Outline of the biological treatment using iron and manganese oxidizing bacteria Groundwater often contains iron, manganese and arsenic originating from natural geological layer and / or mineral resources. We have applied biological iron and manganese removal system utilizing iron and manganese oxidizing bacteria (IRB hereafter) to arsenic removal. The conventional biological iron and manganese treatment by IRB colonizes indigenous IRB in groundwater on filter media by continuous percolation of groundwater. The IRB oxidizes dissolved ferrous iron and manganese ions to particulate form, which are subsequently deposited on the filter media or filtered out by the underlying filter media.
In FY 2005, simultaneous removal of iron and arsenic was achieved in our pilot-scale test treating groundwater that contained arsenic, iron and manganese of geological origin. Removal of manganese, however, was low then. Biological oxidation of manganese reportedly occurs in the region shown in the pH-Eh diagram (Fig.1). The low redox potential of the raw water probably inhibited the oxidation and removal of the manganese. In FY2006, preaeration stage (cascaded aeration with a total drop of one meter) was placed before the biological reactor to facilitate the removal of manganese. Preaeration of the raw water was optimized based on the redox condition of the groundwater, which was characterized by analyses of redox-sensitive chemical species and electrochemical Eh measurements by platinum electrode. As a result, generally more than 80 % of iron, manganese and arsenic were removed from the raw water in FY 2006 pilot-scale test (Table 1).

Fig.1 Field of activity of IRB

Table 1 Removal of Fe, Mn and As in FY2006 per days after the reactor was started

The removal of manganese later started was decreased because we deliberately reduced the preaeration rate on 67th days after the reactor was started. Further work is under way to establish the optimum operation condition for the simultaneous removal of iron, manganese and arsenic in the IRB system.

Features of the IRB system
The system consists of biological reactor, borehole pump, raw water pump and backwashing pump, tanks and sludge condensation facilities. Because the generation of the sludge is minimal, the space required for the system is relatively small, leading to the reduction of the capital cost. The exchange of the filter media is basically unnecessary. Usage of electricity can be minimized if the raw water tank was placed at the higher altitude than the biological reactor. The IRB system also nitrifies ammonium to nitrite and nitrate, making it easier to achieve desired degree of disinfection in the chlorination process.

Constraints of the IRB system
High concentration of phosphate ion in the raw water may reduce the removal of arsenic.

Examples of practical application
IRB system for iron and manganese removal is used for drinking water treatment in Jyoyo-shi, Kyoto Prefecture, and Yamatokoriyama-shi, Nara Prefecture.
Pilot scale test of the IRB system for simultaneous removal of iron, manganese and arsenic from groundwater is now under way in Kyoto Prefecture. A temporary treatment system using IRB was built for the treatment of wastewater from a construction site in Hyogo Prefecture.
In UK, arsenic removal by IRB was tested for groundwater spiked with trivalent arsenic(Lehimas et al., 2001).

1) Lehimas G. F. D., Chapman J. I., Bourgine F. P., Arsenic removal from groundwater in conjunction with biological-iron removal. J. CIWEM, 15: 190-192 (2001).
2) Mouchet, P. From conventional to biological removal of iron and manganese in France. Journal of American Water Works Association. vol. 84 (4), 158-167 (1992)