Environmental Engg‑II (CE-702 (B)) - Unit 2 Short Notes

UNIT 2: WASTEWATER TREATMENT PROCESSES & UNIT OPERATIONS

A. Preliminary & Primary Treatment

1. Screening & Racks

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

• Types:

  • Coarse screens: Bar spacing 50-150 mm.

  • Fine screens: Bar spacing 6-50 mm.

  • Micro-screens: Bar spacing < 6 mm (used for tertiary treatment).

• Design Considerations:

  • Approach velocity: 0.6-1.0 m/s (coarse), 0.6-1.5 m/s (fine) to prevent debris settling or forcing through.

  • Bar spacing: Selected based on downstream process requirements.

  • Screen opening area: Should be 1.5-2 times the gross area of the channel to allow for peak flows.

• Head Loss Through Bar Screens:

$$h_f = \frac{1}{2} \beta \left( \frac{b}{s} \right)^{4/3} \frac{V^2}{2g} \sin^2 \theta$$

Where:

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

*   $\beta$ = shape factor (0.7-1.0, depends on bar profile)

*   $b$ = bar thickness (m)

*   $s$ = clear spacing between bars (m)

*   $V$ = velocity of flow through screen openings (m/s)

*   $\theta$ = angle of inclination from horizontal (°)

> [!TIP] Head loss increases rapidly with reduced bar spacing ($s$). Regular cleaning is critical to avoid excessive losses.

2. Grit Removal

• Purpose: Remove sand, gravel, cinders, and other inorganic particles (specific gravity ~2.65) to prevent abrasion, deposition in pipes, and accumulation in digesters.

• Types of Grit Chambers:

  • Horizontal flow: Simple, rectangular; velocity controlled by weir or orifice.

  • Aerated: Creates a spiral flow; separates lighter organic solids from heavier grit.

  • Vortex (mechanical/hydraulic): High efficiency, compact.

• Design of Rectangular Grit Chamber:

  • Settling velocity ($$\displaystyle V_s $$): Determined by particle size and specific gravity using Stokes' law (for laminar flow, Re<1).

  • Flow velocity ($V$): Set to keep grit in suspension but allow settling. Typically 0.15-0.3 m/s.

  • Detention time ($t$): 60-90 seconds for horizontal flow chambers.

  • Channel dimensions: Based on $$\displaystyle Q = A \times V $$, where $$\displaystyle A = B \times D $$ (width × depth). Length $$\displaystyle L = V \times t $$.

  [!TIP] Grit characteristics vary; design $$\displaystyle V_s $$ range (e.g., 0.016-0.022 m/s) is often provided, not a single value.

3. Primary Sedimentation (Primary Clarifiers)

• Purpose: Remove settleable organic and inorganic solids (primary sludge) and floating material (scum). Reduces load on secondary treatment.

• Removal Efficiencies: ~50-70% Suspended Solids (SS), ~25-40% BOD.

• Design of Rectangular Tank:

  • Detention period ($t$): 1.5-2.5 hours (for plain settling).

  • Overflow rate ($O.R.$): $$\displaystyle O.R. = \frac{Q}{A_l} $$ (m³/m²-day). Typical: 30-50 m³/m²-day.

  • Weir loading rate: $$\displaystyle Q / L_w $$ (m³/m-day). Typical < 125 m³/m-day to prevent short-circuiting.

  • Dimensions: Length:Width ratio = 3:1 to 5:1. Depth: 2-5 m.

• Types of Settling:

  • Discrete: Particles settle as individual units (e.g., sand in grit chamber). Settling velocity constant.

  • Flocculent: Particles agglomerate during settling, increasing $$\displaystyle V_s $$ (e.g., primary sedimentation).

  • Hindered/Zone: High concentration, particles settle as a mass (interface). Occurs in secondary clarifiers.

  • Compression: Particles in sludge layer compress under weight (sludge thickening).

• Septic Tank vs. Imhoff Tank:

B. Secondary (Biological) Treatment

1. Activated Sludge Process (ASP)

• Principle: Aerobic microorganisms (activated sludge) consume organic matter. Mixture is settled; sludge is recycled.

• Components: Aeration tank, secondary clarifier, sludge recycle system.

• Flow Diagram (Configurations):

  • Plug Flow: Longitudinal flow, concentration gradient.

  • Complete Mix: Uniform concentration (stirred tank).

  [!DIAGRAM]

  DiagramCANVAS: Sketch showing flow diagram of ASP with aeration tank, clarifier, sludge recycle, and waste sludge lines. Label components. Show plug flow (long rectangular tank) and complete mix (circular/ square tank) configurations.

• Key Parameters:

  • F/M Ratio (Food to Microorganism): $$\displaystyle F/M = \frac{Q \times L_0}{V \times X} $$ (kg BOD/kg MLSS-day). Controls process stability.

  • SVI (Sludge Volume Index): $$\displaystyle SVI = \frac{Settled \ Sludge \ Volume \ (mL/L)}{MLSS \ (g/L)} $$ (mL/g). Indicates sludge settleability. Ideal: 50-150.

  • MLSS (Mixed Liquor Suspended Solids): Concentration of solids in aeration tank (mg/L).

  • MCRT (Mean Cell Residence Time): $$\displaystyle MCRT = \frac{V \times X}{(Q_w \times X_w) + (Q_e \times X_e)} $$ (days). Equivalent to SRT.

• Oxygen Requirements:

$$O_2 \ required = (BOD_5 \ removed \times 1.47) + (Nitrifed \ NH_3-N \times 4.57) + (Endogenous \ Respiration)$$

*   Actual air needed = $$\displaystyle O_2 $$ required / (Oxygen transfer efficiency × 0.21 × density of air).

2. Trickling Filters

• Principle: Wastewater distributed over a packed bed (rocks, plastic media). Microbial film (zoogleal slime) oxidizes organic matter.

• Types (by Organic Loading):

  • Standard rate: 0.08-0.32 kg BOD/m³-day.

  • High rate: 0.32-1.0 kg BOD/m³-day.

  • Super high rate: > 1.0 kg BOD/m³-day.

• Design Calculations:

  • Surface loading rate ($$\displaystyle q_s $$): $$\displaystyle q_s = \frac{Q}{A} $$ (L/m²-day or m³/m²-day).

  • Organic loading ($$\displaystyle L_o $$): $$\displaystyle L_o = \frac{Q \times L_0}{V} $$ (g BOD/m³-day).

  • Depth ($D$): 0.6-2.0 m (standard), up to 10 m (deep tower filters).

  • Volume ($V$): $$\displaystyle V = A \times D $$.

  • Diameter ($$\displaystyle D_f $$): $$\displaystyle A = \frac{\pi D_f^2}{4} $$.

• NRC Formula for Efficiency:

$$E = \frac{100}{1 + 0.0088 \sqrt[3]{L_o}}$$

Where $E$ = % BOD removal, $$\displaystyle L_o $$ = organic loading (kg/ha-day). **Use consistent units.**

> [!TIP] NRC formula is empirical; ensure units match (kg/ha-day). For $$\displaystyle L_o $$ in g/m²-day, use $$\displaystyle E = \frac{100}{1 + 0.44 \sqrt[3]{L_o}} $$ (approx).

3. Oxidation Ditches

• Principle: Modified ASP with oval-shaped channel and surface aeration rotors. Operates in extended aeration mode (high SRT).

• Working (with sketch):

  DiagramCANVAS: Neat sketch of oxidation ditch. Show oval channel, multiple rotor/aerator units positioned along the channel, flow direction arrows, influent and effluent points, and return sludge line.

• Configurations:

  • Carousel: Single oval channel with multiple rotors.

  • Pasveer: Deep, narrow channel with single long rotor.

  • Biodisc: Rotating disc contactors (less common).

• Advantages: Simplicity, robustness, good nitrification/denitrification potential, less sludge production.

4. Biological Treatment Fundamentals

• Importance of Micro-organisms:

  • Bacteria: Form flocs; primary degraders (heterotrophs for BOD, autotrophs for nitrification).

  • Protozoa: Consume free bacteria, improve effluent clarity.

  • Metazoa (rotifers, worms): Consume floc particles, indicate healthy sludge.

• Kinetics (Monod):

$$r = \frac{k_{max} \cdot S}{K_s + S}$$

Where $r$ = substrate utilization rate, $S$ = substrate concentration, $$\displaystyle k_{max} $$ = max rate, $$\displaystyle K_s $$ = half-velocity constant.

C. Advanced & Tertiary Treatment

1. Nutrient Removal (Nitrogen & Phosphorus)

• Need for Nitrogen Removal: To prevent eutrophication (algal blooms) in receiving water bodies.

• Biological Nitrification-Denitrification:

  • Nitrification (Aerobic): $$\displaystyle NH_4^+ + 2O_2 \rightarrow NO_3^- + H_2O + 2H^+ $$ (by Nitrosomonas, Nitrobacter).

  • Denitrification (Anoxic): $$\displaystyle NO_3^- \rightarrow N_2 $$ (by heterotrophs using organic carbon as electron donor).

  • Process: Requires separate anoxic zone (no DO, NO3 present) followed by oxic zone (DO > 2 mg/L). Carbon source needed for denitrification (may require methanol addition if influent C/N low).

• Other Methods:

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

  • Breakpoint chlorination: Chlorine dose > Cl:NH3 ratio 7.6:1.

  • Ion exchange: Selective resin.

2. Advanced Wastewater Treatment (AWT)

• Definition & Difference: Treatment beyond conventional secondary to meet specific reuse/discharge standards (e.g., potable reuse, sensitive ecosystems). Conventional: BOD/SS removal. AWT: Targeted removal of nutrients, dissolved solids, pathogens, emerging contaminants.

• Important AWT Processes (Tabular):

• Ultrafiltration (Detailed):

  • Principle: Pressure-driven membrane process. Pore size 0.01-0.1 µm removes macromolecules, colloids, bacteria, some viruses.

  • Membrane: Polymeric (PSF, PES) or ceramic. Modules: hollow fiber, spiral wound, tubular.

  • Applications: Tertiary treatment, pretreatment to RO, water reuse.

  • Advantages: High removal efficiency, compact, no chemicals (for separation), consistent effluent quality.

  • Disadvantages: Membrane fouling, high capital/operational cost, concentrate disposal.

3. Disinfection

• Methods:

  • Chlorination: Most common. Forms hypochlorous acid (HOCl). CT concept: $C \times T$ (concentration × contact time) determines kill rate.

  • Chlorination-Dechlorination: Chlorine followed by sulfur dioxide or bisulfite to remove residual chlorine toxic to aquatic life.

  • UV Radiation: Damages microbial DNA. No residual, no chemical by-products.

  • Ozone: Strong oxidant. Very effective, but no residual, high cost.

• Chlorine Demand: Amount of chlorine consumed by reactions with organics/inorganics before free chlorine residual appears.

UNIT 2: AIR POLLUTION CONTROL & FUNDAMENTALS

A. Air Pollutants & Sources

1. Sources & Characteristics of Specific Pollutants

• Sulphur Dioxide (SO₂):

  • Sources: Fossil fuel combustion (power plants, industries), smelting of sulfide ores.

  • Characteristics: Colorless, pungent odor, soluble in water → forms sulfurous/sulfuric acid (acid rain).

• Hydrogen Sulphide (H₂S):

  • Sources: Sewage treatment plants, petroleum refining, natural gas processing, anaerobic decomposition.

  • Characteristics: Colorless, rotten egg odor (detectable at very low ppm), toxic, flammable.

2. Major Air Pollutants (Criteria Pollutants) - Tabular

3. Aerosols

• Definition: System of solid or liquid particles suspended in a gas (air). Size range: 0.001 µm to 100 µm.

• Types & Characteristics:

B. Meteorology & Dispersion

1. Meteorological Factors Influencing Dispersion

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

• Temperature: Affects buoyancy of plume and stability.

• Atmospheric Stability: Determines vertical mixing. Classes: A (very unstable) to F (very stable). Stable → poor dispersion.

• Mixing Height: Height of the mixed layer (top of plume can rise). Higher mixing height → greater volume for dilution.

• Humidity & Precipitation: Can cause plume scavenging (rainout, washout).

2. Temperature Lapse Rate

• Environmental Lapse Rate (ELR): Actual vertical temperature profile in atmosphere. Varies with time/location.

• Adiabatic Lapse Rate (ALR): Temperature change of a parcel of air moving vertically without heat exchange.

  • Dry ALR ($$\displaystyle \Gamma_d $$): ~9.8°C/km (for unsaturated air).

  • Moist ALR ($$\displaystyle \Gamma_m $$): ~4-9°C/km (for saturated air; latent heat release reduces cooling).

• Stability: If ELR < ALR → atmosphere is stable (inversion possible). If ELR > ALR → unstable.

3. Gaussian Plume Model

• Equation (for continuous point source, steady state, flat terrain):

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

Where:

*   $C$ = concentration (g/m³)

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

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

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

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

*   $z$ = receptor height (m)

• Location of Max Pollution: On plume centerline ($$\displaystyle y=0 $$, $$\displaystyle z=0 $$ ground level), but maximum ground-level concentration occurs at some downwind distance.

• Assumptions: Steady state, constant wind, no deposition/chemical reaction, flat terrain, point source.

• Parameters: $$\displaystyle \sigma_y, \sigma_z $$ from Pasquill-Gifford curves/tables based on $x$ and stability class (A-F).

C. Air Pollution from Specific Sources & Control

1. Power Plant Emissions Estimation (Coal-Fired)

• Given: Plant capacity (MW), efficiency ($\eta$), coal properties (CV, ash%, S%), recovery factors.

• Steps:

  1. Coal consumption rate: $$\displaystyle \dot{m}_c = \frac{Plant \ Output \ (MW) \times 3.6 \times 10^6 \ MJ/MW-h}{\eta \times CV \ (MJ/kg)} $$ (kg/h or kg/day).

  2. Fly ash & Bottom ash: Total ash = $$\displaystyle \dot{m}_c \times Ash\% $$. Fly ash = Total ash × % discharged as flyash (often 60-80%). Bottom ash = Total ash - Fly ash.

  3. Particulate Matter (PM): Similar to ash calculation, but use PM% in coal or ESP efficiency.

  4. SO₂: $$\displaystyle \dot{m}_{SO_2} = 2 \times \dot{m}_c \times S\% \times \frac{32}{32} $$ (since 1 kg S → 2 kg SO₂). Or $$\displaystyle \dot{m}_{SO_2} = \dot{m}_c \times S\% \times 2 $$.

• Eco-friendly Ash Utilization:

  • Cement & concrete (partial replacement for cement/clinker).

  • Fly ash bricks/blocks.

  • Structural fills/embankments.

  • Mine reclamation (filling).

  • Agriculture (soil amendment – caution for heavy metals).

2. Control Equipment (Brief)

• Cyclones: Inertial separation of coarse particles (>10 µm).

• Electrostatic Precipitator (ESP): Charges particles, collects on plates. High efficiency for fine particles.

• Baghouses/Fabric Filters: Fabric bags filter particles. Very high efficiency.

• Scrubbers: Wet/dry spray to remove gases (SO₂) and particles.

D. Effects & Miscellaneous

1. Smog

• Photochemical Smog:

  • Formation: NO₂ + sunlight → NO + O; O + O₂ → O₃; O₃ + VOCs → peroxyacetyl nitrate (PAN), aldehydes.

  • Characteristics: Brownish haze, eye irritation, plant damage, rubber cracking. Occurrence: Sunny, warm, stagnant conditions (Los Angeles type).

• Industrial (London-type) Smog:

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

  • Characteristics: Grayish, sulfurous odor, severe respiratory distress. Occurrence: Cold, damp, coal burning (winter).

2. Particulate Matter

• TSPM (Total Suspended Particulate Matter): All particles up to ~50-100 µm. Includes coarse and fine fractions.

• RSPM (Respirable Suspended Particulate Matter) / PM₁₀: Particles ≤ 10 µm aerodynamic diameter. Penetrate to thoracic region.

• PM₂.₅: Fine particles ≤ 2.5 µm. Penetrate to alveoli, enter bloodstream.

• Health Impact: RSPM/PM₂.₅ are more harmful than larger TSPM due to deeper lung penetration. Standards (e.g., NAAQS in India) specify limits for PM₁₀ and PM₂.₅.

UNIT 2: SHORT NOTE TOPICS (Frequently Recurring)

From Wastewater Treatment:

• Diatomaceous Earth Filter: Depth filter using fossilized diatomaceous earth (porous, fine). Used for tertiary polishing to remove very fine colloids, algae, and bacteria. Requires pre-coating and body feed. High effluent quality but high operating cost.

• Ammonia Stripping: Air stripping of ammonia from wastewater. Requires high pH (>10.5) to convert NH₄⁺ to NH₃ gas. Air is blown through packed tower. Limitations: scaling, air pollution (NH₃), sensitivity to temperature.

• Oxidation Ditch Configurations: See Section B.1.3 above. Emphasize Carousel (multiple rotors, oval), Pasveer (deep, single rotor), and operation as extended aeration ASP.

• Septic Tank vs. Imhoff Tank: See comparison table in Section A.3.

From Air Pollution:

• Photochemical Smog: See Section D.1. Focus on photochemical reactions (NOx, VOCs, sunlight), key pollutants (O₃, PAN), and LA-type conditions.

• Temperature Lapse Rate: See Section B.2. Distinguish ELR (observed) vs. ALR (theoretical, dry/moist). Link to stability.

• Gaussian Plume Model: See Section B.3. Write equation, define all terms ($$\displaystyle \sigma_y, \sigma_z, H, U $$), state assumptions, and mention use of Pasquill stability classes.

• Adsorption by Activated Carbon:

  • Principle: Physical/chemical adherence of molecules to porous carbon surface.

  • Isotherms: Freundlich, Langmuir (describe equilibrium).

  • Use: Air (VOCs, odour control) and water (organics, taste/odor). Regeneration by steam/heat.

• Factors Affecting Dispersion: Wind speed/direction, atmospheric stability (lapse rate), mixing height, topography (valleys trap pollution), humidity/precipitation.

• Primary vs. Secondary Pollutants:

  • Primary: Emitted directly from source (e.g., SO₂, CO, PM, NO).

  • Secondary: Formed in atmosphere via chemical reactions (e.g., O₃, H₂SO₄ mist, PAN, sulfate/nitrate PM).

• Effect of Air Pollution on Materials:

  • Corrosion: SO₂, acid deposition on metals.

  • Soot/Deposition: On buildings, monuments (soiling).

  • Degradation: Ozone cracks rubber, fades dyes; acid rain erodes stone (limestone, marble).

UNIT 2: DESIGN & NUMERICAL PROBLEMS (High Weightage)

A. Wastewater Treatment Design

1. Rectangular Sedimentation Tank Design

• Given: Flow $Q$ (m³/d or L/s), detention time $t$ (hrs), overflow rate $O.R.$ (m³/m²-d), weir loading.

• Steps:

  1. Volume $$\displaystyle V = Q \times t $$ (use consistent time units).

  2. Surface area $$\displaystyle A_l = Q / O.R. $$ (ensure $Q$ in m³/d if $O.R.$ in m³/m²-d).

  3. From $$\displaystyle A_l = L \times B $$, choose $L:B$ ratio (3:1 to 5:1). Calculate $L$ and $B$.

  4. Depth $$\displaystyle D = V / A_l $$.

  5. Check weir loading: $$\displaystyle Q / L_w $$ (where $$\displaystyle L_w $$ = total weir length ≈ 0.8L to 0.9L). Should be < 125 m³/m-d.

2. Grit Chamber Design (Rectangular)

• Given: $Q$, particle size $d$, specific gravity $SG$, settling velocity range $$\displaystyle V_s $$ (min, max), flow velocity $V$ (set).

• Steps:

  1. Channel cross-sectional area $$\displaystyle A_c = Q / V $$ (ensure $Q$ in m³/s, $V$ in m/s → $$\displaystyle A_c $$ in m²).

  2. Choose depth $D$ (typically 0.5-1.0 m). Then width $$\displaystyle B = A_c / D $$.

  3. Length $$\displaystyle L = V \times t $$, where $t$ = detention time (60-90 s). Or ensure $$\displaystyle L > 10 \times $$ channel width for good distribution.

  4. Check: $$\displaystyle V_s $$ (min) < $V$ < $$\displaystyle V_s $$ (max) to ensure grit settles but organics don't.

3. Trickling Filter Design

• Given: $Q$, BOD of influent $$\displaystyle L_0 $$ (mg/L), surface loading $$\displaystyle q_s $$ (L/m²-d or m³/m²-d), organic loading $$\displaystyle L_o $$ (g BOD/m³-d or kg/ha-d), depth $D$.

• Steps:

  1. Surface area: $$\displaystyle A = Q / q_s $$ (consistent units).

  2. Volume: $$\displaystyle V = A \times D $$.

  3. Organic loading check: $$\displaystyle L_o = \frac{Q \times L_0}{V} $$ (ensure units match given $$\displaystyle L_o $$).

  4. Diameter: $$\displaystyle A = \pi D_f^2 / 4 $$ → $$\displaystyle D_f = \sqrt{4A/\pi} $$.

  5. Efficiency (NRC): Use formula with $$\displaystyle L_o $$ in kg/ha-day or g/m²-day (convert carefully).

4. Alum Dosing Calculation

• Given: Dose $D$ (mg/L), flow $Q$ (MLD or L/s).

• Calculation: Mass per day = $D \times Q$.

  • If $D$ in mg/L, $Q$ in MLD (million L/day): Mass (kg/day) = $D \times Q$.

  • Example: $$\displaystyle D=28.6 $$ mg/L, $$\displaystyle Q=18 $$ MLD → Mass = 28.6 × 18 = 514.8 kg/day.

B. Air Pollution Calculation

1. Power Plant Emission Estimation

• Given: Capacity $P$ (MW), efficiency $\eta$, coal CV (MJ/kg), ash% $A\%$, S% $S\%$, % fly ash of total ash, recovery % for ash.

• Steps:

  1. Coal consumption: $$\displaystyle \dot{m}_c = \frac{P \times 3.6 \times 10^6}{\eta \times CV} $$ (kg/h or kg/s). (Since 1 MW = 1 MJ/s, 1 MW-h = 3600 MJ).

  2. Total ash: $$\displaystyle \dot{m}_{ash,total} = \dot{m}_c \times A\% $$.

  3. Fly ash generated: $$\displaystyle \dot{m}_{FA,gen} = \dot{m}_{ash,total} \times (\% \text{ as fly ash}) $$.

  4. Fly ash recovered: $$\displaystyle \dot{m}_{FA,rec} = \dot{m}_{FA,gen} \times \text{recovery}\% $$.

  5. Bottom ash generated: $$\displaystyle \dot{m}_{BA,gen} = \dot{m}_{ash,total} - \dot{m}_{FA,gen} $$.

  6. Bottom ash recovered: $$\displaystyle \dot{m}_{BA,rec} = \dot{m}_{BA,gen} \times \text{recovery}\% $$.

  7. SO₂ emission: $$\displaystyle \dot{m}_{SO_2} = 2 \times \dot{m}_c \times S\% $$ (assuming 100% conversion of S to SO₂). If given recovery efficiency for SO₂ (e.g., FGD efficiency), subtract accordingly.

2. Gaussian Plume Application

• Typical Question: Identify parameters, find max concentration location, or calculate concentration given all parameters and $$\displaystyle \sigma_y, \sigma_z $$ from tables.

• Key: Max ground-level concentration on centerline ($$\displaystyle y=0, z=0 $$) occurs at $$\displaystyle x_{max} \approx H / \sigma_z \times \text{some factor} $$. Usually given or asked conceptually.

• Calculation: Plug values into Gaussian equation. Ensure $U$ is at effective stack height $H$.