Effects of dosing iron- and alum-containing waterworks sludge on sulfide and phosphate removal in a pilot sewer

Reusing waterworks aluminium (Al)- or iron (Fe)-sludge instead of chemical coagulants such as Al- or Fe-salts, is a credible solution for sulfide and phosphate control in sewer bulk phase for environmental and economic reasons. We comprehensively evaluated and compared the effects of direct dosing of waterworks Fe-/Al-sludge in pilot-scale sewer rising mains, particularly focusing on removal of sulfides and phosphate and underlying possible mechanisms. Changes in other sewage characteristics were also examined. Waterworks Fe-sludge dosing was effective for sulfide removal at a ratio of 0.29 +/- 0.06 mg S/mg Fe, but exhibited limited effect on phosphate removal. Likewise, Al-sludge was effective for phosphate removal at ratio of 0.29 +/- 0.01 mg P/mg Al, but with limited effect on sulfide removal. The mixing of the sludge stream with raw wastewater, i.e. dilution effect, was primarily responsible for observed reduction in soluble chemical oxygen demand (sCOD) concentrations, under both Fe-/Al-sludge dosing. The Fe-/Al-sludge dosing did not cause any increment in dissolved methane (CH4) and nitrous oxide (N2O) formation, nor release of other metals. Combined spectroscopic, spectrometric, and microscopic analyses suggest a precipitation reaction between sulfide and ferric ions in Fe-sludge, is likely to be the dominant mechanism for sulfide removal when dosing Fe-sludge. In terms of phosphate removal with Al-sludge dosing, ligand-exchange processes between surface hydroxyl (-OH) groups and PO43- ions, favouring the formation of both inner- and outer-sphere surface Al-phosphate complexes, appears to be the dominant mechanism. These findings showed the potential multiple benefits of dosing waterworks Fe-/Al-sludge in sewers. Further system-wide, long-term studies including a comprehensive cost-benefit analysis are warranted for optimisation.