Dechlorination: Using Sodium Bisulfite Safely

Why this matters (risk in one sentence)

Oxidants like free chlorine and chloramine can irreversibly damage many membrane materials and downstream chemistries; sodium bisulfite is a common and effective reducer, but it must be dosed and monitored correctly to prevent oxidation events on one side and overfeed side effects on the other.

Where sodium bisulfite fits

• Most common: ahead of RO/UF membranes after chlorination, or after carbon where residual oxidant can break through.
• Also used: dechlorinating rinse waters, neutralizing oxidants before discharge (site-specific compliance), protecting specialty resins and catalysts.
• Alternative reducers: activated carbon (physical adsorption), sodium thiosulfate (some lab/low-flow applications), catalytic carbon for chloramine (system-dependent).

What you’re actually doing (simple chemistry)

Sodium bisulfite (SBS, typically supplied as an aqueous solution) reduces oxidants to chloride (and related non-oxidizing species), consuming dissolved oxygen in the process. In practice, you treat it as an electron donor that “removes oxidizing power,” which is why it pairs well with ORP control.

• Free chlorine removal: fast reaction; control is usually straightforward with good mixing.
• Chloramine removal: slower and more variable; may need longer contact time and more careful monitoring.
• Key implication: stable, repeatable dosing depends as much on injection design and instrumentation as on chemical selection.

Key decision factors (engineering + commercial)

• Oxidant type: free chlorine vs. chloramine; “total chlorine” reading may hide chloramine challenges.
• Flow variability: stable flow allows fixed ratio dosing; variable flow often needs paced dosing (flow-proportional) or ORP trim.
• Contact time & mixing: injection point location, static mixer, pipe length, and turbulence matter.
• Materials compatibility: pump head/diaphragm, elastomers, and tubing must match SBS chemistry and site conditions.
• Monitoring philosophy: chlorine residual, ORP, or both; define the primary control variable and the verification check.
• Procurement constraints: preferred packaging (IBC/drum/bulk), shelf life expectations, documentation (SDS/COA), and origin/traceability.

Step-by-step implementation

1) Define your “safe” target

Start by defining what “dechlorinated” means for your equipment warranty and site SOP: many membrane and resin systems require non-detect or very low residual oxidant at the membrane inlet. Confirm the required limit with your membrane OEM and site engineering.

2) Choose a control method

• Flow-proportional dosing (common): set a dose ratio (mg/L as product) based on influent oxidant and adjust with periodic verification.
• ORP-trim dosing (robust): flow pacing plus an ORP feedback loop to correct for oxidant variability and reagent strength drift.
• Verification-only (not recommended for critical membranes): manual dosing with spot tests can miss transient chlorine breakthroughs.

3) Size and place the injection point

• Injection location: upstream of a mixing zone (elbow, static mixer, or sufficient straight pipe run with turbulence).
• Avoid dead legs: SBS can degrade in stagnant lines and create control lag.
• Quill/check valve: use appropriate backflow protection and an injection fitting matched to pressure and pipe material.
• Contact time: especially important for chloramine; allow reaction time before the membrane/inlet analyzer.

Dosing logic (procurement-friendly, operator-usable)

Dosing should be based on measured oxidant, not guesswork. If your plant uses “total chlorine” analyzers, verify whether chloramine is present and whether your test method is suitable at your operating pH.

Practical dosing approach

1. Measure oxidant at the dosing point: free chlorine and (if relevant) total chlorine.
2. Set an initial SBS dose: start conservatively, then verify at the membrane inlet / protected point.
3. Confirm with two indicators: (a) residual chlorine test and (b) ORP trend stability (if used).
4. Lock in a ratio: flow-paced dosing ratio plus periodic adjustment for seasonal changes and disinfectant program changes.

Monitoring: what to measure and where

Build monitoring around control and verification. Control keeps the process stable; verification proves you’re meeting the protection requirement.

Overfeed vs underfeed: failure modes you can actually see

Underfeed (oxidation breakthrough)

• Symptoms: intermittent free chlorine at membrane inlet; ORP spikes; accelerated membrane degradation; increased salt passage / reduced rejection over time.
• First checks: analyzer calibration, SBS strength/age, pump output, injection quill blockage, and whether upstream disinfection dosing changed.

Overfeed (excess reducer)

• Symptoms: persistently low ORP beyond normal operation, “sulfite smell,” increased biological activity downstream (system-dependent), oxygen depletion in some circuits, occasional corrosion side effects in specific metallurgy/service.
• First checks: verify actual chlorine is already near-zero upstream (e.g., carbon doing most work), then reduce SBS and confirm no breakthrough occurs.

Handling, storage & safety (EHS-aligned)

• Documentation: maintain the latest supplier SDS, site risk assessment, and emergency response instructions in the dosing area.
• Ventilation: store and handle in a well-ventilated area; avoid confined, poorly ventilated chemical rooms.
• Secondary containment: bunding for drums/IBCs and clearly labeled transfer points.
• Segregation: keep away from oxidizers and incompatible materials per SDS and site rules.
• PPE: follow SDS and site requirements; include splash protection for transfers and line breaks.
• Spill control: neutralization/cleanup procedure should be pre-approved by EHS; train operators before first delivery.

Specification & acceptance checks (what procurement should require)

“Sodium bisulfite” can mean different commercial forms. You want to define your acceptance criteria so operations receives consistent chemistry and predictable dosing performance.

Supply form options

• Sodium bisulfite solution (SBS): commonly supplied as an aqueous solution (concentration varies by market and supplier).
• Sodium metabisulfite (SMB) solid: often dissolved on-site to make a bisulfite solution; reduces freight water but adds make-down equipment and operator steps.

Minimum RFQ specification (copy/paste ready)

• Product: Sodium bisulfite (aqueous) or sodium metabisulfite (solid) with on-site make-down (state preference).
• Intended use: Dechlorination ahead of RO/UF (or describe exact use).
• Quality documents: SDS (current revision), COA per batch/lot, traceability to manufacturer.
• COA items (typical): assay / active content, specific gravity (solutions), pH (solutions), appearance/clarity, insolubles (where applicable).
• Packaging: drum / IBC / bulk (state preference), sealing/closures, labeling, palletization if relevant.
• Logistics: Incoterms, lead time, shelf life, storage temperature guidance, country of origin, and HS code support if needed.

Troubleshooting playbook

Problem: “Chlorine still shows up sometimes”

• Check for flow surges (pump pacing lag) and poor mixing (injection into laminar zones).
• Confirm the oxidant program didn’t change (e.g., switch from free chlorine to chloramine).
• Check SBS age/quality (storage temperature, time in tank, dilution water quality if you dilute).

Problem: “ORP is very low but chlorine is already zero upstream”

• Carbon may be removing most chlorine; SBS setpoint may be too high for current conditions.
• Reduce SBS gradually while verifying no breakthrough at the protected point.
• Ensure ORP probe is clean, calibrated, and located after adequate mixing.

Problem: “Biofouling/biogrowth signs increase downstream”

• Confirm there is no unintended creation of a strongly reducing environment.
• Review pretreatment steps (SDI/turbidity control) and CIP frequency.
• Coordinate with your membrane program (biocide strategy must be compatible with dechlorination approach).

RFQ notes (what to include when you contact us)

• Water source & disinfectant program: free chlorine vs chloramine; typical and max residual.
• Flow range: minimum/normal/peak flow and whether it fluctuates rapidly.
• Protection requirement: target residual at membrane inlet (and OEM limits if known).
• Instrumentation: existing analyzers (free/total chlorine, ORP), and where they’re installed.
• Packaging: drum/IBC/bulk, storage constraints, preferred delivery cadence.
• Compliance: documentation needs (SDS/COA), site rules, export/import needs, and delivery country/city.

Educational content only. Always follow site EHS rules and the supplier SDS for safe handling and use. Setpoints, limits, and acceptance criteria are site- and OEM-specific; validate with your equipment supplier and engineering team.