Abstract:

Vertical flow reactors (VFRs) are downward-flow gravel filter beds which remove Fe at neutral and moderately low pH. Oxygenated water enters the top of the reactor and flows vertically through an open water body where reduced Fe oxidises and precipitates as hydroxide onto a non-reactive gravel bed at the base of the reactor. Dissolved As and divalent trace elements adsorb onto the hydroxide ochre layer. The accumulating ochre layer is periodically removed from the system. VFRs require a smaller footprint than conventional oxidising and settling ponds for the same removal effectiveness and can remove Fe at moderately low pH through filtration of nanoparticulate Fe hydroxide.

Two parallel VFRs with a combined volume of 14,600 m³ were installed at the closed Globe Progress Gold Mine in New Zealand to remove Fe and As from multiple subsoil drains. Data for the first year of operation show the flow rate ranged from 20 L/s to 85 L/s with an average of 41 L/s. Performance results show an average DO increase of 88% through a cascade, an average Fe removal of 98% (with an average discharge of 0.12 mg/L), an average As adsorption of 96% (with an average discharge of 0.0085 mg/L), and a sludge accumulation rate of 88 m³/y. All these results exceed modelled performance. VFRs, until now, have only been used globally in short-term small-scale trials. Success of this system may show that the technology can be applied widely. Design of other full-scale VFRs are already underway in the UK.

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Megan Williams