Experimental Study on an Novel Environment-Friendly Coagulant for Treating Drinking Water ()
1. Introduction
Potable water is the most precious resource [1], so it is an important task to research on drinking water treatment. Water treatment for human consumption is a technically overcome issue in developed countries, but it is still a lethal difficulty in less developed regions. Coagulation is the commonly used method for purifying water [2]. With the mature technology, polyaluminium chloride (PAC) has large market demand as the water treatment coagulant in the world [3,4]. However, its secondary pollution is bad for people’s health. For instance, aluminum ions produced by PAC hydrolysis will lead the person who drink this kind of water suffer from anemia, hair loss and brain dementia .In addition, it has been found to be a risk factor in Alzheimer’s and other disease [5]. The drinking water quality standard of stacking-type permission content is 0.1 mg-Al/L in the western developed countries while higher than 0.2 mg-Al/L in many other countries. Therefore, the important issue urgently to be solved in water treatment field is how to develop new environmental coagulants with satisfactory coagulation effect and little secondary pollution. Because its effect better than a single coagulant. Composite flocculating agent becomes a hotspot of current researches. Nowadays, the synergy between inorganic and organic chemicals is of growing interest. This synergy can be obtained by using two components, or three and even more [6,7].
By using [Al2(OH)nCl6-n]m·[Fe2(OH)NCl6-N]M (PAFC), chitosan (CTS), modified starch (MS) etc potions as raw materials, a new water treatment compound flocculating agent was developed. Chitosan (CTS), a deacetylated form of chitin [8], is the third-largest developing biological resource following the starch, cellulose [9]. The raw materials for preparing CTS is abundant, non-toxic and easily biodegradable [10]. Starch resources are rich, and the modified starch has better adsorption performance, so it has wide application prospects in water treatment field. The chitosan itself is a good natural flocculating agent [11]. The current investigation keeps on the direction: treating face water to improve its quality and make it more adequate to human consumption. The aim of this study was to investigate and characterize the effect of PCS as a novel, environmentally friendly composite coagulant.
2. Material and Method
2.1. Raw Water
The raw water was collected from the Yangtze Water in Wuhan, China. This choice of studying an actual surface water avoided the need to simulate turbidity with different physicochemical procedures, such as kaolin addition. The average characteristics of the raw water are given in Table 1.
Table 1. Raw water characterization.
2.2. Chemicals
PAFC, PAC and PFS were produced by Nanjing Fine Chemical Company of China. CTS (the concentration of Al2O3 is about 0.3 g/g) was obtained from Qingdao Biochemical Company of China. Al-pillared montmorillonite 3 wt% was purchased from Zhongxiang, Hubei, China. Glacial acetic acid was supplied by Chemical Plant of Hubei University. Aluminum single element standard solution is 100 mg/L according to the national standard material center. All chemicals are of analysis pure grade.
2.3. The Preparation of Single Coagulant Working Liquid
The PAFC was diluted with water to prepare 0.1 wt% PAFC water-soluble working liquid. This formed solution was stirred for 5 minutes at room temperature. A few powder chitosan was dissolved it in glacial acetic acid and distilled water. Prepare 0.001 wt% CTS water-soluble working liquid. Prepare 3 wt% MS, 0.1 wt% PFS and 0.1 wt% PAC water-soluble working liquid respectively.
2.4. Standard Jar-Test Trial and Turbidity Measurements
6 L of raw water was equally put into six beakers marked from 1 to 6. The corresponding amount of coagulant was added, and beaker was put into a Jar-test apparatus (MY3000-6C). Stirring for 2 minutes at different high speed, 10 min slow mixing at different speed and 30 min of sedimentation, extracting the supernatant fluid from the center of the baker, 2 cm from surface to determine the concentration of Al3+ with Aluminum ion concentration detector HANNA HI93712 which was supplied by HANNA Instruments in Italy and turbidity in water samples was measured with a turbidimeter which was also supplied by HANNA Instruments.
Trials with different coagulant concentrations were carried out, varying pH conditions between 5 and 9. And difference of dosage and/or compounding ratio cause difference of pH variation through the coagulation process.
3. Results and Discussion
3.1. Confirming the Optimal Formule of the Composite Coagulant
The orthogonal experimental was conducted to confirm the optimum formule of the composite coagulant. Table 2 was the orthogonal layout of the composite coagulant. Table 3 was the results of the orthogonal experimental. k1, k2, k3 were the mean removal efficiencies of Al3+ each factor under three levels. And ki, kii, kiii were the mean removal efficiencies of turbidity each factor under three levels. As shown in Table 3, the optimum formula was A2B2C2 on removing Al3+ and was A2C2B2 on removing turbidity.
Table 3 showed that the removal rate of and the concentration of aluminum ion and turbidity achieved 71.3% and 97.2% respectively by using A2B2C2. So the best proportion of this coagulant was V(0.1 wt% PAFC):V (0.001 wt% CTS):V(3 wt% MS) = 25:6:8.