Trihalomethanes (CHX3) where among the first disinfection byproducts to be discovered in chlorinated water. These substances are formed during chlorine disinfection and disinfection by chlorinated disinfectants. Trihalomethanes can be divided up into trichloromethane (chloroform, CHCl3), bromine dichloromethane (BDCM, CHBrCl2), chlorine dibromomethane (CHBr2Cl) en tribromomethane (CHBr3). Although these substances consist of either chlorinated or brominated methanes, these are not formed in a reaction between chlorine and methane. The substances form during a reaction between chlorine and organic matter in the water.
The trihalomethane concentration in surface water in summer exceeds the concentration present in winter. This is caused by raises in temperature and in the organic matter content of the water. Trihalomethane concentrations in surface water usually exceed groundwater concentrations. This is caused by a variation of the types of organic matter in the water.
When bromine is present, tribromomethanes are more likely to form. Each country has a different trihalomethane content in its drinking water.
Lab tests show that trihalomethanes form during a reaction between propanon (a byproduct of ozone) and chlorine. Propanon is immediately oxidated to trichloropropanon. When pH values are high, hydrolysis may cause chloroform to form from propanon.
When bromine is present, brominated propanon is formed, causing brominated trihalomethanes to form. Trihalomethanes are formed during hydrolysis reactions of various trihalogenic disinfection byproducts and transition products, such as trihaloacetonnitrils, trihaloacetyldehydes and brominated trihalo acetic acids.
What health effects do trihalomethanes cause?
Trihalomethanes are suspected to damage the liver, kidneys and central nervous system. They are also considered carcinogenic.
What are the standards for trihalomethanes?
In the United States the standard was reduced from 100 to 80 μg/L in the ‘Stage 1 disinfectants and disinfection byproducts rule’. (EPA, 2001)
The WHO has a separate standard considering trihalomethanes:
- Bromine dichloromethane (BDCM) - 60 μg/L
- Bromoform - 100 μg/L
- Chloroform - 200 μg/L
What are halogenic acetic acids?
Halogenic acetic acids (HAA) are an important type of chlorinated disinfection byproducts. Acetic acids consist of three hydrogen atoms that are fixed to a COOH-group. H-atoms of halogenic acetic acids are partly replaced by halogen atoms. HAA are non-volatile compounds. HAA can ocasionally be found in the water in higher concentrations than trihalomethanes (THM). This is determined by the pH value of the water. When the pH value is lower, more HAA are formed and when the pH value is higher, more THM are formed.
The composition of naturally present organic matter (NOM) also determines the amount of THM or HAA that is formed.
Like THM, HAA concentrations in surface water in summer exceed concentrations in winter and surface water contains more HAA than groundwater. HAA contribute to THM formation; during the biological decomposition of HAA, THM is formed.
HAA can also be formed during a reaction between propanon and chlorine. When pH values are low, trichloropropanon is oxidized further to form tetra-, penta-, and hexachloropropanon. When these compounds are hydrolysed, mono-, di-, and trichloro acetic acids will form.
What health effects do halogenic acetic acids cause?
Halogenic acetic acids are suspected to raise the risk of cancer.
What are the standards for halogenic acetic acids?
In the United States the EPA has established a standard of 80 μg/L for halogenic acetic acids. (EPA, 2002)
The WHO does not dictate any standards for halogenic acetic acids. (WHO, 2004)
What are haloacetonnitrils (HAN), halo-aldehydes and haloketons?
These disinfection byproducts are usually present in lower amounts than trihalomethanes (THM) and halogenic acetic acids (HAA). These compounds are usually formed immediately during water disinfection, but are decomposed quickly during hydrolysis reactions or reactions with residual disinfectants. The compounds can also be products of reactions of other disinfection byproducts, such as THM and HAA. When pH values are high, these compounds cannot be formed. Contrary to THM and HAA there is no difference between summer and winter concentrations.
Haloacetonnitrils are formed during a reaction of chlorine and acetonnitril. When the reaction time of the disinfectant in the water is low, these disinfection byproducts are decomposed.
Trichlorine acetaldehyde and brominated aldehyde compounds are the seconds largest group of disinfection byproducts imaginable. Mono- en dichlorine acetaldehyde can be formed during disinfection, but will immediately be oxidized to form trichlorine acetaldehydes. Acetaldehyde is a disinfection byproduct of ozone disinfection. When ozone is combined with chlorine, trihaloacetaldehydes form.
Reaction mechanism of acetaldehyde and chlorine:
CH3CH + HOCl -> CCl3CHO
What is MX?
In 1986 a new disinfection byproduct was discovered; 3-chloro-4(dichloromethyl)-5-hydroxy-2(5H) furanone, otherwise known as MX. About 30% of the total mutagenic activity in water can be accredited to this disinfection byproduct. MX is often present in water and because of its activity and health risk, WHO has placed it on a list of potentially dangerous substances to human health. There are no guidelines for dissolved MX, because of a lack in toxicological data on this substance. For the third edition of the WHO Drinking Water Guidelines (1997) a maximum concentration of 1,8 μg/L MX is advised.
Other disinfection byproducts that are often formed during water chlorination are halonitromethanes, halophenols and halofurans. These are not described further on this website.
Scientists have conducted studies on health effects of exposure to high levels of DBPs on laboratory animals. These studies have shown that several DBPs cause cancer in laboratory animals. In addition, some DBPs cause undesirable effects in the animals’ growth and reproduction. It is, however, difficult to estimate how the results of these high dosage studies on laboratory animals can be applied to low dosage, long-term exposure for humans.
Scientists have also studied the relationship between drinking chlorinated water and cancer rates. Some of these studies suggest an increased cancer risk to those using chlorinated drinking water, while others found no increased risk. Other studies that investigate whether chlorinated drinking water has an effect on reproduction and development also show inconsistent results. At the present time, the U.S. Environmental Protection Agency (EPA) does not believe there is enough evidence to state conclusively that DBPs cause these types of health effects. Research on the health effects of DBPs is not complete and the federal government continues funding research on this topic.