specializing in

water restoration

DRAINABLE LIMESTONE BEDS: Treatment Innovation for Difficult Sites

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.

DLBs treat water with elevated aluminum concentrations using a system footprint that is smaller and with a more flexible layout than other passive treatment technologies. 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.

The first experimental DLB units were installed in 2007 and one of these units is still in operation (the other was dismantled at the end of the project). The first 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. Since these early efforts HE has designed dozens of DLBs totaling tens of thousands of tons of limestone. While these systems require minimal maintenance, HE has pioneered techniques for lowering maintenance costs even further. More efficient methods for flushing solids have been developed that prolong system life and HE is always working to refine limestone cleaning procedures to cut costs. The low long-term costs and compact footprint of DLBs make them an attractive option for many mine water treatment sites.