Water treatment for drinking purposes

Ivan Tikhonov

The article discusses the main methods of water purification for drinking and household needs. Household centralized and decentralized water supply networks are accepted as a source of water supply.  Recommendations on the selection of water treatment equipment and materials are given.

It is no secret that the drinking water that enters our home is not always drinkable. Even if the water meets all the requirements, it does not mean that it is safe to drink.

This article discusses the general issues of cleaning tap water and bringing its quality to drinking water quality standards.

Prepared water from a surface source (usually a river) and an artesian well is considered as a source of water supply.

So, if your house is connected to the central city water supply, the water utility takes water from the river and cleans it.

Here it is necessary to explain what the types of contaminants are found in the water and what cleaning methods are used to remove them. Table 1 shows the types of contamination characteristic of surface and artesian waters and how to remove them.

There are 4 types of pollution in the water:

• ionic
• organic and colloidally dissolved
• coarse impurities
• gases

Table 1

Ionic impurities of water determine the salinity of water. If the salinity of water is more than 1000 mg/l, then such water is considered brackish and is not drinkable. Water from the river in the vast majority of cases has a salinity of less than 1000 mg/l and is fresh. Therefore, this water, as a rule, does not require adjustment of the ionic composition or removal of ions.

Water hardness is also determined by calcium and magnesium ions. The hardness of drinking water is allowed up to 10 mg-eq/l or about 800 mg/l. As we can see, if the water hardness is 10 mg-eq/l, then its salinity is already equal to 800 mg/l. And in the water there are still sodium, sulfate and chloride ions. And as a rule, their number is equal to the number of hardness ions. Thus, if the water hardness is 10 mg-eq/l or more, then such water is automatically considered brackish (not drinkable).

Attention! This water is useless to soften. Because after softening, the water will contain sodium ions instead of calcium and magnesium. No more than 350 mg/l of sodium ions are allowed in drinking water. As we can see in this case, there will be an excess of sodium ions. Of course, such water can be used for technical purposes, but you should not drink it.

In this case, the only solution for drinking needs is the use of reverse osmosis. In the process of reverse osmosis desalination of water, most ions are removed from the water. The quality of water improves.

But in most cases, water from the river does not require adjustment of the ionic composition. Therefore, in Table 1, we write that the adjustment of salt content and hardness is not required.

Also, iron, heavy metal and silica ions should be attributed to ionic impurities.

Let us keep in mind that silica is very difficult to remove from water and let us hope that for most surface sources the concentration of silica is no more than 10 mg/l, at least for the middle part of the European Russia.

There is practically no iron in river water, because water has constant contact with air and iron is oxidized and precipitates. Iron in surface water is only found in wetlands. But nevertheless, iron can appear in the water after it has been cleaned. The fact is that as a result of coagulation of river water, part of the bicarbonates passes into carbon dioxide and, accordingly, the pH of the water decreases. As a result, the carbon dioxide balance is disturbed, and the water becomes corrosive. If the water supply pipes are made of ferrous metal, then the consumer will have iron in the water. The permissible concentration of iron in drinking water is 0.3 mg/l. but it is better when it does not exist at all. Accordingly, we write in the table that the water of the surface source will require additional treatment of water from iron. Heavy metals can also enter drinking water through leaky pipes in the water supply system. They will also require additional cleaning.

Organic and colloidally dissolved impurities are the main pollutants of surface water. In fact, they are organic acids (humic), as well as various colloidal suspensions, including those formed by silicic acid and (or) iron. These contaminants are not actually filtered out and do not precipitate. They are partially susceptible to adsorption on activated carbon. But the main method of cleaning is coagulation, i.e. consolidation of these contaminants for the purpose of their subsequent filtration or precipitation and removal.

Coarse impurities are non-water-soluble particles with a size greater than 0.46 microns. These particles can be almost completely removed by filtering through special fillers. Since surface water has contact with the atmosphere, these particles are in excess in the water. At the cleaning stage, they are removed from the water, but can get back into it in the water supply system, especially if the pipes are leaky.

The surface water contains practically no gases such as hydrogen sulfide, ammonia and chlorine. Since there are practically no gases in the air, then based on the equality of partial pressures, the surface water of large rivers will also not contain these gases. Unless the water is contaminated with sewage or is swampy water.

After cleaning, the water is disinfected with chlorine. The residual chlorine content in drinking water should not exceed 0.5 mg/l. This is done to ensure that the water has a residual decontamination effect in the water supply pipeline system, so as not to get secondary bacterial contamination. But what kills bacteria harms a person, so it is necessary to clean such water from residual chlorine.

It is necessary to note one more very important point. In the surface water, even after coagulation, a certain amount of organic compounds remains. The products of dissociation of chlorine in water oxidize organic matter and become organochlorine compounds. These compounds are the strongest carcinogens. In particular, it is said about the possibility of a multiple increase in the risk of developing malignant formations.

Taking into account the above sad, in general, the installation of post-treatment of drinking water for an apartment with a central water supply from a surface source can be presented in two types (using reverse osmosis and without reverse osmosis).

The basic principle of selecting a water treatment technology is that pollutants must be consistently removed from the water from larger to smaller ones. That is, first coarse impurities must be removed from the water.

To do this, at the first stage of cleaning, it is necessary to put a cartridge filter for mechanical water purification. This filter has a filter cartridge made of polypropylene. Water filtration is dead-end, i.e. all contamination from the source water remains on the surface of the cartridge. The smallest filter rating of the cartridge is 1 µm. I.e. particles with a size greater than 1 µm are retained by the cartridge.

Then iron, heavy metals and organic matter must be removed from the water. To remove dissolved iron and heavy metals, an ion exchange resin must be used. There should be a little ion exchange resin. The fact is that if the entire cartridge is filled only with resin, then completely softened water will go first, which can be the cause of, for example, bitter tea, etc. Because when such water is boiled, caustic soda will form in it and, accordingly, the pH will increase. Moreover, after 150-200 liters of purified water, the resin will be completely depleted of sodium ions and will work only for the removal of heavy metals. But the removal of heavy metals does not require such a large amount of resin. Therefore, the resin will only take up valuable space of household filter. In this case, it is better to use a combined loading. As the main option of a combined loading, a mixture of an activated carbon and resin in a ratio of 70/30 % is used. In this case, the resin will be sufficient to remove iron and heavy metal ions and the activated carbon will work to remove organic matter from the water, colloidal impurities, and chlorine.

At the third stage, it is necessary to install a finishing filter or a so-called water-conditioning filter. This filter should only contain activated carbon.

As a result, water after purification may contain a small amount of organic matter, as well as halogen-containing compounds. Such water is formally drinkable. But for the complete removal of organic matter from the water, it is necessary to additionally pass water through a reverse osmotic membrane. As a result, we get very high quality water. There is an opinion that such water is supposedly dead. In fact, this is not the case. If you think in such categories, then on the contrary, it is just the most living water.

Now let’s look at another very important question. The performance of household purification systems.

If you look at the characteristics provided by the manufacturers of such systems, you see that the performance of a standard cleaning system with standard cartridge diameters (about 60 mm) is 2.0-2.5 l/min.

Let’s look at this statement from the point of view of a chemical technologist.

The linear rate of water filtration through the cartridge is initially known for each filter material and is determined on the basis of long-term practical observations. So the linear filtration rate for activated carbon is 6-10 m/h. that is, water through a cross-section of a certain area should move at a speed of 6-10 m/h.

If the filtration rate is faster, then there is no complete process of adsorption of pollutants by coal.

In the case of a cartridge with a diameter of 60 mm filled with coal, we obtain that when a water flow rate of 2.5 l/min (0.156 m3/h) is passed through it, the linear filtration rate is obtained:

0,156/(0,06*0,06*0,785)=55,2 m/h

Where the denominator is the area of a circle with a diameter of 60 mm.

The linear filtration rate is obtained in 5! times more than necessary. Accordingly, there is no complete water purification.

In fact, the performance of such systems should be five times less. I.e., it is necessary to limit the consumption of treated water after such treatment plants at a level of no more than 0.5 l/min (maximum 1 l/min).

As a result, we get a weak trickle and the kettle with a volume of one and a half liters will be filled in three minutes.

In this situation, it is quite possible to consider an ordinary and cheap water filter pitcher. Due to the design, it slowly and therefore effectively cleans tap water. At the same time, the waiting time for purified water will be only slightly longer than when using multi-stage filters.

And the tempting desire to increase the flow of water through a multi-stage filter will only lead to a deterioration in the quality of treated water.

Of course, the cup water filter pitcher cartridge must be changed every month and large multi-stage filter cartridges one time in 3-6 months. But the cost of a pitcher cartridge and the cost of all three cartridges for a multi-stage filter differs by an order of magnitude.

Therefore, my personal opinion is that if you put a multi-stage post-treatment of water for the house, then only with reverse osmosis. At least it makes real sense to get high-quality water. If you only need to remove heavy metals from the water and the smell of chlorine with a slight conditioning effect, then a regular pitcher filter is enough.

If the source of water supply is an artesian well, then a very large number of different variations in the treatment of such water are possible. The main trend is that in artesian water, as a rule, there is an increased salt content, but due to the lack of contact with the atmosphere, there is no organic matter in it. This water is very suitable for reverse osmosis treatment. At the same time, the salt content decreases, and the membrane is not clogged with organic matter.

It is also important to note that the formation of hydrogen sulfide and ammonia is possible in artesian water, because of the lack of contact with atmospheric air. Here we can talk about aeration of water or dosing of chlorine, but in this case, it should be decided separately for each specific episode.