Saturday, November 16, 2019
Influence of Al3+ Addition on Activated Sludge
Influence of Al3+ Addition on Activated Sludge Influence of Al3+ addition on the flocculation and sedimentation of activated sludge: Comparison of single and multiple dosing patterns Mariam Touati Abstract In this research, single and multiple dosing of Al3+ were used to study the flocculation and sedimentation of activated sludge (AS). It was found that in the single Al3+ dosing condition; sludge flocculation is improved but sedimentation is declined. Adding the same amount of Al3+ during multiple dosing attained a similar reduction in turbidity. Interaction energy had a major role during single dosing conditions, whereas multiple dosing was mostly effective in reducing EPS content; this is regarding the differences in the performance of flocculation. Introduction The widely most used process for wastewater treatment is the activated sludge (AS) system. The water quality of the sewerage is affected in a large way by the solid-liquid separation in the system, which depends efficient sludge flocculation and sedimentation. 70% 90% of the secondary clarifiers in the AS systems however have encountered solid-liquid separation issues. Two of the main causers of these issues are the reproduction of filamentous bacteria and deficient sludge flocculation. Dosing of flocculants is one of the most effective solution to poor flocculation performance. Cations act as flocculants when added to wastewater and ease flocculation. High cation concentration helps improve biological flocculation. Extracellular polymeric substances (EPS) consist of polysaccharides, proteins, humic compounds, and other cellular ingredients. EPS is considered an important component of AS flocs. They play a major role in maintaining AS flocs structure and function. In most of the cases, dense, strong, and large flocs are desirable for AS settling and compaction. AS flocs have microorganisms in addition to organic and inorganic particles ingrained in the EPS, and the suspended solids accumulate into them. By bridging with multivalent cations, EPS is able to bind to the microbial cells, which are likely to have an influence on the EPS content in the system. An increase in monovalent cations in the AS declines sludge properties and damages floc, while addition of multivalent cations improves sludge flocculation. It was found that, compared to monovalent and bivalent cations, trivalent cations help improve flocculation performance of the AS under both wastewater treatment plant (WWTP) and laboratory conditions. In the present study, Al3+ was added to AS systems according to single and multiple dosing patterns, and a concentration gradient was set under single dosing conditions. The research objectives were as follows: To investigate the influence of Al3+ dosage on AS flocculation and sedimentation. To determine the more efficient dosing pattern. To reveal the mechanisms governing the influence of Al3+ on AS flocculation and sedimentation. Material and methods AS cultivation The AS used in this study was cultivated in two parallel sequencing batch reactors (SBRs) numbered R-S (single dosing) and R-M (multiple dosing), and each had a working volume of 4 L. The following steps were taken: Each reactor was supplied with a paddle mixer, which operates at 100 rads/min, and that is to prevent the AS from settling. To maintain the dissolved oxygen (DO) in the range of 2-3 mg/L, air was added. sludge retention time (SRT) was controlled around 10 d. The temperatures were kept at 22 Ã ±1Ã ¢Ã Ã °c in the reactors. The SBRs were working at a cycle time of 12 h, and 2 cycles were performed each day. The time used for filling, aerating, settling and decanting was 20, 640, 40 and 20 minutes respectively. Both the influent and effluent volumes per cycle were maintained at 2 L. Tap water was used to synthesize the artificial wastewater. Glucose, NH4Cl and KH2PO4 were used as sources of carbon (C), nitrogen (N) and phosphorus (P) respectively. The relative proportion of chemical oxygen demand (COD), N and P (COD: N: P ratio) in the influent was maintained at 100: 5: 1, with the COD concentration at 700 Ã ± 30 mg/L. Al3+ dosing test Single dosing test The AS in reactor R-S was cultivated without adding Al3+ in the influent. 500 mL of AS sample were added into 5 identical 250 mL beakers, each receiving 100 mL, to which a different Al3+ dosage was added. The Al3+ dosages were 0.125, 0.5, 1 and 1.5 meq/L, respectively, in beakers designated as S-0.125, S-0.5, S-1.0, S-1.5, respectively. No Al3+ was added into beaker S-0, which was the blank control of the experiment. All of the Al3+ in the single dosing test was added once. At 50 rad/min for 5 min then 117 rad/min for 5 min, the samples were mixed. Multiple dosing test In this test, Al3+ at a concentration 0.1 meq/L was added to the influent of reactor R-M and introduced to the reactor in each operation cycle. The mixing pattern was in accordance with single dosing test. EPS extraction protocol A 25-mL sample of AS suspension was centrifuged at 6000 g for 10 min at 4Ã ¢Ã Ã °C, and the supernatant that was carefully collected was bulk solution. A NaCl solution with the same conductivity as the AS sample was prepared; then it was applied to re-suspend the AS material in the tube. Immediately, the AS suspension was oscillated by a vortex mixer for 1 min and then sheared using an ultrasonication instrument at 0.4 W/mL for 30 sec. The suspension was centrifuged at 4000 g for 10 min at 4Ã ¢Ã Ã °C, and the supernatant was collected as LB-EPS. The AS sample left in the tube was re-suspended to its original volume of 25 mL with the NaCl solution, and then oscillated for 1 min and sheared at 0.5 W/mL for 1 min. Subsequently, the suspended sample was centrifuged at 4000 g for 15 min at 4Ã ¢Ã Ã °C, from which the supernatant collected was TB-EPS, and the AS sample left in the tube was the pellet. The final step, the bulk solution, LB-EPS and TB-EPS extracted were filtrated through a 0.45 micro-meter cellulose acetate membrane. Analytical technique The total organic carbon (TOC) contents of the bulk solution, LB-EPS and TB-EPS were determined using a TOC/TN analyzer. The Al3+ concentrations in the bulk solution were analyzed using inductively coupled plasma optical emission spectrometry. After settling for 30 min, 100 mL of AS suspension were suctioned and transferred into a graduated cylinder for measuring the SVI. The turbidity of the supernatant was measured. The total interaction energy of AS was measured and calculated. Results and discussions Basic characteristics of the AS system Influence of Al3+ on AS interaction energy Single dosing According to the total interaction energy curves of the single dosing test, the energy barrier and Al3+ concentration are inversely proportional, as in, the energy barrier decreased as the Al3+ concentration increased. Comparison between single and multiple dosing patterns Multiple Al3+ dosing compressed the double electrical layers through charge neutralization. In addition to zeta potential, other factors also had an effect on AS flocculation, which reimbursed the relatively weak zeta potential reduction following multiple Al3+ dosing and ultimately equilibrated the effluent turbidities. Other factors contributed to the lower turbidity related to multiple Al3+ dosing. These observations suggested that the investigation should not be restricted to zeta potential only, and that the total interaction energy should also be taken into consideration. Multiple Al3+ dosing can promote AS flocculation by compressing the double electrical layers. It was found out that clarifying how dosing patterns affects flocculation performance in terms of total interaction energy is relatively difficult. To evaluate the influence of Al3Ã ¾ on AS flocculation, the role of EPS, besides that of interaction energy, must be examined. Influence of Al3+ on the EPS content Single dosing test High EPS content could worsen sludge flocculation. Sludge flocs become denser and more compact with Al3+ addition. LB-EPS play an important role in the formation of AS flocs But, an excess of LB-EPS might have a negative effect on the bonding of cells and could deteriorate the floc structure, eventually causing undesirable solid-liquid separation. EPS and supernatant turbidity decrease as Al3+ dosage increased. High LB-EPS content may result in high water content in the sludge flocs, leading to worsened sludge compactness and increased floc porosity, all causing poor flocculation The addition of Al3+ was found to lighten AS flocculation by counteracting the negative effect of LB-EPS. Al3+ reduces the suspended solids in the supernatant and improves effluent turbidity. Comparison between single and multiple dosing Both the LB-EPS and TB-EPS contents that were extracted in the multiple dosing test were much lower than those extracted in the single dosing test. Which means that in the multiple dosing test, Al3+ augmented effectively the densification of sludge through the ion bridging effect. Single dosing of Al3+ shown greater effect on the interaction energy, whereas multiple dosing of Al3+ reduced the EPS content more effectively. The eventual flocculation performances achieved by the two dosing patterns were equal. Al3+ distribution characteristics in the AS Distribution characteristics As the single dose increased the Al 3+ concentration found in the sludge increased as well. Because of Al 3+ high charge valence and solubility, almost all of it dosed constrained with the sludge. Therefore, no Al 3+ was found in the bulk solution. The amount of negative charges in different sludge components is in the following sequence: pellet > TBEPS > LB-EPS. No Al3+ could be detected in sludge components other than the pellet when the single dosage was less than 0.125 meq/L. When equal amounts of Al 3+ was added during single and multiple dosing, the Al 3+ accumulations in the sludge were similar, the Al 3+ spreading in different sludge components were different. As seen in figure 2. Al3+ could be detected in the bulk solution in the multiple dosing test, but not in the single dosing test. According to all these findings, it was concluded that the bonding capacity between Al 3+ and the sludge was different with the dosing pattern. Single dosing Single Al3+ dosage and the amount of Al3+ in the pellet are directly proportional, as the first increased the second increases as well, respectively. This caused an increase in the sludge flocs density and a decrease in the content of suspended solids, which leads to the improvement of effluent turbidity. Comparison between single and multiple dosing Pellet Al 3+ concentration has an essential positive effect on sludge flocculation. The time for the reaction between multivalent cations and the AS is critical to flocculation performance. The decrease in the sludge surface charges declined due to the increased Al3+ accumulation in the pellet. Influence of Al3+ on AS settleability Single dosing A higher single dosage of flocculant could increase the nucleation rate and the amount of small aggregates, and through the bridging function, it strengthen the interaction energy among the organics on the sludge surface. Comparison between single and multiple dosing EPS content has a significant effect on sludge settleability. Both sludge flocculation and sedimentation were enhanced by multiple additions of Al3+. Al3+ achieved floc densification by the constant recognition of the sludge structure as well as removing water. The key difference between the two dosing patterns was the variation in the reaction time. Engineering significance In case of the failure of AS flocculation and/or sedimentation in waste water treatment process (WWTP), a solution could be to add flocculants only once. Multiple dosing, which is continuous addition of flocculants in WWTPs, extends the reaction time and at the same time promotes flocculation and sedimentation performances. Therefore, it is preferable that the concentrations of multivalent cations in the pellet, in a practical operational procedure, be continuously monitored and observed; where as soon as these concentrations decrease, low concentration of flocculants must be added continuously, in order to maintain the flocculation and sedimentation of the sludge. Conclusion Sludge flocculation and turbidity elimination ware enhanced when the single dosing increased, whereas sludge sedimentation deteriorated. Better sludge sedimentation was accomplished with multiple dosing, even though there was no big difference in the reduction of turbidity between single and multiple dosing. Regarding reducing the interaction energy barrier, single dosing was more adequate, while in terms of decreasing the EPS content, multiple dosing was more effective.