An iron-facilitated chemical and biological process for phosphorus removal and recovery during wastewater treatment

Abstract

Phosphorus (P) is an important pollutant of concern in wastewater that causes eutrophication and algal blooms in water body. On the other hand, P is a valuable natural resource for agricultural and industrial use. With the rapid depletion of mineral phosphorus on earth, there is a need to recover phosphorus from wastewater. In this study, a new chemical and biological process facilitated with iron dosing has been developed for P removal and recovery during wastewater treatment. The system consists of a main stream identical to the conventional activated sludge process in an aerobic sequencing batch reactor (SBR) for P removal and a side stream of sludge recirculation through an anaerobic SBR (AnSBR) for P release and recovery from the P-rich sludge.

In the aerobic SBR treating a synthetic domestic wastewater, Fe(III) (FeCl3) was dosed to remove P by precipitation and adsorption. Fe(III) dosing at a Fe/P molar ratio of 1.5:1 could reduce the P concentration from more than 10 mg/L to below 1 mg/L in the final effluent. Compared to other dosing periods, dosing Fe(III) right before the SBR settling could achieve the best result in sludge flocculation and P removal. Meanwhile, organic removal was well maintained as 90% of the chemical oxygen demand (COD) was degraded in the aerobic SBR. In the AnSBR, phosphate precipitated with ferric iron in the sludge was released owing to microbial Fe(III) reduction, and a positive correlation was found between the phosphate and ferrous iron concentrations in the sludge suspension. Chemical tests showed that significant P release from Fe(III)-P occurred only if the acidic condition and the reducing condition were combined. For the AnSBR sludge, a higher organic loading, lower pH and higher biomass concentration resulted in a higher level of Fe(III) reduction and P release. Organic acidogenesis prevailed in the reactor and lowered the pH to ~4.5, which facilitated the P release from the solid phase into the liquid phase. With a solids retention time (SRT) of 10 days, the anaerobic supernatant contained a phosphate concentration of up to 70 mg/L, while the settled sludge was returned to the aerobic SBR. The phosphate could be readily recovered from the supernatant with Fe-induced precipitation by aeration and pH adjustment, and the overall P recovery could be achieved at about 70%. In addition to the treatment performance, the speciation of P in the aerobic sludge and the anaerobic sludge also was investigated. A significant change in the immediately available P and the redox-sensitive P was found in the sludge through the aerobic-anaerobic cycle. Such chemical transformation is believed to be crucial to the P removal and recovery during the wastewater treatment process.