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Report on: Waste water treatment Submitted to: Dr. Sahar Mohamed El Marsafy Chemical Engineering Department Faculty of Engineering Cairo University Prepared by: Name: Amr El-sayed Ahmed Bassam Amr Ibrahim Dina Atef Shehata Mai Galal Moawad Serag Saaid Sabery Sherif Ezz El-din Sherif Magd El-din section: 3 1 2 4 2 2 2 June , 2009 B.N: 3 40 9 13 28 36 38 Table of contents Summary List of figures List of tables Aim of work 1. Introduction 1.1 drinking water treatments 1.2 industrial waste water treatments 2. Drinking water treatment 2.1 treatment and conditioning 2.2 water treatment steps 2.3 disinfecting water 2.4 emergency disinfection measures 2.5 ultraviolet light disinfection 2.6 distillation disinfection 2.7 filtration 2.8activated carbon filtration 2.9 oxidizing filtration 2.10 neutralization filtration 2.11 reverse osmosis 2.12 fiber filters 2.13 water softeners 3 industrial water treatment 3.1 treatment of industrial waste water 3.2 solid removal 3.3 oils and grease removal 3.4 removal of biodegradable organics 3.5 activated sludge process 3.6 trickling filter process 3.7 treatment of other organics 3.8 treatment of acids and alkalis 3.9 treatment of toxic materials References page II III III IV 1 2 3 3 4 5 6 7 8 9 10 11 11 11 13 14 16 16 16 16 19 20 20 22 22 23 24 I Summary Water is extremely vital for man-kind, either for daily usage or in industrial processes; regarding limited water supplies Water treatment is necessary to acquire maximum benefit of available water. Various processes of water treatment are used to remove existing contaminants from the used water: Drinking water treatment helps ensuring health and safety, Industrial waste water treatment is used to reduce and filter harmful contaminants before unleashing in the public sewer system. No water is 100 percent pure. It contains contaminants from natural and man-made sources, such as minerals, gases, bacteria, metals and chemicals. Many of these contaminants are harmless. However, some impurities can adversely affect your health. Others damage equipment, stain laundry and fixtures and emit odors. And for the treatment of water for drinking and day using purposes, the process should be carried in a manner that insure the complete removal of any harmful impurities, that can have bad effects on public health, as mentioned above, no water is 100 percent pure, but the report presented, stood on many methods that can be used to guarantee producing pure water safe to be used for drinking and daily activities. II List of figures and tables : Figures: page Fig.1 API 18 Fig.2 Typical parallel plate separator 19 Fig.3 19 Fig.4 & Fig.5 trickling filter 21 Tables: Table 1 Typical Water Quall Problems and Recommended Treatment Systems (page .12) Table 2 Recommended Amounts of Laundry Bleach for Well Disinfection Height of standing (page. 13) III Aim of Work Effort is boosted towards studying state of the art in waste water treatment, and the drinking water treatment, and to show both methods and techniques used for each case, also to stand on the most reliable data and figures that show how each method work. As for choosing whether or not to carry on recycling waste water from industrial sources, it’s very important for any industrial unit or plant, to investigate the probability of reusing the waste water resulting from the industrial process, and through further search and analysis, calculating coast and benefits, reusing waste water can become effective for the whole process, and help save any nearby natural water sources. And for the treatment of water for drinking and day using purposes, the process should be carried in a manner that insure the complete removal of any harmful impurities, that can have bad effects on public health, as mentioned above, no water is 100 percent pure, but the report presented, stood on many methods that can be used to guarantee producing pure water safe to be used for drinking and daily activities. IV 1. Introduction In a modern society, water is one of the things in life we often take for granted. When we turn on the tap, we expect water that is clean, safe and suitable for all household tasks. But in recent years, there has been a growing concern about contamination of our water supplies. We hear unsettling news accounts that make us question the safety of our water. This publication provides a few guidelines for deciding whether you need to install a home water treatment system, and if so, how to select a system appropriate to your specific need. 1.1. DRINKING WATER TREATMENT No water is 100 percent pure. It contains contaminants from natural and man-made sources, such as minerals, gases, bacteria, metals and chemicals. Many of these contaminants are harmless. However, some impurities can adversely affect your health. Others damage equipment, stain laundry and fixtures and emit odors. The first step in eliminating exposure to water-borne contaminants is to assess your situation. If your water comes from a public or municipal system, it is regularly tested for contaminants regulated by federal and state standards, such as microbial pathogens, radioactive elements and certain toxic chemicals. These are the contaminants that affect the safety of water and may cause health problems. Since public and municipal systems are regulated, a home water treatment system is seldom needed for health protection. Water quality problems such as hardness, corrosively, foaming, staining or bad tastes, smell or color are undesirable. However, these factors do not necessarily make the water unhealthful. 1 1.2. Industrial wastewater treatment It covers the mechanisms and processes used to treat waters that have been contaminated in some way by anthropogenic industrial or commercial activities prior to its release into the environment or its re-use. Most industries produce some wet waste although recent trends in the developed world have been to minimize such production or recycle such waste within the production process. However, many industries remain dependent on processes that produce wastewaters 2 2. DRINKING WATER TREATMENT Methods used to improve the quality of water are referred to as treatment or conditioning. What is the difference? Water treatment refers to systems that reduce harmful contaminants in the water, dealing with health and safety of the water. High levels of coliform, nitrates, arsenic, lead and pesticides are examples of harmful contaminants that must be treated before water is safe to drink. 2.1. Treatment and Conditioning Systems Water conditioning refers to water problems that effect water taste, color, odor, hardness and corrosivity rather than health and safety. The presence of high levels of magnesium, calcium, iron, manganese and silt are common contaminants that require water conditioning. It is not uncommon to use both treatment and conditioning methods to improve water quality. Here is a list of possible treatment and conditioning methods: Mechanical or Sedimentation Filtration Activated Carbon Filtration Oxidation Filtration Neutralizing Filtration Reverse Osmosis or Membrane Filtration Distillation Ultra-violet Treatment Water Softener or Cation Exchange Chlorination Disinfection 3 2.2. Water Treatment Steps 1. Have water tested. 2. Remove fine sand, silt, clay and other particles, using a mechanical filter or sedimentation. 3. Treat bacterial contamination, using chlorination or other forms of disinfection. 4. Remove hydrogen sulfide gas and other odor-causing substances, using chlorination, an oxidizing filter, or activated carbon. 5. Remove insoluble iron and manganese particles using a mechanical filter; a water softener, for small amounts of dissolved iron and manganese; an oxidizing filter for higher amounts of dissolved iron and manganese; or a chlorinator followed by a mechanical filter or an activated carbon filter for very high amounts of dissolved iron and manganese. 6. Treat for hardness using a water softener. 7. Neutralize acidity using a neutralizing filter. 8. Remove volatile organic chemicals, trihalomethanes, certain pesticides and radon using an activated carbon filter. 9. Remove heavy metals, such as lead, mercury, arsenic or cadmium, with reverse osmosis units or a distiller. Let's look at several common treatment systems and the contaminants they are capable of removing to provide safe water. 4 2.3. Disinfecting Water Drinking water should be free of coliform bacteria. The EPA drinking water standards indicate that water should contain less than one coliform organism in 100 milliliters. If your coliform test was reported as contaminated, take these steps: 1. Disinfect your well 2. Resample the well and have a second coliform test run. 3. If possible, locate and correct the source of bacteria contamination. There are a number of ways of disinfecting water but remember, treatment of a water supply is a safety factor, not a corrective measure. Don't install a permanent means of disinfection unless you are sure the contamination originates from the groundwater and that it is not a temporary condition. The use of chlorine is the oldest and most common disinfection method for private water supplies. Chlorine is inexpensive and readily available, reliable, easy to use and monitor, and effective against most pathogenic bacteria, virus and cyst organisms. It also kills non-pathogenic iron, manganese and sulfur bacteria. Chlorine is also a strong oxidizing agent which causes a problem mineral such as soluble iron and manganese to change to an insoluble precipitate so it can be filtered from the water. For use in the home, chlorine is readily available as sodium hypochlorite commonly known as household bleach. This product contains 5 percent available chlorine. Chlorine is also available as calcium hypochlorite, which is sold in the form of dry pellets. In this form it has about 70 percent available chlorine. 5 2.4. Emergency Disinfection Measures During an emergency and to disinfect small quantities of water for drinking, cooking, and brushing teeth, temporary methods are sometimes necessary. These methods are not recommended for general or regular use. Boiling - First clear the water by letting it settle and filter through a clean cloth. Boil the water vigorously for one full minute; boiling will kill all bacteria. Over boiling, however, can concentrate chemical impurities such as nitrates. Store the boiled water in sterilized containers if possible. The boiled water will have a "flat" taste which you can eliminate by aeration. Chlorine Bleach - Add 10 drops of chlorine bleach to one quart of water mix the treated water thoroughly and let it stand for 30 minutes. If the water does not have a slight chlorine odor, repeat the dosage and allow it to stand for another 15 minutes. If the water tastes too strongly of chlorine, expose it to the air for a few hours or pour it from one container to another several times. Granular Calcium Hypochlorite - Add a heaping teaspoon of calcium hypochlorite to 2 gallons of water. Use one pint of this chlorine solution in 200 gallons of water. Chlorine Tablets (Halazone) - Add 1 tablet per quart of water. Be sure to read the manufacturer's directions carefully. Iodine Tablets - Add 1 tablet per quart of water - check label for quantity to use. 6 2.5. Ultraviolet Light Disinfection Ultraviolet light is a relatively new method of disinfecting private water systems. Ultraviolet radiation adds nothing to the water and does not produce any taste or odor. The UV light is produced by a mercury vapor lamp which produces a disinfecting dose rated in microwatt-seconds per square centimeter (MWs/cm2). Values of 20,000 MWs/cm2 will kill most types of pathogenic bacteria. However, viruses are more resistant and variable and may need up to 45,000 MWs/cm2... To be effective as a disinfection treatment, ultraviolet radiation must pass through every particle of water. The thinner the water film and the slower the water flow, the more effective the system will be. Also, the water cannot have any turbidity, suspended soil particles, or organic matter. As a consequence, ultraviolet light treatment should only be attempted on clear water. A profiler is recommended on ultraviolet systems as is periodic inspection and lamp cleaning. As a result, without regular inspection and maintenance, water treated by UV cannot be guaranteed to be bacteria free. In fact, one major distributor of the system adds this disclaimer of liability to its literature: "This product is designed for use only on water known to be of acceptable bacterial quality. Its intended use is as a safety device on private and non-municipal potable supplies already identified as safe to drink. If you think your water may be unsafe, please contact your local public health agency." The cost will be at least $900 for a 47,000 microwatt-second household UV treatment system with a flow rate of 8 gallons per minute. 7 2.6. Distillation Disinfection Distillation provides another water disinfection option as a pointof-use system. Distillers are also used to reduce nitrates, remove dissolved salts like chlorides, sulfates and carbonates of sodium, potassium and magnesium, organic matter and other soluble and suspended materials. Distillation units boil water, making steam that is condensed and collected as purified water. Home distillers vary in design. However, the countertop singlebatch version is most common. These distillers cost $250 to $1,200. All home electric distillers use 100-120 volt a.c. current. The water output of a home distiller ranges from 3 to 12 gallons per day. The power consumption of these systems varies from 3 to 5 kilowatt hours of electricity per gallon of distilled water produced. Thus the electric cost of distilled water can be high. There are drawbacks to distillation. The most serious is that liquids with organic molecules whose boiling point is less than that of water will be carried with vapor into the condensate chamber and distillate reservoir. Chloroform, phenol and trichloroethylene have been found in the finished water. Because distilled water is mineral free, it tends to taste flat and is hard on metals. Draw water directly from the distiller and not through any metal piping. Distiller tanks, if not properly used and maintained, can also become notorious breeders of bacteria because of the presence of warm water. To remove bacteria and concentrated salts, disinfect and clean regularly. 8 2.7. Filtration Filtration represents a broad category of treatment systems used to remove particles, taste, odor, some organics and minerals, and some bacteria from the water. Filtration systems fall into several categories: Mechanical or Sedimentation Filtration Oxidation Filtration Neutralizing Filtration Reverse Osmosis or Membrane Filtration In many cases, these filtration systems are an integral component of other water treatment systems. To remain effective, all filtration systems must be regularly inspected and maintained. Mechanical or sedimentation filters simply retain debris as water passes through the filter unit. Mechanical filters are most effective for removing particles such as sand, silt, ferric iron, algae and some bacteria. Their effectiveness will depend on the particle size and the exit clearance of the filter. 9 2.8. Activated Carbon Filtration Activated carbon filtration is a common treatment to remove offensive tastes and odor, color, chlorine and volatile organic chemicals, pesticides and trihalomethanes (a group of suspected carcinogens). Activated carbon will not remove bacteria, dissolved metals such as iron, lead, manganese and copper, or chlorides, nitrates and fluorides. Activated carbon filters, usually made up of granulated, powdered or block carbon, act like a sponge with a large surface area to absorb contaminants in the water. Activated carbon, made from coal and nutshells, has a tremendous surface area - as much as 125 acres per pound of carbon. The efficiency of an activated carbon filter is dependent on the amount of carbon in it and the flow rate of water through the filter. The typical carbon filter is about 10 inches high and 3 inches in diameter - enough charcoal to treat about 1,000 gallons of water. Home-use activated carbon filters are most commonly sold as faucet-mounted, stationary and line bypass systems. The stationary system is connected directly to the cold water faucet. The line bypass system has a separate faucet, but is tapped into the cold water pipe for its water supply. These systems are typically installed under the sink and can be purchased for $50 to $375. Pour-through carbon filters are also on the market. However, due to the small amount of carbon and minimal water contact time with the carbon, this system has limited effectiveness. This is also true of the faucet-mounted carbon filter costing about $25. A carbon unit for radon removal resembles a water softener tank and costs about $1,500 plus installation. Without periodic replacement, carbon filters may provide a breeding ground for bacteria. To assure maximum effectiveness, replace any carbon filter frequently, following the manufacturer's recommendations. 10 2.9. Oxidizing Filtration Oxidizing filters are used mainly to remove iron, manganese and hydrogen sulfide. Manganese greensand is a common chemically reactive medium designed to remove iron and manganese that is in solution. The greensand will also act as a filter and catch iron and manganese precipitates that have been oxidized. The unit works by providing oxygen to the iron and manganese from the greensand bed. As a result, these minerals change from their soluble to insoluble form. The precipitated minerals become trapped as rust particles within the greensand filter bed. To remain effective, manganese greensand filters must be periodically backwashed to thoroughly remove iron precipitate, and regenerated when the oxygen is depleted. 2.10. Neutralizing Filtration Neutralizing filters are typically used for pH modification, or treating acidic water. A neutralizing filter is normally a pressure filter tank filled with limestone chips. As the water passes through the filter bed, calcium carbonate is dissolved into the water and the water pH is increased, reducing its acidity. 2.11. Reverse Osmosis Filtration Reverse Osmosis or "R.O." filtration systems for home water treatment are relatively new, although the process has been used extensively for industrial processes. Reverse osmosis treatment decreases dissolved minerals in the water. It successfully treats water with high salt content, and dissolved minerals such as nitrate, sulfate, calcium, magnesium, potassium, manganese, aluminum, fluoride, silica, boron and bicarbonate. R.O. is also effective with some taste, color and odor-producing chemicals, certain organic contaminants, and specific pesticides. 11 Some systems treat all the water in the house, while others primarily improve safety and quality of drinking water. Before buying water-treatment equipment, have your water supply tested by a recognized, certified water-testing lab. You need to identify the type and level of contaminants if you are to get the rightsystem. Table 1. Typical Water Quall Problems and Recommended Treatment Systems Recommended Treatment Problem Systems Disinfection Bacteria and other Disinfection microorganisms Taste and odor Carbon filter Oxidizing filter followed by carbon Hydrogen sulfide gas (rotten filter; chlorination followed by egg odor) sediment filter Sediment (suspended particles) Fiber filter Hardness (calcium and Softener magnesium) Softener for up to 5 milligrams per liter); Iron filter; chlorination Dissolved iron followed by sand filter and carbon filter Neutralizing filter or chemical-feed pH (acid or alkaline conditions) pump Organic chemicals (pesticides, Carbon filter fuel products) Metals (lead, mercury, arsenic, cadmium), and other minerals Reverse osmosis unit; distillation (nitrate, sulfate, sodium) 12 2.12. Fiber Filters Fiber filters contain spun cellulose or rayon. They remove suspended sediment (or turbidity). The water pressure forces water through tightly wrapped fibers around a tubular opening leading to the faucet. These filters come in a variety of sizes and meshes from fine to course, with the lower micron rating being the finer. The finer the filter, the more particles are trapped and the more often the filter must be changed. Fiber filters may not remove all contaminants. If taste and odor problems remain, use a carbon filter after the fiber filter. Fiber filters and replacement cartridges range in price from a few dollars to several hundred dollars. Remember, filters do not purify or soften water - they only remove some suspended particles and dissolved organic compounds that cause disagreeable odors and tastes. . Table 2. Recommended Amounts of Laundry Bleach for Well Disinfection Height of standing Height of 4-inch 6-lnch 8-inch 12-inch 24-inch standing water well well well well well (feet) 50 1 quart 2quarts 1 gallon 2 gallons 8 gallons 2 16 100 2 quarts 1 gallon 4 gallons gallons gallons 4 32 200 1 gallon 2 gallons 8 gallons gallons gallons 13 2.13. Water Softeners Hard water is caused by dissolved calcium and magnesium in the water. Hard water interferes with laundering, washing dishes, bathing, and personal grooming. It also affects appliances. For example, scale builds up in water heaters, increasing the costs of heating water and reducing the life of the appliance. The calcium and magnesium that cause hardness are reported as grains per gallon, milligrams per liter (mg/ L), or parts per million (ppm). Hard water, when used with soap, causes soap deposits that will not dissolve. Water is softened by passing through a bed of ion-exchange resin. The softening process exchanges calcium and magnesium ions in the water for sodium ions in the resin. About 15 mg of sodium are added per gallon for each grain of hardness reduced. When the sodium is used up, the softener needs to be regenerated. This is done by backwashing to clean the ion-exchange material, brining with salt (sodium chloride) to replace sodium ions, and rinsing to remove any excess salt. A water softener removes small amounts of dissolved iron (5 to 10 ppm). However, if there is oxidized iron or iron bacteria in the water, the ion exchange resin becomes coated or clogged and loses its softening ability. In this case, use an iron filter or chlorination to remove iron. The size water softener needed depends on the hardness of water, the quantity to be softened, and the length of time between recharging. There are three types of ion-exchange softeners for the home. 14 MANUAL. Each step for recharging the unit must be activated by hand. Salt is added directly to the single tank of this softener. SEMI-AUTOMATIC. The homeowner sets the switches when the system needs recharging. The system completes the process by itself. A second tank is needed for the brine system. AUTOMATIC. All steps of the recharging process are controlled by a timing mechanism that the homeowner sets, based on water usage. Some models can measure water usage or remaining softening capacity and recharge themselves only when needed. Most water softeners have a fully automatic recharging feature. These softeners also require a second tank for the brine solution. Water softeners can be installed in various ways. Most people soften hot and cold water but bypass outside water lines. The increased sodium in softened water is a concern to people on a sodium-restricted diet. Therefore, some water softener installations bypass the cold water line in the kitchen only. Water softeners can be rented or purchased. Renting a softener or ion-exchange resin tank is convenient since the user does not worry about maintenance or regeneration. The dealer regularly replaces the ion-exchange resin tank, so a second tank for the brine solution for recharging is not needed. A water softener can cost $500 to over 41,500, but owning the equipment could be more economical in the long run than renting it. The cost of the water softener is balanced against the savings of soft water. Using soft water reduces the quantity of cleaning products needed by as much as 500 percent. The home's plumbing system and water-using appliances will last longer. Other benefits include the time saved in cleaning and removing scale and better results in laundry, dish washing, and personal grooming 15 3. INDUSTRIAL WATER TREATMENT Most industries produce some wet waste although recent trends in the developed world have been to minimize such production or recycle such waste within the production process. However, many industries remain dependent on processes that produce wastewaters 3.1. Treatment of industrial wastewater The different types of contamination of wastewater require a variety of strategies to remove the contamination. 3.2. Solids removal Most solids can be removed using simple sedimentation techniques with the solids recovered as slurry or sludge. Very fine solids and solids with densities close to the density of water pose special problems. In such case filtration or ultrafiltration may be required. Alternatively, flocculation may be used, using alum salts or the addition of polyelectrolytes. 3.3. Oils and grease removal Many oils can be recovered from open water surfaces by skimming devices. However, hydraulic oils and the majority of oils that have degraded to any extent will also have a soluble or emulsified component that will require further treatment to eliminate. 16 Dissolving or emulsifying oil using surfactants or solvents usually exacerbates the problem rather than solving it, producing wastewater that is more difficult to treat. The wastewaters from large-scale industries such as oil refineries, petrochemical plants, chemical plants, and natural gas processing plants commonly contain gross amounts of oil and suspended solids. Those industries use a device known as an API oil-water separator which is designed to separate the oil and suspended solids from their wastewater effluents. The name is derived from the fact that such separators are designed according to standards published by the American Petroleum Institute (API). The API separator is a gravity separation device designed by using Stokes Law to define the rise velocity of oil droplets based on their density and size. The design is based on the specific gravity difference between the oil and the wastewater because that difference is much smaller than the specific gravity difference between the suspended solids and water. The suspended solids settles to the bottom of the separator as a sediment layer, the oil rises to top of the separator and the cleansed wastewater is the middle layer between the oil layer and the solids. Typically, the oil layer is skimmed off and subsequently re-processed or disposed of, and the bottom sediment layer is removed by a chain and flight scraper (or similar device) and a sludge pump. The water layer is sent to further treatment consisting usually of a dissolved air flotation (DAF) unit for additional removal of any residual oil and then to some type of biological treatment unit for removal of undesirable dissolved chemical compounds. 17 API(Fig.1) Parallel plate separators are similar to API separators but they include tilted parallel plate assemblies (also known as parallel packs). The parallel plates provide more surfaces for suspended oil droplets to coalesce into larger globules. Such separators still depend upon the specific gravity between the suspended oil and the water. However, the parallel plates enhance the degree of oil-water separation. The result is that a parallel plate separator requires significantly less space than a conventional API separator to achieve the same degree of separation. 18 Typical parallel plate separator (fig 2) 3.4. Removal of biodegradable organics Biodegradable organic material of plant or animal origin is usually possible to treat using extended conventional wastewater treatment processes such as activated sludge or trickling filter. Problems can arise if the wastewater is excessively diluted with washing water or is highly concentrated such as neat blood or milk. The presence of cleaning agents, disinfectants, pesticides, or antibiotics can have detrimental impacts on treatment processes. Fig.3 19 3.5. Activated sludge process Activated sludge is a biochemical process for treating sewage and industrial wastewater that uses air (or oxygen) and microorganisms to biologically oxidize organic pollutants, producing a waste sludge (or floc) containing the oxidized material. In general, an activated sludge process includes: An aeration tank where air (or oxygen) is injected and thoroughly mixed into the wastewater. A settling tank (usually referred to as a "clarifier" or "settler") to allow the waste sludge to settle. Part of the waste sludge is recycled to the aeration tank and the remaining waste sludge is removed for further treatment and ultimate disposal. 3.6. Trickling filter process A trickling filter consists of a bed of rocks, gravel, slag, peat moss, or plastic media over which wastewater flows downward and contacts a layer (or film) of microbial slime covering the bed media. Aerobic conditions are maintained by forced air flowing through the bed or by natural convection of air. The process involves adsorption of organic compounds in the wastewater by the microbial slime layer, diffusion of air into the slime layer to provide the oxygen required for the biochemical oxidation of the organic compounds. The end products include carbon dioxide gas, water and other products of the oxidation. As the slime layer thickens, it becomes difficult for the air to penetrate the layer and an inner anaerobic layer is formed. The components of a complete trickling filter system are: fundamental components: 20 A bed of filter medium upon which a layer of microbial slime is promoted and developed. An enclosure or a container which houses the bed of filter medium. A system for distributing the flow of wastewater over the filter medium. A system for removing and disposing of any sludge from the treated effluent. The treatment of sewage or other wastewater with trickling filters is among the oldest and most well characterized treatment technologies. A trickling filter is also often called a trickle filter, trickling biofilter, biofilter, biological filter or biological trickling filter. Fig .4 (Trickling filter) Fig .5 A schematic cross-section of the contact face of the bed media in a trickling filter a typical complete trickling filter system 21 3.7. Treatment of other organics Synthetic organic materials including solvents, paints, pharmaceuticals, pesticides, coking products and so forth can be very difficult to treat. Treatment methods are often specific to the material being treated. Methods include Advanced Oxidation Processing, distillation, adsorption, vitrification, incineration, chemical immobilisation or landfill disposal. Some materials such as some detergents may be capable of biological degradation and in such cases; a modified form of wastewater treatment can be used. 3.8. Treatment of acids and alkalis Acids and alkalis can usually be neutralized under controlled conditions. Neutralization frequently produces a precipitate that will require treatment as a solid residue that may also be toxic. In some cases, gasses may be evolved requiring treatment for the gas stream. Some other forms of treatment are usually required following neutralization. Waste streams rich in hardness ions as from de-ionization processes can readily lose the hardness ions in a buildup of precipitated calcium and magnesium salts. This precipitation process can cause severe furring of pipes and can, in extreme cases, because the blockage of disposal pipes. A 1 meter diameter industrial marine discharge pipe serving a major chemicals complex was blocked by such salts in the 1970s. Treatment is by concentration of de-ionisation waste waters and disposal to landfill or by careful pH management of the released wastewater. 22 3.9. Treatment of toxic materials Toxic materials including many organic materials, metals (such as zinc, silver, cadmium, thallium, etc.) acids, alkalis, non-metallic elements (such as arsenic or selenium) are generally resistant to biological processes unless very dilute. Metals can often be precipitated out by changing the pH or by treatment with other chemicals. Many, however, are resistant to treatment or mitigation and may require concentration followed by landfilling or recycling. Dissolved organics can be incinerated within the wastewater by Advanced Oxidation Processes. 23 References: 1. ^ European Environment Agency. Copenhagen, Denmark. "Indicator: Biochemical oxygen demand in rivers (2001)." 2. ^ a b Tchobanoglous, G., Burton, F.L., and Stensel, H.D. (2003). Wastewater Engineering (Treatment Disposal Reuse) / Metcalf & Eddy, Inc. (4th ed.). McGraw-Hill Book Company. ISBN 0-07-041878-0. 3. ^ a b c d Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons. LCCN 67019834. 4. ^ American Petroleum Institute (API) (February 1990). Management of Water Discharges: Design and Operations of Oil-Water Separators (1st ed.). American Petroleum Institute. 5. ^ a b Beychok, Milton R. (December 1971). "Wastewater treatment". Hydrocarbon Processing: 109–112. ISSN 0818-8190. 6. http://en.wikipedia.org/wiki/Water_treatment( cited in 19 June 2009 ) 24