Water Treatment Processes

Q1 — Treatment stages table (blank cells)

Coagulation, chemical / reagent: Alum, Al₂(SO₄)₃ (provides Al²⁺ which hydrolyses to Al(OH)₃).

Flocculation, what is removed: Small destabilised particles collide and grow into larger flocs that can settle under gravity.

Sedimentation, NSW example: Prospect WTP (Sydney Water) or any major NSW drinking-water plant.

Filtration, what is removed: Remaining fine particles; dissolved organic compounds affecting taste, odour and DBP precursor load (via activated carbon).

Disinfection (alternative), reagent: Chloramines (formed by reacting Cl₂ with NH₃).

Fluoridation, what is achieved: Raises fluoride concentration to ~0.6–1.1 mg L⁻¹ to reduce dental caries in the community.

Reverse osmosis, NSW/Australian example: Perth Seawater Desalination Plant (Water Corporation), Kwinana, Western Australia.

Fluoridation, what is achieved: Raises fluoride concentration to ~0.6–1.1 mg L⁻¹ to reduce dental caries in the community; practised across all major NSW reticulated water supplies.

Q2 — Chlorine species graph

(a) Approximately 95–97% at pH 6 (HOCl dominates strongly at low pH — accept 90–99%).

(b) As pH increases from 5 to 10, the percentage of total chlorine present as HOCl decreases steeply (from ~100% to ~0%). The relationship is sigmoidal — the steepest decrease occurs between pH 7 and pH 8.5. 1 mark for “decreases/falls”; 1 mark for describing the sigmoidal or steep nature / quantifying with reference to graph.

(c) At pH 7–7.5 the graph shows HOCl still represents roughly 50% or more of total chlorine, maintaining adequate disinfection. At pH 9 nearly all chlorine is present as OCl⁻, which is a much weaker disinfectant. Allowing pH to rise would require a much higher chlorine dose to achieve the same pathogen inactivation, increasing cost and the risk of DBP formation. 1 mark for reading HOCl% from the graph at each pH; 1 mark for linking higher pH to lower HOCl fraction; 1 mark for practical implication (effectiveness/dose/cost).

(d) The crossover at pH 7.5 represents the point at which HOCl and OCl⁻ are present in equal concentrations — each constitutes 50% of total chlorine. Chemically, this is the pKa of hypochlorous acid (HOCl ⇌ H⁺ + OCl⁻); at pH = pKa, the equilibrium concentrations of the acid and its conjugate base are equal. 1 mark per correct point.

Q3 — Cause-and-effect chain

Boxes (left to right after the filled cause):

1. More organic precursors (NOM) enter the treatment plant and reach the filtration/disinfection stages.
2. During chlorination, HOCl reacts with the dissolved organic matter to form disinfection by-products (DBPs), including trihalomethanes (THMs).
3. THM concentrations may exceed safe regulatory limits (e.g. NHMRC guidelines), posing a potential cancer risk.

Overall outcome: The plant must optimise organic-matter removal before chlorination (e.g. increase activated carbon filtration) and/or consider alternatives such as chloramines to minimise DBP formation while still maintaining effective disinfection.

Q4 — Murray–Darling case study (5 marks)

Award up to 5 marks for identifying three challenges with explanation and matching treatment stage. Accept any three from:

• High turbidity/suspended sediment → coagulation/flocculation stage must use higher alum dose and more thorough mixing to form flocs from the elevated particle load; sedimentation time may need extending.
• High organic matter load (NOM from algal blooms/decaying matter) → activated carbon filtration must be optimised or extended, and chlorine dose must be carefully managed to avoid excessive THM/DBP formation at the disinfection stage.
• Elevated nutrients (phosphate, nitrate) from agricultural runoff → conventional coagulation with alum can assist with phosphate removal (phosphate co-precipitates with Al(OH)₃), but tertiary treatment (chemical precipitation or biological denitrification) may be needed for nitrate.
• Cyanobacterial toxins (e.g. microcystins from blue-green algae) → some toxins pass through conventional treatment; activated carbon adsorption is critical; UV disinfection or ozone may be added as a specific measure.
• Variable/high organic load increasing DBP risk during chlorination → pre-chlorination organic removal via activated carbon essential; chloramines may be substituted to reduce THM formation.

Mark 1 mark each for challenge clearly identified; 1 mark for matching treatment stage with justification — any two full challenge+stage pairs = 4 marks; third partial credit = 5th mark.

Q5 — Disinfection methods comparison

Feature	Free chlorine	Chloramines	UV radiation	Reverse osmosis
Active species / mechanism	HOCl (hypochlorous acid)	NH₂Cl and related species (formed from Cl₂ + NH₃)	UV light inactivates pathogens directly	Semi-permeable membrane removes dissolved salts and contaminants
DBP (THM) risk	High when NOM present	Lower than free chlorine; fewer halogenated DBPs	No chlorinated DBPs formed	Very low (post-RO water has very low NOM)
Residual in distribution?	Yes	Yes (useful residual)	No residual	Not applicable (desalination, not disinfection)
Speed of action	Fast	Slower than free chlorine	Fast	Not applicable
Main limitation	DBP risk with high NOM	Slower disinfection action	No residual protection in distribution	Very high energy cost; produces brine waste
Main advantage	Cost-effective, strong residual	Fewer DBPs; useful residual	No chlorine-based DBPs	Removes dissolved salts; climate-independent supply