Water softening vs antiscalant addition in reverse osmosis systems
As reverse osmosis technology has evolved, the pretreatment methods have evolved with it. One of the major improvements has been the enhancement of scale inhibition chemistries.
With the ability to inhibit/disperse common scale forming compounds such as calcium carbonate, barium/strontium sulfate, calcium phosphate and metal oxides, antiscalants have been replacing water softening equipment with more frequency. In addition to the broad spectrum protection antiscalant’s provide, when considering large flow applications, the cost of softening becomes prohibitive.
The table below is a basic overview of the costs associated to operating a water softening system compared to that of applying antiscalant chemistries at various concentrations of water hardness.
| Pretreatment: Antiscalant | |||
|---|---|---|---|
| Hardness (ppm) | Capital Equipment Cost | Antiscalant Cost ($ per year) | Present Worth |
| 10 | $2,500 | $1,314 | $7,888 |
| 20 | $2,500 | $1,314 | $7,888 |
| 50 | $2,500 | $1,314 | $7,888 |
| 100 | $2,500 | $1,533 | $8,786 |
| 250 | $2,500 | $1,752 | $9,684 |
| Pretreatment: Softening | |||
| Hardness (ppm) | Capital Equipment Cost | Salt Cost($ per year) | Present Worth |
| 10 | $25,000 | $1,794 | $32,356 |
| 20 | $25,000 | $3,588 | $39,712 |
| 50 | $50,000 | $8,970 | $86,779 |
| 100 | $50,000 | $22,424 | $141,943 |
| 250 | $50,000 | $44,800 | $233,688 |
Assumptions:
- Antiscalant dosages range from 3 – 4 mg/L depending on the water hardness.
- Present worth calculations were amortized over a 5 year period at 7% interest.
- Rock salt for softener regeneration costs $0.16 per pound.
- Softener system with resin capacity of 40ft3, for hardness 50 ppm or above design contained two softeners.
- Feed flow rate of 100 GPM, and an RO recovery of 75%.
When comparing antiscalant addition to softening, the main drawback of softening is the cost.