Content, Speciation And Influencing Mechanism Of Residual Al During Drinking Water Treatment Using Al-based Coagulants

Based on comprehensive analysis of numerous literatures, the fractionation and measurement of residual Al speciation was determined for drinking water treatment. Effect of coagulant type, synthesis condition, coagulant dosage, initial pH and hydraulic condition on coagulation performance, residual Al concentration, residual Al speciation distribution and residual Al ratio were investigated with respect to the coagulation treatment of humic acid-kaolin simulated water and reservoir water in this research. The optimal dosage and initial pH were confirmed for coagulation of two raw waters using various Al-based coagulants from the coagulation removal performance and residual Al concentration point of view. Optimum hydraulic condition was also clarified for seasonal reservoir water treatment using Al-based coagulants. The main factors influencing residual Al content and speciation distribution were also analyzed. In addition, relationship between coagulation performance (floc operational parameters) and residual Al speciation concentration was discussed in this paper. The main conclusions were summarized as follows:1. During humic acid-kaolin simulated water coagulation, turbidity and UV254removal efficiencies could reach about90%at the tested dosages for AICl3. Al2(SO4)3and polyaluminum chloride (PAC) coagulation. At higher dosage, PAC gave better coagulation effect. Residual total Al content and residual Al ratio of PAC (0.9mg/L and-3.0%, respectively) were greatly lower than those of AICl3and A12(SO4)3. PAC could effectively decrease the content of dissolved Al speciation (about0.6mg/L) with higher toxicity. At initial pH between6.0-7.0, turbidity removal rate of AICI3Al2(SO4)3and PAC was94%,91.5and90.5%, respectively, while UV254removal rate of AICI3, Al2(SO4)3and PAC was87%,88.5%and82%. respectively. PAC exhibited the least residual Al concentration and residual Al ratio regardless of pH variation. The optimal coagulation performance and lower residual Al content could be achieved at dosage of10～12mg/L. However, favorable pH range for efficient coagulation property and lower residual Al content was6.0-7.0and7.0～8.0. respectively. In addition, dissolved Al was the predominant content (60%-80%) in the effluent. In total dissolved Al, proportion of dissolved organic Al was markedly higher than that of other Al speciation. Dissolved nomomeric Al fraction was mainly present in dissolved inorganically bound Al.2. With respect to the coagulation treatment of reservoir water in spring and fall, PAC had the advantages of higher coagulation removal efficiency, stronger charge neutralization ability, lower residual Al concentration, lower residual Al ratio and wider suitable pH range compared to conventional AICI3. Charge neutralization was examined to be more efficient at acid conditions for AICl3and PAC coagulation. The organic material in treated water was mostly hydrophobic, and this material was removed better with higher dosages and in neutral or weakly alkaline conditions. It was not easy for Al fraction in PAC to transfer and remain in coagulated effluent and residual Al ratio of PAC was lower than that of AICl3. However, higher dissolved organically bound Al content was achievable for PAC coagulation. Coagulation performance of PAC could be optimized at basicity (B)=2.0, dosage=12～15mg/L and pH=6.0. The majority of total Al existed most in dissolved Al form, among which, dissolved organic Al was the predominant speciation. Most dissolved monomeric Al was inorganically monomeric Al. Lower values for concentration of various residual Al species and residual Al ratio were achievable at dosage=12-15mg/L and pH=7.0～8.5. Additionally, residual Al ratio was lowest at B of2.0.3. Adsorption bridging and sweep flocculation were main and effective mechanisms for coagulation treatment of reservoir water with low temperature and low turbidity in winter using polvaluminum silicate chloride (PASiC). With OH-/Al3-molar ratio=2.0and Si/Al molar ratio=0.05in PASiC coagulant. PASiC exhibited beneficial coagulation property and relatively lower content of residual Al. Regardless of B value and Si/Al ratio, the most efficient turbidity and organic matter removal rate were achieved at dosage of12-15mg/L and initial pH of6.0-7.0for PASiC coagulation. Yet. dosage=10～15mg/L and pH1=7.0-8.5were beneficial to the reduction of residual Al content. It was suitable for PASiC to remove macromolecular organics with hydrophobic and aromatic characteristics in reservoir water. In addition, majority of residual Al in the effluent existed in form of soluble or dissolved Al fraction. Monomeric Al was almost the major speciation in dissolved Al and dissolved inorganically-bound monomeric Al was the only component in dissolved monomeric Al. Compared to the coagulation of reservoir water in spring and fall using PAC, the optimum coagulant dosage was comparatively higher for the coagulation of reservoir water in winter using PASiC4. With respect to the coagulation treatment of high algae-laden reservoir water in summer, coagulation characteristics of polyaluminum chloride-poly-diallyl dimethyl ammonium-chloride (PAC-PDMDAAC) were largely influenced by hydroxyl to Al molar ratio and Al to PDMDAAC mass ratio. PAC-PDMDAAC performed better coagulation performance and relatively lower residual Al speciation concentration at B=2.0and A1/PDMDAAC=4:1. The optimal coagulant dosage and initial pH for PAC-PDMDAAC coagulation were6mg/L and6.0. respectively. Concentration of various residual Al species in finished water was dramatically affected by coagulant dosage and initial pH for the composite PAC-PDMDAAC Coagulation conditions of dosage=6-8mg/L and initial pH=7.0～8.5were favorable for the control of residual Al speciation concentration. Majority of the total Al in treated water was dissolved Al. In the dissolved Al fraction, organically bound Al forms were present at a higher proportion. There existed almost no dissolved organically bound nomomeric Al fraction in treated water.5. Concentrations of total Al, total dissolved Al, dissolved monomeric Al and dissolved organically bound Al showed different variation tendency along with the variation of hydraulic conditions during coagulation. For the coagulation treatment of reservoir water using PAC and PASiC, concentrations of various residual Al speciation could be effectively reduced at conditions of rapid mixing speed=200r/min, rapid mixing time=l min. slow mixing speed=40r/min. slow mixing time=15min and settlement time=30min. For PAC-PDMDAAC coagulation in high algae-laden reservoir water in summer, the rapid mixing speed, rapid mixing time, slow mixing speed, slow mixing time and settlement time for minimal residual Al content were 100～200r/min,1min,40～50r/min,15～20min,respectively. The effect of settlement procedure on residual Al concentration was comparatively lower compared to the coagulation process. Considering actual water treatment process and residual Al speciation concentration, settlement time should be determined at30min for seasonal reservoir water treatment using various Al-based coagulants.6. The moderate (optimum) floc size, strength factor, growth rate and turbidity removal efficiency were favorable to the reduction of total Al and suspended Al content for one coagulant under various pH conditions. For PAC-PDMDAAC (MR=2:1) coagulation, concentration of total Al and suspended Al could be reduced effectively at floe size of335～347μm, floc growth rate of35.2～35.7μm/min,floc strength of30and turbidity removal rate of75.2%～76.1%. In case of MR=4:1,the optimum parameters for residual Al content minimization were:floc size=342～381μm, floc growth rate=8.5～47.7μm/min, floc strength=26and turbidity removal efficiency=78.6%～80.9%. In case of MR=8:1, content of of total Al and suspended Al could be minimized at floc size of278～300μm, floc growth rate of32.1～33.7μm/min, floc strength of33and turbidity removal rate of74.6%～75.1%. Under these conditions, residual Al content was mainly affected by Al solubility due to pH variation. For different coagulation system under same pH condition, comparatively lower corresponding residual Al content was achieved at larger floc size, higher floc growth rate, lower floc strength and higher turbidity removal efficiency. And residual Al concentration was primarily related to coagulation removal performance for different coagulation system under the same pH condition. Residual Al content and floc operational parameters (coagulation efficiency) exhibited a linear correlation.