Environmental Engg‑II (CE-702 (B)) - Unit 5 Short Notes

UNIT 5: WASTEWATER TREATMENT & AIR POLLUTION CONTROL

I. FUNDAMENTALS OF WASTEWATER TREATMENT SYSTEMS

A. Unit Operations vs. Unit Processes

• Unit Operations: Physical methods of treatment where only physical forces are applied. No chemical or biological change occurs.

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

• Unit Processes: Chemical or biological methods of treatment where the quality of wastewater is changed by chemical or biological reactions.

  • Examples: Coagulation/Flocculation, Disinfection, Activated Sludge, Trickling Filter, Oxidation Ditch.

[!TIP] Common exam question: "Elaborate various types of unit operations." List them with a brief one-line function.

B. Preliminary Treatment

• Racks and Screens:

  • Coarse Screens: Bar spacing 50-150 mm. Remove large debris.

  • Fine Screens: Bar spacing 6-50 mm. Remove smaller materials.

  • Head Loss Through Bar Screen: Estimated by Kirchmer's Formula:

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

    Where, $h$ = head loss (m), $\beta$ = blockage coefficient (~0.5 for clean, ~0.8 for clogged), $W$ = total width of bars, $b$ = clear spacing, $\theta$ = angle of inclination, $V$ = approach velocity.

*   **Materials:** Mild steel, stainless steel, plastic.

• Grit Chamber:

  • Purpose: Remove sand, gravel, cinders, and other inorganic solids to prevent abrasion and deposition in pipes/equipment.

  • Design Principle: Settling velocity ($$\displaystyle V_s $$) of grit particles (0.2 mm, SG 2.65) is controlled by adjusting horizontal flow velocity ($$\displaystyle V_h $$) to ~0.3 m/s. Particles with $$\displaystyle V_s > V_h $$ settle.

  • Types: Horizontal flow, Aerated (to keep organic solids in suspension).

  • Design Parameter (Horizontal Flow):

$$ \text{Length } (L) = \frac{V_h}{V_s} \times H $$

    Where $H$ = depth of flow.

• Flow Equalization: Needed to dampen peak flows and equalize load from varying sources (e.g., industrial discharges). Uses a detention basin with mixing/aeration to prevent septicity.

II. PRIMARY TREATMENT

A. Sedimentation (Primary Clarification)

• Theory of Settling:

  1. Discrete Settling: Particles settle as discrete units (low concentration, no flocculation). $$\displaystyle V_s $$ = constant.

  2. Flocculent Settling: Particles flocculate during settling, increasing $$\displaystyle V_s $$ (common in primary tanks).

  3. Zone Settling: High concentration, particles settle as a mass (hindered settling, sludge blanket).

  4. Compression Settling: Very high concentration, consolidation under weight of overlying solids.

• Design Parameters:

  • Detention Period: 1.5 - 2.5 hours.

  • Surface Overflow Rate (SOR) / Surface Loading Rate: $$\displaystyle \frac{Q}{A} $$, where $Q$ = flow (m³/d), $A$ = surface area (m²). Typical: 20-30 m³/m²·d.

  • Weir Loading Rate: $$\displaystyle \frac{Q}{L_w} $$, where $$\displaystyle L_w $$ = total weir length (m³/m·d). Should be < 125-250 m³/m·d to avoid short-circuiting.

• Tank Types: Rectangular (with mechanical scrapers) or Circular (with rotating scrapers).

• Performance: Removes ~50-70% suspended solids and ~25-40% BOD.

III. SECONDARY (BIOLOGICAL) TREATMENT

A. Attached Growth Processes

• Trickling Filters:

  • Types:

    • Standard Rate: Depth 1.8-3.0 m, organic loading 0.08-0.32 kg BOD/m³·d. No ponding.

    • High Rate: Depth 1.0-2.0 m, organic loading 0.32-1.0 kg BOD/m³·d. Requires recirculation.

    • Super High Rate: Depth < 1.0 m, very high loading, high recirculation.

  • Components: Filter media (rocks, slag, plastic), Distributor (rotating arms), Underdrain (collects effluent).

  • Design (NRC Formula for Standard Rate):

$$ D = 0.116 + 0.141 \log_{10} L_s $$

    Where, $D$ = depth (m), $$\displaystyle L_s $$ = organic loading in lb BOD/ac-ft·d.

    **Surface Area (A):** 

$$ A = \frac{Q \times \text{BOD}_{influent}}{L_s} $$

    Where $$\displaystyle L_s $$ = surface loading (L/m²·d or g BOD/m²·d).

*   **Efficiency (NRC Formula):**

$$ E = \frac{100}{1 + 0.0086 \sqrt{L_s}} $$

(E in %, $$\displaystyle L_s $$ in lb/ac-ft·d)

• Oxidation Ditch:

  • Working Principle: An oval-shaped channel equipped with mechanical surface aerators (rotors). It's a modified extended aeration activated sludge system (Suspended Growth, but often classified under attached due to configuration).

  • Neat Sketch Features: Oval channel, multi-pass or single-pass flow, rotor/aerator, inlet/outlet structure, sludge return.

  • Configurations: Carrousel (single rotor), Multichannel (multiple loops, better control).

B. Suspended Growth Processes

• Activated Sludge Process (ASP):

  • Definition: A biological process where a mass of microorganisms (activated sludge) is maintained in suspension and aerated. The mixture is then settled and a portion of sludge is recycled.

  • Schematic: Influent + Recycled Sludge → Aeration Tank (with aeration) → Mixture → Secondary Clarifier → Effluent + Sludge Recycle.

  • Configurations: Plug Flow (long rectangular tank), Complete Mix (square tank, uniform concentration).

  • Oxygen Requirement Calculation:

    • Based on BOD Removal: $$\displaystyle O_2 $$ reqd = $1.47 \times \Delta \text{BOD}$ (kg O₂/kg BOD removed) - accounts for synthesis.

    • Based on Endogenous Respiration: $$\displaystyle O_2 $$ reqd = $0.15 \times \text{MLVSS}$ (kg O₂/kg MLVSS·d) for steady-state.

    Total O₂ demand = O₂ for BOD oxidation + O₂ for endogenous respiration.

• Septic Tank vs. Imhoff Tank:

  | Feature | Septic Tank | Imhoff Tank | | :--- | :--- | :--- | | Primary Function | Sedimentation & Anaerobic Digestion (combined in one chamber) | Sedimentation (upper) & Sludge Digestion (lower, separate chamber) | | Sludge Handling | Sludge accumulates in same tank; requires frequent desludging. | Digested sludge settles in lower compartment; easier removal, less frequent. | | Effluent Quality | Poorer (contains more solids, BOD) as it comes from upper compartment only. | Better (effluent from upper compartment after solids settle). | | Scope | For individual houses/small communities. | For small towns/clusters (2-5,000 people). | | Performance | Simple, low cost, but poor efficiency. | More efficient, better sludge stabilization. |

IV. TERTIARY / ADVANCED WASTEWATER TREATMENT (AWT)

A. Need for AWT: Secondary effluent still contains nutrients (N, P), pathogens, and refractory organics. Required for discharge to sensitive water bodies or reuse (agriculture, industry, groundwater recharge). B. Conventional vs. Advanced:

*   **Conventional:** Preliminary → Primary → Secondary.

*   **Advanced:** Conventional + Tertiary (Nutrient removal, Filtration, Disinfection) + sometimes Quaternary (Specialized removal like trace organics).

C. Important AWT Processes:

V. SPECIAL TOPICS IN WASTEWATER ENGINEERING

• Role of Micro-organisms:

  • Bacteria: Workhorses (decompose organics - heterotrophs; nitrifiers - autotrophs). Form flocs.

  • Protozoa: Feed on free bacteria, improve effluent clarity (ciliates, flagellates).

  • Metazoa (Rotifers, Worms): Feed on sludge flocs and smaller organisms; indicate good sludge age/health.

• Nitrogen Removal:

  1. Biological (Nitrification-Denitrification): Most common. Requires alternating aerobic/anoxic zones.

  2. Ammonia Stripping: As above.

  3. Ion Exchange: For concentrated streams.

  4. Breakpoint Chlorination: For disinfection by-product control.

VI. AIR POLLUTION FUNDAMENTALS

A. Sources & Characteristics

• Stationary Sources (Power Plants):

  • Flyash: Fine particulate carried by flue gas. Collected by ESPs/Baghouses.

  • Bottom Ash: Coarse ash from furnace bottom.

  • SO₂ Emission Calculation:

$$ \text{SO}_2 \text{ (kg/h)} = \frac{2 \times \%S \times \text{Coal consumption (kg/h)} \times 32}{100} $$

    (Factor 2 accounts for S → SO₂, 32 is molecular weight of S).

• Specific Pollutants:

  | Pollutant | Primary Sources | Key Characteristics/Effects | | :--- | :--- | :--- | | Sulphur Dioxide (SO₂) | Fossil fuel combustion (coal, oil), smelting. | Colorless, pungent, soluble in water → acid rain. Respiratory irritant. | | Hydrogen Sulphide (H₂S) | Sewage, petroleum refining, kraft mills, volcanic. | Colorless, rotten egg smell, toxic (olfactory fatigue). |

B. Aerosols: Suspensions of solid/liquid particles in gas.

C. Meteorological Factors:

• Wind Speed & Direction: Primary diluting/transporting mechanism.

• Atmospheric Stability: Determines vertical mixing.

  • Stable: Inversion, poor dispersion.

  • Neutral: Normal lapse rate, good dispersion.

  • Unstable: Convective, excellent dispersion.

• Temperature Lapse Rate (ELR vs. ALR):

  • Environmental Lapse Rate (ELR): Actual temperature decrease with height in atmosphere (variable, avg ~6.5°C/km).

  • Adiabatic Lapse Rate (ALR): Temperature change of a parcel of air rising/falling without heat exchange.

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

    • Saturated ALR (SALR): ~5-6°C/km (saturated, latent heat release).

  • Stability Condition: If ELR < DALR → Stable. If ELR > DALR → Unstable.

• Mixing Height: Height of the lowest layer of the atmosphere where pollutants are vertically mixed. Determined by stability.

• Humidity & Precipitation: Rain scavenges particles/gases (wet deposition).

VII. AIR POLLUTION DISPERSION & MODELING

A. Gaussian Plume Model (Steady-State, Point Source)

• Standard Equation (for ground-level release, flat terrain, no deposition):

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

Where:

*   $C$ = concentration (g/m³)

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

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

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

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

• Location of Maximum Pollution: On the plume centerline ($$\displaystyle y=0, z=H $$) for elevated release. For ground-level release, maximum occurs at some $$\displaystyle z>0 $$.

• Assumptions: Steady state, constant wind speed/direction, Gaussian distribution in $y$ and $z$, no chemical transformation/deposition, flat terrain.

• Limitations: Not valid for complex terrain, long travel times (chemical transformation), low wind speeds, or very close to source.

VIII. AIR POLLUTANT CLASSIFICATION & EFFECTS

A. Primary vs. Secondary Pollutants

B. Particulate Matter:

• TSPM (Total Suspended Particulate Matter): All particles up to ~50 µm.

• RSPM (Respirable Suspended Particulate Matter) / PM₁₀: Particles with aerodynamic diameter ≤ 10 µm. Can penetrate to thoracic region (alveoli). Major health concern.

C. Photochemical Smog:

• Formation Mechanism:

  1. NO₂ + sunlight (UV) → NO + O

  2. O + O₂ → O₃ (Ozone)

  3. O₃ + VOCs (hydrocarbons) → complex reactions → peroxyacetyl nitrate (PAN), aldehydes, etc.

• Constituents: O₃, PAN, NO₂, aldehydes, aerosols.

• Characteristics: Brownish haze, occurs in sunny, warm climates (Los Angeles, Delhi), eye irritation, plant damage.

D. Effects of Air Pollution:

• Human Health: Respiratory diseases (asthma, bronchitis), cardiovascular issues, cancer (benzene, PAHs), acute toxicity (CO).

• Materials: Corrosion (SO₂, acid rain), soiling, degradation of paints, textiles, buildings.

• Vegetation: Leaf injury, reduced photosynthesis, growth inhibition (O₃, SO₂).

• Climate: Global warming (CO₂, CH₄), cooling (sulfate aerosols), ozone depletion (CFCs).

IX. SOLID WASTE FROM COMBUSTION (Power Plant Application)

A. Flyash & Bottom Ash Generation Calculation:

Given:

• Power Plant Capacity = $P$ MW

• Efficiency = $\eta$ (decimal)

• Coal Calorific Value = $CV$ MJ/kg

• Ash Content in coal = $A\%$

• Flyash Fraction = $f$ (fraction of ash carried by flue gas)

• Recovery Efficiency: Flyash = $$\displaystyle R_f\% $$, Bottom Ash = $$\displaystyle R_b\% $$

Steps:

1. Coal Consumption Rate:

$$ \dot{m}_{coal} = \frac{P \times 10^3 \text{ kW/MW} \times 3600 \text{ s/h} \times 24 \text{ h}}{\eta \times CV \times 10^6 \text{ J/MJ}} \text{ (kg/h)} $$

(Simpler: Daily coal = $$\displaystyle \frac{P \times 24 \times 3600}{\eta \times CV \times 1000} $$ tonnes/day if $CV$ in kJ/kg).

2. Total Ash Generated:

$$ \dot{m}_{ash,total} = \dot{m}_{coal} \times \frac{A}{100} \text{ (kg/h or t/d)} $$

3. Flyash Generated (before recovery):

$$ \dot{m}_{flyash,raw} = \dot{m}_{ash,total} \times f $$

4. Bottom Ash Generated (before recovery):

$$ \dot{m}_{bottom,raw} = \dot{m}_{ash,total} \times (1-f) $$

5. Recovered Flyash:

$$ \dot{m}_{flyash,rec} = \dot{m}_{flyash,raw} \times \frac{R_f}{100} $$

6. Recovered Bottom Ash:

$$ \dot{m}_{bottom,rec} = \dot{m}_{bottom,raw} \times \frac{R_b}{100} $$

7. Unrecovered (to disposal): Raw - Recovered.

B. Eco-friendly Disposal/Utilization:

• Flyash:

  • Cement/Concrete Industry (Flyash bricks, concrete additive): Most prevalent. Improves workability, long-term strength, reduces permeability.

  • Construction: As fill material, in road bases, embankments.

  • Agriculture: Soil amendment (lime content, micronutrients), but concerns about heavy metals.

  • Mine Filling.

• Bottom Ash:

  • Construction Aggregate: Road sub-base, concrete aggregate (after processing).

  • Structural Fill.

  • Landscaping.