A compact Passive Treatment Process for AMD Using Rice Husk and Rice Bran

An energy and cost saving biological passive treatment system for acid mine drainage (AMD) developed by Japan Oil, Gas and Metals National Corporation (JOGMEC) is a vertical flow anaerobic process that utilizes sulfate-reducing bacteria(SRB) with rice husk and limestone as the substrate and rice bran as the organic resource of SRB. In addition to this sulfate reducing process, an aerobic process of iron oxidation and removal that utilizes iron-oxidizing bacteria is applied to treat high iron content AMD. Iron scale from the aqueduct of drainage treatment facilities has been used as source of the iron-oxidizing bacteria. This passive treatment process is called “JOGMEC Process”. In Japan, flat areas that can be used for mine drainage treatment are limited due to geographic conditions. Therefore, it is necessary to introduce a compact passive treatment system with a higher flow rate (shorter hydraulic retention time (HRT)) compared to the other conventional anaerobic processes that utilize SRB. Then, the laboratory tests have been carried out under short HRT conditions about 25 ~ 50 hours. Since 2014, bench-scale tests have been carried out at an abandoned mine site in Japan. Concentrations of dissolved metals in AMD of the mine are 35-40 mg/L iron (mainly ferrous ion), 18 mg/L zinc, and 8 mg/L copper, respectively, and the value of pH is 3.5. As the AMD contains high concentration of iron, it is firstly removed with the aerobic process of iron oxidation and removal, and then the other metals have been removed with the anaerobic process in the subsequent stage. The results of bench-scale tests show that the metals have been continuously removed for about 300 days under a flow rate of 80 mL/min (the nominal HRT in anaerobic process: 50 hours). During the test term, atmospheric temperature dropped to around -15°C in the winter, and the temperature inside of the reactor of the anaerobic process dropped to around 5°C. The performance of metals removal has been stable even in the lowest temperature during the test term. At present, the metals removal has stably continued for about half a year under challenging conditions that a flow rate has been set to 160 mL/min (the nominal HRT in anaerobic process: 25 hours). Since rice bran, which contains high amount of carbon as sugar, protein and lipid, would be easily decomposed by bacteria, it seems that much organic acids have supplied to SRB and the metals removal has stably continued under short HRT and low temperature conditions. The results suggest that “JOGMEC Process” is able to treat AMD under shorter HRT compared to the conventional biological processes.