Salt Dosage vs. Capacity relationship

[YungPlantEng]

We have a series of smaller softener units that take pre-softened water as an inlet and soften it additionally to make it "super soft". Essentially our bulk softener softens to a maximum of 5 ppm leakage and then these additional polishers are there in case the main softeners failed and went above 5 ppm.

Running through these systems with our vendor there is the ability to adjust salt dosage & capacity. I have trouble understanding what adjusting the "salt dosage" actually does. Does this reduce the amount of super-saturated brine drawn up in order to reduce the overall concentration during regeneration?

My understanding is that you can alter the salt dosage to improve your efficiency while risking hardness leakage. I wanted to adjust the actual capacity before each regen (flowmeter based system) understanding that the capacities were originally calculated assuming 17 gpg (300 ppm) hardness at the inlet. I thought we could just re-calculate the capacity to regen less often and base it off of say a 25 ppm inlet since that would be several times over the worst case scenario. The vendor said that what I would be doing is changing the salt dosage but one of us certainly seems misunderstood.

I'm basically asking for an understanding of what salt dosage is and how it is figured into the capacity calculation. I also wanted to understand if the resin bed would still be spent if very low ppm water was going through it (Would my capacity be 10x if my inlet hardness is 10x less then what was originally calculated?)

Thanks for any help!

 

[pierreick]

Hi,
Think twice before modifying the parameters on the softener, based on experience. You don't want leakage of hardness, right?
Get support from specialist.

How to Calculate Softener Capacity

Water softeners are ion exchange systems designed to remove scale-forming calcium and magnesium ions prior to boiler, cooling, and reverse osmosis systems. The amount of hardness a softener can remove between regenerations is known as the softener capacity and can be expressed either in grains...

www.chemaqua.com

Pierre

 

[bimr]

https://www.dupont.com/content/dam/water/amer/us/en/water/public/documents/en/IER-AmberLite-Water-Conditioning-Manual-45-D01951-en.pdf

If you review chart 4.4.1 Co-current Design Figure 1., it shows leakage (meaning effluent hardness concentration) at different salt dosages. At 70 g/L salt dosage, you would get 20 mg/L leakage. If you upped the dosage to 240 g/L, you would get 3.5 g/L when operating at 1,000 TDS. Note that effluent hardness leakage is dependent on the influent hardness.

If you review chart 4.4.1 Co-current Design Figure 2., resin operating capacity. At 70 g/L salt dosage, you would get 21 kgr operating capacity per cubic feet of resin. If you upped the dosage to 240 g/L, you would get 37 kgr operating capacity per cubic feet of resin.

Upping the salt dosage is usually not economical. You have increased the salt dosage by 3.2 times while only getting 1.76 times more capacity. Changing the concentration of a reactant can shift the equilibrium position to favor the side that consumes the added reactant. However, the chemical efficiency is lower.

Increasing the salt dosage decreases the overall chemical efficiency, but will improve (lessen) the hardness leakage and will provide a longer run cycle in terms of gallons processed.

The resin bed will still be spent if low ppm water is passing through it.

It appears that your system is set up with serial softening units. The first is the working bed and the second is the polisher. The reasoning for this setup is increasing the chemical efficiency and getting the zero soft water effluent for a low pressure boiler. I would suspect that the raw water hardness is high as well.

Some systems with this setup alternate the lead/lag softeners. Alternating the lead/lag softener will ensure that the trailing polishing unit never exhausts and ensures zero hardness effluent.

 

[YungPlantEng]

So the resin bed is spent the same regardless of the influent concentration?

I'll just work through it with the corporate SME engineer... Your answer is very confusing. It also didn't resolve what the salt dosage actually corresponds to. Is it just used for the calculation for the capacity for the flowmeter? Does it actually change the timers on the brine draw?

 

[pierreick]

https://www.chemaqua.com/Portals/CA/Linked_Documents/Tech_Bulletins/TB4-006.pdf

Let you study this document.
Pierre

 

[bimr]

So the resin bed is spent the same regardless of the influent concentration?

The resin bed has a defined total bed capacity (capacity in grains) calculated by multiplying the total cubic feet of resin times the resin operating capacity. The resin operating capacity is from the manufacturer's chart 4.4.1 Co-current Design Figure 2., resin operating capacity. You can use any salt dosage from a minimum of 70 g/L to a maximum of 240 g/L of salt dosage.

The water softener operating run time (in water meter totalizer gallons) is calculated by taking the resin bed capacity and dividing by the influent hardness. If the influent hardness is lower, the run time (gallons treated) is larger. If the influent hardness is higher, the run time (gallons treated) is less.



1. Convert water hardness from ppm to grains per US gallon

Total Hardness (ppm) = Total Hardness (gpg) Example: 118 ppm TH = 6.9 gpg;
17.1 17.1

2. Calculate softener capacity in grains

Ft³ resin X resin exchange capacity (typically 30,000 grains/ft³) = Grains capacity
Example: 6 ft³ resin X 30,000 grains/ft³ = 180,000 grains of capacity

(Note that 30,000 grains/ft ³ is a salt dosage of 135 g/L)

3. Calculate softener capacity in gallons

Softener Capacity (grains) = Max softener capacity in gallons;
Total Hardness (grains per gallon)

Example: 180,000 grains = 26,087 gallons;
6.9 grains/gallon

Your answer is very confusing. It also didn't resolve what the salt dosage actually corresponds to. Is it just used for the calculation for the capacity for the flowmeter? Does it actually change the timers on the brine draw?

You haven't read the document that I provided you. The g/L (grams/liter of salt per cubic feet of resin) is the salt dosage. You can use a salt dosage from a minimum of 70 g/L to a maximum of 240 g/L. Changing the salt dosage will change the total resin bed operating capacity and results in a change of run time (assuming the influent hardness if constant) as reflected in the flow meter totalizer. Changing the salt dosage also effects the hardness leakage and the operating efficiency.

To change the salt dosage, you have to increase/decrease the salt applied by changing the amount of salt applied to the resin bed. Usually the salt is applied at 10% concentration, so you would be increasing/decreasing the salt pumping rate from the brine tank. The brine draw cycle time will not change only the quantity (rate in gpm) of salt that is applied during the brine draw cycle time is changed.

If you review chart 4.4.1 Co-current Design Figure 2., resin operating capacity. At 70 g/L salt dosage, you would get 21 kgr operating capacity per cubic feet of resin. If you upped the dosage to 240 g/L, you would get 37 kgr operating capacity per cubic feet of resin.

 

[EdStainless]

We weren't making boiler feedwater but very clean process water.
Our upper limit was 4uS/cm so we wanted feed water at 1.5-2.
We had four softeners (parallel/series) that we regened at about mid-range salt levels.
This gave us a bit more capacity without using astronomical amounts of salt.
This water then went through mixed bed polish bottles.
To minimize makeup the water in use was recirculated through mixed bed polish bottles that re-gened offsite.
When we got to 3.5 we would stop recirculating and begin a slow dump to drain.

 

[YungPlantEng]

@bimr That still doesn't explain what the salt dosage is. Does it affect the brine draw, etc.?

I don't want to change salt dosage. I want the actual influent hardness (20 ppm) to be accurately reflected in the "gallons treated" capacity that determines when the system is regenerated. The technician is saying I can't do that without changing the salt dosage which makes no sense. The vendor's corporate engineer is saying I can.

@EdStainless
We use conductivity on our main softener system resin to determine when it's exhausted. I did have some desire with messing with the output of that sensor but figured I would get away with changing salt dosage instead and just using extra water on increased regen intervals. But yes that's very similar to our main system where these all require various diffe

 

[bimr]

The water softener cycle (gallons throughput) is based on a calculation using the following parameters:

1. applied salt dosage in lbs/ cubic feet or g/L
2. resin volume in softener
3. raw water hardness

If any of these parameters change, it will be reflected in the gallons throughput. The stoichiometric equations above will calculate the throughput using the listed parameters.

If you don't use the stoichiometric amount as defined by the stoichiometric equations above, you can end the water softener cycle early or late. If you end the water softener cycle early in the run, the softening process will be less efficient and you will not use all of the available capacity. If you end the water softener cycle later in the run, the softening process will be more efficient, but hardness will breakthrough.

In your last post, you said you don't want to change the salt dosage, then you turn around and say you want to change the salt dosage?????????

 

[EdStainless]

When you regen the resin reaches an equilibrium with the brine.
The stronger (more concentrated) the brine the further it pushes the reactions and the more capacity you get.
I have seen people that have significant variations in influent seasonally.
They monitor that and adjust the brine concentration so that they get roughly the same capacity in gallons.

 

[bimr]

Ed says: “When you regen the resin reaches an equilibrium with the brine.”

That is an incorrect statement and will confuse the poster more than he already is. When salt is applied, the high stoichiometric salt ratio drives the reversible chemical reaction one way, and then the chemical reaction reverses after the regeneration is over.

I outlined the calculations above if the poster wants to calculate the cycle time.


There may be some seasonal variety if you are operating on surface water rather than well water. However, the variation will not be extreme. Again, you are just going to confuse the poster even more.

In the poster’s last post, he said he doesn’t want to change the salt dosage and then says he wants to change it. Go figure.