| It has important practical significance for human health, improving water treatment process and environmental protection that developming new type of flocculant. With the devolepment of biotechnology, microbial flocculant emerges as the times require. People belive microbial flocculant would take place of the traditional flocculants in the fields of water treatment, food processing, pharmaceutical industry for their advantages.Two flocculant-producing strains were isolated from activated sludge and soil by means of the common screening and purication method. The bacterium was identified as Pseudomonas fluorescens according to its morphological physiological and biochemical characters, as well as 16S r DNA sequence (GenBank accession NO.AM259177) homology comparision. The other mildew was identified as Aspergillus niger according to abserving thalli modalidy and microorganism coloration, and conserve the strains by cryopreservation.The experiment conditions were optimized for the strains with kaolin suspension as flocculation object. The best condition of production of microbial flocculant of Pseudomonas fluorescens were: (1) The optimum inoculation volume is about 1/50; (2) The optimum carbon source for this strain is grapevine 20g·L-1, and the best nitrogen source is yeast extract 0.5g·L-1; (3) The optimum cultural conditions were decided: the initial pH value was 4.0, the cultural temperature was 30℃, the rotation speed was 160-180rpm and the cultural time was 48h. The best conditions for Aspergillus niger producting flocculant were as follow: (1)The optimum inoculum concentration is about 9.33×108/L;(2)The optimum carbon source for this strain is sucrose 30g·L-1 and the best nitrogen source is urea 2.1g·L-1; (3)The optimum cultural conditions of Aspergillus niger were decided: the initial pH value was 6-7, the cultural temperature was 30℃, the rotation speed was 180rpm and the cultural time was 120h.The flocculation activity of Pseudomonas fluorescens and Aspergillus niger for kaolin clay suspension could arrive at 98.8% and 93.5% at the best culture condition, respectively.By the way of ethanol precipitation, two kinds of microbial flocculant (designated PF-2 and AN-2) were separated from the culture broth. It was confirmed that most composition of the two microbial flocculants was extrapolysaccharide by the way of UV spectrum, IR spectrum and phenol-sulfate acid method. The results showed that the flocculating mechanism of PF-2 resulted from the combination of kaolin suspending solution with PF-2 by electrovalent bond, and deposit by bridge and adsorption.The wastewater purication experiments demonstrated that the PF-2 and AN-2 had good purifying effect in the treatment of papermaking midcourse wastewater. Under the optimal treatment conditions, when the wastewater was treated by PF-2, the removal rate of CODCr and turbidity can reach 80.0% and 92.0%, respectively. When the wastewater was treated by AN-2, the removal rate of CODCr and turbidity can reach 71.5% and 85.4%, respectively. Experiment showed that microbial flocculant have broad prospects in treating papermaking midcourse wastewater.
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