The Degradation Of Trichloroacetaldehyde In Drinking Water As A Disinfection By-Product

The provisions of items of the current "drinking water health standards"(GB5749-2006) increased to 106 items, with many disinfection indicators classified as regular items. Disinfection process is the last process of the drinking water treatment. While the disinfection process inactivates the pathogenic microorganisms in the water to ensure the biological safety of drinking water, it may also generates a series of halogenated compounds, namely the disinfection by-products. Besides trihalomethane and haloacetic acid, halogenated acetaldehyde is another big part of disinfection by-products, which is given priority to with trichloroacetaldehyde(chloral, in the form of chloral hydrate in water).Considering the widespread presence of chloral in drinking water in China, this paper focused on chloral as the topic. It investigated the degradation properties of chloral under hydrolysis, oxidation, reduction UV-photolysis, volatilization, ultrasonic degradation, microwave degradation, the activated carbon adsorption and reverse osmosis removal. This study is aimed at improving understanding of the degradation and removal properties of chloral, and providing a scientific basis for improving the safety of drinking water.The results showed that hydrolysis of chloral can be significantly accelerated by the increase of the p H and temperature. Chloral can be oxidized by high levels of residual chlorine, but can’t be oxidized by chloramine. Chloral can be dechlorinated by iron powder, with the best reduction effect identified in the solution with p H at 4.0~6.0. Under the real condition of the water distribution system, the degradation levels of choral hydrolysis, oxidation and reduction are perhaps not significant because of the limitations of p H, temperature, residual chlorine, etc..The UV-photolysis experiments showed that ultraviolet light irradiation of chloral was significantly accelerated by the increase of the p H and lamp power. Under the condition of similar experiments, the reaction rate of chloral photolysis is quicker than that of hydrolysis. Unlike the main products of hydrolysis(i.e. chloroform), the main product of chloral UV-photolysis is chloride. It’s indicated that UV-photolysis of chloral degrades more thoroughly. The interferents on chloral UV-photolysis included I-, NO3-, Cl- and humic acid. The I- and NO3- promote its photolysis, whereas humic acid inhibit the process, and there is no obvious effect of Cl-. This suggests that the existence of ultraviolet absorption peak material will affect the reaction rate of chloral UV-photolysis, making the reaction rate either increase or decrease. UV-photolysis is also a good way of removing choral without introducing harmful byproducts.Furthermore, boiling water(by heating or microwave) and treating water with reverse osmosis are the quick and efficient means of removing choral from water. Boiling can efficiently remove choral in the tap water, and reverse osmosis method is not affected by p H and the initial concentration. Adsorption has a limited removal efficiency on choral, and can be readily affected by adsorbent and flow velocity. In contrast, stirring and ultrasonication proven to be lack of capacity in removing choral from water.