Research on Disinfection By-products in Drinking Water
Research on Disinfection By-products in Drinking Water

Application Engineer: Shi Xiao

Water is one of the indispensable substances in people’s lives and the source of life. With the development of society, people’s living standards have been continuously improved, and people’s requirements for the quality of drinking water have also been continuously improved. It only requires cleanliness and hygiene, but more importantly, "safety". "Safety" means the improvement of water quality. The "Drinking Water Hygiene Standard Inspection Method" (GB/T575-2006) implemented in 2007 covers pathogenic microorganisms, inorganic substances, organic compounds, and environmental hormones. The turbidity of the water is changed from the previous "3" is changed to "1", so that the water we drink in the future will be clearer, cleaner and safer. At the same time, there are new requirements for the disinfection of drinking water.

Disinfection by-products (DBPs) are compounds produced by the reaction between disinfectants and organics in the water when disinfecting drinking water.

So what is drinking water?

Drinking water: refers to drinking water and domestic water for human life. It includes groundwater (generally referred to as well water) and surface water (river, river, lake, reservoir, spring water, etc.).

In the process of drinking water disinfection, the disinfection by-products (DBPs) produced by the reaction of disinfectants and natural organic matter (NOMs) are mainly produced by the four main disinfection methods of chlorine, chloramines, chlorine dioxide, and ozone. Drinking water disinfection is one of the most effective public health measures in the 20th century, providing an effective guarantee for preventing drinking water epidemics. However, the disinfection and sterilization of drinking water is accompanied by a chemical reaction between the disinfectant and some natural organic matter, environmental organic pollutants and bromine or iodide contained in the source water, resulting in a variety of disinfection by-products, posing a potential threat to human health. Epidemiological studies have shown that there is a potential correlation between chlorinated drinking water and the incidence of bladder cancer, rectal cancer and colon cancer. In addition, drinking water DBPs may also cause reproductive and developmental side effects. Therefore, DBPs have become one of the hotspots of drinking water safety research. There are many types of drinking water disinfection by-products, which vary with the changes of disinfectants, disinfection technology and the chemical composition of the source water.

1. Types and characteristics of commonly used drinking water disinfectants

There are four disinfectant indicators stipulated in the domestic drinking water hygiene standards: chlorine, chloramine, chlorine dioxide and ozone. The disinfection mechanism and effect of various disinfectants are different, and the scope of application is also different.

1.1 Chlorine disinfection

Chlorination disinfection is the longest used and most extensive disinfection method. Since its inception in 1908, it has played a major role in preventing the spread of water-borne diseases. Chlorination disinfection includes two disinfectants: liquid chlorine and sodium hypochlorite. Liquid chlorine disinfection is currently the most cost-effective and widely used drinking water disinfection process in public water supply systems. It has the advantages of mature technology, strong sterilization ability, long duration, and low price. About 99.5% of drinking water plants in my country use chlorine. Disinfection process. Sodium hypochlorite reduces the hazards and technical requirements in the operation of liquid chlorine, but it is possible to introduce inorganic by-products, such as chlorate (ClO3-), chlorite, and bromate. Due to the long history of chlorination disinfection, the research on the by-products of chlorine disinfection is also more in-depth. Chlorinated disinfection by-products, mainly trihalomethanes, halogenated acetonitriles (HANs), cyanogen halides (XCNs), halogenated picrines, halogenated acetaldehydes (HATs), halogenated phenols (HHBs), halogenated ketones (HKs) ), halogenated nitromethanes (HNMs), halogenated hydroxyfurans (CHFs). Among these chlorinated DBPs, trihalomethanes (such as chloroform) have been identified as carcinogens. The drinking water safety regulations of the United States stipulate that bromodichloromethane, dichloroacetic acid, bromate, etc. are listed as suspected carcinogens. Most of the other DBPs are generally toxic and have irritating or anesthetic effects on human organs. A large number of epidemiological investigations have shown that long-term drinking of chlorine disinfected drinking water can increase the risk of digestive and urinary system cancer, and there is a statistical correlation between the two.

1.2 Chloramine disinfection

The DBPs produced by the chlorination disinfection process have attracted more and more attention. In order to control the concentration of THMs and by-products in drinking water, many water plants have begun to switch from chlorination disinfection to chloramine disinfection. Compared with chlorine, chloramine has good penetrating ability and long duration of stability, which can better prevent the growth of microorganisms in the drinking water distribution system network; in addition, chloramine disinfection can also significantly reduce chlorine smell and chlorine Phenolic taste. However, due to its low disinfection ability, chloramine is often used as a secondary disinfectant, combined with other strong oxygen (such as chlorine, ozone) for drinking water disinfection. Under the same conditions, the yield of DBPs, especially THMs, is obvious. However, recent studies have found that chloramine disinfection may produce some more potentially harmful nitrogen-containing disinfection by-products (NDBPs), such as cyanogen chloride, nitrosodimethylamine, and halogenated acetamide.

1.3 Chlorine dioxide disinfection

The chlorine dioxide disinfection method is an efficient, fast, long-lasting and safe drinking water disinfection method. Chlorine dioxide has a strong oxidizing ability, and it has a good killing effect on all pathogenic microorganisms spread by water. Chlorine dioxide does not react with ammonia nitrogen in water, has a good sterilization effect, and has a small dosage and fast action. The oxidation and disinfection capacity is less affected by pH and ammonia nitrogen in the water. It has a wide range of applications and can improve the color and taste of the water. . However, chlorine dioxide disinfection technology is expensive, which limits the popularization and application of this disinfection method in my country.

Chlorine dioxide is a strong oxidant rather than a chlorinating agent. It mainly reacts with organic pollutants by oxidation rather than substitution, so it does not produce organic halides such as trihalomethane THMs. Compared with chlorine or chloramine disinfection, chlorine dioxide disinfection process produces more haloacetic acid HAAs (mainly DCAA, CBAA and DBAA). The inorganic by-products ClO2-, ClO3- and BrO3- of chlorine dioxide disinfection are potentially toxic at high doses or high concentrations, and ClO2- can cause hemolytic anemia.

1.4 Ozone disinfection

Ozone disinfection is increasingly used in drinking water treatment. Ozone sterilization is to oxidize and decompose the internal glucose of bacteria to inactivate the bacteria to death. The sterilization performance test shows that ozone has obvious inactivation effect on almost all bacteria, viruses, fungi, protozoa, and oocysts.

As an alternative to chlorine disinfection, ozone disinfection is increasingly used. At room temperature (20℃), the half-life of ozone in tap water is 20min; therefore, when applying ozone disinfection, auxiliary disinfectants such as chlorine, chloramine, and chlorine dioxide need to be used to maintain continuous disinfection in the pipe network. The sterilization effect of ozone is stronger than that of liquid chlorine and chlorine dioxide, but because ozone is extremely unstable, it needs to be prepared on site during use, which increases the cost of disinfection. At present, only some water companies in a few countries use the ozone disinfection process. Compared with chlorination disinfection, the total amount of DBPs produced is also much less. However, the source water containing bromide ions can form bromate by-products during the ozonation process. When the source water contains a relatively high concentration of organic matter, DBPs such as aldehydes, carboxylic acids, ketones, phenols, and bromates (when the source water contains a relatively high concentration of bromide) are produced; among them, carboxylic acids account for 26%; In addition, there are still 63% of unknown organic matter to be discovered. Formaldehyde can cause human nasopharyngeal cancer, nasal cavity cancer and sinus cancer, and can cause leukemia. Bromoacetic acid is considered to have stronger DNA damage ability than chloroacetic acid; in addition, bromate has strong carcinogenicity, and the Cancer Research Bureau has listed it as a suspected human carcinogen with a higher probability.

2. Discussion on the detection methods of disinfection by-products

Sensitive and accurate analytical methods are important methods for studying DBPs in drinking water. The complexity and diversity of DBPs components have brought new challenges to analysis and testing. In the detection and analysis, different detection methods are selected according to the different characteristics of DBPs.

Because the concentration of DBPs in water is very low, it is not possible to directly inject samples. Although some DBPs are enriched and concentrated, it is still difficult to directly determine them. Indirect determination after derivatization is required to obtain satisfactory results. Therefore, the sample pretreatment method plays an important role in obtaining accurate test results.

Common separation and enrichment methods in sample pretreatment include: liquid-liquid extraction (LLE), solid phase extraction (SPE), solid phase microextraction (SPME), supercritical fluid extraction (SFE), and purge-trap method. LLE is a classic sample enrichment method, which is efficient and fast. Wei Jianrong et al. [1] used methyl tert-butyl ether organic solvent as the extractant to extract and enrich the water sample, and used ECD as the detector to determine the trihalomethane, haloacetonitrile, halogenated ketone and trihaloethyl in drinking water by gas chromatography. 12 kinds of disinfection by-products such as aldehyde, 12 kinds of substances have good linearity, and the recovery rate is 71%-123%. SPE makes up for the deficiencies of LLE, can reduce the use of reagents and environmental pollution, and at the same time improve the extraction efficiency. Obtain high-purity concentrated isolate, so it is more and more widely used.

Chromatography occupies a dominant position in the analysis methods of drinking water DBPs. With the development of chromatographic technology and combined technology, the advantages of chromatographic methods in the detection of DBPs are becoming more and more obvious, and the scope is becoming wider and wider. At present, the determination of trihalomethane, trihaloacetic acid, haloacetonitrile, halogenated ketones, trihaloacetaldehyde and other substances are usually analyzed by GC and GC/MS. For the separation and analysis of organic compounds and various ions with high boiling point, high relative molecular weight and poor thermal stability, reversed-phase high performance liquid chromatography is generally used, and the detector is generally equipped with ultraviolet detection and fluorescence detector. With the development of chromatographic mass spectrometry technology, LC-MS technology has become a common technique for identification and analysis of DBPs. Current research shows that most of the unknown DBPs are high-polarity and high-molecular-weight substances, which can predict LC/MS The application of /MS in the discovery and detection of DBPs will increase day by day. Ionic or easily ionizable DBPs such as oxyhalides, haloacetic acids, etc. are generally detected by ion chromatography. Yang Chunying [2] used ion chromatography to simultaneously determine 5 kinds of DBPs, chlorite, chlorate, bromate, dichloroacetic acid and trichloroacetic acid in drinking water. The method has a good linear recovery rate of 97.6%～105.6%. At present, most haloacetic acids are determined by derivatization gas chromatography, and sample pretreatment is cumbersome. Ion chromatography can avoid this problem, and it will play a greater role in the detection of haloacetic acids in the future.

3 Conclusion

In summary, the harm of DBPs has generally attracted attention, but there is a lack of in-depth research on the rare DBPs. At present, it is necessary to establish a new standardized method with good selectivity, high sensitivity, and multi-component analysis at one time. Through systematic and in-depth research, disinfectants can reduce the types and content of DBPs while eliminating water-borne diseases, ensuring the safety of human drinking water and protecting human health.

[1] Wei Jianrong. Study on the detection method for the determination of disinfection by-products in drinking water by liquid-liquid extraction and gas chromatography [J]. China Health Inspection, 2004/14(5): 542-544

[2] Yang Chunying, Simultaneous determination of 5 disinfection by-products in drinking water by ion chromatography [J]. Analytical Chemistry. 2007 35 (11): 1647-1650

 

 The OIC-600 ion chromatograph produced by Beijing Hankeyao Instrument Co., Ltd. provides an overall solution for the detection of chlorite, chlorate and bromate in the above-mentioned disinfection by-products according to the national standard method.

 

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