Study On Enhanced Coagulation Treatment Of Urban Rivers By Magnetic Mediators

The deterioration of urban rivers seriously affected its functional application and posed a threat to the health of residents around. Coagulation is commonly used to improve the quality of river waters. However, the application of coagulation was restricted by low sedimentation rate of flocs and agent residual. Magnetic particles was employed to enhance coagulation efficiency in this paper. Relevant research results can provide a technical reference for its practical application. The operating parameters were optimized, and the effect of water quality changes on both traditional coagulation and magnetic coagulation were investigated in this study. Furthermore, the mechanism of magnetic coagulation was studied to guide the treatment of three kinds of actual urban river.PAC, AS, PFS and FC were evaluated in the aspects of charge of hydrolysates, coagulation efficiencies, floc characteristics and reagents cost, respectively. Results showed that PAC was the suitable coagulant. The dosing ratio of PAC, MP and PAM were optimized in the process of treating synthetic wastewater. Results showed the dosage of PAC reduced from 30 mg/L to 10-15 mg/L and flocs sedimentation time shortened to 2 min by adding magnetic particles. The effects of dosing strategy including “three stage dosing”, “two stage dosing” and “one stage dosing” on coagulation efficiencies and flocs sedimentation were discussed. In the case of PAC/MP+PAM that MPs are uniformly dispersed, the best removal of turbidity, UV254 and TP were obtained(96.66%, 80.86% and 98.47%, respectively). While the rapid and slow stirring speed changed in the range of 100-300r/min and 30-90r/min, the removal rates of turbidity, UV254 and TP fluctuated in 74.78-95.22%, 55.81-61.79% and 77.14-90.07%, respectively. Therefore, the efficiency of magnetic coagulation was influenced by hydraulic conditions observably. Response surface method was employed to confirm the optimal combination of rapid and slow stirring speed were 198 r/min and 81 r/min.Magnetic coagulation was more adaptable to the attack of acid or alkali. The turbidity removal could be improved from 24.51% and 92.19% to 59.92% and 97.06%, respectively, at pH= 5 and pH = 9 compared to conventional coagulation. When PAC dosage was 5mg/L, effluent turbidity could be reduced from 33.7 NTU to 22.6 NTU through magnetic coagulation in seriously polluted river water(the proportion of domestic wastewater was 20%). The effluent UV254 was reduced 51.11% at low concentration of suspended solids by magnetic coagulation,but only reduced 7.47% at high suspension density. Therefore, the assisted UV254 removal efficiency of magnetic coagulation was weakened by the increasing concentration of suspended solids. The coagulation effect was seriously affected by algae and its extracellular organic matter. The effluent turbidity was 27.1 NTU, 22.8 NTU, 19.6 NTU and 14.3NTU, respectively, at four different combinations of EOM and MP according to only EOM, neither MP nor EOM, both MP and EOM, only MP. Therefore, the effect of EOM on coagulation could be reduced or even eliminated by adding MP. Magnetic coagulation performed a better anti-shock loading capability..Analysis of the mechanism of magnetic particles assisted coagulation showed that heterogeneous nucleation during the coagulation could promote the growth of flocs more tightly. The effluent quality(except nitrogen contained) of magnetic coagulation treatment three kinds of typical polluted river water(the black and odorous water, high turbidity water and low temperature and low turbidity water) could meet the standard of Ⅳ class water derived from “environmental quality standards for surface water”.