Study On Characteristics Of Colloids In Coagulation Process Of Water And Wastewater Treatment

Coagulation, sedimentation and filtration are used to remove colloidal and larger particles in water treatment of conventional processing methods. In wastewater reuse treatment, the advanced treatment of secondary effluent will also be applied to the water treatment (coacervation→flocculation→sedimentation→filtration→disinfection), but the water quality of secondary effluent was quite different from surface water. The basis of surface water's turbidity is inorganic, such as bedload; when the basis of secondary effluent's turbidity is colloids and zoogloea. So it's improper to use parameter of water treatment on advanced treatment indiscriminately. It's necessary to do some research on the characteristic of secondary effluent, especially on colloid's characteristics in coagulation process.This investigation aims to study on colloid's characteristic of secondary effluent and surface water by coagulation process, which comes from Beishiqiao sewage plant and Xi'an water treatment plant, respectively. Mainly include: Do jar tests on these two water substance, the influence of coagulation process on colloid's characteristic was analyzed by measuring floc size distribution, molecular weight distribution and Zeta potential of their particle. The coherent condition and change situation of particle in two water substance in coagulation process were analyzed through online monitoring of PDA2000. Screening and comparison was carried on to its coagulant and the optimum dosage, so as to determine the optimum operating parameters. The results were shown as following:(1) The zeta potential of surface water is about -33mv～-23mv; the zeta potential of secondary effluent is about -45mv～-30 mv. After adding different amounts of PAC, CPAM, the zeta potential of surface water and secondary effluent all has had the change. There was obvious isoelectric point in surface water after adding coagulant, and the isoelectric point was more ahead of time after adding CPAM than adding PAC alone. The zeta potential of secondary effluent didn't have a change of potential symbol along with the increase of coagulant dosage, and there was no obvious isoelectric point. Analyzing the reasons, may be because of the function of "specific adsorption".(2) In coagulation process, floc size distribution of particle in water has also had the change in varying degrees. The particle-size distribution of raw water is scattered, and the particle size of secondary effluent is greater than that of surface water. The average grain size of secondary effluent is 734.6μm, when the surface water is 52.55μm. The particle-size distribution was more focused after coagulation, there was obvious distributed sector and respective peak value. The particle size distribution of secondary effluent distributes at 1.2μm—2.3μm and 37μm—116μm two intervals, when the particle size distribution of surface water distributes at 1.7μm—4μm, and there is a peak value at 2.5μm, its volume distribution greater than 30%.Compare to surface water ,there was a large interval of matter in secondary effluent can not be removed by conventional coagulation process.(3) The result of study on molecular weight distribution showed: in secondary effluent, the organic matter which molecular weight(MW) less than 2kDa accounted for 45.61 %,which MW greater than 0.45μm accounted for 25.43%and the rest interval accounted for 28.96%.In surface water , the organic matter which MW less than 2kDa accounted for 14.81%, which MW in 2kDa-10kDa accounted for 59.26% and the organic matter which in rest interval was very little. The dissolved organic matters in secondary effluent were mainly focused on the fraction of lower than MW2kDa, The dissolved organic matters in surface water were mainly focused on the fraction of 2kDa-10kDa.(4) By online monitor on coagulation process of secondary effluent, we found that: at the coagulant dose of PAC40mg/L, CPAM0.2mg/L, the FI curve rose slowest and the FI index peak value was minimality in coagulation process; at the coagulant dose of PAC40mg/L, CPAM0.2mg/L, the FI index rose to peak value rapidly and maintain stability; at the coagulant dose of PAC50mg/L, although the FI index rose to peak value rapidly, but it could not maintain stability and there were many drop phenomenon. By online monitor of coagulation process of surface water: at the coagulant dose of PAC30mg/L, CPAM0.15mg/L, the FI index rose rapidly at the beginning, but it tended to decline rapidly afterward; at the coagulant dose of PAC50mg/L, CPAM0.25mg/L, the FI index rose quickly at start and reached maximum 1.72 at 130s, then the FI index tended to decline slow. (5)The optimum dosage of secondary effluent is PAC50mg/L, CPAM0.25mg/L.The optimum dosage of surface water is PAC30mg/L, CPAM0.15mg/L.The efficacy of PAC and CPAM for removal of inorganic matter (turbidity) in secondary effluent and surface water is good, but the efficacy of PAC and CPAM for removal of organic matter (turbidity) is relatively weak.