Evaluation of the Swim-Bed Attached-Growth Process for Nitrification of Hanoi Groundwater Containing High Levels of Iron

Abstract

A swim-bed attached-growth bioreactor (BF reactor) using a novel acryl-fiber biomass carrier (Biofringe: BF) for treating Hanoi groundwater, which is polluted by high levels of ammonium, has been developed. The swim-bed technology is aerobic and combines the advantages of fix-bed attached-growth processes, which can retain high amounts of slowly growing nitrifiers, and moving-bed attached-growth processes, which avoid clogging problems. Experiments were conducted in 7.7-l reactors, using synthetic Hanoi groundwater, containing 30 mg-N/l of ammonium. Two reactors (BF1 and BF2) were used to investigate the ammonium removal capacities; BF1 was fed influent containing 5 mg/l of iron and BF2 was fed influent without iron. Maximum ammonium removal rates of 0.24 and 0.48 kg-N/m³/d, corresponding to hydraulic retention times (HRTs) of 3 and 1.5 hours were achieved for BF1 and BF2, respectively and nitrification efficiencies are close to 98% for the both reactors. The ferrous form of iron was oxidized to the ferric form as a hydroxide (Fe(OH)₃), which was mostly washed out. This resulted in a high iron removal efficiency (98%) with effluent suspended solid (3-6 mg/l) containing a low volatile suspended component (20%). Nitrification efficiency decreased sharply due to a decrease in temperature from 25 to 15°C, but efficiency quickly recovered following 1 day of operation, demonstrating that the attached-immobilized nitrifiers in BF reactor were able to adapt to the decrease in temperature. Nitrifying bacteria communities from BF reactors were investigated with rRNA-based molecular techniques, and ammonia oxidizers as Nitrosomonas were found in both reactors. Ammonia oxidizers identified as a Nitrosospira sp. were also found in BF2.