Environmental Engineering
Comprehensive guide to environmental engineering principles, water treatment, and pollution control.
Water Treatment
Water treatment processes remove contaminants from water to make it suitable for its intended use. Key processes include:
1. Coagulation and Flocculation:
Coagulation destabilizes colloidal particles, while flocculation promotes their aggregation. The required coagulant dose can be determined through jar tests.
2. Sedimentation:
The settling velocity of a particle in water can be calculated using Stokes' Law:
v_s = g × (ρ_p - ρ_w) × d² / (18 × μ)
Where:
- v_s = settling velocity (m/s)
- g = gravitational acceleration (9.81 m/s²)
- ρ_p = particle density (kg/m³)
- ρ_w = water density (kg/m³)
- d = particle diameter (m)
- μ = dynamic viscosity of water (kg/m·s)
3. Filtration:
The head loss through a clean filter can be calculated using the Kozeny-Carman equation:
h_L = (k × L × v × (1-ε)² × μ) / (ρ_w × g × ε³ × d²)
Where:
- h_L = head loss (m)
- k = Kozeny constant (typically 5)
- L = filter depth (m)
- v = approach velocity (m/s)
- ε = porosity of the filter medium
- d = effective size of the filter medium (m)
4. Disinfection:
The required chlorine dose for disinfection can be calculated using the Ct concept:
Ct = C × t
Where:
- Ct = product of disinfectant concentration and contact time (mg·min/L)
- C = disinfectant concentration (mg/L)
- t = contact time (min)
Wastewater Treatment
Wastewater treatment removes contaminants from wastewater before it is discharged to the environment. Key processes include:
1. Primary Treatment:
Removes settleable solids through physical processes. The overflow rate for a primary clarifier is:
v_o = Q / A
Where:
- v_o = overflow rate (m³/m²·day)
- Q = flow rate (m³/day)
- A = surface area of the clarifier (m²)
2. Secondary Treatment:
Biological processes remove dissolved and colloidal organic matter. For an activated sludge process, the mean cell residence time (MCRT) is:
θ_c = (V × X) / (Q_w × X_r + Q_e × X_e)
Where:
- θ_c = mean cell residence time (days)
- V = volume of the aeration tank (m³)
- X = mixed liquor suspended solids concentration (mg/L)
- Q_w = waste sludge flow rate (m³/day)
- X_r = return activated sludge concentration (mg/L)
- Q_e = effluent flow rate (m³/day)
- X_e = effluent suspended solids concentration (mg/L)
3. Tertiary Treatment:
Advanced processes remove specific contaminants not removed by secondary treatment, such as nutrients, heavy metals, and pathogens.
Air Pollution Control
Air pollution control techniques reduce the emission of pollutants into the atmosphere. Key concepts include:
1. Gaussian Dispersion Model:
Predicts the concentration of pollutants downwind from a point source:
C(x,y,z) = (Q / (2π × u × σ_y × σ_z)) × exp(-y²/(2σ_y²)) × [exp(-(z-H)²/(2σ_z²)) + exp(-(z+H)²/(2σ_z²))]
Where:
- C = concentration (g/m³)
- Q = emission rate (g/s)
- u = wind speed (m/s)
- σ_y, σ_z = dispersion coefficients (m)
- H = effective stack height (m)
- x, y, z = coordinates (m)
2. Control Equipment Efficiency:
The collection efficiency of air pollution control equipment is:
η = (C_in - C_out) / C_in × 100%
Where:
- η = collection efficiency (%)
- C_in = inlet concentration (g/m³)
- C_out = outlet concentration (g/m³)
3. Control Technologies:
- Particulate control: Cyclones, electrostatic precipitators, fabric filters
- Gaseous pollutant control: Absorption, adsorption, thermal and catalytic oxidation
- NOx control: Selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR)
- SOx control: Flue gas desulfurization (FGD), fuel switching