📖 In-Depth Theory
Potable Water
POTABLE WATER is water that is SAFE TO DRINK — it has sufficiently low levels of dissolved substances and microbes.
Potable water is NOT the same as PURE water:
Pure water contains ONLY H₂O molecules — no dissolved salts, minerals, microbes.
Potable water contains low but acceptable levels of dissolved minerals — which are actually needed for good health (calcium, magnesium, fluoride etc.).
GLOBAL WATER AVAILABILITY:
Only ~3% of Earth's water is freshwater.
Of this, most is locked in ice caps — less than 1% is accessible freshwater.
Water scarcity affects billions of people.
SOURCES OF WATER IN THE UK:
SURFACE WATER — rivers, lakes, reservoirs.
GROUND WATER — aquifers underground, accessed by wells and boreholes.
SEAWATER — can be desalinated (expensive and energy-intensive).
Treating Fresh Water to Make it Potable
UK fresh water treatment process:
STEP 1 — SEDIMENTATION:
Water flows into large settling tanks.
Large particles and suspended solids SINK to the bottom (sedimentation).
Alternatively, a coagulant (e.g. aluminium sulfate) is added → particles clump together (flocculation) → settle faster.
STEP 2 — FILTRATION:
Water passes through sand and gravel filters.
Removes finer particles and some microorganisms.
STEP 3 — CHLORINATION:
Small amounts of CHLORINE (or ozone/UV) added to kill harmful microorganisms (bacteria, viruses).
Chlorine is very effective and maintains protection as water travels through pipes.
RESULT: potable water — not pure, but safe to drink.
PURIFYING SEAWATER — DESALINATION:
TWO METHODS:
1. DISTILLATION — boil seawater, condense steam → pure water. Expensive (high energy).
2. REVERSE OSMOSIS — force seawater through membranes that block dissolved salts. Energy-intensive.
Used in water-scarce regions (Middle East, some islands).
Not widely used in UK — cheaper freshwater sources available.
Waste Water Treatment
WASTE WATER includes:
SEWAGE — water from toilets, drains, containing human waste, bacteria, organic matter.
AGRICULTURAL WASTE — nitrates, pesticides.
INDUSTRIAL WASTE — may contain heavy metals, chemicals.
Must be treated before being released back into rivers or the sea.
SEWAGE TREATMENT PROCESS:
STEP 1 — SCREENING:
Large solids removed by screens/grilles.
STEP 2 — SEDIMENTATION:
Remaining solids sink to form SLUDGE.
Liquid (effluent) is separated.
STEP 3 — BIOLOGICAL TREATMENT:
Effluent undergoes AEROBIC DIGESTION — air is pumped through, aerobic bacteria break down organic matter → CO₂ + H₂O.
Some plants also use anaerobic digestion.
STEP 4 — SLUDGE TREATMENT:
SLUDGE undergoes ANAEROBIC DIGESTION — bacteria digest organic matter WITHOUT oxygen.
Produces BIOGAS (mainly methane) — can be used as fuel.
Remaining solids (digestate) can be used as fertiliser.
FINAL STEP — CHLORINATION:
Effluent disinfected with chlorine before discharge to rivers/sea.
⚠️ Common Mistake
Potable water is NOT the same as PURE water. Potable = safe to drink (low levels of dissolved minerals and microbes). Pure = only H₂O. Tap water is potable but not chemically pure — it contains dissolved calcium, chlorine, fluoride etc. Distilled water is pure but not ideal for drinking long-term (lacks minerals).