State indicators for monitoring the anaerobic digestion process

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<ul><li><p>Denmark, Building 113, DK-2800, Kgs. Lyngby, Denmark</p><p>ology, F</p><p>a r t i c l e i n f o</p><p>Article history:</p><p>Monitoring and control are important strategies for achieving</p><p>ideal indicator should reflect the current process status and</p><p>be straightforward to measure. Moreover, its response to the</p><p>process imbalances should be significant compared to back-</p><p>groundfluctuations.Thecommonindicators for themonitoring</p><p>Biogas production is the most commonly monitored indi-</p><p>1994). The low biogas production results not only fromprocess</p><p>inhibition but also from low reactor loading. pH is relatively</p><p>straightforward to measure and is often the only online liquid</p><p>stated measured parameter. A pH decrease can indicate an</p><p>accumulation of VFA. In a reactor with low buffering capacity,</p><p>* Corresponding author. Tel.: 45 45251429; fax: 45 45932850.</p><p>Avai lab le a t iencedi rec t .com</p><p>els</p><p>wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 7 3e5 9 8 0E-mail address: (I. Angelidaki).a better process stability and higher conversion efficiencies in</p><p>anaerobic digesters. Monitoring is a requirement for process</p><p>control. The lack of suitable process indicators results in the</p><p>limited control and optimization of anaerobic digestion. An</p><p>cator, since it indicates the overall process performance and</p><p>can be measured by a number of robust online sensors.</p><p>However, it can poorly indicate an imbalanced state and often</p><p>decreaseswhen the process is already damaged (Moletta et al.,digesters effectively.</p><p> 2010 Elsevier Ltd. All rights reserved.</p><p>1. Introduction of the biogas process are gas production, biogas composition,pH, alkalinity and volatile fatty acids (VFA) (Hawkes et al., 1993).Received 24 February 2010</p><p>Received in revised form</p><p>6 June 2010</p><p>Accepted 14 July 2010</p><p>Available online 23 July 2010</p><p>Keywords:</p><p>Anaerobic digestion</p><p>Monitoring</p><p>Volatile fatty acids</p><p>Dissolved hydrogen</p><p>Biogas0043-1354/$ e see front matter 2010 Elsevdoi:10.1016/j.watres.2010.07.043a b s t r a c t</p><p>Anaerobic process state indicators were used to monitor a manure digester exposed to</p><p>different types of disturbances, in order to find the most proper indicator(s) for monitoring</p><p>the biogas process. Online indicators tested were biogas production, pH, volatile fatty acids</p><p>(VFA), and dissolved hydrogen. Offline indicators tested were methane and hydrogen</p><p>content in the biogas. A CSTR reactor with 7.2 L working volume was operated at a varying</p><p>hydraulic loading rate (HRT 10e20 days) for 200 days. During this period, the reactor was</p><p>overloaded with extra organic matter such as glucose, lipid, gelatine, and bio-fibers, in</p><p>order to create dynamic changes in the process state. Biogas production increased in</p><p>response to the increase in organic load with a slight decrease in methane content. pH was</p><p>relatively stable and did not show clear response to hydraulic load changes. However, pH</p><p>changes were observed in response to extra organic load. Individual VFA concentrations</p><p>were an effective indicator, with propionate persistent for the longest time after intro-</p><p>duction of the disturbance. Dissolved hydrogen was very sensitive to the addition of easily</p><p>degradable organics. However, it responded also to other disturbances such as slight air</p><p>exposure which had no impact on process performance. A combination of acetate,</p><p>propionate and biogas production is an effective combination to monitor this type ofAdvanced Water Management Centrec Laboratory of environmental biotechnniversity of Queensland, St Lucia, QLD 4067, Australia</p><p>rench National Institute for Agronomic Research, Avenue des Etangs, 11100 Narbonne, FranceDepartment of Environmental Engineering, Technical University ofb , The UKanokwan Boe , Damien John Batstone , Jean-Phillippe Steyer , Irini Angelidaki *aState indicators for monitoringprocess</p><p>a a,b</p><p>journa l homepage : www.ier Ltd. All rights reservedthe anaerobic digestion</p><p>a,c a,</p><p>ev ier . com/ loca te /wat res.</p></li><li><p>pHcanbeauseful indicator.However, thepHresponsehas low</p><p>sensitivity in a well-buffered system (Bjornsson et al., 2000).</p><p>Biogas composition is a traditional parameter where low</p><p>methane percent (i.e. high carbon dioxide content) could</p><p>indicate inadequate process performance. However, the</p><p>carbondioxide content is dependent on pH and, consequently,</p><p>fluctuation of pH can affect the gas composition without</p><p>decreasing methane production (Hansson et al., 2002).</p><p>Hydrogen content of biogas is a very sensitive indicator and is</p><p>connected to the imbalance between microbial groups in the</p><p>digestion process (Molina et al., 2009; Steyer et al., 2002). The</p><p>hydrogen content in biogas can easily be measured online</p><p>using a semiconductor sensor (Hornsten et al., 1991). However,</p><p>dissolved hydrogen may be more appropriate than gaseous</p><p>hydrogen, as it is not delayed by liquidegas transfer and could</p><p>2. Material and methods</p><p>The experiment was carried out in a 9-L CSTR reactor with</p><p>a 7.2 L working volume. Cattle manure (3%TS, 2%VS) was used</p><p>as substrate for the reactor. The reactor was operated at 55 Cat a varying hydraulic loading (10e20 days HRT) for 200 days</p><p>and was fed four times per day using a peristaltic pump</p><p>(Watson Marlow) controlled by a timer and relay.</p><p>To compare the indicators responses, hydraulic and</p><p>organic load disturbances were introduced. For hydraulic</p><p>disturbances, the feed volume was increased by increasing</p><p>the feed duration. For organic overload, different organic</p><p>compounds, besides the daily manure feed, were added into</p><p>the reactor as summarised in Table 1. Rapeseed oil and gela-</p><p>tor</p><p>8 Mixed with feed and fed 4 times</p><p>wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 7 3e5 9 8 05974better correlate to the VFA concentration (Pauss and Guiot,</p><p>1993). Dissolved hydrogen increases together with VFA accu-</p><p>mulation during sudden increase of organic load (Bjornsson</p><p>et al., 2001b). VFA is widely suggested as process indicator,</p><p>since it is the main pre-methanogenic intermediate (Jacobi</p><p>et al., 2009; Molina et al., 2009). VFA accumulation in anaer-</p><p>obic reactors indicates process imbalance (Ahring et al., 1995).</p><p>Moreover, individual VFA concentrations give specific infor-</p><p>mation for process diagnosis (Ahring et al., 1995; Cobb andHill,</p><p>1991). Total VFA concentration can be measured online by</p><p>titration (Feitkenhauer et al., 2002), or indirectly where light</p><p>spectroscopy is correlated to total VFA concentrations, by</p><p>using near infrared spectroscopy (NIR) (Holm-Nielsen et al.,</p><p>2008; Jacobi et al., 2009). However, to measure individual VFA,</p><p>online monitoring is more complex. The online monitoring of</p><p>individual VFAhas beenbased on sample filtration followed by</p><p>analysis in a gas chromatograph (Pind et al., 2003), or using</p><p>headspace extraction followed by analysis in a gas chromato-</p><p>graph (Boe et al., 2007).</p><p>Many of the studies cited above assessed only a limited</p><p>number of indicators, and often in processes operating under</p><p>unstressed state. Moreover, lack of an online sensor for indi-</p><p>vidual VFA limits the evaluation of this important indicator.</p><p>The aim of this study is to assess the suitability of different</p><p>anaerobic process indicators. A range of indicators, including</p><p>biogas production, pH, individual VFA, dissolved hydrogen,</p><p>and gas phasemethane and hydrogen contentwere compared</p><p>under different types of disturbances.</p><p>Table 1 e Summary of extra organic load added to the reac</p><p>Day Amount added (g/day)</p><p>Lipid Glucose Gelatine</p><p>77 85 e e</p><p>112 157 e e</p><p>126 e 25 e</p><p>137 e 50 e</p><p>142 e e 25</p><p>161 e 50 e</p><p>168 e 100 e</p><p>185e187 4 e e188e196 4 40 etine were used to represent lipid and protein, respectively.</p><p>Glucose was used to represent easily degradable carbohydrate</p><p>while bio-fiber containing arabinoxylans (Ispaghula Husk,</p><p>Vi-Siblin; Edwards et al., 2003) was used to represent slowly</p><p>degradable carbohydrate.</p><p>During operation, the responses of different process indi-</p><p>cators were measured. Online indicators were biogas</p><p>production, pH, volatile fatty acids (VFA), and dissolved</p><p>hydrogen. Offline indicators were percent methane and</p><p>hydrogen in the biogas. Biogas production was measured by</p><p>an automated displacement gas metering system with</p><p>a 100 mL reversible cycle and registration (Angelidaki et al.,</p><p>1992). The water used in gas meter was acidified to pH 3 by</p><p>HCl added NaCl to prevent CO2 dissolution. Gas production</p><p>data was recorded automatically every 6 h. pH was measured</p><p>online by a mini CHEM-pH Process Monitor (TPS Pty Ltd.,</p><p>Australia). The meter was calibrated against pH 4.00 and pH</p><p>6.88 buffers every second week. The pH was recorded auto-</p><p>matically every 10 min. Individual VFA concentrations were</p><p>measured by an online VFA monitoring system based on ex-</p><p>situ VFA extraction (Boe et al., 2007). The reactor had a liquid</p><p>circulation loop from which a 40 mL liquid sample was</p><p>pumped into an extraction chamber, acidified, added with</p><p>salt, and was heated in order to extract the VFA into gas phase</p><p>before injecting into a gas chromatograph (GC) for analysis.</p><p>The signal output from the GC was then sent to data pro-</p><p>cessing system for integration. The VFA concentrations were</p><p>analysed and recorded automatically every 6 h.</p><p>during experiment.</p><p>Method of addition</p><p>Bio-fiber</p><p>e Added once, directly into the reactor</p><p>e Mixed with feed and fed 4 times</p><p>e Added once, directly into the reactor</p><p>e Added once, directly into the reactor</p><p>e Added once, directly into the reactor</p><p>e Added once, directly into the reactor</p><p>e Added once, directly into the reactor8 Mixed with feed and fed 4 times</p></li><li><p>Bjornsson et al. (2001a). The system applied a liquid-to-gas</p><p>wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 7 3e5 9 8 0 5975membraneextractionforextractingdissolvedhydrogenfromthe</p><p>liquid content. Thedissolvedhydrogendiffused throughaTeflon</p><p>membrane immersed in the reactor. The diffused hydrogenwas</p><p>then oxidized at the surface of a Palladium-Metal Oxide semi-</p><p>conductor (Pd-MOS) sensor. The picoammeter converted the</p><p>resultingoxidationcurrent toasignal.Thesignaloutput fromthe</p><p>picoammeter was then recorded in the data processing system.</p><p>The results from hydrogen sensors were presented here as</p><p>relative numbers of signal outputs compared to the initial signal,</p><p>since it was found unreliable to calibrate absolute concentration</p><p>of dissolved hydrogen inmanure against water.</p><p>Gas phase methane and carbon dioxide were measured</p><p>offlinebyagaschromatograph(Mikrolab, Arhus)equippedwith</p><p>thermal conductivity detector and a glass column 20m 3mmID packed with Poropack Q (10/80). The temperature of the</p><p>injector, the detector and the oven was isothermal at 55 C.Heliumwas used as a carrier gas with the flow rate 40mL/min.</p><p>Gas phase hydrogen was measured by a gas chromatograph</p><p>(Mikrolab, Arhus) equippedwith thermal conductivity detector</p><p>and a packed column 4.5 m 3 mm ID Molsieve 5A 10/80. Theinjector and detector temperature was 90 C. The temperatureprogramwas isothermal at 80 C.Nitrogenwasusedas a carriergas with the flow rate 20 mL/min.</p><p>Online data processing was done by a programmable logic</p><p>control (PLC) system (Versamax PLC, GE Fanuc Automation</p><p>Europe S.A, Luxembourg), with a PC interface. All calculations,</p><p>including peak area calculation of the GC were managed</p><p>within the PLC. The interface and data logging on the PLCwere</p><p>using GE Cimplicity HMI 6.1 (HMI, GE Fanuc Automation</p><p>Europe S.A, Luxembourg).</p><p>3. Results</p><p>All the measured indicators showed response to the changes</p><p>in hydraulic and organic load. During the start-up period (day</p><p>0e20), very high VFA concentrations, up to 70 mM, were</p><p>observed. Biogas production and VFA levels increased while</p><p>pH changed by 0.5e1 unit. Acetate and butyrate were themost</p><p>dominant VFA. After day 20, acetate and butyrate decreased</p><p>relatively quickly while propionate was the most persistent.</p><p>3.1. Response to lipid additionDuring day 122e128, the dissolved hydrogen was measured</p><p>by an online hydrogen micro-sensor (Unisense A/S, Aarhus,</p><p>Denmark). The sensor principle is based on hydrogen diffusion</p><p>from the liquid through a sensor tip silicone membrane, to the</p><p>platinum anode which is polarized against an internal refer-</p><p>ence. The flow of electrons from the oxidizing anode to the</p><p>internal referencereflects linearly thehydrogenpartialpressure</p><p>around the sensor tip and is in the pico-amp range. A picoam-</p><p>meter converted the resulting oxidation current to a signal. The</p><p>signal output from the picoammeter was then recorded in the</p><p>data processing system (Unisense A/S, Aarhus, Denmark).</p><p>During day 157e200, the dissolved hydrogen was measured</p><p>by the online hydrogen measuring system developed byTwo lipid additions were introduced by adding 85 g and 157 g</p><p>of rapeseed oil directly into the reactor at day 77 and day 110,respectively (Fig. 1). No increase in biogas production and only</p><p>a small increase of VFA were observed after the first addition.</p><p>While after the second one a drop of both biogas production</p><p>and methane percent, but no clear response in both pH and</p><p>VFA were observed. After the second addition, most of the oil</p><p>came out undigested with the effluent from the top of reactor</p><p>and biogas production returned slowly to normal levels.</p><p>3.2. Response to glucose addition</p><p>Four glucose additions were introduced by adding 25, 50, 50</p><p>and 100 g of glucose directly into the reactor at day 126, 137,</p><p>161 and 168, respectively. The results from the first two</p><p>additions are shown in Fig. 2, and the results from the last two</p><p>additions are shown in Fig. 3.</p><p>At approximately 1 day after the 25 g glucose was added,</p><p>biogas production increased shortly (Fig. 2a), pH droppedwhile</p><p>dissolved hydrogen increased (Fig. 2b), and VFA concentration</p><p>increased slightly while methane percent did not show signifi-</p><p>cant response (Fig. 2c and d). Hydrogen content in biogas</p><p>increased slightly during the same period that dissolved</p><p>hydrogen increased. However, the values were very low and</p><p>fluctuated. At day 123, dissolved hydrogen showed some</p><p>response fewminutesafter the reactorwasopened to repair the</p><p>effluent tube.Additionof 50g glucoseat day 137 showedsimilar</p><p>response to theaddition of 25 g glucose, however,with stronger</p><p>response of VFA, where butyrate, iso-valerate and valerate</p><p>increased slightly at both day 137 and 161. At day 161, the dis-</p><p>solved hydrogen increased sharply, along with a pH drop,</p><p>a slight increase in hydrogen content and a slight decrease in</p><p>biogas methane content (Fig. 3b and d).</p><p>After the addition of 100 g glucose, biogas production</p><p>increased while methane content decreased (Fig. 3a and d).</p><p>Acetate and butyrate increased significantly and followed by</p><p>an increase of iso-butyrate, iso-valerate, valerate and propio-</p><p>nate concentrations (Fig. 3c). pH values dropped and dissolved</p><p>hydrogen increased sharply (Fig. 3b). Dissolved hydrogen</p><p>dropped back very quickly, while pH slowly increased over</p><p>severalhours.VFAtook longer time todecreaseback tonormal.</p><p>Also, the gas phase hydrogen content increased slightly.</p><p>3.3. Response to protein addition</p><p>Proteinwasaddedatday 142byadding25g gelatinedirectly into</p><p>the reactor (Fig. 2). Only biogas production and acetate concen-</p><p>tration slightly increased while the rest of VFA and...</p></li></ul>