Optimization of donnan dialysis for alum recovery from potable water treatment residues

Abstract

Treatment of effluent water to meet fresh water discharge limits is very essential. Aluminium sulphate (alum) is the most widely used coagulant during the pre-treatment process, however, it generates a large amount of residue. Subsequent discharge of these residues from potable water treatment plants (PWTPs) to landfill sites and river bodies, without treatment, poses a great threat to the ecosystem and human health. In essence, the rising concern of managing residues, associated with the disposal cost, toxicity and stringent legislation, calls for more robust and effective technologies. In response, this study comes in handy owing to the green chemistry benefits of aluminium recovery from PWTPs for reuse. Primary recovery methods include acid treatment and alkalization. Although these two recovery processes ensue a minimum of 60% recovery, organics and heavy metals solubilize during the process. Donnan dialysis as a separation, recovery and concentrating technology is investigated in this project. The aims and objectives were to optimize the recovery of aluminium using Donnan dialysis with respect to phase conditions, to evaluate the inhibition effect of selected metals on aluminium transport and finally, to establish the organic transport in Donnan dialysis. Using a statistical approach, the feed phase conditions such as feed flowrate (0.64-2.21 mL/s), feed concentration (100-3300 mg/L), and pH (1.3-3.7) were considered against sweep phase conditions of acid concentration (0.25-1 N) and flowrate (0.64-2.21 mL/s). The response surface methodology’s face-centered central composite design (FC-CCD) statistical method was adapted for the selection of influential factors and establishing the relationship between selected conditions. The FC-CCD used had three levels and six center points for analysis. The effect of Ca, Mg, Mn, Fe, Cu, Zn and Pb on Al permeation through the Nafion 117 membrane was studied at constant flow and concentration conditions. Once the effects in the binary inhibition study were completed, aluminium recovery from a residue obtained from a local PWTP was conducted. Simultaneously, the rejection of organics by the membrane was also assessed during the aluminium recovery process. Preliminary experiment validation experiments showed a high deviation of ±6.4 mg/L at the feed phase, 7.33% deviation at the sweep phase and mass balance closure greater than 95%. Furthermore, study on the water transport across the membrane was directly proportional to the acid concentration. Comparing HCl and H2SO4, HCl had a lower Van’t Hoff factor, hence, was used in proceeding experiments. A one factor at a time experiment to determine the final range of feed concentration to use showed that a maximum of 2000 mg/L was required to meet a 50% recovery target. The FC-CCD experiment showed that the ascending order of the effects of factors was sweep flowrate < feed flowrate < sweep concentration < feed concentration. The sweep flowrate had a negative influence on aluminium permeation and was statistically insignificant (p > 0.05). Quadratic and predictive models developed at different time intervals were statistically significant at a 95% confidence level. Also, a high recovery of 94% and high concentrating effect at the sweep phase was 1.65 in the 2:1 feed to sweep phase volume experiment. Analysis of FC-CCD combinative study of feed concentration, pH of feed phase and feed flowrate showed that a high feed concentration (> 1000 mg/L) at a high pH (> 2.5) will yield an Al-recovery > 60%. At a 95% confidence level, the statistical analysis showed that the pH was the most significant factor. The interacting factors for the statistically significant model was feed concentration-feed flowrate and feed concentration-pH. The one-on-one inhibitive study at equal phase flowrates and feed concentration revealed that Fe gave the highest inhibition while the least transport across the Nafion 117 membrane was Mn2+. In descending order, Fe2+ > Ca2+ > Zn2+ > Mg2+ > Cu2+ . The rejection of organics is limited to 24- 32 hours where a maximum of 98% rejection was achieved under the synthetized solution and acid digested residue runs. In conclusion, Donnan dialysis by RSM has proven to be feasible for the recovery of aluminium from potable water treatment residue. Also, the FC-CCD adapted from the RSM is seen to be very promising, economical and a reliable alternative statistical tool to determine the most influential factor and predict and obtain the optimal operation conditions for a system. Therefore, there are economic, sustainable and research prospects of DD coupled with RSM towards recovery of metal salts and heavy metals from PWTP residues in large scale implementation.