When Is Desalination Needed For Household Use?

I grew up living in a rural environment surrounded by different water sources, all with different salt levels. We could use some for showering, some for drinking, others for washing vehicles and some were too salty for anything! It always got me thinking, what were the individual cut-offs for each water use? what made some water too salty to do certain things like drinking or washing?

As a general rule, desalination is needed above 1700mg of salt per litre of water. This is the concentration stipulated by the European Union, the highest acceptable level amongst reputable global organizations. The EU figure refers specifically to drinking water, other water uses have different salt tolerances.

When making a decision on whether desalination is needed you must first consider what you will be using the water for around the house. This guide divides common water uses into six categories: drinking water, pet water, house plants, showering, washing clothes and car washing.

Drinking Water: 1700mg per litre

Household drinking water is the most common query, this applies to both humans and pets, for this, we can use the directive from the EU of under 2500 μs cm-1 conductivity (European Union 1998), which translates to 1700mg of salt per litre. However, this figure from the EU needs to be considered with the World Health Organization which put the maximum level at 1000mg/L and the United States Environmental Protection Authority which put the maximum at 500mg/L! The reason for this stark difference comes down to what the figure represents as an actual maximum level

We can look at these various global organizations on a scale between 500mg/l and 1700mg/l:

• European Union: 1700mg/L, The maximum that humans can tolerate in terms of taste and bodily function (European Union 1998).
• World Health Organization: 1000mg/L, The upper limit human taste buds can handle before it becomes unpalatable (World Health Organization 2022).
• US EPA: 500mg/L, The target amount for municipal water supplies where taste and hydration levels are deemed optimal (US EPA 2009).

So when coming to a decision on whether you need to desalinate your drinking water there is no clear-cut answer on when you need to desalinate, for a day-to-day household supply anything above 500mg/L could potentially warrant desalination however, for a camping trip you would perhaps be willing to tolerate higher levels for a short period of time.

Pet Water

As a general rule, if you shouldn’t drink, neither should your pets, pets and in particular, cats and dogs are sensitive to salt. You have probably seen dogs drinking from random water sources, including slightly brackish locations and even the ocean. Just because they do it DOES not mean it is good for them! the American Kennel Club’s advice on the topic is:

“In most cases, a few mouthfuls of salt water may only cause diarrhea. Consuming large amounts of salt water, however, can be fatal. When a dog ingests salt water, the excess salt draws water from the blood into the intestines, leading to diarrhea, vomiting, and dehydration.”

American Kennel Club: Is It Dangerous for Dogs to Drink Salt Water? Sept 2020

So, it is quite simple really, your drinking water should be the same as what your pets drink! Anything saltier requires desalination before drinking, simple distillation or an under-sink RO system are the two most popular methods.

Houseplants

Houseplants can vary a lot in their salt tolerances, however, generally speaking, regular tap water is fine to water your house plants. The general rule for plants is that they can tolerate up to 1500mg of salt per litre and as most tap water around the world sits between 500mg and 1000mg of salt per litre, it will be fine for your plants.

There are some common houseplants however that are much more susceptible to salt and others that are much more resistant. One popular variety of houseplants that cannot handle much salt is the ‘Spider Plant’ Although there is no cutoff salt level, tap water and particularly softened tap water will lead to brown leaf formation and generally poor plant health. The recommendation with these plants is to actually water with distilled or rainwater due to low salt levels, an under-sink reverse osmosis desalination system would also produce suitable water for salt-sensitive plants like the Spider Plant,

One more salt-resistant variety of houseplant is the bougainvillaea with its beautiful pink flowers. These plants can handle salt levels up to 8ds/m (6400 mg/l TDS)! This high tolerance for salt in water and soil enables bougainvillaeas to thrive all over the world, no desalination of tap water is required for them!

Showering & Bathing

Generally speaking, you can shower in waters of any salinity, we frequently swim in the ocean for instance, so there is no risk involved. However, salty water leaves residue on the skin once the water evaporates and it also doesn’t enable products like soap and shampoo to function effectively. So, this salt level is much more subjective than other categories. One way to look at it is to see what is considered a ‘salty bath’ for health benefits etc, work out the approximate concentration and use that as an upper level. Healthline recommends adding one cup of salt to a standard bathtub to help treat skin inflammation issues.

A standard bathtub is around 300L, when 2/3 full it is 200L of ~1000mg salt/L tap water.

Adding 1 cup of salt (112,800.00 mg) to the bath

Equals a new concentration of approximately 1500-1600mg/L of salt per litre.

Based on the above calculation, any water above 1500-1600mg/L will start to feel salty. The most common way to deal with this would be an in-line reverse osmosis desalination system which would drop salt levels to more comfortable levels. Just to reiterate, this is a subjective number based on comfort, rather than any clear-cut cause for concern.

Washing Clothes

Technically you can wash clothes in salty water, sailors have done it for centuries. The issue once again is comfort, as the water evaporates the residual salt can make the material stiff and even itchy. You have probably noticed this phenomenon when wearing swimming shorts at the beach. Unless you are hand washing clothes, another issue is corrosion or staining of the actual washing machine. Most washing machines use 201 stainless steel which provides some corrosion resistance however with continued use in slightly salty or brackish (>10,000mg/L salt) environments it will start to corrode. Regarding comfort there is no clear-cut limit, so a similar approach will be used as with bathing salt levels, what is considered ‘salty’ by experts on the topic?

Howstuffworks.com recommends adding 1/2 a cup of salt to a regular washing machine cycle to help prevent colours from bleeding from new clothes, keeping them fresh and bright. Putting the numbers together we come up with:

1/2 a cup of salt = 56400mg

An average washing machine cycle uses 14-20 gallons of water (53-75L). We will use the midpoint of 64L of 1000mg/l tap water for calculation purposes.

The resultant concentration = 1880mg/L

Based on the above assumptions and numbers, you can use saltier water sources, unsuitable for drinking, for washing clothes. Clothes can still be washed at higher salt levels however you may start to notice comfort issues.

Car Washing

For general washing purposes, we will use the max salt level a car can be washed with before being corroded as a benchmark. Cars are made of various types of regular steel, all susceptible to corrosion when exposed to moisture and salt. Technically water of any salt concentration will cause corrosion, however being realistic, you can wash your car with tap water for years without too many problems. This comes down to personal preference and how much you value your car! It has been observed that mild steel has a linear increase in corrosion rates from a chloride concentration of 100mg/l up to 5000mg/l. With chloride concentration in tap water around the globe frequently exceeding that lower 100mg/l threshold you will observe slow corrosion taking place on your vehicle, particularly in areas where the water takes longer to dry. So, definitely don’t wash your car with salty or brackish water! however, as a general rule, it will be ok to wash your vehicle with tap water. If you have a high-value car or a prized possession, it is recommended to wash it at specialized car washes that have desalination systems installed or install a reverse osmosis unit on your at-home water source.