Effects of hardness; zeolite softener

Effects of hardness

• Wastes soap: Ca and Mg form an insoluble scum with soap, which makes washing difficult and increases soap consumption.
• Scale in boilers, heaters and pipes (CaCO$_3$, CaSO$_4$): it reduces heat transfer, wastes fuel and can cause boiler explosions; it also reduces the carrying capacity of pipes.
• Bad for the textile, paper, dyeing, laundry and food industries; harms fabrics and skin.
• Bad taste and poor cooking (pulses and vegetables do not soften well), and a scum forms on tea.
• Very hard water may cause digestive upset and, in some people, kidney stones.

Zeolite (base-exchange) water softener

Zeolite is a hydrated sodium aluminium silicate (Na$_2$Z), either natural (green sand) or synthetic (permutit), which exchanges its sodium ions for Ca and Mg ions. Water passes down through the zeolite bed in a closed pressure vessel.

   raw hard water / brine (regeneration)
             |
     +-------v-------+
     |:::::::::::::::|  zeolite bed
     |:::::::::::::::|  (on gravel support)
     +-------|-------+
             v
   soft water out / waste brine to drain

Softening:

$$Ca(HCO_3)_2 + Na_2Z \rightarrow CaZ + 2NaHCO_3$$ $$MgSO_4 + Na_2Z \rightarrow MgZ + Na_2SO_4$$

Regeneration (when the bed is exhausted): the bed is washed with 10 % common-salt brine:

$$CaZ + 2NaCl \rightarrow Na_2Z + CaCl_2$$

and then rinsed with water. Both temporary and permanent hardness are removed.

Advantages

• Gives water of nearly zero hardness; the final hardness is easy to control by bypassing part of the flow.
• No sludge is formed; the unit is compact, can be automatic and is easy to operate.
• Removes both carbonate and non-carbonate hardness; regeneration is cheap and quick.

Disadvantages

• The sodium content of the treated water increases (a problem for people with heart or kidney trouble), and the total dissolved solids are not reduced.
• Cannot treat turbid or coloured water, or water containing iron and manganese, because the bed gets clogged or fouled.
• Not economical for very hard water; regeneration needs salt and produces brine waste.
• The water may become corrosive and alkaline (sodium bicarbonate).