Environmental Engg‑II (CE-702 (B)) - Unit 3 Short Notes

UNIT 3: WASTEWATER TREATMENT & AIR POLLUTION CONTROL

1.0 Fundamentals of Wastewater Treatment

Unit Operations and Processes

• Unit Operations: Physical processes where only physical forces are involved. No chemical or biological change occurs.

  • Examples: Screening, Grit removal, Sedimentation, Flotation, Filtration, Adsorption.

• Unit Processes: Chemical or biological processes that bring about a change in the wastewater quality.

  • Examples: Chemical coagulation, Disinfection, Biological oxidation (Activated sludge, Trickling filters).

Role of Micro-organisms in Biological Treatment

• Heterotrophic Bacteria: Decompose organic matter (BOD/COD) using organic carbon as energy source. Primary agents in carbonaceous BOD removal.

• Autotrophic Bacteria: Obtain energy from inorganic compounds (e.g., ammonia, nitrite). Crucial for nitrification (ammonia → nitrite → nitrate).

• Protozoa & Metazoa: Consume bacteria and suspended solids, improving effluent clarity and acting as indicators of sludge health.

• Process: Microorganisms form flocs (activated sludge) or biofilms (trickling filters) that can be separated from treated water.

2.0 Preliminary Treatment

Racks and Screens

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

• Types:

  • Coarse Screens: Bar spacing 50-150 mm.

  • Fine Screens: Bar spacing 6-25 mm.

  • Microscreens: Bar spacing < 6 mm.

• Head Loss through Bar Screens: Estimated by Kern's formula:

$$h_f = \frac{1}{2} \frac{b}{a} \frac{v^2}{2g} \left( \frac{\sin \theta}{\cos^2 \theta} \right) \left( \frac{n}{n - \frac{b}{a}} \right)^{4/3}$$

Where:

*   `a` = clear spacing between bars (m)

*   `b` = bar thickness (m)

*   `v` = approach velocity (m/s)

*   `θ` = angle of screen with horizontal

*   `n` = number of bars

> [!TIP] For exams, remember the simplified form: $$\displaystyle h_f = \beta \frac{v^2}{2g} $$, where $\beta$ is a loss coefficient depending on screen geometry.

Grit Chamber

• Purpose: Remove sand, gravel, cinders, and other inorganic solids (grit) to prevent abrasion and deposition in pipes and tanks.

• Design Principle: Based on horizontal flow velocity ($$\displaystyle V_h $$) and settling velocity ($$\displaystyle v_s $$) of target grit particles.

  • $$\displaystyle V_h $$ is kept low enough to allow grit to settle but high enough to prevent organic matter from settling.

  • Standard $$\displaystyle V_h $$: 0.15 - 0.3 m/s.

• Settling Velocity ($$\displaystyle v_s $$): For discrete particles, calculated by Stokes' Law (for laminar flow, Re < 1):

$$v_s = \frac{g (s-1) d^2}{18 \nu}$$

Where `s` = specific gravity, `d` = diameter (m), `ν` = kinematic viscosity (m²/s).

For turbulent flow, use Newton's or intermediate regime formulas.

• Rectangular Grit Chamber Design: Key parameters are length (L), width (W), and depth (D).

  • Detention time: 60-90 seconds.

  • $$\displaystyle L = V_h \times t $$ (where t is detention time).

  • $$\displaystyle W = Q / (D \times V_h) $$.

  • $D$ is chosen to keep $$\displaystyle V_h $$ within limits.

3.0 Primary Treatment

Sedimentation

• Purpose: Remove settleable organic and inorganic solids (primary sludge) and floating matter (scum) by gravity.

• Types of Settling:

  1. Discrete Settling: Particles settle as isolated entities (e.g., sand in grit chamber). Settling velocity constant.

  2. Flocculent Settling: Particles flocculate during settling, increasing mass and settling velocity (most common in primary tanks).

  3. Hindered Settling: High particle concentration creates a "blanket" where settling is hindered by upward flow of displaced water.

  4. Compression Settling: Under high load, sludge at the bottom is compressed by weight of overlying sludge.

Design of Rectangular Sedimentation Tank

• Key Parameters:

  • Surface Loading Rate (SLR): $$\displaystyle q_o = Q / A $$ (m³/m².day). Typical: 100-200 m³/m².day for plain sedimentation.

  • Detention Time (t): $$\displaystyle t = V / Q $$. Typical: 1.5-2.5 hours.

  • Length:Width Ratio: 3:1 to 5:1.

  • Depth: 2-4.5 m.

• Design Steps:

  1. Determine Q (peak flow).

  2. Select q_o and t.

  3. Calculate Area $$\displaystyle A = Q / q_o $$.

  4. Choose L:W ratio → find L and W.

  5. Calculate Volume $$\displaystyle V = A \times D $$ and check $$\displaystyle t = V/Q $$.

  6. Provide inlet/outlet devices to distribute flow and prevent short-circuiting.

Septic Tank vs Imhoff Tank

Chemical Coagulation (Alum Dosing)

• Purpose: Add coagulants (e.g., alum - $$\displaystyle Al_2(SO_4)_3.14H_2O $$) to destabilize colloidal particles and form larger flocs for removal in sedimentation/filtration.

• Alum Dose Calculation:

$$\text{Alum required (kg/day)} = \frac{\text{Dose (mg/L)} \times \text{Flow (ML/day)}}{1000}$$

> [!TIP] Example: Dose = 28.6 mg/L, Flow = 18 ML/day → Alum = $$\displaystyle (28.6 \times 18) / 1000 = 0.5148 $$ tonnes/day.

4.0 Secondary Treatment Processes

Trickling Filters

• Working Principle: Wastewater is sprayed over a packed bed of rocks/plastic media. A biological film (slime) grows on media. As wastewater trickles down, organic matter is oxidized by aerobic microorganisms in the film. Excess film sloughs off and is removed in a settling tank.

• Types:

  • Standard Rate: Low organic loading (40-100 g BOD/m².day), deeper (2-3 m), lower efficiency (80-85% BOD removal).

  • High Rate: High organic loading (100-400 g BOD/m².day), shallower (1-2 m), requires recirculation, higher efficiency.

• Design Parameters:

  • Surface Loading Rate (Organic Loading): $$\displaystyle L_s = \frac{Q \times L_0}{A} $$ (g BOD/m².day), where $$\displaystyle L_0 $$ = influent BOD (mg/L).

  • Depth: 0.6-2.4 m (high rate to standard).

• Efficiency Determination (NRC Formula):

$$E = 100 \left[ 1 - \frac{1}{1 + k_d \left( \frac{V}{Q} \right)} \right]$$

Where:

*   `E` = % BOD removal

*   `k_d` = filter rate constant (day⁻¹), depends on temperature and filter type.

*   `V/Q` = hydraulic detention time in filter (days).

Activated Sludge Process

• Description: Wastewater is aerated in an aeration tank with a dense suspension of microorganisms (activated sludge). The mixture flows to a secondary clarifier where sludge is settled. Part of the sludge is recycled (Return Activated Sludge - RAS) to maintain biomass; excess is wasted (Waste Activated Sludge - WAS).

• Flow Diagram Sketch: [Influent] → [Aeration Tank (Air diffusers)] → [Secondary Clarifier] → [Effluent] & [RAS] → Aeration Tank, [WAS] → Sludge Handling.

• Oxygen Requirements Calculation:

  • Theoretical Oxygen Demand: For BOD oxidation: $$\displaystyle CH_2O + O_2 \rightarrow CO_2 + H_2O $$. 1 mg BOD ≈ 1 mg O₂.

  • Practical Requirement: $$\displaystyle O_2 $$ required (kg/day) = $$\displaystyle Q \times (L_0 - L_e) \times \text{Factor} $$.

  • Factor accounts for endogenous respiration, nitrification (if occurring), and inefficiencies. Typical: 1.3 - 1.5 for carbonaceous BOD removal.

  • Air Supply: $$\displaystyle Air (m³/min) = \frac{O_2 \text{ required (kg/day)} \times 1000}{0.21 \times 60 \times 24 \times \rho_{air}} $$, where $$\displaystyle \rho_{air} \approx 1.2 $$ kg/m³.

Oxidation Ditch

• Working: A modified extended aeration activated sludge system. It's an oval-shaped, continuous-flow channel (ditch) equipped with surface aerators (rotors or brushes) that provide both aeration and circulation. It operates at very long SRT (> 20 days), promoting complete oxidation and nitrification.

• Sketch:

  DiagramSEARCH: oxidation ditch configuration with rotor and clarifier

• Configurations:

  • Single Ditch: One oval channel with one or more aerators.

  • Tandem Ditch: Two or more ditches in series.

  • Parallel Ditch: Multiple ditches operating in parallel for redundancy.

  • Carrousel System: A branded type with multiple rotor positions.

5.0 Advanced/Tertiary Treatment

Nitrogen Removal

• Biological Nitrification-Denitrification:

  1. Nitrification (Aerobic): $$\displaystyle NH_4^+ + 1.5O_2 \rightarrow NO_2^- + 2H^+ + H_2O $$ (by Nitrosomonas); $$\displaystyle NO_2^- + 0.5O_2 \rightarrow NO_3^- $$ (by Nitrobacter).

  2. Denitrification (Anoxic): $$\displaystyle NO_3^- \rightarrow N_2 $$ (gas) by heterotrophic bacteria using organic carbon as electron donor. Requires anoxic zone (DO ≈ 0 mg/L).

  • Process Configurations: Bardenpho, Modified Ludzack-Ettinger (MLE), SBR, Oxidation Ditch with anoxic zones.

• Ammonia Stripping:

  • Principle: Raise pH (> 10.5) using lime to convert ammonium ($$\displaystyle NH_4^+ $$) to free ammonia ($$\displaystyle NH_3 $$). Air is blown counter-current through the wastewater in a stripping tower, volatilizing $$\displaystyle NH_3 $$. The air is then absorbed in an acid (e.g., $$\displaystyle H_2SO_4 $$) to recover ammonia as ammonium sulfate.

  • Limitation: Scaling/fouling, high chemical cost, sensitivity to temperature.

Advanced Wastewater Treatment (AWT)

• Definition: Treatment beyond secondary (biological) level to remove nutrients (N, P), non-biodegradable organics, pathogens, and total dissolved solids (TDS) to very high standards for reuse or sensitive discharge.

• Comparison with Conventional Treatment:

  | Feature | Conventional (Primary + Secondary) | Advanced (Tertiary) | | :--- | :--- | :--- | | Goal | BOD, SS removal to ~30 mg/L. | N, P, TDS, pathogens to < 5-10 mg/L. | | Processes | Screening, Sedimentation, Biological (AS/TF). | Chemical precipitation, Filtration, Membrane, Disinfection, Adsorption. | | Cost | Moderate. | High (capital & O&M). | | Reuse | Limited (irrigation). | High (industrial, potable, groundwater recharge). |

• Important AWT Processes (Tabulation):

  | Process | Purpose | Key Mechanism | | :--- | :--- | :--- | | Chemical Precipitation | Phosphorus removal | Addition of alum/ferric chloride/lime to form insoluble phosphates. | | Sand/Multimedia Filtration | Remove residual SS | Physical straining. | | Activated Carbon Adsorption | Remove organics, taste/odor | Adsorption onto porous carbon. | | Membrane Filtration | Remove TDS, colloids, pathogens | Ultrafiltration (UF), Reverse Osmosis (RO). | | Disinfection | Pathogen kill | Chlorination, Ozonation, UV. | | Ion Exchange | Remove specific ions (e.g., $$\displaystyle Na^+ $$, $$\displaystyle Cl^- $$) | Exchange with resin ions. |

Filtration Methods

• Diatomaceous Earth Filter:

  • Medium: Filter aid made from fossilized diatoms (siliceous). Forms a thin precoat on a septum.

  • Operation: Body feed of DE slurry added during filtration to maintain precoat thickness. Removes very fine particles (1-3 µm).

  • Application: Polishing filter after coagulation/filtration for high-quality effluent.

• Ultrafiltration (UF):

  • Mechanism: Membrane filtration with pore size 0.01-0.1 µm. Removes colloids, macromolecules, bacteria, and viruses. Operates on size exclusion.

  • Driving Force: Pressure (1-10 bar).

  • Advantages: No chemical addition, compact, high removal efficiency.

  • Limitations: Membrane fouling, high capital cost, concentrates (retentate) require disposal.

6.0 Air Pollution Fundamentals

Air Pollutants

• Classification & Characteristics:

  | Pollutant | Primary Sources | Key Characteristics & Health Effects | | :--- | :--- | :--- | | Sulphur Dioxide (SO₂) | Fossil fuel combustion (coal/oil), smelting. | Colorless, pungent, soluble in water → acid rain. Irritates respiratory tract, aggravates asthma. | | Hydrogen Sulphide (H₂S) | Anaerobic decomposition (sewage, swamps), petroleum refining. | Colorless, rotten egg smell, toxic (olfactory fatigue). Respiratory paralysis at high conc. | | Particulate Matter (PM) | Combustion, construction, agriculture. RSPM (Respirable, <10µm) vs TSPM (Total Suspended, <100µm). | RSPM penetrates deep lungs (alveoli), causes bronchitis, asthma, lung cancer. TSPM includes larger particles (soot, dust). |

• Primary vs Secondary Pollutants:

  • Primary: Emitted directly from source (e.g., $$\displaystyle SO_2 $$, $$\displaystyle NO_x $$, CO, PM).

  • Secondary: Formed in atmosphere by chemical reactions of primary pollutants (e.g., $$\displaystyle O_3 $$, $$\displaystyle H_2SO_4 $$, $$\displaystyle HNO_3 $$, $PAN$).

• Aerosols: Solid or liquid particles suspended in gas (air). Types:

  • Dust: >1 µm, settles quickly.

  • Fume: <1 µm, formed by condensation.

  • Fog/Mist: Liquid droplets.

  • Smoke: From incomplete combustion.

  • Smog: Mixture (e.g., photochemical smog).

Meteorological Factors

• Temperature Lapse Rate (ELR vs ALR):

  • Environmental Lapse Rate (ELR): Actual rate of temperature decrease with height in the atmosphere. Varies with time/place (average ~6.5°C/km).

  • Adiabatic Lapse Rate (ALR): Temperature change of a parcel of air rising/falling without heat exchange. Dry ALR = 9.8°C/km. Saturated ALR ≈ 5-6°C/km (due to latent heat release).

  • Stability: If ELR < ALR → stable (inversion, poor dispersion). If ELR > ALR → unstable (good dispersion).

• Factors Influencing Dispersion:

  1. Wind speed & direction.

  2. Atmospheric stability (lapse rate).

  3. Mixing height (depth of turbulent layer).

  4. Topography (valleys trap pollutants).

  5. Precipitation (scavenges pollutants).

Air Pollution Dispersion Models

• Gaussian Plume Model:

  • Assumptions: Steady-state, constant emissions, flat terrain, no deposition/chemical reaction, wind speed constant with height.

  • Equation for Ground-Level Concentration ($C$) at downwind distance x:

$$C(x,y,z=0) = \frac{Q}{2\pi \sigma_y \sigma_z U} \exp\left(-\frac{y^2}{2\sigma_y^2}\right) \left[ \exp\left(-\frac{H^2}{2\sigma_z^2}\right) + \exp\left(-\frac{H^2}{2\sigma_z^2}\right) \right]$$

Where:

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

*   `U` = wind speed (m/s)

*   `H` = effective stack height (m)

*   `σ_y`, `σ_z` = dispersion parameters (depend on downwind distance `x` and atmospheric stability).

*   `y` = lateral distance from plume centerline.

*   `z` = vertical height.

*   For **ground-level source (H=0)**, equation simplifies.

• Location of Maximum Concentration: For a ground-level source, maximum occurs directly downwind at x where $$\displaystyle \sigma_z $$ is smallest (initially). For an elevated stack, maximum ground-level concentration occurs at a downwind distance $$\displaystyle x_{max} \approx H / \tan \theta $$, where $\theta$ is the vertical spread angle. The maximum concentration itself is:

$$C_{max} \approx \frac{2Q}{\pi H^2 U} \cdot \frac{\sigma_y}{\sigma_z} \text{ (at } y=0\text{)}$$

Specific Air Pollution Phenomena

• Photochemical Smog:

  • Formation: In sunny, stagnant conditions, $$\displaystyle NO_x $$ and VOCs (from vehicles/solvents) react in presence of sunlight to form ozone ($$\displaystyle O_3 $$), PANs, aldehydes, etc.

  • Characteristics: Brownish haze, eye irritation, plant damage, rubber cracking. Typical in Los Angeles-type basins.

Industrial Emission Estimation

• Power Plant Calculations (Flyash, Bottom Ash, SO₂):

  1. Coal Consumption Rate: $$\displaystyle M_c = \frac{\text{Plant Output (MW)} \times 24 \times 3600 \times 10^6}{\eta \times \text{CV (MJ/kg)} \times 10^6} $$ kg/day.

     • η = overall efficiency (decimal).

     • CV = calorific value (MJ/kg).

  2. Ash Content: $$\displaystyle M_{ash} = M_c \times \frac{\text{% Ash}}{100} $$ kg/day.

  3. Flyash & Bottom Ash: Given % flyash in total ash. $$\displaystyle M_{flyash} = M_{ash} \times \frac{\text{% Flyash}}{100} $$. $$\displaystyle M_{bottom} = M_{ash} - M_{flyash} $$.

  4. Recoverable Ash: Apply recovery % to flyash and bottom ash separately.

  5. SO₂ Emission: $$\displaystyle M_{SO2} = 2 \times M_c \times \frac{\text{% S}}{100} \times \frac{32}{32} $$ (since S → SO₂, molecular weight ratio ≈ 2).

     • More precisely: $$\displaystyle M_{SO2} = 2 \times \frac{32}{32} \times M_c \times (\%S/100) $$.

Air Pollution Control Methods

• Adsorption by Activated Carbon:

  • Mechanism: Organic vapors/gases adhere to porous surface of activated carbon via van der Waals forces.

  • Process: Fixed bed adsorber. Carbon can be regenerated by steam/heat.

  • Application: Control of VOCs, odours, $$\displaystyle H_2S $$, $$\displaystyle SO_2 $$ (with impregnation).

• Other Methods:

  • Stripping: Mass transfer from liquid to gas phase (e.g., ammonia stripping from wastewater).

  • Filtration: Particulate control (baghouse, ESP, cyclone).

7.0 High-Frequency Short Note Topics

Photochemical Smog

• Definition: A type of smog formed by the reaction of sunlight with nitrogen oxides ($$\displaystyle NO_x $$) and volatile organic compounds (VOCs).

• Formation: $$\displaystyle NO_2 \xrightarrow{h\nu} NO + O $$; $$\displaystyle O + O_2 \rightarrow O_3 $$; $$\displaystyle O_3 + VOC \rightarrow PANs $$, aldehydes, etc.

• Characteristics: Brownish haze, high oxidant content ($$\displaystyle O_3 $$, PAN), occurs in sunny, warm afternoons, causes eye/respiratory irritation, plant damage. Typical of Los Angeles.

Temperature Lapse Rate (ELR vs ALR)

Gaussian Plume Model

• Purpose: Predict pollutant concentration downwind of a continuous point source (stack).

• Core Equation (for elevated stack, ground-level receptor):

$$C(x,y,z=0) = \frac{Q}{2\pi \sigma_y \sigma_z U} \exp\left(-\frac{y^2}{2\sigma_y^2}\right) \left[ \exp\left(-\frac{(H)^2}{2\sigma_z^2}\right) + \exp\left(-\frac{(H)^2}{2\sigma_z^2}\right) \right]$$

*   `Q`: emission rate (g/s)

*   `U`: wind speed (m/s)

*   `H`: effective stack height (physical height + plume rise)

*   `σ_y`, `σ_z`: lateral & vertical dispersion coefficients (from Pasquill-Gifford curves, depend on `x` and stability class).

• Location of Max Concentration: For a given x, max occurs at y=0 (plume centerline). The downwind distance of peak ground-level concentration for an elevated stack is approximately $$\displaystyle x_{max} \approx H / \tan \theta $$, where $\theta$ relates to $$\displaystyle \sigma_y/\sigma_z $$ ratio.

Diatomaceous Earth Filter

• Medium: Filter aid composed of fossilized diatom skeletons (silica).

• Operation: A precoat is formed on a filter septum (cloth/mesh). During filtration, a slurry of DE (body feed) is added continuously to replenish the precoat and prevent clogging.

• Pore Size: Very fine (1-3 µm). Removes particles much smaller than sand filters.

• Application: Polishing filter for high-purity water/wastewater effluent, often after coagulation/sedimentation. Common in swimming pools and pharmaceutical industries.

Ultrafiltration (UF)

• Mechanism: Pressure-driven membrane separation with pore size 0.01-0.1 µm.

• Removes: Colloids, macromolecules, bacteria, viruses, some endotoxins. Does not remove dissolved ions/salts (unlike RO).

• Driving Force: Hydraulic pressure (1-10 bar).

• Configuration: Spiral wound, hollow fiber, tubular.

• Advantages: No chemical addition, compact, high and consistent quality.

• Limitations: Membrane fouling/scaling, high energy cost, concentrate disposal.

Ammonia Stripping Method

• Principle: Convert soluble ammonium ion ($$\displaystyle NH_4^+ $$) to volatile ammonia gas ($$\displaystyle NH_3 $$) by raising pH > 10.5 (using lime), then strip with air.

• Process: Air is blown counter-current through a packed tower (stripper) containing high-pH wastewater. $$\displaystyle NH_3 $$ is transferred to air stream. The air is then passed through an acid (e.g., $$\displaystyle H_2SO_4 $$) absorber to recover ammonia as ammonium sulfate fertilizer.

• Key Issues: Scaling/fouling from carbonate precipitation, high lime cost, temperature sensitivity (stripping efficiency drops in winter), odour control needed.

RSPM vs TSPM

Factors Affecting Air Pollution Dispersion

1. Wind Speed & Direction: Higher speed → faster dilution. Direction determines impact zone.

2. Atmospheric Stability: Determined by lapse rate. Stable (inversion) → poor vertical mixing, high concentrations. Unstable → good mixing.

3. Mixing Height: Height of the turbulent boundary layer. Low mixing height (night, winter) traps pollutants near ground.

4. Topography: Valleys, basins trap pollutants (e.g., Los Angeles, Delhi). Hills cause downwash.

5. Precipitation: Rain/snow efficiently scavenges particles and soluble gases ("wet deposition").

6. Surface Roughness: Urban areas (buildings) increase turbulence, enhance dispersion but create recirculation zones.

Primary vs Secondary Air Pollutants

Effect of Air Pollution on Materials

• Corrosion: $$\displaystyle SO_2 $$, $$\displaystyle NO_x $$, acid rain accelerate corrosion of metals (steel, copper), limestone, marble (e.g., historical monuments).

• Soiling & Discoloration: Deposition of soot, dust, and aerosols on buildings, fabrics, paintings.

• Degradation: Ozone and other oxidants cause cracking of rubber, fading of dyes, deterioration of paints and plastics.

• Erosion: Acidic deposition leaches protective layers from stone and concrete.

• Economic Impact: Increased maintenance cost, loss of cultural heritage, reduced lifespan of materials.

Adsorption by Activated Carbon

• Mechanism: Physical adsorption via van der Waals forces on highly porous carbon (surface area 500-1500 m²/g).

• Factors Affecting Capacity:

  • Nature of adsorbate (polarity, molecular size).

  • Temperature (decreases with increase).

  • Carbon properties (surface area, pore size distribution).

• Process: Fixed-bed column. Breakthrough curve indicates when carbon needs regeneration.

• Regeneration: Thermal (steam/heat), chemical, or vacuum. Not 100% effective; carbon loses mass each cycle.

• Applications: Removal of VOCs, odours, taste/odor compounds, $$\displaystyle H_2S $$, residual chlorine, pesticides from air and water. Used in water treatment, air scrubbers, respirators.