环境生物技术-

1、环境生物技术沈阳大学07环境工程2009-9-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,

2、 in which the 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

3、the appropriate 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 microor

4、ganisms can be maintained 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 we

5、ll as enhanced biological 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-anoxi

6、c zone denitrification ;simultaneous 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 a

7、nd aeration systems, the SBR process 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. Wast

8、ewater treatment is performed by a time 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 development

9、s3.1.5.1 Pure oxygen-activated sludge process3.1.5.2 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 organ

10、ic carbon predominantly as a single soluble substance (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.

11、Municipal wastewater and wastewater from most food processing 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 gr

12、ow biomass. In industrial wastewater treatment plants, missing 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 waste

13、water is measured as the biochemical oxygen demand (BOD5, or BOD20, 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 su

14、bstances in the wastewater are considered as environmental factors that may 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

15、, or circular and have mixers in the center or propellers （螺旋桨） (Fig. 3.12). Rectangular 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 b

16、e installed in almost any type of tank. Tanks for vertical shaft surface aerators are either 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 appropr

17、iate mixers must be installed. Mixing can also be performed by rotating bridges on which diffusers 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 nitrifi

18、cation and denitrification in practice is performed mainly in closed-loop tanks equipped with horizontal-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 diffuse

19、rs arranged in fields in closed-loop tanks .3.3.2 Carbon removal processes In industrialized countries today, removal 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 impo

20、rtant.In designing, provisions should be made for a sludge that settles well a sufficiently high volumetric（体积的，容积的） oxygen 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,

21、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 wastewater 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 med

22、ia trickling filter（滴滤池） as the first stage followed by an activated sludge plant. Often, intermediate（中间的） settling to remove trickling 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

23、 clean water may be lower than of fine bubble（微泡） diffused air systems, surface aeration is in some respects preferable. First, no problems with diffuser clogging or destruction have to be considered; And second, under process conditions the aeration efficiency of fine bubble systems is lower than i

24、n 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 these 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-st

25、age 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 dilution of ammonia nitrogen to a concentration equivalent to the desired effluent concentration of nitrate nitrogen. Consequently, the organics are d

26、iluted 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 wastewater characteristics, the dilution ratio, and the process configuration. Often anaerobic contact tanks （厌氧接触池）for enhanced biological phosphate （磷

27、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） tanks with distinct compartments（隔室） for denitrification and nitrification, e.g., pre-anoxic zone denitrification, step-feed process, o

28、r 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., intermittent（间歇的） nitrificationdenitrification, alternating（交替的） nitrificationdenitrification (Bio-Denitro process), or intermitten

29、t nitrificationdenitrification with intermittent wastewater feeding (JARV process). Closed-loop tanks in which anoxic zones for denitrification and aerobic zones for nitrification are established at the same time (simultaneous nitrificationdenitrification). Activated sludge plants for nitrification

30、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 the final effluent quality. To overcome such problems, implementing（实施，实现） provisions（供给） for some denitrification e

31、ven 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 aerobic zone. Since biodegradation of organic carbon follows first-order kinetics, the anoxic zone may be subdivi

32、ded (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 shown in Section 3.3.1. Since it is often difficult to keep the dissolved oxygen concentration in the out

33、let（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 merely has to push the flow through the dividing wall（隔板）. Process control may be limited to a

34、utomatic 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 outlet of a group of aeration tanks, the signals can be used for additional（附加的） control

35、 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 already off, the set-point for aeration control can be lowered. If ammonia increases

36、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-point for aeration control above the usual 1.52.0 mg L1 of dissolved oxygen. Th

37、is 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 internal recycling（回收，循环） flow can be increased.3.3.3.3 Step-feed denitri

38、fication 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 distributed to each denitrification zone (Fig. 3.17). Although the first

39、 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 and complete denitrification in the anoxic zones, the effluent n

40、itrate 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 not be equal to the inverse number（倒数） of units.

41、 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 (QIR,n) (Eqs. 40 and 41). In existing plants,

42、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 contained in the wastewater flow (x Q) i

43、s 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 considered an advantage, since t

44、he 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 = 0.4 Q, Q2 = 0.33 Q,

45、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 Q

46、IR,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

47、then have to be increased 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 diff

48、erences between the pre-anoxic zone 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

49、some head loss occurs as the water flows 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 proce

50、ss. In both processes, dissolved oxygen that enters 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 w

51、ith a non-aerated zone. 3.3.3.4 Simultaneous nitrification 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 th

52、e load of any wastewater treatment plant fluctuates diurnally, the concentrations of nitrate and ammonia vary inversely when the aerator setting is constant. To achieve the desired nitrogen removal, a process control is therefore required. Pasveer in 1964 16 was the first to report on simultaneous d

53、enitrification in an oxidation ditch. He achieved this by setting the optimal immersion depth of the surface aerator so as to create a sufficiently large anoxic zone. At the Vienna Blumenta plant (Section 3.1.3), the oxygen uptake rate was continuously measured with a special (homemade) respirometer

54、. The respirometer output was used to switch the appropriate number of aerators to achieve the desired nitrogen removal 61. The first real process control for simultaneous denitrification was developed by Ermel 62. A continuous sample flow was separated from the mixed liquor by ultrafiltration and d

55、iverted to a nitrate monitor. At the Salzgitter Bad plant the two closed-loop aeration tanks are operated in parallel. Each tank is equipped with three mammoth rotor surface aerators. One rotor in each tank is operated continuously to create sufficient circulating flow. The two other rotors of each

56、tank are automatically switched on if the nitrate concentration in the sample flow drops to, e.g., SNO3 = 3 mg L1. The aerators were stopped if, e.g., SNO3 reached 6 mg L1 and were switched on again after the set point of SNO3 = 3 mg L1 was reached. Figure 3.20 shows a daily time course of ammonia a

57、nd nitrate levels measured in one of the aeration tanks. Due to the still-low denitrification rate during the period of high ammonia load (8 to 12 AM) the concentration of ammonia rises to about 4 mg L1 N. As the denitrification rate increases after noon, the off-periods of the two aerators become s

58、horter and the ammonia concentration decreases. A more sophisticated control system based on monitoring both nitrate and ammonia was used in the wastewater treatment plant in Hildesheim 63. In Salzgitter Bad an ORP controller was tested as a very simple, low-cost control method. Since the oxidationr

59、eduction potential (ORP) drops sharply, if at zero DO nitrate reaches zero, the controller switches the additional aerators on at a certain slope of ORP. The additional aerators are then operated for a preset period of time. Figure 3.21 shows the time course of ORP. The effluent during the time when

60、 the ORP controller was used was as good as when the nitrate controller was used 64. At about the same time, similar experiments with ORP were conducted in Canada 65. Some newer plants for simultaneous nitrification and denitrification are equipped with propellers (in addition to surface aerators) t