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Water Chemistry and Treatment

Jun 03 2012 06:50 PM | Chris Haslego in Utilities ****-

Sizing Conventional Water Softners

Assume 60 gallons of water per day per person (natural average) if iron is present: multiply ppm x 2 and add hardness to arrive at compensated hardness. Iron is more readily exchanged than hardness and this method compensates the softener capacity.

Example: Number of Persons x 60 x Comp. Hardness = Grains Capacity Used Per Day.

Multiply: Grains per Day x 3 Days = Approximate Capacity of Softener Required. The number of days may be extended to arrive at a catalog unit which will not exhaust before regeneration.

4 persons in household
18 grains hardness
1.5 ppm iron (clear water)

Compensated Hardness = 1.5 x 2 + 18 = 21 grains

4 x 60 x 21 = 5040 grains/day x 3 days = 15,120 grains capacity ML-15S (15,000 grain capacity) is too small. Timer can only be set for: Once/6 days, Twice/6 days, Three times/6days. (Cannot be set for every 4th or 5th day.)

ML-22D (22,000 gr.) = 4.36 Days (too large) ML-25S (25,000 gr.) = 4.96 Days (too large) ML-30D (30,000 gr.) = 5.95 Days (too small)

Due to limitation of time clock there is not an efficient choice. The closest time clock unit would be a ML-22D. Consider metered (MP) unit: MP-25S (4.96 days average) or MP-36D (7.143 days average).

Reduced Salt Setting Conventional Softners

Reduced salt dosage, 6 pounds per cubic foot of resin versus maximum salt dosage, 15 pounds per cubic foot of resin may be used to gain salt efficiency. Marlo softeners are set up to take advantage of this savings wherever it is possible to use it.

The reduced salt dosage, minimum salt setting, may not be used if iron is present in the water. With reduced salt the resin is not fully regenerated which means the resin bed retains some hardness and iron would also be retained. The iron will eventually oxidize within the resin bed and iron fouling will result in lost capacity.

Minimum salt setting can be used to obtain a more exact sizing of a unit based on 20,000 grains per cubic foot of resin versus 30,000 grains at maximum salting. This results in a smaller, less costly unit plus salt savings can be 30% or more.

Example: 3 persons, 15 grains hardness, no iron

3 x 60 x 15 - 3240 grains per day
15,000 grain unit at minimum salt = 10,500 gr. cap./ 3240 = 3.24 days
(25,000 grain unit at maximum salt = 25,000 gr. cap. / 3240 = 7.70 days)
(25,000 grain unit at minimum salt = 16,500 gr. cap. / 3240 = 5.09 days)

ML-15S is best sizing.

Common Symbols Used in Water Treament



Calcium, Ca++ Nitrate, NO3-
Magnesium, Mg++ Chloride, Cl-
Iron, Fe++ Sulfate, SO4-
Manganese, Mn++ Bicarbonate, HCO3-
Sodium, Na+ Calcium Carbonate, CaCO3-
Copper, Cu++ Hydroxide, OH-
Hydrogen Sulfide, H2S
Carbon Dioxide, CO2
Oxygen, O2

This article is shared with permission from Marlo Incorporated. Original author: Tony Ramirez, ramirezt"at"marlo-inc.com

Utilities Articles



I am looking for treatment solution to avoid negative LSI in permeate water as water nature becomes corrosive with -ve LSI.

Any solution?

Zeeshan Amin
Apr 24 2014 09:17 AM

Hi Chris,

Great article, especially for  engineers involved with RO and make up water treatment of cooling towers.

I need to calculate neutralizing acid