Acid Mine Drain (AMD) treatment to achieve very low residual heavy metal concentrations
Mount Emmons mine wastewater treatment plant currently treats on average around 450,000 gallons per day of acid mine drain (AMD) water collected into several collection ponds. The plant uses standard lime hydroxide precipitation process at pH 10.7 to precipitate heavy metals and remove AMD contaminants with the subsequent flocculation - dissolved air flotation (DAF) and sand filtration.
Possible opening of the new molybdenum mine at the same site would require replacement of the current large DAF and sand filters with the small footprint equivalents. The pilot studies with the small footprint, hybrid centrifugal - dissolved air flotation (GEM System) and walnut filters have been performed. The results of the pilot study are described in this manuscript. GEM System and the walnut filters can be used as a replacement for current technologies.
Keywords: acid mine drain treatment; precipitation, flocculation-flotation; filtration, walnut vs. sand filters
Mount Emmons mine (i.e. 'client') wastewater treatment plant currently treats large amounts of acid mine drain (AMD) water collected into several collection ponds. The treatment plant currently operates 5 days per week, approximately 9 hours per day, to treat water that is collected throughout the previous day. The total flow of water varies seasonally, but on average the plant treats about 450,000 gallons per day.
The plant uses standard lime hydroxide precipitation process to precipitate heavy metals and remove AMD contaminants. The process occurs at a pH of 10.7 in order to provide waste stream with an environment (approximately 14-20 minutes residence time) that will maximize the precipitation and eventual removal of cadmium, with good removal of zinc, iron, manganese, lead, silver and aluminum.
After pH adjustment with lime in AMD reactor, the wastewater stream with the precipitated metal hydroxides is pumped into a Sand Trap, where cationic granular high molecular weight flocculants are injected to the stream to begin the flocculation of precipitated particles prior to DAF treatment. An anionic surfactant is also injected to the stream while in the Sand Trap to help produce hydrophobic surfaces and more bubbles to enhance flotation in the DAF tank.
After addition of chemicals in Sand Trap, the flocculated wastewater is pumped to the client’s DAF Systems. Currently, client operates two out of their 3 existing DAF systems at one time, leaving one extra system in case repairs or tank cleanings are being performed. The DAF Systems each operate at approximately 400-500 gallons per minute. The flow varies depending on the level in the collection ponds, and client uses a Variable Flow Drive pump to pump necessary amount of water to DAF while pond influent level changes.
The current DAF Systems are very large systems that rely on a tank with only 0.6 GPM/ft2 HRT, and consequently high amount of retention time for flotation. The DAF systems also rely on anionic surfactant chemicals added to the stream for the formation of bubbles. Plant personnel explained that the main reason they need 3 DAF’s is because there are many heavy metal hydroxides particles that sink to the bottom of the tank, and tanks periodically need to be shut off and drained to remove sludge sediment from the bottom of the tanks.
The floated solids are skimmed from the surface and drop into an auger system which transfers the sludge to large tanks. The sludge contains approximately 2-3% solids to water ratio, and is processed through client’s filter press for sludge thickening. Because the DAF sludge is difficult to dewater, client adds approximately 400 ppm of flocculant to the sludge to allow it to dewater properly. After filter press, sludge has approximately 16% dry solids. Because the sludge contains high concentration of zinc, manganese, and cadmium which are all environmentally hazardous, client mixes sludge into cement, which are eventually hauled away to be disposed safely.
Client’s DAF effluent is processed through four (4) large Sand Filters. In addition, tests are currently performed with a Walnut Filter, which is similar to a standard industrial sand filter. Client has problems using existing sand filters because there are still traces of manganese and ferrous hydroxide and other particles that end up in the DAF effluent and foul the sand filters to the point where it becomes very difficult to backwash, often requiring maintenance or shut down. These instances will result in plant effluent water being out of compliance.
From the sand filters, wastewater is stored and eventually flows into the Coal River. The nearby town of Crested Butte, CO is directly downstream from the Mount Emmons Mine. The mine is heavily monitored by state and city agencies, to make sure the mine eliminates heavy metals that are hazardous to aquatic life. In 2010 prior to this pilot study local environmentalist agencies and groups argued that the permits for opening a new molybdenum mine will be granted only if a superb modernized wastewater treatment facility is available.
Upcoming plant modernization – goals
- If possible replace large DAF units with smaller footprint system for solid/liquid separation; space will be needed if new molybdenum mine opens
- Replace sand filters with walnut filters to save on backwash water
- Test for possible use of high density sludge (HDS) process to achieve higher solids in sludge and save on sludge disposal costs
- Improve on current process (identify why after 2 days of no operation at start up more heavy metals are present in the wastewater