What’s wrong with water softeners?

Cambridge has hard water and it scales up our appliances. Some of them we can clean with a bit of effort, but some are not so easy. You can clean the heating element in your kettle just by soaking in hot dilute vinegar – but getting at the heat exchanger in your hot water cylinder is a tricky job. The obvious answer is a water softener. 

There are two problems.

1. Water softeners use extra water and we are already water stressed in this area.
2. Water softeners use a lot of salt and this is bad for the environment. It has to go somewhere: usually it ends up in our rivers.

How bad is this and what can we do about it?

How much extra water does a water softener use? 2% to 10% or even more.

Softeners vary and some are more efficient than others. The Harvey Water Softener claims to use only 2% extra which would be close the theoretical minimum. They have a nice explanation here [1].

However, other water softeners may use 10% extra or more. GE says a water softener uses 35-65 US gallons of water on each recharge [2]. Call it 50 – that will be 190 litres (The Harvey Water Softener uses 17 litres). It does not say how often it does a recharge - it depends on how hard your water is and how much water you use. However from reading about other softeners you can expect this to be at least once a week. Assuming 2 people in the household, and once a week recharge, this is an extra 13.6 litres per person per day, roughly 10% of average water use [3]. Anecdotally I have heard of a water softener using an extra 15 litres/person/day.

How much salt does a water softener use?

This also depends on the water softener. Based on the statistics here [4] a conventional water softener will use 150-200 kg salt each year (assuming two people). That is an awful lot more than you use normally. A modern softener such as a Harvey uses somewhat less – say 120 kg/year. However this is still a lot.

Most of this salt runs via the sewers into rivers.

What is the environmental impact?

Rainwater has very little sodium. Even near the coast it typically has less than 10 mg/l of sodium [5]. Hence natural fresh water is low sodium and few plants and animals are tolerant of high sodium levels. Households with septic tanks are advised not to discharge the brine from a softener into it [6].

Considering the salt used by the softener diluted by average use, the concentration of salt works out at about 430 mg sodium/litre [7] which is more than twice the EU standard for potable water (200 mg sodium/litre). However there are no standards for sodium levels for surface waters (i.e. rivers and lakes). The Water Framework Directive does not mention sodium except for groundwater [8]. The water discharged from the sewers into rivers is diluted by flow from rain or other sources. However, in periods of low rainfall there may be very little other flow. Fortunately, in this country, not many people have water softeners.

Some US states have banned water softeners.

In the US, some states have banned salt based water softeners because of the environmental impact. A 2018 study found that over a period of 25 years at least a third of U.S streams and rivers had become saltier. Not only would this affect drinking water but also agriculture as these water supplies could become too salty to be used in irrigation systems. Of course water softeners are only one source of this extra salt [9].

Removing sodium from water is expensive

Water treatment to remove dissolved salts from water is possible but it is expensive and requires energy. For example one method is electrocoagulation, whereby an electric current forces the salt to react with metals (iron or aluminium) to form insoluble particles that can be removed more easily [10]. 

Are there health benefits/risks of softened water

You are generally advised not to drink softened water – hence ideally the kitchen tap is excluded from the softening process. This is mainly because softened water is slightly salty. This is particularly bad for some people who need a low sodium diet. In addition, if there is lead in the pipe this is likely to dissolve into softened water whereas it will not dissolve into hard water.

Drinking hard water has another advantage in that it contains minerals including calcium and magnesium which are good for you.

We also use water for washing. Hard water can be bad for dry skin and hair and it can exacerbate eczema. When combined with soap it forms scum and you need more soap than with soft water. There are no downsides with washing in soft water [11].

Is my DHW cylinder really going to scale up?

Cylinders can and do scale up. In very bad cases it can reduce the volume of the cylinder significantly. Also scale on the coil (heat exchanger) or immersion heater reduces efficiency which means it takes more energy to heat your water. There are designs to minimise the problem, but the best way to avoid scale (other than using a water softener) is to set the hot water temperature as low as possible. Scaling increases the hotter you go and is much faster above 60°C, so try to keep below that. You are generally advised to heat to 60°C occasionally to sterilise the cylinder but once every one or two weeks is fine.

You will find adverts for magnets that supposedly reduce limescale but there is little evidence that they work in hot water cylinders [10]. I have found a recent study that concludes magnets do help to remove scale in the presence of air bubbles [12]. However there should not be air in your cylinder.

What can I do?

There are salt free water softeners / water ‘conditioners’ of various types on the market. Magnet based solutions are probably ineffective – at least there is little evidence that they work [13] [14].

Water conditioners do not remove scale, they promote precipitation into a form that does not build up on surfaces. This reduces scaling up of kettles and DHW cylinders but does nothing about the scum you get with soap [15]. This is a partial solution that you might consider.

Or you can live with the hard water and perhaps try to keep your hot water cylinder below 60°C most of the time (this also saves energy).

If you do decide to go with a conventional water softener, choose one that is economical in water demand. At least for the moment, the water usage is more critical than the salt.

[1] Why does it take 17 litres of water to regenerate a Harvey Water Softener (Harvey Water Softeners)

[2] Water Softener – how it works (GE)

[3] Calculations for water use by softener

Assumptions:

50 US gallons used per recharge = 190 litres

One recharge per week 

2 people

140 litres/person/day normal use

then

190/7/2/140 = 10%

[4] How much salt will my softener use? (Hydroworks)

[5] Rainwater (Science Direct)

[6] Water Softeners and Septic tanks or sewage treatment systems (WTE Sewage Treatment Systems)

[7] Calculations for the salt concentration after diluting with typical use.

Assumptions

120 kg/year salt used.

2 people

140 litres/person/day normal use

Plus 10% for the water softener

Sodium mass as a fraction of salt = (23/(23+35) = 40%

then

120e6*0.4/365/(140*1.1*2) = 430 mg/litre

[8] The Water Framework Directive (Standards and Classification) Directions (England and Wales) 2015

[9] The Environmental Damage Caused by Salt Softeners (Halcyon water conditioners)

[10] (Rahat Alam, Saif Ullah Khan, Muhammad Usman, Mohammad Asif, Izharul Haq Farooqi) A critical review on treatment of saline wastewater with emphasis on electrochemical based approaches (Process Safety and Environmental Protection) 2022

[11] Hard Water vs. Soft Water: Which One Is Healthier? (Healthline)

[12] Do magnets remove limescale from water pipes? (Naked Scientists) 2009

[13] (Maziar Naderi, Vida Past , Amir Hossein Mahvi) Magnetic treatment as a suppressive method for CaCO3 scale deposition in hard waters in the presence of air bubbles, (Desalination and Water Treatment) 2024

[14] The Truth About Salt-Free Water Softeners (Fresh Water Systems) 2024