Chemical dosing is used to treat sewage effluent at wastewater treatment works. It involves the automatic injection of reagents into wastewater/treated effluent to treat odour, reduce phosphorus concentration, correct pH levels, remove COD/organics, and aid in settling sludge and solids in water. Chemicals are also used for disinfection.

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 control logic for phosphate removal is the closed loop, and sometimes a feed-forward system is added on top of that which is more predictive of changes in water chemistry and smart. Chemicals are sometimes dosed according to flow and sometimes as per a specific set point in the water such as turbidity, Dissolved Oxygen, N levels, P levels or pH. Sometimes complex algorithms such as Feed Forward Control are used which are based on predictive models.

Jar tests are normally done to optimise the dosing rate/pH.

Figure Above: Jar Testing at Carrington Power Station UK to Determine Optimal Dose of Ferric, Polymer and Test the Effect of Chlorine on the Flocculant in the Clarifiers

What type of reagents are used?

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

Ferrous dosing

Ferrous dosing systems are mainly used in sewers where anaerobic bio-reactions occur regularly. These reactions produce hydrogen sulphide, a gas that is characterised by its rotten egg smell and corrosivity/toxicity. 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.

Figure Above: Ferric Dosing Station At Carrington Power Station. Ferric is aggressive to SS and hence the chemical dosing pumps which were equipped with SS ball NRV’s Failed During Commissioning

Magnesium oxide/hydroxide dosing

Chemical reactions that cause hydrogen sulphide emissions require acidic to neutral pH levels between 6.8 and 7.2. If the pH level goes higher or lower than that, then conditions are no longer suitable. Magnesium oxide dosing works by raising the pH level to an eight or higher, and this reduces the production of odorous gases. Magnesium also binds N and P forming Struvite solids.

Calcium nitrate dosing

Anaerobic sulphate reducing bacteria in untreated wastewater undergo a biological process that produces free-ranging sulphides that bond with hydrogen molecules to form a highly toxic and corrosive hydrogen sulphide gas. When calcium nitrate is added to wastewater, it provides an alternative oxygen source, thus creating a new reaction pathway unfavourable to SRB’s. This decreases hydrogen sulphide production, which then reduces the presence of odours. We have a complete page on septic odours if that is interesting to the reader.