Disinfection By-Products (DBP’s)
Introduction
The disinfection of water containing NOM = Natural Organic Matter results in the formation of DBP’s = Disinfection By Products. Some of these DBP’s can be carcinogenic, therefore, potable water treatment engineers have to be extra careful to remove NOM from drinking water sources.
Chemical drinking water disinfection with substances such as chlorine has been applied for more than a century. During the seventies, scientists discovered the possibility of formation of disinfection byproducts by means of gas chromatography testing. Disinfection byproducts can be harmful to human health. After this discovery, extensive research took place on the origin of disinfection byproducts, on the health effects and on procedures to prevent the formation of these products during the disinfection process.
What are disinfection byproducts?
Disinfection byproducts are chemical, both organic and inorganic substances that can form during a reaction of a disinfectant with naturally present organic matter in the water.
How are disinfection byproducts formed?
Disinfection byproducts can form when disinfectants, such as chlorine, react with naturally present compounds in the water. The formation of these products mainly takes place during reactions in which organic substances, such as humic acid and fulvine acid, play a part. These materials end up in water during the decomposition of plant matter.
In 1971 the American scientist Bellar discovered that chloroform was abscent in the Ohio river water which was used for drinking water production . However, chloroform appeared to be present in drinking water originating from drinking water purification plants. This proves the formation of disinfection byproducts during chlorination.
Little information can be found on the chemical structures of humic acids and fulvine acids. The mechanism of formation of disinfection byproducts therefor remains unclear. Research is difficult, because of the extensive number of substances that make up organic matter.
Which factors influence the formation of disinfection byproducts?
The types of disinfection byproducts that are formed depend on a number of factors:
– The type of disinfectant
– The disinfection dose
– The disinfection residual
When the dose and residual of the disinfectant are higher, more disinfection byproducts are formed. To prevent halogenic disinfection byproducts from forming, alternative disinfectants are applied today. However, disinfection byproducts may still form.
– Circumstances of disinfection: reaction time , temperature and pH
When the reaction time is shorter, higher concentrations of trihalomethanes (THM) and halogenic acetic acids (HAA) may be formed. When the reaction time is longer, some temporary forms of disinfection byproducts may become disinfection endproducts, such as tribromine acetic acid or bromoform. Haloacetonitrils (HAN) and haloketons (HK) are decomposed.
When temperatures increase, reactions take place faster, causing a higher chlorine concentration to be required for a proper disinfection. This causes more halogenic disinfection byproducts to form. An increase in temperatures also enhances the decomposition of tribromine acetic acids, HAN and HK (haloacetonitriles = HANs, and haloketones = HKs).
When pH values are high, more hypochlorite ions are formed, causing the efficacyof chlorine disinfection to decrease. At higher pH values, more THM is formed, whereas more HAA is formed when pH values are lower. At high pH values, HAN and HK are decomposed by hydrolysis, because of an increase in hydrolysis reactions at higher pH values.
The levels of trihalomethanes in drinking water are often higher in the distribution network than at drinking water production companies. When hydrolysis takes place many disinfection byproducts become trihalomethanes.
– The constituents of water
– Concentrations and properties of naturally present organic matter (NOM) in the water
NOM is the predecessor of a disinfection byproduct. The level of organic matter is usually registered as the “total organic carbon” concentration or the “dissolved organic carbon” concentration. The composistion and concentration of naturally present organic matter determine the types and concentrations of disinfection byproducts that will eventually be formed. Naturally present organic matter contains compounds, such as humic acids, fulvine acids, hydrophobic acids, hydrofobic neutral substances, transfilic acids, transfilic neutral substances, hydrophilic acids and hydrophilic neutral substances.
Seasons influence the naturally present organic carbon concentration, causing the concentrations of disinfection byproducts to vary.
The concentrations of disinfection byproducts in surface water vs groundwater may be different.
Table 1: disinfection byproducts of various disinfectants
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Reference: Lenntech
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