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What is the cost of feeding soft vs hard water to an RO?
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In our previous blog What is the Impact of an RO System on a Boiler Water Treatment Program? We discussed the importance of Reverse Osmosis to feed your steam boiler system. The savings come in the form of reduced water, chemical and improved steam efficiencies through greater cycles of concentration. 
Recently a customer was considering a significant expansion on their RO systems for boiler make-up and process water to the facility. They planned to move to 100 gpm permeate systems and the current water softeners were not big enough to support the new RO. Like all mechanical rooms space was at a premium and a bulk brine silo would also need to be brought in. I ran some simple calculations to determine costs of feeding Anti-Scalant with hard water, the savings would be substantial; plus no capital costs on new equipment and construction.
How does Reverse Osmosis work?
RO systems work by forcing water through semi-permeable membranes to reduce the amount of Total Dissolved Solids (TDS) in the feed water and flushing them to drain. The clean useable RO water that is left is referred to as Permeate Water. A waste stream of water called Concentrate is critical in the operation because it prevents the membranes from fouling and loading up with solids.  
How to decide between feeding soft or hard water?
Regardless of the water quality it is important there is no chlorine in the water. Chlorine attacks the membranes which leads to TDS break through and costly premature replacement.  
Soft water feed to the RO will protect the membranes from calcium build-up and loss of water flow, rejection, and pump failure. When feeding with clean, soft water you can expect maximum life cycles on the membranes. The downfall is that water goes to drain. Circumstances change with temperatures and water quality but no matter how many gallons of permeate you produce, approximately 30% goes to drain as concentrate. That concentrate was softened and used salt that accounts for excess regenerations.
Hard water feed to the RO can only be accomplished with the help of a chemical Anti-Scalant that is introduced into the feed line to the RO. The chemical creates a time delay between the bicarbonate and the calcium magnesium. The delay allows the water to pass through the membrane before scale is formed. Typical feeds of the Anti-Scalant is about 4ppm, which is 3.3lbs of product/100,000 gallons. So very little product is needed vs the amount of salt needed to soften the water.
Clean in Place Required
The downfall is the RO unit will need to go through a Clean in Place (CIP), periodically and the RO needs to be designed to do so. This process involves back feeding warm acid for scale removal of calcium and iron and warm caustic for organics that will eventually accumulate on the membranes. This process involves the RO to be offline for 4-8 hours depending on size, the cleaning frequency depends on water quality but quarterly to bi-annually can be expected.
Environmental Benefit
There is a significant environmental benefit on not softening the water, avoiding the chloride discharge from the brine used in regeneration of the water softeners. Local municipalities are struggling with Chloride levels coming to the water treatment facility and the discharge to local lakes and streams. The levels are high enough to be damaging to wildlife. Chlorides are very soluble and hard to remove without costly upgrades to facilities. Locally here in Dane County, WI - Madison Metro Sewerage had been offering rebates to businesses that can show chloride savings.Below are a few scenarios that compare costs related to hard vs soft water make-up. In large and moderate volume permeate with relation to different hardness levels. As you will see the higher the hardness the more salt needed.
5,000,000 Gallons of Permeate
w/ 25 Grains Hardness
  5,000,000 Gallons of Permeate
w/ 8 Grains Hardness
   Soft Water Hard Water     Soft Water Hard Water
 Lbs of Salt  64,484  0    Lbs of Salt 20,635   0
 Regen Gallons  201,190 0    Regen Gallons 64,381   0
 Anti-Scalant  0 23.5 Gallons    Anti-Scalant  23.5 Gallons
 CIP Needed  No  Yes    CIP Needed  No   Yes
 Total Cost  $13,901.00  $1,124.50*    Total Cost $4,448.80  $1,124.50* 
 * CIP cost not included frequency varies     * CIP cost not included frequency varies 
 150,000 Gallons of Permeate
w/ 25 Grains Hardness
  150,000 Gallons of Permeate
w/ 8 Grains Hardness
   Soft Water  Hard Water     Soft Water  Hard Water 
 Lbs of Salt   1,935  0    Lbs of Salt   619  0
 Regen Gallons  6,036  0    Regen Gallons   1,931  0
 Anti-Scalant   0 0.71 Gallons     Anti-Scalant   0  0.71 Gallons
 CIP Needed   No Yes    CIP Needed   No Yes 
 Total Cost $416.88   $33.74*    Total Cost  $133.47  $33.74* 
 * CIP cost not included frequency varies    * CIP cost not included frequency varies 
Cost Estimate parameters:
  • Twin Alt Softener with 10% reserve
  • 10lbs/CF Softener Media
  • $10/ 50# of Salt delivered and loaded to Brine Tank
  • $5/1,000 gallons of combined water supply and Sewer
  • 30% Concentrate to drain 
Remember regardless of the size and usage of the RO a CIP will need to take place and the costs associated with the labor will need to be factored in. It really comes down to water quality and the potential savings. It’s safe to say if your RO unit is at or above the 30 gpm permeate it may to time to at least look at the options of Anti-Scalant.  
This is very simple calculation, if you would like to learn more please contact me or any of the Water Treatment Professional at Watertech of America, Inc.
Jason Waack
Territory Manager

Jason has over 15 years experience in the water treatment industry. He has been a Territory Manager with Watertech for 5 years, managing accounts in the central part of Wisconsin.

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