Economic evaluation of upgrading aeration systems

Transcription

1 Copyright s prt of the Jnury 1984, JURNAL ATER PLLUTIN CNTRL FEDERATIN, shington, D. C Printed in U. S. A. Economic evlution of upgrding ertion systems Michel K. Stenstrom, Hmid R. Vzirinejd, A. S. Ng

2 Economic evlution of upgrding ertion systems Michel K. Stenstrom, Hmid R. Vzirinejd, A. S. Ng Aertion my be the most importnt opertion in wstewter tretment. The ctivted sludge process, currently the most populr method of biologicl wstewter tretment, requires efficient nd relible ertion systems. ther types of tretment processes, such s lgoons, often require ertion to supplement the nturlly occurring oxygen trnsfer. In-strem ertion, prcticed in severl Europen countries to mitigte the effects of oxygendemnding wste dischrged into rivers nd lkes, is n extreme exmple. A vriety of ertion systems re populr with design engineers. Numerous systems hve been developed nd evluted over the lst 50 yers. Mny of these systems hve been unsuccessful nd re rrely found tody, lthough the successful ones hve become quite common. Ech system hs specil dvntges nd disdvntges, nd the process ofsystem selection is often subjective one, bsed on the experience nd confidence of the engineer or user with prticulr system. The ertion system is seldom chosen on n entirely rtionl bsis nd consequently mny modern dy designers use obsolete technology in order to comply with client's request, to mtch n existing system for component comptibility, or to conform with "trdition." The spirling costs of electricity nd other energy forms re now cusing engineers nd their clients to reevlute the design of ertion systems. The economics of n ertion system, prticulrly operting costs, re contributing much more hevily to system selection. For this reson, trnsfer efficiency is incresingly importnt. The object of this pper is to evlute the economics of replcing n older, less efficient ertion system with new, efficient system such s fine bubble diffuser system. Current energy svings re put in perspective with the time-vlue of money (interest rtes), operting costs, nd future costs projections. Upgrding the ertion system for hypotheticl 13.1-m;/ min (5-mgd) tretment plnt is evluted with respect to stndrd efficiencies, process vribles, projected operting costs, nd process flexibility, using investment costs obtined from seven other upgrding projects. CNVERSIN T PRCESS RATES Before n evlution of ertion system lterntives cn be mde, field oxygen trnsfer rtes must be determined. Mnu- 20 A decision to upgrde ertion systems should be bsed on mny opertionl fctors s well s on economics. fcturers usully dvertise stndrd trnsfer rtes determined under stndrd conditions (in tpwtert zero dissolved oxygen (D) concentrtion, 20 C, 36% reltive humidity, nd t brometric pressure corresponding to men tmospheric pressure). Conversion of the stndrd oxygen trnsfer rte to the field rte is done through the use of lph, bet, nd thet fctors, nd the desired dissolved oxygen concentrtion s follows: here: (STR#T f -2o) TRf = C*.20 (Tfwf C*_2o - C) (1) TRf = trnsfer rte t field conditions, = correction fctor for chnge in mss trnsfer coefficient for wstewter contminnts, STR = trnsfer rte t stndrd conditions, B = temperture correction fctor, = 1.024, Tf = field temperture ( C), Tf, w f = correction fctors for oxygen content in ir due to brometric pressure nd humidity, = correction fctor for equilibrium dissolved oxygen concentrtion, C*.Z = equilibrium dissolved oxygen concentrtion t 20 C, nd C = operting dissolved oxygen concentrtion. The Americn Society of Civil Engineers (ASCE) oxygen trnsfer subcommittee hs formulted proposed stndrd procedure' for mking this conversion nd their work should be consulted before using this eqution. Additionl comment on the use of lph, bet, nd thet fctors follows. Alph nd bet fctors were intended to ccount for chnges in oxygen trnsfer rte with wstewter chrcteristics; however, it is now well known tht the lph fctor for n ertion system is s much function ofthe ertion device s wstewter chrcteristics. Gilbert' nd Stenstrom nd Gilbert' nd others hve noted the effects ofertion device type on lph fctors. Their reviews indicte lph fctors for fine bubble diffusers hve frequently been mesured in the rnge of 0.3 to 0.7, nd depend very strongly on degree of tretment. Corse bubble diffusers hve higher lph fctors, with the reported rnge ofvlues for municipl wstewters verging bout 0.8. Alph fctors for the vrious types of surfce ertors hve been reported from 0.6 to 1.2, nd depend on energy density, s well s on other fctors. Unfortuntely, mny current texts on tretment plnt design still use uniform lph fctors for different ertion devices. Journl PCF, Volume 56, Number 1

3 Process Design Tble 1-Hypotheticl plnt specifictions. Plnt size 13.1 m3/min (5.0 mgd) perting D concentrtion 1.0 mg/l Averge lph fctor (fine) 0.4 Averge lph fctor (corse) Kg 02/wire-kh STR (fine) STR (corse) Design exmple retrofit of fine bubble system Note.-Number of new fine bubble diffusers = (7.0 Ib 02/wire-hp-hr) 1.2 Kg 02/wire-kh (2.0 Ib 02/wire-hp-hr) It ppers tht lph fctors re lso highly dependent on turbulence nd power intensity. It ws shown by Stenstrom nd Hwng tht for identicl wstewters thelph fctor cn chnge by s much s 50% depending on power input level. Alph fctors for surfce ertors nd turbine ertors frequently increse with incresing power per unit volume. Consequently, design engineers must be cutious nd conservtive when specifying nd mesuring lph fctors. If more thn one type of ertion device is considered for proposed design, seprte lph fctors should be determined nd specified for ech device. Extreme cre must be exercised when using "bucket" lph fctors for full-scle devices. It ws demonstrted tht improper ppliction of lph fctors determined on smll scle cn be worse thn simply guessing or ssuming lph fctors. It is extremely importnt thn the ertion device used for lph fctor testing be s similr s possible to the device to be used in the full-scle instlltion. ECNMICS AND TIME-VALUE F MNEY There re number of lterntive wys of expressing the time-vlue of money-for exmple, present worth nd nnul cost. The present worth method is used here becuse it is prticulrly suitble when vrying pyments nd benefits re ssessed, lthough identicl conclusions would be obtined if the nnul cost method were used. The benefit-cost rtio is used to compre lterntives, nd represents the rtio of the net present worth of benefits to costs. (Note lso, tht txes re neglected in subsequent evlutions.) The methods used here nd other methods re described in detil by Grnt et L, 5 nd by DeGrmo et l. 6 ECNMICS EXAMPLE PRBLEM The following hypotheticl exmple illustrtes the evlution of the economic merits of different oxygen trnsfer rtes nd systems costs for upgrding n existing plnt, using low efficiency spirl roll system, to high efficiency fine bubble difruser system. This lterntive system ws selected becuse it is the most commonly used by tretment plnt mngers. It is usully less cpitl intensive becuse existing blowers cn normlly be reused. Tble 1 shows the bsic ssumptions for the 13.1 m3/min (5 mgd) hypotheticl tretment plnt. An operting D concentrtion of 1.0 mg/l is selected s the design bsis nd STRs of4.2 Kg Z/wire-khr (7.0 lb Z/wire-hp-hr) for the fine bubble Tble 2-Cpitl costs for upgrding to fine bubble diffusers. Loction Project size (mgd) Yer Number of diffusers Number of diffusers/ mgd (5) riginl cost $/mgd $/Diffuser (5) (7) Current cost $/mgd $/Diffuser ($) constructed Referonce Modgen, UK' [Tnk 1150 $15,725 $ Houck7 18 Tnks xford, UK' /Tnk , , Houck7 8 Tnks xford, UK' /Tnk , , Houck7 4 Tnks Tllmn Islnd, /Tnk , , Houck7 New York, N.Y. 1 Tnk Sepulved, L /Tnk , , Birke 9 Tnks hittier Nrrows, /Tnk , , Yunt9 Los Angeles County 3 Tnks Los Angeles County' ,800-45,400 Yunt9 This study' /Tnk , Tnk ' All costs re for wetted tnk prts only, nd exclude cost of blowers nd ir heders, except for the 1969 xford job which includes cost of blowers, piping nd controls, nd for hittier job which includes the cost of n ir filter. Vribility in cost cn occur becuse of the type of heders used (steel versus stinless), nd how much of the min ir distribution system, including vlves, re replced. riginl cost updted to 1979 by Houck7. Project under construction t the time of this writing, costs reported originlly indexed to Includes the cost of n ir filter. Also one-third of the diffusers were 9-in. pltes tht hve been converted to domes using rtio of 0.8. ' Projected costs for eight tretment plnts. 'Projected costs. Jnury

4 Stenstrom et l. Tble 3-Cost indices. Tble 5-Summry of fifth-yer clening costs. ENR' ENR Yer construction building EPA CE plnt` M & S (5) Tsk Person hours Skill fctor Extension Dome trnsport unloding nd loding (4 hr/ domes) Tnk dewtering nd gross clening (12 hr/1000 domes) "Engineering News-record" index for the fourth qurter of ech Diffuser removl nd yer collection (50 hr/1000 EPA ntionl verge index for 5-mgd plnts for the fourth qurter domes) of ech yer. Dome firing (10 hr/1000 CE plnt cost index, published in Chem. Eng. domes) M & S equipment cost index published in Chem. Eng. Reinstlltion (75 hr/1000 domes) diffuser system nd 1.2 Kg Z/wire-khr (2.0 lb Z/wire-hphr) for n existing corse bubble diffuser system re ssumed. The STR for the fine bubble system is representtive of n verge cse fine bubble diffuser system operting t low energy density (0.008 to 0.13 k/m' or 0.3 to 0.5 hp/1000 cu ft) with blower/motor efficiency of70%. The 2.0 lb 02/wire-hp-hr STR is representtive of well designed nd operted spirl roll system. Averge lph fctors of 0.4 for the fine bubble nd 0.8 for the corse bubble system re ssumed. A bet fctor of0.95 ws lso ssumed. The TR for the fine bubble system is 1.44 Kg Z/wire-khr (2.38 lb Z/wire-hp-hr) nd the TR for the corse bubble system is 0.83Kg Z/wire-khr (1.36 lb Z/wirehp-hr). The cpitl costs to upgrde severl tretment plnts re shown in Tble 2. These figures were obtined by survey of plnts in the Los Angeles re, nd from published dt. Estimtes were updted to December, 1982 using the "Engineering News- Record" cost estimtes shown in Tble 3. The other cost indices re provided s comprison. The plnt dt re included to show the cost vribility nd rnge of recent projects. The cost selected for the exmple problem is slightly higher thn the men cost for the recent projects. This ws done in nticiption of the infltion tht will occur between the time of this writing Tble 4-Annul operting costs. Summry nnul operting costs excluding energy costs Tsk (1) Person hours Skill fctor Extension Air filter mintennce (2 hr/1000/month) Air flow nd hed loss monitoring (2 hr/1000 diffusers/month) Hours subtotl Cost t $14/hr $1650 Replcement ir filters (2/1000/yr, t $50/ech) $ 270 Totl $1920 Hours subtotl Cost t $14/hr Kiln costs Diffuser nd gsket replcement costs ssuming 7% loss nd $10/diffuser stone. Totl $6388. $1000. $1900. $9288. (Jnury, 1983), nd publiction. An informl survey of mnufcturers indictes tht the lowest cost for purchse nd instlltion of the diffusers nd wetted tnk prts, if obtined through competitive bidding, is pproximtely $35 to $40 per diffuser. Actul cost estimtes for ny instlltion should be obtined considering the site-specific spects ofech instlltion. The estimtes provided here should only be used for scoping or plnning purposes. The nnul operting costs except those for energy re shown in Tble 4. Items such s blower mintennce were not considered becuse it ws ssumed they would be the sme for both systems. These costs were estimted bsed on surveys of the Los Angeles re plnts nd the dt presented by Houck nd Boon.' It is lso routinely noted tht fine bubble ertion efficiency declines over time becuse ofgs-side nd liquid-side clogging, nd rnges of 5 to 10% decline per yer re reported. This clogging problem would mke it necessry to clen the diffusers every five yers, which would entil shut-down of n ertion bsin nd clenup with removl nd refiring of ll diffusers. The estimted expenses for clenup shown in Tble 5 were lso developed from survey ofthe Los Angeles re plnts nd the work of Houck nd Boon.' They re slightly higher thn the "rule of thumb" estimtes of $3/diffuser. Power cost ws ssumed to be $0.07/kh, which is typicl forpower in the Los Angeles re. Benefits due to energy svings were clculted from the difference in horsepower consumption of the two systems. In developing this exmple it ws ssumed tht mny components of the corse bubble system, such s blowers, ssocited electricl controls, nd primry ir heder could be slvged nd need not be upgrded or replced. Also it ws ssumed tht the interest rte for both principl nd for discounting of 22 Journl PCF, Volume 56, Number 1

5 Process Design Tble 6-Comprisons of vrious lterntives for economic evlution. Project life (yers) (1) Aertion ssumption Infltion ssumption Benefit cost-rtio Net worth (dollrs) (5) 20 Constnt None Constnt None Declining None (5%/yr) 20 Declining None (10%/yr) 20 Declining Incresing (10%/yr) (5%/yr) 5 Declining Incresing (10%/yr) + D control' (5%/yr) 20 Declining Incresing (10%/yr) + D control' (5%/yr) ' It is ssumed tht D control provides 25% horsepower svings. future benefits nd costs is 12% for ll cses, unless otherwise specified. Tble 6 shows the present worth nd benefit-cost rtio of seven probble investment lterntives, using different scenrios for power cost, declining ertion system efficiency, nd project life. The first row of Tble 6 shows the most bsic cse tht ssumes constnt ertion efficiency nd constnt power costs with 20-yer project life. The net present worth of upgrding the system with fine bubble diffusers is $ The benefitcost rtio is 2.02, which is the rtio of present worth of benefits divided by present worth of costs. Positive vlues of present worth nd benefit-cost rtios greter thn unity indicte economiclly fvorble projects, for the ssumptions stted. Row 2 in Tble 6 is identicl to the exmple in Row 1 except for project life. The benefits decline if the project is required to recover ll cpitl costs in five yers. This decline results from higher principl pyments. Investment is just recovered in the 5-yer period. Row 3 in Tble 6 shows the economic benefits if ertion efficiency of the fine bubble diffuser system declines by 5%/yr with 5-yer clenings, which restore efficiency to its originl vlue. Row 4 is identicl to Row 3 except tht ertion efficiency declines by 10% per yer. Declining efficiency ffects the net worth, but the project is still quite fvorble. Row 5 shows n even more extreme cse, which ssumes n infltion increse of 5% per yer for ll costs, including power. If infltion is ssumed the retrofit project becomes more economicl. The most economicl lterntive, presented in the finl two rows in Tble 6, requires the ddition of D control system tht sves 25% ofthe power costs. It ws ssumed in developing this exmple tht cpitl costs would increse by $ nd tht nnul operting costs would increse by $2000. Tble 7 shows the economics ofthe retrofit project for three economic/process ssumptions: 5%/yr infltion nd constnt efficiency, 10%/yr declining efficiency nd no infltion, nd 10%/ yr declining efficiency with 5%/yr infltion, for project periods from 2 to 10 yers. The pybck period for the 5%/yr infltion, zero-decline ssumption is less thn 4 yers. Pybck periods rnge from 5 to 7 yers depending on the economic ssumptions. Tble 8 shows the present worth of energy svings, neglecting ll costs for the three process/economic ssumptions. These Tble 7-Economic comprisons of vrying project life with different infltion nd clogging ssumptions. Infltion nd efficiency ssumptions' Project life (yers) Benefit-cost rtio Constnt efficiency nd 5%/yr infltion Net worth (dollrs) Benefit-cost rtio Declining efficiency nd zero infltion Net worth (dollrs) (5) Benefit-cost rtio (6) Declining efficiency nd 5%/yr infltion Net worth (dollrs) (7) ' here efficiency declines, rte of 10%/yr is ssumed. Jnury

6 Stenstrom et l. Tble 8-Present worth of energy svings from incresed trnsfer efficiency. Net present worth (dollrs)' Investment period (yers) Const. E/5% Inf. net worth (dollrs) Declining E/% Inf. net worth (dollrs) " Declining E/5% Inf. net worth (dollrs) e here efficiency declines 10%/yr is ssumed. svings were included to show the present worth of potentil syings tht cn be used to predict the economiclly justified investment for ech project life nd ssumption. ther operting costs, including clening cost, were neglected. The net present worth cn be directly compred to the present investment cost of n lterntive. This tble is included so tht comprisons my be mde to investment costs which re different from those ssumed. Tble 9 presents n nlysis of clening frequency. Houck nd Boon' reported tht diffusers re most frequently clened t 6- or 7-yer intervls. A 5-yer period ws ssumed for the previous nlyses. Tble 7 shows the benefit-cost rtios for vrying clening frequency from 2 to 15 yers. Three yers is the most economicl period. A decline of 10%/yr in trnsfer efficiency is sufficiently lrge to wrrnt further developments in clening. An in-situ technique to clen the outside of the diffusers s well s internl stone fouling, would be very economicl if it could be performed t little cost. Figures 1, 2, 3, nd 4 further illustrte the economics of upgrding the ertion system for different process nd economic conditions. Figure 1 shows the benefit-cost rtio for power cost vrying from $0.03 to $0.13/kh while vrying the fine bubble diffuser lph fctor from 0.3 to 0.7. Project life of 20 yers nd 10%/yr declining ertion efficiency with 5-yer clening frequency were used in ll 4 figures. Unless otherwise noted, Tble 9-Clening frequency nlysis. Clening frequency (yers) (1) No infltion 5% Infltion Note-Assuming 10%/yr decline in efficiency with 100% restortion fter clening. the ssumptions presented in Tble 1 were lso used for Figures 1 through 4. Figure 2 shows similr contours for identicl conditions except for vrying power costs nd interest rtes. Agin the project is highly fvorble except t very low power costs nd very high interest rtes. Figures 3 nd 4 show similr contours with 12% interest rtes, $0.07/kh power costs, nd chnging lph fctors for both ertion systems. These figures show the effects of infltion. Figure 4 includes 5%/yr infltion while Figure 3 does not. Infltion lwys mkes the ertion upgrding project more fvorble, becuse energy cost is the primry operting cost. CNCLUSINS The preceding exmple nd discussion shows the economic results of upgrding low-efficiency corse bubble system to t 3Y F 0 U w Figure 1-Benefit-cost rtio for vrying power cost nd fine bubble lph fctor. 24 Journl PCF, Volume 56, Number 1

7 Process Design 0.90 L 3 Y 0 F 0 U. S F y y J m m m w Q U CC H U Q Q S Figure 2-Benefit-cost rtio for vrying power cost nd interest rte. high-efficiency fine bubble system. The exmple is typicl becuse it included well designed diffuser system nd gs flow regime nd the ssumed cost of the upgrde ws verge to modertely high. This exmple is not intended to be typicl of ny prticulr ertion system, but to the best of the uthors' knowledge, is ccurte nd representtive of rel project. The economics ofthe proposed project vry from fvorble to very fvorble, nd illustrte the dvntges of upgrding low-efficiency ertion systems. F y Y to J m U U Q Q S J I I i I i i I INTEREST RATE (% PER YEAR) I I I I I I I I I 1 I ALPHA FACTR, FINE BUBBLE SYSTEM Figure 3-Benefit-cost rtio for vrying corse bubble nd fine bubble lph fctors without infltion L I i I I I i I I I Figure 4-Benefit-cost rtio for vrying corse bubble nd fine bubble lph fctors with 5%/yr infltion. The upgrding project is very fvorble when compred to most public works projects." Declining interest rtes, or use of tx free bonds to obtin lower interest rtes, mke the project even more fvorble. The decision to upgrde should not be mde solely on n economic bsis, but must include opertionl fctors s well. Flexibility is prticulrly importnt. The bility to control nd mintin the desired D concentrtion cn bejust s importnt s the efficiency of the ertion system. The nlyses show tht clening, required for economicl opertion, should be done every 3 yers. The most drmtic result is the utomtic D control. Control would require smll cpitl investment when compred to n entire retrofit, nd svings from this step lone mount to nerly 70% of the energy svings of the entire retrofit. This exmple is not intended to promote the use of fine bubble ertion systems, but to promote the use of higher efficiency systems in generl. In mny pplictions, low-speed mechnicl ertors cn provide energy svings similr to those for fine bubble ertion systems nd should lso be evluted. If dequte clening nd mintennce of fine bubble diffusers cnnot be provided, the lterntive higher efficiency lterntives should be used becuse indequte mintennce of the finebubble system will nullify most of the energy svings. ACKNLEDGMENTS Authors. Michel K. Stenstrom is ssocite professor, Hmid R. Vzirinejd nd Adm S. Ng re postgrdute reserch engineers from the ter Resources Progrm t the School of Engineering nd Applied Science, University of Cliforni, Los Angeles. Correspondence should be ddressed to Michel K. Stenstrom, University ofcliforni, Los Angeles, 7619 Boelter Hll, Engineering Systems Deprtment, School of Engineering nd Applied Science, Los Angeles, CA Jnury

8 Stenstrom et l. REFERENCES 1. Billod, C. R., "Proposed Stndrd for Mesurement of xygen Trnsfer Rte in Clen ter." In "Aertion Systems Design, Testing, pertion nd Control.". C. Boyle (Ed.), Mdison, is. (Aug. 1982). 2. Gilbert, R. G., "Mesurement ofalph nd Bet Fctors." In "Proceedings: orkshop Towrd n xygen Trnsfer Stndrd.". C. Boyle (Ed.), U. S. Environ. Prot. Agency, report EPA-600/ (April 1979). 3. Stenstrom, M. K., nd Gilbert, R. G., "Effects of Alph, Bet, nd Thet Fctor Upon the Design, Specifiction nd pertion of Aertion Systems." ter Res., 15, 643 (1981). 4. Stenstrom, M. K., nd Hwng, H. J., "The Effect ofsurfctnts on Industril Aertion Systems." Proceedings of the 34th Purdue In- dustril ste Conference, Ann Arbor Science, Ann Arbor, Mich., 34, 902 (1980). 5. Grnt, E. L., etl., "Principles of Engineering Economy." (7th Ed.), John iley nd Sons, New York, N. Y. (1982). 6. DeGrmo, P., et l., "Engineering Economy." (6th Ed.), Mcmillin, New York, N. Y. (1979). 7. Houck, D. K., nd Boon, A. G., "Survey nd Evlution of Fine Bubble Diffuser Aertion Equipment." U.S. Environ. Prot. Agency, report EPA-600/ Birk, R., Personl Communiction, Bureu of Snittion, City of Los Angeles, Los Angeles, Clif. (Jn. 1983). 9. Yunt, F.., Personl Communictions, Snittion Districts of Los Angeles County, hittier, Clif. (Jn. 1983). 10. DeGrmo, P., et l., "Engineering Economy." (6th Ed.), Mcmillin, New York, N. Y. (1979). 26 Jouml PCF, Volume 56, Number 1