Novel Online Monitoring and Alert System for Anaerobic Digestion Reactors

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  • Published: September 14, 2011

    r 2011 American Chemical Society 9093 dx.doi.org/10.1021/es202245f | Environ. Sci. Technol. 2011, 45, 90939100

    ARTICLE

    pubs.acs.org/est

    Novel Online Monitoring and Alert System for Anaerobic DigestionReactorsFang Dong, Quan-Bao Zhao, Wen-Wei Li,*, Guo-Ping Sheng, Jin-Bao Zhao, Yong Tang, Han-Qing Yu,*,

    Kengo Kubota, Yu-You Li, and Hideki Harada

    Department of Chemistry, University of Science and Technology of China, Hefei, 230026 ChinaDepartment of Civil Engineering, Tohoku University, Sendai 980-8579, Japazn

    bS Supporting Information

    INTRODUCTION

    Anaerobic digestion (AD) processes are sensitive to environ-mental fluctuations and frequently suffer from operation instabil-ity. Thus, real-time monitoring and robust control of AD proces-ses are essential to avoid possible instability due to disturbance.For this purpose, intensive studies have focused on fast, reliable,and online monitoring of AD processes as well as the develop-ment of efficient feedback alert and control strategies.1 A numberof indexes have been frequently adopted in the process monitor-ing, including volatile fatty acids (VFAs), pH, redox potential,biogas production rate, and composition. Compared with theeasy-to-measure parameters like pH and redox potential, VFAand biogas production are widely considered as the two mostcrucial and direct indicators of the system status.2,3 In a normallyoperating AD system, VFAs produced by acidogens and aceto-gens can be immediately utilized by the methanogens formethane production, leaving a low VFA concentration.4 Thus,the VFA accumulation is usually interpreted as the result ofmethanogenesis inhibition or organic overloading, and implies arisk of process upset.5 However, VFA alone is insufficient toreveal the overall reactor status. In this context, the biogascomposition and production rate can also provide usefulinformation.6,7 Therefore, a combined detection of both liquid-and gas-phase parameters is considered as an efficient strategy toprovide comprehensive insight into an AD process.810

    A variety of techniques for online monitoring of VFAsare available, such as fluorescence spectroscopy,11,12 near-infrared(NIR) spectroscopy,13 titration,14 and gas chromatography.15,16

    Among them, titration is the most practical option, attributed toits high simplicity, rapidness, and cost-effectiveness.14,1719 Onthe other hand, NIR spectroscopy technique is gaining a greatpopularity for online monitoring of gas-phase parameters.20

    Thus, incorporating titration and NIR spectroscopy techniquesmight offer a relatively simple way to achieve a comprehensiveand accurate monitoring of AD process.

    In addition to parameter detection, effective analysis of theobtained data is of equal importance for reactor status evaluationand diagnosis. Because of the high complexity of AD systems andthe unclear interrelations of many involved parameters, it isdifficult to extract useful hidden information from the largedata sets of individual parameters and to find out the truesituation as well as potential risks. For AD process evaluationor diagnosis, a common practice is to set threshold values forsome individual indicators like pH and VFA, and to judge thereactor status on the basis of the detected values.8,2123However,such an evaluation is usually inaccurate, because information

    Received: June 30, 2011Accepted: September 14, 2011Revised: September 11, 2011

    ABSTRACT: Effective monitoring and diagnosis of anaerobic digestion pro-cesses is a great challenge for anaerobic digestion reactors, which limits theirstable operation. In this work, an online monitoring and alert system for upflowanaerobic sludge blanket (UASB) reactors is developed on the basis of a set ofnovel evaluating indexes. The two indexes, i.e., stability index S and auxiliaryindex a, which incorporate both gas- and liquid-phase parameters for UASB,enable a quantitative and comprehensive evaluation of reactor status. A series ofshock tests is conducted to evaluate the response of the monitoring and alertsystem to organic overloading, hydraulic, temperature, and toxicant shocks. Theresults show that this system enables an accurate and rapid monitoring anddiagnosis of the reactor status, and offers reliable early warnings on the potentialrisks. As the core of this system, the evaluating indexes are demonstrated to be ofhigh accuracy and sensitivity in process evaluation and good adaptability to theartificial intelligence and automated control apparatus. This onlinemonitoring and alert system presents a valuable effort to promotethe automated monitoring and control of anaerobic digestion process, and holds a high promise for application.

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    about the relationship between the individual parameters and thereactor status is still lacking and thus such an evaluation reliesheavily on the professional knowledge and experiences ofoperators. Moreover, these parameters can only reveal thecurrent reactor status, but it is actually, in most cases, too latefor an effective process control once the threshold values arereached. Indeed, no effective method for AD reactor diagnosis andrisk prediction is currently available. As such, it is imperative topropose and apply objective indexes for AD reactor diagnosis. Suchindexes should be ideally accurate and sensitive to environmentalfluctuations, reveal the change dynamics of reactor status, and beadaptive to online monitoring, autoalert, and control systems.

    In this work, an effective online monitoring and alert system isdeveloped for an upflow anaerobic sludge blanket (UASB)reactor. Particularly, two new evaluating indexes, stability indexS and auxiliary index a, are proposed for a quantitative assessmentof reactor status, which for the first time take into account thechanging dynamics of both liquid- and gas-phase parameters ofUASB reactors. On the basis of a comprehensive investigationinto the relationship between the indexes and reactor perfor-mance, a database of S and a under various disturbances isestablished to offer a foundation for online diagnosis and earlywarning of AD process.

    MATERIALS AND METHODS

    UASB Reactor andOnlineMonitoring andAlert System. Inthis work a bench-scale Plexiglas-made UASB reactor was used,which consisted of a reaction zone of 2.0 L and a gassolidsseparator zone. The online monitoring system consisted of a gas-phase detection unit and a liquid-phase detection unit thatconnected to a computer through a data acquisition card (PCI-1602, ICPDASCo., China), as illustrated in Figure 1. The liquid-phase detection unit was comprised of an automatic sampleloading and ejecting device and an online titration device,controlled by computer through 485 serial ports. A precise meterpump (BT100-1F, Longer Co., China) was used as the sampleloading and ejecting device. The online titration device consistedof an inject pump (TJ-3A, Longer Co., China), a pH glass

    electrode (LE409, Mettler Toledo Co., USA), and a titrationcell. A magnetic stirrer was used for the liquid mixing in thetitration cell. The gas-phase detection unit consisted of methaneand carbon dioxide sensors (AGM10 and AGM32, Onward Co.,China) and a gas flow measurement device for monitoring ofbiogas production rates.The software of alert system was designed on a LabView

    virtual instrument platform (National Instruments Co.,USA),which incorporates multifunctions of data acquisition, analysis,recording, and display. Specifically, the indexes S and a profileswere calculated, recorded, and displayed real-time based on theLabView software.Experimental Operations. The seed sludge for the UASB

    reactor was taken from an anaerobic digester in a local citrate-processing wastewater treatment plant. The pH and volatilesuspended solids (VSS) of seed sludge were 7.2 and 42.2 g/L,respectively. The reactor temperature was maintained at 35 (1 C using a heating jacket, except during the temperature shocktesting period. The reactor was operated at a fixed loading rate of10 g/L chemical oxygen demand (COD) and 24-h hydraulicretention time (HRT) except as otherwise specified. Sucrose-richsynthetic wastewater was used as the feedstock, which contained3000 mg/L NaHCO3 as the buffer and sufficient amountof inorganic nutrients as follows (in mg/L): NH4HCO3, 405;K2HPO4 3 3H2O, 155; CaCl2, 50; MgCl2 3 6H2O, 100; FeCl2, 25;NaCl, 10; CoCl2 3 6H2O, 5; MnCl2 3 4H2O, 5; AlCl3, 2.5;(NH4)6Mo7O24, 15; H3BO3, 5; NiCl2 3 6H2O, 5; CuCl2 3 5H2O,5; and ZnCl2, 5.After the reactor reached a steady state in terms of COD

    removal and methane production, four consecutive shock testswere conducted to investigate the response of the UASB reactorto various operating shocks, including organic overloading,hydraulic loading, temperature, and toxicant shocks (Table 1).The organic overloading shock was imposed by switching thefeed COD concentration to six fold while keeping the flow rateunchanged. The hydraulic loading shock was provided byelevating the flow rate while accordingly reducing the feedCOD and bicarbonate concentrations to maintain a constantorganic loading rate (OLR). The temperature shock wasachieved by directly shifting the reactor temperature from 37to 5 C. For the toxicant shock, toxicants that need a relativelyhigh concentration to induce significant toxic effect, e.g., formaldehyde,

    Figure 1. Schematic of online monitoring and alert system for a UASBreactor (dotted lines: data; solid lines: liquid or gas pipes).

    Table 1. Operation Conditions for Shock Tests

    test duration operation condition

    6-fold organic overload shock 24 h influent COD of 60 000 mg/L

    influent NaHCO3 of 3000 mg/L

    HRT of 24 h

    5 C temperature shock 96 h influent COD of 10 000 mg/Linfluent NaHCO3 of 3000 mg/L

    HRT of 24 h

    temperature of 5 C6-fold hydraulic shock 24 h influent COD of 1667 mg/L

    influent NaHCO3 of 500 mg/L

    HRT of 4 h

    toxicant shock slug dose influent COD of 10 000 mg/L

    influent NaHCO3 of 3000 mg/L

    HRT of 24 h

    chloroform dosage of 80 mg/L

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    and surfactant, which shows chronic toxicity,24 were not chosenhere in order to avoid OLR interference.25 In this study,chloroform was selected as the toxicant, because of its signifi-cant toxic effect on anaerobic microorganisms even at a very lowlevel.26 The toxicant shock was imposed through slug dose ofchloroform at 80 mg/L.An interval of over two months was adopted between every

    two shock tests to ensure a stable operation before anothershocking test. The variations of the gas- and liquid-phase para-meters during the entire test period were measured online andthe indexes S and a were calculated.Monitoring and Analysis. The VFA concentration and pH

    were measured online using the liquid-phase detection unit withfive-point titration technique.19 The volume and composition ofmethane and carbon dioxide were detected online by the gas-phase detection unit. All detections were conducted once perhour except during the temperature shock test, in which a 4-hmeasurement frequency was adopted. The pH electrode andtitration cell were cleaned or washed regularly to prevent possiblefouling. The monitoring data were collected by the data acquisi-tion card and real-time recorded by computer. VSS and CODconcentrations were measured according to the StandardMethods.27

    Evaluating Indexes. Two evaluating indexes are proposedhere for reactor status evaluation. The stability index S is used todescribe the VFA accumulation in the liquid phase, while theauxiliary index a is used to evaluate the variation trend of

    methane production in the gas phase, as follows:

    S 1QCH4

    dVFAdt

    100 1

    a dQCH4dt

    2

    where (dVFA)/(dt) (mmol/L/h) is the change rate of total VFAconcentration at time t and QCH4 (mmolCH4/L/h) is methaneproduction rate (MPR).The signs of indexes also serve as useful signals for reactor state

    evaluation. To benefit an automatic process diagnosis, a diag-nosis software based on the statistic analysis and logic judgmentprograms is designed for data analysis. In this study, if themajority (i.e., 70%) of the index data show the same sign in atime span of at least 5 h, this sign would be automatically assignedto the index. Specifically, if a change of sign occurs, the systemwill not give an immediate judgment. Instead, it would evaluatewhether such a sign change is caused by a real change of reactorstatus or just a temporary fluctuation according to the statistics ofthis point and the subsequent points in at least 5 h.On the basis of the signs and values of both indexes, the

    current state and possible risks of reactor can be determined. Inthis study, the dangerous state is defined as sustaining VFAsaccumulation and corresponding inhibition in methane produc-tion. Under this state, the majority of S would be positive and awould be negative according to the eqs 1 and 2, and a higher value

    Figure 2. Index variations under 6-fold organic overloading shock: (A) S and a; and (B) total VFA concentration and MPR.

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    of S implies a higher degree of risk. To better reflect the degree ofdanger and stability, several threshold values are set here. Underthe premise of the positive S coupled with negative a, the alertsystem will give alarm if the absolute value of S exceeds 50.0.Notably, this threshold for alarm is only a type of emergencyalarm that indicates a serious situation. Actually, even at thebeginning of shock, the diagnosis system has already detected theoccurrence of shock according to signs of S and a. In a specialcase, if both indexes are close to zero, i.e., |S| is less than 4.0, and|a| is less than 1.5, this refers to a typical steady state. Or, if bothindexes have frequently changed signs (positive and negativesigns occur alternately and both occurrences are below 70%), thiscan be regarded as an unsteady state. In this way, the systemstatus can be diagnosed and a warning signal will be given by thealert system in advance if a dangerous state or trend is identified.

    RESULTS

    Reactor Performance under Steady-State Conditions.Thegas production rate and compositions as well as the effluent pH,COD, and VFAs during the reactor steady-state operation periodwere monitored to evaluate the reactor performance. The averagegas production rate was in a range of 407.7431.5 mL/h. Themethane and carbon dioxide contents were 65.568.0% and20.127.0%, respectively. The effluent pH was 7.147.21. Theeffluent COD and VFA concentrations were 41.596.0 and39.095.5 mg/L, respectively.Organic Overloading Shock.The typical response of the AD

    process to an organic overloading shock is as follows:28 increasein VFA concentration, sludge volume index, and MPR; decreasein COD removal efficiency and methane content; change of pHdepending on the buffer capacity of the feed. According to thevariations of operating conditions and S and a values, the organicoverloading shock test could be divided into 4 phases (Figure 2).When the organic overloading shock was applied in phase 1

    (P1, hours 014), initially no obvious inhibition was observed atthis stage. The S value was positive because of a rising VFAconcentration. Meanwhile, a also showed positive values as theMPR kept increasing. In phase 2 (P2, hours 1524), the shockcondition continued and anobvious inhibition occurred. S remainedpositive values, but a turned negative as theMPR started to decline.The shock was stopped since phase 3 (P3, hours 2537), and thereactor rapidly restored, with S and a values changed to negative andpositive, respectively. In phase 4 (P4, after hour 38), the reactorbecame restabilized, and both S and a values were close to zero.A lasting warning signal was given by the alert system

    throughout P2 according to the identification result of diagnosissoftware, because most S showed positive values while a wasmostly negative. The absolute value of S exceeded 1000.0 at hour24, indicating a high risk of reactor collapse due to severe VFAaccumulation. From hour 25, the organic overloading shock wasterminated, and the S and a in P3 changed to negative and

    positive values, respectively. Such changes suggest that the shockstrength was weakened or stopped, resulting in a reduced VFAconcentration and recovery of the methane production capacity.Accordingly, the alert stopped with the recovery of reactor status.In P4, both the S and a values approached zero. The MPR andVFA concentration became stabilized, and the reactor wastending toward stable operation.Hydraulic Loading Shock. The variations of S and a during

    the 6-fold hydraulic loading shock are shown in Figure S1(Supporting Information). Most of the S showed negative values,and the absolute values were all below 3.0 in P1 (hours 124).Alternative positive and negative values of a were observedalmost in half-and-half, with all the absolute values below 1.4.These results suggest that the MPR remained stable in P1. Afterhour 24 (P2), the hydraulic shock stopped. The value of Smostlybecame positive and its absolute values were below 2.4 duringhours 2536. The total VFA concentration increased slightly.The a values also showed positive and negative fluctuation, andthe absolute values fell to below 0.8. TheMPR remained stable inthis stage. After hour 37, both the absolute values of S and a wereclose to zero, and the reactor restored stable performance. Nosignificant change in both indexes was observed during thehydraulic loading shock, indicating that the reactor performancewas less affected by such a hydraulic loading shock. This wasconsistent with previous studies, in which the COD removalvaried only slightly under the condition of decreased HRT whileunchanged OLR.29

    Temperature Shock. The changes of S and a during thetemperature shock are shown in Figure S2. During the tempera-ture shock (P1, hours 196), most of S exhibited positive values,indicating an increase in VFA concentration. The absolute valuesof S mounted from 3.0 up to a maximum of 60.0 at hour 28,suggesting an apparently increasing trend of VFA accumulation.Meanwhile, negative values were found for a in P1, indicating aconsiderable suppression of methane production. Accordingly,warning signals were given by the alert system throughout P1.Noticeably, the signs of a here were apparently different fromthose in the hydraulic and organic overloading shocks, whichmostly showed positive values. With temperature shock, themethane production was almost completely inhibited duringhours 6096 and the absolute values of a were below 0.1. Withthe ending of temperature shock after hour 96 (P2), the S valuechanged drastically from positive to negative. A rapid drop in thetotal VFA concentration was observed. At the same time, the avalue changed from negative to positive and a gradual restorationof methane production was observed. As a consequence, the alertsignal automatically terminated. Although both S and a valuesfluctuated between positive and negative values within P3 (hours188264), they became stable and approached zero inP4 (after hour265), indicating that the UASB reactor restored stable performance.Toxicant Shock.The response of S and a to the toxicant shock

    is illustrated in Figure S3. Both S and a exhibited violent

    Table 2. Operation Conditions and Shock Types of Tests in Literature

    shock type duration shock condition references

    4-fold organic overload shock hours 03 influent COD increased from 5000 to 20 000 mg/L 254-fold hydraulic shock hours 03 HRT decreased from 12 to 3 h 25toxicant shock slug addition 10 mg chloroform/L was dosed 25

    temperature shock days 070 temperature increased from 37 to 55 C 30temperature shock days 3038 temperature increased from 55 to 65 C 31

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    responses to the toxicant shock in the initial 4 h (P1). The S indexmostly showed positive values above 50.0, while a mostly

    exhibited negative values during this period. This indicates arapid VFA accumulation and a severe inhibition in methaneproduction. A warning signal was given by the system in P1. Afterhour 5 (P2), a remained negative and the biogas productiondropped to near zero, while S value changed from positive tonegative in P2. Because the VFA could not be consumed by themethanogens, as evidenced by the completely ceased gas pro-duction, this means that the VFA production process was alsoinhibited. The slight decreased VFA concentration in the reactormight be attributed to the effluent discharge. The sharp increasein the absolute value of S and the ceased methane production athour 12 indicate a severe system collapse. After hour 12, a wasnear zero and no positive value of a was observed, analogue tothat under other shock-ceasing conditions. This further confirmsthe full stop of methane production and collapse of the UASBsystem.Verification. To further evaluate the practicability of the two

    indexes S and a, the experimental data of shock tests reported inprevious studies were used for verification. The operating con-ditions and shock types in these tests are summarized in Table 2,whereas the calculated results of S and a are shown in Figure 3.As shown in Figure 3C, the S and a calculated from the

    literature data show changing profiles highly similar to those inthis study, except for the toxicant shock test. For the toxicantshock test by Tay and Zhang,25 the calculated values of aincreased for a short period before dropping to zero again,indicating the recovery of methane production and stableperformance after the ending of shock. In contrast, in our studyan irreversible collapse of reactor occurred due to the muchhigher dosage of chloroform, which had caused unrecoverabletoxic damage. Generally, these two evaluating indexes demon-strate good applicability to various AD systems.

    DISCUSSION

    Merits of the Monitoring and Alert System. The con-structed monitoring and alert system shows several distinctadvantages over the conventional systems. First, comprehensivebut reasonably simple online monitoring of AD process isrealized, which offers the basis for a rapid and accurate evaluationof AD reactor status. Second, the alert system demonstrates highcapacity in process diagnosis and early warning, attributed tothe introduction of the two new indexes, S and a. Third, thesystem, which is characterized by automated execution, mo-dularized design, and easy use, holds great promise for futurepractical application. All monitoring processes are automaticallyexecuted with software program. Simple and commerciallyavailable components are adopted for the hardware of this systemto facilitate modularization. Most of all, no complex operationand maintenance are required for this system, and customizedservices for reactor monitoring and online data processing can beachieved by using the virtual instrument software program.The two indexes, S and a, are proposed for diagnosis of AD

    reactor status. They can reflect reactor status and changing trendaccurately, and have a rapid response to the operating conditiondisturbance. On the basis of the diagnosis results, an early alertcan be given by the system to prevent the upcoming risks.Accuracy of Indexes.The S and a indexes, which incorporate

    the key operating parameters in both liquid- and gas-phase, offermeaningful information on anaerobic reactor status. Specifically,a combined evaluation of values and signs of both indexes revealsthe hidden pattern of dynamic change in VFA accumulation and

    Figure 3. Profiles of S and a based on data in literature: (A) 4-fold organicoverloading shock;25 (B) 4-fold hydraulic loading shock;25 (C) toxicantshock;25 (D) 55 C temperature shock;30 and (E) 65 C temperature shock.31

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    methane production. Based on the experimental results of theshock tests and literature data verification, a database about thecorrelation between the two indexes and anaerobic reactor statuswas established (Table 3). Such a combination of different signsand magnitudes of the two indexes corresponds well to differentreactor status and shock types. Here, the absolute values of S anda reflect the intensity of changes or disturbance in anaerobicreactors, whereas their signs indicate a specific reactor status andshock type.As shown in Table 3, positive S is found under many different

    operating conditions and reactor status, implying that the valueof S alone is insufficient to clearly identify reactor status.Noticeably, the signs of a also vary significantly and offer valuablesupplementary information. On the basis of a combined analysisof S and a at the initial stage of shock, it is possible to preliminarilydistinguish the shock type and find out whether anaerobes aresignificantly inhibited. Generally, a positive S and positive acorrespond to an organic overloading shock, while a positive Sand negative a indicate a temperature or toxicant shock. At thelate stage of an organic overloading shock (Figures 2A and 3A), apositive S and negative a are obtained regardless of the shockintensity, which suggests a methanogenesis-inhibited state. Incontrast, a negative S and positive a at the late stage of shockmeans consumption of VFA by methanogens and a gradualadaptation and reactor recovery after shock. In addition, thechanging trends of S and a are also informative. If S value changesfrom positive to negative, while a value remains negative for along time, the inhibition of both VFA and methane production issuggested.Apparently, there exist some relationships between the in-

    dexes of S and a and the reactor status. Compared with theconventional individual parameters, such as pH, VFA, and MPR,which rely on their preset threshold values for judgment andusually lead to subjective and incorrect conclusions, these twointegrated evaluating indexes proposed in this study offer moreobjective, accurate, and comprehensive assessment on the statusand shock types of anaerobic reactors.Sensitivity of Indexes. Both S and a demonstrate high

    sensitivity, mainly attributed to an integrated analysis of both theabsolute value and the sign as well as the use ofMPR as a regulatoryfactor. With this regulatory factor, the changes of S can be properlyand significantly magnified or buffered, so that the reactor changing

    trend can be more clearly revealed. This ensures a high sensitivity ofthe indexes and a capability of the system for early warning. Forexample, in the organic overloading shock test (Figure 2), thepositive S and a in P1 corresponded to the VFA accumulation andMPR increase. Such an MPR increase was attributed to theuninhibited methanogenesis at the beginning. As a consequence,the increase in S was weakened by the methane production, whichserves as a buffer against the factor dVFA/dt. Thus, S showed nodrastic increase. The moderate change of S was consistent with thereactor status. In P2, however, the MPR severed as amplifier of S,because methanogenic inhibition occurred. At this phase, the Sremained positive, while a shifted to negative. The increase indVFA/dt was magnified by the MPR, leading to a maximum S ofover 1000.0 at the end of shock. This was consistent with thesignificantly deteriorated reactor performance. Accordingly, alertwas given by the system throughout P2. It is worthwhile to note that,however, the shock had already been detected by the system even atthe beginning of shock.For both S and a, their signs are dependent on the transient

    changing trend of the monitoring parameters. The sign willchange and respond immediately once the reactor becomesinstable or undergoes changing state, even when the conven-tional evaluating parameters are still below their threshold values.For example, in the organic overloading shock test, the VFAconcentration, methane production, and pH all showed littlechanges at the initial shock stage (within 1 h): the VFAconcentration increased slightly from 2.0 to 2.2 mmol/L, themaximum MPR increased from 6.4 to 7.5 mmol/L/h, and pHand methane component kept at around 7.3 and 66.0%, respec-tively. However, both S and a values changed substantially. S anda jumped rapidly from near zero to 5.6 and 3.2, respectively, witha positive sign (the sign can change instantaneously as a result ofshock). These changes suggest a changing reactor status and offerhints to the type and level of the encountered shocks. Thepositive sign of S and a clearly suggested an organic overloadingshock. As a consequence of diagnosis results, alert was given bythe system. The time of earliest alert given was at hour 15, whichwas 5 h earlier than the occurrence of severe acidification (pH

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