Isolation Of A Flocculant-Producing Bacterium XNJA And Optimization Of The Flocculating Conditions

When it comes to the age of Industrial Era, flocculants have been widely used in a variety of industrial fields, including sewage disposal, food fermentation, potable water purification, and industrial downstream processes. In general, according to their different accomplishments and property, flocculants can be categorized into chemically synthesized flocculants (organic and inorganic flocculants) and natural flocculants (chitosan, algin, and microbial flocculants). As traditional flocculant is criticized of its toxicity, which can produce negative effects on human health and ecological environment, researches on new form of flocculants have come to the top of scientists’ agenda both domestic and abroad. A particularly new form of flocculant, named as bioflocculant has come to its existence since late 1980s.Bioflocculant is in fact a special macromolecule secreted by microorganisms, and it can help the substance in liquid to sink more quickly and more efficiently. These substances include solid particles, bacteria, cells, and colloidal particles. Because it is degradable and will not produce secondary pollution, it is thought as the representative of "the third generation", safe, and high efficient flocculant.Twelve bacterial strains capable of flocculating were isolated from the activated sludge of a sewage treatment plant in Suzhou, Jiangsu province. Among them, one bacterial strain, designated as XNJA, was selected for further study because it showed the highest flocculating activity. Based on the morphological, biochemical characteristics and comparative analysis of the 16S rRNA gene, strain XNJA was identified as Serratia sp.Strain XNJA grows well at pH6.0～10.0, and the optimal pH and temperature for its growth is at 7.0 and 30℃, respectively.The optimal carbon and nitrogen source for its growth is glucose and peptone, respectively.It was found that the medium composition and other cultural conditions have significant effects on bio-flocculant production. The optimal cultivation conditions of strain XNJA are as follows:glucose as its carbon source, urea as its nitrogen source, with a temperature of 30℃ and initial pH value of 7-8. The flocculation activity of strain XNJA on kaolin solution could be reached 80.5% after optimization of the cultivation conditions. The addition of Al3+ did not have significant effect on the flocculation activity, while other metal ions such as Ni+、Hg+、Fe3+、Mg2+ showed inhibitory effects on the flocculation activity.The cell-free supernatant has high flocculating activity, indicating that the bio-flocculant produced by XNJA is an extra-cellular polymer. The bioflocculant produced by strain XNJA is highly purified according to the method described by Baror and Shilo and Salehizadeh et al. The bioflocculant is a white powder. A yield of 1.46 g l-1 purified bioflocculant was obtained and it also showed the high flocculation activity (about 79.6%).It can be learned from chemical analyses on purified bioflocculant that the bio-flocculant is a proteoglycan composed of carbohydrate and protein. Infrared spectrometry further indicates the presence of carboxyl, hydroxyl, and amino groups, and it is a typical heteropolysaccharide. Its efficient flocculation activity suggests that it has great potential use in the future industrial applications.