Environmental Engineering-I (CE-602) - Unit 4 Short Notes

UNIT 4: WATER SUPPLY & WASTEWATER ENGINEERING

PART A: WATER SUPPLY ENGINEERING

1.0 Population Forecasting & Water Demand Estimation

1.1 Population Forecasting Methods

• Arithmetic Increase Method: Assumes constant population increase.

$$P_n = P_0 + n \cdot \bar{x}$$

Where, $$\displaystyle \bar{x} = \frac{\text{Increase in population over } n \text{ decades}}{n} $$

*Suitable for old, established towns with steady growth.*

• Incremental Increase Method: Average of incremental increases is added to present population.

$$P_n = P_0 + n \cdot \bar{x} + \frac{n(n-1)}{2} \cdot \bar{y}$$

Where, $$\displaystyle \bar{y} = \text{Average of incremental increases} $$.

*Suitable for towns with increasing growth rate.*

• Logistic Curve Method (High Priority)

  • Concept: S-shaped curve modeling growth with a saturation limit ($$\displaystyle P_s $$).

  • Logistic Equation:

$$P_t = \frac{P_s}{1 + e^{a + b \cdot t}}$$

    Where $$\displaystyle P_t $$ = population at time $t$, $$\displaystyle P_s $$ = saturation population, $a, b$ = constants.

*   **Determination of $$\displaystyle P_s $$, $a$, $b$**:

    From three known populations at times $$\displaystyle t_0, t_1, t_2 $$:

$$P_s = \frac{2P_0 P_1 P_2 - P_1^2(P_0 + P_2)}{P_0 P_2 - P_1^2}$$

    Then solve for $a, b$ using the equation at two time points.

> [!TIP] Past papers often ask to find $$\displaystyle P_s $$, coefficients, and future population using given data for three periods.

1.2 Water Demand Concepts

• Per Capita Demand ($q$):

$$q = \frac{\text{Total water supplied}}{\text{Population}} \text{ (lpcd)}$$

*Factors affecting*: Climate, living standards, industrial activities, pressure, metering, leakage.

• Types of Water Demand:

  • Domestic (cooking, bathing, flushing)

  • Industrial & Commercial

  • Institutional (schools, hospitals)

  • Public Uses (street washing, gardening)

  • Fire Demand (High Priority)

  • Losses & Wastages (leakage, unauthorized use)

• Fire Demand Estimation:

  • Kuichling's Formula:

$$Q = 3182 \sqrt{P}$$

    Where $Q$ = fire flow (lit/min), $P$ = population.

*   **Boston's Formula (National Board of Fire Underwriters)**:

$$Q = \frac{1}{2} \left( 1.17 \sqrt{P} + 2 \right) \times 10^3 \text{ (lit/min)} \text{ for } P>50,000$$

    Simplified: $$\displaystyle Q = 10200 \sqrt{P} $$ (lit/min) for large cities.

*   **Buston's Formula**:

$$Q = 5663 \sqrt{P}$$

> [!TIP] Fire demand is asked in combination with total draft calculation. Use appropriate formula based on population size.

1.3 Total Water Requirement & Draft Calculation

• Average Daily Demand (ADD) = $q \times P$

• Maximum Daily Demand (MDD) = $1.8 \times \text{ADD}$ to $2.0 \times \text{ADD}$

• Maximum Hourly Demand (MHD) = $1.5 \times \text{MDD}$ to $3.0 \times \text{ADD}$

• Coincident Demand: Fire demand is added to MDD or MHD, whichever is larger, but not both simultaneously.

$$\text{Total Draft} = \text{Max}( \text{MDD}, \text{MHD}) + \text{Fire Demand}$$

2.0 Sources of Water & Intake Structures

2.1 Sources of Water

2.2 Intake Structures

• Purpose: To draw water from source and convey to treatment plant with minimal obstruction.

• Types: River intake, Lake intake, Canal intake.

• Factors Governing Selection:

  • Source type, depth, location, silt content, flood levels, accessibility.

• Canal Intake Working:

  DiagramCANVAS: A canal with a intake tower having coarse screen at inlet, bell-mouth entry, and a pipeline to pump house. Gates for control.

  • Components: Intake well, screens (coarse/fine), bell-mouth, gate valves, connecting conduit.

  • Working: Water enters through screens into well, flows by gravity or pumped to treatment plant.

3.0 Water Quality & Treatment Processes

3.1 Water Quality Standards & Analysis

• Physical Parameters: Temperature, colour, odour, turbidity, TDS.

• Chemical Parameters: pH, hardness, chlorides, fluorides, heavy metals, nitrates, iron, manganese.

• Biological Parameters: Pathogens (bacteria, virus, protozoa), Indicator Organisms (Coliforms, E. coli).

• Water Quality Standards (BIS/WHO): Limits for each parameter (e.g., Turbidity < 1 NTU, pH 6.5-8.5, Coliforms 0/100 mL).

• Waterborne Diseases: Cholera (V. cholerae), Typhoid (S. typhi), Dysentery (Shigella), Jaundice (Hepatitis A), Giardiasis.

• Microbiological Examination:

  • Coliform Index: Number of coliforms per 100 mL. Zero indicates safe water.

  • Most Probable Number (MPN) Test: Statistical method to estimate coliform density from positive/negative tube results.

3.2 Preliminary & Primary Treatment: Screening & Sedimentation

• Theory of Sedimentation: Gravity settling of particles. Based on Stokes' Law.

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

Where $v$ = settling velocity, $d$ = particle diameter, $$\displaystyle \rho_p, \rho $$ = densities, $\mu$ = viscosity.

*Temperature Effect*: $\mu \downarrow$ as $T \uparrow$ $\Rightarrow$ $v \uparrow$.

• Factors Affecting Sedimentation: Particle size/density, flow velocity, tank dimensions, temperature.

• Design of Sedimentation Tanks:

  1. Surface Overflow Rate (OFR):

$$OFR = \frac{Q}{A} \text{ (m}^3/\text{m}^2/\text{day)}$$

    Typical: 32-40 m³/m²/day for plain, 60-100 for coagulated.

2.  **Detention Time**:

$$t = \frac{V}{Q} \text{ (hours)}$$

    Typical: 3-4 hrs (plain), 2-3 hrs (coagulated).

3.  **Dimensions**: L/B = 2:1 to 5:1, Depth = 3-4.5 m, Length ≤ 100 m.

• Coagulation-Cum-Sedimentation Tank Design (Numerical):

  Given: Population, water supply rate, OFR, detention time.

  Steps:

  1. Calculate $$\displaystyle Q_{avg} $$ (m³/sec).

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

  3. Volume $$\displaystyle V = Q \times t $$.

  4. Find $L, B, D$ satisfying L/B and depth limits.

3.3 Coagulation & Flocculation

• Definition: Addition of chemicals to destabilize colloidal particles and form larger flocs.

• Common Coagulants:

  • Alum (Al₂(SO₄)₃·18H₂O):

$$\text{Al}_2(\text{SO}_4)_3 \cdot 18\text{H}_2\text{O} + 6\text{Ca(HCO}_3)_2 \rightarrow 2\text{Al(OH)}_3\downarrow + 3\text{CaSO}_4 + 6\text{CO}_2 + 24\text{H}_2\text{O}$$

*   **Chlorinated Copperas (FeCl₃)**: $$\displaystyle \text{FeCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{Fe(OH)}_3\downarrow + 3\text{HCl} $$

• Factors Affecting Coagulation: pH (optimum 6.5-7.5 for Alum), temperature, mixing intensity/duration (flocculation), coagulant dose, alkalinity.

3.4 Filtration

• Theory of Filtration Mechanisms:

  • Straining: Physical removal of large particles.

  • Sedimentation: Inertial impaction in pores.

  • Adsorption: Van der Waals forces.

  • Biological: Schmutzdecke layer in SSF.

• Slow Sand Filter (SSF):

  • Construction: Sand (0.15-0.30 m) over gravel, underdrain system.

  • Working: Slow gravity flow (0.1-0.2 m/hr). Schmutzdecke (biological layer) forms on top, responsible for purification.

  • Cleaning: Scraping top 1.5-2 cm sand.

  • Advantages: Simple, excellent pathogen removal, low cost.

  • Disadvantages: Large area, slow, frequent cleaning.

• Rapid Sand Filter (RSF):

  • Construction: Sand (0.6-0.9 m, effective size 0.4-0.6 mm) over graded gravel, underdrain system.

  • Working: Higher rate (100-200 m³/m²/day). Requires coagulation pretreatment.

  • Design:

    • Area $$\displaystyle A = \frac{Q}{\text{Rate of filtration}} $$

    • Number of units: $$\displaystyle N = \frac{\text{Total Area}}{\text{Area per unit}} $$ (1 unit standby).

  • Backwashing:

    • Expanded Depth ($$\displaystyle h_e $$):

$$h_e = \frac{D}{1 - n_e}$$

        Where $D$ = original depth, $$\displaystyle n_e $$ = expanded porosity.

    *   **Backwash Velocity** ($$\displaystyle V_b $$):

$$V_b = \frac{g d^2 (\rho_s - \rho)}{18 \mu \phi} \cdot \frac{(1 - n)^2}{n^3}$$

        Where $\phi$ = shape factor (0.85 for sand).

    *   **Methods**: Upflow only, upflow with air scouring.

• Comparison: SSF vs RSF

  | Feature | Slow Sand Filter (SSF) | Rapid Sand Filter (RSF) | | :--- | :--- | :--- | | Rate of Filtration | 0.1-0.2 m/hr | 100-200 m³/m²/day (~5-10 m/hr) | | Pretreatment | Not essential | Coagulation-flocculation essential | | Mechanism | Biological (Schmutzdecke) | Physical (straining, adsorption) | | Area Required | Large | Small | | Cleaning | Scraping top sand | Backwashing (upflow) | | Cost | Low capital, high land | High capital, low land |

3.5 Disinfection

• Definition: Killing/inactivating pathogenic microorganisms.

• Break Point Chlorination:

  • Chlorine Demand: Chlorine consumed by organic matter, ammonia, etc.

  • Process: Chlorine dose increased until all demand is satisfied (breakpoint). Further addition (superchlorination) provides free chlorine residual for disinfection.

  • Curve: Plot of chlorine residual vs. dose. Breakpoint where residual appears after initial fall.

• Other Methods: Ozone (strong oxidant, no residual), UV (physical, no chemical).

4.0 Water Distribution Systems & Storage

4.1 Distribution Systems Layouts

4.2 Pumps & Pumping Stations

• Types: Centrifugal (most common), Reciprocating (high head, low discharge).

• Selection Factors: Required head, discharge, efficiency, cost, maintenance.

• Pump Power Calculation (BHP):

$$BHP = \frac{\rho g Q H}{102 \cdot \eta_p \cdot \eta_m}$$

Where $\rho$ = density, $g$ = gravity, $Q$ = discharge, $H$ = total head (static lift + friction loss), $$\displaystyle \eta_p $$ = pump efficiency, $$\displaystyle \eta_m $$ = motor efficiency.

> [!TIP] Total head $$\displaystyle H = H_s + h_f $$. Friction loss $$\displaystyle h_f $$ from Darcy-Weisbach or Hazen-Williams.

4.3 Service & Distribution Reservoirs

• Differentiation:

  • Storage Reservoir (Source): Raw water storage near source (dam, reservoir).

  • Service/Distribution Reservoir (City): Treated water storage within distribution system.

• Functions of Distribution Reservoirs:

  • Balancing hourly demand variations.

  • Emergency storage (fire, breakdown).

  • Pressure maintenance (elevation provides head).

• Location: At highest point in distribution area, central for uniform pressure.

• Storage Capacity Determination - Mass Curve Method (24-hr pumping):

  1. Plot cumulative inflow (pumping) vs. time (mass curve).

  2. Draw line from start to end with slope = average demand.

  3. Maximum vertical deviation between pumping curve and demand line = required storage.

  DiagramCANVAS: Mass curve with pumping line above demand line, vertical lines showing storage requirement at peak.

4.4 Distribution System Appurtenances

• Valves: Gate (isolation), Check (prevent backflow), Air Relief (release air), Pressure Reducing.

• Fire Hydrants: Connection for fire hoses.

• Water Meters: Measure consumption.

• Sewer Appurtenances (for contrast): Manholes (access), Lamp holes (inspection), Flushing gates, Inverted siphons.

PART B: WASTEWATER ENGINEERING (SEWAGE & SEWERAGE)

5.0 Wastewater Characteristics & Analysis

5.1 Definition and Characteristics

• Physical: Temperature (higher than water supply), colour (greyish), odour (rotten egg if stale), solids (suspended, dissolved, settleable).

• Chemical:

  • pH ~ 7-8.

  • Organic Matter: BOD (Biochemical Oxygen Demand), COD (Chemical Oxygen Demand). BOD < COD.

  • Nitrogen (organic, ammonia, nitrites, nitrates), Phosphorus, Chlorides, Sulfates.

• Biological: Pathogens (bacteria, viruses, helminths), Indicator Organisms (Coliforms).

• Significance of Analysis: Design treatment units, assess pollution load, compliance with discharge standards.

5.3 Decomposition of Organic Matter

• Aerobic Decomposition: With oxygen, produces CO₂, H₂O, nitrates, stable organics.

• Anaerobic Decomposition: Without oxygen, produces CH₄, CO₂, H₂S, organic acids, sludges.

• Relative Stability: Measure of completeness of sewage decomposition. High stability = less oxygen demand in receiving water.

5.4 Population Equivalent (PE)

• Definition: Number of persons producing same pollutant load (usually BOD) as given sewage flow.

$$PE = \frac{\text{BOD load from sewage (kg/day)}}{\text{Standard BOD contribution per person (0.06 kg/day)}}$$

> [!TIP] Standard BOD per capita = 0.06 kg/day (60 g/day).

6.0 Sewage Flow & Sewer Design

6.1 Variation in Sewage Flow

• Average Dry Weather Flow (ADWF): Normal flow excluding stormwater.

• Maximum Dry Weather Flow (MDWF): Peak flow in dry season, typically 1.5-2.0 × ADWF.

• Minimum Dry Weather Flow: Nighttime flow, 1/3 to 1/2 of ADWF.

• Wet Weather Flow: Combined flow of sewage and storm runoff.

6.2 Estimation of Sewage Flow

• Design Discharge for Combined System:

$$Q_{design} = Q_{sewage} + Q_{storm}$$

Where:

*   $$\displaystyle Q_{sewage} = \text{Peak sewage flow} = k \times (P \times q) / 86400 $$ (m³/sec)

*   $$\displaystyle Q_{storm} = \frac{C \cdot i \cdot A}{360} $$ (Rational method, m³/sec)

    $C$ = runoff coefficient, $i$ = rainfall intensity (mm/hr), $A$ = area (hectares).

> [!TIP] For separate systems, design only for MDWF. For combined, add storm runoff.

6.3 Sewer Design & Hydraulics

• Manning's Formula:

$$V = \frac{1}{n} R^{2/3} S^{1/2}$$

Where $V$ = velocity (m/s), $n$ = roughness (0.013-0.015 for concrete), $R$ = hydraulic radius (m), $S$ = slope.

$$Q = A \cdot V$$

• Design of Circular Sewers (Partial Depth):

  • For given $D$, $S$, $n$, find $d$ (depth of flow) and $Q$.

  • Use tables or iterative calculation of area $A$ and wetted perimeter $P$ for segment.

  • Limiting Velocities:

    • Self-Cleansing Velocity ($$\displaystyle V_{min} $$): Prevents deposition. Min 0.6-0.9 m/s for small sewers.

    • Non-Scouring Velocity ($$\displaystyle V_{max} $$): Prevents erosion. Max 2.5-3.0 m/s.

• Shape, Size & Gradient: Circular most common. Size 150-300 mm (laterals) to 1.5-2.0 m (mains). Gradient to achieve $$\displaystyle V_{min} $$.

7.0 Sewerage System Components & Construction

7.1 Types of Sewer Networks

7.2 Sewer Appurtenances (Detailed)

• Manholes:

  • Purpose: Access for inspection, cleaning, junction, change of direction/slope.

  • Types: Normal, Drop, Junction.

  • Spacing: 30-100 m (straight), less at junctions.

  • Sketch:

    DiagramCANVAS: Circular/rectangular chamber with benching, steps, cover at ground.

• Lamp Holes:

  • Purpose: Simple vertical pipe for visual inspection (limited use).

  • Sketch:

    DiagramCANVAS: Small diameter pipe (150 mm) with cover, extending to surface.

• Others: Flushing gates (clean dead ends), Inverted siphons (under obstacles), Street inlets (catch drains).

7.3 Construction Techniques

• Trenching: Open cut (common), tunnelling (under roads).

• Laying & Jointing: Lowering pipes, bedding, jointing (rubber gasket, cement mortar for concrete).

• Materials: Concrete, PVC, Cast Iron.

• Quality Control & Safety: Testing (water tightness, deflection), shoring, barricades, dewatering.

8.0 Wastewater Treatment & Disposal

8.1 Primary Treatment: Sedimentation

• Same design parameters as water sedimentation (OFR, detention time).

• Typical Removal: 50-60% suspended solids, 25-35% BOD.

8.2 Secondary Treatment: Biological Methods

• Activated Sludge Process (High Priority):

  • Principle: Aerobic microorganisms consume organic matter.

  • Components:

    1. Aeration Tank: Wastewater + recycled sludge + air/oxygen.

    2. Secondary Clarifier: Settles activated sludge.

    3. Sludge Recycling: Return settled sludge to aeration tank.

  • Process Description:

    Wastewater → Grit chamber → Primary sedimentation → Aeration tank (4-8 hrs) → Clarifier → Treated effluent. Sludge from clarifier partly recycled, partly wasted.

  • Variations: Trickling Filter (fixed film), Rotating Biological Contactors (RBCs).

8.4 Disposal Methods

• Disposal by Dilution:

  • Oxygen Sag Curve (High Priority):

    DiagramCANVAS: Graph of DO deficit (D) vs. distance downstream. Zones: A (degradation), B (critical), C (recovery).

    • Zone of Degradation (A): BOD exerted, DO drops.

    • Critical Point (B): Max DO deficit ($$\displaystyle D_c $$), lowest DO.

    • Zone of Recovery (C): Reaeration > deoxygenation, DO recovers.

  • Streeter-Phelps Equation (Conceptual):

$$D = \frac{K_d L_0}{K_a - K_d} \left( e^{-K_d t} - e^{-K_a t} \right) + D_a e^{-K_a t}$$

    Where $D$ = DO deficit, $$\displaystyle L_0 $$ = ultimate BOD, $$\displaystyle K_d $$ = deoxygenation constant, $$\displaystyle K_a $$ = reaeration constant, $t$ = time, $$\displaystyle D_a $$ = initial deficit.

    > [!TIP] Numerical problems ask for $$\displaystyle D_c $$ and its location. $$\displaystyle D_c $$ occurs at $$\displaystyle t_c = \frac{1}{K_a - K_d} \ln \frac{K_a}{K_d} $$.

• Land Treatment: Slow rate (irrigation), rapid infiltration, overland flow.

• Disposal after Treatment: Irrigation, industrial reuse, groundwater recharge.

9.0 INTEGRATED & CALCULATION-FOCUSED TOPICS

9.1 Numerical Design Problems - Key Formulas & Steps

1. Logistic Curve:

   • $$\displaystyle P_s = \frac{2P_0 P_1 P_2 - P_1^2(P_0 + P_2)}{P_0 P_2 - P_1^2} $$

   • Solve $$\displaystyle P_t = \frac{P_s}{1 + e^{a + b t}} $$ for $a, b$ using two $(t, P)$ pairs.

2. Fire Demand:

   • Kuichling: $$\displaystyle Q = 3182 \sqrt{P} $$ (lit/min)

   • Boston: $$\displaystyle Q = 10200 \sqrt{P} $$ (lit/min) for $$\displaystyle P>50,000 $$

3. Coagulation Dosage:

   • For Alum: $$\displaystyle \text{Dose (mg/l)} = \frac{\text{Alum required (mg/l)}}{1000} \times \text{Flow (MLD)} \times 10^6 $$

   • Lime requirement (if alkalinity < coagulant demand): Use reaction stoichiometry.

4. Sedimentation Tank:

   • $$\displaystyle A = Q / OFR $$, $$\displaystyle V = Q \times t $$, then $L, B, D$.

5. Rapid Sand Filter:

   • $$\displaystyle A = \frac{Q_{design}}{\text{Filtration rate}} $$, $$\displaystyle N = \lceil A / A_{unit} \rceil + 1 \text{ standby} $$.

   • Backwash: $$\displaystyle V_b $$ from formula, $$\displaystyle h_e = D/(1-n_e) $$.

6. Pump BHP:

   • $$\displaystyle H = H_s + h_f $$ (Darcy: $$\displaystyle h_f = f \frac{L}{D} \frac{V^2}{2g} $$).

   • $$\displaystyle BHP = \frac{\rho g Q H}{102 \cdot \eta_p \cdot \eta_m} $$.

7. Sewer Design (Circular, partial flow):

   • Use Manning: $$\displaystyle Q = \frac{1}{n} A R^{2/3} S^{1/2} $$. For segment, $$\displaystyle A = \frac{R^2}{2}(\theta - \sin\theta) $$, $$\displaystyle P = R\theta $$, $$\displaystyle R = A/P $$.

   • Iterate for $d/D$ to get $Q$ or $V$.

8. BOD Calculations:

   • 5-day BOD (Dilution):

$$BOD_5 = \frac{(D_0 - D_5) f}{P}$$

    $$\displaystyle D_0, D_5 $$ = DO initial/final (mg/l), $f$ = dilution factor, $P$ = % dilution/100.

*   **Temperature Correction**:

$$BOD_{T2} = BOD_{T1} \cdot \theta^{(T_2 - T_1)}$$

    $\theta$ = 1.135 for $$\displaystyle K_d $$, 1.024 for $$\displaystyle K_a $$, 1.047 for BOD.

*   **Ultimate BOD**:

$$L_0 = \frac{BOD_5}{1 - 10^{-k \cdot 5}}$$

9. Mass Curve for Storage:

   • Plot cumulative pumping vs. time. Draw demand line. Max vertical gap = storage.

10. Oxygen Sag Curve:

    • $$\displaystyle D_c = \frac{K_d L_0}{K_a - K_d} \left( \frac{K_a}{K_d} \right)^{-K_d/(K_a-K_d)} $$

    • $$\displaystyle t_c = \frac{1}{K_a - K_d} \ln \frac{K_a}{K_d} $$

    • $D$ at $$\displaystyle t_c $$ = $$\displaystyle D_c $$. $$\displaystyle BOD_5 $$ at critical point: $$\displaystyle L_t = L_0 (1 - e^{-k_d t_c}) $$.

9.2 Comparative Studies & Short Notes - Key Points

• SSF vs RSF: See table in 3.4.

• Surface vs Groundwater: See table in 2.1.

• Disinfection Methods: Chlorination (residual, by-products), Ozone (strong, no residual), UV (no chemical, no residual).

• Distribution Layouts: See table in 4.1.

• Waterborne Diseases: Link pathogens to diseases (Cholera-Vibrio, Typhoid-Salmonella).

• Sewer Appurtenances: Manholes (large, access) vs Lamp holes (small, inspection only).

• Chemical Parameters of Sewage: BOD, COD, pH, nitrogen forms, chlorides.

• Disposal Methods: Dilution (requires DO analysis, oxygen sag) vs Land Treatment (soil filtration, reuse potential).