给水排水专业外语论文

1、the activated-sludge processabstract-this paper introduces the composition and principle of activated-sludge process,recent developments and future developments tendencyt also provides a design scheme and a examplekey words-activated-sludge,aeration tank,computer simulatioredesign scheme1n troducti

2、onthe activated-sludge process is a biological method of wastewater treatment that is performed by a variable and mixed community of microorganisms in an aerobic aquatic envir onment. these microorga nisms derive en ergy from carb on aceous organic matter in aerated wastewater for the production of

3、new cells in a process known as synthesis, while simultaneously releasing energy through the conversion of this organic matter into compounds that contain lower energy, such as carbon dioxide and water, in a process called respiration. as well, a variable number of microorganisms in the system obtai

4、n energy by converting ammonia nitrogen to nitrate nitrogen in a process termed nitrification. this con sortium of microorga nisms, the biological comp orient of the process, is know n collectively as activated sludgethe overall goal of the activated-sludge process is to remove substances that have

5、a dema nd for oxygen from the system. this is accomplished by the metabolic reactions (syn thesis-respiratio n and n itrificaction) of the microorga nisms, the separati on and sett i i ng of activated-sludge solids to create an acceptable quality of sec on dary wastewater effluent, and the collectio

6、n and recycling of microorganisms back into the system or removal of excess microorganisms from the system.2.the principle of the activated-sludge process2.1the components of the activated-sludge processbefore begi nning a discussi on of the biological comp orient of the system, an overview of the p

7、hysical components that comprise the activated-sludge process would seem to be in order. this will help the reader gain a better understanding of the environment in which a mixed comm unity of microorga nisms metabolizes orga nic matter, settles to form a thickened sludge, and is recycled back into

8、or removed from the systemaccording to activated sludge, manual of practice(water environment association, 1987),the activated-sludge process con tains five esse ntial interrelated equipme nt components. the first is an aeration tank or tanks in which air or oxygen is introduced into the system to c

9、reate an aerobic environment that meets the needs of the biological community and that keeps the activated sludge properly mixed at least seven modifications in the shape and number of tanks exist to produce variations in the pattern of flow.second, an aeration source is required to ensure that adeq

10、uate oxygen is fed into the tank(s) and that the appropriate mixing takes place. this source may be provided by pure oxygen, compressed air or mechanical aeration. just as there are modifications in the shape and number of aeration tanks that can be used in the activated-sludge process, different eq

11、uipment systems exist to deliver air or oxygen into aeration tanks.third, in the activated-sludge process, aerati on tanks are followed by sec on dary clarifiers. in sec on dary clarifiers, activated-sludge solids separate from the surro unding waterwater by the process of flocculation (the formatio

12、n of large particle aggregates, or flocs, by the adherence of floc-forming organisms to filamentous organisms) and gravity sedimentation, in which flocs settle toward the bottom of the clarifier in a quiesce nt envir onment. this separation leads ideally to the formatio n of a sec on dary efflue nt

13、(wastewater havi ng a low level of activated-sludge solids in suspension) in the upper portion of the clarifier and a thickened sludge comprised of flocs, termed return activated sludge, or ras, in the bottom portion of the clarifier. next, return activated sludge must be collected from the secondar

14、y clarifiers and pumped back to the aeration tank(s) before dissolved oxygen is depleted .in this way, the biological community n eeded to metabolize in flue nt organic or inorganic matter in the wastewater stream is replenished.finally, activated sludge containing an overabundance of microorganisms

15、 must be removed, or wasted (waste activated sludge, or was), from the system this is accomplished with the use of pumps and is done in part to control the food-to-microorganism ratio in the aeration tank(s).2.2the basic process of the activated-sludge processthe process involves air or oxygen being

16、 introduced into a mixture of primary treated or scree ned sewage or in dustrial wastewater (called wastewater from now on) combi ned with organisms to develop a biological floc which reduces the organic content of the sewage. this material, which in healthy sludge is a brown floc, is largely compos

17、ed of saprotrophic bacteria but also has an important protozoan flora mainly composed of amoebae,spirotrichs,peritrichs including vorticellids and a range of other filter feeding species other importa nt con stituents in elude motile and sede ntary rotifers in poorly man aged activated sludge, a ran

18、ge of mucilagi nous filame ntous bacteria can develop including sphaerothus natans which produces a sludge that is difficult to settle and can result in the sludge blanket decanting over the weirs in the settlement tank to severely contaminate the final effluent quality. this material is often descr

19、ibed as sewage fungus but true fun gal comm un ities are relatively un comm on.the combi nation of wastewater and biological mass is commonly known as mixed liquor. in all activated sludge plants, once the wastewater has received sufficient treatment, excess mixed liquor is discharged into settling

20、tanks and the treated supernatant is run off to undergo further treatment before discharge. part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new wastewater entering the tank this fraction of the floc is called return activated sludge (r.a.s.). e

21、xcess sludge is called surplus activated sludge(s.a.s.) or waste activated sludge(w.a.s). s.a.s is removed from the treatment process to keep the ratio of biomass to food supplied in the wastewater in balanee. s.a.s is stored in sludge tanks and is further treated by digestion, either un der an aero

22、bic or aerobic conditions prior to disposal.many sewage treatment plants use axial flow pumps to transfer nitrified mixed liquor from the aeration zone to the anoxic zone for denitrification. these pumps are often referred to as internal mixed liquor recycle pumps (imlr pumps). the raw sewage, the r

23、as, and the nitrified mixed liquor are mixed by submersible mixers in the anoxic zones in order to achieve denitrification.activated sludge is also the name given to the active biological material produced by activated sludge plants.4.developments of activated-sludge process4.1 recent developments o

24、f activated-sludge processan innovative activated sludge system without excess sludge product!on was introduced to the three existing treatment plants receiving of petrochemical wastewater totally about 17.5ton codcr/day and 9,600 m3/day in japan. in the system, simultaneous sludge treatment and was

25、tewater treatment is possible in the same aeration tank. a part of sludge retur ning from the sec on dary settli ng tank is ozon ated to cha nge it to more biodegradable compounds and the ozonated sludge is then put into the aeration tank for biological degradati on. the degree of excess sludge redu

26、ction is con trolled by the changing sludge mass to be ozonated the kinetics and stoichiometries of ozonated sludge were incorporated into activated sludge model no.1 (asm1) to predict mlss concentration and oxygen uptake rate in the each aeration tank. the prediction by the process & hydraulic

27、models matched very reas on ably in the dyn amic conditions with the cha nging in flue nt loadi ng rate and the ozon atio n. the system has bee n dem on strated successfully by minimal excess sludge withdrawing for more than four years the water qualities in the effluent also kept at acceptable leve

28、ls and below the local regulation.4.2future developments of activated-sludge processtill now,numerous modifications of activated-sludge process have evolved in the last 10 to 20 years.ma ny new tech no logies appear7such as mbr,computer modeli ng. nearly all of the various modificati ons are based o

29、n the same fun dame ntal prin ciples of biological treatment.because the design and operation of the activated-sludge process is becoming more complex,a mathematical model of the activated-sludge process has been derived which considers the fate of bacteria which flocculate, bacteria which do not fl

30、occulate, and two forms of ciliated protozoa. computer simulation techniques have been used to study the population dynamics of these organisms in a single completely-mixed and a series of completely-mixed activated-sludge reactor systems; in both cases steady-state solutions were obtained. at stead

31、y state, the concentration of soluble substrate in the effluent is determined by the growth rate (fixed by the sludge-wastage rate) of the sludge bacteria. the concentration of dispersed bacteria in the effluent is similarly determined by the growth rate of the ciliated protozoa. the model predicts

32、that the habit of ciliated protozoa would have a considerable effect on effluent quality. a plant containing only free-swimming ciliates would produce a fairly turbid effluent whereas a plant containing attached ciliates would produce a highly clarified effluent. activated-sludge plants which contai

33、n no protozoa would be expected to deliver very turbid effluents although the concentration of soluble substrates would be precisely the same in all three cases. it was possible to simulate successions of organisms which are qualitatively similar to those observed in practice when an activated-sludg

34、e plant is set into operation. the results of the model predictions are discussed in the light of full-scale and experimental-scale observati ons.5.design of activated-sludge design scheme5.1 activated sludge process variablesthe main variables of activated sludge process are the mixing regime, load

35、ing rate, and the flow scheme.5.2mixing regimegen erally two types of mixing regimes are of major in terest in activated sludge process: plug flow a nd complete mixing. in the first one, the regime is characterized by orderly flow of mixed liquor through the aeration tank with no element of mixed li

36、quor overtaking or mixing with any other element. there may be lateral mixing of mixed liquor but there must be no mixing along the path of flowin complete mixing, the contents of aeration tank are well stirred and uniform throughout. thus, at steady state, the effluent from the aeration tank has th

37、e same composition as the aerati on tank con tents.the type of mixing regime is very important as it affects (1) oxygen transfer requirements in the aerati on tank, (2) susceptibility of biomass to shock loads, (3) local environ mental conditions in the aeration tank, and (4) the kinetics governing

38、the treatment process5.3loading ratea loadi ng parameter that has bee n developed over the years is me hydraulic retention time(hrt), q, d q = vqv= volume of aerati on tank, m3, and q= sewage in flow, m3/d5.4flow scheme the flow scheme involves: the pattern of sewage addition the pattern of sludge r

39、eturn to the aeration tank and the pattern of aeration.sewage addition may be at a single point at the inlet end or it may be at several points along the aeration tank. the sludge return may be directly from the settling tank to the aeration tank or through a sludge reaeration tank aeration may be a

40、t a uniform rate or it may be varied from the head of the aeration tank to its end5.6design considerationthe items for con sideratio n in the desig n of activated sludge plant are aerati on tank capacity and dimensions, aeration facilities, secondary sludge settling and recycle and excess sludge was

41、ting.5.7aeration tank the volume of aeration tank is calculated for the selected value ofqc by assuming a suitable value of mlss concentration，x.vx = yqqc(so s)1 + kdqcalternately, the tank capacity may be designed fromf/m = qso / xvhence, the first step in designing is to choose a suitable value of

42、qc (or f/m) which depends on the expected winter temperature of mixed liquor, the type of reactor, expected settling characteristics of the sludge and the nitrification required. the choice generally lies between 5 days in warmer climates to 10 days in temperate ones where nitrification is desired a

43、longwith good bod removal, and complete mixing systems are employed.the second step is to select two interrelated parameters hrt, t and mlss concentration. it is seen that economy in reactor volume can be achieved by assuming a large value of x. however, it is seldom taken to be more than 5000 g/m3.

44、 for typical domestic sewage, the mlss value of 2000-3000 mg/l if conventional plug flow type aeration system is provided, or 3000-5000 mg/l for completely mixed types considerations which govern the upper limit are: initial and running cost of sludge recirculation system to maintain a high value of

45、 mlss, limitations of oxygen transfer equipment to supply oxygen at required rate in small reactor volume, increased solids loading on sec on dary clarifier which may necessitate a larger surface area, design criteria for the tank and minimum hrt for the aeration tank.the length of the tank depends

46、upon the type of activated sludge plant. except in the case of extended aeration plants and completely mixed plants, the aeration tanks are designed as long narrow channels. the width and depth of the aeration tank depends on the type of aeration equipment employed the depth control the aeration eff

47、iciency and usually ranges from 3 to 4.5 m. the width controls the mixing and is usually kept between 5 to 10 m. width-depth ratio should be adjusted to be between 1.2 to 22. the length should not be less than 30 or not ordinarily ion ger tha n 100 m.5.80xygen requirementsoxygen is reqiured in the a

48、ctivated sludge process for the oxidation of a part of the influe nt orga nic matter and also for the en doge nous respirati on of the micro-orga nisms in the system. the total oxygen requirement of the process may be formulated as follows: o2 required (g/d) = q(so - s) -1.42 qwxrfwhere, f = ratio o

49、f bod5 to ultimate bod and 1 42 = oxygen dema nd of biomass (g/g)the formula does not allow for n itrificatio n but allows only for carb on aceous bod removal.5.9aeration facilitiesthe aeration facilities of the activated sludge plant are designed to provide the calculated oxygen demand of the waste

50、water against a specific level of dissolved oxygen in the wastewater.5.10sludge recyclethe mlss con centrati on in the aerati on tank is con trolled by the sludge recirculati on rate and the sludge settleability and thickening in the secondary sedimentation tank.qr= xq xr-xwhere qr= sludge recircula

51、tion rate, m3/dthe sludge settleability is determined by sludge volume index (svi) defined as volume occupied in ml by one gram of solids in the mixed liquor after settling for 30 min. if it is assumed that sedimentation of suspended solids in the laboratory is similar to that in sedimentation tank,

52、 then xr = 106/svi. values of svi between 100 and 150 ml/g indicate good settling of suspended solids. the xr value may not be taken more than 10,000 g/rrpunless separate thickeners are provided to concentrate the settled solids or sec on dary sedime ntatio n tank is designed to yield a higher value

53、 5.11 excess sludge wastingthe sludge in the aeration tank has to be wasted to maintain a steady level of mlss in the system. the excess sludge qua ntity will in crease with in creasi ng f/m and decrease with increasing temperature. excess sludge may be wasted either from the sludge return line or d

54、irectly from the aeration tank as mixed liquor. the latter is preferred as the sludge concentrati on is fairly steady in that case. the excess sludge gen erated under steady state operation may be estimated byqc= vxqwxror qwxr = yq (so - s) - kd xv6.design exampledesign of completely mixed activated

55、 sludge systemdesign a completely mixed activated sludge system to serve 60000 people that will give a final effluent that is nitrified and has 5-day bod not exceeding 25 mg/l. the following design data is available.sewage flow = 150 l/person-day = 9000 m3/daybods = 54 g/person-day = 360 mg/l ; bodu

56、 = 1.47 bod5total kjeldahl nitrogen (tkn) = 8 g/person-day = 53 mg/lphosphorus = 2 g/person-day = 13.3 mg/lwinter temperature in aeration tank = 18°cyield coefficient y = 0.6 ; decay constant kd = 0.07 per day ; specific substrate utilization rate = (0.038 mg/l)-1 (h)at 18°cassume 30% raw

57、bod5 is removed in primary sedimentation, and bod5 going to aeration is, therefore, 252 mg/l (0.7 x 360 mg/l).design:(a) selection of qcf t and mlss concentration.considering the operating temperature and the desire to have nitrification and good sludge settling characteristics, adopt qc = 5d. as th

58、ere is no special fear of toxic inflows, the hrt, t may be kept between 3-4 h, and mlss = 4000 mg/l.(b) effluent bods：(1/5 + 0.07)substrate concentration, s = 1 (1/qc + kd)二s = 12 mg/l.assume suspended solids (ss) in effluent = 20 mg/l and vss/ss =0.8.if degradable fraction of volatile suspended sol

59、ids (vss) =0.7 (check later), bod5 of vss in effluent = 0.7(0.8x20) = 11 mg/l.thus, total effluent bod5 = 12 + 11 = 23 mg/l (acceptable).(c) aeration tank:vx = yqqc(s0 s) where x = 0.8(4000) = 3200 mg/l1 + kdqcor 3200 v = (0.6"5)(9000)(252-12)1 + (0.07)(5)v= 1500 m3detention time, t = 1500 x 24 = 4h9000f/m = (252-12)(9000) = 0.4