DRAINABLE LIMESTONE BEDS: The Flushing Project
A Drainable Limestone Bed (DLB) is an oxic, limestone-based treatment technology intended for acidic waters with elevated concentrations of iron, aluminum and manganese. DLBs were developed as part of an innovative treatment research grant from the PA DEP Bureau of Abandoned Mine Reclamaton. Existing oxic limestone-based treatment systems were monitored and observed, then a pair of identical experimental treatment systems were constructed. By adjusting several variables including flush duration, flush interval, limestone gradation and influent distribution, an optimal combination of these variables was found.
One of the two original experimental DLBs is still in operation (the other was dismantled at the end of the research project). Known as the “West Box”, it consists of a 30 cubic yard roll off container filled with 30 tons of limestone. The West Box is able to reliably treat water at twice the acidity loading rate as Vertical Flow Ponds. Since its construction in October 2007, the West Box has generated over 3 tons of alkalinity and continues to function with zero operation and maintenance.
A surprising benefit of DLBs is the ability to remove manganese in a single treatment step. Typically, AMD is treated to raise pH, generate alkalinity and remove iron before a second treatment cell (typically an oxic limestone bed) is used to remove manganese. DLBs can remove manganese while simultaneously raising pH and generating alkalinity. A full scale (1,000 ton) DLB was constructed in 2010 that has been lowering influent manganese concentrations from 28 mg/L to less than 1 mg/L.