Chemical-Physical Methods of Wastewater Treatment

Our wide range of processes, from precipitation to sorption and nanofiltration, are designed to meet the specific challenges of these industries. The complexity of the wastewater composition often requires multi-stage treatment combining different chemical-physical processes. By skilfully combining these technologies, cost-effective and highly efficient solutions can be achieved for the treatment of a wide range of wastewater types.

Precipitation

Precipitation is a chemical process that separates a previously soluble substance from a fluid. A common method is to create a precipitation reaction by adding suitable agents. Through precipitation, heavy metals, for instance, transform to not easily soluble metal hydroxides. Other situations may require precipitation to carbonates or sulfides.

Anions can often be precipitated as calcium, iron, and aluminum salts. The separation of fluoride ions, for instance, is achieved through precipitation with milk of lime. During wastewater treatment in the treatment plant, adding salts like iron(II) sulfate, iron chloride or aluminum chloride lowers the phosphate concentration. The phosphate precipitation can either be integrated as simultaneous precipitation into the biological treatment stage or added as a subsequent separate process step.

Ion Exchanger

Ion exchangers are materials that can replace the ions of one solution with other ions. The cation exchanger, for instance, replaces calcium ions with sodium ions. Once the ion exchange is exhausted and the calcium ions are completely saturated, the ion exchanger needs regeneration. This success of this process is based on the principle of displacement: the higher the ions’ charge, the stronger the ion-binding to the ion exchanger. If both types of ions are charged the same, the one with the larger radius will be the one with the stronger ion-binding force. During the ion exchange process, the stronger-binding ion will displace the lesser-binding ion.

Ion exchangers are suitable for the removal of heavy metals and anions and are therefore often used as ‘policing filters’ after precipitation and flocculation. In addition, they assist with water softening, changing the water’s salt content, and water desalination; particularly important for the semiconductor industry that uses them to produce extremely clean, demineralized water known as ultra pure water.

Ion exchangers are also used in the treatment of wastewater from galvanic coating systems that contain metal ions or for the purification of phenol-containing wastewater from the chemical industry. Both the metal ions and the phenols can be recovered.

Adsorption is the accumulation of substances on the surface of a solid body, which is a physical process where molecules stick to boundary surfaces through the van der Waal force. If chemical bonding binds substances to the surface of a solid body, the process is called chemisorption. In contrast to adsorption, chemisorption is non-reversible.

Wastewater treatment uses activated carbons to bind soluble water contents that could not be sufficiently removed with lower-priced methods such as biological wastewater treatment, precipitation and flocculation. Colorants from textile dying plants, for instance, often can only be completely removed through adsorption on activated carbon. Anthropogenic trace elements such as pharmaceutical residues and polar organic substances like adsorbable, organically-bound halogens (AOX) also bind to activated carbon.

Doped activated carbon can also be employed to remove arsenic and heavy metals. Granulated iron hydroxide is another ideal agent to remove toxic metalloid arsenic from potable water, contaminated ground water and industrial wastewaters. In this process, the iron hydroxide reacts with the arsenate ions to form iron arsenate. This method is efficient as well as cost-effective.

Redox reactions are frequently utilized in chemical wastewater treatment and in the treatment of potable water. Oxidation processes with ozone and hydrogen peroxide efficiently remove chlorinated hydrocarbons and pesticides from potable water. 

In wastewater treatment, oxidation processes are used to remove difficult biodegradable compounds. Particularly efficient is photochemical purification, which forms hydroxyl radicals from hydrogen peroxide or ozone through UV-light exposure. These Advanced Oxidation Processes (AOP) are used to degrade drug substances like antibiotics, cytostatic drugs, hormones and other anthropogenic trace substances. 

In addition, ozone aids in the oxidization of iron and manganese in well water. Reduction processes are required to transform heavy metal ions, for instance, into easily dissoluble sulfides.

Wastewater Treatment with Membrane Technology

With membrane filtration, dissolved and undissolved substances can be separated from the wastewater and concentrated. Separation takes place under pressure; the membrane retains particles or molecules of a certain size with a certain pore size. The various processes are used for water treatment, wastewater purification, process water recycling and the concentration of valuable substances for recovery.

Microfiltration is used to separate particles, bacteria and yeasts. It is therefore used for cold sterilisation and the separation of oil-water emulsions, among other things.

Nanofiltration can retain viruses, heavy metal ions, large molecules and very small particles. The process is used for water softening and in drinking water treatment.

Ultrafiltration is an important process for drinking water and wastewater treatment. It is used to separate particles, microorganisms, proteins and turbidity from the water, including in the membrane bioreactor (MBR). Ultrafiltration is used, for example, to purify circulating water in swimming pools. As the formation of clogging deposits on the membrane can now be avoided, existing wastewater treatment plants are increasingly being supplemented with ultrafiltration as a polishing step. When retrofitting older, conventional wastewater treatment plants, ultrafiltration can be used directly in or after the aeration tank to replace downstream treatment steps or to increase the purification performance of biological wastewater treatment.

Reverse osmosis is an important process step, for example in the concentration of landfill wastewater, in drinking water treatment in rural regions that are not connected to the mains, in seawater desalination or in the decalcification of boiler water in power plants. The concentration of substances dissolved in liquids is increased through a semi-permeable membrane by using pressure to reverse the osmosis process: If the pressure is higher than the respective osmotic pressure, the molecules of the solvent diffuse to the side of the membrane on which dissolved substances are already less concentrated. This process is also used to obtain ultrapure water.

Neutralisation is used in wastewater technology to adjust the pH value. Acids or bases are added as required, particularly after processes such as precipitation and flocculation and to neutralise industrial wastewater.