Effect of cascade biological pretreatment of raw water with algae on conventional water treatment process

A cascade biological pretreatment reactor(CBPR) was used to improve the removal rates of ammonia, small molecule organic compounds and microcystins from eutrophic raw water. The pilot-scale installation was run continually based on the conventional water treatment process(CWTP) and the process combined with biological pretreatment(BPTP). The effluents of each reactor were analyzed. The results show that CBPR removes more than 75% of ammonia and helps to increase the DOC removal rate from 30.3% to 49.2% in the coagulation-sedimentation reactor. Compared with CWTP, the removal rates of extracellular microcystin-LR and total microcystin-LR are increased 47.4% and 43.1% in BPTP, respectively. The CBPR removes effectively small particles with diameter less than 5µm and increases big particles with diameter more than 7µm, which enhances the particles removal in coagulation-sedimentation and filtration reactors. The CBPR degrades effectively small molecular organic compounds, decomposes big molecular organic compounds into small ones and promotes the removal of medium molecular organic compounds by CWTP.