The Sewickley seam is located approximately 80 to 120 feet above the Pittsburgh seam. Both seams dip to the west from their outcrop. Due to this westward dip, the mine pool elevation in the Pittsburgh seam mines, west of the current Sewickley seam active mining operations, is at a higher elevation than that of the Sewickley seam.
Morgantown Energy Producing Co., LLC (MEPCO), a bituminous coal mining and processing firm in the northern Appalachian coal mining region of Monongalia County, W.Va., and in Greene County, Pa., controls the Sewickley seam reserve overlying the subject mine pools. In order for MEPCO to mine the majority of its Sewickley seam reserves, the company must pump the water from the Shannopin, Humphrey, Pursglove and Osage mine pools in sufficient quantity to lower the water level to an elevation below that of the Sewickley seam.
To accomplish this, MEPCO currently operates two acid mine drainage (AMD) treatment facilities at the Shannopin Mine Steele Shaft site in Dunkard Township, Greene County, Pa. The original AMD treatment facility (Phase 1) began operation in 2004 and draws polluted underground water to the surface through a steel shaft that remains from earlier mining operations. The plant treats the water and then releases it into nearby Dunkard Creek at a rate of about 3,500 gallons per minute (gpm).
To further maximize water recovery and minimize the associated sludge volumes, N.A. Water Systems, a business unit of Veolia Water Solutions & Technologies North America, was selected to design and procure a 4,000 gpm AMD treatment plant at the site. The new AMD treatment plant (Phase 2) is using the innovative technology, DenseSludge™ to maximize water recovery and minimize sludge volumes. The net results of the process are improved process control (particularly pH control), reduced gypsum deposition (i.e. less scaling) on system components, and drier sludge of less volume. The water removed from the sludge can then be discharged instead of being reinjected into the mine pool for re-treatment.
The DenseSludge Process
Conventional water treatment for metals removal and pH adjustment can produce significant volumes of sludge for disposal. DenseSludge technology, however, can reduce sludge generation by up to 90% and improves sludge management by creating a dewatered material containing up to 70% solids.
Typical AMD streams contain high sulfate concentrations that combine with calcium ions in lime-based treatment systems to form gypsum crystals. Compared to that generated in conventional lime treatment systems, the physical form of this gypsum is radically altered when the DenseSludge process is used. The characteristics of the metals particles are also changed in this process, as the metal hydroxides are converted to metal oxides.
The dense sludge particles settle faster, dewater more readily, are more easily pumped, and hold much less water than conventional precipitates. In some cases, because the metal content of the sludge is more concentrated, it can be economically recovered. In general, conventional and DenseSludge treatment processes are very similar. The changes required to implement the DenseSludge process are minor, and typically much of the existing equipment is used.
Adding Alkali
The most significant change between DenseSludge technology and conventional metals treatment is the method by which the alkali source is added. In conventional treatment systems, the alkali is added directly into the influent to achieve a desired pH setpoint. Generally, that setpoint is the pH at which the minimum solubility occurs for the target metal(s), or at the pH where discharge limitations can be reliably met.
In the DenseSludge process, the alkali source is combined with recycled sludge before being combined with the influent. The alkali source can be lime, caustic, ammonia or any other neutralization agent that readily reacts and can be continuously metered. The sludge particles react with the alkali to provide attraction sites for the removal of metals. This causes the gypsum crystals to grow. The continued recirculation of sludge ultimately converts the metal hydroxides to oxides through a series of steps.
The method of feeding the alkali (proportioning valve, proportioning weir box, or metering pump) is the same for both conventional and dense sludge systems. In some cases the pH setpoint can be lowered with the DenseSludge system and still maintain optimum metals precipitation.
Sludge Conditioning
The sludge for recirculation is withdrawn from the solids settling unit and pumped to the sludge conditioning tank where it combines with the alkali source. The resultant mixture of sludge and alkali is then directed to the neutralization tank where it combines with the influent. The demand for alkali depends upon the system pH setpoints and a probe in the neutralization tank provides continuous measurement and feedback to the proportioning device. Many operators of treatment plants that have been converted to the DenseSludge process report a savings in alkali consumption.
The sludge generated in the solids settling unit is recirculated constantly at a rate sufficient to meet the constraints of the DenseSludge process. In addition to continuous recycle, a certain quantity of sludge is removed from the system each day to maintain the equilibrium of the system. For best results this blowdown system should be designed to handle the maximum anticipated loadings on a continuous basis. Then the system can be operated intermittently to maintain the density of the recirculated sludge or to keep this density within a target range of 15%-25% solids. This compares to a typical range of 1%-3% solids with conventional systems.
For AMD derived from coal mining sites, such as MEPCO’s Shannopin Mine Steele Shaft site, 20% solids or less provides a good working density with lime alkali and ultra high molecular weight (UHMW) cationic polymer used as settling aid. For applications involving other metals, 20%-30% solids in the underflow from a clarifier/thickener is achievable.
The technology is generally suited for acidic waste streams containing soluble metals. Although there is no strict guideline for pH characteristics, the wastewater must contain metals in the soluble form. In some cases, as with ferric iron, this dictates an influent pH below 3.5.
When evaluating the process for use with acid mine drainage, if an appreciable amount of ferric iron could be present in the influent and the pH is low enough for it to be present in a soluble state, a two-stage neutralization system should be utilized to optimize the DenseSludge process. Because of the chemistry of the process, metals that are not dissolved when they reach the process cannot be densified, since they have already been precipitated.
MEPCO’s Phase 2 AMD Facility
Start-up of the Phase 2 AMD plant at MEPCO’s Steele Shaft facility commenced in August 2007. The mine water processed through the Phase 2 plant is accessed via an existing air intake/emergency escape shaft, which serves to contain and provide mounting for either one or two new vertical turbine pumps. This shaft is located near the existing air ventilation exhaust shaft that contains the vertical turbine pump that supplies mine water to the initial (Phase 1) AMD treatment facility.
This project consists of four concrete tanks to clarify mine water runoff. Sludge is recirculated through the treatment system from the bottom of the clarifier. The recirculated sludge is pumped from the clarifier back to the sludge conditioning tank where it is mixed with lime as an alkali source. The resultant mixture of sludge and alkali overflows the sludge conditioning tank into a ferrous oxidation process where it combines with pre-aerated influent AMD mine water. The recirculation flow rate is manually controlled using a variable frequency drive (VFD) to maintain the required sludge recycle ratio.
Sophisticated Automation
MEPCO wanted the treatment plant to be highly automated. The treatment system includes a PC-based SCADA system which controls the start/stop of the plant process equipment and provides monitoring of the process, including equipment status, monitoring setpoints and process points. The plant’s SCADA system can be configured to allow plant automation with little need for operator intervention, or it can be configured to operate in a fully manual mode. The automatic configuration requires minimal operator input to operate the plant, yet gives the operator the required information necessary to ensure that the system is in compliance with all regulatory requirements.
The SCADA system provides operations personnel insight into the total process through extensive process graphic displays. The structure of these displays allows operators to navigate through the plant and AMD feed pump station, viewing equipment and process status. The SCADA system also allows operators to control the entire plant, or discrete pieces of equipment, through pop-up control windows. The graphic displays reflect real-time conditions such a flow rates, liquid levels, system pressures, pump and equipment condition and alarm occurrences, allowing quick assessment of the process status at all times. Operators can also call up real-time and historical trend displays to pinpoint irregular or unusual conditions.
An important function is the system’s automatic sludge wasting feature. Non-intrusive instrumentation installed on the sludge recirculation piping provides real-time measurement of the density of the sludge. The plant’s sludge density is maintained in a very tight operating range, with high and low setpoints of 1.13 and 1.11 specific gravity (sg). If density rises above 1.13 sg, sludge is automatically wasted from the process until it drops to 1.11 sg. At that point, the system automatically stops wasting and flushes out the waste lines. Depending on the current status of the process, automatic sludge wasting can take place multiple times in a day, or not at all.
In addition, a series of interlocks have been established to keep operations running smoothly. For example, if the plant’s lime system runs out of lime or the line becomes clogged, the whole system shuts down once pH drops to 7.1. An interlock has also been established for the plant’s makeup water system. If the plant’s primary makeup water source is interrupted, the system automatically switches to a backup source.
A Valuable AMD Treatment Technique
MEPCO’s new AMD treatment system operates in an automatic mode and is staffed 24/7. As with the Phase 1 system, treated effluent from the new clarifier discharges directly into nearby Dunkard Creek. Periodically, excess sludge is wasted from the recirculation loop to maintain the proper density. This sludge is transferred to a borehole for injection into a mine cavity.
AMD is one of the mining industry’s greatest environmental challenges. The Department of Energy estimated in 2006 that more than 1.3 trillion gallons of acid mine water can be found in abandoned coal mines mostly beneath Fayette, Greene and Washington counties in Pennsylvania and in Monongalia County in West Virginia. At the MEPCO site in Greene County, the DenseSludge process is proving to be a valuable AMD treatment technique, reducing waste generation considerably and optimizing overall treatment system performance.
Zick is director of the mining market for N.A. Water Systems, a Veolia Water Solutions & Technologies company. He can be reached at robert.zick@veoliawater.com or 412-809-6688.
