Sorption technology
Sorption technologies are used in water treatment to clarify water and improve its organoleptic qualities by removing dissolved organic substances and gases.
In water treatment technology, activated coals and other (more often artificial sorbents) are used to reduce water colour and odour. In particular, they are effective against natural humic acids, a wide range of industrial organic compounds, including phenols, organochlorine, etc., effectively reduce the chlorine content, indicators of total organic carbon.
These processes allow using it in sugar, food and alcohol industries, in complex chemical productions. Some craftsmen use activated carbon to clean moonshine from sea liquor oils.
The concept of adsorption is applied to liquid and gas media, including water. It is the process of absorption of gases, vapours, substances from a solution or gas mixture by the surface layer of a solid body (liquid). These absorbers are called adsorbents and the absorbed substance is called adsorbate.
Mechanism of adsorption
Let’s look at this process with an example of activated carbon. If we consider its particle under a microscope, we can see many protrusions, depressions and peaks, which significantly increase its surface and form its porosity. There are several types of pores that are classified by size: macro, meso and micro.
There are two types of adsorption:
- Physical Reaction
The physical is a mechanism that occurs due to the presence of special (vanderwaals) interactions. They cause the attraction of molecules to the adsorbent surface. This physical process is of reversible nature. It means that part of the sorbed substance can return to water under certain conditions (temperature increase, change of water composition, etc.). Thanks to this, activated coals can be partially recovered by backflushing with water.
- Chemical Reaction
If the process is complicated by chemical reaction, then chemical adsorption (chemo sorption) occurs, it is not characterized by back reactions. This means that if substances are fixed to the surface, they will not return to the water. It is worth noting that both processes are not separated in time.
Sorbents
The most popular sorbent for water treatment is activated carbon. It is obtained from natural plant materials (wood, coconut shells, apricot stones, etc.) or minerals (bituminous and hard coal). The production technology is based on burning in special furnaces and subsequent activation. Activation is the process of opening closed pores in the coal structure.
There are three activation methods:
- treatment with solutions of certain substances and subsequent high-temperature treatment without air access (to avoid combustion);
- treatment with hot steam and/or carbon dioxide at 800 – 850 oC;
- Hot steam treatment with a limited amount of air for combustion of a certain amount of coal.
Modern sorbents for water treatment
Nowadays, activated carbon is used for sorption water treatment. Usually granular forms are used as they are more convenient to use. Depending on the quality of the raw materials and production technology, the specific surface area of such coals reaches 800 – 1300 m²/year.
One of the largest manufacturers is American company Calgon Carbon. The products are supplied to Ukraine. Mainly in water treatment technologies are coals obtained from coconut shells.
It is worth noting that there are alternatives to coals. These include new synthetic sorbents such as HumiSorb, a component of the Ecomix multifunctional mixture that is capable of comprehensively purifying water from organic compounds, salts of hardness and iron.
HumiSorb equipment
Modern water treatment equipment is performed as cylinder filters with capacity from 0.6 to 25 – 30 cubic meters per hour. Productivity of such systems is determined by the volume of activated carbon and, accordingly, the size of the filter.
The principle of operation of such systems consists in the fact that pre-cleaned from mechanical particles water from the pipeline through the distribution system is fed into the filter housing (cylinder) and distributed over the thickness of the load. Then, it is discharged through the central pipe to the customer.
After a certain time, the system is regenerated. It involves flushing the load at high reverse current velocities. High-quality activated coals, depending on water quality, can last more than five years if used correctly.
After this period, the material is replaced. On an industrial scale its thermal regeneration is possible, which provides for firing in furnaces at 800 – 850 oC to remove organic particles and reactivation.
Another modification of the coal filter is the cartridge main filter. It is a classic BB20 or BB10 type flask, which is mounted in a pipeline. As a filter element, a cartridge with granulated activated carbon or a carbon block of compressed powder is installed.
Such solutions have quite a small resource (50-60 m3 for BB20 filter) and when exhausted, they must be replaced. Such a resource is enough for water treatment in an apartment with 2-3 residents for about 3 months. Widely used in household and production of drinking water (spill points, kiosks, etc.).
History
Activated carbon has been used for water treatment for a long time. The first mention of it was found in Sanskrit in ancient India. Ancient Romans used it to purify water. At the end of the 18th century, people began using activated carbon for industrial purposes in the pharmacological and sugar industries. And already in the XX century it became widely used in water purification for water clarification.
