Advantages of using limestone include low cost, ease of use, and formation of a dense, easily handled, sludge. Disadvantages include slow reaction time, loss in efficiency of the system because of coating of the limestone particles with iron precipitates, difficulty in treating Acid Mine Drainage with a high ferrous-ferric ratio, and ineffectiveness in removing manganese. Hydrated lime is normally the neutralizing agent of choice by the coal mining industry because it is easy and safe to use, effective, and relatively inexpensive. Caustic soda is especially effective for treating low flows in remote locations and for treating Acid Mine Drainage having a high manganese content. Major disadvantages are its high cost, the dangers involved with handling the chemical, poor sludge properties, and freezing problems in cold weather.
Ammonia costs less than caustic soda and has many of the same advantages.
However, ammonia is difficult and dangerous to use and can affect biological conditions downstream from the mining operation. The possible off-site impacts are toxicity to fish and other aquatic life forms, eutrophication and nitrification. Fish species generally exhibit low tolerance to unionized ammonia and toxicity levels can be affected by Ph, temperature, dissolved oxygen and other factors. Ammonia use is not allowed in all States and, where permitted, additional monitoring is required. Initial design and construction costs may be significant, ranging into tens of thousands of dollars.