Centralized iron-dosing into returned sludge brings multifaceted benefits to wastewater management. (15th September 2021) - Hu, Zhetai
Highlights: A new Fe-dosing strategy proposes centralized dosage into returned sludge than sewage. Centralized Fe-dosing acidifies sludge and resulted in free nitrous acid accumulation. The new strategy achieves less biomass production and shortcut nitrogen removal. The new strategy still provides the same benefits as common mainstream Fe-dosing. Abstract: Iron salts (i.e. FeCl3 ) are the most used chemicals in the urban wastewater system. Iron is commonly dosed into sewage or the mainstream system, which provides multiple benefits such as enhanced phosphorus removal and improved sludge settleability/dewaterability. This study reported and demonstrated a new approach that dosed FeCl3 into returned sludge in order to bring two more benefits to wastewater management: short-cut nitrogen removal via the nitrite pathway and less biomass production. This approach is achieved based on our findings that with similar amount of FeCl3, centralized iron dosing into a sidestream sludge unit generated iron concentration two orders of magnitude higher than the common mainstream dosing (e.g. 10–40 mg Fe/L-wastewater), leading to sludge acidification (pH = 2.1) with Fe (III) hydrolysis. Together with accumulated nitrite in the supernatant of the sludge, ppm-level of free nitrous acid was generated and thus enabled sludge disintegration, cell lysis, and selective elimination of nitrite-oxidizing bacteria (NOB). Long-term effects on nitrifying bacteria and overall reactor performance were evaluated using two laboratory reactor experiments for over one year. The experimental reactor showed stable nitrite accumulation with an average NO2 − /(NO2 − + NO3 − ) ratio above 80% and ∼30% observed biomass yield reduction compared to those in control reactors. In addition, the centralized sludge dosing strategy still provided benefits such as improved settleability and dewaterability of sludge and enhanced phosphorus removal. Graphical abstract: Image, graphical abstract