UA5 C3 Liquid and Solid Emissions Obj 5.docx
Transcript
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 d...
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.
