The vertical flow pond (VFP) has emerged the passive treatment system of choice for many of the mine drainage discharges throughout the Appalachian Region. The VFP consists of a layer of compost over a layer of limestone in a single pond. In theory, the VFP should function as a reaction vessel with metal deposition occurring in ponds and wetlands located beyond the VFP. However, in practice the VFP serves as a depositional vessel as well. Unfortunately, metal precipitates cannot be removed effectively from the VFP without near complete exhumation. As a result, many of the VFP’s have or will fail prematurely and operation and maintenance of these systems tends to be much more labor intensive than originally intended.
In a typical VFP, the purpose of the organic layer is to reduce any ferric iron to errous iron and to strip the dissolved oxygen as the AMD passes through. In the ferrous form, iron should not form a precipitate within the VFP. Intended as a reducing zone, sulfate reduction and consequent alkalinity generation should occur in the organic layer. Next, the AMD enters a layer of limestone where dissolution of the limestone results in an increased pH and alkalinity, allowing for the future precipitation of precipitates.
A number of VFP’s have been studied in detail and the chemical reactions that do or do not occur are becoming better understood as well as the conditions under which these reactions take place. Most of the VFP’s, especially those treating large flows, do not function completely in accordance with the theory. Although not universal, here are some of the problems with the VFP’s.
Being a stubborn lot, many designers cling to the VFP concept and have engineered hybrid VFP’s in an attempt improve on performance and longevity. Some of these hybrids, such as the addition of extensive flushing systems, have improved performance, but have also increased operation and maintenance obligations.
Separating the systems into individual components may be a way to go. For instance, PA BAMR designed a limestone upflow pond with an automatic flushing siphon to deal with aluminum in discharges. The limestone amended organic substrate system of R. C. Thomas sets up the proper conditions for sulfate reduction to occur and shows promise. Basically, we need to feel that we are not constrained by existing designs and start anew.
We can no longer afford to continue to construct passive treatment systems that are doomed for failure. At this point in time, many studies have been conducted on constructed VFP’s and there is a fair understanding what works and what fails. It is now time to engineer systems for success and minimal operation and maintenance. Based on chemistry and flow conditions, we can predict with a fair amount of certainty the chemical changes that will occur when the drainage is subject to different reagents, such as compost or limestone. We also have a fair understanding of how to enhance the removal of metal precipitates. Armed with this knowledge the following actions are proposed:
In summary, funds and encouragement are needed to promote research and development of new and improved methods for passive treatment.
The development of the flow chart is anticipated to cost about $20,000 for compilation of data and assembling a group of experts. Development and testing of new approaches will be much more expensive. Funding of several projects at levels of $100,000 or so should be anticipated.
The flow chart should be developed in a period of 6 months to 1 year. New approaches will probably require several years for pilot tests followed by full-scale systems.