Pilot system construction report: Dapo Coal AMD site of Huaxi, Guiyang.

Abstract

The ecological treatment system consists of three (3) components which constitute the treatment train. The first component removes iron by facilitating oxidation and nucleation of small iron particles. This occurs in two or more open ponds depending on the flow volume to be treated. The chemical characteristics of the AMD determine if aeration is needed. This can be achieved by passing the AMD flow over a cascade. In the first open pond the precipitates accumulate. Depending on the precipitate formation rate, the nucleation sites can be provided by installing sediment curtains into the pool. The relatively clear colorless acidic AMD is collected in the second open pond. This first step is the most important step, as effective removal of precipitated iron is needed to minimize coating of organic matter in the following component the ARUM (Acid Reduction Using Microbiology) ponds. The second component is the construction of the ARUM ponds. For these ponds to function a microbial active sediment needs to be constructed and a living floating vegetation cover is placed over the surface of the pond. The sediment construction requires two types of organic matter easily degradable organic matter and highly recalcitrant material (slow or non degradable). The easily degradable organics will serve as food sources to the microbes and the recalcitrant material will provide the structure for the biofilms to housing the microbes. In these ponds low oxygen conditions prevail. The root mass suspended over the sediment of the living floating islands, provides organic acids or the easily degradable organic matter, to the sediment. The detailed microbial activity of ARUM has only be assessed pragmatically, based on the scientific and microbiological knowledge, but not quantified. Geochemical simulations using PHREEQC (http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc) of the pore water which evolved in constructed sediments indicated that conditions for bio-mineralization are indeed given. The saturation indexes suggesting that relatively stable metals are formed in these constructed sediments. Furthermore in multiple empirical field and laboratory tests reliably increases in pH and decreases in Eh have been documented. Thus these ponds achieve significant reduction of metals in the AMD or ARD. A third component of the treatment train might be a biological polishing pond. However this is not always needed and depends on the location of the discharge of the system and the location where compliance with the discharge regulations is expected. A biological polishing pond is an open pond in which through adsorption to algal cell walls elements are removed from the water. These would be elements, which would need to be oxidized in order to be adsorbed and which were not removed in the precipitation pond, were many metals are removed together with the precipitating iron. In most system constructed to date, such a pond was not needed. Depending on the geochemical characteristics of the AMD, all three components of the treatment train can be added in sequence. The determination of the sequence of the components, the sizing of the ponds is the main objective of the feasibility study, as this aspect is always site specific. Unfortunately for this project the elemental composition of the AMD and the two treatment components, iron oxidation / precipitation and ARUM, could not yet be determined, as the analytical instrumentation was out of order. It is recommended that this aspect will be addressed, once the pilot system is constructed and taken into operation. From this data adjustments and alterations may well be needed. The Dapo site was selected based on economic and topographic criteria detailed in the site selection report (attached as Appendix 1). The pilot system is constructed based on the data obtained during the feasibility study. These data are key to designing effective functioning of the treatment. The oxidation rate of the AMD determines if an oxidation cascade is needed or not and it determines the size of the ponds for the iron precipitates to settle. In addition, tests have to be carried out to determine the suitability of the locally available organic matter to construct the ARUM sediments. Finally with the running of the treatment system through two seasons (two dry and two rainy seasons) the design criteria are developed for the full treatment system of the site. With these data the scale up of the system is facilitated to achieve complete treatment of the effluents from the adit. It is important to recognize, that pilot system implementation is a stepwise procedure, which, when carried out systematically, will provide the groundwork for other treatment system with similar AMD characteristics.