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1、Chapter 14 Biological And Chemical Conversion Technologies,固体废物的全过程管理,Introduction,The purpose of this chapter is to introduce and review the biological and chemical processes that can be used to transform the organic fraction of MSW into gaseous，liquid，and solid conversion products.The major focus
2、of this chapter is on the biological processes，because they have been used most commonly for the transformation of organic waste materials.,14-1 Biological principles,To continue to reproduce and function properly，an organism must have:a source of energy;carbon for the synthesis of new cell tissue，i
3、norganic elements(nutrients)such as nitrogen，phosphorus，sulfur，potassium，calcium，and magnesium.,Nutritional requirements for microbial growth,Carbon and energy sources.Two of the most common sources of carbon for cell tissue:organic carbon;Organisms that use organic carbon for the formation of cell
4、tissue are called heterotrophs.carbon dioxide;Organisms that derive carbon from carbon dioxide are called autotrophs.,The energy needed for cell synthesis may be supplied by light or by a chemical oxidation reaction.phototrophs：Those organisms that are able to use light as an energy source.Phototrop
5、hic organisms may be either heterotrophic(certain sulfur bacteria)or autotrophic(algae and photosynthetic bacteria).Chemotrophs：organisms that derived their energy from chemical reactions.chemotrophs may be either heterotrophic(protozoa原生动物,fungi真菌,and most bacteria细菌)or autotrophic(nitrifying bacte
6、ria),Table14-1 General classification of microorganisms by source of energy,Nutrient and growth factor requirements.Nutrients，rather than carbon or an energy source，may at times be the limiting material for microbial cell synthesis and growth.The principal inorganic nutrients needed by microorganism
7、s arenitrogen(N)，sulfur(S)，phosphorus(P)，potassium(K)，magnesium(Mg)，calcium(Ca)，iron(Fe)，sodium(Na)，chlorine(Cl).Minor nutrients of importance include zinc(Zn)，manganese(Mn)，molybdenum(Mo)，selenium(Se)，cobalt(Co)，copper(Cu)，nickel(Ni)，tungsten(W).,Microbial nutrition and biological conversion proces
8、ses.The major objective in most biological conversion processes is the conversion of the organic matter in the waste to a stable end product.In accomplishing this type of treatment，the chemoheterotrophic organisms are of primary importance because of their requirement for organic compounds as both c
9、arbon and energy source.,Chemoheterotrophic organisms may be further grouped according to their metabolic type and their requirement for molecular oxygen.respiratory metabolism:Organisms that generate energy by enzyme-mediated electron transport from an electron donor to an external electron accepto
10、r(such as oxygen)are said to have a respiratory metabolism.fermentative metabolism:does not involve the participation of an external electron acceptor.,Types of microbial metabolism,respiratory metabolismaerobic respiration(有氧呼吸):molecular oxygen is used as the electron acceptor in respiratory metab
11、olism.anoxic respiration(无氧呼吸):In environmental engineering，processes that make use of these organisms,such as nitrate and sulfate which can function as electron acceptors for some respiratory organisms in the absence of molecular oxygen,are often referred to as anoxic.,obligate anaerobic(专性厌氧):Orga
12、nisms that generate energy by fermentation and that can exist only in an environment that is devoid of oxygen.facultative anaerobes（兼性厌氧微生物）:organisms has the ability to grow in either the presence or the absence of molecular oxygen.,Table14-2 Typical electron acceptors in bacterial reactions,Microo
13、rganisms are commonly classified，on the basis of cell structure and function，as eucaryotes(真核生物)，eubacteria（真细菌），and archaebacteria（原始细菌），as shown in Table 14-3.The procaryotic（原核的）groups(eubacteria and archaebacteria)are of primary importance in biological conversion of the organic fraction of soli
14、d wastes and are generally referred to simply as bacteria.The eucaryotic（真核的）group includes plants，animals，and protists（原生生物）.,Types of microorganisms,Table 14-3 Classification of microorganisms,Eucaryotes（真核生物）important in biological conversion of organic wastes include(1)fungi（真菌），(2)yeasts（酵母菌），(
15、3)actinomycetes（放线菌）.,Bacteria（细菌）.Typically，bacteria are single cellsspheres，rods，or spirals.Bacteria are ubiquitous in nature and are found in aerobic(in the presence of oxygen)and anaerobic(in the absence of oxygen)environments.Because of the wide variety of inorganic and organic compounds that c
16、an be used by bacteria to sustain growth，bacteria are used extensively in a variety of industrial operations to accumulate intermediate and end products of metabolism.,Fungi真菌.Fungi are considered to be multicellular，nonphotosynthetic，heterotrophic protists.Most fungi have the ability to grow under
17、low-moisture conditions，which do not favor the growth of bacteria.In addition，fungi can tolerate relatively low pH values.Because of their ability to degrade a wide variety of organic compounds over a broad range of environmental conditions，fungi have been used extensively in industry for the produc
18、tion of valuable compounds，such as organic acids，various antibiotics，and enzymes.,Yeasts(酵母菌).Yeasts are fungi that cannot form filaments(mycelium)and are therefore unicellular.In general，wild yeasts are of little value，but cultured yeasts are extensively to ferment sugars to alcohol and carbon diox
19、ide.Actinomycetes.The actinomycetes are a group of organisms with intermediate properties between bacteria and fungi.In industry，this group of microorganisms is used extensively for the production of antibiotics.Because their growth characteristics are similar，actinomycetes are often grouped with fu
20、ngi for discussion purposes.,Environmental conditions of temperature and pH have an important effect on the survival and growth of microorganisms.In general，optimal growth occurs within a fairly narrow range of temperature and pH values，although the microorganism may be able to survive within much b
21、roader limits.,Environmental requirements,Table 14-4 Some typical temperature ranges for various bacteria,TemperatureAccording to the temperature range in which they function best，bacteria may be classified aspsychrophilic，mesophilic，thermophilic.Typical temperature ranges for bacteria in each of th
22、ese categories are presented in Table 14-4.,pH The hydrogen ion concentration，expressed as pH，is not a significant factor in the growth of microorganisms，in and of itself，within the range from 6 to 9(which represents a thousandfold difference in the hydrogen ion concentration).pH 6.5-7:the optimum p
23、H for bacteria growthpH 9.0 4.5Undissociated molecules of weak acid or bases can enter the cell more easily than hydrogen or hydroxide ions and,by altering the internal pH,damage the cell.,Moisture content is another essential environmental requirement for the growth of microorganisms.The moisture c
24、ontent of the organic wastes to be converted must be known，especially if a dry process such as composting is to be used.,The biological conversion of an organic waste requires the biological system to be in a state of dynamic equilibrium.To establish and maintain dynamic equilibrium，the environment
25、must be free of inhibitory concentrations of heavy metals，ammonia，sulfides，and other toxic constituents.,Aerobic biological transformations,In many cases the ammonia，NH3，produced from the carbonaceous oxidation of organic matter is oxidized further to nitrate，NO3-(a process known as nitrification).,
26、The general aerobic transformation of solid waste can be described by means of the following equation.,Process microbiology.The biological conversion of the organic fraction of municipal solid waste under anaerobic conditions is thought to occur in three steps(Fig.14-1).The first step:the enzyme-med
27、iated transformation of high-molecular-mass compounds into compounds suitable for use as a source of energy and cell tissue.The second step:the bacterial conversion of the compounds resulting from The first step into identifiable lower-molecular-mass intermediate compoundsTe third step:the bacterial
28、 conversion of the intermediate compounds into simpler end products,principally methan and carbon dioxide.,Anaerobic biological transformations,Theoretical stages,Biochemical pathways.It is important to note that methane bacteria can only use a limited number of substrates for the formation of metha
29、ne.Currently，it is known that methanogens use the following substrates:CO2+H2，formate，acetate，methanol，methylamines，and carbon monoxide.,4H2+CO2 CH4+2H2O 4HCOOH CH4+3CO2+H2O CH3COOH CH4+CO2 4CH3OH 3CH4+CO2+2H2O4(CH3)3N+6H2O 9CH4+3CO2+4NH3 4CO+2H2O CH4+3CO2,In anaerobic fermentation,The two important
30、 pathways:4H2+CO2 CH4+2H2O CH3COOH CH4+CO2,Environmental factors.To maintain an anaerobic treatment system that will stabilize an organic waste efficiently，the nonmethanogenic and methanogenic bacteria must be in a state of dynamic equilibrium.void of dissolved oxygen,free from inhibitory concentrat
31、ions of free amonia and such contents as heavy metals and sulfides,The PH should range from 6.5-7.5(sufficient alkalinity should be present to)A sufficient amounts of nutrients such as N,PTemperature,Gas production.The general anaerobic transformation of solid waste can be described by means of the
32、following equation.,CaHbOcNd nCwHxOyNz+mCH4+sCO2+rH2O+(d-nx)NH3 Where s=a-nw-m r=c-ny-2s,Biological process selection Aerobic and anaerobic processes both have a place in solid waste management.Each process offers different advantages.anaerobic processes:the operation is more complex offer the benef
33、it of energy recovery in the form of methane gas and thus are net energy producersAerobic processes:net energy users because oxygen must be supplied for waste conversion relatively simple operation and，if properly operated，can significantly reduce the volume of the organic portion of MSW.,14-2 Aerob
34、ic composting,Aerobic composting is the most commonly used biological process for the conversion of the organic portion of MSW to a stable humus-like material known as compost.Applications of aerobic composting include yard waste，separated MSW，commingled MSW，co-composting with wastewater sludge.,Pro
35、cess description All aerobic composting processes are similar in that they all incorporate three basic steps:preprocessing of the MSW，aerobic decomposition of the organic fraction of the MSW，product preparation and marketing.,Fig.14-2,Windrow，aerated static pile，and in-vessel are the three principal
36、 methods used for the composting of the organic fraction of MSW(Fig.14-2).,Process microbiology During the aerobic composting process a succession of facultative and obligate aerobic microorganisms is active.In the beginning phases of the composting process，mesophilic bacteria are the most prevalent
37、.(15-45)After the temperatures in the compost rise，thermophilic bacteria predominate，leading to thermophilic fungi，which appear after 5 to 10 days.(45-70)In the final stages，or curing period as it is sometimes known，actinomycetes and molds appear.,Critical parameters:moisture content，C/N ratio，and t
38、emperature.For most biodegradable organic wastes,once the moisture content is brought to a suitable level(50-60 percent)and the mass aerated,microbial metabolism speeds up.The aerobic microorganisms,which utilize oxygen,feed upon the organic matter and develop cell tissue from nitrogen,phosphorus,so
39、me of the carbon,and other required nutrients.Much of the carbon served as a source of energy for the organisms and is burned up and respired as carbon dioxide.,Design and operational considerations,Particle size.Most materials that comprise the organic fraction of MSW tend to be irregular in shape.
40、This irregularity can be reduced substantially by shredding the organic materials before they are composted(Fig.14-4).Particle size influences the bulk density，internal friction and flow characteristics，and drag forces of the materials.,Carbon-to nitrogen ratio.The most critical environmental factor
41、 for composting is the carbon-to nitrogen ratio(C/N ratio).The optimum range for most organic wastes is from 20 to 25 to 1.in general，all of the organic nitrogen present in most organic compounds will become available，whereas not all of the organic carbon will be biodegradable.,Blending and seeding.
42、Two design factors that may affect the blending of organic fraction of municipal solids waste for composting are C/N ratio and moisture content.Laboratory analyses are usually required to determine how the various organic materials should be blended for aerobic composting.Seeding involves the additi
43、on of a volume of microbial culture sufficiently large to effect the decomposition of the receiving material at a faster rate.,Moisture content.The optimum moisture content for aerobic composting is in the range of 50 to 60 percent.Moisture can be adjusted by blending of components or by addition of
44、 water.When the moisture content of compost falls below 40 percent，the rate of composting will be slowed.,Mixing/Turning.Initial mixing of organic wastes is essential to increase or decrease the moisture content to an optimum level.Mixing can be used to achieve a more uniform distribution of nutrien
45、ts and microorganisms.Turning of the organic material during the composting process is a very important operational factor in maintaining aerobic activity.,Temperature.Aerobic composting systems can be operated in either the mesophilic，30 to 38（85 to 100），or the thermophilic，55 to 60(131 to 140)，tem
46、perature regions.A typical temperature profile range observed in windrow composting is illustrated in Fig.14-5.,Control of pathogens(病原体).The destruction of pathogenic organisms is an important design element in the compost process，as it will affect the temperature profile and aeration process.Elimi
47、nation of all pathogenic microorganisms can be accomplished by allowing the composting waste to reach a temperature of 70C for 1 to 2 hours.,TABLE14-8 temperature and time of exposure required for destruction of some common pathogens and parasites,Air requirements.In processes with forced aeration，s
48、uch as the aerated static pile and the in-vessel systems，the total air requirement and air flow rate are essential design parameters.,pH control.Control of pH is another important parameter in evaluating the microbial environment and waste stabilization.The pH value，like the temperature，of the compo
49、st varies with time during the composting process.The initial pH of the organic fraction of MSW is typically between 5 and 7.,degree of decomposition.A suitable methodology for the measurement of the degree of decomposition is not available.However，several methodologies have been proposed.The labora
50、tory analysis of chemical oxygen demand(COD)and the lignin test provide a quick check for determining the degree of decomposition.,Control of odor.The majority of the odor problems in aerobic composting processes are associated with the development of anaerobic conditions within the compost pile.In
