Environmental Engg‑II (CE-702 (B)) - Unit 1 Short Notes

I. WASTEWATER TREATMENT: FUNDAMENTALS & PRELIMINARY PROCESSES

Unit Operations vs. Unit Processes

• Unit Operations: Physical processes that change the physical state of wastewater without chemical/biological transformation. Energy input is often mechanical.

  • Examples: Screening, grit removal, sedimentation, filtration, flotation.

• Unit Processes: Chemical or biological processes that transform the constituent characteristics of wastewater.

  • Examples: Coagulation/flocculation, disinfection, oxidation, biological oxidation (activated sludge, trickling filters).

[!TIP] Exam Focus: This is a classic 6-mark question. Clearly distinguish with examples. Often asked as "Elaborate various types of unit operations."

Preliminary Treatment

1. Racks & Screens

• Purpose: Remove large floating solids (rags, sticks, plastics) to protect pumps and downstream equipment.

• Types:

  • Coarse Screens: Opening 25-50 mm. Manually cleaned.

  • Fine Screens: Opening 6-25 mm. Mechanically cleaned (chain-driven, raked).

• Materials: Mild steel, stainless steel, or high-density polyethylene (HDPE).

• Head Loss Through Bar Screens:

  The empirical formula for clean screens:

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

> Where:

> *   $$\displaystyle h_f $$ = head loss (m)

> *   $K$ = bar shape coefficient (0.8-1.0 for sharp-edged)

> *   $W$ = width of flow (m)

> *   $b$ = clear spacing between bars (m)

> *   $V$ = approach velocity (m/s)

> *   $\theta$ = angle of inclination from horizontal.

>

> For clogged screens, an additional factor (1.5 to 2.0) is applied.

> \boxed{h_f = K \left( \frac{W}{b} \right)^{4/3} V^2 \sin\theta}

2. Grit Removal

• Purpose: Remove sand, gravel, cinders, and other inorganic solids to prevent abrasion, deposition in pipelines, and accumulation in digesters.

• Grit Chamber Design (Rectangular, Horizontal Flow):

  • Key Parameter: Settling velocity ($$\displaystyle V_s $$) of target grit particle (usually 0.2 mm, SG=2.65).

  • Design Velocity (Flow-through velocity): $$\displaystyle V = 0.3 $$ m/s (typical). Maintained by a proportional weir.

  • Channel Dimensions: Determined from $$\displaystyle Q = W \times D \times V $$, where $Q$ = flow, $W$ = width, $D$ = depth.

  • Length ($L$): $$\displaystyle L \geq V_s \times t $$, where $t$ = detention time (30-60 sec).

• Factors Affecting Removal: Particle shape factor (spherical vs. flat), organic matter adhesion (making grit lighter).

[!TIP] Common Pitfall: Grit chambers are designed for inorganic particles. Organic solids (specific gravity ~1.2) should not settle; high velocities prevent their deposition.

Flow Equalization & Measurement

• Need: To dampen flow and load fluctuations from industrial discharges or diurnal patterns, ensuring uniform loading on downstream treatment units.

• Types of Basins:

  • In-line: Mixed with wastewater flow.

  • Side-stream: Bypass flow for equalization.

  • Combined: Mix of both.

• Measurement: Parshall flumes, Venturi meters, weirs (Cippoletti, rectangular) are common.

II. PRIMARY TREATMENT

Sedimentation (Primary Settling)

• Theory: Gravity settling of suspended solids. Governed by Stokes' Law for laminar conditions (Re < 0.2):

$$ V_s = \frac{g (\rho_p - \rho) d^2}{18 \mu} $$

Where $$\displaystyle V_s $$ = settling velocity, $$\displaystyle \rho_p $$, $\rho$ = particle & fluid density, $d$ = diameter, $\mu$ = viscosity.

*   **Hindered Settling:** At high concentrations (>~1000 mg/L), particles interfere, reducing effective $$\displaystyle V_s $$. Described by **Kynch theory**.

• Types of Tanks:

  • Horizontal Flow: Rectangular, with mechanical scrapers.

  • Radial Flow: Circular, central inlet, peripheral weir.

• Key Design Parameters:

  • Detention Period ($$\displaystyle t_d $$): 1.5 - 2.5 hours.

  • Surface Loading Rate / Overflow Rate ($$\displaystyle q_o $$): $$\displaystyle q_o = Q/A $$ (m³/m².day). Typical: 20-40 m³/m².day. This is the critical design parameter.

  • Weir Loading Rate: $$\displaystyle Q/L_w $$ (m³/m.day). < 125-250 m³/m.day to prevent weir choking.

• Design of Rectangular Tank (with scrapers):

$$ \text{Volume } (V) = Q \times t_d $$

$$ \text{Area } (A) = \frac{Q}{q_o} $$

$$ \text{Length-to-Width Ratio } (L:W) = 4:1 \text{ to } 5:1 $$

$$ \text{Depth } (H) = 2 - 4 \text{ m} $$

Septic Tanks & Imhoff Tanks (Comparison)

[!TIP] Exam Focus: Direct comparison questions are frequent. Highlight the two-story separation as Imhoff's key advantage.

III. SECONDARY TREATMENT: ATTACHED GROWTH PROCESSES

Trickling Filters

• Types:

  • Standard Rate: Depth 2-3 m, organic loading 0.08-0.32 kg BOD/m³.d, recirculation optional.

  • High Rate: Depth 1-2 m, organic loading 0.32-1.0 kg BOD/m³.d, mandatory recirculation (R=2-5).

  • Super High Rate: Depth <1 m, organic loading >1.0 kg BOD/m³.d, high recirculation.

• Design Parameters:

  • Surface Loading Rate ($$\displaystyle q_s $$): $$\displaystyle q_s = \frac{Q}{A} $$ (L/m².day or m³/m².d). Standard: 1.0-4.0 m³/m².d.

  • Organic Loading Rate ($$\displaystyle L_o $$): $$\displaystyle L_o = \frac{Q \times L_0}{A} $$ (kg BOD/m³.d or g BOD/m².d).

  • Depth ($H$): 0.6-2.4 m (high rate shallower).

  • Recirculation Ratio ($R$): $$\displaystyle R = \frac{\text{Recirculated Flow}}{\text{Influent Flow}} $$.

• Performance & Efficiency (NRC Formula):

$$ E = \frac{100}{1 + 0.0044 \sqrt{\frac{A}{Q \times L_0}}} $$

Where $E$ = % BOD removal, $A$ = area (m²), $Q$ = flow (m³/d), $$\displaystyle L_0 $$ = influent BOD (mg/L).

• Design Calculation Steps (from Dec 2024 paper):

  1. Given $Q$, $$\displaystyle L_0 $$, $$\displaystyle q_s $$, $$\displaystyle L_o $$.

  2. Area $$\displaystyle A = Q / q_s $$.

  3. Depth $$\displaystyle H = L_o / (q_s \times L_0) $$ (since $$\displaystyle L_o = q_s \times L_0 \times H $$).

  4. Volume $$\displaystyle V = A \times H $$.

  5. Diameter (circular): $$\displaystyle D = \sqrt{\frac{4A}{\pi}} $$.

  6. Efficiency $E$: Use NRC formula.

• Media: Stones (standard), slag, plastic modules (high rate), redwood slats.

Oxidation Ditches

• Working Principle: Modified extended aeration activated sludge system in a continuous loop channel. Oxygen is supplied by surface aerators (brush or disc type). Long SRT (>15 days) leads to complete oxidation and stable sludge.

• Configuration & Layouts:

  • Shape: Oval or race-track.

  • Arrangement: Single ditch or multiple ditches in series/parallel.

  • Typical Process Train: Oxidation ditch → Secondary clarifier → Sludge return/wasting.

• Process Sketch Components:

  1. Channel: Concrete or earthen, typically 3-5 m deep.

  2. Surface Aerator: Provides oxygen and mixing.

  3. Secondary Clarifier: Circular or rectangular.

  4. Sludge Return & Wasting Pumps.

• Operational Characteristics:

  • MLSS: 3000-6000 mg/L (higher than conventional ASP).

  • SRT: >15 days (often 20-30 days).

  • F/M Ratio: Very low (0.05-0.1 kg BOD/kg MLSS.d).

IV. SECONDARY TREATMENT: SUSPENDED GROWTH PROCESSES

Activated Sludge Process (ASP)

• Definition & Mechanism: A biological process where wastewater is mixed with a dense suspension of microorganisms (activated sludge) in an aeration tank. Microorganisms consume organic matter, form flocs, and are separated in a clarifier. Part of the sludge is recycled.

• Process Description with Sketch:

  1. Aeration Tank: Wastewater + return sludge mixed. Aeration provided by diffused or mechanical aerators.

  2. Secondary Clarifier: Settles biomass. Supernatant is effluent.

  3. Sludge Return: Settled sludge pumped back to aeration tank.

  4. Sludge Wasting: Excess sludge removed to maintain MLSS.

• Variations:

  • Conventional: Influent added at one end.

  • Step Aeration: Influent added at multiple points along tank length.

  • Complete Mix: Influent rapidly mixed throughout tank.

  • Extended Aeration: Long SRT (>15 days), low F/M. Similar to oxidation ditch.

• Oxygen Requirements Calculation:

$$ O_2 \text{ required} = \frac{Q \times (L_0 - L_e)}{1000} \times (1 - Y) + \text{for endogenous respiration} $$

Where:

*   $Q$ = flow (m³/d), $$\displaystyle L_0 $$, $$\displaystyle L_e $$ = influent/effluent BOD (mg/L).

*   $Y$ = yield coefficient (0.4-0.6 kg VSS/kg BOD removed).

*   Endogenous respiration: $0.05-0.1$ kg O₂/kg VSS.d × mass of MLSS.

• Process Control Parameters:

  • MLSS (Mixed Liquor Suspended Solids): 2000-4000 mg/L (conventional).

  • SVI (Sludge Volume Index): $$\displaystyle SVI = \frac{\text{Settled volume (mL/L) in 30 min} \times 1000}{MLSS (mg/L)} $$. Ideal: 50-150 mL/g. >150 indicates bulking.

  • F/M Ratio (Food to Microorganism): $$\displaystyle F/M = \frac{\text{BOD load (kg/d)}}{MLSS \times V \text{ (kg)}} $$. Typical: 0.2-0.4 kg BOD/kg MLSS.d.

  • SRT (Solids Retention Time / Sludge Age): $$\displaystyle SRT = \frac{\text{Mass of MLSS in system (kg)}}{\text{Mass of sludge wasting per day (kg/d)}} $$. Critical for process stability.

Microorganisms in Biological Treatment

• Bacteria: Primary floc formers (e.g., Sphaerotilus, Zoogloea). Decompose organic matter.

• Protozoa: Feed on free bacteria, indicate sludge health (ciliates = good, flagellates = young sludge).

• Metazoa (Rotifers, Worms): Indicate well-settling, old sludge.

• Sludge Age (SRT): Average time biomass remains in system. Controls microbial population, nitrification potential, and sludge yield.

V. NUTRIENT REMOVAL & ADVANCED WASTEWATER TREATMENT (AWT)

Biological Nitrogen Removal

• Need: Prevent eutrophication in receiving water bodies.

• Nitrification: Autotrophic oxidation of ammonia to nitrate.

  • Step 1 (Nitrosomonas): $$\displaystyle NH_4^+ + 1.5O_2 \rightarrow NO_2^- + H_2O + 2H^+ $$

  • Step 2 (Nitrobacter): $$\displaystyle NO_2^- + 0.5O_2 \rightarrow NO_3^- $$

  • Conditions: High DO (>2 mg/L), pH 7.5-8.5, temperature >15°C, long SRT (>10 days).

• Denitrification: Heterotrophic reduction of nitrate to nitrogen gas under anoxic conditions.

  • $$\displaystyle NO_3^- + \text{organic matter} \rightarrow N_2 + CO_2 + H_2O $$

  • Conditions: No DO, presence of organic carbon (or methanol addition).

• Process Configurations: Single sludge (with anoxic zone), two-sludge system, Bardenpho (4-tank process: anaerobic-anoxic-aerobic-anoxic), oxidation ditch modifications (single or multiple channels with alternating oxic/anoxic zones).

Advanced Wastewater Treatment (AWT)

• Definition: Treatment beyond conventional secondary (biological) to achieve very high removal of organics, nutrients, and pathogens for reuse or sensitive water bodies.

• Difference from Conventional: AWT targets specific constituents (TSS, BOD, N, P, pathogens, TDS) to meet stringent standards (e.g., <1 mg/L BOD, <1 mg/L TSS, <0.1 mg/L TP). Conventional focuses on BOD/TSS removal (~85-90%).

• Important AWT Processes (Tabular Form):

Coagulation & Chemical Dosing

• Alum Dose Calculation:

$$ \text{Dose (mg/L)} = \frac{\text{Coagulant mass (mg)}}{\text{Flow (L)}} $$

$$ \text{Mass per day (kg/day)} = \frac{\text{Dose (mg/L)} \times \text{Flow (ML/day)} \times 10^3}{10^6} = \frac{\text{Dose} \times \text{Flow (ML/d)}}{1000} $$

> **Example (Nov 2023):** Dose = 28.6 mg/L, Flow = 18 ML/d.

> 

$$ \text{Mass/day} = \frac{28.6 \times 18}{1000} = 0.5148 \text{ kg/d} \approx 515 \text{ g/d} $$

> \boxed{\text{Mass (kg/d)} = \frac{\text{Dose (mg/L)} \times \text{Flow (ML/d)}}{1000}}

VI. AIR POLLUTION: FUNDAMENTALS & SOURCES

Major Air Pollutants (Tabular Form)

Aerosols

• Definition: Solid or liquid particles suspended in gas (air). Size: 0.001 µm to 100 µm.

• Types & Characteristics:

  | Type | Formation | Typical Size | Settling Velocity | | :--- | :--- | :--- | :--- | | Dust | Mechanical disintegration | 1-100 µm | Fast | | Fume | Condensation of vapor (metals) | 0.01-1 µm | Very slow | | Mist | Condensation of liquid droplets | 0.5-10 µm | Slow | | Smoke | Incomplete combustion | 0.01-1 µm | Very slow | | Fog | Water droplets (condensation) | 1-20 µm | Slow |

Primary vs. Secondary Pollutants

• Primary Pollutants: Emitted directly from a source.

  • Examples: SO₂, NOx, CO, VOCs, lead, primary PM.

• Secondary Pollutants: Formed in the atmosphere by chemical/photochemical reactions of primary pollutants.

  • Examples: Ozone (O₃) (from NOx+VOCs+sunlight), sulphate aerosols (from SO₂), nitrate aerosols (from NOx), PAN (Peroxyacetyl nitrate, from VOCs+NOx).

VII. AIR POLLUTION: METEOROLOGY & DISPERSION

Meteorological Factors Influencing Dispersion

1. Wind Speed & Direction: Primary driver of horizontal transport. Higher speed → greater dilution.

2. Temperature Lapse Rates:

   • Environmental Lapse Rate (ELR): Actual vertical temperature gradient in atmosphere (~6.5°C/km).

   • Adiabatic Lapse Rate (ALR): Temperature change of a rising/falling air parcel due to expansion/compression.

     • Dry ALR: ~9.8°C/km (unsaturated).

     • Saturated ALR: ~5-6°C/km (saturated).

   • Inversion: Temperature increases with height (ELR < ALR). Traps pollutants near ground.

3. Atmospheric Stability Classes (Pasquill-Gifford): A (very unstable) to F (very stable/strong inversion). Determined by solar radiation and wind speed. Unstable → high turbulence → good dispersion.

4. Turbulence & Mixing Height: Vertical eddies mix pollutants. Mixing height is the top of the turbulent boundary layer; above it, little mixing occurs.

Gaussian Plume Model (Point Source)

• Standard Equation (for continuous, steady-state plume):

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

Where:

*   $C$ = concentration (g/m³)

*   $Q$ = emission rate (g/s)

*   $U$ = wind speed (m/s) at plume height

*   $$\displaystyle \sigma_y, \sigma_z $$ = horizontal & vertical dispersion coefficients (m), functions of downwind distance $x$ and stability class.

*   $H$ = effective stack height (physical height + plume rise).

*   $z$ = vertical coordinate (ground at z=0).

• Location of Maximum Ground-Level Concentration:

  • Directly downwind ($$\displaystyle y=0 $$) at ground level ($$\displaystyle z=0 $$).

  • Occurs at distance $$\displaystyle x_{max} $$ where $$\displaystyle \frac{d}{dz} \left[ \exp\left(-\frac{(z-H)^2}{2\sigma_z^2}\right) + \exp\left(-\frac{(z+H)^2}{2\sigma_z^2}\right) \right]_{z=0} = 0 $$.

  • For $$\displaystyle H > 0 $$, max concentration is at $x$ where $$\displaystyle \sigma_z $$ is minimum relative to $H$. Typically a few km downwind.

• Assumptions: Steady state, constant wind speed/direction, flat terrain, no chemical reactions, no deposition, point source, Gaussian distribution in y & z.

• Limitations: Not valid for complex terrain, very short distances (<10H), low wind speeds, or chemical transformation.

VIII. AIR POLLUTION: INDUSTRIAL SOURCES & CONTROL

Flyash & Bottom Ash from Thermal Power Plants (Estimation)

• Given: Plant capacity $P$ (MW), efficiency $\eta$, coal CV (MJ/kg), ash content $$\displaystyle A_c $$, % flyash ($F\%$), recovery % for flyash ($$\displaystyle R_f\% $$) and bottom ash ($$\displaystyle R_b\% $$).

• Step 1: Coal Consumption Rate:

$$ \text{Coal (kg/s)} = \frac{P \times 10^6 \text{ W}}{\eta \times \text{CV (J/kg)}} $$

$$ \text{Coal (t/day)} = \frac{P \times 86.4}{\eta \times \text{CV (MJ/kg)}} \quad (86.4 = 86400 \text{ s/day} / 1000) $$

• Step 2: Total Ash Generated:

$$ \text{Total Ash (t/day)} = \text{Coal (t/day)} \times \frac{A_c}{100} $$

• Step 3: Split:

$$ \text{Flyash (raw, t/day)} = \text{Total Ash} \times \frac{F}{100} $$

$$ \text{Bottom Ash (raw, t/day)} = \text{Total Ash} - \text{Flyash} $$

• Step 4: Recoverable Ash (after collection system efficiency):

$$ \text{Recoverable Flyash} = \text{Flyash} \times \frac{R_f}{100} $$

$$ \text{Recoverable Bottom Ash} = \text{Bottom Ash} \times \frac{R_b}{100} $$

• Eco-friendly Disposal/Utilization:

  • Cement & Concrete: Flyash as pozzolanic material (replaces cement).

  • Bricks & Blocks: Flyash bricks ( Class F flyash).

  • Structural Fill/Embankments: Road construction, mine reclamation.

  • Agriculture: Soil amendment (lime content, nutrients).

[!TIP] Exam Focus: Dec 2024 paper had a full 8-mark question on this. Structure answer: 1) Coal consumption calc, 2) Total ash, 3) Split into flyash/bottom ash, 4) Apply recovery %, 5) List 3-4 utilizations.

Photochemical Smog

• Formation Mechanism:

  1. Emissions of NOx and VOCs from vehicles/industry.

  2. Sunlight (UV) photolyzes NO₂: $$\displaystyle NO_2 + h\nu \rightarrow NO + O $$.

  3. Free O reacts with O₂ to form Ozone (O₃).

  4. VOCs react with OH radicals → complex organic peroxy radicals → convert NO to NO₂ without consuming O₃, allowing O₃ to accumulate.

  5. Other products: PAN (Peroxyacetyl nitrate), aldehydes, particulates.

• Occurrence: "Los Angeles Type" smog. Sunny, warm, stagnant conditions. Basin topography (e.g., LA, Delhi, Beijing).

• Constituents: O₃ (major), PAN, NOx, VOCs, aldehydes, aerosols.

• Effects: Eye irritation, respiratory problems, plant damage, material degradation (rubber cracking).

Air Pollution Effects on Materials

• Corrosion: SO₂ and particulates accelerate corrosion of metals (steel, copper).

• Soiling: Deposition of soot and dust on buildings, monuments (loss of aesthetic value).

• Degradation:

  • Stone: Sulphate formation from SO₂ causes gypsum crusts → spalling (e.g., marble, limestone).

  • Metals: Tarnishing, pitting.

  • Paints & Polymers: Chalking, embrittlement, fading (UV + pollutants).

  • Textiles & Leather: Weakening, discoloration.

Air Pollution Control Methods (Brief)

• Adsorption by Activated Carbon: Physical adsorption of VOCs and odors on porous carbon surface. Used in air strippers, canisters. Regenerated by heating.

• Other Methods:

  • Gravitational Settling Chambers: Remove large particles (>50 µm).

  • Cyclones: Centrifugal force for medium particles (>10 µm).

  • Wet Scrubbers: Spray liquid to remove gases/particles (SO₂, acid gases).

  • Electrostatic Precipitators (ESP): Corona discharge charges particles, collected on plates. >99% efficiency for fine particles.

  • Fabric Filters (Baghouses): Fabric bags filter particles. High efficiency for PM.