Home » Chemicals Used In Waste Water Treatment
Chemicals used in water treatment are fascinating in what they can accomplish. Chemical dosing is used to treat sewage effluent at wastewater treatment works for example. 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.
Other uses of chemicals in water treatment include Ammonia and Phosphate dosing in water steam cycles to precent corrosion. Anti-scalants are dosed in RO systems to precent scaling. Corrosion inhibitors are dosed in cooling water circuits to delay or stop corrosion. It is a fascinating world of applied chemistry…
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/effluent parameters.
Ferrous/Ferric 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.
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.
Ferric or Aluminium Dosing
Ferric and Aluminium compounds are dosed either to reduce SS through coagulation/flocculation or to bind dissolved Phosphorus. I have had excellent results in phosphorus removal using PAC on a site in Ireland.
Disinfectant
Final effluent is normally disinfected using an oxidising biocide like Chlorine to reduce fecal bacterial numbers. A chemical free way to reduce MPN numbers is to use UV light. Ozone is an expensive way to kill bacteria but it is more environmentally friendly as the break down product of O3 is O2 so the water is oxygenated after the bacteria/viruses are killed.
Sodium Per-Carbonate SPC is a relatively environmentally friendly biocide. It is used as a bleaching agent in laundry detergents such as Vanish.
Flocculant
Flocculants are sometimes used to help in the settling of colloidal and suspended solids. They are also used in thickening settled sludge which helps in its dewatering. Green flocculants like alginates can be used or Hydrex from Veolia. Tanafloc is gaining popularity in the market too. SNF is one of the best suppliers of flocculants.
Starch-based flocculants are produced by several companies and are a focus of ongoing research as a more environmentally friendly alternative to synthetic polymers.
ACAT (Applied Chemicals International Group) offers flocculants for various applications, including some potentially bio-based options within their product range.
Cargill Deutschland (Germany) produces specific cationic starches, such as C*bond HR 35.849, used in industries like paper manufacturing.
Univar Solutions distributes the Caflon Floc™ range of water-based dispersion polymers, which may include starch-derived products.
Magnetite/Microsand
Tertiary Solids Removal clarifiers such as the Co-Mag help produce very clear effluent and can also remove P from water. Magnetite/Microsand is sometimes added to create heavier, more settleable flocs.
Sodium Carbonate/Bicarbonate Dosing
Some waters as in the north of England and Ireland are soft and have not much alkalinity to them. When Nitrification happens, the pH can dip to below 6. In such cases alkalising chemicals are used or buffering chemicals such as Na2CO3 which is soda ash or NaHCO3 which is sodium bicarbonate to increase the pH.
Active Carbon
Activated Carbon is sometimes added to waste water effluent to remove trace residuals of organics such as pharmaceuticals, PFAS, and other persistent organic pollutants such as those used in Personal Care Products. Look at my page for Active Carbon suppliers and applications of Activated Carbon.
Methanol Dosing for Anaerobic Reactors
Anaerobic waste water treatment processes sometimes require the addition of a carbon source for the Heterotrophic bacteria that convert Nitrates to Nitrogen. Sometimes iron salts are also added to capture sulphide in-situ to precent H2S formation in the biogas.
Chemical Dosing Using Electro-Chemistry
Some suppliers offer chemical free dosing of coagulants using sacrificial Iron or Aluminium anodes. These systems offer some advantages but they are prone to fouling/oxidation and this results in inconsistent dosing rates. Three of the electro-coagulation companies that I am familiar with are:
Kolina – UK
Power and Water – UK
VentilAqua – Portugal
Nijhuis Saur Industries – Dutch
Morselt – Dutch
Sometimes magnesium sacrificial anodes are used for recovery of Struvite.
Global Suppliers of Water Treatment Chemicals
Eco-Lab (Formerly known as Nalco)
Kemira – Finland
SNF – France
Veolia – UK and France
Kurita – Japanese
DOW Chemical Company – USA
B&V Chemicals UK
Solenis (acquired companies like BASF Flocculants, Drew, Hercules, Diversey, Ashland, Betz Laboratories and Stockhausen).
Lonza – Switzerland
Clarion – USA
Chem-Aqua – USA and Europe
Genesys RO – RO Anti-scalants
Companies like DeNora, Xylem, OVIVO and Evoqua sometimes offer systems that generate disinfectants like chlorine using electro-chlorination.
Univar
Challenge for Young Chemists
If you could think of a way to get Dissolve Oxygen to stay under water at higher concentrations than 10 ppm, you could win a Nobel prize. The application would be to save billions in air blower costs that is one of the main OPEX costs in running an STP.
Biography of the Author
Rami Elias Kremesti is a chartered chemist and water treatment specialist based out of High Wycombe in the UK. He is the MD of Kremesti Environmental Consulting Ltd.
He earned his M.Sc. in chemistry from the University of North Texas and his B.Sc. also in chemistry from the American University of Beirut. Rami is a passionate environmentalist and finds deep meaning in his work to protect the environment especially its water resources.
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Rami Elias Kremesti Portrait
