The Experimental Study Of Microcystis Aeruginosa Removal By Coagulation With PAC

Deterioration of water quality caused by algae pollution has become an important problem for the safety of drinking water. A large number of algal cells will not only increase coagulant consumption and plug the filter, but also produce stinking odour and disinfection by^-products. As a conventional unit in water treatment, coagulation can remove algae and humic substance efficiently. Microcystis aeruginosa, which is generally found in a variety of water bodies in eutrophication, is selected as a model algae. This paper studied the removal efficiency of Microcystis aeruginosa by coagulation with PAC (Polyaluminum chloride), then explored the influence factors from coagulation technological conditions and water quality conditions, and furthermore investigated the effectiveness of clay minerals as coaguation aid for algal removal. The main conclusions are as follows:(1) PAC dosage should be increased with the increase of the concentration of Microcystis aeruginosa. When the concentration of the algae OD680 nm was 0.060, the residual turbidity achieved 0.80 NTU and the removal effect of Chla was 84.0% with 20 mg/L PAC after 30 minutes, with the concentration of residual Al below 0.2 mg/L (the national standard limit). pH played an important role in algal removal, and the best removal effect was achieved at pH 7 8. The algal removal efficiency was increased by 8.0% with the addition of 1.0 mg/L KMnO4 for pre^-oxidation. However, the residual turbidity raised with the decline of algal removal efficiency, when the dosage of KMnO4 was higher than 0.5 mg/L or the interaction time with algal cells longer than 30 minutes.(2) The co^-existing anions Cl^-, H₂PO₄^-, SO₄^2- and HCO₃^- inhibited the algal removal efficiency. The maximal algal removal efficiency was reduced by 47.6%,23.6%,31.7% and 50.2%, respectively. While minor change was observed with the addition of NO₃^- and F^-. The inhibition of Humic acid (HA) and cationic surfactant CTMAB on algal removal was firstly increased and then declined with the increase of their concentration. When the concentration of HA or CTMAB was 6 mg/L or 5 mg/L, the maximal algal removal efficiency was reduced by 51.4% or 21.8%, respectively. HA and CTMAB also help raised algal removal rate in a higher concentration (10 mg/L). However, anion surfactant SDS and nonionic surfactant Triton X-100 had little side effects on removal of algae. (3) Clay minerals (kaolinite, bentonite, sepiolite and diatomite) improved the removal efficiency of Microcystis aeruginosa as aid coagulants, among which the effect of sepiolite was best. With 20 mg/L sepiolite, the removal efficiency increased from 84.0% to 91.6%. At the same time, clay minerals decreased the concentration of residual Al, while increased the residual turbidity. At different water conditions, both sepiolite and kaolinite improved the algal removal efficiency. With Cl^-,H₂PO₄^-,SO₄^2-,HCO₃^-,HA and CTMAB in water bodies, the removal efficiency increased 1.7% 10.9% by adding 30 mg/L sepiolite, and increased 3.0% 14.3% by adding 30 mg/L kaolinite.