Can hydrogen peroxide be used effectively in place of sodium bisulfite for dechlorination of seawater RO feedwater? We are concerned about dissolved oxygen levels in the concentrate that is disposed back into the sea…

Q: Can hydrogen peroxide be used effectively in place of sodium bisulfite for dechlorination of seawater RO feedwater? We are concerned about dissolved oxygen levels in the concentrate that is disposed back into the sea…

Sodium sulfite  and sodium metabisulfite both work very well for dechlorination at a stoichiometric ratio of 1:1:

$${SO{_3}{^2}{^-} + HOCl \rightarrow SO_4{^2}{^-} + Cl^- + H^+}$$$${HSO{_3}{^-} + HOCl \rightarrow SO{_4}{^2}{^-} + Cl^- + 2H^+}$$

This works out to about 1.5 ppm sodium bisulfite — to 1 ppm hypochlorite (active) by mass, and typically a slight excess is used to increase the rate of reaction.
If you use sodium metabisulfite, the ratio is about 1.35 ppm to 1 ppm active hypochlorite by mass (when calculating required quantity as powder) – sodium metabisulfite powder hydrolyzes to bisulfite when dissolved into water:

$${Na_2S_2O_5 + H2O \rightarrow 2HSO{_3}{^-} + 2Na^+}$$

Excess sulfite will scavenge dissolved oxygen – however, the solubility of oxygen is inversely proportional to temperature, so there will be little impact in cool water if you slightly overdose sulfite.

Hydrogen peroxide reacts with free chlorine at a pH > 7 and typically requires a mass ratio of 0.5 ppm: 1 ppm for dechlorination but will substantially increase DO. Given that you are using this with seawater, peroxide will be substantially consumed by organics and a substantial excess will be required. Peroxide would not seem like an efficient method for your application.

Another issue that comes to mind is that seawater contains bromide salts, some of which will react to form hypobromous acid upon hypochlorite addition. Sulfites reduce both chlorine and bromine, thereby deactivating them. By contrast, any unreacted bromide salts would react with peroxide to form hypobromous acid – so even if the peroxide were an effective dechlorinator in seawater, its use could potentially create a new problem.