Ways to purify tap water: Charcoal Filtered Water vs Reverse Osmosis vs Distilled Water vs Purified Water vs Demineralised Water
Written by William Fryer - MA Oxon
There are a number of methods available to purify/clean tap water. They range from methods you can use at home to produce a single jug of clean water, all the way up to massive industrial operations making clean water from sewage or the ocean’s salt water. The three main methods for creating clean drinking water in the home are: Carbon Filtration, Reverse Osmosis and a Water Distiller.
This article will define the different methods and types of water produced. Before we discuss each method it’s important to think about the type and nature of contaminants. We have a list of 79 contaminants which can be found in tap water but most water contains: minerals - typically metal salts, things like calcium carbonate which forms scale; volatile organic compounds (VOCs) these are organic compounds which have a boiling point below water and need to be removed by carbon filtration; potentially viruses and bacteria which are killed by boiling; chlorine, present in a lot of tap water as a cheap disinfectant; possibly fluoride which needs to be removed by distillation. In short there is a complex cocktail of different impurities and different methods work for different impurities.
Gas masks filter out poisonous chemicals using carbon filtration; the same process that some water filtration systems use. Activated charcoal (carbon) is a chemical compound that is very porous and has an incredibly high surface area. That high surface area is very good at absorbing different types of impurities. When impure water passed through it the porous surface of the activated carbon scrubs the unwanted properties from the water. The microscopic holes in the surface of the activated charcoal filter captures impurities which cleans the water.
Activated carbon is also used in a medical setting to treat poisonings and overdoses, where it absorbs the undesired substance from the body. The problem with carbon filtration is that eventually the activated carbon is used up and needs to be replaced, this is why you need to keep replacing activated charcoal filters. Another problem with activated charcoal filters is they are very good at absorbing Volatile Organic Compounds but not much else. VOCs include chemicals like oils, herbicides and pesticides. Any rogue minerals, fluoride or scale will NOT be removed by charcoal filtration. In a way this is a problem. most tap water is mainly contaminated by rogue minerals, fluoride and scale but very little in the way of Volatile Organic Compounds.
However VOCs have a strong influence on taste so when you filter tap water in this way it often seems to taste a lot cleaner even though only one class of chemicals has been extracted. Activated charcoal filtration is basically the minimum standard for purified drinking water. Charcoal filtration is a profitable business. Units range from £10-£400 and sellers make a lot of money selling replacement filters. Claims of special technologies by filter manufacturers are usually unfounded.
Osmosis is a process where two liquids try to find equilibrium between their two chemical states. When one liquid is placed next to another with a semi-permeable membrane between them the solvent flows from the liquid with a low concentration Into the liquid with the high concentration so both liquids end up with similar levels of solute and solvent. Reverse osmosis turns this process backwards. By pressurising one of the liquids it can push the liquids out of equilibrium, where one liquid will contain all the chemical solute and the other will be free of it. Reverse osmosis is a good way to purify water.
Typically you end up with water that is fairly pure, in fact it is pure enough for window cleaners to use so they avoid the smears associated with using tap water. It is NOT however the gold standard. RO units are typically fairly expensive to buy and fit but once fitted there is very little in the way of running costs. Outside of industrial settings, problems come from the disposal of the unneeded solution and the amount and size of equipment needed to pressurise the liquid. Reverse Osmosis units can take up an entire cupboard in a household setting and may need a plumber to fit them. Reverse osmosis will NOT kill Bacteria, Viruses or Fungal spores.
Distillation is the only process by which you can achieve absolutely pure water. Most people know that boiling water kills off bacteria, viruses and germs, a water distiller takes this process one step further.
With a water distiller the water is boiled continuously and turns to steam. The boiling kills off any bugs and the steam that comes off is absolutely pure water vapour. This water vapour is then condensed back into water while it circulates through a heat exchanger in the head of the water distiller. The steam is more pure than the tap water because the water has a lower boiling point than the contaminants so any nasty chemicals get left behind. VOCs can still get through because they typically have a lower boiling point, which is why Megahome distillers use an activated charcoal filter at the end of the process to filter out VOCs. Water distillation is the only way to get absolutely pure water with EVERYTHING, including nasties like Fluoride, removed. Water distillation is the method used by laboratories to produce pure water. It is also the process by which rain is produced - water evaporates from the oceans, condenses in clouds and falls as rain. Water distillers are typically fairly cheap to buy but can take a bit more work in terms of maintenance and they cost about 9 to 10p per litre of water for electricity.
"Purified water" is a poorly defined term. Some people might talk about purified water if they have taken water from a river and run it through a sand filter to remove a lot of the sediment. Similarly they may define purified water as distilled water, which is pure water. The water can be said to have been purified if it has been through a process which removed impurities but if you want absolutely pure water then distillation is the only method.
Demineralised water or Deionised water
Demineralised and deionised water are the same thing because most minerals are in fact ions. This type of water is very similar to distilled water in that it has had the bulk of its mineral content removed, usually in a large industrial-type process. However, any bacteria or viruses present will not have been killed off because the water has not been boiled and certain chemicals, for example VOCs will still be present. This makes demineralised water not really suitable for consumption because there is still a (very small) health risk. Demineralised water is typically used in car batteries because the water has had ions removed. Distilled water could also be said to be a type of demineralised or deionised water. And you can use distilled water in car batteries too.
Charcoal filtration is very effective at removing VOCs. A slow rumbling boil is the best method for killing viruses, bacteria and any biological contamination. Distillation is the overall most effective method of purification because it removes the most contaminants, kills biological contamination but it needs to be combined with charcoal filtration to remove VOCs. In order of effectiveness:
1. Water distiller with charcoal filter.
2. Reverse osmosis.
3. Any water filter.