Coagulation Effect And Floe Properties Of Ferric Based Coagulants In The Yellow River Reservoir Water Treatment

The quality of the Yellow River reservoir water is considered to be seasonal vaired. A series of ferric based coagulants were prepared to treat the Yellow River reservoir under different conditions:reservoir water in spring and fall, high algae-laden water in summer and low-temperture low turbidity in winter. Coagulation tests were carried out to determine the suitable coagulants for each water quality conditions. The influence of coagulants prepare conditions (B value and molar ration), pH and hydraulic conditions on coagulation effects was investigated. Pilot scale tests were carried out to study the coagulation effect on coagulation-sedimentation-filtration process and coagulation-flotation-filtration process. The floc properties, such as floc size, floc growth rate, fractal dimension, floc strength and floc recovery capability, were investigaged. Main research contents and the results are as follows:1. In the treatment of spring and fall reservoir water, five coauglantswere assessed by their turbidity and organic matter removal. The results showed that, polyferric chloride (PFC) and polyferric aluminum chloride (PFAC) obtained better organic matter removal effects than other coagulants. The influence of dosage, B value, Al/Fe molar ratio and pH on coagulation was investigated. For PFC, when B=0.5, the best coagulation effects were obtained, and the optimum dosage range was8-12mg/L. For PFAC, when B=1.5and Al/Fe=7:1, the removal of turbidity and organic matter were better in the dosage range of6-10mg/L. In the experimental pH conditions, the residual turbidity decreased with the pH increased, and the organic matter removal efficiency increased first and then decreased, with the optimum pH range of5.00-5.50and5.50-6.50, respectively.2. Two organic-inorganic composite coagulants, PFC-PDMDAAC and PFAC-PDMDAAC were used to treat the high algae-laden water. The influence of dosage, composite ratio, pH and dosage method on turbidity, organic matter and algae removal were investigated. The results showed that, compared with inorganic coagulants, the orgainic-inorganic composite coagulants gave better coagulation effects. The composite coagulants performed well in a wider pH range of5.0-8.0. In the composite coagulants, the organic components and the inorganic components work cooperative and give better coagulation effects than use them together. The composite ratio of PFAC-PDMDAAC were optimized to be (A+Fe)/PDMDAAC 4:1-8:1, and the Fe/PDMDAAC for PFC-PDMDAAC was4:1. The optimum dosages of PFC-PDMDAAC were4mg/L. and for PFAC-PDMDAAC is3mg/L.3. In the treatment of low-temperture low-turbidity water, two inorganic composite coagulants PFASiC and PFSiS were used. and the influence of dosage, Si content and pH on turbidity and organic matter removal were investigated. The results showed that, the silicate composite coagulants gave better effects than inorganic coagulants. The optimum dosage range of PFSiS was10-12mg/L, and the optimum pH range is5.50-6.00. For PFASiC, when Si/(Fe+Al) molar ratio was0.05, best coagulation effects were obtained, and the optimum pH range was5.00-6.25.4. The influence of hydraulic conditions on the Yellow River water treatment was assessed by a series of orthogonal tests. The results showed that, the optimum hydraulic conditions of each coagulants were as follows. PFC and PFAC:rapid mixture speed200rpm. rapid mixture time30s. slow stirring speed40rpm. slow stirring time15min:PFASiC:150rpm,30s,40rpm.25min:PFAC-PDMDAAC:300rpm,60s,40rpm,15min.5. The pilot scale results showed that, in the treatment of high algae-laden water on coagulation-sedimentation-flitration process, the optimum dosages of PFAC and PFAC-PDMDAAC were4-6mg/L and2-5mg/L respectively. Compared with PFAC. PFAC-PDMDAAC gave better sedimentation effect and could improve the quality of sand-filtration effluent. In the treatment of low-temperature low-turbidity water, floatation gave better separation effect than sedimentation. In the floatation process. PFASiC performed better than PFAC, with the optimum dosage of1-5mg/L. The dosing costs of PFAC-PDMDAAC and PFASiC decreased29.9%and15.9%, compared with PFAC.6. During the coagulation of the Yellow River water, the variation of floc size was determined by a laser fraction instrument to investigate the floc growth. A series of shear forces were applied on the flocs, and the breakage and regrowth of floes were studied. The fractal dimension of flocs was calculated by the relationship between the scattered light intensity and the scattering vector. The results showed that:(1) The properties of flocs formed by PFC and PFAC were influenced by dosage, pH and coagulant prepared conditions. At larger dosage, the size of flocs was larger and the floe growth rate is faster. Under the pH=7.00condition, the PFC flocs grow faster and the formed flocs were bigger. The strength of PFC flocs increased with the pH decreased. PFAC flocs gave faster growth rate and larger floc size at pH=9.00. When B=0.5, the PFC flocs gave faster growth rate, larger floc size and the flocs were much stronger. When B=1.5and Al/Fe=7:1, PFAC flocs gave faster growth rate and larger floc size. The strongest flocs appeared at B=1.5and Al/Fe=5:1. The recovery capability of floes was related to the B value, Al/Fe molar ratio, pH and the applied shear force. Compared with PFAC flocs, PFC flocs gave better recovery capability.(2) In the treatment of low-temperature low-turbidity water, floes formed by silicate inorganic composite coagulants were much bigger and stronger than those formed by inorganic coagulants. For PFASiC, when Si/(Fe+Al)=0.05, flocs gave faster grow rate and larger size. Floc strength increased with the increase of Si content. For PFSiS flocs, the pH condition gave larger influence to growth rate, but litter influence to floc size. When pH is bigger than5.50, PFSiS flocs is much stronger and hard be broken, but the broken flocs recovered badly. For PFASiC, flocs formed at pH=5.50were the largest. The flocs formed at pH=4.00gave best recovery capability, while worst recovery capability was observed at pH=7.00. In the formation of PFASiC, the rapid mixture speed and rapid mixture time affected the floc size significantly, and the slow stirring gave litter influence.(3) In the treatment of high algae-laden water, the organic content in composite coagulants, pH and dosage method influence the floc properties. The growth of flocs slowed down when PDMDAAC was added, but the floc sizes were increased. Compaered with flocs formed by inorganic coagualnts, those formed by organic-inorganic coagulants were much stronger and gave better recovery capabilities. Flocs growth rates increased with the increase of pH, but the floc sizes were similar at experimental pH conditions. PFAC+PDMDAAC gave largest floc size, stronger flocs and better recovery capability.(4) During the coagulation, floc fractal dimension increased in the first mintues, and then decreased. The increase of fractal dimension indicated the formation of compact structures, and the decrease of fractal dimension indicated that the bridging and swept dominate the formation of flocs, resulting incompact flocs. The fractal dimension increased after breakage, which means the change in structure during. Generally, biger flocs had smaller fractal dimensions.