UA5 C3 Liquid and Solid Emissions Obj 4.docx

The most abundant liquid on Earth is water. While nearly three quarters of the Earth’s surface is covered with water, only a small amount (about 0.01%) is fresh surface water (lakes and streams). Water found in soil and rock formations is known as groundwater. Water consumption demands are met by us...

The most abundant liquid on Earth is water. While nearly three quarters of the Earth’s surface is covered with water, only a small amount (about 0.01%) is fresh surface water (lakes and streams). Water found in soil and rock formations is known as groundwater. Water consumption demands are met by using surface water and groundwater drawn from wells or springs. Plants and industrial operations use a tremendous amount of water. Water discharged from these plants is known as effluent. Water alone is not usually considered to be a contaminant. However, many of the thousands of materials suspended or dissolved in it are contaminants. When water is used for any purpose, it is usually of much lower quality when it is disposed. Potentially hazardous materials get into water either by direct dumping, or by various processes where water carries, cleans, or unintentionally dissolves a contaminant. When undesirable material is dissolved or suspended in water, the solution or mixture has the potential to be a liquid pollutant. In some cases, and only if properly controlled, there are hazardous materials that can be released without creating a problem. Challenges arise when materials are released in an uncontrolled manner, such as a spill or a leak. In an energy plant, there are many places where liquid wastes could originate such as: • Cooling tower blowdown • Boiler blowdown • Waste from boiler waterside or fireside cleaning • Ash handling slurries • Water treatment equipment • Plant floor drains • Flue gas scrubbers • Oil storage locations If these liquids are not handled properly, liquid pollution could result. Materials become pollutants when they begin to have adverse effects on other occupants of the ecosystem. The effects of pollutants vary considerably depending on: The type and quantity of the pollutant. The susceptibility of the local environment to the pollutant. Other conditions that may accentuate or lessen the effects of the pollutant. When hazardous liquid spills occur, they could cause: Fires or explosions. Exposure of humans, animals, and plants to toxic fumes, vapours, or clouds. Hazardous byproducts formed by reactions in the environment. Evidence of water pollution may show up as an unexpected increase or decrease in the population of one or more species. As well, sickness, deformity, and abnormal behaviour can occur in an otherwise normal population. Thousands of dead fish found along the banks of a river are an indicator of a hazardous spill. Solid pollutants may be picked up or dissolved by runoff due to precipitation, and then become liquid pollutants. Industrial dusts may become liquid pollutants in this way. Like gaseous and solid emissions, the allowable types and concentrations of liquid pollutant emissions from energy plants are usually regulated with a license or permit system. Permits will also reference allowed disposal processes and other permits that may be required. Power Engineers must ensure that the plants they operate meet these requirements. Thermal Pollution in Liquid Reservoirs One of the main factors to consider when choosing the location of a new power plant is the availability of a cooling water supply of sufficient quantity and quality. A condensing steam turbine requires large amounts of cooling water for use in the condenser. The water picks up considerable heat as it travels through the condenser and other related equipment. Then, it may discharge to its source at a higher temperature. This process causes thermal pollution. Other contributors to warm water discharge are industries with high cooling loads, including: • Steel mills • Pulp mills • Chemical processing plants • Cold storage facilities • Air conditioning cooling systems As water gets warmer, its ability to dissolve oxygen decreases (this is the “de-aeration principle”). The reduced percentage of dissolved oxygen can have lethal effects on many kinds of water life. These effects have been proven both by experiment and actual observation. While in tropical regions, aquatic life may survive in water as warm as 35°C, North American aquatic life cannot adapt to such a warm environment. Even in moderate quantities, power plant effluent may asphyxiate fish passing through it, especially in narrow streams. Elevated temperatures are common in areas below sewer discharge points. Some sections of rivers and lakes now remain free of ice year-round. This interrupts the normal migration patterns of waterfowl, partly because of accessibility to open water. Thermal stratification can occur in lakes where wind and currents are not sufficient to cause mixing of incoming warm water. Stratification may trap other pollutants in layers at varying depths in a lake. These thermally separated layers are called thermoclines. In lakes, thermoclines can have layers with insufficient oxygen. The release of thermal effluent into natural waters is regulated both at the federal, provincial and territorial levels. Regulations usually focus on limiting thermal releases, in order to not exceed the permitted maximum weekly average temperature (MWAT). To ensure that energy plants have some flexibility in their effluent streams, artificially created lakes provide initial cooling to the released water. These lakes are generally located between the plant and the ultimate point of discharge. Liquid wastes are not pollutants, unless they escape containment, enter the environment, and cause adverse effects. Proper treatment before the liquid waste enters the environment can prevent pollution. Occasionally, effluent can be cleaned up to the point that part of it can be reused rather than dumped. This process also reduces the amount of fresh source water required. Some plants,notably in the pulp and paper and oil sand industries, are now being designed so that most or all of their wastewater can be reused, producing “near zero effluent.” The following principles and devices can be used to reduce the effects of liquid pollution: • pH control • settling ponds • vacuum filters • grease traps pH Control Effluents dumped into an existing stream should have a similar pH value to the receiving water. A deviation from this existing pH can damage life in the stream. Industries generating strongly alkaline or acidic effluents must neutralize the effluent before it is released. Even within cities or municipalities with common sewer collection systems, companies are required to maintain their wastewater discharge between given pH values. Sewer line sampling monitors can detect sources of pH deviation. Fines are levied against the offenders. Companies that produce high or low pH waste streams monitor and neutralize their effluent before it enters the sewer system or waterway. One common method of control uses a dilution tank or pond. If monitors detect a surge of acid, a chemical pump adds enough alkaline material to bring the effluent back to the desired pH. If the system shows an alkaline deviation, an acid pump is used to bring the pH under control. When the flow is neutralized, it can be discharged. Settling Ponds Some industries discharge particulate laden water from the process. In years past, it was common practice to dump the discharge into the nearest body of water and forget about it. The downstream effects did not concern management. However, through responsible management, regulatory requirements, and environmental awareness, this practice has rapidly changed. One method that improves this problem is the use of settling ponds or tanks. Effluent is allowed to flow slowly through a settling pond, where particulate matter settles to the bottom. The clean effluent is then dumped into the body of water. Very fine particles or those in colloidal solution are too small to settle out in a settling pond. Industries that generate large quantities of colloidal material must use coagulants to help cluster these small particles together so that they will settle. This process is also used in some water supplies to cause accelerated settling of fine material. Occasionally, effluent can be cleaned up to the point that part of it is cycled back to the water intake rather than being dumped. This process also reduces the amount of water required from the source. Some plants, notably in the pulp and paper industry, are now being designed so that most or all of their wastewater can be reused, thus attaining “zero effluent” targets. Oil sands mining operations in northern Alberta continually recycle over 80-95 percent of the water they use. A positive side effect is that some of the settled material formerly lost with effluent flow can be retained. This material may be returned to the process, where it is used rather than lost. This could create increased profits because of improved efficiency. Vacuum Filters Another method to capture particles is the vacuum filter. Water flows inward through a fine mesh filter, formed into the shape of a horizontal cylinder. The cylinder is partially submerged in an effluent tank. The cylinder slowly rotates, and material gathers on the outside of the filter. A vacuum is maintained inside the cylinder and causes dewatering of the filtered particles as they rotate above the water line. The material that is on the filter is scraped or blown off by an air jet and collected. The cleaned filter then rotates down into the effluent tank again, to filter more water. The filtered water on the inside of the cylinder is either pumped away for recycling or dumped if no further treatment is required. Grease Traps Some plant effluent may contain materials that float on the surface of the water, such as oils and greases. When the density of the materials is much lower than that of water, the material floats readily and can be separated by a simple skimming process, which may be accomplished using grease traps. In a grease trap, water carrying oil or grease enters the chamber where a calm area allows separation to occur by gravity. The grease floats on the top and clean water is conducted away through the bottom outlet pipe. The grease or oil is then removed as it accumulates on top of the water. Large units may have several chambers in series to assure better capture. A surface skimming mechanism may also be used for continuous oil extraction. When the density difference between the water and the material is minimal, or when the material is emulsified with the water, a long calm period may not be enough to cause separation. In some cases, additives may be used to help break the emulsion so that separation can occur by gravity. Another method is to use centrifugal separation. A centrifuge spins material at high speed, multiplying gravitational force hundreds or even thousands of times. This process is similar to the way that cream is separated from milk in a separator. Other Methods When liquids spill, they can travel great distances. This adds to the cost of control, clean up, and disposal. The use of evaporators or crystallizers can eliminate the liquid component of the waste. These devices leave behind a solid material that is easier to handle and cheaper to dispose. The keys to limiting the effects of an uncontrolled release of liquid are to contain or immobilize the material and neutralize or lower its hazard potential. Effective isolation may require a specific process to be housed in its own room or building with a dedicated water supply, ventilation, and effluent treatment. Containment dikes or catch basins are used to collect any excursions from the process area until neutralized. Increased training and vigilance on the part of operators is a most effective method of pollution control. Earthen or concrete berms are often used to contain releases from large vessels. Berms must be adequate in height so that bermed-in areas are capable of holding 110 percent of the vessel volume, according to Environment and Climate Change Canada and the United States Environmental Protection Agency (USEPA). This volume requirement may increase, depending on local precipitation patterns and frequency of containment inspections. The 110 percent standard may not be sufficient for larger storm events. It is the responsibility of the owner or operator to determine if additional containment capacity is needed to contain rain. Preventive Measures – Thermal Pollution Cooling ponds are effective at preventing thermal pollution. If cooling water can be supplied in unlimited quantities, and there is sufficient flat space around the power plant, a cooling pond system might be the answer. The arrangement shown in Figure 6 uses two ponds in series, although only one might be used at a time. When both ponds are used in series, then valves A, C, F, and G are open, while B, D, and E are closed. High temperature water enters through A and its temperature is recorded with a thermometer at location “a”. The water is then sprayed into the main pond where it is discharged in a fine spray above the surface. The droplets that are exposed to the atmospheric air not only cool down, but also have a chance to enrich themselves with oxygen. A barrier, as shown, maximizes the time the water spends in the main pond. The water exits through valve C and its temperature is recorded at location “b.” The water then enters the suction of the auxiliary pump. The spraying process is repeated in the auxiliary pond and the temperature is recorded at location “c.” Finally, the cooled water is discharged into the river or lake. Cooling Towers When a plant site cannot accommodate the installation of cooling ponds, cooling towers may be used instead. These towers allow most of the cooling water required by the plant to be recycled. Although the towers can cut down on thermal pollution, there are other considerations. If nearby industrial flue gas emissions contain SOX, the water vapour discharging from cooling towers can contribute to localized acid rain. This causes damage to aquatic and plant life, and may also damage metal structures. Another concern is the discharge of chemically treated water into the environment. The various chemicals used to keep the cooling tower water from developing bacteria and mold, or to prevent wood decay and metal damage, can have adverse effects on downstream aquatic organisms. The plant must treat this effluent before it can be discharged. Environment and Climate Change Canada defines “disposal” and “treatment” as follows: Disposal: The final disposal to landfill, land application or underground injection, either on the facility site or at a location off the facility site; transfer to a location off the facility site for storage or treatment prior to final disposal; or movement into an area where tailings or waste rock are discarded or stored, and further managed to reduce or prevent releases to air, water or land, either on the facility site or at a location off the facility site. The disposal of a substance is different from a direct release to air, water or land. Treatment: Subjecting a substance to physical, chemical, biological or thermal processes at a location off the facility site prior to final disposal. Several methods of disposal have been used through the years. Landfill was a quick and easy way to hide thousands of drums of hazardous material. The risk was not well assessed, as numerous old landfill sites are now leaking dangerous materials into waterways. Encasement and burial at sea has been used, but this merely moves the hazard to another location. Deep well injection has been used to dispose of hazardous liquids into subsurface rock formations thousands of feet below the Earth’s surface. This practice is a vast improvement over landfills, but again the hazard is only moved to another location. The USEPA has restrictions regarding the land disposal of hazardous wastes (including Industrial and Municipal Waste Disposal Wells). The restrictions prohibit hazardous waste disposal unless the “waste has been treated to become non-hazardous or the disposer can demonstrate that the waste will remain where it has been placed for as long as it remains hazardous, which has been defined as 10,000 years by regulation.” Diluting liquid waste in order to stay within regulatory limits is not permitted. There are other technologies and strategies that treat liquid waste to make disposal more effective. These treatment methods include: • Physical treatment • Chemical treatment • Biological treatment • Thermal treatment Physical Treatment Non-hazardous liquid waste can often be treated by dewatering and sedimentation, using settling ponds. Water can be removed from solid components by centrifuges, filtration, or other similar processes. Centrifuges work on the same principle as cyclone separators, but can be more complex in operation. Centrifuges are used in industries such as wastewater treatment, oil and gas, and sewage treatment. Chemical Treatment In a chemical treatment, chemicals can be added to liquid waste to neutralize highly basic or acidic liquids. Chemicals that cause precipitation, coagulation, and flocculation of matter are commonly used in wastewater and water treatment plants. There are also many types of oxidation processes. These processes can change organic and inorganic compounds into less hazardous forms. Biological Treatment Industrial effluent and wastewater that have significant organic components may be treated by anaerobic digestion, which is a biological treatment. This reduces the amount of organic matter and the amount of waste. Thermal Treatment The only certain way to destroy many organic materials is through complete combustion. This is accomplished in high temperature incinerators, which is a form of thermal treatment. Environment and Climate Change Canada states: “incineration is a type of thermal treatment that is recognized as an effective and environmentally sound disposal method for a wider range of wastes”. Incinerators can destroy liquid waste; however, they might produce gaseous emissions that require further treatment. Even with incineration, close control and vigilance is necessary to assure complete destruction of the waste. Handling Liquid Waste Transportation of hazardous liquids for disposal must be done carefully, in compliance with regulations, and with attention to the possible consequences. Crude oil, gasoline, liquefied petroleum gas, solvents, fuel oils, lubricating oils, heat transfer oils, and hundreds of liquid products present unique hazards if spills or leaks occur. Spills in lakes, streams, oceans, or environmentally fragile areas cause environmental damage, even if relatively small amounts are involved. Each liquid must be handled according to information on current material safety data sheets. Neutralizing cleanup and disposal procedures should be followed. Appropriate clothing, tools and containers must be employed and personnel working on the cleanup must be thoroughly decontaminated and monitored for effects—even after the cleanup is complete. Operation of industrial plants provides an opportunity for solid and liquid waste to harm the environment. Knowing from where these emissions and effluents originate is the first step in controlling them. An understanding of how waste treatment and handling systems are designed to work, and to what waste product they are best applied, is essential. With this knowledge, Power Engineers can operate these systems properly and according to regulatory requirements. Careful selection, operation, and monitoring of emission and effluent control systems can minimize the impact of plant operation on the environment.