The online generation and application of ferrate(VI) for sewage treatment—A pilot scale trial

Abstract

Ferrate(VI) is an oxidant; under acidic conditions, the redox potential of ferrate(VI) salt is the strongest among all oxidants used for water and wastewater treatment. Ferrate(VI) is also a coagulant; during the oxidation process, ferrate(VI) ions are reduced to Fe(III) ions or ferric hydroxide, which simultaneously generates a coagulant in a single dosing and mixing unit process. The superior performance of ferrate(VI) as an oxidant/disinfectant and coagulant in water and wastewater treatment has been extensively studied. However, challenges have existed to the implementation of ferrate(VI) technology in water and wastewater treatment practice due to the instability of the ferrate(VI) solutions and a high preparation cost of a solid ferrate(VI). It would be an ideal approach to generate ferrate(VI) in situ and apply the generated ferrate(VI) directly for wastewater treatment. This paper reports the online preparation and use of ferrate(VI) for sewage treatment at a pilot scale applied in a wastewater treatment plant in the UK. The technology has been demonstrated to be promising in terms of removing suspended solids, phosphate, COD and BOD at a very low dose range, 0.005–0.04 mg l −1 as Fe 6+ in comparison with a normal coagulant, ferric sulphate at high doses, ranging between 25 and 50 mg l −1 as Fe 3+. In terms of the similar sewage treatment performance achieved, the required dose with ferrate(VI) was 100 times less than that with ferric sulphate. However, the full operating cost needs further assessment before the ferrate(VI) technology could be implemented in a full scale water or wastewater treatment.