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Post by Robot on Jul 27, 2004 10:15:37 GMT -5
Nutrients: Nutrients are usually thought of as compounds of nitrogen or phosphorus, although certainly other elements, such as iron, magnesium, and potassium are also necessary for bacterial and plant growth.
Nitrogen occurs primarily in the oxidized forms of nitrates (NO3-) and nitrites (NO2-) or the reduced forms of ammonia (NH3) or "organic nitrogen"-- where the nitrogen is part of an organic compound such as an amino acid, a protein, a nucleic acid, or one of many other compounds. All of these can be used as nutrients, although the organic nitrogen first needs to decompose to a simpler form .
Phosphorus is biologically important in the form of phosphate, the most highly oxidized state of the element. The most biologically available form is dissolved orthophosphate, (PO4-3). (In solution, there are up to three hydrogens attached to the molecule, each one decreasing the negative charge of the ion by one. How many hydrogens are attached depends on the pH.) There are also condensed forms of phosphate, with more than one phosphorus atom per ion, such as pyrophosphate and polyphosphates. There are also organic phosphates, and all of these forms can be either dissolved or particulate (i.e., insoluble). The sum of all the forms is known as total phosphorus.
Significance: These nutrients are important in natural waters because, in excess, they can cause nuisance growth of algae or aquatic weeds. In wastewater treatment, a deficiency of nutrients can limit the effectiveness of biological treatment processes. In some plants treating industrial wastewaters, ammonia or phosphoric acid must be added as a supplement.
Measurement: Ammonia can be measured colorimetrically, by the Nessler or phenate methods, after distillation from an alkaline solution to separate it from interferences. It can also be determined by an electrode method, sometimes without distillation, since there are fewer interferences. Organically-bound, reduced nitrogen can be determined by the same methods after a digestion (the Kjeldahl digestion) which converts the nitrogen in those compounds to ammonia. The combination of ammonia and organic nitrogen is known as "Total Kjeldahl Nitrogen," or TKN. (TKN analysis is used for measuring protein content of animal feeds, as well.) Nitrite is determined colorimetrically. Nitrate can also be determined this way; the most popular way is by first reducing nitrate to nitrite chemically using cadmium, then analyzing the nitrite. There is an electrode method for nitrate, but it is not considered too accurate. Finally, ammonia (as the positively charged ammonium ion, NH4+), nitrate, and nitrite can be measured by ion chromatography, as well.
Phosphate can be measured by ion chromatography, also. Greater sensitivity, at lower cost, is obtained by colorimetric methods which measure dissolved orthophosphate. Some insoluble phosphates and condensed phosphates-- so called "acid-hydrolyzable phosphate"-- can be included by heating the sample with acid to convert these forms to orthophosphate. If the organic phosphate is to be included, to measure "total phosphate", then the sample must be digested with acid and an oxidizing agent, to convert everything to the orthophosphate form.
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Post by Robot on Jul 27, 2004 10:16:35 GMT -5
Chlorine: The pure element exists as the molecule, Cl2, which is a gas or a liquid at normal temperatures, depending on the pressure. When dissolved in water, most of it reacts to form hypochlorous acid (HOCl) and hydrochloric acid (HCl) which make the water more acidic. The HOCl dissociates, to some extent, to form H+ and OCl-, called hypochlorite ion. (The HCl dissociates completely.) If there is enough alkalinity to react with the hydrogen ions produced and keep the pH around neutral, most of the chlorine will be in the form of hypochlorous acid and hypochlorite ion. Disinfection can be done using solutions of sodium hypochlorite, which produce the same substances in solution. Hypochlorite ion is not considered as strong a disinfectant as HOCl, so the pH can affect the disinfectant efficiency. Dissolved chlorine, hypochlorous acid, and hypochlorite ion, taken together, are all known as "free chlorine". Free chlorine can react with ammonia in solution to form compounds called chloramines, which are weaker disinfectants than free chlorine, but have the advantage of not being used up by side reactions to the extent that free chlorine is. Free chlorine (and chloramines) also react with organic nitrogen compounds to form organic chloramines, which are even weaker disinfectants. The chloramines are termed "combined chlorine," and the sum of the free and combined forms are called "total chlorine." (Note that a large enough amount of chlorine can oxidize ammonia to nitrogen gas; this can be used as a chemical means of destroying ammonia.)
Significance: Chlorine is the most commonly used disinfecting agent for drinking water and wastewater. It is coming into some disfavor because of toxic and carcinogenic byproducts, such as chloroform, which are formed when it reacts with organic matter present in the water. Unless reduced to chloride, chlorine itself is toxic to aquatic life in receiving waters. Pure chlorine liquid or gas is also a storage and transportation hazard because of the possibility of accidental releases to the atmosphere. Some treatment plants are switching to hypochlorite solution because it is safer to handle. Others are eliminating it entirely and using UV light or ozone for disinfection.
Measurement: There are several choices for chlorine measurement, some of which can distinguish between free chlorine and the various chloramines. There are titrations involving visual, color-indicator endpoints, as well as electrochemically measured endpoints. Some of them can be used to differentiate among the various forms of chlorine depending on whether iodide ion is added to the testing mixture. The indicator known as DPD (full name, N,N-diethylparaphenylenediamine) can be used to measure free or total chlorine both colorimetrically or as a titration indicator. "Amperometric titration" is a sensitive electrochemical method.
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Post by Robot on Jul 27, 2004 10:19:18 GMT -5
Oil and Grease is the name given to a class of materials which can be extracted from water using certain organic solvents. They can be of biological origin (animal fat, vegetable oil); they can be "mineral" (petroleum hydrocarbons); or they can be synthetic organic compounds.
Fats and greases from restaurants and food processing industries can clog sewers, causing blockages and backups. Petroleum products can be toxic and flammable, and can coat surfaces and interfere with biodegradation by microorganisms in wastewater treatment plants. They are mostly biodegradable, especially biological oils and greases, but are a problem due to forming a separate phase from the water.
Measurement: The major method of analysis is liquid-liquid extraction. Currently, the chlorofluorocarbon known as CFC-113 is used, but is due to be phased out in favor of the hydrocarbon, hexane, because of the damage done by CFC's to the stratospheric ozone layer. In the procedure, the sample is acidified, and then shaken several times with the solvent. The solvent portions are combined and evaporated, and the residue is measured by weight. In a CFC solution, the concentration of the oil/grease can also be measured by infrared spectrophotometry without having to evaporate the solvent. To determine petroleum hydrocarbons alone, the extract solution can be treated with the material, silica gel, which absorbs the more polar biological compounds. A newer method, solid phase extraction, passes the water sample through a small column or filter containing solid sorbent material which absorbs the oil and grease. It is then desorbed from the sorbent using a solvent and analyzed as above.
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Post by Robot on Jul 27, 2004 10:20:20 GMT -5
A flame atomic absorption spectrophotometer in useMetals: Chemically, metals are classified as elements which tend to lose electrons in a chemical reaction. As solids, they have easily movable electrons, which makes them good conductors of electricity and reflectors of light. In compounds, they tend to be positively charged, because they have lost electrons (which carry a negative charge), and they tend to bind with non-metals. This tendency makes some of them, such as iron and magnesium, biologically useful as part of biochemically active compounds like enzymes. Others, such as lead, cadmium, and mercury are highly toxic because they interfere with the normal operation of these biological compounds. The US EPA lists nine metals used in industry (arsenic, cadmium, chromium, copper, lead, mercury, nickel, silver, and zinc) as toxic "priority pollutant" metals. Measurement: There are numerous colorimetric methods for metals. Most of them are more useful in a purer medium, such as drinking water, than they are in wastewater, because of the presence of interfering substances. The most popular methods in use today involve one form or another of atomic spectroscopy, as described previously. Another technique, X-ray spectroscopy, is useful primarily for solid samples. There are also electrochemical methods, like polarography and "anodic stripping voltametry" (ASV) which are quite sensitive; but due to their complexity, they are usually thought of as being confined mostly to research purposes.
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Post by Robot on Jul 27, 2004 10:20:53 GMT -5
Cyanide: Cyanide is the name of an ion composed of carbon and nitrogen, CN-. It is used in the mining and metal finishing and plating industries-- usually as the sodium or potassium salts, NaCN or KCN-- because of its ability to bind very strongly to metals to form water-soluble complex ions. This same property makes it highly toxic to living things because it prevents the normal activity of biologically important, metal-containing molecules. It is, however, biodegradable by some bacteria in low concentrations; and they can become acclimated to higher concentrations if given enough time. For unacclimated microorganisms in a wastewater treatment plant, however, a cyanide "dump" by an industry can lead to inhibition or even death, which can cause a severe "plant upset."
Measurement: Cyanides are usually measured by a sensitive colorimetric/ spectrophotometric procedure which can detect levels down to about 5 parts per billion in water. Since much of the cyanide in a sample is likely to be bound to metal ions, a digestion/distillation procedure is necessary to measure "total" cyanide. Cyanide can also be measured by ion chromatography or an electrode method, though the latter is not considered too accurate.
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Post by Robot on Jul 27, 2004 10:21:33 GMT -5
Toxic Organic Compounds: An organic compound is any compound which contains carbon, with the exception of carbon monoxide and carbon dioxide, carbonates, or cyanides. Organic compounds contain chains and/or rings of connected carbon atoms, often with other elements attached. There are millions of possible compounds, with many useful properties. Many are biologically active, since all living things are made up of organic molecules. Industries use and produce thousands of organic compounds in manufacturing such items as plastics, synthetic fibers, rubber, pharmaceuticals, pesticides, and petroleum products. Some of the compounds are starting materials; some are solvents; some are byproducts.
The US EPA lists 116 of them as toxic "priority pollutants"; many states have longer lists. One of the major groupings is volatile organic compounds (VOC's), many of which are chlorine-containing solvents. There are also petroleum hydrocarbons and starting materials for plastics, dyes, and pharmaceuticals. The "semi-volatile" group include solvents, PAH's (polycyclic aromatic hydrocarbons, like naphthalene and anthracene which are coal tar constituents), as well as pesticides (especially chlorinated pesticides) and PCB's (polychlorinated biphenyls, which were formerly used in electrical transformers and other products).
Measurement: Most of these are analyzed routinely by gas chromatography (GC), often followed by mass spectrometry (MS) for identification. HPLC is also used for some analytes. A technique which is becoming available for field measurements for some of these compounds is immunoassay, sometimes called ELISA, for "enzyme-linked immunosorbent assay." This method, which produces a color reaction related to the concentration of the target compound, or family of compounds, is portable, relatively inexpensive and does not require a great deal of training. It is in use more for surveying hazardous waste sites, however, than for water analysis.
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Post by Robot on Jul 27, 2004 10:22:23 GMT -5
Pathogenic microorganisms: Sewage contains large numbers of microbes which can cause illness in humans, including viruses, bacteria, fungi, protozoa and worms (and their eggs or ova). They originate from people who are either infected or are carriers. While many of these can be measured directly by microscopic techniques (some after concentration), the analyses most commonly performed are for so-called "indicator organisms." These organisms, while not too harmful themselves, are fairly easy to test for and are chosen because they indicate that more serious pathogens are likely to be present. For instance, wastewater treatment plants are often required to test their effluents for the group known as "fecal coliforms," which include the species E. coli, indicative of contamination by material from the intestines of warm-blooded animals. Water supplies test for a more inclusive group called "total coliforms", and in some cases, for general bacterial contamination (heterotrophic plate count, or HTP.)
Measurement: The two most commonly used methods of analysis for indicator organisms are the multiple tube fermentation technique and the membrane filter procedure.
In the first method, a number of tubes containing specific growth media are innoculated with different amounts of the sample and incubated for a particular time at a prescribed temperature. The appearance of colors, fluorescence, or gas formation indicates the presence of bacteria belonging to the target group.
The number of organisms per 100 mL in the original sample is estimated from most probable number (MPN) tables, which list the values of MPN for different combinations of positive and negative results in tubes which contained different initial volumes of the sample. Often, positive results must be confirmed by further innoculation of small amounts of material from the positive tubes into tubes containing a different media, which can extend the test to several days. The second technique involves filtering a known volume of sample through a membrane filter (made of a material such as cellulose acetate) which has a small enough pore size to retain the bacteria. The filter is then placed in a dish of sterile nutrient media, either soaked into an absorbent pad or in a gel such as agar, and sealed. The dish is incubated for the prescribed time and temperature. The media contain a colored indicator which will identify the target bacteria. Each bacterium in the original sample will result in a colony after incubation, which is large enough to see without a great deal of magnification. The concentration in the sample can be determined by direct count of the colonies, knowing the volume of sample used. In some cases, these colonies require further confirmation.
Detection and enumeration of HTP or of specific pathogenic bacteria, such as Salmonella, E. coli, or Enterococcus can be done by similar methods, but utilizing specific growth media for each type. Viruses are usually measured by concentration, followed by addition to cultures of cells which they infect and counting the number of plaques formed due to cell destruction. Pathogenic protozoa and ova of multicelled organisms are determined by concentration and direct counting under the microscope, often with the aid of fluorescent staining compounds.
Besides, direct observation, identification of pathogenic microorganisms can be done by standard techniques used in clinical laboratories involving observing reactions in a battery of different indicating media. Some newer methods use chromatography to identify patterns of compounds which serve as "fingerprints" for certain bacteria; DNA analysis is another recent innovation. Most wastewater treatment plants, however, confine their testing to simply counting the numbers indicator bacteria.
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Post by Robot on Jul 27, 2004 10:24:12 GMT -5
How is Wastewater Treated to Remove Pollutants?
Physics, Chemistry, Microbiology and Engineering are all involved in purifying wastewater so that it can be safely returned to the environment.
Wastewater treatment plants can be divided into two major types: Biological and Physical/Chemical.
Biological plants are more commonly used to treat domestic or combined domestic and industrial wastewater from a municipality. They use basically the same processes that would occur naturally in the receiving water, but give them a place to happen under controlled conditions, so that the cleansing reactions are completed before the water is discharged into the environment.
Physical/chemical plants are more often used to treat industrial wastewaters directly, because they often contain pollutants which cannot be removed efficiently by microorganisms-- although industries that deal with biodegradable materials, such as food processing, dairies, breweries, and even paper, plastics and petrochemicals, may use biological treatment. And biological plants generally use some physical and chemical processes, too.
A physical process usually treats suspended, rather than dissolved pollutants. It may be a passive process, such as simply allowing suspended pollutants to settle out or float to the top naturally-- depending on whether they are more or less dense than water. Or the process may be aided mechanically, such as by gently stirring the water to cause more small particles to bump into each other and stick together, forming larger particles which will settle or rise faster-- a process known as flocculation. Chemical flocculants may also be added to produce larger particles. To aid flotation processes, dissolved air under pressure may be added to cause the formation of tiny bubbles which will attach to particles.
Filtration through a medium such as sand as a final treatment stage can result in a very clear water. Ultrafiltration, nanofiltration, and reverse osmosis are processes which force water through membranes and can remove colloidal material (very fine, electrically charged particles, which will not settle) and even some dissolved matter. Absorption (adsorption, technically) on activated charcoal is a physical process which can remove dissolved chemicals. Air or steam stripping can be used to remove pollutants that are gasses or low-boiling liquids from water, and the vapors which are removed in this way are also often passed through beds of activated charcoal to prevent air pollution. These last processes are used mostly in industrial treatment plants, though activated charcoal is common in municipal plants, as well, for odor control.
Some examples of chemical treatment processes, in an industrial setting, would be converting a dissolved metal into a solid, settleable form by precipitation with an alkaline material like sodium or calcium hydroxide.
Dissolved iron or aluminum salts or organic coagulant aids like polyelectrolytes can be added to help flocculate and settle (or float) the precipitated metal. converting highly toxic cyanides used in mining and metal finishing industries into harmless carbon dioxide and nitrogen by oxidizing them with chlorine destroying organic chemicals by oxidizing them using ozone or hydrogen peroxide, either alone or in combination with catalysts (chemicals which speed up reactions) and/or ultraviolet light In municipal treatment plants, chemical treatment-- in the form of aluminum or iron salts-- is often used for removal of phosphorus by precipitation. Chlorine or ozone (or ultraviolet light) may be used for disinfection, that is, killing harmful microorganisms before the final discharge of the wastewater. Sulfur dioxide or sulfite solutions can be used to neutralize (reduce) excess chlorine, which is toxic to aquatic life. Chemical coagulants are also used extensively in sludge treatment to thicken the solids and promote the removal of water.
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Post by Robot on Jul 27, 2004 10:27:32 GMT -5
A typical treatment plant consists of a train of individual unit processes set up in a series, with the output (effluent) of one process becoming the input (influent) of the next process. The first stages will usually be made up of physical processes that take out easily removable pollutants. After this, the remaining pollutants are generally treated further by biological or chemical processes. These may 1) convert dissolved or colloidal impurities into a solid or gaseous form, so that they can be removed physically, or 2) convert them into dissolved materials which remain in the water, but are not considered as undesirable as the original pollutants. The solids (residuals or sludges) which result from these processes form a side stream which also has to be treated for disposal. A common set of processes that might be found at a municipal treatment plant would be: Preliminary treatment to remove large or hard solids that might clog or damage other equipment. These might include grinders (comminuters), bar screens, and grit channels. The first chops up rags and trash; the second simply catches large objects, which can be raked off; the third allows heavier materials, like sand and stones, to settle out, so that they will not cause abrasive wear on downstream equipment. Grit channels also remove larger food particles (i.e., garbage). Primary settling basins, where the water flows slowly for up to a few hours, to allow organic suspended matter to settle out or float to the surface. Most of this material has a density not much different from that of water, so it needs to be given enough time to separate. Settling tanks can be rectangular or circular. In either type, the tank needs to be designed with some type of scrapers at the bottom to collect the settled sludge and direct it to a pit from which it can be pumped for further treatment-- and skimmers at the surface, to collect the material that floats to the top (which is given the rather inglorious name of "scum".) The diagram below shows the operation of a typical primary settling tank.
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Post by Robot on Jul 27, 2004 10:28:46 GMT -5
Secondary treatment, usually biological, tries to remove the remaining dissolved or colloidal organic matter. Generally, the biodegradation of the pollutants is allowed to take place in a location where plenty of air can be supplied to the microorganisms. This promotes formation of the less offensive, oxidized products. Engineers try to design the capacity of the treatment units so that enough of the impurities will be removed to prevent significant oxygen demand in the receiving water after discharge. There are two major types of biological treatment processes: attached growth and suspended growth. In an attached growth process, the microorganisms grow on a surface, such as rock or plastic. Examples are 1) open trickling filters, where the water is distributed over rocks and trickles down to underdrains, with air being supplied through vent pipes, 2) enclosed biotowers, which are similar, but more likely to use shaped, plastic media instead of rocks, and 3) so-called rotating biological contacters, or RBC's, which consist of large, partially submerged discs which rotate continuously, so that the microorganisms growing on the disc's surface are repeatedly being exposed alternately to the wastewater and to the air. The most common type of suspended growth process is the so-called activated sludge system (see diagram below). This type of system consists of two parts, an aeration tank and a settling tank, or clarifier. The aeration tank contains a "sludge" which is what could be best described as a "mixed microbial culture", containing mostly bacteria, as well as protozoa, fungi, algae, etc. This sludge is constantly mixed and aerated either by compressed air bubblers located along the bottom, or by mechanical aerators on the surface. The wastewater to be treated enters the tank and mixes with the culture, which uses the organic compounds for growth-- producing more microorganisms-- and for respiration, which results mostly in the formation of carbon dioxide and water. The process can also be set up to provide biological removal of the nutrients nitrogen and phosphorus (see below). After sufficient aeration time to reach the required level of treatment, the sludge is carried by the flow into the settling tank, or clarifier, which is often of the circular design. (An important condition for the success of this process is the formation of a type of culture which will flocculate naturally, producing a settling sludge and a reasonably clear upper, or supernatant layer. If the sludge does not behave this way, a lot of solids will be remain in the water leaving the clarifier, and the quality of the effluent wastewater will be poor.) The sludge collected at the bottom of the clarifier is then recycled to the aeration tank to consume more organic material. The term "activated" sludge is used, because by the time the sludge is returned to the aeration tank, the microorganisms have been in an environment depleted of "food" for some time, and are in a "hungry", or activated condition, eager to get busy biodegrading some more wastes. Since the amount of microorganisms, or biomass, increases as a result of this process, some must be removed on a regular basis for further treatment and disposal, adding to the solids produced in primary treatment. (The type of activated sludge system that has just been described is a continuous flow process. There is a variation in which the entire activated sludge process take place in a single tank, but at different times. Steps include filling, aerating, settling, drawing off supernatant, etc. A system like this, called a sequencing batch reactor, can provide more flexibility and control over the treatment, including nutrient removal, and is amenable to computer control.)
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Post by Robot on Jul 27, 2004 10:29:20 GMT -5
Nutrient removal refers to the treatment of the wastewater to take out nitrogen or phosphorus, which can cause nuisance growth of algae or weeds in the receiving water.
Nitrogen is found in domestic wastewater mostly in the form of ammonia and organic nitrogen. These can be converted to nitrate nitrogen by bacteria, if the plant is designed to provide enough oxygen and a long enough "sludge age" to develop these slow-growing types of organisms. The nitrate which is produced may be discharged; it is still usable as a plant nutrient, but it is much less toxic than ammonia. If more complete removal of nitrogen is required, a biological process can be set up which reduces the nitrate to nitrogen gas (and some nitrous oxide). There are also physical/chemical processes which can remove nitrogen, especially ammonia; they are not as economical for domestic wastewater, but might be suited for an industrial location where no other biological processes are in use. (These methods include alkaline air stripping, ion exchange, and "breakpoint" chlorination.)
Phosphorous removal is most commonly done by chemical precipitation with iron or aluminum compounds, such as ferric chloride or alum (aluminum sulfate). The solids which are produced can be settled along with other sludges, depending on where in the treatment train the process takes place. ("Lime", or calcium hydroxide, also works, but makes the water very alkaline, which has to be corrected, and produces more sludge.). There is also a biological process for phosphorus removal, which depends on designing an activated sludge system in such a way as to promote the development of certain types of bacteria which have the ability to accumulate excess phosphorus within their cells. These methods mainly convert dissolved phosphorus into particulate form. For treatment plants which are required to discharge only very low concentrations of total phosphorus, it is common to have a sand filter as a final stage, to remove most of the suspended solids which may contain phosphorus.
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Post by Robot on Jul 27, 2004 10:29:53 GMT -5
Disinfection, usually the final process before discharge, is the destruction of harmful (pathogenic) microorganisms, i.e. disease-causing germs. The object is not to kill every living microorganism in the water-- which would be sterilization-- but to reduce the number of harmful ones to levels appropriate for the intended use of the receiving water. The most commonly used disinfectant is chlorine, which can be supplied in the form of a liquefied gas which has to be dissolved in water, or in the form of an alkaline solution called sodium hypochlorite, which is the same compound as common household chlorine bleach. Chlorine is quite effective against most bacteria, but a rather high dose is needed to kill viruses, protozoa, and other forms of pathogen. Chlorine has several problems associated with its use, among them 1) that it reacts with organic matter to form toxic and carcinogenic chlorinated organics, such as chloroform, 2) chlorine is very toxic to aquatic organisms in the receiving water-- the USEPA recommends no more than 0.011 parts per million (mg/L) and 3) it is hazardous to store and handle. Hypochlorite is safer, but still produces problems 1 and 2. Problem 2 can be dealt with by adding sulfur dioxide (liquefied gas) or sodium sulfite or bisulfite (solutions) to neutralize the chlorine. The products are nearly harmless chloride and sulfate ions. This may also help somewhat with problem 1.
A more powerful disinfectant is ozone, an unstable form of oxygen containing three atoms per molecule, rather than the two found in the ordinary oxygen gas which makes up about 21% of the atmosphere. Ozone is too unstable to store, and has to be made as it is used. It is produced by passing an electrical discharge through air, which is then bubbled through the water. While chlorine can be dosed at a high enough concentration so that some of it remains in the water for a considerable time, ozone is consumed very rapidly and leaves no residual. It may also produce some chemical byproducts, but probably not as harmful as those produced by chlorine.
The other commonly used method of disinfection is ultraviolet light. The water is passed through banks of cylindrical, quartz-jacketed fluorescent bulbs. Anything which can absorb the light, such as fouling or scale formation on the bulbs' surfaces, or suspended matter in the water, can interfere with the effectiveness of the disinfection. Some dissolved materials, such as iron and some organic compounds, can also absorb some of the light. Ultraviolet disinfection is becoming more popular because of the increasing complications associated with the use of chlorine.
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Post by Robot on Jul 27, 2004 10:31:55 GMT -5
Sludge from primary settling basins, called primary or "raw" sludge, is a noxious, smelly, gray-black, viscous liquid or semi-solid. It contains very high concentrations of bacteria and other microorganisms, many of them pathogenic, as well as large amounts of biodegradable organic material. Because of the high concentrations, any dissolved oxygen will be consumed rapidly, and the odorous and toxic products of anaerobic biodegradation (putrefaction) will be produced. The greasy floatable skimmings from primary treatment are another portion of this putrescible solid waste stream.
In addition to the primary sludge, wastewater plants with secondary treatment will produce a "secondary sludge", consisting largely of microorganisms which have grown as a result of consuming the organic wastes. While not quite so objectionable, due to the biodegradation which has already taken place, it is still very high in pathogens and contains much material which will decay and produce odors if not treated further.
Ultimately, the sludge must all be disposed of. The way in which this is done depends on the quality of the sludge-- and determines how it needs to be treated. The most desirable final fate for these solids would be for beneficial use in agriculture, since the material has organic matter to act as a soil conditioner, as well as a some fertilizer value. This requires the highest quality "biosolids", free of contamination with toxic metals or industrial organic compounds, and low in pathogens. At a somewhat lower quality, it can be used for similar purposes on non-agricultural land and for land reclamation (e.g., strip mines). Poorer quality sludge can be disposed of by landfilling or incineration.
One commonly used method of sludge treatment, called digestion, is biological. Since the material is loaded with bacteria and organic matter; why not let the bacteria eat the biodegradable material? Digestion can be either aerobic or anaerobic. Aerobic digestion requires supplying oxygen to the sludge; it is similar to the activated sludge process, except no external "food" is provided. In anaerobic digestion, the sludge is fed into an air-free vessel; the digestion produces a gas which is mostly a mixture of methane and carbon dioxide. The gas has a fuel value, and can be burned to provide heat to the digester tank and even to run electric generators. Some localities have compressed the gas and used it to power vehicles. Digestion can reduce the amount of organic matter by about 30 to 70 percent, greatly decrease the number of pathogens, and produce a liquid with an inoffensive, "earthy" odor. This makes the sludge safer to dispose of on land, since the odor does not attract as many scavenging pests, such as flies, rodents, gulls, etc., which spread pathogens from the disposal site to other areas-- and there are fewer pathogens to be spread.
A liquid sludge, which might contain 3 to 6% dry weight of solids, can be dewatered to form a drier sludge cake of maybe 15 to 25 percent solids, which can be hauled as a solid rather than having to be handled as a liquid. Equipment used to dewater sludge includes centrifuges, vacuum filters, and belt presses or plate-and-frame presses. Chemical coagulants are commonly added to help form larger aggregates of solids and release the water. Further processes such as composting and heat drying can produce a drier product with lower pathogen levels. Another approach involves treatment with lime (calcium oxide), which kills pathogens due to its highly alkaline nature as well as the heat that is generated as it reacts with the water in the sludge; this also results in a drier product. A final disposal method which eliminates all of the pathogens and greatly reduces the volume of the sludge is incineration. This is not considered a beneficial use, however, and is becoming less popular due to public concerns over air emissions.
Sludges from physical-chemical treatment of industrial waste streams containing heavy metals and non-biodegradable toxic organic compounds often must be handled as hazardous wastes. Some of these will end up in hazardous waste landfills, or may be chemically treated for detoxification-- or even for recovery of some components for recycling.
Recalcitrant organic compounds can be destroyed by carefully controlled high-temperature incineration, or by other innovative processes, such as high-temperature hydrogen reduction.
In the handling and treatment of both wastewater and sludge, a prime concern is odor control.
In the sewers, prevention of anaerobic conditions and sulfide formation is an important consideration in preventing odors. Hydrogen sulfide is also the major cause of sewer corrosion. If the water is warm and the flow is not rapid enough to aerate the water and scour the pipes, addition of chemicals such as nitrate, hydrogen peroxide or iron compounds can be helpful. In processes where odors are inevitable, the areas must be contained and ventilated. The air then has to be passed through some type of treatment system, such as an activated charcoal bed, a chemical scrubber (often using hypochlorite solution), a compost pile type biofilter-- or the air can even be used as part of the air supply for an activated sludge system.
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Post by Robot on Jul 27, 2004 10:33:53 GMT -5
Computers in Water Pollution Control Computers find extensive use in the field of water pollution control. Software stream models are available to predict the effects of pollutants entering the waterways. These can be used by regulatory agencies to set limits on allowable discharges. The movement and treatment of pollutants in groundwater as a result of spills and leaks can be similarly modeled. Programs are available to help deal with the complexities of designing wastewater collection systems (sewers) and predicting the effects of storm water flows. Models are used by engineers to design the types of biological, chemical, and physical wastewater treatment processes described above, so that they will remove pollutants to the required levels. These programs can also be used by operations staff to optimize the treatment processes, and even for partial automation by accepting real-time information from sensors and using the information to control pumps, valves, aerators, etc. Software is available for scheduling equipment maintenance, tracking plant performance, preparing reports for regulatory agencies, and managing industrial pretreatment programs. High volume analytical laboratories can also use laboratory information management systems (LIMS) to keep track of samples, store analysis and quality control results, and produce reports. In many cases, instruments can be connected directly to the LIMS to reduce transcription errors. Here is a link to my own listing with descriptions of over 50 such programs. www.geocities.com/RainForest/5161/profsoft.htm#OPREP
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Post by Robot on Jul 27, 2004 10:36:23 GMT -5
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