Screening Of Microbial Flocculant-Producing Strains And Study On Flocculating Activity

Organic and inorganic flocculants have been used widely due to their low pricesand good effect. However, their disadvantages have drawn people's attention.Aluminium remaining in drinking water easily leads to senile dementia. Themonomer of polyacrylamide can't be degraded and it's both neurotoxic andoncogenic for human body. Microbial flocculants are safe and biodegradablemacromolecular flocculants secreted by microorganisms. Their degradation productsare harmless to the ecosystem and humans. The development of a new biodegradablebioflocculant with strong flocculating activity is attracting wide research interest.Therefore, researchers are showing a growing concern on the development of MBF.Eight strains of microbial flocculant-producing microorganisms are isolatedfrom soil. After rescreening, Two strain named C1 and C5 with high and stableflocculating activity is obtained. The factors affecting the production andflocculating activity of microbial flocculant, such as carbon sources, nitrogensources, complex nitrogen sources, inorganic salts, initial pH value and culturetemperature are investigated. The results of C1 indicates that the optimum carbonsources, nitrogen sources, initial pH value and culture temperature are 2% glucose,0.2% beef extract, pH 6.0 and 30℃, respectively. The strain of C1 produces a highactivity flocculant, with flocculating rate of 90.4% against 0.4% kaolin claysuspension under the optimum cultural conditions. The results of C5 indicated that the optimum carbon sources, nitrogen sources, initial pH value and culturetemperature are 1% glucose, 0.2% glutamic acid, pH 7.5 and 36℃. The strain of C5produced a high activity flocculant, with flocculating rate of 92.0% against 0.4%kaolin clay suspension under the optimum cultural conditions.Through orthogonal test of C1, the optimal flocculating conditions are found tobe: 2.0 mL of microbial flocculant dosage, pH7.0, 4mL of 1% CaCl₂ and 60 min ofsettling time. Through the orthogonal test of C5, the optimal flocculating conditionsare found to be: 2.5 mL of microbial flocculant dosage, pH6.0, 5 mL of 1% CaCl₂and 100 min of settling time.The flocculants are extracted from culture broth using acetone solvent. Theflocculants of C1 is mainly composed of amylose. The flocculants of C5 is mainlycomposed of amylose and protein. That is determined by Phenol-Sulfuric acidreaction and Ninhydrin reaction. The flocculants has heat stability. Thedecolorization rate of Methylene Blue by the treatment of microbial flocculantsachieve to 98.5% and 99.0%. It has good flocculating effect on decoloring and claysuspension. And through following test, the optimal technologic conditions arefound. The results show that the flocculating mechanism resulted from thecombination of protein with kaolin clay by bridge at the active position of theflocculation and charge effect neutralization.In order to decrease the cost of microbial flocculant, the cheap substitutive ofethanol for carbon source are developed for C1 and C5, which are based on fermentmedium. The flocculanting activities are up to 84.4% and 82.8%. The optimumsubstitutive mediums of C1 for nitrogen source are bean cake,soybean powder andbean sprout extract. The flocculanting activity are 83.0%,84.1% and 83.0%. Theoptimum substitutive medium of C5 for nitrogen source is bean sprout extract. Theflocculanting activity achieves to 81.8%.