Novel Online Monitoring and Alert System for Anaerobic Digestion Reactors

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<ul><li><p>Published: September 14, 2011</p><p>r 2011 American Chemical Society 9093 | Environ. Sci. Technol. 2011, 45, 90939100</p><p>ARTICLE</p><p></p><p>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,*,</p><p>Kengo Kubota, Yu-You Li, and Hideki Harada</p><p>Department of Chemistry, University of Science and Technology of China, Hefei, 230026 ChinaDepartment of Civil Engineering, Tohoku University, Sendai 980-8579, Japazn</p><p>bS Supporting Information</p><p> INTRODUCTION</p><p>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</p><p>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</p><p>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</p><p>Thus, incorporating titration and NIR spectroscopy techniquesmight offer a relatively simple way to achieve a comprehensiveand accurate monitoring of AD process.</p><p>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</p><p>Received: June 30, 2011Accepted: September 14, 2011Revised: September 11, 2011</p><p>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.</p><p>;iName=master.img-000.jpg&amp;w=177&amp;h=134</p></li><li><p>9094 |Environ. Sci. Technol. 2011, 45, 90939100</p><p>Environmental Science &amp; Technology ARTICLE</p><p>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.</p><p>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.</p><p>MATERIALS AND METHODS</p><p>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</p><p>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</p><p>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</p><p>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</p><p>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,</p><p>Figure 1. Schematic of online monitoring and alert system for a UASBreactor (dotted lines: data; solid lines: liquid or gas pipes).</p><p>Table 1. Operation Conditions for Shock Tests</p><p>test duration operation condition</p><p>6-fold organic overload shock 24 h influent COD of 60 000 mg/L</p><p>influent NaHCO3 of 3000 mg/L</p><p>HRT of 24 h</p><p>5 C temperature shock 96 h influent COD of 10 000 mg/Linfluent NaHCO3 of 3000 mg/L</p><p>HRT of 24 h</p><p>temperature of 5 C6-fold hydraulic shock 24 h influent COD of 1667 mg/L</p><p>influent NaHCO3 of 500 mg/L</p><p>HRT of 4 h</p><p>toxicant shock slug dose influent COD of 10 000 mg/L</p><p>influent NaHCO3 of 3000 mg/L</p><p>HRT of 24 h</p><p>chloroform dosage of 80 mg/L</p><p>;iName=master.img-001.jpg&amp;w=227&amp;h=204</p></li><li><p>9095 |Environ. Sci. Technol. 2011, 45, 90939100</p><p>Environmental Science &amp; Technology ARTICLE</p><p>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</p><p>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</p><p>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</p><p>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</p><p>methane production in the gas phase, as follows:</p><p>S 1QCH4</p><p> dVFAdt</p><p> 100 1</p><p>a dQCH4dt</p><p>2</p><p>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</p><p>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</p><p>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</p><p>Figure 2. Index variations under 6-fold organic overloading shock: (A) S and a; and (B) total VFA concentration and MPR.</p><p>;iName=master.img-002.jpg&amp;w=319&amp;h=336</p></li><li><p>9096 |Environ. Sci. Technol. 2011, 45, 90939100</p><p>Environmental Science &amp; Technology ARTICLE</p><p>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.</p><p>RESULTS</p><p>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, resp...</p></li></ul>