Dechlorination Calculations and System DesignChlorine and chloroamines are commonly used as potable water disinfectants and found at levels of < 0.2 to over 5 mg/l as chlorine in most potable waters. Chlorine will destroy ion exchange resins and many water treatment membranes so it should be removed from potable waters to be treated using ion exchange resins or membrane processes. The most common methods for removing chlorine and chloroamines from potable water are passing the water through an activated carbon filter or injection of a sulfite solution. We will consider use of the sulfite solution methods in this memo.
The basic equations for the reactions are:
chlorine NaHSO3 (bisulfite) + Cl2 →NaHSO4 + 2 HCl
chloroamines (they vary in composition!) 3NaHSO3 + 3 H2O + NH3Cl6 →3NaHSO4 + 5HCl + NH4Cl
Our sulfite products have the following compositions:
|Product||% as SO3||% as metabisulfite||% as bisulfite|
In theory you could use 1.6 mg/l per mg/l chlorine, for practical work dose at 3 mg/l bisulfite per mg/l chlorine.
Chloroamines, which have a variable composition, use a practical dose rate of 6 mg/l of bisulfite per mg/l of chloroamine.
Note that overdosing is not a problem and is to be preferred to under dosing!
As the target is total chlorine removal, the chemical pump must be sized to provide a sufficient dose at maximum flow rate for total chlorine removal with about 25 seconds reaction time before city water hits the ion exchange resin or membrane unit. The simplest equipment would thus consist of an appropriate sized chemical pump set for the maximum flow rate running constantly with enough pipe downstream to obtain the needed reaction time. In place of piping, an in line mixer could be employed. Example System Design-Specification input – 5 mg/l chlorine flow rate – maximum 25 gpm (36,000 gpd) PCT 5405 used – 39% active bisulfite
- bisulfite – PT 5405 dose calculation
5 mg/l chlorine x 3 = 15 mg/l bisulfite
15 mg/l bisulfite/0.39 = 38.5 mg/l PCT 5405
- chemical pump selection
25 gpm x 38.5 mg/l x 0.012 = 11.6 lb/day PCT 5405
11.6 lb/day/10.8lb/gal = 1.1 gal/day/24 hr/day = 0.05 gph
no flow control – use an LMI A141-812 rated 0.5 gph, speed = 35% stroke = 30%
For larger installations where the flow could be expected to vary substantially, an electrical contacting water meter with a flow proportional chemical pump would be used. Jumping our flow rate to a maximum of 100 gpm (144,000 gpd) the following items are calculated.
- chemical pump selection
100 gpm x 38.5 mg/l x 0.012 = 46.2 lb/day PCT 5405
46.2 lb/day/10.8 lb/gal = 4.3 gal/day/24 hr/day = 0.18 gph
using a 5 gal/contact water meter, one selection would be to use the LMI
AD81-812 rated 0.21 gph, speed = controlled by the water meter with a pulse multiply
of 5, stroke = 100%
- water meter selection
Carlon JSJ200, 5 gallons/contact
Test the treated water downstream of the injection point and adjust the feed rate to obtain “Not Detected” chlorine, adjust feed rate accordingly.
Pleased contact Corporate Engineering if you would like assistance in sizing larger installations or wish to use other chemical pumps and water meters.