Decreasing Hydraulic Retention Time Affects the Performance of Constructed Anaerobic Wetland for Acid Mine Drainage Remediation

Abstract

Acid mine drainage treatment through constructed wetlands offers several advantages over active systems, however the design and operation of these systems can be a real challenge for engineers and managers due to the various complex biological processes involved, which are sometimes not fully understood. In anaerobic wetlands, Hydraulic Retention Time (HRT) is one of the critical factors in system performance, as affects microbial activity and physicochemical processes. Very low HRT decrease the ability of SulphateReducing Bacteria (SRB) to generate alkalinity and precipitate metals/metalloids in the form of sulphides, and the higher flow rates involved could lead to system aeration and pollutions flushing. This research evaluated the effect of varying HRT (5.4, 3.2 and 2.7 d) on AMD treatment efficiency in a small-scale anaerobic constructed wetland prototype. AMD was transported from the abandoned tailings at Mesapata, Ancash-Peru and stored in a 20L tank, then fed continuously to the wetland using a peristaltic pump. Water samples were collected at the inflow and outflow of the wetland (three times per month) for analysis of total metals/metalloids, acidity and sulphates. Daily pH and conductivity measurements were also taken. The results showed a high removal capacity of the wetland for Al, Fe, Cu, Zn, As, Cd and acidity under HRT of 5.4 d. However, at lower HRT the efficiencies decreased (acidity and Fe) and in some cases the efficiencies were negative (Al, Co, Ni, Cd). Conversely, As and Pb showed significant efficiencies despite increasing HRT. The results demonstrate the different effect of HRT on individual behaviour and metal/metalloid removal from microcosm wetlands, which should be taken into account by professionals when setting their AMD treatment targets.