Experimental Research On The Performance Of Enhanced Flocculation Based On Bottom Sludge Addition

Bottom sludge enchanced flocculation(SEF) is a high efficiency water treatment technology that enhances coagulation by controlling the manner of sludge addition. It is generally considered to be stable, low cost and far superior decontamination compared to traditional flocculation(TF). SEF has been widely applied for water treatment, but the relevant research has limited knowledge of its efficiency illustration and macro theoretical explanations. To date we have not achieved a deeper clarification of the intrinsic flocculation mechanism from the microscopic point of view, nor efficiently controlled this kind of flocculation. This yields a lack of systematic theoretical support for this technology to effectively guide its application.This study established a coagulation test platform of image acquisition and processing to research SEF. We ran controlled trials with polymeric aluminium as the coagulant and anionic polyacrylamide(PAM) to represent high polymer electrolyte(PE) as long-chain molecule aids. The enhancement of flocculation by SEF compared with TF in the absence/presence of polymer electrolyte was investigated first, included the effect of sludge dosages and addition positions. Concomitantly, the effect characteristics after bottom sediment dosing were also analyzed, included performance based on flocculation by sludge dosing without other coagulants addition, the sustainability of the intensifying action and floc characteristics by SEF, and we present the floc growth mechanism of SEF. And then, with different charge states, the effect of stirring conditions on the SEF performance was studied. Finally, in order to further enhance the efficiency of SEF based on sludge activation, we studied the effect of sludge treatment by low-frequency ultrasonic and acid.Our results showed that Kaolin flocs formed in the charge neutralization stage had higher regeneration factor than 88%, the intensifying effect of SEF is observed with residual turbidity decrease of 29.7%~44.3%. Kaolin flocs formed in the stage of Zeta potential across the isoelectric point(IEP) with lower regeneration factor than 80% result in the increase of residual turbidity with significant increases in small floc frequency and fractal dimension from flocs when stirring stops. The flocs formed by colloids of humic acid-kaolin had lower regeneration factor than 80%, SEF during charge neutralization and restabilization resulted in a increase of residual turbidity compared with TF. Addition of PAM improved the regeneration performance of flocs after breakage. At PAM dosages over 0.1 mg/L at the state of far IEP and over 0.2 mg/L at the state of near IEP, the SEF of the humic acid-kaolin system showd the intensifying effect. In addition, floc size in SEF was larger, and the flocculation rate was faster. The study also found re-flocculation after breakage had an indication effect on SEF, and when the regeneration factor was greater than 1, the intensifying effect was observed. Increase of sludge dosages can enhance SEF further, but it showed some instability at the state of near IEP. The condition without bridging effect by long-chain molecules should give priority to the site after coagulant dosing at the state of near IEP, and with bridging effect the preference for the site before coagulant dosing at the state of far IEP.The effect of sludge addition alone on colloidal systems of source water were investigated, the results indicated that particles in bottom sludge could adsorb colloids in source water. The persistence performance of SEF was also investigated, the results showed incomplete stability enhancement in the first cycle, but the intensifiying effect continued through six cycles, floc fractal dimension and small floc frequency when stirring stops remained stable, thus verifying the feasibility of long-running. Floc characteristics of SEF were also investigated. Compared with TF, the results demonstrated that flocs from SEF possess better settleability and higher mass fractal dimension, SEM analysis also showed tighter surface structure and significant increase of aluminum content which means a reduction in the risk of aluminum contamination. Multifractal analysis showed more uniform floc probability distribution in the late period of flocculation. Besides, we proposed the floc growth and aggregation mechanism of SEF which is of great theoretical significance in guiding practical applications.The effect of mixing conditions on the intensifying effect of SEF was invstigated. The results showed that a too low or too high value of pre-crushing strength was not conducive to turbidity removal. Under four dosage conditions, the residual turbidity of flocculation with stirring rate of 200 r/min was higher than 250 r/min, besides, higher pre-crushing rate of 300 r/min and 400 r/min led to an increase in the frequency of small flocs when stirring stops. Without bridging effect by long-chain molecules, higher mixing strength and longer mixing time result in a production of more small flocs, and so are not conducive to the intensifying effect of SEF. With bridging effect by long-chain molecules, increase of mixing strength and time can lead to thorough breakage of molecular chains, and thus increase the chances of broken flocs coming into contact with particles in raw water and the brokenness of flocs formed in SEF. This facilitates the reorganization of flocs and enhances the intensifying effect of SEF. In the absence of bridging effect by long-chain molecules, the use of higher strength of slow mixing increased floc breakage, causing the loss of intensifying effect. However, in the presence of bridging effect by long-chain molecules, the higher strength can increase collisions and aggregation between particles leading to improvement of intensifying effect.The further intensifying of SEF by bottom sludge treatment with low frequency ultrasound and acid was explored based on the idea of sludge activation. The results showed that, far from the IEP with bridging effect by long-chain molecules, SEF can be further improved with residual turbidity decrease of 9.5%~24.9% and better floc morphology. And when sludge treated by acid activation, further intensifying did not occur in the presence of bridging effect by long-chain molecules, but in the absence of bridging effect, optimum p H range of acid activation is 5.0~2.0, residual turbidity decreased by 19.9%~47.9% at the state of far IEP and 8.3%~36.3% at the state of near IEP.