Overview of Acid Mine Drainage Treatment Chemicals
Since the passage of the Surface Mining Control and Reclamation Act in 1977, coal mine operators have been required to meet land reclamation performance standards established by federal and state regulatory programs. Operators must also meet water quality standards established in the Clean Water Act of 1972, which regulates discharges into waters of the United States. Control of AMD is a requirement imposed on operators by both SMCRA and CWA. In addition to the surface mining permit, each mining operation must be issued a National Pollutant Discharge Elimination System permit under CWA.
Pollutant discharge levels are determined by the
U.S. Environmental Protection Agency's technology-based standards, or the
discharge levels may be based on the more stringent water quality-based standards
for discharges released into streams with designated uses. If a Total Maximum Daily
Load has been developed for a stream, then load allocations from a specific
point discharge may be assigned. If AMD problems develop during mining or after
reclamation and discharge limits are exceeded, a plan to treat the discharge
must be developed. Treatment of AMD includes neutralization of acidity and precipitation
of metal ions to meet the relevant effluent limits. In most cases, a variety of
alternative treatment methods can be employed to meet the limits specified.
Water quality discharge permits on surface mines usually require monitoring of pH, total suspended solids, and iron and manganese concentrations. Other parameters may be requested by the regulatory authority in a particular mining situation. However, in order for an operator to make a selection of an AMD treatment system, one must determine (in addition to the above parameters) the flow rate, the receiving stream’s flow and quality, availability of electrical power, the distance from chemical addition to where the water enters a settling pond and the settling pond’s volume for water retention time.
After evaluating these variables over a period of time, the operator can consider the economics of different chemicals and alternative AMD treatment systems. Special circumstances, like the treatment of trace elements in the water (like selenium or arsenic), may require a specialized treatment system that includes filtration techniques or reverse osmosis, which are generally much more expensive to install and operate than traditional alkaline addition and precipitation.
Typical AMD chemical treatment systems consist of an inflow pipe or ditch, a storage tank or bin holding the treatment chemical, a means of controlling its application rate, a settling pond to capture precipitated metal oxyhydroxides and a discharge point. The latter is the point at which NPDES compliance is monitored. The amount of chemical needed for neutralization can be calculated by multiplying the flow (gpm), the AMD’s acidity (mg/L) and a factor of .0022. The product is the tons of acid that require neutralization per year (calcium carbonate equivalent). This value (tons of acid/yr) can then be multiplied by a conversion factor for each chemical to determine the amount of the chemical needed.