Patent Application
20070045189
The invention pertains to methods for treating acid mine drainage wastewater to reduce dissolved and suspended particles therein to result in a product water that is suitable for discharge or for use as makeup water to a power plant or the like.
Acid mine drainage (AMD) water is created by surface mining, deep mining, or refuse piles when pyrite is exposed to air. Pyrite commonly occurs in mineral seams (e.g., gold, copper, coal, etc.) and in the rock layers adjacent to these seams. It is exposed to air during the mining and mineral recovery process. When pyrite interacts with oxygen and water, ferrous and ferric iron and sulfuric acid are created. The low pH of the resultant water solubilizes many undesirable heavy metal species such as iron, manganese, and aluminum as well as lead, zinc, cadmium, and mercury. In addition, the water can also include high levels of suspended solids. Often, these waters contain excessive Ca+2 and SO4−2 ion concentrations that under certain conditions precipitate as CaSO4 on surfaces that come into contact with the water.
Various approaches have been used to remediate AMD water. For example, chemicals may be added to precipitate dissolved metals contained in the water to subsequently coagulate and separate the precipitated solids from the AMD prior to mechanical filtration systems such as mixed media filters, etc.
Water reuse applications and pollution standards for discharge water are becoming more stringent, increasing the need for current AMD water treatment to further reduce total dissolved solids and sulfate levels below the values that can currently be achieved by the now commonly accepted methods of remediation. For example, in one planned AMD water remediation project the treated water will be used as makeup water to a power plant. In this case, total dissolved solids (TDS) must be reduced to below 330 ppm, SO4−2 to less than 60 ppm, and Ca+2 below 50 ppm, and Fe, Mn, and Al must be reduced to less than 0.1 ppm each.
In accordance with the invention, a method is provided for treating acid mine water wherein lime or other neutralizing agent is first used to neutralize the acid water and precipitate metals, treating the neutralized water in a clarifier to produce clarifier effluent having reduced dissolved solids, adding an oxidizing agent to the clarifier effluent to oxidize remaining dissolved metal species to form a clarifier effluent with suspended metal particles therein and treating the clarifier effluent via microfiltration to form a microfiltration filtrate having a reduced content of suspended particles. Microfiltration filtrate is then further purified by a reverse osmosis membrane system.
In other exemplary embodiments of the invention, a cartridge filter is employed upstream from the reverse osmosis station and downstream from the microfiltration unit. Additionally, scale control agents and the like can be fed to the system to prevent fouling of the reverse osmosis membranes. Especially efficacious are the phosphonate calcium sulfate control agents, which are fed to the RO (Reverse Osmosis) unit so as to inhibit the formation of calcium sulfate scale on RO membranes.
The invention will be further described in the following detailed description and accompanying drawings.
Turning to
Discharge from decarbonation/aeration tank 6 flows over an internal weir prior to entering a flow splitter wherein a lime sludge mixture is added from sludge densification tank 16 for neutralization of the water. Following lime addition, the flow is directed to the two parallel reaction tanks 8, 10. Each of these tanks is equipped with a surface aerator to provide sufficient oxygen transfer for further iron oxidation. Discharge from the tanks 8, 10 flows over an internal weir prior to entering a flume wherein the waters are combined with a polymer coagulant from source 20 and makeup water dilution station 22. As is conventional in the art, the clarifier influent flume and center well of clarifier 18 provide sufficient flocculation time for the polymer to promote particle agglomeration.
Effluent from the clarifier is directed in effluent line 24 to clarifier effluent tank 32. Solids that settle in the clarifier, forming sludge or underflow, are recycled to the sludge densification tank via line 28, or possibly pumped to a mine borehole or the like for disposal through line 26 and 27.
The lime system, in one exemplary embodiment, comprises a silo 12 and slaker 14 where water is introduced to provide a lime slurry. Additional water can be added to the slurry to provide about a 5-20% consistency lime slurry which overflows into the densification tank 16 where it is mixed with recycled sludge from the clarifier. This lime and sludge mixture is added to the AMD flow by gravity, following the decarbonation/aeration tank to neutralize acidity.
An oxidizing agent such as sodium hypochlorite is fed from tank 30 to the clarifier effluent upstream from clarifier effluent tank 32. This will help to ensure oxidation and removal in downstream microfilter 34 of the remaining Fe and Mn in the clarifier effluent since either of these metal species could be problematic in the reverse osmosis unit 48. The AMD from clarifier effluent tank 32 is directed via a pump to the microfiltration unit 34 wherein particulate matter larger than 0.1 μm will be retained by the MF membranes. Filtrate is directed to filtrate tank 40 and MF concentrate (reject) will travel through line 36 to sump 38 from which it will be recycled back to the inlet of the decarbonation tank.
Feed to the RO station 48 will be taken from the filtrate tank 40, pressurized, and sent to the RO machines. In one embodiment of the invention, about 30-80% of the RO feed will pass through the RO membranes resulting in low dissolved solids permeate or product water that, as shown, is directed to line 50 to effluent tank 52 and subsequently to treated water tank 54 where it is pumped through product line 99 for discharge or use, for example, as makeup water for a power plant or the like.
The reject or concentrate from the RO membranes, containing the rejected ions, is directed to line 56 and forwarded to waste line 27 which can be in communication with a borehole or other waste containment site.
In order to minimize fouling and scaling on the RO membranes, pH may be adjusted via addition of acid from acid tank 44 to the RO feed. Similarly, a dechlorination chemical, such as sodium bisulfite, may also be fed to the RO feed from source 42 to protect the membranes from the harmful effects of chlorine. Also, as shown, a skid feed 98 may be provided to provide a source of cleaning treatment for the RO unit with a mechanism for quickly connecting and disconnecting the unit to the RO feed.
In accordance with one embodiment of the invention, RO permeate water quality will have a maximum level of: 60 ppm SO4−2, 10 ppm silica, 50 ppm Ca+2, 0.1 ppm Al, 0.1 ppm Fe, 25 ppm Mg, 0.1 ppm Mn and 300 ppm total dissolved solids.
Turning now to
As shown in
As shown, permeate from the upstream parallel RO units 64, 66 is directed through lines 72, 74 for conjoint flow through permeate line 76 which collects permeate from intermediate RO unit 78 and downstream RO unit 80 forming permeate exit line 84 that can lead for example into a treated water tank or other reservoir or process line such as shown in
Treatment of AMD water with RO membrane technology is challenging. Particulate fouling from suspended solids, metal fouling from aluminum, iron, and manganese and mineral scale fouling from calcium sulfate are all problems that must be overcome in order to employ RO technology as part of the process. Without proper treatment and process control, the presence of any one of these fouling sources can affect performance, maintenance and overall RO membrane life.
As to the RO antiscalants that may be fed to the RO feedwater, we have found that calcium sulfate scale forming species are commonly encountered in AMD and must be properly treated to enhance RO membrane performance. Phosphonate antiscalants (including water soluble salts thereof) perform well in this regard. These compounds should be fed in an amount adequate to keep the membrane surfaces free of foulants. For example, the phosphonate antiscalant may be fed in an amount of about 0.1-50 ppm with a more preferred amount being between about 1-20 ppm. The phosphonate antiscalants are brought into contact with the RO membranes preferably by feeding them to the filtrate from the MF unit, but if the MF experiences calcium based scaling, the antiscalants may also be added prior to the MF.
Exemplary phosphonates have a carbon to phosphorous bond as shown in the following:
wherein M is a water soluble cation or H.
The preferred phosphonate, hexamethylene diamine-N,N, N′, N′-tetra(methylene phosphonic acid)-K salt form was tested in the field at a 2 ppm actives level used in the RO feed of AMD having a calcium sulfate concentration of about 6-8 times saturation.
In order to assess the efficacy of proposed acid mine drainage systems in accordance with the invention, studies were undertaken with an abandoned eastern state location mine and its associated acidic, aqueous mine water. Prior to the commencement of the study, a clarification system had been installed in which the acid mine wastewater was pumped out of the mine, neutralized, and clarified to protect against environmental damage that could occur if the acidic mine water escaped directly from the mine.
Recent environmental regulations dictate that discharge from the mine wastewater should have reduced sulfate and total dissolved solids (TDS) standards respectively of less than 850 ppm sulfate and less than 2500 ppm TDS. Further, since the mine water is ultimately to be used as plant makeup water in a zero liquid discharge facility, water quality would have to be further improved to contain less than 300 ppm TDS, less than 60 ppm SO4−2, with Ca+2 below 50 ppm and Fe, Mn, and Al each being below 0.1 ppm.
Samples of the eastern mine water were analyzed and found to be as follows:
The analysis indicated that the AMD water from the mine was characterized by relatively low pH (2.8 to 6.5), high sulfate (2,220 to 3,580 ppm) and total dissolved solids (3,376 to 5,500 ppm), and high levels of metals, primarily iron (169 to 372 ppm), aluminum (0.4 to 4.7 ppm), and manganese (4.7 to 8.7 ppm).
The existing treatment system utilized a clarifier and associated equipment to aerate and raise the pH of the wastewater with lime, yielding a reduction of total suspended solids (TSS) to an average of 35 ppm with oxidation and subsequent precipitation and removal of Fe and Mn to levels of 3.0 and 2.0 ppm, respectively.
Two alternative treatment programs were envisioned for the pilot study. In one program, the clarifier effluent was to be diverted to a new clarifier effluent tank with sodium hypochlorite being fed thereto to assure oxidation and removal of iron and Mn in downstream microfilters. Reverse osmosis (RO) was to be employed downstream from the microfiltration units.
The other proposed alternative system was to take the clarifier effluent and use it as feed to a sand filter and then to an RO system.
The proposed systems can be summarized as follows:
With regard to System 1, a sand based continuous backwash upflow filter was employed downstream from the clarifier. The filter produced a continuous filtrate stream and a continuous concentrate stream and did not need to be shut down for backwash cycles. Sand was backwashed internally in the filter tank using filtered water that was redistributed back on top of the sand bed.
In System 1, the filtrate from the sand filter was fed to a multimedia (MMF) type cartridge filter with filtrate then fed to a RO (Reverse Osmosis) membrane system. The RO membrane system consisted of three spiral wound hollow fiber membranes available from the GE Osmonics Division.
Standard procedure was to flush the RO system with RO permeate whenever it would shut down. The RO feed was dosed with sodium bisulfite to scavenge residual chlorine and an antiscalant was fed upstream from the RO system in order to prevent calcium sulfate scaling.
The following RO parameters were measured and logged: temperature, prefilter inlet pressure, concentrate pressure, prefilter outlet pressure, feed pressure, feed flow, permeate flow, concentrate flow, feed conductivity and permeate conductivity.
The RO system was always run at a set recovery which was adjusted by regulating the permeate and concentrate flow rates with adjustments to the feed pump discharge valve and the concentrate back pressure valve. No subsequent adjustments to pressure were made once the concentrate and permeate flow rates were set for a certain recovery. Fouling was therefore monitored by observing increases in pressure, at a given permeate flow rate and recovery rate. If there is no major change in temperature or feed TDS (measured in this test as conductivity), the pressure should remain constant.
During the course of the test, the RO was run at different recoveries. Pursuant to verifying the full-scale equipment's design recovery of 65%, the target or goal was to achieve continuous steady state operation at 75% recovery. It is standard practice to exceed the design performance of the commercial process during a pilot. This is especially important in wastewater and/or streams where the membranes are at risk for mineral scaling (as is the subject stream), in order to establish some margin between design operating parameters and critical levels. Hence, the pilot was also run at 75% recovery.
In System 2, the sand filter and multimedia filter were not used. Instead, in its place upstream from the RO membrane system, a microfiltration (MF) unit was used. This MF unit was a hollow fiber MF module. Physical characteristics of the membrane are described below
An ancillary pretreatment system was installed upstream of the MF system. This equipment supplemented the basic MF system with the capacity to provide oxidation with sodium hypochlorite.
The MF media was supplied in modules using MF class hollow fiber PVDF membrane, 0.1 micron pore size, TMP Trans Membrane Pressure) ˜2.5 bar, pH range 1-10 operational.
Pilot Run Conclusion
The overall evaluation of pretreatment processes, post clarifier and pre-RO, led us to select System 2. As we experienced, the clarifier effluent was subject to carry over of suspended solids, as well as unique, high concentration dissolved chemistry. Specifically, dissolved metals, such as iron and manganese, present in the effluent were detrimental to the RO membranes. When soluble in the clarifier effluent, the metals were not removed by conventional sand filtration since they are not particles. With oxidation, these species precipitate but are not large enough to be captured effectively by a sand and/or multimedia filter. Microfilters (MF) provided a more positive protection for the RO membranes. Since space may be of concern in many applications, the smaller spatial requirements of the MF systems is also an advantage.
Except for the need to conduct frequent cleanings, the MF system demonstrated stable operation at proposed operating conditions when the upstream clarification process was in control, consistently protecting the RO from potentially catastrophic membrane fouling. We did find that with fouling from suspended particulate inhibited by the MF, scaling of the RO membranes presented a potential problem. However, the use of phosphonate scale control agents (e.g., hexane tetramethylenephosphonic acid), were found effective in inhibiting membrane scaling from CaSO4 at RO recovery rates of about 65% and greater.
Whereas we have shown and described herein certain embodiments of the present invention, it is intended that these be covered as well as any change or modification therein which may be made without departing from the spirit and scope of the invention.
