What is Chemical Dosing in Wastewater Treatment

Chemical dosing is used to treat sewage effluent at wastewater treatment works. It involves the automatic injection of reagents into a wastewater network to treat odour, reduce phosphorus concentration, correct pH levels, and aid in settling sludge and solids in water.

How is chemical dosing done?

To accurately and consistently inject chemicals into wastewater, you will need a pump technology that delivers chemicals in a way that optimises their usage and increases their effectiveness. A chemical dosing system that employs either metering or peristaltic pumps attached to pipework built into a cabinet containing a control system provides the most effective solution for this process.

The chemicals used in phosphate removal are very expensive. Therefore, it is the interest of treatment plants to exercise strict control of their dosing regimen so that they remain compliant and still make profits. The most reliable and cost-effective approach to phosphate removal is the closed loop, feed-forward system.

What type of reagents are used in chemical dosing?

There are different types of chemicals used in dosing systems, but your environmental permit determines what substance you use.

Ferrous dosing unit

Ferrous dosing systems are mainly used in sewers where bio-reactions occur regularly. These reactions produce hydrogen sulphide, a gas that is characterised by its rotten egg smell. Ferrous dosing reduces the impact of hydrogen sulphide odours by starting another chemical reaction. It utilises ferrous chloride, which binds with sulphides in the wastewater, forming a stable insoluble precipitate. The key benefits of this dosing system are that it eliminates the formation of hydrogen sulphide odours and lessens the corrosion of wastewater facilities.

Magnesium hydroxide dosing unit

Chemical reactions that cause hydrogen sulphide emissions require pH levels between 6.8 and 7.2. If the pH level goes higher or lower than that, then conditions are no longer conducive. Magnesium dosing units work by raising the pH level to an eight or higher, and this reduces the production of odorous gases.

Calcium nitrate dosing unit

The bacteria in untreated wastewater initiate a biological process that develops free-ranging sulphides that bond with hydrogen molecules to form a highly toxic hydrogen sulphide gas. When calcium nitrate is added to wastewater, it provides an alternative oxygen source, creating a new reaction pathway. This decreases hydrogen sulphide production, which then reduces the presence of odours.

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