Bisulphite/metabisulphite for dechlorination in RO: dosage, mixing and verification (SWRO and BWRO)

Bisulphite (SBS) / Metabisulphite (SMBS) for RO dechlorination: dosage and control (SWRO and BWRO)

If there is free chlorine reaching the polyamide membranes, the risk is oxidation and loss of rejection. DuPont indicates that dechlorination before the membranes is mandatory to protect the membranes and that degradation can occur after cumulative exposure.

SBS vs SMBS: which one is actually used in the plant?

In industrial operation, it is most common to dose sodium metabisulphite (SMBS) and, when dissolved in water, it forms sodium bisulphite (SBS). DuPont describes it with the hydrolysis reaction.

Practical translation: in day-to-day life you will talk about “bisulphite”, but many times you are preparing and dosing SMBS.

Dosage: quick rule and “by-the-book” basis”

Calculation of SMBS dose for dechlorination: rule of thumb 3× free chlorine and theoretical reference 1.34× (mg/L as Cl2 )

DuPont gives two key figures for removing free chlorine:

  • Theoretical (stoichiometric): 1,34 mg SMBS delete 1,0 mg/L free chlorine.

  • Practice (with margin): normally used 3,0 mg SMBS by 1,0 mg/L chlorine to ensure full dechlorination.

Ready-to-use formula

If your free chlorine (FAC) is X mg/L as Cl₂:

  • SMBS dose (mg/L) ≈ 3 × X (DuPont recommended rule of thumb)

If your analysis gives you “total chlorine” or chloramines are present, the calculation may change. In this case, it is advisable to measure what species of “chlorine” is present before applying quick rules.

Injection and mixing point (where all is gained or lost)

Declorination is rapid, but the mix rules. DuPont recommends static mixers to ensure proper contact and mixing.

Recommended injection point (practical):
DuPont indicates as a recommended point downstream of the cartridge filters (so that the chlorine protects the cartridges).

Control and verification: not “by eye”.”

ORP as chlorine alarm (and typical threshold)

DuPont suggests monitoring the absence of chlorine with an ORP electrode downstream of the mixing line and mentions typical thresholds. 175-200 mV to activate shutdown when chlorine is detected.

Measure chlorine and bisulphite (better than ORP only)

Hach recalls that measuring chlorine and bisulphite helps to protect membranes from oxidation and that in situ analysis avoids loss of analyte in the sample.
In addition, Hach warns that a control based on ORP alone may be affected by pH interferences and lead to overdosing bisulphite.

 

RO dechlorination checklist: measure free chlorine, dose SMBS, ORP as alarm (175-200 mV) and adjust with chlorine and sulphite test.

Operational rule: ORP can serve as an “alarm”, but to fine-tune the dosage, it is best to residual chlorine + residual bisulphite.

Common mistakes (the most common ones in the field)

  • SWRO (seawater): the chlorination chemistry changes to bromide (HOBr is formed), and the measurement of “free chlorine” may include interferences from brominated species. DuPont explains this in the seawater section.
  • BWRO (brackish): there is usually less bromide, but the risk of oxidation of polyamide by free chlorine remains the same: dechlorinate before membranes.

SWRO vs BWRO (what changes)

  1. Inconsistent sampling:
    1. Change sampling point
    2. Do not purge line
    3. Measure immediately after a manoeuvre (backwash, start-up, switchover) without noting it2. Interpret isolated SDI
  2. Cross SDI with:
    1. ∆P of filters/UF
    2. frequency of cartridge change
    3. ∆P of RO
    4. and, if you do standardisation, with standardised flow and salt passage (Toray insists on monitoring and standardisation as an operating discipline).
  3. Confusing “high SDI” with “bad membranes”.”

Quick checklist

  • Measure free chlorine before RO

  • Calculate SMBS ≈ 3 × chlorine (mg/L) as a starting point

  • Inject with good mix (ideal static mixer)

  • Check for absence of chlorine (ORP as alarm)

  • Fine-tuning with test chlorine + bisulphite (better than ORP only)

  • Monitor solution life and avoid storage of dechlorinated water.

Frequently Asked Questions (FAQ)

What is the difference between SBS and SMBS?

SMBS (sodium metabisulphite) is normally prepared and dosed in solution at the plant; when dissolved, bisulphite is formed. In daily operation it is common to speak of “bisulphite” although the product is SMBS.

As a reference, DuPont indicates a theoretical dose of 1.34 mg SMBS per 1 mg/L free chlorine and that in practice a higher dose is often used to ensure full dechlorination.

DuPont recommends the injection point downstream of the cartridge filters so that the chlorine protects the cartridges; and recommends good mixing (e.g. static mixers).

Yes: although dechlorination is rapid, DuPont indicates that good mixing is required to ensure that the reaction is complete, and that static mixers are recommended.

ORP can serve as an alarm, but for fine tuning it is better to measure residual chlorine and residual sulphite/bisulphite. Hach recommends checking ultra-low levels of chlorine and sulphite to optimise dechlorination and protect membranes.

DuPont mentions the use of ORP to monitor the absence of chlorine and cites typical ranges in the order of 175-200 mV as the alarm/stop threshold in some control schemes.

DuPont explicitly states: dechlorinated water should not be stored in tanks, because the oxidation barrier is lost and the risk of biological growth increases.

Beyond chemical consumption, the practical way is to measure residual sulphite/bisulphite and confirm that you are not “overdoing it” to maintain stable control (and avoid pH mismatches/interferences). Hach emphasises directly measuring chlorine and sulphite at very low levels to adjust dosing.

Recommended external sources

Main reference:

2. Chlorine measurement (check “chlorine = 0”)

3. ORP control (when it is useful and why it is not enough on its own)

4. Direct measurement of ultra-low chlorine and sulphite/bisulphite (very pro)

5. Stability/lifespan of the solution (for your “common mistakes” section)

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