To develop the proper acid mine drainage (AMD) treatment system suitable for domestic environment, the characteristics of eighty-eight AMDs were analyzed. Using statistical analysis, domestic AMDs were categorized with respect to their pH and ionic strength. It was found that AMDs with high ionic strength and low pH have high potential to be hazardous to the environment and the ecosystem. Based on this observation, the focus turned to the development of a treatment system that could effectively mitigate the heavy metal toxicity and acidity of AMDs with high ionic strength and low pH.
In preliminary studies, artificial AMD was designed to have high ionic strength and low pH and was used to obtain reproducible data in the laboratory scale experiments.
The characteristics of the conventional components of constructed wetlands including limestone and organic substrates were investigated in continuous and batch systems. It was observed that limestone beds could not effectively remove Zn, Cd, and Mn since these metals require high pH to precipitate as metal hydroxides. It was found that the formation of metal hydroxide precipitates on the surface of limestone prevented the further dissolution of limestone. The control of oxidation-reduction potential under 100 mV could reduce the formation of hydroxide precipitates on the surface of limestone. Cow manure, oak compost, mushroom compost, and sludge cake from a municipal wastewater treatment plant were selected as organic substrates. Upflow reactors packed with cow manure or oak compost effectively removed toxic metals in AMD for 25 days. Treatment system packed with oak compost showed highest metal removal efficiency mainly by ion exchange between metal ions and Ca. The treatment system packed with cow manure removed heavy metals by sulfate reduction and ion exchange. After 25 days, the metal removal efficiencies of both systems decreased rapidly. It was observed that in both systems the effluent Ca concentration went down to the influent level and oxidation-reduction potential increased. It also observed that the sulfate reduction activity was rapidly decreased after 10 days. From these results, it was postulated that establishing and maintaining a reductive environment would be the key factor for both ion exchange and sulfate reduction. To generate more reductive environment, various types of microbial granules were investigated as an external source of sulfate reduction and decomposition of organic matters. Sulfate reduction, carbon utilization pattern and other physical characteristics of three microbial granules were investigated. From these experiments microbial granules sampled from OB beverage cooperation was chosen as the microbial source. Desulfobulbus and Desulfovibrio species were the major sulfate reducing bacteria in the granule. The sulfate reducing and carbon utilization activities of the granule were stably maintained from pH 3 to pH 8.
Three laboratory scale upflow anaerobic reactors were operated for about 250 days to determine the effect of the microbial granule in the treatment of artificial acid mine drainage. The treatment system with cow manure and the granule showed highest metal removal efficiency. Except Mn, all the toxic metals were removed under the regulated level (removal efficiency > 99 %). The removal efficiency of Mn was around 85 %, which is the highest value reported up to now. The sulfate concentration of the effluent was less than 3 mM, and the majority of the reduced sulfur existed as soluble sulfide, HS". Thus the introduction of the microbial granule in the treatment system made it possible to treat the hazardous artificial AMD effectively and stably.
The Il-Kwang mine located in the suburbs of Pusan was selected for a field test of the developed treatment system. The size of the main treatment unit was 1.5 m (length) X 1.0 m (width) x 1.0 m (depth) and was composed of a gravel bed, a limestone bed, an organic substrates bed with or without the microbial granules. All systems were operated with high Fe loading conditions (90-95 gdm) based on the laboratory experimental results. The system composed of mushroom compost, cow manure and the microbial granules removed the various metal ions effectively (removal efficiency > 95 %) for up to three months. The pH of the effluent was maintained around pH 7. Total organic carbon of the effluent was under 100 mg/1 and volatile fatty acids were not detected. The treatment efficiency was recovered to this level when the flow of AMD resumed after winter. The sub-merged system composed with mushroom compost and cow manure showed comparable metal removal efficiency as the system with microbial granules. It was postulated that the characteristics of mass transfer and diffusion of reduced materials throughout the sub-merged system would be better than those of the microbial granule amended system. This also suggested that the operational method and design structure as well as the composition of treatment systems would affect the treatment efficiency. Through the laboratory scale and field experiment, it was shown that the introduction of external microbial sources with a high content of sulfate reducing bacteria to the conventional wetland was the most feasible method to effectively treat the acid mine drainages with high ionic strength and low pH for a long time.
본 연구는 산도가 높고, 다양한 중금속을 함유하고 있는 산성광산폐수를 효율적으로 처리할 수 있는 경제적이고 자연친화적인 처리기술을 개발하기 위한 것이다.
산성광산폐수는 광업활동이 진행되는 기간뿐만 아니라 광산이 휴ㆍ폐광된 이후에도 수십 년~수 세기에 걸쳐 지속적으로 발생하며, pH가 낮고, 독성 중금속의 농도가 높기 때문에 주변 생태계에 심각한 영향을 미치고 있다.
현재까지 다른 나라에서 개발된 산성광산폐수 처리기술 중 인공소택지를 이용한 처리법은 다른 방법에 비해 경제적이고 효율적인 것으로 평가되어 왔다. 그러나 소택지 내부 물질이 수리학적으로 불안정하고, Mn 제거효율이 매우 낮으며, 넓은 부지 면적이 소요되고, 개발된 기술의 적용성이 낮은 단점을 가지고 있다. 본 연구에서는 기존에 개발된 인공소택지를 이용한 처리법이 갖고 있는 이러한 단점을 개선하기 위해 황산염환원균의 활성이 우수한 미생물원을 처리 시스템에 도입하였고, 황산염환원균의 활성을 극대화할 수 있도록 처리시스템을 설계하였다.
실험실 규모 연구에서는 처리시스템의 현장 적용성을 높이기 위해 국내에서 발생하는 광산폐수의 화학적 특성을 분석하여 인공광산폐수를 제조, 사용하였다. 국내 석탄광산폐수는 pH에 따라 4개의 그룹으로 나뉘어 졌으며, 금속광산폐수는 Ionic Strength에 따라 4개의 그룹으로 나뉘어졌다. 인공광산폐수는 pH가 낮고, Ionic Strength가 높게 제조되어 국내에서 발생하는 산성광산폐수보다 잠재적 생물학적 독성이 높았다.
처리시스템은 자갈층, 석회석층, 그리고 유기물 및 미생물 granule층으로 구성되어, 있으며 폐수는 상향류 방식으로 공급되었다. 석회석층은 유입수의 pH를 약 5까지 끌어올렸으며, 산화환원전위를 낮춰서 혐기적 환경이 형성되기 유리한 조건을 만들었다. 유기물 및 미생물 granule층은 황산염 환원반응 및 이온교환 반응을 통해 pH를 약 7까지 끌어올릴 수 있었고, 장기간에 걸쳐 혐기적 환경을 유지함으로써, 높은 중금속 제거 효율을 보였주었다. 실험실 규모 실험에서는 기존에 발표된 최대 부하량보다 더 높은 중금속 부하 조건에서 약 250일 동안 Mn을 제외한 모든 유입수 내 중금속을 99 % 이상 제거할 수 있었고, Mn의 제거 효율도 80 %로 기존에 다른 처리시스템의 최대효율 (약 40 %)보다 월등히 우수한 처리결과를 보여주었다. 부산 일광광산에서 수행된 소규모 현장 실험에서도 미생물 granule이 도입된 처리시스템은 대부분의 중금속을 95% 이상 제거하였으며, Mn의 제거효율도 약 80 %로 우수한 결과를 보여주었다. 유출수의 pH 는 중성 범위 내에서 안정화되었으며, 산화환원전위값이 낮게 유지됨으로써, 광산폐수에 존재하고 있는 중금속들이 금속황화물 형태로 침전, 제거되었다.
이상의 결과는 기존의 연구결과보다 106 ~ 108 배 높은 황산염환원활성을 유지함으로써 산도와 중금속을 동시에 효율적으로 제거할 수 있었기 때문으로 판단된다. 이러한 산성광산폐수 처리시스템의 개발은 처리시스템 건설에 소요되는 부지를 획기적으로 줄일 수 있고, 기후변화와 지형적 특성에 따른 중금속 및 산도의 변화에도 안정적인 처리효율을 유지할 수 있으므로, 기후변화가 비교적 심하고, 인공소택지 건설에 필요한 부지 확보가 어려운 국내 산성광산폐수 처리에 적합할 것으로 판단된다.
