How to Use SDIC (Sodium Dichloroisocyanurate) in Water Treatment

Overview

SDIC is a stabilized chlorine donor containing 56–60% available chlorine. It dissolves in water to release hypochlorous acid (HOCl), the active biocidal agent. Unlike sodium hypochlorite, SDIC releases chlorine slowly and maintains residual longer due to the cyanuric acid stabilizer. Widely used in cooling water, swimming pools, and drinking water disinfection.

Preparation & Dissolution

Tablets: Drop into chlorinator/feeder or dissolve in bucket. Do NOT crush tablets.

Granules/Powder: Dissolve in clean water to make 1–2% stock solution:

1. Add granules to water (never water to granules)
2. Stir gently until dissolved (5–10 min)
3. Use within 24 hours

Dose calculation: To get 1 ppm free Cl₂, add approximately 1.7 ppm SDIC (60% available chlorine).

Dosing Guide

Application	Free Cl₂ Target	SDIC Dose (approx.)
Cooling tower continuous	0.3–0.5 ppm	0.5–1.0 ppm product
Cooling tower shock	2–3 ppm (30 min)	3.5–5.0 ppm product
Swimming pool	1–3 ppm	1.7–5.0 ppm product
Drinking water	0.2–0.5 ppm residual	0.3–0.9 ppm product
Emergency disinfection	5 ppm (30 min contact)	8.5 ppm product

Application Procedure

1. Determine system volume (m³)
2. Calculate required SDIC: Volume × target Cl₂ (ppm) × 1.7 = grams of SDIC
3. Dissolve in bucket of water or use automatic feeder
4. Add to system — inject at a point with good mixing
5. Test free chlorine after 30 minutes using DPD test kit
6. Adjust dose to maintain 0.3–0.5 ppm free Cl₂ residual
7. For continuous dosing, use timer-controlled feeder or metering pump with SDIC solution

Monitoring & Control

Parameter	Frequency	Target
Free chlorine	2x daily minimum	0.3–0.5 ppm (cooling) / 1–3 ppm (pool)
pH	Daily	7.0–7.6 optimal for HOCl
Cyanuric acid	Monthly	< 50 ppm (pool), not critical for industrial
Total bacteria count	Monthly	< 10⁴ CFU/mL

Common Mistakes

• Mixing SDIC with acid: Releases chlorine gas — potentially lethal. NEVER mix with HCl, H₂SO₄, or any acid.
• Mixing with other pool chemicals: Do NOT mix SDIC with calcium hypochlorite or liquid bleach — can cause fire or explosion.
• Ignoring pH: At pH > 8.0, only 20% of chlorine is in the active HOCl form. Maintain pH 7.0–7.6 for effectiveness.
• Cyanuric acid buildup: In pools, cyanuric acid accumulates over time and "locks" chlorine. Dilute with fresh water if CYA > 80 ppm.

Storage & Handling

• Shelf life: 24–36 months in sealed container at room temperature
• Temperature: Store < 40°C. Keep dry — moisture accelerates decomposition.
• Container: HDPE or lined fiber drums. Keep away from organics, acids, and other oxidizers.
• Safety: Strong oxidizer. Causes eye/skin burns. Store separately from other chemicals. Fire risk if contaminated with organics.