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Waste Water Treatment


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  • 1. Global Environmental Lab (Private) Limited (GEL)Waste Water TreatmentTheorySubmitted by: Mr. Gulfam Raza HaiderySubmitted to: Mr. Umer MehmoodAbstract: The document includes brief introduction to waste water, waste watertreatment methods, role of microorganisms in water as well as in activated sludge. Aconcise description about the poisonous substances, which can be harmful for themicroorganisms, has also been added.
  • 3. 3Chapter 1ORGANIC MATTERS IN H2O1.1 CLASSIFICATION OF ORGANIC MATTER (OM)1. Natural organic matters (NOM)2. Anthropogenic organic matters3. Particulate organic matters(POM)4. Dissolved organic matters(DOM)1.1.1 Sources of natural organic matter The OM is derived from natural sources; it is often referred to as natural organicmatter (NOM). The OM that is of natural origin is derived primarily from plant and/ or microbial residues.On land, plants grow; sometimes shed leaves, and die, leaving roots within theupper soil layers and ‗litter‘ on the soil surface. Microorganisms also flourishwithin the soil. When they die their biomass adds to the soil organic content. Organic matter is also produced in situ within a water body. Wetlands, bothnatural and constructed, are a prime example. There, the luxuriant growth ofvegetation produces a thick mat of aerial material and roots that, upon death, aredeposited in the water. Other water bodies, rivers, lakes, and oceans, support the growth of aquaticplants and animals to a smaller degree and their organic remains also becomepart of the total aquatic system like algae, bacteria and other microorganisms. Microscopic animals also release soluble organic matter from their bodies. Humic material (HM) is a form of environmental organic matter of plant ormicrobial origin.1.1.2 Sources of Anthropogenic organic matters Besides the natural sources, there are human inputs that contribute to theorganic matter water. These include large volumes of poorly defined wastes,such as domestic sewage or mill effluent, that are sometimes discharged directlyor after treatment into rivers, lakes, and oceans. Besides the bulk effluents,anthropogenic sources also supply specific organic compounds—agriculturalchemicals, medicinal, and products or byproducts of industrial processes.
  • 4. 4 Release of nitrilotriacetic acid (NTA) into a waste stream from a detergent-manufacturing facility is an example of an indisputably anthropogenic event. Trichloromethane (chloroform) is a chlorinated hydrocarbon that we mightassume is produced only via industrial processes.1.1.3 Chemistry of dissolved organic matter About 50% of DOM is made up by Humic Material (HM). As an approximation, the remaining DOM consists of following;o low molar mass acids(25%) such as; oxalic acids citric acids formic acids acetic acidso Neutral compounds much of which is carbohydrate material (15%)o On a global scale it is estimated that around 10% of microbial activity inwater goes to the production of DOM.1.2 HUMIC MATERIALHumic material (HM) is a form of environmental organic matter of plant or microbialorigin.1.2.1 Subdivision of humic materialHumic material (also called humate or humus) is subdivided in an operational sense intothree classes or categories; Fulvic acid (FA) is the fraction of humic matter that is soluble in aqueoussolutions that span all pH values. Humic acid (HA) is insoluble under acid conditions (pH 2) but soluble atelevated pH. Humin (Hu) is insoluble in water at all pH values.1.2.2 Forms of humic materialsHumic materials as a group are found in the aqueous and terrestrial environments in avariety of forms and associations. Free HM consists of soluble or insoluble forms of the material itself. Complexes HM is chemically bound to metals, other inorganic species such asphosphate, or organic molecules.The complexes HM are either in solution or in particulate form.
  • 5. 5 Surface-bonded HM is chemically bonded to other solids such as clay mineralsor iron and aluminium oxides.In this way the surface of the inorganic material is altered so that its chemicalproperties are determined largely by the organic coating. The HM can then reactin a manner similar to that of the pure material itself.1.3 TOXICITY OF SPECIFIC COMPOUNDSOrganic matter in water is of environmental importance for several reasons. For onething, particular compounds may be toxic in varying degrees to living organisms,including humans. Following are all well-known; Polyaromatic hydrocarbons Polychlorinated biphenyls Dioxins
  • 6. 6Chapter 2WASTE WATER TREATMENT METHODS2.1 METHODS OF WASTE WATER TREATMENTWastewater refers to any water which has been used for some human activity and thushas been contaminated and left it unsuitable for further use.OrWastewater treatment is a process wherein the contaminants are removed fromwastewater - both domestic and industrial, in order to produce waste stream or solidwaste suitable for safe discharge or reuse.They can be broadly categorized into three different groups (on the basis of tasksinvolved).1. Physical method2. Chemical method3. Biological method2.1.1 Physical methodWhen it comes to physical wastewater treatment, following physical processes are usedfor the treatment of water instead of resorting to chemicals or biological means. SedimentationWherein coarse screening of waste water is done to remove contaminatingobjects after allowing them to settle at the base, when heavy contaminants settledown, the removal of cleared effluent or waste stream becomes relatively easy. AerationWherein air is added to the wastewater physically in order to provide oxygen tothe contaminated water. FiltrationWherein the contaminated water is passed through various filters to separate thecontaminating solids from the water. Sand filter is by far the most common filterused in this process.
  • 7. Filtration problems BiofoulingBiofouling or biological fouling is the accumulationof microorganisms, plants, algae, or animals on wetted surfaces. Suchaccumulation is referred to as epibiosis when the host surface is anotherorganism and the relationship is not parasitic. Effect of biofoulingBiofouling essentially breaks down water filtration systems in water purificationfacilities. Causes of biofouling In water, Natural Organic Matter (NOM) can still bind to metal ions and minerals. These bound molecules are not necessarily stopped by the purification process,but do not cause harm to any humans, animals, or plants. However, because of the high level of reactivity of natural organic matter,byproducts that do not contain nutrients can be made. These byproducts are much larger and can induce biofouling. The largermolecules clog the water purification filters Treatment of biofouling The byproduct problem could be treated by the disinfection technique known aschlorination, which often breaks down residual material clogging systems. Water with natural organic matter could be disinfected with ozone-initiated radicalreactions.The ozone has very strong oxidation characteristics. It can form hydroxyl radicals(OH) when it decomposes, which will react with the natural organic matter to shutdown the problem of biofouling. Anti-fouling is the process of removing or preventing these accumulations fromforming. In industrial processes, bio-dispersants can be used to controlbiofouling.2.1.2 Chemical methodAs opposed to physical treatment of water, chemical treatment involves the use ofchemicals to get rid of contaminants in it.Various methods are used to treat water chemically, they are following; ChlorinationIn this process, chlorine - a strong oxidizing chemical, is used to kill the bacteriawhich lead to decomposition of water.
  • 8. 8 OxidizationThis is a chemical water treatment method, wherein oxidizing agents - such asozone, are used to treat polluted water. These oxidizing agents make waterreusable by hampering the biological growth process of bacteria which happensto be the main cause of decomposition of water. NeutralizationWhen it comes to industrial wastewater treatment, a chemical process known asNeutralization is quite common.This process involves adding acid or base to the water to adjust its pH value andbring it back to neutral level. Lime (CaO), commonly known as quicklime or burnt lime, is one of thebest examples of base used in the process of neutralization to neutralizeacid wastes. Polyvalent metals, i.e. metals having more than one valence, are veryoften used as coagulating chemicals in sewage treatment. Iron and other metals containing compounds like ferric sulfate andaluminum sulfate are some of the best examples of coagulants.2.1.3 Biological methodIn biological water treatment processes, bacteria and other such microorganisms areused to biochemically decomposes the wastewater and stabilizes the end product.Biological water treatment is further categorized into two sub-divisions as following; Aerobic processIn the aerobic process, bacteria consume the organic matter and helps convert itto carbon dioxide in the presence of oxygen. Anaerobic processIn the anaerobic process, on the other hand, sludge is fermented at a particulartemperature in the absence of oxygen.References:
  • 9. 9Chapter 3PRELIMINARY WASTE WATER TREATMENT STEPS3.1 BACKGROUND OF PRIMARY TREATMENTThe first waste water treatment systems, introduced by the end of the 19thcentury, weredesigned as units for the separation of solids and liquid by means of gravity settling: aprocess known as the primary treatment of waste water.A large fraction of the organic material in waste water is not settle able and therefore isnot removed by primary treatment.3.2 BACKGROUND OF SECONDARY TREATMENTSecondary treatment was introduced in the first decades of the 20thcentury, With theobjective of improving the treatment efficiency of waste water treatment plants.Secondary treatment is characterized by the use of biological methods to remove theorganic material present in the waste water.In search of an efficient waste water treatment system, the activated sludge processwas developed in 1914 by Lockett and Ardern at the University of Manchester. Theynoted that aeration of municipal sewage resulted in an increased removal rate oforganic material, while at the same time the formation of macroscopic flocs wasobserved, which could be separated from the liquid phase by settling, forming abiological sludge.The capacity of the sludge to increase the removal rate of organic material led to thecommon denomination ―activated sludge‖.The basic principle of the activated sludge process has not changed since thefirst application: organic material is still placed in contact with activated sludge inan aerobic environment and let the end to come.3.3 MODES OF PROCESSIn its original version, the activated sludge process was operated as a batch process:The first important advance in the development of the activated sludge process was thetransformation of the original sequential batch process into a continuous process,through the addition of a settling tank after the biological reactor.
  • 10. 103.4 WASTE WATER TREATMENT AND PUIRFICATION STAGESTreatment and purification stages are categorized as following;1. Primary treatment2. Secondary treatment3. Disinfection4. Sludge treatment3.4.1 Primary treatmentPrimary treatment uses both physical as well as chemical method. Screening chamberThe incoming wastewater, called influent, passes through screens consisting ofupright bars, spaced one to three inches apart. These bars remove large piecesof trash including rags, sticks, newspaper, soft drink cans, bottles, plastic cupsand other similar items. This protects the main sewage pumps and otherequipment. The garbage is transported to landfills. The main sewage pumps thenlift the wastewater from the screening chamber to the surface level of the plant.The wastewater enters primary settling tanks, also called sedimentation tanks,for one to two hours. CoagulationThe materials which are suspended or found in the colloidal form in raw water orinfluent are removed by coagulation.Substance that is used to carry out coagulation is called coagulant. The most important coagulants are;o Al2(SO4)3·14H2O or Al2(SO4)3·18H2O (alum)o FeCl3o FeCl3 (with lime)o Fe2(SO4)3 (with lime)o FeSO4·7H2O (copperas) (with lime) Alum or aluminium sulphateWhen alum is added to waste water in alkaline medium,aluminium hydroxide isprecipitated out as reaction shows following;K2SO4.Al2 (SO4)3·24H2O +3Ca(OH)2 3CaSO4+2Al(OH)3+K2SO4+24H2OSo, suspended particles get adsorbed on the surface of gelatinuous aluminiumhydroxide.
  • 11. 11 Ferric saltsFerric salts are commonly used as coagulants but they are difficult to handlebecause an insoluble ferric oxide is produced in the pH range from 3 to 13. LimeThe water may contain calcium and magnesium salts which make water hard. Itis treated by adding lime in waste water as shown by reactions; Partially treated waste water and primary sludgeThe flow of the water is slowed, allowing heavier solids to settle to the bottom ofthe tank and the lighter materials to float. The settled solids, called primary sludge, are then pumped through cyclonedegritters — devices that use centrifugal force to separate out sand, grit (such ascoffee grinds) and gravel. This grit is removed, washed and taken to landfills.o The degritted primary sludge is pumped to the plants sludge handlingfacilities for further processing.o The partially treated wastewater from the primary settling tanks then flowsto the secondary treatment system.
  • 12. 123.4.2 Secondary treatmentSecondary treatment uses chemical, physical and biological methods.The partially treated wastewater from the primary settling tanks then flows to thesecondary settling tanks which are termed to be as aeration tanks (also called biologicalreactors or bubbling tanks).Secondary treatment is also called the activated sludge process.AerationAir pumped into large aeration tanks by means of aeration equipments which are infollowing forms and any of them can be used according to process feasibility ; Surface aerators Compressors connected to submerged air diffusersIn this process, air is passed through partially treated waste water and following effectsoccurs such as; Removal of dissolved foul smelling H2S Removal of Organosulpur compounds Removal of Volatile organic compounds Some organic materials are oxidized with air and CO2 is produced Removal of remaining organic materials by passing water over activated carbon Aeration process also oxidizes water soluble Fe+2to Fe+3which then formsinsoluble Fe(OH)3 and is separated as solid
  • 13. 13Aeration equipments mix the wastewater and sludge that stimulates the growth ofoxygen-using bacteria and other tiny organisms that are naturally present in thesewage.These beneficial microorganisms consume most of the remaining organic materials thatare polluting the water and this produces heavier particles that will settle later in thetreatment process.Wastewater passes through these bubbling tanks in three to six hours, producingsludge which is called activated sludge.The aerated wastewater then flows to the final settling tanks called clarifier which aresimilar to the primary settling tanks. Here the heavy particles and other solids settle tothe bottom as secondary sludge. Some of this sludge is re-circulated back to theaeration tanks called return sludge which stimulates the activated sludge process.The returned sludge contains millions of microorganisms that help maintain the right mixof bacteria and air in the tank and contribute to the removal of as many pollutants aspossible.The remaining secondary sludge is removed from the settling tanks and added to theprimary sludge for further processing in the sludge handling facilities.Wastewater passes through the settling tanks in two to three hours and then flows to adisinfection tank.3.4.3 DisinfectionEven after primary and secondary treatment, disease causing organisms may remain inthe treated wastewater.ChlorinationTo disinfect and kill harmful organisms, the wastewater spends a minimum of 15-20minutes in chlorine-contact tanks. Hypochlorous acid HOCl is used as disinfectingagent.Hypochlorous acid is not stable thus it cannot be stored. So it is generated by dissolvingthe water with any of the following; Molecular chlorine gasCl2 + H2O HOCL + H++ Cl- Sodium hypochlorite
  • 14. 14 Calcium hypochloriteThe treated wastewater, or effluent, is then released into local waterways.Harmful effects of chlorinationReaction of chlorine with dissolved ammonia and organic matters produce detrimentaleffects.Hypochlorous acid reacts with dissolved ammonia to form chloramines NH2Cl, NHCl2and the most important nitrogen trichloride NCl3 which is powerful eye-irritant. Thereaction is given below;NH3 + 3HOCl NCl3 + 3H2OThe alkaline pH can prevent the formation of chloramines.Production of toxic organic compounds in chlorination If phenol is present in water, chlorinated phenols are formed which are toxic intaste and offensive in odor. When Hypochlorous acid reacts with humic acid, chloroform CHCl3 is formed.Chloroform is a liver carcinogen. To avoid the formation of toxic compounds withchlorine, ozone or chlorine dioxide is used for the disinfection of water.3.5 TERMINOLOGIES InfluentThe incoming wastewater, called influent. EffluentThe treated wastewater, or effluent. Primary sludgeThe settled solids in the sedimentation tanks during primary treatment are calledprimary sludge. Activated sludgeThe settled solids in aeration tanks can be termed as activated sludge.The activated sludge process is named so because it involved the production ofan activated mass of microorganisms capable of aerobically stabilizing theorganic content of a waste. Secondary sludgeThe settled solids in final settling tanks (clarifiers) are called secondary sludge. Return sludgeRe-circulated sludge from clarifier to aeration tank is called return sludge.
  • 15. 15Chapter 4ACTIVATED SLUDGE4.1 INTRODUCTIONThe activated sludge process was developed in England in 1914.Activated sludge is probably the most versatile of the biological treatment processescapable of producing an effluent with any desired Biological Oxygen Demand (BOD).4.2 FORMATION OF ACTIVATED SLUDGEThe wastewater contains some suspended and colloidal solids and when agitated in thepresence of air, the suspended solids form nuclei on which biological life develops andgradually build up to larger solids which are known as activated sludge.Activated sludge is a brownish floc-like substance consisting of organic matter obtainedfrom the wastewater and inhabited by myriads of bacteria and other forms of biologicallife.Activated sludge with its living organisms has the property of absorbing or adsorbingcolloidal and dissolved organic matter.The biological organisms utilize the absorbed material as food and convert it to inertinsoluble solids and new bacterial cells. Much of this conversion is a step-by-stepprocess.Some bacteria attack the original complex substances to produce simpler compoundsas their waste products. Other bacteria use the waste products to produce still simplercompounds and the process continues until the final waste products can no longer beused as food for bacteria.4.3 WHY RETURN SLUDGE IS NEEDED?The generation of activated sludge or floc in wastewater is a slow process and theamount so formed from any volume of wastewater during its period of treatment is smalland inadequate for the rapid and effective treatment of the wastewater which requireslarge concentrations of activated sludge.Such concentrations are built up by collecting the sludge produced from each volume ofwastewater treated and re-using it in the treatment of subsequent wastewater flows.The sludge so re-used is known as returned sludge.The purpose of return sludge is to maintain a concentration of activated sludge in theaeration tank sufficient for the desired degree of treatment.
  • 16. 16Ample return sludge pump capacity should be provided since the return sludge volumemay range from 10 to 50 percent of the volume of wastewater being treated andsometimes more. For a conventional plant, the percentage is usually between 20 and30 percent.The surplus, or excess activated sludge, is then permanently removed from thetreatment process and conditioned for ultimate disposal.4.4 REMOVAL OF ACTIVATED SLUDGEExcess activated sludge should be wasted as required to maintain the desired solidsconcentration in the aeration tank.This can be done by either withdrawing mixed liquor directly from the aeration tank or towaste from the sludge return line.4.5 TERMINOLOGIES On-site sludgeSludge produced in septic tanks is termed as on-site sludge. Off-site sludgeActivated sludge in the aeration tanks is referred as off-site sludge.
  • 17. 17Chapter 5MICROORAGNISMS IN ACTIVATED SLUDGE5.1 CLASSIFICATION OF LIVING CELLSAll living cells can be classified into following two types; ProkaryoticProkaryotic cells lack a nucleus and other membrane-bound structures. EukaryoticEukaryotic cells possess these structures. The nucleus is the primary membrane-bound structure in eukaryotic cells.5.2 CLASSIFICATION OF MICROORGANISMSBased upon cellular structure and function, microorganisms are commonly classified as; Prokaryotes Eukaryotes5.2.1 Prokaryotic organisms in the activated sludge processThe predominate type of bacteria present will be determined by following factors; The nature of the organic substances in the wastewater The mode of operation of the plant The environmental conditions present for the organisms in the processThe prokaryotes consist of following two types;1. Eubacteria or ―true‖ bacteria2. Archaebacteria or ―ancient‖ bacteriaThe eubacteria and archaebacteria are the most important microorganisms in biological,wastewater treatment plants.Together, these two prokaryotes, commonly, are referred to as bacteria.5.2.2 Eukaryotic organisms in the activated sludge processThere are four important eukaryotic organisms in the activated sludge process.1. Fungi2. Protozoa3. Rotifers*
  • 18. 184. Nematodes*.These free-living (non-disease-causing) eukaryotes enter wastewater treatment plantsthrough inflow and infiltration (I/I) as soil and water organisms.* Rotifers and nematodes are collectively called metazoan.FungiFungi are relatively rare in activated sludge. When present, most of the fungi tend to beof the filamentous forms which prevent good floc formation and therefore decrease theefficiency of the plant.Following factors stimulate fungi growth; High carbohydrate waste Unusual organic compounds Low pH Low dissolved oxygen concentrations Nutrient deficienciesMost fungi are strict aerobes and can tolerate a low pH and a low nitrogen environment.Although fungi grow over a wide range of pH values (2–9), The optimum pH for most species of fungi is 5.6. The nitrogen nutrient requirement for growth of fungi is approximately one-half asmuch as that for bacteria. In the activated sludge process filamentous fungi may proliferate and contributeto settle-ability problems in secondary clarifiers. The proliferation of filamentous fungi is associated with low pH (<6.5) and lownutrients.
  • 19. 19ProtozoaProtozoa are unicellular organisms.In the activated sludge process, protozoa are placed commonly in five groups accordingto their means of locomotion. These groups are following; Amoebae Flagellates Free-swimming ciliates Crawling ciliates Stalked ciliatesAmoebaeFlagellatesFree-swimming ciliatesCrawling ciliatesStalked ciliates
  • 20. 20Ciliated protozoaCiliated protozoa are the most important groups of protozoa in the activated sludgeprocess.They possess short hair-like structures or cilia that beat in unison to produce a watercurrent for locomotion and food gathering—that is, to bring bacteria into their mouthopening.Ciliated protozoa provide the following benefits to the activated sludge process; Add weight to floc particles and improve their settleability Consume dispersed cells and cleanse the waste stream Produce and release secretions that coat and remove fine solids (colloids,dispersed cells, and particulate material) from the bulk solution to the surface offloc particles Recycle nutrients (nitrogen and phosphorus) through their excretionsRotifers and nematodesRotifers and nematodes are multicellular microscopic animals (metazoan)RotifersNematodesThose also provide numerous benefits to the activated sludge process. The metazoanburrow into floc particles. The burrowing action promotes acceptable bacterial activityfor the degradation of substrates in the core of the floc particle by permitting thepenetration of dissolved oxygen, nitrate (NO-3), substrates, and nutrients.
  • 21. 215.3 SUMMARYActivated sludge is a biological contact process where following microorganisms arecommonly found; Bacteria Fungi Protozoa Small organisms such as;o Rotifers ando Nematode wormsThe bacteria are the most important group of microorganisms for they are the onesresponsible for the structural and functional activity of the activated sludge flocs. Alltypes of bacteria make up activated sludge.References: Waste Water Bacteria by Micael H.Gerardi
  • 22. 22Chapter 6GROWTH OF BACTERIA IN WASTE WATER TREATMENT PLANT6.1 CELLELAR COMPOSITIONS OF BACTERIAThe chemical composition of bacteria is as following; 80% water approximately 20% dry material.Of the dry material, 90% is organic approximatelyA simple organic formula for a bacterial cell that includes nitrogen is C5H7O2N. 10% is inorganicInorganic compounds such as ionized ammonia (NH4+), ammonium salts,nitrate (NO3−) and nitrite (NO2−) are most often used.Although the inorganic composition of bacterial cells is relatively small, a shortage ofany inorganic element can limit bacterial growth and wastewater treatment plantefficiency.6.2 ESSENTIAL ELEMENTS FOR BACTERIA GROWTHThe growth of bacteria in wastewater treatment plants is affected by many factorsincluding the presence of available nutrients like following; Major elementsThe major elements (macroelements) in the composition of bacterial cellsinclude C, H, N, O, P and S. These elements are required in large quantities. Minor elementsThe minor elements (microelements) such as Ca, Fe, K, Mg and Na arerequired in small quantities. Trace elementsThe trace elements including Co, Mn, Mo, Ni and Zn are required in relativelysmall quantities for most bacteria.These wastewaters often are nutrient deficient. The most commonly occurring deficiencies for nutrients in industrialwastewaters are the major elements nitrogen and phosphorus,
  • 23. 23While Deficiencies for minor and trace elements (calcium, cobalt, iron and nickel) dooccur.6.3 SPECIFIC BACTERIA AND CERTAIN ELEMENTS Methane-forming bacteriaSome bacteria such as methane-forming bacteria require major element likesulfur and some minor and trace elements such as Co, Fe and Ni in quantities2–5 times greater than other bacteria.Some bacteria including a small group of methane-forming bacteria useatmospheric or molecular nitrogen (N2). HalophilesHalophiles require large quantities of chlorine (Cl) and sodium. Former of vitamin B12Bacteria that synthesize vitamin B12 require Co in large amounts. Gram-positive bacteriaCalcium is required in large amounts by Gram-positive bacteria for the synthesisof cell walls. HeterotrophsNearly all bacteria obtain carbon from organic compounds AutotrophsNearly all such bacteria obtain carbon from carbon dioxide Oxygen andhydrogen requirements for cellular synthesis are often satisfied together by theavailability of organic compounds. Aerobic and facultative anaerobic bacteriaSulfur is available to aerobic and facultative anaerobic bacteria in the oxidizedform as sulfate (SO42−).Sulfur is available to anaerobic bacteria in the reduced form as sulfide (HS−).Some bacteria are capable of using sulfur-containing amino acids as a source ofsulfur.Phosphorus is available to bacteria as phosphate. The form of phosphate(H2PO4−, HPO42−, or PO43−) used by bacteria is pH-dependent.Consequently, nutrient addition to biological treatment units may be required whensoluble, cBOD-rich industrial wastewaters are being treated.
  • 24. 246.4 FACTORS AFFECTING BACTERIA GROWTHThe growth of bacteria in wastewater treatment plants and consequently treatmentefficiency is influenced by a variety of followings; Nutritional factorsNutritional factors include the availability of followings;o Substrateso Nutrients Physical factorsPhysical factors include followings;o pHo Temperatureo response to free molecular oxygen6.4.1 Physical factors6.4.1.1 pHIt is studied by caterogization as following; Optimum pHBacteria have an optimum pH at which they grow best.For most bacteria the optimum pH usually is near neutral (pH 7) and mostbacteria do not grow at values ±1 unit of their optimum pH and cannot toleratepH values below 4 or above 9.5. Operational pHMost biological treatment units operate at pH values near neutral (6.8 to 7.2) andthese units may experience operational problems at pH values below or above anear neutral pH value.o Operational problems at pH values lower than 6.8Operational problems that may occur in biological treatment units thatexperience pH values lower than 6.8 include the following: Decreased enzymatic activity Increase in hydrogen sulfide (H2S) production Inhibition of nitrification Interruption of floc formation Undesired growth of filamentous fungi and some Nocardioformso Operational problems at pH values higher than 7.2Operational problems that may occur in biological treatment units thatexperience pH values higher than 7.2 include the following: Decreased enzymatic activity
  • 25. 25 Increase in ammonia (NH3) production Inhibition of nitrification Interruption of floc formation6. Groups of bacteria w.r.t. acidity or alkalinityThere are three groups of bacteria with respect to the conditions of acidity or alkalinitythat they can tolerate. These groups include following; AcidophilesAcidophiles or acid-loving organisms grow at pH values lower than 5.4.Thiobacillus and Sulfolobus grow at pH values lower than 2, and many fungiprefer pH values lower than 5. NeutrophilesNeutrophiles grow at pH values from 5.4 to 8.5.Most bacteria in wastewater treatment plants are neutrophiles. AlkalinophilesAlkalinophiles or base-loving organisms grow at pH values from 7 to 11.5.The nitrifying bacteria,Nitrosomonas and Nitrobacter are alkalinophiles. Effect of pH upon the activity of bacteriaIn addition to the effect that pH has upon the activity of bacteria, there are two pH-related operational concerns. First, pH affects the degree of ionization of substrates, nutrients, and toxicwastes and their transportation into bacterial cells. Second, the use of substrates and production of wastes by bacteria maysignificantly change the pH of a biological treatment unit.The change in pH may result in undesired bacterial activity and inefficient treatment ofwastewater or sludge.Examples of pH change in biological treatment units due to bacterial activity include thefollowing: Denitrifying bacteria increase the pH of a biological treatment unit through therelease of hydroxyl ions (OH−). Nitrifying bacteria decrease the pH of an aeration tank through the use anddestruction of alkalinity. Organotrophic bacteria decrease the pH of a biological treatment unit through theproduction of carbonic acid (H2CO3) when they release carbon dioxide. Fermentative bacteria decrease the pH of an anaerobic digester through theproduction of fatty acids.
  • 26. 26 Methane-forming bacteria increase the pH of an anaerobic digester through useof fatty acids, especially acetate.6.4.2 TemperatureTemperature exerts two significant effects upon a bacterial population. First, it affects the rate of diffusion of substrates and nutrients into bacterial cells. Second, it affects the rate of enzymatic activity. With increasing temperature therate of diffusion of substrates and nutrients into bacteria cells increases, and therate of enzymatic activity increases.Therefore, with increasing bacterial activity during warm wastewater temperatures, anoperator of a wastewater treatment plant can decrease solids (bacteria) inventory andstill maintain acceptable treatment of wastewater.However, with decreasing bacterial activity during cold wastewater temperatures, anoperator of a wastewater treatment plant may need to increase solids inventory in orderto maintain acceptable treatment of wastewater.Optimum TemperatureThe impact of temperature upon bacterial activity is significant.For every 10°C rise in temperature, enzymatic activity nearly doubles.However, once the optimum temperature for enzymatic activity and cellular growth hasbeen exceeded, enzymes become denatured (damaged) and can no longer efficientlycatalyze biochemical reactions.Temperature rangeThere are three groups of bacteria with respect to the minimum and maximumtemperatures that they remain active.
  • 27. 27Bacterial group Range of temperature Best growth temperaturePsychrophiles(cold-loving) −10°C to 30°C 12°C to 18°CMesophiles*120°C to 50°C 25°C to 40°CThermopiles*2(heat-loving) 35 to 75°C 50°C to 65°C*1 Mesophiles are common inhabitants of the gastrointestinal tract of humans (bodytemperature approximately 37°C) and enter wastewater treatment plants in largenumbers in human feces. They are present in very large numbers in the activatedsludge process and the mesophilic anaerobic digester.*2Thermophiles are common inhabitants of thermophilic anaerobic digesters andthermophilic composting operations.6.4.3 Response to free molecular oxygenBacteria grow in the presence or absence of free molecular oxygen and can be placedin three groups according to their need for or response to free molecular oxygen. Thesegroups are following; AerobesAerobes require oxygen for the degradation of substrate.Examples of aerobic bacteria in activated sludge process include;o Filamentous organisms haliscomenobacter hydrosis and sphaerotilusnatanso Floc former zoogloea ramigerao Nitrifying bacteria nitrosomonas and nitrobacter AnaerobesAnaerobic bacteria do not use free molecular oxygen for the degradation ofsubstrates.These organisms include;o Sulfate-reducing bacteria that use sulfate (SO42−)
  • 28. 28o Methane-forming bacteria (O2 intolerant) that use carbon dioxide. Facultative anaerobesThe term ―facultative‖ implies the ability to live under different conditions.Facultative anaerobic bacteria have the ability to use free molecular oxygen oranother molecule such as nitrate (NO3−) to degrade substrate.Denitrifying bacteria including bacillus, escherichia and pseudomonas arefacultative anaerobic bacteria.With respect to the quantity of oxygen necessary in activated sludge process to ensureacceptable biological activity by aerobe and facultative anaerobes, there are fouractivities of concern.References:Waste-water treatment technologies, United Nations New York, 2003Waste-water Bacteria by Michael H. GeradiWastewater Engineering Treatment, Disposal, and Reuse by Metcalf & Eddy, 1991,McGraw-Hill, New YorkWastewater Treatment by Sundstrom, D. W. and Klei, H. E., 1979, WastewaterTreatment
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