Bioflocculant production by different microbial species and their potential application in dairy wastewater treatment

The aim of this study was to characterize the bioflocculants secreted from Bacillus subtilis (B. subtilis), Aspergillus oryzae (A. oryzae) and Rhizopus oligosporus (R. oligosporus). For precise investigation of bioflocculants, fourier transform infrared (FTIR) spectroscopy was performed, and pH and temperature stability, and decolorization efficiency were evaluated. In addition, the effects of bioflocculants use on dairy wastewater properties, including chemical oxygen demand (COD) and total suspended solids (TSS), were assessed. The experiments showed that 5-day fermented B. subtilis, A. oryzae, and R. microsporus var. oligosporus were able to produce 2.51, 2.24, and 2.15 g/l of bioflocculants, respectively. The produced bioflocculants differed in terms of performance rate. The order of performance rate at 20-40 °C was R. oligosporus > B. subtilis > A. oryzae. FTIR analysis revealed differences between the chemical structures of the three bioflocculants and the involvement of N-H bands, C-O group, and carboxylic acids and their derivatives in these bioflocculant structures. Thermostability analysis of bioflocculants indicated that R. oligosporus produced more stable bioflocculants than others. It was observed that the increasing of pH caused an increase in the flocculating activity of bioflocculants produced by B. subtilis and A. oryzae. In contrast, bioflocculants from R. oligosporus showed better flocculation performance in acidic conditions. In the case of dairy wastewater, the addition of all tested bioflocculants caused a significant decrease in COD, TSS, and dyes and the best results belonged to bioflocculants from R. oligosporus.