 Q42 — Ans: A,C,D	MSQ: Which of the following statements about composting are correct?

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O. Mini-Check — Lesson 3.2 (5 Questions)

Q1 (MCQ): Per CPCB National Ambient Air Quality Standards (2009), the annual average standard for PM2.5 is:
(A) 60 µg/m³ (B) 100 µg/m³ (C) 40 µg/m³ (D) 80 µg/m³

A1: (C) 40 µg/m³. PM2.5 annual NAAQS = 40 µg/m³; 24-hour = 60 µg/m³. PM10 annual = 60 µg/m³; 24-hour = 100 µg/m³. PM2.5 gets the stricter (lower) limit because finer particles cause greater health harm.

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Q2 (MCQ): A water sample has a BOD₅ of 15 mg/L. Its COD value, measured on the same sample, would most likely be:
(A) Less than 15 mg/L (B) Exactly 15 mg/L (C) Greater than 15 mg/L (D) Cannot be determined without further tests

A2: (C) Greater than 15 mg/L. COD measures ALL organic matter (biodegradable + non-biodegradable); BOD measures only biodegradable organic matter. Therefore COD ≥ BOD always for the same sample.

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Q3 (MCQ): Per the Noise Pollution (Regulation and Control) Rules, 2000, the permissible ambient noise level for a silence zone during daytime (0600–2200 hrs) is:
(A) 40 dB(A) (B) 50 dB(A) (C) 55 dB(A) (D) 45 dB(A)

A3: (B) 50 dB(A). Silence zone day limit = 50 dB(A); night limit = 40 dB(A). The 45 dB(A) is the residential night limit; 55 dB(A) is the residential day limit. Silence zones (around hospitals, schools, courts) are the most restrictive category.

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Q4 (MSQ): Which of the following statements about sewage treatment processes are correct? Select all that apply.
(A) Primary treatment removes suspended solids by physical sedimentation
(B) The UASB reactor is an aerobic secondary treatment process
(C) Tertiary treatment includes disinfection by chlorination or UV
(D) Primary treatment typically achieves 30–40% BOD reduction
(E) Secondary treatment using the activated sludge process is biological

A4: (A), (C), (D), and (E).
– (A) ✓ Primary = physical screening and sedimentation.
– (B) ✗ UASB = anaerobic (not aerobic). It uses anaerobic bacteria and produces methane.
– (C) ✓ Tertiary includes disinfection (chlorination, UV, ozonation).
– (D) ✓ Primary treatment: ~30–40% BOD reduction.
– (E) ✓ Activated sludge = aerobic biological secondary treatment.

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Q5 (MCQ): In the pollution control hierarchy, which approach should be pursued first before considering treatment or spatial buffer planning?
(A) Installing filtration equipment at sensitive receptor locations
(B) Establishing green buffer zones between source and receptor
(C) Reducing or eliminating the pollution at its source
(D) Relocating the sensitive receptor away from the pollution source

A5: (C) Source reduction. The correct hierarchy is: Source reduction (prevent generation) → Treatment (capture what cannot be eliminated) → Buffer planning (separate source from receptor spatially) → Receptor protection (last resort). Source reduction is always preferred as it addresses the root cause rather than managing the symptom.

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