Sources and Types of Water Pollution
Water pollution occurs when harmful substances contaminate bodies of water — rivers, lakes, groundwater, and oceans — impairing their chemical, physical, or biological quality. Understanding pollution requires identifying both the contaminants and their chemical behavior in aquatic systems.
Major categories of water pollutants include: - Nutrients (nitrogen and phosphorus from agricultural runoff and sewage) → cause eutrophication - Pathogens (bacteria, viruses, protozoa from human and animal waste) - Heavy metals (lead, mercury, cadmium, arsenic) from industrial discharges and mining - Organic compounds (pesticides, pharmaceuticals, petroleum products, industrial solvents) - Suspended solids (silt from construction, erosion) - Thermal pollution (heated water from power plant cooling, reduces dissolved oxygen) - Acidification from acid rain or acid mine drainage
Chemistry of Eutrophication
Eutrophication is the over-enrichment of water with nutrients, particularly nitrates (NO₃⁻) and phosphates (PO₄³⁻). This triggers explosive algal growth ("algal blooms"). When the algae die and decompose, microbial respiration consumes dissolved oxygen:
Organic matter + O₂ → CO₂ + H₂O + nutrients
The resulting hypoxic or anoxic conditions (called "dead zones") kill fish and other aquatic life. The Gulf of Mexico dead zone, fed by Mississippi River agricultural runoff, covers thousands of square kilometers each summer.
Phosphorus control is often more important than nitrogen because phosphorus is the limiting nutrient in most freshwater systems — reducing phosphate inputs can halt eutrophication even when nitrogen is abundant.
Drinking Water Treatment Chemistry
Producing safe drinking water involves a sequence of physical and chemical processes.
Coagulation and Flocculation
Raw water typically contains fine suspended particles too small to settle by gravity. Coagulants — typically aluminum sulfate (Al₂(SO₄)₃, "alum") or iron(III) chloride (FeCl₃) — are added to neutralize the negative charges on particle surfaces.
Al₂(SO₄)₃ + 6 H₂O → 2 Al(OH)₃↓ + 3 H₂SO₄
The Al(OH)₃ floc forms sticky, gelatinous particles that trap colloidal material and bacteria. Gentle stirring (flocculation) encourages particles to collide and aggregate into larger flocs that settle in sedimentation tanks.
Filtration
Settled water passes through sand and gravel filters, removing remaining particles and some microorganisms. Activated carbon filters adsorb dissolved organic compounds, chlorine taste/odor, and trace pesticides.
Disinfection
This is the most critical step for public health. The three main chemical disinfectants are:
Chlorination (most common worldwide): Cl₂ + H₂O ⇌ HOCl + H⁺ + Cl⁻
Hypochlorous acid (HOCl) is the active germicidal species — it penetrates bacterial cell walls and oxidizes essential enzymes. At pH 7.5, roughly 50% of dissolved chlorine exists as HOCl. At pH 9, it shifts almost entirely to the less effective hypochlorite ion (OCl⁻).
A key challenge is the formation of disinfection byproducts (DBPs): when chlorine reacts with natural organic matter (humic acids from decaying vegetation), it produces trihalomethanes (THMs) such as chloroform (CHCl₃) and haloacetic acids (HAAs), which are potential carcinogens. Removing organic matter before chlorination minimizes DBP formation.
Ozonation (O₃) is a more powerful oxidant than chlorine, effective against viruses and protozoa (including Cryptosporidium, which is chlorine-resistant). Ozone leaves no persistent residual, so a small amount of chlorine is typically added afterward to maintain disinfection through distribution.
UV disinfection uses germicidal UV light (254 nm wavelength) to damage microbial DNA without adding chemicals, thus producing no DBPs. It is increasingly used as a secondary or primary disinfectant.
pH Adjustment and Corrosion Control
Water is adjusted to pH 7–8 using lime (Ca(OH)₂) or sodium hydroxide (NaOH) to minimize pipe corrosion. In the Flint, Michigan water crisis (2014–2019), failure to add orthophosphate corrosion inhibitors led to lead leaching from old service lines into drinking water — a catastrophic public health failure rooted in basic water chemistry.
Wastewater Treatment
Wastewater (sewage) requires treatment before discharge. The process typically has three stages:
Primary treatment: Physical removal of large solids by screening and sedimentation. Removes ~50% of suspended solids.
Secondary treatment: Biological treatment using microorganisms to consume dissolved organic matter. In the activated sludge process, aerobic bacteria oxidize organics: Organic compounds + O₂ → CO₂ + H₂O + biomass
This reduces Biological Oxygen Demand (BOD) — the oxygen required to biologically oxidize the organic content — by 85–95%.
Tertiary treatment: Advanced processes for specific pollutants: - Nitrogen removal: Nitrification (NH₄⁺ → NO₃⁻ by Nitrosomonas bacteria) followed by denitrification (NO₃⁻ → N₂ by facultative anaerobes) - Phosphorus removal: Chemical precipitation with alum or ferric salts, or biological phosphorus removal - Micropollutant removal: Advanced oxidation processes (ozone + UV or ozone + H₂O₂) to degrade pharmaceuticals and persistent organic pollutants
Emerging Contaminants
Pharmaceuticals and personal care products (PPCPs) — including antibiotics, hormones (17α-ethinylestradiol from birth control pills), and antidepressants — are present at nanogram-per-liter concentrations in surface waters worldwide. Conventional treatment removes them incompletely. Microplastics (particles < 5 mm) are now detected in tap water, bottled water, and even the deepest ocean trenches. These represent the frontier of water treatment chemistry.