Characteristics and mechanisms of phosphorus removal by dewatered water treatment sludges and the recovery

The use of novel industrial by-products (IBPs) to remove phosphorus (P), instead of high-cost P removal techniques, is one of the sustainable solutions to protect aquatic life from excessive P discharges. One of such IBPs is dewatered drinking water treatment works sludges generating from using aluminium or iron salts as coagulant during the drinking water treatment process. Previous studies have shown that the sludges hold promise as a novel adsorbent for the removal of P from wastewaters; however, comprehensive investigation into factors affecting the P removal and the recovery is lacking. Therefore, the main aim of this study is to contribute to a mechanistic understanding of P removal and retention by dewatered water treatment sludge (DWTS), and the associated coagulant recycling and P recovery from the P-saturated sludge used as substrate in a constructed wetland system. Seventeen DWTSs were collected from different areas in the UK to study the combined effect of sludge inherent properties and solution chemistry; and the P equilibrium and kinetic adsorption behaviour using batch experiments. Results revealed that the metal content (Al, Fe, Aloxalate and Feoxalate) and specific surface area components had the most significant explanation for the variance of: (i) P-uptake at different initial P concentrations; (ii) the adsorption maxima; and (iii) the Freundlich constant. Overall, giving the combined effect of intrinsic sludge properties and solution chemistry, dewatered waterworks sludges with high reactive metal content (Al and Fe), Ca and SO42- ions, and total specific surface area, would be the best choice for P retention in practical applications. Phosphorus retention by two Al- and two Fe-DWTS were modelled under various operation conditions of hydraulic retention time and influent P concentration, using a continuous feeding system. Four design equations for P retention were developed and these successfully predicted discrete P retention, maximum P loaded to the sludge, accumulative amount of P retention, and lifespan at the required P saturation degree. The model results revealed that the lifespan of ferric sludge is about four years to reach its saturation point, if the flow rate of 190 (l/capita.d) and inflow P concentration of 5 mg/l are used. IV With regards to coagulant recycling and P recovery using electrodialysis (ED) technology, P saturation degree influenced negatively on Fe and P recovery where their percentages dropped from 70 ± 8%, 49 ± 3% to 17 ± 2, 6 ± 1% when P saturated sludge increase from 0% to 100% respectively. The normalised values of recovered Fe to permeated dissolved organic carbon (DOC) were between 29 and 290. Most of the recovered coagulants were comparable in performance with commercial coagulant in term of DOC removal (42 to 59%), Turbidity, and UV254 absorbance. Overall, the results have shown that DWTS has great potential not only for P removal but also for coagulant and P recovery. However, further research is needed before the developed models can be applied at field scale, and also to enhance the ED recovery for further benefits