Environmental Engg‑II (CE-702 (B)) - Unit 4 Short Notes

1.0 WASTEWATER TREATMENT PROCESSES

1.1 Fundamentals

Unit operations are physical processes that do not involve chemical or biological change (e.g., screening, sedimentation). Unit processes involve chemical or biological transformations (e.g., coagulation, activated sludge).

[!TIP]

Common pitfall: Confusing unit operations (physical separation) with unit processes (chemical/biological conversion). In exams, clearly distinguish with examples.

1.2 Preliminary Treatment

Screening

• Types:

  • Coarse screens: bar spacing 50–150 mm, remove large debris.

  • Fine screens: bar spacing 6–25 mm, remove smaller solids.

• Materials: Stainless steel, mild steel (coated).

• Design criteria: Approach velocity 0.6–1.0 m/s, head loss limited to 0.2–0.3 m.

Head loss through bar screens (empirical):

$$ h_f = \frac{\beta}{2g} \left( \frac{W}{b} \right)^{4/3} v^2 \sin^2 \theta $$

\boxed{h_f = \frac{\beta}{2g} \left( \frac{W}{b} \right)^{4/3} v^2 \sin^2 \theta}

Where:
$$\displaystyle h_f $$ = head loss (m)
$\beta$ = blockage coefficient (0.5–1.0)
$W$ = total width of bars (m)
$b$ = clear spacing between bars (m)
$v$ = approach velocity (m/s)
$\theta$ = angle of screen inclination (°)

Grit Removal

• Purpose: Remove sand, gravel, cinders to prevent abrasion and accumulation in tanks.

• Types:

  • Horizontal flow: simple, low maintenance, velocity 0.2–0.3 m/s.

  • Aerated: creates spiral flow, velocity 0.3–0.5 m/s.

  • Vortex: high removal efficiency, compact.

• Design parameters:

  • Settling velocity of target grit: $$\displaystyle V_s = \sqrt{\frac{g(\rho_p - \rho)d}{\rho C_d}} $$ (turbulent flow) or Stokes' law for laminar.

  • Flow velocity maintained by proportional weir.

  • Channel dimensions from $$\displaystyle Q = V \times A \times L $$? Actually, for horizontal grit chamber: $$\displaystyle Q = V_s \times A \times L $$? No, design: $$\displaystyle V = V_s $$ for particles to settle. So area $$\displaystyle A = Q / V $$, and length $L$ from detention time (typically 30–60 s).

Numerical example: Given $$\displaystyle d = 0.2 $$ mm, $$\displaystyle \rho_p = 2.65 $$, $$\displaystyle V_s $$ range 0.016–0.022 m/s, $$\displaystyle Q = 10,000 $$ m³/day, $$\displaystyle V = 0.3 $$ m/s.

Convert $Q$ to m³/s: $$\displaystyle 10,000 / 86400 = 0.1157 $$ m³/s.

Area $$\displaystyle A = Q / V = 0.1157 / 0.3 = 0.3857 $$ m².

Take $$\displaystyle V_s = 0.016 $$ m/s (conservative), detention time $$\displaystyle t = L / V = 30 $$ s → $$\displaystyle L = V \times t = 0.3 \times 30 = 9 $$ m.

Depth $$\displaystyle D = A / L = 0.3857 / 9 = 0.0429 $$ m ≈ 43 mm. But typical depth 0.9–1.5 m, so adjust $$\displaystyle V_s $$ or $t$. Usually, use $$\displaystyle V_s $$ for smallest particle, and $t$ based on $$\displaystyle L/V = 30–60 $$ s. So with $$\displaystyle V=0.3 $$ m/s, $$\displaystyle L=9 $$ m for 30 s, then $$\displaystyle A = Q/V = 0.3857 $$ m², so width $$\displaystyle W = A/D $$, with $$\displaystyle D=0.9 $$ m → $$\displaystyle W = 0.3857/0.9 = 0.428 $$ m. But channel is rectangular, so dimensions: width ≥ 0.5 m, depth 0.9–1.5 m, length 10–25 m. Provide range.

1.3 Primary Treatment

Sedimentation (Primary Clarifiers)

• Types of settling:

  • Discrete: particles settle individually (e.g., sand).

  • Flocculent: particles flocculate during settling (e.g., organic solids).

  • Hindered: high concentration, particles interfere (zone settling).

  • Compression: particles in contact, compress under weight (sludge thickening).

• Tank design:

  • Shape: rectangular (common) or circular.

  • Detention period: 1.5–2.5 hours.

  • Surface loading rate: 20–30 m³/m²·day (or 0.8–1.2 m³/m²·h).

  • Overflow rate = $Q/A$.

• Removal efficiency:

  • Settleable solids: 50–70%.

  • BOD: 25–35% (mostly particulate BOD).

Septage Treatment

• Septic tank:

  • Construction: Two-chamber tank with baffles, inlet/outlet tees.

  • Working: Anaerobic digestion of sludge, scum layer, effluent to soak pit or secondary treatment.

  • Advantages: Simple, low cost, no energy.

  • Limitations: Requires desludging every 2–3 years, not for large flows, groundwater contamination risk.

• Imhoff tank:

  • Construction: Two-story; upper sedimentation chamber, lower digestion chamber with slope.

  • Working: Solids settle in upper chamber, slide to lower for anaerobic digestion. Effluent from upper chamber.

  • Advantages: Better solids reduction, less odor, sludge digestion in situ.

  • Limitations: Complex construction, not for high flows, requires periodic cleaning.

Comparative analysis:

[!TIP]

In exams, compare based on: flow capacity, treatment efficiency, maintenance, cost. Imhoff tank is an improvement over septic tank for better sludge digestion.

1.4 Secondary Treatment (Biological Processes)

Trickling Filters

• Types:

  • Standard rate: depth 2–3 m, organic loading 0.08–0.32 kg BOD/m³·day (80–320 g/m³·day).

  • High rate: depth 0.5–1 m, organic loading 0.4–1.6 kg BOD/m³·day.

  • Super high rate: organic loading >1.6 kg BOD/m³·day.

• Design parameters:

  • Surface loading rate: 10–40 m³/m²·day (standard), up to 100 m³/m²·day (high rate).

  • Organic loading: mass of BOD applied per unit volume per day.

  • Depth: 1–3 m (standard), 0.5–1 m (high rate).

  • Volume: $$\displaystyle V = \frac{Q \times L}{1000} $$? Actually, $$\displaystyle V = \frac{Q \times L}{1000} $$? No, volume $$\displaystyle V = A \times D $$, where $$\displaystyle A = Q / \text{surface loading rate} $$, $D$ = depth.

• Efficiency calculation (NRC formula):

$$ E = \frac{100}{1 + 0.44 \sqrt{W/V}} $$

\boxed{E = \frac{100}{1 + 0.44 \sqrt{W/V}}}

Where:
$E$ = BOD removal efficiency (%)
$W/V$ = organic loading in lb BOD/1000 ft³/day.
Metric conversion: If $$\displaystyle L_v $$ in kg BOD/m³/day, then $$\displaystyle W/V \approx L_v / 0.016 $$, so $$\displaystyle E = \frac{100}{1 + 3.5 \sqrt{L_v}} $$.

• Advantages: Simple, low energy, good for small communities, resistant to shock loads.

• Disadvantages: Large land area, odor, fly nuisance, clogging.

Activated Sludge Process

• Process description:

  DiagramCANVAS: Schematic showing wastewater → aeration tank (with diffused air) → secondary clarifier → effluent. Sludge return from clarifier to aeration tank. Waste sludge from clarifier.

• Components:

  • Aeration tank: mixed liquor aerated, MLSS 2000–4000 mg/L.

  • Secondary clarifier: solids-liquid separation.

  • Sludge return system: return activated sludge (RAS) to maintain MLSS.

• Oxygen requirements:

  • Method 1 (BOD load): $$\displaystyle O_2 = 1.42 \times \text{BOD}_{\text{removed}} $$ (stoichiometric, 1 mg BOD ≈ 1.42 mg O₂).

  • Method 2 (Endogenous respiration):

$$ O_2 = Q (L - E) \times 1.42 + 0.1 \times V \times X $$

\boxed{O_2 = Q (L - E) \times 1.42 + 0.1 \times V \times X}

Where:
$$\displaystyle O_2 $$ = daily oxygen requirement (kg/day)
$Q$ = flow (m³/day)
$L$ = influent BOD (mg/L)
$E$ = effluent BOD (mg/L)
$V$ = aeration tank volume (m³)
$X$ = MLSS concentration (mg/L)
$0.1$ = endogenous respiration rate (mg O₂/mg MLSS·day) at 20°C.

• Variations:

  • Oxidation ditch: modified activated sludge with continuous loop.

  • Extended aeration: SRT > 20 days, MLSS high, effluent quality better.

  • Contact stabilization: separate contact tank for adsorption.

Oxidation Ditch

• Working principle: Continuous oval or circular channel with surface or submerged rotors for aeration and mixing. Hydraulic retention time 20–30 hours, SRT 10–30 days.

• Configurations: Carrousel (single rotor), oval, dual ditch.

• Operational parameters:

  • DO: 1–2 mg/L.

  • SRT: 10–30 days.

  • MLSS: 2000–4000 mg/L.

Role of Microorganisms

• Bacteria:

  • Floc formers: Zoogloea, Pseudomonas – produce extracellular polymers for floc.

  • Filamentous: Sphaerotilus – excess causes bulking.

• Protozoa:

  • Free-swimming (e.g., Paramecium) – consume free bacteria.

  • Ciliated (e.g., Vorticella) – attached to floc, indicate good sludge.

• Metazoa: Rotifers, nematodes – indicate healthy sludge, consume excess bacteria.

• Food chain dynamics: Bacteria → protozoa → metazoa; controls bacterial population, improves floc formation and effluent clarity.

1.5 Advanced/Tertiary Treatment

Need and Overview

• Limitations of secondary treatment: Inadequate removal of nutrients (N, P), pathogens, TDS, micropollutants.

• AWT vs conventional: AWT adds barriers for specific contaminants; conventional focuses on BOD, solids.

• Typical AWT processes:

Filtration Processes

• Diatomaceous earth filter:

  • Principle: Depth filtration using porous diatomaceous earth (fossilized algae).

  • Operation: Precoat layer formed, body feed maintains thickness.

  • Applications: Turbidity removal, polishing step.

• Ultrafiltration (UF):

  • Principle: Pressure-driven membrane with pore size 0.01–0.1 μm.

  • Characteristics: Removes macromolecules, colloids, viruses; operates at low pressure (1–10 bar).

  • Applications: Water reuse, pretreatment for RO, removal of bacteria.

Nutrient Removal

• Nitrogen removal:

  • Biological nitrification-denitrification:

    • Nitrification (aerobic):

$$ \text{NH}_4^+ + 1.5 O_2 \rightarrow \text{NO}_2^- + 2H^+ + H_2O \quad (\text{Nitrosomonas}) $$

$$ \text{NO}_2^- + 0.5 O_2 \rightarrow \text{NO}_3^- \quad (\text{Nitrobacter}) $$

  Conditions: DO > 2 mg/L, pH 7.5–8.5, SRT > 10 days.  

- **Denitrification** (anoxic):  

$$ 2\text{NO}_3^- + 5\text{CH}_2\text{O} + 2H^+ \rightarrow N_2 + 5CO_2 + 7H_2O $$

  Requires carbon source (methanol or influent COD).  

• Chemical/Physical methods:

  • Ammonia stripping: Raise pH > 10, air strip NH₃.

  • Ion exchange: Resins exchange NH₄⁺ for Na⁺.

  • Breakpoint chlorination: Chlorine doses to oxidize NH₃ to N₂.

• Phosphorus removal (if applicable):

  • Biological (EBPR): Alternating anaerobic/aerobic conditions, PAOs store P.

  • Chemical: Alum (Al₂(SO₄)₃), lime (Ca(OH)₂), ferric chloride.

Other AWT Processes

• Adsorption (activated carbon):

  • Principle: Adsorbates adhere to carbon surface via van der Waals forces.

  • Isotherms:

    • Langmuir: monolayer adsorption, $$\displaystyle \frac{x}{m} = \frac{aP}{1+bP} $$.

    • Freundlich: multilayer, $$\displaystyle \frac{x}{m} = k P^{1/n} $$.

  • Applications: Removal of organics, taste/odor control.

• Reverse osmosis (RO): Semi-permeable membrane, pressure > osmotic pressure, removes ions, molecules.

• Disinfection: Chlorination, UV, ozone (if not covered earlier).

2.0 AIR POLLUTION

2.1 Sources and Characteristics of Air Pollutants

• Major pollutants:

  • SO₂: Fossil fuel combustion (power plants), volcanic emissions. Characteristics: colorless, pungent, soluble, causes acid rain, respiratory irritation.

  • H₂S: Sewage, petroleum refining, natural gas. Characteristics: rotten egg smell, toxic (paralyzes olfactory nerve at high conc.), flammable.

  • NOₓ: Vehicles, power plants. Forms ozone, acid rain.

  • CO: Incomplete combustion, vehicles. Binds hemoglobin, reduces oxygen transport.

  • Particulates: TSPM (<100 μm), RSPM/PM₁₀ (<10 μm), PM₂.₅ (<2.5 μm). Sources: combustion, construction, dust. Health: respiratory, cardiovascular.

  • Aerosols: Solid/liquid particles suspended in gas. Types:

    • Dust: >1 μm, from mechanical disruption.

    • Fumes: <0.1 μm, from condensation of vapors.

    • Mist: liquid droplets, from atomization.

    • Smoke: from incomplete combustion, contains soot, gases.

    • Fog: water droplets, natural.

Characteristics table:

[!TIP]

Remember: Primary pollutants are emitted directly (SO₂, CO, PM). Secondary form in atmosphere (O₃, PAN, sulfate aerosols).

2.2 Meteorological Factors

• Temperature lapse rate:

  • Environmental lapse rate (ELR): Actual temperature decrease with height (average 6.5°C/km).

  • Adiabatic lapse rate (ALR): Temperature change of rising/falling air parcel without heat exchange.

    • Dry ALR: 9.8°C/km.

    • Saturated ALR: 4–6°C/km (due to latent heat release).

• Wind speed/direction: Higher wind increases dispersion; direction determines downwind impact.

• Atmospheric stability: Classes A–F (Pasquill):

  • A: Very unstable (sunny, windy) → high dispersion.

  • D: Neutral (cloudy, moderate wind).

  • F: Stable (night, calm) → low dispersion, high concentrations.

• Mixing height: Height of turbulent layer; diurnal: high in afternoon, low at night.

• Humidity and precipitation: Scavenging of soluble gases (SO₂) and particulates by rain.

2.3 Air Pollution Dispersion Models

• Gaussian plume model: Assumes steady-state, constant wind, flat terrain, no deposition/chemical reaction.

  Equation for continuous point source:

$$ C(x,y,z) = \frac{Q}{2\pi \sigma_y \sigma_z U} \exp\left(-\frac{y^2}{2\sigma_y^2}\right) \left[ \exp\left(-\frac{(z-H)^2}{2\sigma_z^2}\right) + \exp\left(-\frac{(z+H)^2}{2\sigma_z^2}\right) \right] $$

\boxed{C(x,y,z) = \frac{Q}{2\pi \sigma_y \sigma_z U} \exp\left(-\frac{y^2}{2\sigma_y^2}\right) \left[ \exp\left(-\frac{(z-H)^2}{2\sigma_z^2}\right) + \exp\left(-\frac{(z+H)^2}{2\sigma_z^2}\right) \right]}

Where:
$C$ = concentration (g/m³)
$Q$ = emission rate (g/s)
$$\displaystyle \sigma_y, \sigma_z $$ = dispersion parameters (m) from Pasquill-Gifford curves.
$U$ = wind speed (m/s)
$H$ = effective stack height (m)
$x$ = downwind distance (m)
$y$ = lateral distance (m)
$z$ = vertical height (m)

• Maximum concentration location: For ground-level release ($$\displaystyle H=0 $$), max at $x$ where $$\displaystyle \sigma_z $$ is minimum? Actually, for elevated release, max ground-level concentration at $$\displaystyle x_{\text{max}} \approx \frac{H}{\sqrt{2} \sigma_z} $$? More precisely, for given stability, find $x$ where $C$ at ground ($$\displaystyle z=0 $$) is max. Often approximated by $$\displaystyle x_{\text{max}} \approx \frac{U H}{\sigma_z^2} $$? Not simple. In exams, given $$\displaystyle \sigma_y, \sigma_z $$ as functions of $x$, differentiate $C(x,0,0)$ w.r.t $x$ and set to zero.

• Factors affecting dispersion: Wind speed (higher → lower $C$), stability (unstable → larger $\sigma$), emission height (higher → better dispersion), terrain (valleys trap pollutants).

2.4 Smog Formation

• Photochemical smog:

  • Formation: NOₓ + VOCs + sunlight → ozone (O₃), peroxyacetyl nitrate (PAN), aldehydes.

  • Constituents: O₃, PAN, NO₂, hydrocarbons.

  • Effects: Eye irritation, respiratory problems, plant damage, material degradation.

• Sulfurous smog (London type):

  • Formation: SO₂ + particulates + fog → sulfuric acid mist.

  • Causes: Coal burning, winter inversion.

  • Effects: Respiratory diseases, high mortality (1952 London smog).

• Comparison:

2.5 Air Pollution from Power Plants

• Flyash and bottom ash generation:

  • Coal consumption:

$$ \text{Coal consumed (kg/day)} = \frac{\text{Power output (W)} \times 86400}{\text{Efficiency} \times \text{CV}} $$

• Ash generated: $$\displaystyle \text{Ash} = \text{Coal consumed} \times \text{ash content} $$.

• Flyash: typically 60–80% of ash (entrained with flue gas).

• Bottom ash: remainder (collected at bottom).

• Recoverable: given percentages (e.g., 75% flyash, 90% bottom ash).

• Eco-friendly disposal/utilization:

  • Cement and concrete additive (flyash as pozzolana).

  • Bricks and tiles (flyash bricks).

  • Structural fill, road base.

  • Agriculture (soil amendment, but heavy metal concern).

Numerical example (Dec 2024):

Given: 800 MW plant, efficiency 40%, CV = 21 MJ/kg, ash content 38%, 60% ash as flyash, 75% flyash recoverable, 90% bottom ash recoverable.

Power output = 800 MW = 800 × 10⁶ W.

Energy output per day = 800 × 10⁶ × 86400 = 6.912 × 10¹³ J.

Coal energy input = 6.912 × 10¹³ / 0.4 = 1.728 × 10¹⁴ J.

Coal mass = 1.728 × 10¹⁴ / (21 × 10⁶) = 8228.6 kg/day? Wait, 21 MJ/kg = 21 × 10⁶ J/kg, so coal mass = 1.728e14 / 21e6 = 8228.6 kg? That's only 8.2 tonnes/day for 800 MW? That's too low. Mistake: 800 MW is power, but over day, energy = 800 MW × 24 h = 19200 MWh = 19200 × 3.6e9 J = 6.912e13 J, correct. But coal CV 21 MJ/kg = 21e6 J/kg, so coal = 6.912e13 / 0.4 / 21e6 = 6.912e13 / 8.4e6 = 8228 kg? That seems low because typical coal consumption for 800 MW is about 3000–4000 tonnes/day. Check: 800 MW × 24 h = 19200 MWh. If CV 21 MJ/kg = 5.833 kWh/kg (since 1 kWh = 3.6 MJ, so 21/3.6 = 5.833 kWh/kg). Efficiency 40%, so coal required per kWh = 1/(5.833×0.4) = 0.428 kg/kWh. Then for 19200 MWh = 19.2e6 kWh, coal = 19.2e6 × 0.428 = 8.22e6 kg = 8220 tonnes/day. Yes, 8220 tonnes/day. So ash = 8220 × 0.38 = 3124 tonnes/day. Flyash = 0.6 × 3124 = 1874 tonnes/day. Bottom ash = 0.4 × 3124 = 1250 tonnes/day. Recoverable flyash = 0.75 × 1874 = 1405 tonnes/day. Recoverable bottom ash = 0.9 × 1250 = 1125 tonnes/day.

2.6 Air Pollution Control Methods

• Particulate control:

  • Electrostatic precipitator (ESP): Charges particles, collects on plates. Efficiency >99%.

  • Baghouses (fabric filters): Fabric bags filter particles, efficiency >99%.

  • Cyclones: Inertial separation,效率 70–90% for large particles.

  • Scrubbers: Wet removal, also for gases.

• Gaseous control:

  • Adsorption: Activated carbon for VOCs, odors.

  • Absorption: Scrubbers with liquid (e.g., NaOH for SO₂).

  • Catalytic conversion: Catalytic converters for NOₓ, CO, hydrocarbons.

• Ammonia stripping: For ammonia removal from wastewater or air; raise pH, air strip.

2.7 Short Note Topics (Integrated)

• RSPM vs TSPM:

  • TSPM: Total suspended particulates (all sizes up to 100 μm).

  • RSPM: Respirable suspended particulates (PM₁₀, <10 μm), penetrate deep lungs.

• Factors affecting dispersion:

  • Meteorological: wind speed, stability, mixing height, humidity.

  • Topographic: valleys, hills, buildings (urban canyon effect).

• ELR vs ALR:

  • ELR: Actual environmental temperature gradient.

  • ALR: Temperature change of air parcel rising adiabatically (dry or saturated).

• Primary vs secondary pollutants:

  • Primary: Emitted directly (SO₂, CO, NOₓ, PM).

  • Secondary: Formed by atmospheric reactions (O₃, PAN, H₂SO₄, nitrate aerosols).

• Effect on materials:

  • Corrosion: SO₂, NOₓ form acids, corrode metals, stone.

  • Soiling: Particulates deposit on surfaces, buildings, monuments.

• Adsorption by activated carbon:

  • Principle: Physical adsorption on porous carbon (high surface area).

  • Isotherms: Langmuir (monolayer, homogeneous), Freundlich (heterogeneous, multilayer).

  • Applications: Water/air purification, decolorization, recovery of solvents.

3.0 NUMERICAL PROBLEMS AND DESIGN CALCULATIONS

Key formulas:

1. Grit chamber settling velocity (for design particle):

$$ V_s = \sqrt{\frac{g d (\rho_p - \rho)}{\rho C_d}} \quad \text{or} \quad V_s = \frac{g d^2 (\rho_p - \rho)}{18 \mu} \text{ (Stokes)} $$

\boxed{V_s = \frac{g d^2 (\rho_p - \rho)}{18 \mu}} for laminar flow ($$\displaystyle Re < 1 $$).

2. Sedimentation tank:

   • Area: $$\displaystyle A = Q / v_o $$, where $$\displaystyle v_o $$ = overflow rate (m/s).

   • Volume: $$\displaystyle V = A \times t $$, $t$ = detention time (s).

   • Typical: $$\displaystyle v_o = 0.8–1.2 $$ m³/m²·h, $$\displaystyle t = 2–4 $$ h.

3. Trickling filter:

   • Area: $$\displaystyle A = Q / \text{surface loading rate} $$.

   • Depth from organic loading: $$\displaystyle D = \frac{L_a}{L_v} $$, where $$\displaystyle L_a $$ = organic loading per area (kg BOD/m²·day), $$\displaystyle L_v $$ = organic loading per volume (kg BOD/m³·day).

   • NRC efficiency: $$\displaystyle E = \frac{100}{1 + 0.44 \sqrt{W/V}} $$ (imperial) or $$\displaystyle E = \frac{100}{1 + 3.5 \sqrt{L_v}} $$ (metric, $$\displaystyle L_v $$ in kg BOD/m³/day).

4. Activated sludge oxygen requirement:

$$ O_2 = Q (L - E) \times 1.42 + 0.1 \times V \times X $$

\boxed{O_2 = Q (L - E) \times 1.42 + 0.1 \times V \times X}

5. Alum dose:

$$ \text{Dose (mg/L)} = \frac{\text{Alum required (mg)}}{\text{Flow (L)}} $$

\boxed{\text{Dose} = \frac{\text{Alum (mg)}}{\text{Flow (L)}}}

6. Power plant emissions:

   • Coal consumption: $$\displaystyle \text{Coal (kg/day)} = \frac{\text{Power (W)} \times 86400}{\eta \times \text{CV (J/kg)}} $$.

   • Ash: $$\displaystyle \text{Ash (kg/day)} = \text{Coal} \times \text{ash fraction} $$.

   • Flyash: $$\displaystyle \text{Flyash} = \text{Ash} \times \text{flyash fraction} $$.

   • SO₂: $$\displaystyle \text{SO₂ (kg/day)} = \text{Coal} \times S_{\text{content}} \times \frac{64}{32} $$ (since S → SO₂, molecular weight ratio 2).

7. Gaussian plume: Use equation above; solve for $C$ at given $(x,y,z)$.

4.0 COMPARATIVE ANALYSES AND CONCEPTS

Unit operations vs unit processes:

Septic tank vs Imhoff tank (see 1.3.2 table).

Conventional vs AWT:

Primary vs secondary air pollutants:

RSPM vs TSPM:

ELR vs ALR:

[!TIP]

For short notes, present comparisons in tables – examiners prefer clear, tabular differences. Always define terms first.