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1、环境生物技术2023-9-gying3 Activated Sludge Process3.1.1 Single-stage processFig.Flow diagram of anactivated sludge plant.3 Activated Sludge Process3.1.1 Single-stage processAn activated sludge plant is characterized by four elements:An aeration tank equipped with appropriate aeration equipment,in which th
2、e biomass is mixed with wastewater and supplied with oxygen.A final clarifier,in which the biomass is removed from the treated wastewater by settling or other means.Continuous collection of return sludge and pumping it back into the aeration tank.Withdrawal of excess sludge to maintain the appropria
3、te concentration of mixed liquor.3.1.2 Two-stage processThe two-stage process has several advantages.Harmful substances can be removed in the first stage,which is important for the treatment of industrial wastewater;and in the low-load second stage,due to the high sludge age microorganisms can be ma
4、intained that are able to remove slowly biodegradable organics or to oxidize ammonia.Furthermore,bulking sludge is only rarely observed in either stage.The disadvantages are that about twice as many clarifiers are needed as in the one-stage process and that nitrogen removal,as well as enhanced biolo
5、gical phosphate removal,may be inhibited owing to missing organics,which are removed in the first stage.3.1.3 Single sludge carbon,nitrogen,and phosphorous removalIn the early 1960s three different methods for nitrogen removal were demonstrated:post-denitrification;pre-anoxic zone denitrification;si
6、multaneous denitrification;Today most newer plants are built with means for enhanced biological phosphate removal and/or equipment for simultaneous precipitation.3.1.4 Sequencing batch reactor(SBR)processBecause of the development of reliable automatic process control and aeration systems,the SBR pr
7、ocess today is a perfect alternative to the conventional activated sludge process.The reactor is usually equipped with an aeration system,a mixing device,a decanter to withdraw treated wastewater,an excess sludge removal device,and the process control system.Wastewater treatment is performed by a ti
8、me series of process phases(fill,react,settle,decant).As in the conventional activated sludge process,the SBR process is capable of carbon,nitrogen,and phosphorous removal.It is used for both industrial and municipal wastewater.3.1.5 Special developments3.1.5.1 Pure oxygen-activated sludge process3.
9、1.5.2 Attached growth material in activated sludge aeration tanks3.1.5.3 High-rate reactors3.1.5.4 Membrane separation of mixed liquor3.2 Technological and microbiological aspects3.2.1 Wastewater characteristicsWastewater to be treated can contain organic carbon predominantly as a single soluble sub
10、stance(e.g.,an alcohol),as a mixture of soluble substances,or as a mixture of solids and numerous soluble organic substances.Many industrial wastewaters are mixtures of soluble organic substances,but some may contain mainly one organic substance.Municipal wastewater and wastewater from most food pro
11、cessing industries is always a mixture of soluble and particulate organic matter.Microbial degradation of organic carbon requires certain amounts of nitrogen,phosphorous,calcium,sodium,magnesium,iron,and other essential trace elements to grow biomass.In industrial wastewater treatment plants,missing
12、 elements have to be added.Since domestic wastewater contains all the necessary elements in excess,it is advantageous to treat special industrial wastewater together with municipal wastewater.The concentration of organic matter in wastewater is measured as the biochemical oxygen demand(BOD5,or BOD20
13、,incubation period of 5 or 20 d,nitrification inhibited),chemical oxygen demand(COD),or total organic carbon(TOC).3.2.6 Environmental factors 3.2.6.1 Dissolved oxygenThe dissolved oxygen concentration(DO),the pH,and toxic substances in the wastewater are considered as environmental factors that may
14、inhibit the biological reactions.3.2.6.2 Alkalinity and pH3.2.6.3 Toxic substancesThanks!3.3 设备结构3.3 Plant Configurations3.3.1 Typical Tanks for mixing and aeration Mixing tanks for denitrification can be square,rectangular,or circular and have mixers in the center or propellers（螺旋桨）(Fig.3.12).Recta
15、ngular tanks can be visualized as a series of square tanks.In closed-loop,tanks either propellers or vertical shaft（轴）impellers（推动器）maintain the circulating flow.Fine-bubble diffused-air aeration(曝气)systems can be installed in almost any type of tank.Tanks for vertical shaft surface aerators are eit
16、her square or rectangular in which the length is a multiple of the width(Fig.3.13).Tanks for cyclic（循环）aeration are preferably（更好的，更可取的）circular or of closed loop type.Aeration equipment as well as appropriate mixers must be installed.Mixing can also be performed by rotating bridges on which diffuse
17、rs are mounted.(Fig.3.14).Since the aeration is switched off cyclically（循环的）,only non-clogging aeration systems,e.g.,membrane diffusers for fine bubble aeration are appropriate.Simultaneous nitrification and denitrification in practice is performed mainly in closed-loop tanks equipped with horizonta
18、l-axis（水平的，轴）surface aerators,e.g.,mammoth（巨大的）rotors（旋翼）or vertical shaft surface aerators,as in the carousel（转盘）process.Simultaneous denitrification can also be performed with air diffusers arranged in fields in closed-loop tanks.3.3.2 Carbon removal processes In industrialized countries today,rem
19、oval of both carbon and nitrogen is increasingly required;however,in developing countries and for pretreatment of industrial and trade wastewater,removal of organic carbon only is important.In designing,provisions should be made for a sludge that settles well a sufficiently high volumetric（体积的，容积的）o
20、xygen transfer rate a robust（强壮的）aeration system,especially in developing countries and in industrial plants Sludge settling（沉淀）is improved by plug-flow（阻塞流）type aeration tanks,aeration tanks constructed as a cascade（串联）,and/or use of a selector（选择器）.Two-stage plants may be a choice if the wastewate
21、r is highly concentrated.In Germany,numerous plants that receive wastewater with a high fraction of readily（容易的）biodegradable organics have been operated with a plastic media trickling filter（滴滤池）as the first stage followed by an activated sludge plant.Often,intermediate（中间的）settling to remove trick
22、ling filter sludge was not implemented.Although the aeration efficiency(expressed as kg oxygen transferred per kWh at zero DO)of surface aeration systems measured in clean water may be lower than of fine bubble（微泡）diffused air systems,surface aeration is in some respects preferable.First,no problems
23、 with diffuser clogging or destruction have to be considered;And second,under process conditions the aeration efficiency of fine bubble systems is lower than in clean water,which is not true of surface aeration systems.The weak points of surface aerators are the bearings（轴承）and the gears（齿轮）.If thes
24、e are properly designed,maintenance is limited to lubrication（润滑）and changing gear oil.3.3.3 Nitrogen removal processes3.3.3.1 Introduction The single-stage activated sludge process for nitrogen removal,when the organic matter of the wastewater is used for denitrification,incorporates（合并，一体化，混合）the
25、dilution of ammonia nitrogen to a concentration equivalent to the desired effluent concentration of nitrate nitrogen.Consequently,the organics are diluted by the same ratio.Whether conditions favorable(适宜的)to the development of filamentous（丝状的，细丝质的）bacterial growth are created depends on the wastewa
26、ter characteristics,the dilution ratio,and the process configuration.Often anaerobic contact tanks（厌氧接触池）for enhanced biological phosphate（磷酸盐）removal are considered for the purpose of suppressing（抑制）filamentous growth.The processes for nitrogen removal can be divided into three groups:Subdivided（细分
27、）tanks with distinct compartments（隔室）for denitrification and nitrification,e.g.,pre-anoxic zone denitrification,step-feed process,or post-denitrification process.Completely mixed or closed-loop tanks in which conditions for nitrification or denitrification are established periodically（周期性的）,e.g.,int
28、ermittent（间歇的）nitrificationdenitrification,alternating（交替的）nitrificationdenitrification(Bio-Denitro process),or intermittent nitrificationdenitrification with intermittent wastewater feeding(JARV process).Closed-loop tanks in which anoxic zones for denitrification and aerobic zones for nitrification
29、 are established at the same time(simultaneous nitrificationdenitrification).Activated sludge plants for nitrification only can suffer from a too-low residual alkalinity（碱度，碱性）.Especially if the aeration tank is completely mixed due to denitrification in the final clarifier,sludge can float and harm
30、 the final effluent quality.To overcome such problems,implementing（实施，实现）provisions（供给）for some denitrification even when nitrification only is required is strongly recommended.3.3.3.2 Pre-anoxic zone denitrification The activated sludge tank is divided into two main parts,the anoxic zone and the ae
31、robic zone.Since biodegradation of organic carbon follows first-order kinetics,the anoxic zone may be subdivided(Fig.3.16).Furthermore.the last one or two anoxic compartments（隔室）may also be equipped with aeration installations,for flexibility（灵活性）.The aeration tank may have any configuration,as show
32、n in Section 3.3.1.Since it is often difficult to keep the dissolved oxygen concentration in the outlet（出口）zone as low as desired,a final nonaerated zone,from which the internal recirculation（再循环，回流）flow is withdrawn（收回，撤回）,may be appropriate.If the tank is arranged in a U-shape,the recycling pump m
33、erely has to push the flow through the dividing wall（隔板）.Process control may be limited to automatic control of the aeration intensity,to maintain a preset（预设，预置）dissolved oxygen concentration.If monitors for ammonia and nitrate are installed at the aeration tank outlet（出口，出水口）or even better at the
34、outlet of a group of aeration tanks,the signals can be used for additional（附加的）control measures,the purposes of which may be:Saving energy:If the concentration of ammonia is zero and that of nitrate is in the desired range,aeration of the denitrification cells can be switched off or,if they are alre
35、ady off,the set-point for aeration control can be lowered.If ammonia increases the opposite measures have to be taken.Keeping ammonia low in the winter:If at very low temperatures of the mixed liquor the concentration of ammonia increases,one might try to improve nitrification by raising the set-poi
36、nt for aeration control above the usual 1.52.0 mg L1 of dissolved oxygen.This was successful at some plants.Improving nitrate removal:If the concentration of ammonia is about zero but the concentration of nitrate is at its upper limit,aeration in the denitrification cells can be stopped and/or the i
37、nternal recycling（回收，循环）flow can be increased.3.3.3.3 Step-feed denitrification process The activated sludge tank in the step-feed denitrification process consists of two to three pre-anoxic zone units in series:the return sludge is diverted to the first denitrification zone and the wastewater is di
38、stributed to each denitrification zone(Fig.3.17).Although the first experiments on this process were performed in the UK 英国 and later in Japan 59,the first results for a full-scale（全尺寸，实比）plant were reported by Schlegel 施勒格尔 in Germany.Again,considering complete nitrification in the aeration zones a
39、nd complete denitrification in the anoxic zones,the effluent nitrate concentration for the last denitrification zone can be calculated by mass balance(Eq.39):(x Q)SNH4,N=(Q+QRS+QIR,n)SNO3,e(39)The flow(x Q)is the fraction of the total wastewater flow entering the last denitrificationzone,and x must
40、not be equal to the inverse number（倒数）of units.To increase nitrate removal in the step-feed process,it is necessary to decrease the fraction of wastewater diverted to the last denitrification zone(decrease x)or to increase the return sludge flow and/or the internal recirculation of the last unit (QI
41、R,n)(Eqs.40 and 41).In existing plants,however,internal recirculation is generally not used with the step-feed process.Since increasing the return sludge flow (QRS)can hinder（阻碍）the final clarification（净化，清洁）,it seems more appropriate to decrease x.This is possible only if the organic carbon contain
42、ed in the wastewater flow(x Q)is sufficient to denitrify the incoming nitrate load.Due to the stepwise（逐渐的）dilution of the return sludge with wastewater,the MLSS drops from unit to unit.The result is a higher average concentration of MLSS than in the effluent of the last aeration tank.This is consid
43、ered an advantage,since the MLSS of the effluent determines the size of the final clarifiers.To maintain the same loading rates in the three compartments（隔室）it is possible to chose appropriate tank volumes or,which is easier,to distribute the wastewater flow appropriately.This would occur when Q1=0.
44、4 Q,Q2=0.33 Q,and Q3=0.27 Q.The MLSS would then be,respectively,4.3 kg m3,3.5 kg m3,and 3.0 kg m3.Without internal recirculation,the ratio CCOD/SNO3,which determines the degree of nitrate removal,in the three denitrification zones differs considerably（相当的）.With QIR,1=1.01.5 Q,QIR,2=0.40.5 Q,and QIR,
45、3=0,CCOD/SNO3 takes the same value.In the three-step process with QRS=Q,QIR,3=0,and x=0.27,the denitrification efficiency is 86%(Eq.41).If with more-concentrated（集中的，浓缩的）wastewater a low nitrate concentration must be maintained,QIR,3(Eq.41)has to be selected.QIR,1 and QIR,2 then have to be increased
46、 appropriately.The similarity（相似）of the pre-anoxic zone denitrification process and the step-feed process is obvious when Eqs.38 and 41 are compared.To achieve 86%denitrification efficiency in the pre-anoxic zone process,the recycling ratio must be QR/Q=6.4.The differences between the pre-anoxic zon
47、e process and the step-feed process are illustrated（图解）in Figure 3.18.The advantages of the pre-anoxic zone process are that each of the three tanks is operated independently and that the tanks all have the same water level（含量）and depth.In the step-feed process some head loss occurs as the water flo
48、ws from one tank to the next;therefore,either the water depth differs from tank to tank(same bottomlevel)or the water depth is kept constant(different bottom levels).For maintenance,it is necessary to have a bypass（支路）for each tank in the step-feed process.In both processes,dissolved oxygen that ent
49、ers the denitrification zone removes organic carbon and hence（因此）decreases denitrification.Therefore,non-aerated outlet zones are shown in the pre-anoxic zone tanks.In the step-feed process only the first and second aeration tanks may be equipped with a non-aerated zone.3.3.3.4 Simultaneous nitrific
50、ation and denitrification The key to nitrogen removal by the simultaneous nitrificationdenitrification process is to appropriately set the aerators so as to establish sufficiently large aerobic and anoxic zones simultaneously(Fig.3.19).Since the load of any wastewater treatment plant fluctuates diur