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外文文献翻译
徐雯雯:
外文文献翻译
RecirculatingAquacultureTank
ProductionSystems
AnOverviewofCriticalConsiderations
ThomasM.Losordo,MichaelP.MasserandJamesRakocy
Traditionalaquacultureproductioninpondsrequireslargequantitiesofwater.Approximately1milliongallonsofwaterperacrearerequiredtofillapondandanequivalentvolumeisrequiredtocompensateforevaporationandseepageduringtheyear.Assuminganannualpondyieldof5,000poundsoffishperacre,approximately100gallonsofwaterarerequiredperpoundoffishproduction.InmanyareasoftheUnitedStates,traditionalaquacultureinpondsisnotpossiblebecauseoflimitedwatersuppliesoranabsenceofsuitablelandforpondconstruction.
Recirculatingaquacultureproductionsystemsmayofferanalternativetopondaquaculturetechnology.Throughwatertreatmentandreuse,recirculatingsystemsuseafractionofthewaterrequiredbypondstoproducesimilaryields.Becauserecirculatingsystemsusuallyusetanksforaquacultureproduction,substantiallylesslandisrequired.
Aquaticcropproductionintanksandracewayswheretheenvironmentiscontrolledthroughwatertreatmentandrecirculationhasbeenstudiedfordecades.Althoughthesetechnologieshavebeencostly,claimsofimpressiveyieldswithyear-roundproductioninlocationsclosetomajormarketsandwithextremelylittlewaterusagehaveattractedtheinterestofprospectiveaquaculturists.Inrecentyears,avarietyofproductionfacilitiesthatuserecirculatingtechnologyhavebeenbuilt.Resultshavebeenmixed.Whiletherehavebeensomenotablelarge-scalebusinessfailuresinthissector,numeroussmall-tomedium-scaleeffortscontinueproduction.
Prospectiveaquaculturistsandinvestorsneedtobeawareofthebasictechnicalandeconomicrisksinvolvedinthistypeofaquacultureproductiontechnology.Thisfactsheetandothersinthisseriesaredesignedtoprovidebasicinformationonrecirculatingaquaculturetechnology.
Criticalproductionconsiderations
Allaquacultureproductionsystemsmustprovideasuitableenvironmenttopromotethegrowthoftheaquaticcrop.Criticalenvironmentalparametersincludetheconcentrationsofdissolvedoxygen,un-ionizedammonia-nitrogen,nitrite-nitrogen,andcarbondioxideinthewateroftheculturesystem.Nitrateconcentration,pH,andalkalinitylevelswithinthesystemarealsoimportant.Toproducefishinacosteffectivemanner,aquacultureproductionsystemsmustmaintaingoodwaterqualityduringperiodsofrapidfishgrowth.Toensuresuchgrowth,fisharefedhigh-proteinpelleteddietsatratesrangingfrom1.5to15percentoftheirbodyweightperdaydependingupontheirsizeandspecies(15percentforjuveniles,1.5percentformarketsize).Feedingrate,feedcomposition,fishmetabolicrateandthequantityofwastedfeedaffecttankwaterquality.Aspelletedfeedsareintroducedtothefish,theyareeitherconsumedorlefttodecomposewithinthesystem.Theby-productsoffishmetabolismincludecarbondioxide,ammonia-nitrogen,andfecalsolids.Ifuneatenfeedsandmetabolicbyproductsareleftwithintheculturesystem,theywillgenerateadditionalcarbondioxideandammonia-nitrogen,reducetheoxygencontentofthewater,andhaveadirectdetrimentalimpactonthehealthoftheculturedproduct.
Inaquacultureponds,properenvironmentalconditionsaremaintainedbybalancingtheinputsoffeedwiththeassimilativecapacityofthepond.Thepondnaturalbiologicalproductivity(algae,higherplants,zooplanktonandbacteria)servesasabiologicalfilterthatprocessesthewastes.Aspondproductionintensifiesandfeedratesincrease,supplementaland/oremergencyaerationarerequired.Athigherratesoffeeding,watermustbeexchangedtomaintaingoodwaterquality.Thecarryingcapacityofpondswithsupplementalaerationisgenerallyconsideredtobe5,000to7,000poundsoffishperacre(0.005to0.007poundoffishpergallonofpondwater).
Thecarryingcapacityoftanksystemsmustbehightoprovideforcost-effectivefishproductionbecauseofthehigherinitialcapitalcostsoftankscomparedtoearthenponds.Becauseofthisexpenseandthelimitedcapacityofthe“natural”biologicalfiltrationofatank,theproducermustrelyupontheflowofwaterthroughthetankstowashoutthewasteby-products.Additionally,theoxygenconcentrationwithinthetankmustbemaintainedthroughcontinuousaeration,eitherwithatmosphericoxygen
(air)orpuregaseousoxygen.
Therateofwaterexchangerequiredtomaintaingoodwaterqualityintanksisbestdescribedusinganexample.Assumethata5,000-gallonproductiontankistobemaintainedataculturedensityof0.5poundoffishpergallonoftankvolume.Ifthe2,500poundsoffisharefeda32%proteinfeedatarateof1.5percentoftheirbodyweightperday,then37.5poundsoffeedwouldproduceapproximately1.1poundsofammonia-nitrogenperday.(Approximately3percentofthefeedbecomesammonia-nitrogen.)Additionally,iftheammonianitrogenconcentrationinthetankistobemaintainedat1.0mg/l,thenamassbalancecalculationonammonia-nitrogenindicatesthattherequiredflowrateofnewwaterthroughthetankwouldbeapproximately5,600gallonsperhour(93gpm)tomaintainthespecifiedammonia-nitrogenconcentration.Evenatthishighflowrate,thesystemalsowould
requireaerationtosupplementtheoxygenaddedbythenewwater.
Recirculatingsystemsdesign
Recirculatingproductiontechnologyismostoftenusedintanksystemsbecausesufficientwaterisnotavailableonsiteto“wash”fishwastesoutofproductiontanksinaflow-throughconfigurationorproductionsystemthatuseswateronlyonce.Inmostcases,aflow-throughrequirementofnearly100gallonsperminutetomaintainoneproductiontankwouldseverelylimitproductioncapacity.Byrecirculatingtank
waterthroughawatertreatmentsystemthat“removes”ammoniaandotherwasteproducts,thesameeffectisachievedaswiththeflow-throughconfiguration.Theefficiencywithwhichthetreatmentsystem“removes”ammoniafromthesystem,theammoniaproductionrate,andthedesiredconcentrationofammonia-nitrogenwithinthetankdeterminetherecirculatingflowratefromthetanktothetreatmentunit.Usingtheexampleoutlinedabove,ifatreatmentsystemremoves50percentoftheammonia-nitrogeninthewateronasinglepass,thentheflowratefromthetankwouldneedtobetwicetheflowrequirediffreshwaterwereusedtoflushthetank(93gpm/0.5=186gpm).Akeytosuccessfulrecirculatingproductionsystemsistheuseofcost-effectivewatertreatmentsystemcomponents.Allrecirculatingproductionsystemsremovewastesolids,oxidizeammoniaandnitrite-nitrogen,removecarbondioxide,andaerateoroxygenatethewaterbeforereturningittothefishtank(seeFig.1).Moreintensivesystemsorsystemsculturingsensitivespeciesmayrequireadditionaltreatmentprocessessuchasfinesolidsremoval,dissolvedorganicsremoval,orsomeformofdisinfection.
Wastesolidsconstraints
Pelletedfeedsusedinaquacultureproductionconsistofprotein,carbohydrates,fat,mineralsandwater.Theportionnotassimilatedbythefishisexcretedasahighlyorganicwaste(fecalsolids).Whenbrokendownbybacteriawithinthesystem,fecalsolidsanduneatenfeedwillconsumedissolvedoxygenandgenerateammonia-nitrogen.Forthisreason,wastesolidsshouldberemovedfromthesystemasquicklyaspossible.Wastesolidscanbeclassifiedintofourcategories:
settleable,suspended,floatableanddissolvedsolids.Inrecirculatingsystems,thefirsttwoareofprimaryconcern.Dissolvedorganicsolidscanbecomeaprobleminsystemswithverylittlewaterexchange.
Settleablesolidscontrol:
Settleablesolidsaregenerallytheeasiestofthefourcategoriestodealwithandshouldberemovedfromthetankandfiltrationcomponentsasrapidlyaspossible.Settleablesolidsarethosethatwillgenerallysettleoutofthewaterwithin1hourunderstillconditions.Settleablesolidscanberemovedastheyaccumulateonthetankbottomthroughproperplacementofdrains,ortheycanbekeptinsuspensionwithcontinuousagitationandremovedwithasedimentationtank(clarifier),mechanicalfilter(granularorscreen),orswirlseparator.Thesedimentationandswirlseparatorprocessescanbeenhancedbyaddingsteepinclinetubes(tubesettlers)inthesedimentationtanktoreduceflowturbulenceandpromoteuniformflowdistribution.
Suspendedsolidscontrol:
Fromanaquaculturalengineeringpointofview,thedifferencebetweensuspendedsolidsandsettleablesolidsisapracticalone.Suspendedsolidswillnotsettletothebottomofthefishculturetankandcannotberemovedeasilyinconventionalsettlingbasins.Suspendedsolidsarenotalwaysdealtwithadequatelyinarecirculatingproductionsystem.Ifnotremoved,suspendedsolidscansignificantlylimittheamountoffishthatcanbegrowninthesystemandcanirritatethegillsoffish.Themostpopulartreatmentmethodforremovingsuspendedsolidsgenerallyinvolvessomeformofmechanicalfiltration.Thetwotypesofmechanicalfiltrationmostcommonlyusedarescreenfiltrationandgranularmediafiltration(sandorpelletedmedia).FormoreinformationonthesedevicesseeSRAC453,RecirculatingAquacultureTankProductionSystems:
AReviewofComponentOptions.
Fineanddissolvedsolidscontrol:
Finesuspendedsolids(<30micrometers)havebeenshowntocontributemorethan50percentofthetotalsuspendedsolidsinarecirculatingsystem.Finesuspendedsolidsincreasetheoxygendemandofthesystemandcausegillirritationanddamageinfinfish.Dissolvedorganicsolids(protein)cancontributesignificantlytotheoxy