Chemical-Physical Methods of Wastewater Treatment

Flocculation Process for Removing the Finest Particles

Flocculation creates the conditions for removing even the finest particles from the water, which are present in suspension or in the form of colloidal solutions. If the surface charges are the same, for example, these tiny solid particles cannot come together to form larger agglomerates due to mutual electrical repulsion.

The agglomeration of such particles can be achieved with the help of suitable chemicals, so-called flocculants and flocculation aids. This creates macro-flocs that sediment. Flocculation is used to improve the settling properties and dewatering of sewage sludge. When using iron and aluminum salts as flocculants, phosphate precipitation can occur at the same time.

Precipitation Processes in Wastewater Treatment

Precipitation is a chemical process by which a previously dissolved substance is separated from a solution. A common method is the precipitation reaction through the addition of suitable substances. For example, heavy metals can be precipitated by converting them into sparingly soluble metal hydroxides. Sometimes precipitation as carbonate or sulphide may also be necessary.

Anions can often be precipitated as calcium, iron or aluminum salts. Fluoride ions, for example, are separated by a precipitation reaction with milk of lime. In the course of wastewater treatment in the sewage treatment plant, the phosphate concentration is reduced by adding salts such as iron(II) sulphate, iron chloride or aluminum chloride. This so-called phosphate precipitation can either be integrated into the biological treatment stage as simultaneous precipitation or as a separate downstream process stage.

Ion exchangers are materials that can be used to replace ions in a solution with other ions. For example, a cation exchanger is used to replace calcium ions with sodium ions. When this ion exchanger is exhausted, i.e. completely saturated with calcium ions, it must be regenerated. This process works because the more ions are bound to the ion exchanger, the higher their charge and, with the same charge, the larger their ionic radius. The stronger binding ion displaces the weaker binding ion.

Ion exchangers are suitable for the targeted removal of heavy metals and anions. They are therefore often used as "police filters" after precipitation/flocculation. They are also used for softening, resalination and desalination of water. This is particularly important in the semiconductor industry, where they are used to produce extremely clean demineralized water, known as ultrapure water. Ion exchangers are also used in the treatment of waste water from galvanic coating systems, which contain metal ions, or for the purification of phenol-containing waste water 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. This is typically a physical process in which molecules adhere to the interface through van der Waals forces. If substances are bound to the surface of a solid by chemical bonds, this is referred to as chemisorption. In contrast to adsorption, chemisorption is often not reversible.

Activated carbons are used in wastewater treatment to capture dissolved water constituents that could not be adequately removed using cheaper methods such as biological wastewater treatment, precipitation and flocculation. For example, dyes from textile dyeing plants can often only be completely removed from wastewater by adsorption on activated carbon. Anthropogenic trace substances, such as pharmaceutical residues, and polar organic substances, such as adsorbable organically bound halogens AOX, also bind to activated carbon.

Doped activated carbon can also be used to remove arsenic and heavy metals. Iron hydroxide granulate is also ideal for removing the toxic semi-metal arsenic from drinking water, contaminated groundwater and polluted industrial wastewater. The iron hydroxide ultimately reacts with the arsenate ions to form iron arsenate. This process is both very efficient and cost-effective.

Redox reactions are used in a variety of ways in chemical wastewater treatment and drinking water treatment. For example, radical oxidation with ozone and hydrogen peroxide is used to efficiently remove chlorinated hydrocarbons and pesticides from drinking water.

Oxidation processes are also used in wastewater treatment to remove compounds that are difficult to biodegrade. Photochemical purification, in which hydroxyl radicals are generated from hydrogen peroxide or ozone in the presence of water using UV light, is particularly effective. These so-called Advanced Oxidation Processes (AOP) are also used to destroy active pharmaceutical ingredients such as antibiotics, cytostatics or hormones and other anthropogenic trace substances.

Ozone is also used to oxidize iron and manganese in well water. Reduction processes, on the other hand, are required for the conversion of heavy metal ions into poorly soluble sulphides, for example.

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 neutralize industrial wastewater.