Organic matter in drinking water is easy to cause water pollution, which is an important factor endangering human health. The biological stability of drinking water is an internal factor related to the growth of organic matter, and the water purification process achieves the purpose of removing organic matter through the external way of water treatment, which constitute two important aspects of removing organic matter.

[Keywords:] Biological stability water purification process of organic water pollution

With the rapid development of social economy and the acceleration of urbanization, water crisis has become the second biggest crisis facing mankind after the oil crisis. At present, in China, with the rapid development of industry, especially organic chemical industry, petrochemical industry, medicine, pesticides, insecticides and herbicides, the output and types of organic matter are increasing day by day, and all kinds of production wastewater and domestic sewage directly enter the water body when they fail to meet the standards, and the water pollution problem of source water is becoming more and more serious. However, under the existing conditions, the traditional water production technology widely used in water plants has been difficult to meet the requirements of increasing water quality standards, and the safety of drinking water has attracted widespread attention.

As far as the removal and control of organic matter in drinking water are concerned, biological stability and water purification technology have a certain measure and influence on it. Taking the organic matter in drinking water as the research object, the determination index, removal mechanism and law of organic matter were discussed from the aspects of biological stability and water purification process.

I. Sources, Hazards and Biological Stability of Organic Matter

From the source, the source of organic matter in source water can be divided into two categories. One is natural organic matter, which is the metabolite of natural environment, including humus, microbial secretions, dissolved plant tissues and animal excreta. The other is synthetic organic matter, including pesticides and industrial waste.

The results show that the organics in drinking water have many harmful effects: (1) Some organics are highly toxic persistent organic pollutants or endocrine disruptors, which are carcinogenic, reproductive toxic and sexual, posing a direct threat to human health; (2) Some organic substances are precursors of disinfection by-products, which can form toxic halogenated organic substances during chlorination and disinfection, which is harmful to human health; (3) Biodegradable organics in drinking water will do harm to the water supply network and the water quality of the network. The third kind of harm has become the focus of attention in recent years.

During the period of 1996, the World Health Organization investigated 277 cases of aquatic diseases in Europe. The results show that aquatic diseases caused by microbial regrowth in pipe network system account for 43%. The survey results of 36 cities in China, which account for 42.44% of the national water supply, show that the total number of bacteria in the factory water is only 6.6 /L, but it has risen to 29.2 /L in the pipe network water.

In the conventional water purification process, chlorine gas is generally used for disinfection to maintain a certain residual atmospheric content in the pipe network to control the growth of bacteria. However, studies have shown that some bacteria or Escherichia coli can be repaired and re-grown in the pipe network after chlorine gas disinfection. When the concentration of nutrients in the factory water is high enough, even if the chlorine dosage is increased, it is difficult to inhibit the growth of bacteria. A great deal of research on biofilm growth, bacterial regeneration and E.coli outbreak in water supply network shows that the existence of biodegradable organic matter (BOM) in factory water is the main reason for the growth of heterotrophic bacteria in water supply network, and the concept of biological stability of drinking water is put forward.

Second, the concept and index of biological stability and the growth mechanism of bacteria in water supply network.

(a) Concepts and indicators

The biological stability of drinking water refers to the potential of biodegradable organic matter in drinking water to support the growth of heterotrophic bacteria, that is, when organic matter becomes the limiting factor for the growth of heterotrophic bacteria, the organic nutrient matrix in drinking water is the greatest possibility to support the growth of bacteria. At present, assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC) are generally used as the main evaluation indexes of biological stability of drinking water. More and more studies and experiments have proved that AOC and BDOC, as indicators to measure the content of biodegradable organic matter in drinking water, are closely related to the growth of bacteria in drinking water pipe network. Only by controlling the contents of AOC and BDOC in the factory water within a certain limit can the bacterial regrowth in the pipe network be effectively prevented.

(2) Biological stability and growth mechanism of bacteria in water supply network

The research shows that the biological stability of drinking water is high, which means that the content of organic nutrients needed for the growth of bacteria in water is low, and bacteria are not easy to grow in it. On the contrary, the biological stability of drinking water is low, indicating that the content of organic nutrients needed for the growth of bacteria in water is high, and bacteria are easy to grow in it. The growth (regrowth) of bacteria in tap water and its pipe network can be divided into three categories according to the source: first, more bacteria in the factory water enter the pipe network, resulting in an increase in bacteria in tap water; Secondly, the growth and reproduction of bacteria in the pipe network leads to the increase of bacteria in tap water; Finally, the entry of foreign bacteria in the pipe network. But the second point is the main way of bacteria breeding in tap water and its pipe network under the condition that the factory water is disinfected normally and the pipe network is in good condition.

It is generally believed that the content of organic substrate is the main factor affecting its growth, so reducing the content of biodegradable organic matter in water will play a decisive role in controlling the growth of heterotrophic bacteria.

Thirdly, the removal mechanism of organic matter in the existing water purification process.

As mentioned above, because the organic matter in drinking water has a lot of harm to the pipe network, water pollution and human health, in order to reduce the content of organic matter in drinking water and reduce its harm, people must work hard on how to control and remove organic matter. Among them, water purification process is an important link to control and remove organic matter. Some widely used water purification processes are listed here, and their corresponding removal mechanisms are briefly discussed.

(A) Routine and its strengthening process

The existing conventional water purification process in waterworks generally consists of coagulation, sedimentation (clarification), filtration and chlorination, which was formed at the beginning of last century and has a history of one hundred years, and is still widely used. However, it is difficult to ensure the biological stability of water treated by conventional process, and the removal effect of organic matter is poor.

(2) Biological (pre-) treatment

Biological treatment of drinking water refers to the effective removal of organic pollutants, ammonia nitrogen, nitrate and nitrite in water through the metabolic activities of microbial communities. The mechanism of removing organic matter by biological oxidation includes: (1) direct degradation of small molecular organic matter by microorganisms; (2) Decomposition of macromolecular organic matter by microbial extracellular enzymes; (3) Biosorption flocculation. At present, biological oxidation (pretreatment) technology can effectively remove dissolved organic matter, improve the biological stability of factory water and reduce the dosage of subsequent disinfectants, so it has become a process method for water treatment.

(3) activated carbon adsorption

Activated carbon is a porous hydrophobic adsorbent with developed fine pore structure and huge specific surface area. The polarity and molecular size of organic matter are the main factors affecting the removal of organic matter by activated carbon. Organic substances with low solubility, poor hydrophilicity, weak polarity and small molecules are easily adsorbed by activated carbon. Activated carbon adsorption is mainly used for advanced treatment of drinking water. It is found that activated carbon has a strong adsorption capacity for small and medium molecular weight organic compounds, so it has a good removal effect for AOC and BDOC.

To sum up, biological stability and water purification technology are two important aspects related to drinking water safety. In order to adapt to the trend of accelerating economic development and urbanization, we should make great efforts in technical research, improve the biological stability of drinking water through the research and application of advanced water purification technology, and realize the development and treatment of bio-stable drinking water with low organic nutrient matrix content and without causing heterotrophic bacteria to grow again in the water supply network.