| Flocculation is one of the important units and sometimes it is indispensable in water treatment. The objects of flocculation are hydrophobic colloid and suspended particles made of water-fast material. In order to wipe off the suspended solid and colloid in water, a great number of organic and inorganic flocculants have been synthesized. However, most of them are expensive, micro-toxic and the residual sludge is hard to dispose, which is an important source of second pollution.On basis of keeping up starch's inherent properties, the structure of starch is modified in part with physical, chemical and enzyme method, thus the character of starch is changed and modified starch flocculant is made. Modified starch flocculants have a series of virtues such as non-toxicity, abundance in resources, low price and biodegradability. A modified cationic flocculant is selected in this paper based on the premise that the research progress in the modified starch flocculants and their application in water treatment in recent decade are introduced. The paper analyzes the flocculation effect of modified cationic flocculant which is used respectively in kaolin sample, natural water and industrial wastewater in the city. The results are as follows: (1) Modified starch has good performance in flocculation when it is used in kaolin water, it can remove mostly the turbidity of the kaolin water sample in low concentration. (2) It has poor effect when the modified starch is used directly in the coagulation of natural water. But when it is used as assistant flocculant with alum floc or ferric floc, it works in a certain extent (3) It has obvious effect when it is used in industrial wastewater. Two typical wastewaters are chose. One is dyeing wastewater. After modified starch coagulated the dyeing wastewater, the turbidity removal ratio is 79.7% and COD removal ratio is 59% when its dosage is in its optimal concentration by 0.1mg/L. The other is petrochemical wastewater. When modified starch is used as assistant flocculant with PAC in the concentration by 0.3+300 mg/L, it has its optimal effect. The turbidity removal ratio is 79.9% and the COD removal ratio is 26.9%.The experimental results indicate that modified starch has excellent flocculatingeffect in water and wastewater treatment. With the development of modified starch and improvement of the product exploitation technique, its disposal ability in water treatment is improved gradually. Evaluation on modified starch in effect and cost must rely on comprehensive environmental and economic profits. In a word, modified starch can be used as a kind of promising flocculant agent in water treatment.
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