Enhanced Treatment Of Low-temperature And Low-turbidity Water By Rotating Grid Coagulation And Direct Filtration After Adding Chitosan

In northern China, many water treatment plants face the problem of treatinglow-temperature and low-turbidity raw water. In order to make the effluent meet thestandard, additional coagulant is added, which not only increase the cost, but alsoincrease the risk of residual aluminum concentration. Overdose of coagulantconsumes the alkalinity in raw water, resulting in lower water pH and heavierpipeline corrosion. To solve this problem, water plant upgrading is the most realisticapproach, so the development of simple and practical technology, which can beaccepted willingly by the plants, becomes increasingly urgent.This study developed two ways to strengthen the treatment of low-temperatureand low-turbidity water. One way is the addition of grid on mixing paddles inmechanical flocculation tank; the other is the direct filtration with the addition ofchitoson. The addition of grid could improve the uniformity of energy input.Chitoson, which is an electrolyte with high charge density, can improve theeffectiveness of filtration and reduce the dosage of main coagulants.A series of experiment was conducted to study the coagulation enhancementeffect of rotating grid. The results showed that the addition of grid could improvecoagulant effectiveness obviously. The enhancement of coagulation was mostobvious when the mesh size was5mm and8mm. Appropriate reduction of therotating speed wouldn’t have a significant impact on the coagulation process. At lowtemperatures, the enhancement of rotating grid on coagulation was more obvious.The number of particles, whose size was in the range of2-25μm, tend to decreasedbecause of the addition of grid on the mixing paddles. When3mm,5mm and8mmgrid were applied, the number of particles in sedimentation tank effluent wasobviously lower than that when no grid was added on the mixing paddles.Rotating grid was used in Harbin Mopanshan water treatment plant to enhancethe coagulation. Water quality monitoring results showed that coagulation effectwhen65mg/L coagulant was added to the unit using rotating grid was consistentwith that when80mg/L coagulant was added to the original unit. The application ofrotating grid decreased the coagulant consumption by19.6%, and saved the cost ofcoagulant for302,000Yuan. By controlling the speed of mixing paddle, the stirringmotor power was decreased.The turbidity of raw water in Mopanshan water treatment plant was lower than1NTU in most of the time, which made it suitable for direct filtration. The presentstudy used chitosan to strengthen the filtration process. The results showed that the best treatment effectiveness was achieved when chitosan dosage was4mg/L. Thefiltration effect was not obviously affected by different microflocculation whenchitosan was added, while it was just opposite when adding PAC. Temperature hadan obvious influence on the filtration enhancement of chitosan. Filtration had betterperformance at6℃was better than that achieved at18℃when chitosan was added.Water head loss increased more slowly when chitosan was added than that whenPAC was used. In the same dosage, the ability of chitosan to improve the Zetapotential was4.5times higher than that of PAC. Chitosan enhanced the filtration byincreasing Zeta potential while PAC depended on both increasing Zeta potential andpromoting the formation of large particles whose diameter was in the range of2-5μm and15-56μm.