By Jeff Easton and Jim Woods, WesTech Engineering
Diminishing and increasingly regulated high-quality fresh water sources has pushed the issue of industrial water recycling into the forefront.
Water is required in almost every industrial sector for the processing and manufacture of products. Sources of high quality raw water for commercial plants are becoming progressively scarce. The availability of water from rivers and lakes is not only diminishing, but what is available is increasingly regulated. This scenario has pushed industrial water recycling into the forefront as a high-profile concern.
Cooling water systems, particularly at power plants and oil refineries, are the largest industrial consumers for recycled water, due to their high-volume demand. Other industrial applications include oil and gas drilling, petroleum refining, chemical plants, metal finishers, textile and carpet dying, paper manufacturing, cement manufacturers, and other cooling and process applications.
Many companies are aware of the risks that growing water constraints could place on their operations, and recognize the need to consider unconventional sources of water. The technology, chemistry and processes exist today to feasibly and economically integrate water reuse from unconventional sources into almost any industrial process application.
Unconventional industrial water sources
Unlike conventional water sources like potable supplies, rivers, lakes, surface ponds and fresh water wells, unconventional industrial water sources can originate from wastewater treatment plant effluent, brackish, surface, well, and mine pool water, acid mine drainage, hydraulic fracturing flowback and produced water.
These water sources may contain varying levels of suspended solids, oils and greases, colloidal silica and metals, and dissolved minerals and organics. Contaminants can include dirt and sediments, hardness (such as dissolved calcium and magnesium), heavy metals (like lead, zinc, cadmium, mercury and arsenic), salts, organics and colour.
Since every industrial application requires a different level of finished water quality, understanding the condition of the source water and the finished water quality requirements determines the processes and equipment needed. For most industrial uses of reclaimed water, conventional processes involve secondary treatment, filtration and disinfection steps to achieve a desired level of water quality. Most applications will require multiple processes to achieve the desired finished water quality.
Reuse of municipal wastewater
Recycled municipal wastewater can be used for a broad range of reuse applications, but not for direct drinking water and the manufacturing of food and beverages. Besides traditional uses such as industrial processes, agricultural irrigation, and the irrigation of lawns, landscapes, cemeteries and golf courses, many areas add recycled water to underground storage basins that are used as drinking water supplies.
Water recycling is very important in arid climates, like southern California, where water must be imported from other parts of the state. The Sanitation Districts of Los Angeles County operate the largest engineered wastewater recycling program in the world. The goal is to recycle as much water as possible from their 10 water reclamation plants (WRPs). These play a major role in meeting southern California’s water needs, providing primary, secondary, and tertiary treatment for approximately two million litres per day, 650,000 litres of which are available for reuse.
Utilizing mine pool water/acid mine drainage
New or expanded steam electric power plants frequently need to turn to non-traditional alternative sources of water for cooling. One type of alternative water source is groundwater collected in underground pools associated with coal mines, known as mine pool water. When this water flows from the mine to the surface it is called acid mine drainage (AMD). It contains multiple combinations of acidity, and metals such as arsenic, cadmium, copper, mercury, silver and zinc.
With water sources becoming harder to obtain for industrial applications, these marginal-quality mine pool waters and AMD streams are becoming more attractive for reclamation and reuse. From a cooling perspective, mine pool/AMD water is desirable for industrial water use because it has a relatively consistent and low temperature year round.
Implementing sustainable and financially viable methods to reuse vast quantities of mine pool/AMD water is an area of relatively new, but growing, interest for mining operations. The technologies exist to economically treat any strength of acid mine drainage for industrial reuse.
Recent technological refinements in such processes as CO2 stripping, aeration, thickening, clarification, sludge disposal, ultrafiltration and reverse osmosis are making these systems more streamlined and efficient. This enables full-scale mine pool water/AMD reuse projects to not only control, manage and reuse these contaminated waters, but also to be financially viable.
An advanced surface aeration process is critical in facilitating an economically feasible solution for treatment of the mine pool/AMD wastewater. New impeller designs increase oxygen transfer efficiency, and reduce axial and radial loads. Such a system can produce a minimum efficiency of 3.8 pounds of oxygen per horsepower-hour. This improved transfer efficiency saves significant operational costs over the life of the equipment. Reduced axial and radial loads increase the life of the drive unit and reduce the size of support structures and beams for the surface aerators.
Systems like these are making acid mine drainage reuse more accessible for mining operations, which require systems to not only be financially feasible, but capable of efficiently handling wastewater streams at remote locations, usually within a confined footprint.
The latest developments in high-rate thickeners, used to separate liquids and solids at very high rates, are effective in coal refuse thickening, gold recovery, copper leaching, molybdenum processing, and other mining and chemical applications where mine pool water/AMD is sourced. Separation is effected rapidly because of the system’s hydraulics, which can be in excess of 20 times the hydraulics of conventional thickeners. As a result, the plant area required for this new generation of thickeners is greatly reduced.
Hydraulic fracturing flowback and produced water
As more hydraulic fracturing wells come into operation, so does the stress on surface water and groundwater supplies from the withdrawal of large volumes of water used in the process. This can be as much as 3.8 million litres of fresh water per wellhead to complete the fracturing process alone.
Equally important is the growing volume of wastewater generated from fracturing wells, requiring disposal or recycling. Up to 60% of the water injected into a wellhead during the fracturing process will discharge back out of the well shortly thereafter, as flowback wastewater. For the life of the wellhead, it will discharge up to 380,000 litres per day of produced wastewater.
Because water is the base fluid and biggest component used in hydraulic fracturing, its importance remains a critical factor in the operation and economics of shale oil and gas production. Fresh water and wastewater operating procedures which have been in place since the late 1990s are experiencing increasingly stiffer governmental regulations on water availability and disposal limitations. This is prompting oil and gas executives to reassess their current water use for fracturing, and adopt a more unified, and longer-range perspective on their water life-cycle management.
Fresh water supplies for use in hydraulic fracturing are becoming more expensive and more unobtainable.
Wastewater associated with shale oil and gas extraction can contain high levels of total dissolved solids (TDS), fracturing fluid additives, total suspended solids (TSS), hardness compounds, metals, oil and gas, bacteria and bacteria disinfection agents, and naturally occurring radioactive materials. These contaminants are partially a combination of chemicals and agents inserted deep into the well (3,000 metres and deeper) which facilitate fracturing by modifying the water chemistry to increase viscosity, carry more sand and improve conductivity.
Some drilling operators elect to reuse a portion of the wastewater to replace and/or supplement fresh water in formulating fracturing fluid for a future well or re-fracturing the same well. Reuse of shale oil and gas wastewater is, in part, dependent on the levels of pollutants in the wastewater and the proximity of other fracturing sites that might reuse the wastewater.
Mobile solutions to treat wastewater at the wellhead enable recycling and reuse of flowback wastewater without the need for storing it in surface ponds on-site, or for trucking it for disposal at off-site deep-well injection locations. The drawback of wellhead mobile solutions is that they do not provide continuous processing to handle produced wastewaters, which would need to be processed for potentially 20 years following fracturing.
Since produced wastewater represents 95%, or more, of the wastewater generated during the life cycle of a well, mobile processing systems do not provide a solution adequate to solving the long-term problems of diminished water sourcing.
Brackish surface and well water
Brackish water refers to water supplies that are more saline than freshwater, but much less salty than seawater. This level of salinity in water is measured in TDS. In hydraulic fracturing, saline water is introduced into the process by contacting brackish aquifers.
The two most common desalination technologies are membrane and thermal processes. Membrane processes rely on permeable membranes to separate salts from water. They can be pressure-driven (reverse osmosis) or voltage-driven (electrodialysis).
Reverse osmosis is currently the most common desalination treatment method. The thermal process involves heating saline water to produce water vapour, which is then condensed and collected as fresh water. In a reverse osmosis system, the greater the TDS concentration of the water, the higher the pressure needed for the pumps to push water through the membranes, and, consequently, the higher the energy costs.
Mine pool water/acid mine drainage
The reuse of mine pool water/AMD in hydraulic fracturing for shale oil and gas production is quickly becoming a hot topic of interest. Many current shale oil and gas hydraulic fracturing wells are in close proximity to mine pool water/AMD areas, creating a unique opportunity to beneficially use these wastewater sites for hydraulic fracturing.
According to a 2013 Duke University-led study, much of the naturally occurring radioactivity (radium and barium) in fracturing wastewater might be removed by blending it with wastewater from mine pool water/acid mine drainage. Blending can bind some fracturing contaminants into solids that can then be removed before the water is discharged back into waterways.
Centralized handling of flowback and produced wastewater
Centralized treatment of wastewater has emerged as a viable solution for long-term efficiency in managing water sourcing and wastewater treatment in hydraulic fracturing. Centralized treatment facilities handle both the flowback wastewater and produced wastewater from oil and gas wells within a region, in a radius of 70 km – 80 km. Pipelines connect all wellheads directly with the central treatment plant.
Such centralized plants can be integrated with alternative sources of water to supplement fresh water needs for fracturing, such as from abandoned mines, stormwater control basins, municipal wastewater treatment plant effluent, and power plant cooling water. Centralized water management allows wastewater sourcing to be implemented on an economy of scale that has not before been realized in the shale oil and gas production industry.
Jeff Easton and Jim Woods are with WesTech Engineering Inc. This article appears in ES&E Magazine’s December 2016 issue.