Precursors Analysis And Risk Assessment Of Chloral Hydrate For River Type Water Sources

Chloral hydrate(CH) is the most prevalent disinfection by-product(DBP) in addition to trihalomethanes(THMs) and haloacetic acids(HAAs) in drinking water, which poses a tremendous threat to the health of human beings. However, there is not a sufficient understanding of the precursors of CH and the effective risk assessment methods for CH under the existing process conditions. The CH formation characteristics were analysed in this paper. To screen the main component of CH precursors, separation based on molecular weight, separation based on polarity and three-dimensional excitation and emission matrix(3D-EEM) fluorescence spectroscopy were used. In addition, the CH formation potential(CHFP) of model compounds including twenty amino acids(AAs), bovine serum albumin(BSA), etc. during chlorination, were investigated. Ultimately, a multi-attribute assessment method was established to assess the risk of CH and two cases were studied, aiming to provide effective mothods and technical support for the control and risk assessment of CH.The studies for CH formation characteristics indicated that CH yields increased with the increase of temperature from 5 oC to 35 oC. CH degradation rate was greater than its formation rate when the temperature was 45 oC, and CH yields began to decrease. CH yields decreased with the increase of p H, and it decreased to zero when p H was 11. As Cl2:TOC(mass ratio) was from 1:1 to 4:1, CH yields increased with the increase of chlorine dose. However, CH yields did not increse as Cl2:TOC was greater than 4:1. The positive correlation was detected between CH and conventional DBPs, i.e., THMs and HAAs. Furthermore, the relationship between CH and HAAs was more outstanding than that between CH and THMs. CH yields were proportional to the square of chlorine consumption. The linear relationship between concentration of CH precursors and TOC of raw water was more outstanding compared with UV254 and SUVA254, hence, the concentrations of CH precursors could be predicted by TOC of raw water.The research for analysis of CH precursors suggested that the concentrations of CH precursors in water sources in a city of the south of China were between 19.30 μg/L and 66.70 μg/L, in which the contribution rate to CH precursors from algae and particulate organic matter(POM) was 6.83~33.73% and the contribution rate to CH precursors from dissoved organic matter(DOM) was 62.67~93.17%, so, the majority of CH precursors was from DOM. The organic matter with molecular weight(MW)<1KDa and MW>10KDa were the primary precursors of CH in the summer water sample. The dominating components of CH precursors were aromatic protein-like and soluble microbial by-product-like compounds. Alanine, asparagine, aspartic acid, cysteine, histidine, threonine, tryptophan, tyrosine, BSA and DNA had relatively high CH yields which were higher than 30 μg/mg-C during chlorination. The difference of CH formation kinetics among six model compounds suggested that CH formation was closely related to the structure of organic matter, which paved the way for the research of mechanism of CH formation.The risk of CH was assessed by the establishment of a multi-attribute assessment method. The indicator system included three elements layer: raw water quality, environmental factors and water treament process, including seven sub-indicators: p H, water temperature, TOC, rainfall, pollution condition around the water sources, alkali aluminum dose and chlorine dose. Calculated values of weight corresponding to each indicators were: 0.1, 0.1, 0.2, 0.1, 0.1, 0.08 and 0.32. The classic Simple Additive Weighting Method was empolyed as assessment method to assess the risk of CH, where the full score is 1, the water source with higher score is the one with higher risk of CH. The case studies for water sources X and Y indicated that the CH formation risk of X was higher than that of Y. In additon, the risk degree of CH was average or high for X and Y under the conventional treatment process, and the risk of CH was relatively higher in the April to October for them.