Culture Of Complex Bioflocculant-Producing Strains And Optimization Of Conditions By Using Cassava Starch Wastewater

Cassava starch wastewater which contains a large number of crude protein and a certain amount of carbohydrates, is a typical high concentration organic wastewater in food industry. The high contents of total nitrogen (TN) and total phosphorus (TP) in cassava starch wastewater can be used as the important energy for the growth and metabolism of microbial, thus cassava starch wastewater is an ideal base medium for complex microbial flocculant producing bacteria. In the present study, anaerobic wastewater collected from a cassava starch factory was selected as the raw material of bacteria isolation, and the objectives of this study were:(ⅰ) to isolate and obtain the strains that could use cassava starch wastewater in the production of highly efficient bioflocculant, (ⅱ) to optimize the nutritional conditions and fermentation conditions by using cassava starch wastewater to produce bioflocculant, and (ⅲ) to determine the flocculation performance, structure-property and flocculation mechanism of the complex microbial flocculant. The main conclusions of the present study were shown as below:(1) Two lactic acid bacterial strains LB3 and LB5 which could produce bioflocculant with high flocculating to cassava starch wastewater were screened from anaerobic wastewater. The flocculation rate of complex bacteria fermentation broth against starch wastewater was 74.8%.16S rDNA gene sequence analysis showed that the two strains of LB3 and LB5 could be identified as Lactobacillus casei and Lactobacillus plantarum respectively.(2) The culture conditions which combined strains fermented cassava starch wastewater to produce bioflocculant were optimized through single factor test. The culture conditions were:raw concentration wastewater without adding KH2PO4, with glucose as the optimal carbon source in wastewater culture medium, combined nitrogen source of yeast extract and peptone by 1:2,4.5% of total carbon and nitrogen amount with C/N ratio of 1.5, initial pH value 5.5, two strains inoculum size 5% and the ratio of 1:4 was inoculated sterilization wastewater culture medium,30℃ 150 r/min for 48 h. With the condition above, the flocculation rate of fermentation broth to starch wastewater and Kaolin clay suspension was more than 85%.(3) According to the flocculation characteristics, the best coagulant aid was CaCl2. When flocculant concentration was 20 mL/200 mL, the dosing quantity of 1% CaCl2 was 4 mL/200 mL, stand for 30 min after flocculation, the flocculation rate of fermentation broth to starch wastewater and Kaolin clay suspension was 83.7% and 91.7% respectively.(4) The complex flocculant MBF-LB35 had a strong thermal stability, which polysaccharose and protein contents were 46.81% (w/w) and 5.88% (w/w), respectively, flocculation of the active ingredient mainly a kind of polysaccharide. The flocculating active substance produced by two strains was present in cell surface also belong to polysaccharide. Complex flocculant MBF-LB35 contains large amounts of carboxyl and hydroxyl groups, which could promote the flocculant solubility in water, enhanced extensibility flocculant molecules, and increased the effective length of the molecule. These groups could act as a key role in bridging the suspended particles to form big flocs during the flocculation processes.