Water Pollution Overview

Water Pollution

• Pure water is not found outside a lab.
• Water is considered polluted when it has enough foreign materials to make it unsuitable for its intended use (e.g., drinking, recreation).

Pollutant Classification

• Pollutants can be categorized by their source:

  • Point sources: Easier to manage; pollutants from a single source are collected and treated at a single point.
  • Dispersed (non-point) sources: Harder to manage; pollutants come from many different sources making them harder to control.

• Pollutants can also be categorized by their effects and origin:

  • Pathogenic organisms
  • Oxygen-demanding substances
  • Plant nutrients
  • Toxic organics
  • Inorganic chemicals
  • Sediments
  • Radioactive substances
  • Heat
  • Oil

Water Quality Expression

• Water quality is often expressed in quantitative terms (e.g., mass per unit volume).
• Common units include milligrams per liter (mg/L), parts per million (ppm), parts per billion (ppb), and percentages.

Mass per Unit Volume

• Milligrams per liter (mg/L) is a common measure of concentration.
• If 0.3 grams of salt is dissolved in 1500 mL of water, the concentration is 200 mg/L.
• Concentrations in air are often expressed in micrograms per cubic meter (µg/m³).

Part per Million (ppm)

• 1 ppm = 1 mg of solute per 1,000,000 mg of water.
• 1 mg/L = 1 ppm

Percentage Concentration

• Concentrations above 10,000 mg/L are usually expressed as percentages.
• Percentage = (mass of solute (mg) / mass of solvent (mg)) x 100

Toxic and Radioactive Substances

• Toxic substances can cause immediate harm, chronic illnesses, or passing on harm to future generations (e.g., cancer, mutations).
• Examples of toxic heavy metals are Cadmium (Cd), Chromium (Cr), Lead (Pb), Mercury (Hg), Silver (Ag), Arsenic (As), Barium (Ba), and Selenium (Se).
• Radioactive substances are undesirable and have maximum allowable concentrations for public water.

Radiation

• Radiation from unstable atoms poses a hazard and maximum allowable concentrations are set for radioactive materials.
• Radon, occurs naturally in groundwater.
• Sources include nuclear power plants, industrial/medical radioactive chemicals and uranium ore refining.

Biological Parameters

• The presence or absence of living organisms provides a good indication of water quality.
• Diversity of fish and insects indicates the health of the aquatic environment.
• The disappearance of species and overabundance of others can indicate pollution.
• Examples of biological indicators are bacteria, algae, protozoa, viruses and coliform.

Aerobic and Anaerobic Digestion

• Aerobic digestion requires oxygen; it breaks down waste into carbon dioxide, water, nitrates, and biomass.
• Anaerobic digestion does not require oxygen; it produces methane and carbon dioxide, often called biogas.

Advantages of Aerobic Digestion

• Efficient at breaking down waste products.
• Yields better effluent quality.
• Releases substantial energy.

Anaerobic Digestion Advantages

• Transforms pollutants into methane, carbon dioxide, and smaller bio-solids.
• Biomass growth is less in anaerobic processes compared to aerobic ones.
• More compact than aerobic bio-solids.

Anaerobic Digestion Steps

• Hydrolysis: complex organic matter is broken down into simple, soluble components.
• Fermentation: carbohydrates are broken down by microbes into organic acids, gases, and more microorganisms.
• Acetogenesis: fermentation products are converted into acetate, hydrogen, and carbon dioxide.
• Methanogenesis: acetate, hydrogen, and carbon dioxide are converted into methane by methanogenic bacteria

Aerobic Decomposition

• Microorganisms use organic matter and oxygen to produce stable solids, carbon dioxide, and more organisms.
• Organisms that require oxygen are known as aerobic organisms.

Anoxic Decomposition

• Microorganisms use chemically combined oxygen (like Nitrates and Nitrites) from organic matter to produce nitrogen gas, carbon dioxide, stable solids, and more organisms.

Demineralization

• Water in nature contains minerals that can harm both humans and animals.
• Demineralization removes mineral by using ion exchange.
• Positively charged ions (e.g., calcium, magnesium) are removed by cation beds while negatively charged ions (e.g., sulfate, chloride) are removed by anion beds.

Desalting

• Desalting is used to obtain fresh water from salty water resources (e.g., ocean).
• Greek sailors used evaporation 4th Century BC
• Desalination plants produce over 6 billion gallons of water a day.
• Thermal or membrane processes are used for desalting.

Desalting Processes

• Thermal processes (distillation): heats water to evaporate salt, condensing the fresh water vapor.
• Membrane processes (ED and RO): Use membranes to separate salts from fresh water using pressure and membranes

Electrodialysis

• Electrodialysis uses membranes based on electrical charge.
• Positive and negatively charged ions are attracted to oppositely charged electrodes.

Membrane Processes

• Reverse Osmosis (RO): Uses pressure on salt water to push water through a membrane while salt is left behind
• Electrodialysis (ED): Uses membranes and electrical current to separate ions by charge.

Conclusions

• Depending on the type of water (seawater or brackish), Nanofiltration plants provide approximately 85-95% water recovery while RO systems typically recover 50-80%.