Desalination Plant Chemical Program Design

Chemical Program Overview

Treatment Stage	Chemical	Purpose	Typical Dose
Intake	NaOCl	Biofouling prevention	1–3 ppm Cl₂ (shock)
Pre-chlorination	Sodium bisulfite	Dechlorination before RO	3 ppm (stoichiometric)
Coagulation	FeCl₃ or membrane flocculant	SDI reduction	1–5 ppm
pH adjustment	H₂SO₄	Lower pH for CaCO₃ control	To pH 6.5–7.0
Antiscalant	RO antiscalant	Scale prevention	2–5 ppm
CIP cleaning	Alkaline + acid cleaners	Restore membrane performance	2–4% solution
Post-treatment	Ca(OH)₂ + CO₂	Remineralization	To LSI ~0
Final disinfection	NaOCl	Residual disinfection	0.2–0.5 ppm Cl₂

Pretreatment Design

Goal: SDI < 3, turbidity < 0.5 NTU, zero chlorine.

Options (from basic to advanced):

1. Conventional: Coagulation + sand filter + cartridge filter
2. UF/MF: Direct membrane pretreatment (preferred for new plants)
3. DAF + UF: For high-algae source water

Critical: Remove ALL free chlorine before RO membranes — polyamide membranes are destroyed by oxidants.

Antiscalant Selection for Desalination

Seawater RO: General purpose antiscalant typically sufficient (recovery 35–45%) Brackish water RO: May need high-hardness grade at high recovery (75–90%)

Dose calculation: Based on concentrate composition, not feedwater.

CIP Schedule

Indicator	Action
Normalized permeate flow −10%	Clean (alkaline first, then acid)
ΔP +15%	Clean (check for biofilm)
Salt passage +50%	Clean (check for scaling)
Every 3–6 months	Preventive CIP even if no decline

Post-Treatment

RO permeate is aggressive (low pH, low alkalinity, low hardness). Remineralization required:

• Lime + CO₂: Ca(OH)₂ saturator + CO₂ injection → target LSI ≈ 0, pH 7.5–8.5
• Limestone contactor: Permeate through calcite bed — simpler but slower
• Add NaOCl for final disinfection residual