Hi Avo, running costs for water softeners will vary around the UK & dependant upon;
*How much water you need to generate each day
*How hard your water is (you can find this on your local water utility company web site)
*How cheaply you can acquire granular salt locally (or tablets/blocks)
*Where/from who you locate your water softener i.e. cost/efficiency etc.
My best advice to anyone who has hard water to convert to RO quality is to contact your local softener supplier. The manufacturer of the water softeners will tell you how much salt the machine will use during regeneration (washing the resin with brine water). Once you know this you can work out the Kilogram cost per month per annum etc. They will also tell you how much water can pass through the softener before it needs regeneration with salt/brine. This will depend on the softener resin capacity of the unit you buy i.e. bigger unit, more upfront capital cost.
As you can see, I cant provide definitive answers because of the variables involved. I've been in the water treatment industry for 28 years and have seen 40 inch RO membranes trashed in 4 months by hard water, so economically, as you point out, you have to make a commercial decision as to whether a softener will pay for itself.
As a general rule, if you have very soft water (e.g. Highlands Scotland/Some Parts Wales/Devon/Cornwall etc) a softener may well not pay itself back, however may save money in DI polishing resin. If you have very hard water, a softener will dramatically extend the membrane life/performance and consequently save money on DI resin.
I work primarily on Industrial RO systems and you will never find one without a softener and carbon pre-filtration. However, I do agree entirely with your point that its only worth the expense of a water softener if it pays for itself.
Cheers Sean
