CHAPTER 10

Advanced Water Treatment — Membrane, Ion Exchange, Defluoridation

Q: How much fluoride can activated alumina remove?

Capacity 1-4 mg fluoride per gram of alumina. Breakthrough after 5000-20000 bed volumes. Regenerate with NaOH (removes fluoride) then HCl (restores acidic surface). Cost ₹50-150 per m³ treated water.

Q: How to remove iron from groundwater?

Aeration (spray aerator, cascade) oxidizes ferrous iron (Fe²⁺) to ferric (Fe³⁺), which precipitates as Fe(OH)₃; removed by filtration. Contact time 10-15 min aeration; filter HLR 5-10 m/hr. For > 10 mg/L iron, use multi-stage aeration.

Advanced Treatment

Specifies advanced treatment processes for removing specific contaminants — arsenic, fluoride, iron/manganese, nitrate, TDS, organic micropollutants. Includes reverse osmosis, nanofiltration, ion exchange, activated alumina, aeration-filtration, Nalgonda technique.

⚗ Water TreatmentManual on Water Supply and Treatment3rd Edition (1999) with 2024 revision updates

Key formulas

RO recovery: R = Q_permeate / Q_feed × 100%. Typical 70-80% for brackish water RO.
Flux (RO): J = Q_permeate / A_membrane (L/m²/hr); typical 15-25 LMH at design pressure.
Specific energy (RO): E = (P × Q_feed) / Q_permeate, typical 3-5 kWh/m³ for seawater, 0.5-1.5 for brackish.
Ion exchange capacity (for hardness): C = Q_total / V_resin (eq/L); typical 1.5-2.0 eq/L for cation resin.
Nalgonda coagulation for fluoride: Lime dose + Alum dose ratio 1:8 approximately; contact 15 min, settle 60 min.

Key values & thresholds

RO recovery brackish pct: 70 - 80
RO recovery seawater pct: 40 - 50
RO flux LMH: 15 - 25
RO pressure seawater bar: 55 - 70
RO pressure brackish bar: 10 - 20
RO TDS rejection pct: 95 - 99
RO specific energy seawater kWh per m3: 3 - 5
RO specific energy brackish kWh per m3: 0.5 - 1.5
activated alumina fluoride capacity mg per g: 1 - 4
nalgonda alum dose mg per L per mg F: 150 - 200
nalgonda lime dose mg per L per mg F: 20 - 30
iron removal aeration contact min: 10 - 15
iron removal filter HLR m per hr: 5 - 10
ion exchange capacity eq per L: 1.5 - 2.0

Clause-level requirements

Select process based on contaminant: arsenic (iron co-precipitation, activated alumina, RO), fluoride (Nalgonda alum-lime, activated alumina, RO), iron (aeration + filtration), nitrate (ion exchange, RO), high TDS (RO, nanofiltration).
Reverse Osmosis (RO): membrane rejects 95-99% of dissolved solids. Seawater RO (TDS 35,000 → < 500) at 55-70 bar, recovery 40-50%. Brackish water RO (TDS 2000-10000) at 10-20 bar, recovery 70-80%.
Activated Alumina: batch or continuous; capacity 1-4 mg fluoride per gram; regenerate with NaOH + HCl. Cost ₹50-150/m³ treated water.
Nalgonda technique: alum + lime flocculation/sedimentation; low-cost, suitable for rural defluoridation. Residual aluminum concern; aluminum < 0.2 mg/L acceptable.
Iron removal: aeration (cascade, spray, compressor) + filtration. Removes ferrous iron by oxidation to ferric, precipitates as Fe(OH)₃. Manganese removal similar but requires higher pH (> 8.5) or permanganate.
Ion exchange: cation exchange for hardness (softening); anion exchange for nitrate, sulfate; selective ion exchange for fluoride, arsenic. Regenerate with brine (for hardness) or NaOH/HCl.

Practitioner notes — what goes wrong in the field

RO adoption in India: Chennai Nemmeli (100 MLD seawater), Minjur (100 MLD), Jamnagar (45 MLD for industrial), plus numerous packaged household RO units.
Household RO (2-10 L/hr): widespread in Indian urban households due to hardness concerns. Waste 3-4 L water per L produced — environmental issue.
RO concentrate (reject) disposal: 30-50% of feed water as brine. Disposal options: sea outfall (coastal), evaporation ponds, deep well injection. Inland RO faces disposal challenges.
Activated alumina for fluoride: effective but expensive. Used in Andhra Pradesh (Nalgonda district, Guntur), Rajasthan, Karnataka fluoride endemic zones.
Nalgonda technique: cheaper alternative, rural-suitable. Alum dose 150-200 mg/L per mg/L fluoride. Residual aluminum needs monitoring.
Iron removal: simple aeration (spray aerator, cascade) + filtration typically adequate. For high iron (> 10 mg/L), provide multiple aeration stages.
Manganese removal: trickier than iron — needs pH > 8.5 or oxidants (KMnO₄, chlorine dioxide). Manganese above 1 mg/L causes black staining.
Arsenic removal: iron-based coagulation (FeCl₃) + filtration works well; activated alumina also effective. For West Bengal arsenic-affected areas, community-scale treatment preferred over household units.
Nitrate removal: ion exchange or RO. Blending with low-nitrate source often most economical for municipal supply.
Cost comparison: conventional treatment ₹5-15/m³; RO ₹30-60/m³; activated alumina fluoride removal ₹50-150/m³; Nalgonda ₹10-20/m³. RO is expensive but only option for high-TDS sources.
Energy recovery in RO: turbo-expander or pressure exchanger recovers energy from pressurized concentrate stream. Reduces specific energy by 30-40%.
Membrane fouling: biofilm, scaling, organic fouling. Pretreatment (cartridge filter, antiscalant, biocide) essential. Cleaning (CIP) every 1-3 months extends membrane life.

FAQs

What is RO recovery?

Percentage of feed water recovered as permeate. Brackish water RO: 70-80%; seawater RO: 40-50%. Higher recovery concentrates the reject, increases fouling risk. Balance recovery vs membrane life.

What specific energy for RO?

Seawater RO: 3-5 kWh/m³ (with energy recovery); brackish water RO: 0.5-1.5 kWh/m³. Major operating cost. Energy recovery devices (pressure exchangers) reduce by 30-40%.

How does Nalgonda technique remove fluoride?

Alum + lime added to water; alum hydroxide flocs adsorb fluoride during sedimentation. Dose: 150-200 mg/L alum per mg/L fluoride removed + 20-30 mg/L lime. Contact 15 min + settle 60 min. Residual alum (Al) must be < 0.2 mg/L.

How much fluoride can activated alumina remove?

Capacity 1-4 mg fluoride per gram of alumina. Breakthrough after 5000-20000 bed volumes. Regenerate with NaOH (removes fluoride) then HCl (restores acidic surface). Cost ₹50-150 per m³ treated water.

How to remove iron from groundwater?

Aeration (spray aerator, cascade) oxidizes ferrous iron (Fe²⁺) to ferric (Fe³⁺), which precipitates as Fe(OH)₃; removed by filtration. Contact time 10-15 min aeration; filter HLR 5-10 m/hr. For > 10 mg/L iron, use multi-stage aeration.

Can RO remove arsenic?

Yes — 95-99% rejection. But RO is expensive (₹30-60/m³). For arsenic removal, activated alumina or iron co-precipitation (FeCl₃) is more economical and widely deployed in West Bengal, Bihar arsenic-affected areas.

What is ion exchange for water softening?

Cation exchange resin (sodium-form) exchanges Na⁺ for Ca²⁺ and Mg²⁺ in water, reducing hardness. Capacity 1.5-2.0 equivalents per liter resin. Regenerate with brine (10-15% NaCl) every few days/weeks depending on hardness.

Cross-references

CPHEEO WS Chapters 4 & 7IS 10500IS 13405