35 years of lessons learned designing, building, and monitoring passive treatment systems for mining influenced water (part 1)

Abstract

While natural processes have been remediating acidic water exhibiting elevated dissolved metals for eons, engineers and scientists have been trying to mimic nature in the treatment of mining influenced water for only about 50 years. The technical community has adopted the term “passive treatment” to describe the technology. The design of passive treatment systems is based on the application of multidisciplinary engineering sciences including biogeochemistry, hydraulics, physics, microbiology, and agronomics to name a few. Consequently, there are many opportunities to not quite “get it right” in passive treatment system design, construction, and operation. Fortunately, Mother Nature can be quite forgiving and sometimes a design or operational hiccup can reveal new insights. The case histories presented in this review reflect the learning experiences of a mining engineer whose technical education barely touched on the fundamentals of passive treatment system design. Early system designs were often based on the findings of other practitioners who were gracious enough to share their knowledge at pre-internet conferences and in easy-to-understand publications. Indeed, the current state of practice of passive treatment design rests on the shoulders of a few brilliant pioneers, most of whom are still with us. This review continues the sharing tradition. The common thread among the mini-case histories discussed is that most are from bench- and pilot-scale testing programs, which reinforces the benefits of small-scale testing as an economical way to accumulate valuable lessons learned experience with minimum risk. Key takeaways from the lessons learned by this author are:● Biochemical reactors (BCRs) can be overloaded, but only for a short time to avoid permanent harm to the microbial community.● Other metal removal processes can substitute for sulfate reduction in overloading situations but the beneficial effects may be reversible.● Media material substitution risk in full scale construction can be lessened if bench or pilot data is available.● BCRs are feasible in cold climates.● The age of woody material (fresh vs . moldy) in BCR media is inconsequential.● Allow plants to grow on BCR surfaces with caution.● Excessive aluminum concentrations do not cause BCR media to plug.● Metal adsorption to manganese oxide coatings may be a viable alternative to sulfide precipitation in BCRs.● Mixing bypassed mining influenced water with BCR effluent may offer benefits in a freshet.● A test failure may be due to an improper design and should not condemn the passive treatment concept.● In final design and construction, avoid deviating from the bench- or pilot-scale test conditions.