The Sulfate reduction, Autotrophic denitrification and Nitrification Integrated (SANI) process is a saline sewage treatment process based on Hong Kong’s Seawater Toilet Flushing (SWTF) activity. By integration of sulfur–cycle bioprocesses into the carbon-nitrogen cycle of the traditional activated sludge process, this new process has achieved a significant reduction in excess sludge production as well as in operational cost and land footprint.

However, the requirement of sulfate in the influent for the SANI process may restrict its utilization in continental areas. A possible solution to this conundrum is dosing iron sludge, which is produced during coagulation and odor control in many wastewater treatment plants. The iron sludge from anaerobic digesters, mainly in the form of f...[ Read more ]

The Sulfate reduction, Autotrophic denitrification and Nitrification Integrated (SANI) process is a saline sewage treatment process based on Hong Kong’s Seawater Toilet Flushing (SWTF) activity. By integration of sulfur–cycle bioprocesses into the carbon-nitrogen cycle of the traditional activated sludge process, this new process has achieved a significant reduction in excess sludge production as well as in operational cost and land footprint.

However, the requirement of sulfate in the influent for the SANI process may restrict its utilization in continental areas. A possible solution to this conundrum is dosing iron sludge, which is produced during coagulation and odor control in many wastewater treatment plants. The iron sludge from anaerobic digesters, mainly in the form of ferrous sulfide and ferric iron, could potentially retain sulfur within the system.

The potential application of iron sludge to precipitate sulfide and act as an electron donor in the autotrophic denitrification process was investigated in this study, since it would be the critical issue for the application of an iron-modified SANI process. Preliminary batch tests dosed with ferrous sulfide, ferrous hydroxide and elemental sulfur indicated that ferrous iron not only acted as the electron carrier but also a more powerful electron donor than ferrous sulfide alone for autotrophic denitrification. The long-term operation of the lab-scale Upflow Anaerobic Sludge Blanket (UASB) reactor showed that 83% and 69% average nitrogen removal efficiency could be achieved with hydraulic retention times (HRTs) of 7.2 and 3.6 hours during the steady state，respectively. A concentrated layer of iron sludge formed at the bottom of the reactor due to the high density of iron. Amorphous iron on the surface of the iron sludge acted as the dominant electron donor to improve nitrogen removal. Both the relative quantity of surface iron content and specific surface area of this iron sludge had a positive correlation with nitrogen removal performance.

Key words: Autotrophic Denitrification (AD), Iron sulfide, Ferrous iron, Up flow Anaerobic Sludge Blanket (UASB)