厭氧消化技術(shù)

1、 Overview of Anaerobic BiotechnologyDefinition and Principle of Anaerobic Processes 定義：有機(jī)物在無(wú)氧或氧的前驅(qū)體（如H2O2）的環(huán)境中被礦化。根據(jù)電子接收的情況，可以將厭氧過(guò)程分為厭氧發(fā)酵和厭氧呼吸。Anaerobic Fermentation厭氧發(fā)酵：在黑暗且無(wú)外界電子受體的條件下，有機(jī)物被嚴(yán)格厭氧或兼性微生物通過(guò)內(nèi)部的氧化還原平衡反應(yīng)分解代謝。The product generated during the process accepts the electrons released during the

2、 breakdown of organic matter. Thus, organic matter acts as both electron donor and acceptor. In fermentation the substrate is only partially oxidized, and therefore, only a small amount of the energy stored in the substrate is conserved. The major portion of the adenosine triphosphate (ATP) or energ

3、y is generated by substrate-level phosphorylation.It is important to point out that the major portion (two-thirds) of methane is produced through anaerobic fermentation in which acetate acts as both electron donor and electron acceptor. Methane production through this route is commonly known as acet

4、otrophic (or acetoclastic) methanogenesis. 葡萄糖厭氧發(fā)酵Anaerobic Respiration 厭氧呼吸正好相反，需要額外的電子受體來(lái)接收有機(jī)物分解過(guò)程中產(chǎn)生的電子。 electron acceptors: CO2, SO42, or NO3 Both substratelevel phosphorylation and oxidative phosphorylation generate energy (or ATP). The energy released under such a condition is much greater tha

5、n anaerobic fermentation. organic matter CO2 CH4electronshydrogenotrophic methanogenesisSome anaerobes use CO2 as an electron acceptor reduce hydrogen to acetic acid.The presence of sulfate in an anaerobic environment diverts part of organic matter toward sulfate reduction by a specialized group of

6、anaerobic bacteria known as sulfate-reducing bacteria (SRB). The release of odorous H2S gas is a characteristic of anaerobic environment in which sulfate acts as an electron acceptor. SRB are mostly obligate anaerobes, although studies have shown that some species of SRB are capable of aerobic respi

7、ration. organic matter NO3- N2electronsdenitrificationWhen NO3 acts as an electron acceptor bacteria involved: nitrate-reducing bacteria (NRB)facultative bacteria, capable of aerobic respiration and/or nitrate respiration.Major anaerobic microbes involved in carbon, nitrogen, and sulfur pollution co

8、ntrol and the respective electron donors, electron acceptors The affinity of microorganism for the electron acceptor is in the following order:O2 NO3 MnO2 FeOH SO42 CO2Important Considerations in Anaerobic Biotechnology Volumetric Organic Loading Rate biomass yield, substrate utilization rate, HRT a

9、nd SRT, start-up time, microbiology, environmental factors, reactor configurationVolumetric Organic Loading RateHigh-rate anaerobic reactors such as UASB, EGSB, anaerobic filter, and fluidized bed reactors are capable of treating wastewater at VOLR of 1040 kg COD/m3 day, and on occasion can exceed 1

10、00 kg COD/m3day in fluidized bed reactors.Ci: biodegradable COD concentration (mg/L), Q: wastewater flow rate (m3/day)V is anaerobic bioreactor volume (m3).VOLR to the reactor is dependent on several factors, such as the kinetics of pollutant degradation, biomass level in the bioreactor, and types o

11、f bioreactorBiomass YieldBiomass yield is a quantitative measure of cell growth in a system for a given substrate.X is increase in biomass concentration (mg VSS/L), and S is decrease (consumed) in substrate concentration (mg COD/L).Of note is the biomass yield per mole of ATP, which totals 10.5 g vo

12、latile suspended solids (VSS) for both aerobic and anaerobic bic ATP generation is 38 mol/glucose, anaerobic ATP generation is 4 mol/glucose a significantly lower biomass yield for the anaerobic treatment processAnaerobic degradation of organic matter is accomplished through a number o

13、f metabolic stages in a sequence by several groups of microorganisms. This differs from the aerobic treatment process, in which such synergistic relation does not exist. The yield coefficient of acid-producing bacteria is significantly different from that of methane-producing bacteria. The aerobic t

14、reatmen process gives a fairly constant yield coefficient for biodegradable COD irrespective of the type of substrates.For an anaerobic system, the yield coefficient depends not only on COD removed but also on the types of substrates being metabolized.Carbohydrate and protein have relatively high yi

15、eld coefficients, as the two groups of microorganisms (acidogens and methanogens) are involved in the metabolism of the substrates to methane. The overall yield coefficients for these substrates are the sum of individual yield coefficient of acidogens and methanogens. Acetate and hydrogen on the oth

16、er hand have relatively low yield coefficients as only methanogens are involved in the metabolism of these substrates.Specific Biological ActivityAnaerobic processes have a substrate utilization rate of 0.751.5 kg COD/kg VSSday, which is more than double that of the aerobic treatment process. These

17、are quite reasonable rates, as O2 transfer/diffusion limitation is not an issue in an anaerobic process, unlike anaerobic system. Furthermore, by maintaining a high concentration of diversified group of biomass in close proximity through biomass immobilization or granulation, a good balance of syntr

18、ophic relation between acidogens and methanogens can be achieved.Hydraulic Retention Time and Solids Retention Time HRT indicates the time the waste remains in the reactor in contact with the biomass. The time required to achieve a given degree of treatment depends on the rate of microbial metabolis

19、m. Waste containing simple compounds such as sugar is readily degradable, requiring low HRT, whereas complex wastes need longer HRT.SRT, on the other hand, controls the microbial mass (biomass) in the reactor to achieve a given degree of waste stabilization.SRT is a measure of the biological systems

20、 capability to achieve specific effluent standards and/or to maintain a satisfactory biodegradation rate of pollutants.Maintaining a high SRT produces a more stable operation, better toxic or shock load tolerance, and a quick recovery from toxicity. The permissible organic loading rate in the anaero

21、bic process is also determined by the SRT. HRT is a deciding factor in process design for complex and slowly degradable organic pollutants, whereas SRT is the controlling design parameter for easily degradable organics.Start-Up TimeStart-up is the initial commissioning period during which the proces

22、s is brought to a point where normal performances of the biological treatment system can be achieved with continuous substrate feeding.Start-up time is one of the major considerations in anaerobic processes because of the slow growth rate of anaerobic microorganisms, especially methanogens, and thei

23、r susceptibility to changes in environmental factors. A start-up time of 24 months is quite common at a mesophilic temperature range (37C). Periods exceeding a year may be needed under thermophilic conditions (55C), due to the high decay rate of biomass. The start-up time also depends on the initial

24、 biomass inventory. The more seed used, the shorter the start-up time.MicrobiologyThe microbiology of the anaerobic treatment system is much more complicated than that of the aerobic one. An anaerobic process is a multistep process in which a diverse group of microorganisms degrades the organic matt

25、er in a sequential order resulting a synergistic action.The stability of an anaerobic treatment system is often debated, mainly due to the fragile nature of microorganisms especially methanogens to the changes in environmental conditions.When an anaerobic treatment system fails, it may take several

26、months for the system to return to a normal operating condition.Reactor ConfigurationThe selection of reactor types is based on the requirement of a high SRT/HRT ratio, so as to prevent the washout of slow-growing methanogens.The treatment performance of the selected reactors is, therefore, mainly d

27、ependent on their capability to retain biomass, thus maintaining a high SRT/HRT ratio.Another approach for reactor configuration selection is based on required effluent quality. Because of relatively high half-saturation constants (Ks) for anaerobic microorganisms, CSTRs may not be suitable, as imme

28、diate dilution of the waste leads to low concentrations of organic matters, but still too high to meet the effluent discharge standards, which are below the range of anaerobic degradation.Merits of Anaerobic Biotechnology Less Energy Requirement For the removal of 1 kg COD, 0.50.75 kg O2 is required

29、 during a conventional aerobic treatment process. The higher end of the range can be explained by the O2 requirement for endogenous respiration. The energy input for the transfer of O2 into liquid for most aerators is in the order of 1 kWh/kg O2. The use of anaerobic treatment provides a net financi

30、al gain through energy generation from methane gas, as well as savings realized through the elimination of energy inputs required for aeration. Less Biomass (Sludge) Generation Aerobic wastewater treatment process generates considerable amounts of sludge. Biological oxidation of every kilogram of so

31、luble BOD produces 0.5 kg of sludge. Anaerobic treatment processes, on the other hand, utilize more than 90% of the biodegradable organic matter (COD) for methane production, with only 10% or less converted to biomass. Furthermore, the anaerobic sludge is well stabilized and needs no further treatme

32、nt other than dewatering for final disposal. A higher organic loading rate is no recommended for aerobic treatment processes primarily due to the following: 1. Limited O2 supply/transfer rate, especially in fixed-film reactors, such as a trickling filter and a rotating biological contactor. 2. Limit

33、ation related to the maintenance of high biomass concentrations due to poor settleability, especially in the activated sludge process. Anaerobic treatment processes are not limited by O2 transfer capability, and extremely high concentrations of biomass can be maintained in high-rate reactors such as

34、 UASB, anaerobic filters, and expanded/fluidized bed reactors. Therefore, loading rates 1020 times higher for anaerobic treatment processes are possible.Limitations of Anaerobic Process The presence of sulfate reduces the methane yield due to substrates (such as hydrogen and acetate) diversion to sulfate reduct