There is a thread going in the build section where the OP wanted feedback on the type of filter he should get, sand, cart, or DE. A couple of people chimed in that one of the disadvantages of a sand filter was the chemical dilution that can occur after backwashing and adding top off water. One responder said that with a salt water pool, it could be that a sand filter may not be the best choice since salt would have to be replenished, etc, etc. No doubt that backwashing and refill will reduce the chemicals in the water. But, it got me to thinking about what the real numbers are in terms of gallons lost from backwash and salt loss. For now, I only did an analysis of salt loss. You can do something similar for CYA, and calcium, but I thought the salt would be a good place to start.
First, there have to be some assumptions. You need to figure in the following:
1) g/min lost in each backwash
2) how long do you backwash
3) Optimum salt level
I used 40 gallons/minute for the backwash rate. Obviously this will vary on filter size, pump size, head loss, etc. But I figured 40 would be a decent starting number. How long you backwash in another variable that can/will change depending on a variety of factors. For my example is used 5 minutes (4 minute backwash, 1 minute rinse) since this is usually what I do. Actually, 4 minutes is a lot of backwash time, IMO. I also used 3400 ppm salt as an optimum since most SWCG use salt in this range.
So, 40 g/min for 5 min =200 gal
200 gal=757 L
3400 ppm salt = 3.4 g/L
3.4 x 757 =2574 g of salt
= 2574 g of salt lost per backwash. That equals 31 ppm lost (assuming a ~20,000 gal pool) from the original 3400 ppm assuming one replaces the 200 gallons lost.
2574 grams is 5.7 pounds.
So, for a 40 pound bag of salt at ~$10 a bag, that roughly $1.47 worth of salt per backwash. Not a lot of money lost per backwash. Plus, 31 ppm is not enough salt loss to even come close to changing the salt concentration enough to stop chlorine generation because of low salt even if you didn’t add any.
I invite others to chime in if my math is off someplace or my assumptions are off or to generally throw sticks. My thought with this was to see if the belief that sand filters were not the greatest thing for salt water pools had some merit. As disclaimers, I did not take into account the cost of water to refill after backwash. For most, 200 gallons is a small cost (note I said most, not all).
Just a point of reference for how many inches 200 gallons represents. An 18x32 pool is 576 sq. ft. 200 gallons is around 1/2 inch of water in a pool that size.