Sulfite Demand from Dissolved Oxygen

Calculates oxygen scavenger (sulfite) demand from the dissolved oxygen load, using the stoichiometric ratio of about 7.9 mg sulfite consumed per 1 mg dissolved oxygen. Shows that a rising oxygen load proportionally raises sulfite demand.

$$\text{Sulfite demand (mg/L)}=\text{DO (mg/L)}\times 7.9$$

Variables

Symbol	Meaning	Unit
$\text{DO}$	Dissolved oxygen load entering the system	mg/L
7.9	Stoichiometric ratio, mg sulfite consumed per mg dissolved oxygen	mg sulfite / mg O2

Worked example

Normal (75% condensate return): DO = 2.3 mg/L, sulfite demand = 2.3 x 7.9 = about 18 mg/L. Upset (25% condensate return): DO = 6.4 mg/L, sulfite demand = 6.4 x 7.9 = about 51 mg/L, nearly three times the demand from the same root cause.

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Part of The Blueprint of Water