Experimental Research On The Coagulation Mechanism Of Reclaimed Water

Wastewater reclamation is an effective way to mitigate water crisis and improvewater use efficiency. As a key process in reclaimed water treatment, study on themechanism of coagulation between raw water and coagulants is important, which canfuther improve the coagulation effect and guide the practical wastewater reclaimationprocess. In this paper, the influent of a water reclaimation plant in Tianjin was used asraw water in the jar tests and pilot scale test. Combined with the analysis of polymericaluminum, the following experiments have been studied.（1）The aluminum chloride content is10.2%and the basicity of PACl is72.5%.Aluminum species of Ala、Alb、Alc are32.98%、27.84%、39.18%respectively. Alcis the highest proportion in PACl, which has good briging ability. Alb content is lowerwhich plays a smaller part of charge neutralization. So the PACl coagulant in the testmainly rely on adsorption bridging and the charge neutralization simultaneously tocoagulate particles.（2）There are peaks of H-OH and Al-OH in the PACl infrared spectra, whichcan bond with particles through hydrogen bonding interaction. Because of hydrogenbonding, PACl can trap particles and coagulate them by bridging. Also the positivelycharged PACl will absorb particles through electrostatic adsorption and coagulatethem by charge neutralization. The coagulation mechanism is that PACl first absorbson particles and then occurs brdging and charge neutralization.（3）Through orthogonal test and pilot scale test, the optimal stirring intensity Gvalue is that mixed stirring is300s-1, G values of four flocculation stirring are58.4s-1、43.0s-1、30.0s-1、14.3s-1, and the optimal PACl dosage is14mg/L. Changes in particlesize and Zeta potential of the colloidal particles is measured which shows that chargeneutralization is not the only coagulation mechanism. （4）Orthogonal test shows that the four flocculation optimum stirring residencetime are3min,4min,5min,6min. The mixing time is1min, and total flocculationtime is18min.（5）pH value in the water will not affect PACl’s coagulation with particles. Theconcentration of Cl-can hardly influence the removal efficiency of turbidity. LowSO42-concentration is favorable for turbidity removal. Although higher SO42-concentration will lower the turbidity removal efficiency, it still have a good turbidityremoval result.