CHEM3120 · Environmental and Analytical Chemistry
Water Purification & Desalination
Lecture 25 closes CHEM3120 with the chemistry of making water safe and fresh: coagulation and flocculation to remove suspended clay and colloids, sedimentation and filtration, disinfection, and distillation and reverse-osmosis desalination, assembled into a treatment train. Stokes-law sedimentation is the examinable calculation; the treatment-train logic appears as short-answer and MCQ, and this material is on Prof. Kepert's examinable Block 3 list.
What this chapter covers
- 01Suspended solids, colloids and turbidity in raw water
- 02Coagulation and flocculation: destabilising and aggregating fine particles so they settle
- 03Sedimentation (Stokes' law) and filtration to remove aggregated solids
- 04Stokes settling velocity v = (2/9)·g·(ρ_s - ρ_d)·r²/η
- 05Disinfection of treated water
- 06Distillation and reverse osmosis (RO) for desalination
- 07Assembling a treatment train from these unit processes
Stokes-law settling velocity of a clay particle
- +1Identify the density difference and the squared radius: ρ_s - ρ_d = 1800 - 1000 = 800 kg/m^3; r² = (5.0×10^-6)² = 2.5×10^-11 m².
- +1Substitute into Stokes' law: v = (2/9) × 9.81 × 800 × 2.5×10^-11 ÷ (1.0×10^-3).
- +1Evaluate: (2/9)×9.81 = 2.18; ×800 = 1744; ×2.5×10^-11 = 4.36×10^-8; ÷1.0×10^-3 = 4.4×10^-5 m/s.
Key terms
- Coagulation
- Chemically destabilising fine suspended particles (e.g. with an added electrolyte) so their mutual repulsion is overcome and they can begin to aggregate.
- Flocculation
- Gentle mixing that brings destabilised particles together into larger aggregates (flocs) big enough to settle or be filtered out.
- Sedimentation (Stokes' law)
- Gravity settling of suspended particles; the terminal velocity v = (2/9)g(ρ_s - ρ_d)r²/η rises with the square of particle radius and the density difference.
- Reverse osmosis (RO)
- Desalination by forcing water through a semipermeable membrane against the osmotic pressure, leaving dissolved salts behind; an energy-efficient alternative to distillation.
- Distillation
- Desalination or purification by evaporating water and condensing the vapour, leaving non-volatile salts and solids behind; energy-intensive but robust.
- Treatment train
- The sequence of unit processes (coagulation, flocculation, sedimentation, filtration, disinfection, and desalination if needed) assembled to take raw water to a required quality.
Water Purification & Desalination FAQ
What is the difference between coagulation and flocculation?
They are two stages of the same goal — getting fine particles to settle. Coagulation chemically destabilises the particles, overcoming the surface charge that keeps them apart, usually by adding an electrolyte or coagulant. Flocculation then uses gentle mixing to let those destabilised particles collide and grow into larger aggregates (flocs). Coagulation makes sticking possible; flocculation grows the flocs big enough to remove by sedimentation or filtration.
Why must fine clay be coagulated before sedimentation?
Because Stokes' law makes fine particles settle far too slowly. Settling velocity scales with the square of the radius, so a few-micron clay particle drifts down at only tens of micrometres per second — hours to clear a basin. Coagulation and flocculation combine many small particles into much larger flocs, and because velocity goes as r², even a modest increase in size produces a large jump in settling speed, making sedimentation practical.
How does reverse osmosis differ from distillation for desalination?
Both remove dissolved salts but by different physics. Distillation evaporates the water and condenses pure vapour, leaving salts behind — robust but energy-intensive because it must supply the latent heat of vaporisation. Reverse osmosis instead pushes water through a semipermeable membrane under pressure that exceeds the osmotic pressure, so the salts are held back mechanically. RO is generally more energy-efficient for seawater, which is why it dominates modern desalination.
Can AI help me with the water-purification topics in CHEM3120?
Yes. Sia can work a Stokes-law settling calculation with you step by step, explain the logic of a treatment train, and contrast coagulation/flocculation and distillation/reverse osmosis in the short-answer style this closing block uses. It explains the method and checks your reasoning; it does not do graded assessment for you, and University of Sydney academic-integrity rules apply.
Exam move
This closing chapter blends one key calculation with process reasoning. Secure the Stokes-law routine — substitute in strict SI units and remember velocity scales with r², which is the whole justification for coagulation and flocculation. Then be able to assemble and justify a treatment train (coagulation → flocculation → sedimentation → filtration → disinfection, plus distillation or reverse osmosis for desalination) and to contrast the two desalination routes on energy grounds. Because this is on Prof. Kepert's examinable Block 3 list and lands late in the semester, fold it into your STUVAC revision so it is fresh for the closed-book final, which supplies a formula sheet. Confirm the exam date, room and permitted materials on Canvas and the exam timetable.
Working through Water Purification & Desalination in CHEM3120? Sia is AskSia’s AI Chemistry tutor — ask any CHEM3120 Water Purification & Desalination question and get a clear, step-by-step explanation grounded in how CHEM3120 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
