Environmental Quality: Water, Waste, and Air Pollution Principles
English with a size of 105.52 KBUnderstanding the Dissolved Oxygen Sag Curve
- The Dissolved Oxygen (DO) sag curve represents the variation of dissolved oxygen in a river or stream after the discharge of organic waste.
- When organic matter is discharged, microorganisms consume oxygen to decompose it, causing a drop in DO levels.
- The curve typically has three main zones: the pollution zone, active decomposition zone, and recovery zone.
- The lowest point on the curve is called the critical point, and the corresponding DO level is the critical DO.
- Initially, DO decreases due to high Biochemical Oxygen Demand (BOD) in the pollution and decomposition zones.
- After the critical point, DO starts to recover as the organic matter is consumed and natural reaeration from the atmosphere increases.
- The rate of DO depletion is governed by the deoxygenation rate, while recovery is governed by the reaeration rate.
- The DO sag curve helps in evaluating the self-purification capacity of a water body.
- The curve is influenced by factors like temperature, flow rate, organic load, and turbulence.
- The Streeter-Phelps equation is commonly used to mathematically describe the DO sag curve.
Material Recovery Methods in Waste Management
Electrolysis
Used to recover metals like copper and zinc from industrial waste streams through electrochemical reactions.Precipitation
A chemical method where target materials are converted into insoluble compounds and separated as solids.Filtration and Sedimentation
Physical methods to recover suspended solids or valuable particles from liquid waste.Distillation
Used to recover solvents or volatile substances by separating components based on boiling points.Adsorption
Recovers materials like solvents or gases using adsorbents like activated carbon.Hazardous Waste Treatment Techniques
Incineration
High-temperature combustion to destroy hazardous organic compounds and reduce waste volume.Solidification/Stabilization
Mixing waste with binding agents (like cement) to reduce mobility and toxicity.Chemical Treatment
Uses chemical reactions (neutralization, oxidation, reduction) to detoxify hazardous components.Biological Treatment
Utilizes microorganisms to degrade biodegradable hazardous wastes (e.g., bioremediation).Thermal Treatment (Pyrolysis/Gasification)
Decomposes waste at high temperatures in low or no oxygen to produce syngas or recover energy.BOD Test Procedure in the Laboratory
- Sample Collection: Collect the water or wastewater sample in a clean, airtight BOD bottle (usually 300 mL).
- Initial DO Measurement: Measure the initial dissolved oxygen (DO) using a DO meter or Winkler’s method.
- Incubation: Seal the bottle and incubate it at 20°C for 5 days in the dark to prevent photosynthesis.
- Final DO Measurement: After 5 days, measure the final DO of the sample.
- BOD Calculation: BOD₅ = DO (initial) - DO (final) mg/L
Limitations of the Gaussian Plume Model
- Assumes Steady-State Conditions: It considers constant emission rate and weather conditions, which is unrealistic in real environments.
- Applies Only to Simple Terrain: Not suitable for complex terrains like hills, valleys, or urban areas with tall buildings.
- Limited to Continuous Point Sources: Best suited for stack emissions; not accurate for area or line sources like roads or landfills.
- No Chemical Reactions Considered: Ignores chemical transformations (e.g., SO₂) in the atmosphere.
- Assumes Homogeneous Atmosphere: Assumes uniform wind speed and direction, which rarely exists in real atmospheric conditions.
- Not Valid for Calm or Very High Wind Conditions: Inaccurate when wind speed is near zero or extremely high, as plume behavior deviates.
- Does Not Account for Plume Rise Dynamics Fully: Simplifies plume rise, ignoring effects of temperature gradients and buoyancy variations.