Treatment of waste water


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Treatment of waste water

  1. 1. Treatment of Waste WaterSubmitted to: Sir Ghulam MustafaSubmitted by: Hafiz Muhammad Athar Rizwan 09050607-008 BS Chemistry 3rd Department of Chemistry 1
  2. 2. Treatment of Waste WaterWaste water treatment is the process of removing contaminants from wastewater andhousehold sewage, both runoff (effluents) and domestic. It includes physical, chemical, andbiological processes to remove physical, chemical and biological contaminants. Its objective is toproduce an environmentally-safe fluid waste stream (or treated effluent) and a solid waste (ortreated sludge) suitable for disposal or reuse (usually as farm fertilizer). Using advancedtechnology it is now possible to re-use sewage effluent for drinking water, although Singapore isthe only country to implement such technology on a production scale in its production ofNEWater.Wastewater treatment is the process of taking wastewater and making it suitable for dischargeback into the environment. Wastewater can be formed by a variety of activities, includingwashing, bathing, and using the toilet. Rainwater runoff is also considered wastewater. No matterwhere it comes from, this water is full of bacteria, chemicals, and other contaminants.Wastewater treatment reduces the contaminants to acceptable levels so as to be safe for dischargeinto the environment.Wastewater is any water that has been adversely affected in quality by anthropogenic influence.It comprises liquid waste discharged by domestic residences, commercial properties, industry,and/or agriculture and can encompass a wide range of potential contaminants and concentrations.In the most common usage, it refers to the municipal wastewater that contains a broad spectrumof contaminants resulting from the mixing of wastewaters from different sources.Sewage is correctly the subset of wastewater that is contaminated with feces or urine, but is oftenused to mean any waste water. "Sewage" includes domestic, municipal, or industrial liquid wasteproducts disposed of, usually via a pipe or sewer or similar structure, sometimes in a cesspoolemptier.The physical infrastructure, including pipes, pumps, screens, channels etc. used to conveysewage from its origin to the point of eventual treatment or disposal is termed sewerage. 2
  3. 3. Wastewater or sewage can come from: Human waste (feces, used toilet paper or wipes, urine, or other bodily fluids), also known as blackwater, usually from lavatories; Cesspit leakage; Septic tank discharge; Sewage treatment plant discharge; Washing water (personal, clothes, floors, dishes, etc.), also known as greywater or sullage; Rainfall collected on roofs, yards, hard-standings, etc. (generally clean with traces of oils and fuel); Groundwater infiltrated into sewage; Surplus manufactured liquids from domestic sources (drinks, cooking oil, pesticides, lubricating oil, paint, cleaning liquids, etc.); Urban rainfall runoff from roads, carparks, roofs, sidewalks, or pavements (contains oils, animal feces, litter, fuel or rubber residues, metals from vehicle exhausts, etc.); Seawater ingress (high volumes of salt and micro-biota); Direct ingress of river water (high volumes of micro-biota); Direct ingress of manmade liquids (illegal disposal of pesticides, used oils, etc.); Highway drainage (oil, de-icing agents, rubber residues); Storm drains (almost anything, including cars, shopping trolleys, trees, cattle, etc.); Black water (surface water contaminated by sewage); Industrial waste industrial site drainage (silt, sand, alkali, oil, chemical residues); o Industrial cooling waters (biocides, heat, slimes, silt); o Industrial process waters; o Organic or bio-degradable waste, including waste from abattoirs, creameries, and ice cream manufacture; o Organic or non bio-degradable/difficult-to-treat waste (pharmaceutical or pesticide manufacturing); o extreme pH waste (from acid/alkali manufacturing, metal plating); o Toxic waste (metal plating, cyanide production, pesticide manufacturing, etc.); 3
  4. 4. o Solids and Emulsions (paper manufacturing, foodstuffs, lubricating and hydraulic oil manufacturing, etc.); o agricultural drainage, direct and diffuse.The composition of wastewater varies widely. This is a partial list of what it may contain: Water ( > 95%) which is often added during flushing to carry waste down a drain; Pathogens such as bacteria, viruses, prions and parasitic worms; Non-pathogenic bacteria; Organic particles such as faeces, hairs, food, vomit, paper fibers, plant material, humus, etc.; Soluble organic material such as urea, fruit sugars, soluble proteins, drugs, pharmaceuticals, etc.; Inorganic particles such as sand, grit, metal particles, ceramics, etc.; Soluble inorganic material such as ammonia, road-salt, sea-salt, cyanide, hydrogen sulfide, thiocyanates, thiosulfates, etc.; Animals such as protozoa, insects, arthropods, small fish, etc.; Macro-solids such as sanitary napkins, nappies/diapers, condoms, needles, childrens toys, dead animals or plants, etc.; Gases such as hydrogen sulfide, carbon dioxide, methane, etc.; Emulsions such as paints, adhesives, mayonnaise, hair colorants, emulsified oils, etc.; Toxins such as pesticides, poisons, herbicides, etc.Treatment:There are numerous processes that can be used to clean up waste waters depending on the typeand extent of contamination. Most wastewater is treated in industrial-scale wastewater treatmentplants (WWTPs) which may include physical, chemical and biological treatment processes.However, the use of septic tanks and other On-Site Sewage Facilities (OSSF) is widespread inrural areas, serving up to one quarter of the homes in the U.S. The most important aerobictreatment system is the activated sludge process, based on the maintenance and recirculation of acomplex biomass composed by micro-organisms able to absorb and adsorb the organic mattercarried in the wastewater. Anaerobic processes are widely applied in the treatment of industrial 4
  5. 5. wastewaters and biological sludge. Some wastewater may be highly treated and reused asreclaimed water. For some waste waters ecological approaches using reed bed systems such asconstructed wetlands may be appropriate. Modern systems include tertiary treatment by microfiltration or synthetic membranes. After membrane filtration, the treated wastewater isindistinguishable from waters of natural origin of drinking quality. Nitrates can be removed fromwastewater by microbial denitrification, for which a small amount of methanol is typically addedto provide the bacteria with a source of carbon. Ozone Waste Water Treatment is also growing inpopularity, and requires the use of an ozone generator, which decontaminates the water as Ozonebubbles percolate through the tank.Disposal of wastewaters from an industrial plant is a difficult and costly problem. Mostpetroleum refineries, chemical and petrochemical plants have onsite facilities to treat theirwastewaters so that the pollutant concentrations in the treated wastewater comply with the localand/or national regulations regarding disposal of wastewaters into community treatment plants orinto rivers, lakes or oceans. Other Industrial processes that produce a lot of waste-waters such aspaper and pulp production has created environmental concern leading to development ofprocesses to recycle water use within plants before they have to be cleaned and disposed of.The Primary Treatment Process:1. Screening:Wastewater entering the treatment plant includes items like wood, rocks, and even dead animals.Unless they are removed, they could cause problems later in the treatment process. Most of thesematerials are sent to a landfill.2. Pumping:The wastewater system relies on the force of gravity to move sewage from your home to thetreatment plant. So wastewater-treatment plants are located on low ground, often near a river intowhich treated water can be released. If the plant is built above the ground level, the wastewaterhas to be pumped up to the aeration tanks (item 3). From here on, gravity takes over to move thewastewater through the treatment process. 5
  6. 6. 3. Aerating:One of the first steps that a water treatment facility can do is to just shake up the sewage andexpose it to air. This causes some of the dissolved gases (such as hydrogen sulfide, which smellslike rotten eggs) that taste and smell bad to be released from the water. Wastewater enters aseries of long, parallel concrete tanks. Each tank is divided into two sections. In the first section,air is pumped through the water.As organic matter decays, it uses up oxygen. Aeration replenishes the oxygen. Bubbling oxygenthrough the water also keeps the organic material suspended while it forces grit (coffeegrounds,sand and other small, dense particles) to settle out. Grit is pumped out of the tanks and taken tolandfills.4. Removing sludgeWastewater then enters the second section or sedimentation tanks. Here, the sludge (the organicportion of the sewage) settles out of the wastewater and is pumped out of the tanks. Some of thewater is removed in a step called thickening and then the sludge is processed in large tanks calleddigesters.5. Removing scum:As sludge is settling to the bottom of the sedimentation tanks, lighter materials are floating to thesurface. This scum includes grease, oils, plastics, and soap. Slow-moving rakes skim the scumoff the surface of the wastewater. Scum is thickened and pumped to the digesters along with thesludge.Many cities also use filtration in sewage treatment. After the solids are removed, the liquidsewage is filtered through a substance, usually sand, by the action of gravity. This method getsrid of almost all bacteria, reduces turbidity and color, removes odors, reduces the amount of iron,and removes most other solid particles that remained in the water. Water is sometimes filteredthrough carbon particles, which removes organic particles. This method is used in some homes,too. 6
  7. 7. 6. Killing bacteria:Finally, the wastewater flows into a chlorine contact tank, where the chemical chlorine is addedto kill bacteria, which could pose a health risk, just as is done in swimming pools. The chlorine ismostly eliminated as the bacteria are destroyed, but sometimes it must be neutralized by addingother chemicals. This protects fish and other marine organisms, which can be harmed by thesmallest amounts of chlorine.The treated water (called effluent) is then discharged to a local river or the ocean7. Wastewater Residuals:Another part of treating wastewater is dealing with the solid-waste material. These solids arekept for 20 to 30 days in large, heated and enclosed tanks called digesters. Here, bacteria breakdown (digest) the material, reducing its volume, odors, and getting rid of organisms that cancause disease. The finished product is mainly sent to landfills, but sometimes can be used asfertilizer.The following consist of a list of wastewater treatment technologies: 1.Activated sludge systems 2.Advanced Oxidation Process 3.Distillation1.Activated sludge:Activated sludge is a process for treating sewage and industrial wastewaters using air and abiological floc composed of bacteria and protozoans.The process: The process involves air or oxygen being introduced into a mixture of primarytreated or screened sewage or industrial wastewater (called wastewater from now on) combined 7
  8. 8. with organisms to develop a biological floc which reduces the organic content of thesewage.This material, which in healthy sludge is a brown floc, is largely composed ofsaprotrophic bacteria but also has an important protozoan flora mainly composed of amoebae,Spirotrichs, Peritrichs including Vorticellids and a range of other filter feeding species. Otherimportant constituents include motile and sedentary Rotifers. In poorly managed activatedsludge, a range of mucilaginous filamentous bacteria can develop including Sphaerotilus natanswhich produces a sludge that is difficult to settle and can result in the sludge blanket decantingover the weirs in the settlement tank to severely contaminate the final effluent quality. Thismaterial is often described as sewage fungus but true fungal communities are relativelyuncommon.The combination of wastewater and biological mass is commonly known as mixed liquor. In allactivated sludge plants, once the wastewater has received sufficient treatment, excess mixedliquor is discharged into settling tanks and the treated supernatant is run off to undergo furthertreatment before discharge. Part of the settled material, the sludge, is returned to the head of theaeration system to re-seed the new wastewater entering the tank. This fraction of the floc iscalled return activated sludge (R.A.S.). Excess sludge is called surplus activatedsludge(S.A.S.) or waste activated sludge(W.A.S). S.A.S is removed from the treatment processto keep the ratio of biomass to food supplied in the wastewater in balance. S.A.S is stored insludge tanks and is further treated by digestion, either under anaerobic or aerobic conditionsprior to disposal.Many sewage treatment plants use axial flow pumps to transfer nitrified mixed liquor from theaeration zone to the anoxic zone for denitrification. These pumps are often referred to as internalmixed Liquor recycle pumps (IMLR pumps). The raw sewage, the RAS, and the nitrifiedmixed liquor are mixed by submersible mixers in the anoxic zones in order to achievedenitrification.Activated sludge is also the name given to the active biological material produced by activatedsludge plants. 8
  9. 9. A generalized, schematic diagram of an activated sludge process.2. Advanced oxidation process:Advanced Oxidation Processes (abbreviation: AOPs), refers to a set of chemical treatmentprocedures designed to remove organic and inorganic materials in waste water by oxidation.Contaminants are oxidized by four different reagents: ozone, hydrogen peroxide, oxygen, andair, in precise, pre-programmed dosages, sequences, and combinations. These procedures mayalso be combined with UV irradiation and specific catalysts. This results in the development ofhydroxyl radicals. A well known example of AOP is the use of Fentons reagent.The AOP procedure is particularly useful for cleaning biologically toxic or non-degradablematerials such as aromatics, pesticides, petroleum constituents, and volatile organic compoundsin waste water [1]. The contaminant materials are converted to a large extent into stable inorganiccompounds such as water, carbon dioxide and salts, i.e. they undergo mineralization. A goal ofthe waste water purification by means of AOP procedures is the reduction of the chemicalcontaminants and the toxicity to such an extent that the cleaned waste water may be reintroducedinto receiving streams or, at least, into a conventional sewage treatment. 9
  10. 10. 3. DistillationDistillation is a method of separating mixtures based on differences in their volatilities in aboiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not achemical reaction.Commercially, distillation has a number of applications. It is used to separate crude oil into morefractions for specific uses such as transport, power generation and heating. Water is distilled toremove impurities, such as salt from seawater. Air is distilled to separate its components—notably oxygen, nitrogen, and argon—for industrial use. Distillation of fermented solutions hasbeen used since ancient times to produce distilled beverages with a higher alcohol content. Thepremises where distillation is carried out, especially distillation of alcohol, are known as adistillery. Steps in Wastewater Treatment1. Pretreatment: i. Coarse Screening:Often the first step in wastewater treatment is coarse screening to remove large materials (rags,logs and other large objects) from the wastewater that could interfere with the treatment process.Coarse screens (also known as bar racks) are made up of vertical or inclined bars with openingsone to three inches wide. These screens are raked cleaned from time to time. Some wastewatertreatment plants have automated equipment that clean the screens automatically. At other plants,the screens are manually raked. ii. Grit Chambers:The next step is to remove smaller objects (such as sand, broken glass, silt and pebbles). If theseobjects are not removed, they can damage pumps and other mechanical devices. These objectsalso have a tendency to settle in corners and bends, thus reducing flow capacity and eventuallyclogging pipes and channels. 10
  11. 11. Many grit chambers are concrete-lined ponds, similar to swimming pools, where the wastewaterenters one end and leaves at the opposite end. These grit chambers are typically sized so that thespeed of the wastewater flowing through the chamber is kept well below one foot per second.Flows this slow allow sand- and silt-sized particles to settle out of the wastewater.Another design that is sometimes used is an aerated grit chamber. In am aerated grit chamber, airbubbles are injected into a wastewater basin to force a spiral, or rolling, flow. The air bubbles aresupposed to strip organic material off the surfaces of the inert grit, and the grit settles in thebottom.Each wastewater treatment plant usually has at least two grit chambers. This allows each one tobe closed for cleaning. Mechanical devices are usually installed to clean each grit chamber.Normally, the grit is buried in a sanitary landfill. iii. Comminutors:Comminutors are mechanical devices with revolving cutting bars. Comminutors are placeddownstream of the grit chambers to cut and shred any remaining solids. iv. Flow Equalization:Wastewater seldom flows into wastewater treatment plants at the same rate throughout each day.In many cities, the greatest flows reaching the wastewater treatment plants arrive mid-morning.Such uneven flow volumes reduce wastewater treatment plants efficiency.To even out these periods of high and low flow, large basins are constructed at some wastewatertreatment plants to store the wastewater flow from peak periods and release it for treatment.These basins require aeration and mixing to prevent odors and deposition of solids.2.Primary Treatment:After screening the solids and removing the grit, the wastewater still contains light organicsuspended solids. Some of these can be removed by gravity in a sedimentation tank. These tanksare typically twelve feet deep and hold the wastewater for two or three hours. What settles out is 11
  12. 12. called sludge. The sludge is removed from the primary treatment tank with mechanical scrapersand pumps. Grease, oil, and other floating substances rise to the top, where they are removed bysurface skimming equipment.3.Secondary Treatment:Secondary treatment removes soluble materials that require oxygen for decay, as well as furtherremoval of suspended solids. a. BOD:Many of the pollutants in city wastewater, if not treated and dumped directly into streams, killfish and other wildlife by depriving them of oxygen. These pollutants naturally rot, ordecompose, on their own; but require oxygen to do so. These pollutants steal dissolved oxygenfrom the water as they decompose, and fish and other aquatic wildlife die from oxygenstarvation.The amount of these oxygen-stealing pollutants present in wastewater is measured in terms ofbiochemical oxygen demand (BOD, pronounced "bee-oh-dee"). BOD is the amount of oxygen,usually measured in milligrams of oxygen per liter of wastewater, required to decompose theorganic pollutants found in the wastewater.Because these organic pollutants decompose on their own in the presence of oxygen andcommon microorganisms, these pollutants can be decomposed by merely forcing oxygen into thewastewater and giving these pollutants time to decompose by natural processes. This is theprocedure used in secondary treatment.The two most common secondary treatment methods are trickling filters and activated sludge. b. Trickling Filters:A trickling filter is a bed of coarse stone or perforated plastic material over which wastewater issprayed. The most common design is a bed of stones three to ten feet deep inside a large circular 12
  13. 13. concrete tank. Some tanks are more than 200 feet in diameter. The wastewater is sprayed overthe filter from rotating arms.As the wastewater trickles through the bed, microorganisms establish themselves on the stone orplastic surfaces as slime. The wastewater picks up oxygen as it is sprayed over the filter andpasses over these microorganisms. These microorganisms, in the presence of high amounts ofoxygen, feed on the organic materials in the wastewater.The microbial slime on the filter bed will grow and eventually clog the filter if not washed out.Thus, the flow from the filter is sent to a sedimentation basin to allow these solids to settle out.This sedimentation basin is called a secondary clarifier or a final clarifier to distinguish it fromthe sedimentation basin used for primary treatment. c. Activated Sludge:Activated sludge is another method of providing secondary treatment to wastewater, whereby amixture of wastewater and biological sludge (microorganisms) is agitated and aerated. Thebiological solids are then allowed to settle out.The name "activated sludge" comes from the biological mass formed when oxygen (in the formof air) is continuously injected into the wastewater. In this process, microorganisms arethoroughly mixed with organics under conditions that stimulate their growth. As themicroorganisms grow and are mixed by the agitation of the air, the individual microorganismsclump (or flocculate) together to form a mass of microbes called activated sludge. About eightcubic feet of air are required for every cubic foot of wastewater.In the activated sludge process, wastewater flows continuously into an aeration tank where air isinjected into the wastewater to mix the wastewater with the activated sludge, and also to providethe oxygen needed for the microorganisms to break down the organic pollutants. The mixture ofwastewater and activated sludge is called mixed liquor. The mixed liquor flows to a secondaryclarifier (settling tank) where the activated sludge settles out. Some (usually twenty or thirtypercent) of the settled sludge is returned to the aeration tank (and hence is called return sludge)to maintain a high population of microbes to break down the organics. 13
  14. 14. Since more activated sludge is produced than is needed for return sludge, the excess sludge isremoved and disposed of. d. Secondary Clarifiers:Regardless which method of secondary treatment is used, the end result is a mixture ofmicroorganisms and partially treated wastewater that is essentially free of dissolved andsuspended organic material. The organic material that was dissolved or suspended in thewastewater has now been broken down and consumed by these microorganisms. Thesemicroorganisms, while very small, are still large enough to settle out of the wastewater.Wastewater leaving trickling filters and activated sludge aeration tanks is then sent to secondaryclarifiers (settling tanks) where the microorganisms settle out. At this point, the wastewatertreatment process is nearly completed. e. Disinfection:The next step in secondary wastewater treatment is disinfection. In the United States, the mostcommon method of disinfection is chlorination. Chlorine is injected into the wastewater and thewastewater is held in a basin for about fifteen minutes to allow the chlorine to react with anyremaining pathogens.Since chlorine is toxic to fish, the chlorine is often removed from the wastewater as a last step.The treated wastewater can then be released into a stream.4. Sludge:Both the primary and the secondary treatment processes produce large amounts of sludge. Sludgeis commonly disposed of by storing it in a tank (called a digester) where it undergoes anaerobicdigestion. Methane gas is one by-product of anaerobic digestion, which can be burned off orused as fuel. Another commonly used method of disposal is incineration. 14
  15. 15. 5. Advanced Wastewater Treatment:Although secondary treatment can remove over eighty-five percent of the BOD, suspendedsolids and nearly all pathogens, sometimes additional treatment is required. Sometimes a higherlevel of BOD removal is required. Sometimes other pollutants, such as nitrogen, phosphorus,non-biological chemical oxygen demand (COD), or heavy metals may be present that requireremoval. Besides removing these other pollutants, advanced wastewater treatment (AWT) can beso effective in removing pollutants that the treated wastewater can be reused. In many parts ofthe world, water is too valuable a resource to be thrown away.Some processes used in AWT include filtration, carbon adsorption, phosphorus removal andnitrogen removal. a) Filtration:Secondary treatment processes are highly effective in reducing the BOD in wastewater.However, the secondary clarifiers used to settle out microorganisms in the secondary treatmentprocess are not totally effective. Some of these microorganisms remain in the wastewater after itleaves the secondary clarifier, and they add BOD since the decay of these microorganisms willexert its own oxygen demand.Sand filters, similar to those used for producing drinking water, are sometimes used foradditional removal of microorganisms and other solids. Unlike water treatment sand filters,however, the filters used in wastewater treatment often use large, lightweight aggregates (such ascoal) at the top to improve efficiency and facilitate cleaning. Partially treated wastewater usuallycontains higher concentrations of solids than does water in a water treatment plant, so thesefilters must be designed for greater efficiency and for more frequent cleaning. b) Carbon Adsorption:Even after secondary treatment and filtration, soluble organics may still be present in thewastewater. These remaining materials are called refractory organics. The most practical way toremove refractory organics is by adsorbing them on activated carbon. 15
  16. 16. Adsorption (not to be confused with absorption) is the accumulation of materials on an interface(in this case, the liquid/solid boundary layer). Carbon is activated by heating it in the absence ofoxygen. This activation process creates many small pores in each carbon particle. Activation thusincreases the surface area of each carbon particle, making it more effective as an adsorptionagent.After the adsorption capacity of the carbon has been exhausted, it can be restored by re-heating itin the absence of oxygen. This process drives off the adsorbed organics, which can be consumedin an afterburner. c) Phosphorus Removal:Phosphorus, in wastewater, is considered a pollutant because it encourages the growth of algae.Phosphorus removal usually involves the addtion of ferric chloride, alum or lime to thewastewater, mixing it in a reaction basin, and then sending the mixture to a clarifier to allow thephosphorus-containing precipitate to settle out. d) Nitrogen Control:Nitrogen in any soluble form is a plant nutrient and may need to be removed from thewastewater to control the growth of algae. In addition, nitrogen in the form of ammonia exerts anoxygen demand and can be toxic to fish. Nitrogen can be removed from wastewater by bothbiological and chemical means. The biological process is called nitrification/denitrification andthe chemical process is called ammonia stripping.6. Nitrification/Denitrification:The natural nitrification process can be forced to occur in the activated sludge process bymaintaining a cell detention time of at least fifteen days. Bacteria can convert nitrates into waterand the gases nitrogen and carbon dioxide. Small amounts of organic materials (such asmethanol, or raw or settled sewage) is added to provide a food source for the bacteria for thisdenitrification process. 16
  17. 17. I. Ammonia Stripping:Nitrogen in the form of ammonia can be removed chemically by raising the pH (often, by addinglime) to convert the ammonium ion into ammonia, which can be stripped from the water byblowing large quantities of air through the water. Waste Water treatmentStormwater Treatment and Management:Stormwater treatment includes (1) storage in retention ponds where evaporation and seepage takeplace, and diversion to natural or artificial wetlands , where pollutants are removed by 17
  18. 18. vegetation and sedimentation and water is returned to the atmosphere by evapotranspiration.These methods take advantage of the ability of natural filtration and biological processes to aidin restoring water quality. Under certain circumstances, chemicals may also be introduced astreatment aids.As noted above, the principal method used for stormwater treatment is storage wherein naturalprocesses of sedimentation, evaporation, and nutrient removal take place. Because of the largevolumes of water generated by storms, it usually is not practical to divert these waters totreatment plants such as those used to process municipal and industrial wastewaters. However, anumber of devices can be inserted into stormwater systems to achieve various levels of removalof solids and other constitutents. These devices employ features of some of the components ofwastewater treatment plants described previously. 18