State indicators for monitoring the anaerobic digestion process
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a r t i c l e i n f o
Monitoring and control are important strategies for achieving
ideal indicator should reflect the current process status and
be straightforward to measure. Moreover, its response to the
process imbalances should be significant compared to back-
groundfluctuations.Thecommonindicators for themonitoring
Biogas production is the most commonly monitored indi-
1994). The low biogas production results not only fromprocess
inhibition but also from low reactor loading. pH is relatively
straightforward to measure and is often the only online liquid
stated measured parameter. A pH decrease can indicate an
accumulation of VFA. In a reactor with low buffering capacity,
* Corresponding author. Tel.: 45 45251429; fax: 45 45932850.
Avai lab le a t www.sc iencedi rec t .com
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: email@example.com (I. Angelidaki).a better process stability and higher conversion efficiencies in
anaerobic digesters. Monitoring is a requirement for process
control. The lack of suitable process indicators results in the
limited control and optimization of anaerobic digestion. An
cator, since it indicates the overall process performance and
can be measured by a number of robust online sensors.
However, it can poorly indicate an imbalanced state and often
decreaseswhen the process is already damaged (Moletta et al.,digesters effectively.
2010 Elsevier Ltd. All rights reserved.
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
Received in revised form
6 June 2010
Accepted 14 July 2010
Available online 23 July 2010
Volatile fatty acids
Biogas0043-1354/$ e see front matter 2010 Elsevdoi:10.1016/j.watres.2010.07.043a b s t r a c t
Anaerobic process state indicators were used to monitor a manure digester exposed to
different types of disturbances, in order to find the most proper indicator(s) for monitoring
the biogas process. Online indicators tested were biogas production, pH, volatile fatty acids
(VFA), and dissolved hydrogen. Offline indicators tested were methane and hydrogen
content in the biogas. A CSTR reactor with 7.2 L working volume was operated at a varying
hydraulic loading rate (HRT 10e20 days) for 200 days. During this period, the reactor was
overloaded with extra organic matter such as glucose, lipid, gelatine, and bio-fibers, in
order to create dynamic changes in the process state. Biogas production increased in
response to the increase in organic load with a slight decrease in methane content. pH was
relatively stable and did not show clear response to hydraulic load changes. However, pH
changes were observed in response to extra organic load. Individual VFA concentrations
were an effective indicator, with propionate persistent for the longest time after intro-
duction of the disturbance. Dissolved hydrogen was very sensitive to the addition of easily
degradable organics. However, it responded also to other disturbances such as slight air
exposure which had no impact on process performance. A combination of acetate,
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
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
journa l homepage : www.ier Ltd. All rights reservedthe anaerobic digestion
ev ier . com/ loca te /wat res.
pHcanbeauseful indicator.However, thepHresponsehas low
sensitivity in a well-buffered system (Bjornsson et al., 2000).
Biogas composition is a traditional parameter where low
methane percent (i.e. high carbon dioxide content) could
indicate inadequate process performance. However, the
carbondioxide content is dependent on pH and, consequently,
fluctuation of pH can affect the gas composition without
decreasing methane production (Hansson et al., 2002).
Hydrogen content of biogas is a very sensitive indicator and is
connected to the imbalance between microbial groups in the
digestion process (Molina et al., 2009; Steyer et al., 2002). The
hydrogen content in biogas can easily be measured online
using a semiconductor sensor (Hornsten et al., 1991). However,
dissolved hydrogen may be more appropriate than gaseous
hydrogen, as it is not delayed by liquidegas transfer and could
2. Material and methods
The experiment was carried out in a 9-L CSTR reactor with
a 7.2 L working volume. Cattle manure (3%TS, 2%VS) was used
as substrate for the reactor. The reactor was operated at 55 Cat a varying hydraulic loading (10e20 days HRT) for 200 days
and was fed four times per day using a peristaltic pump
(Watson Marlow) controlled by a timer and relay.
To compare the indicators responses, hydraulic and
organic load disturbances were introduced. For hydraulic
disturbances, the feed volume was increased by increasing
the feed duration. For organic overload, different organic
compounds, besides the daily manure feed, were added into
the reactor as summarised in Table 1. Rapeseed oil and gela-
8 Mixed with feed and fed 4 times
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,
1993). Dissolved hydrogen increases together with VFA accu-
mulation during sudden increase of organic load (Bjornsson
et al., 2001b). VFA is widely suggested as process indicator,
since it is the main pre-methanogenic intermediate (Jacobi
et al., 2009; Molina et al., 2009). VFA accumulation in anaer-
obic reactors indicates process imbalance (Ahring et al., 1995).
Moreover, individual VFA concentrations give specific infor-
mation for process diagnosis (Ahring et al., 1995; Cobb andHill,
1991). Total VFA concentration can be measured online by
titration (Feitkenhauer et al., 2002), or indirectly where light
spectroscopy is correlated to total VFA concentrations, by
using near infrared spectroscopy (NIR) (Holm-Nielsen et al.,
2008; Jacobi et al., 2009). However, to measure individual VFA,
online monitoring is more complex. The online monitoring of
individual VFAhas beenbased on sample filtration followed by
analysis in a gas chromatograph (Pind et al., 2003), or using
headspace extraction followed by analysis in a gas chromato-
graph (Boe et al., 2007).
Many of the studies cited above assessed only a limited
number of indicators, and often in processes operating under
unstressed state. Moreover, lack of an online sensor for indi-
vidual VFA limits the evaluation of this important indicator.
The aim of this study is to assess the suitability of different
anaerobic process indicators. A range of indicators, including
biogas production, pH, individual VFA, dissolved hydrogen,
and gas phasemethane and hydrogen contentwere compared
under different types of disturbances.
Table 1 e Summary of extra organic load added to the reac
Day Amount added (g/day)
Lipid Glucose Gelatine
77 85 e e
112 157 e e
126 e 25 e
137 e 50 e
142 e e 25
161 e 50 e
168 e 100 e
185e187 4 e e188e196 4 40 etine were used to represent lipid and protein, respectively.
Glucose was used to represent easily degradable carbohydrate
while bio-fiber containing arabinoxylans (Ispaghula Husk,
Vi-Siblin; Edwards et al., 2003) was used to represent slowly
During operation, the responses of different process indi-
cators were measured. Online indicators were biogas
production, pH, volatile fatty acids (VFA), and dissolved
hydrogen. Offline indicators were percent methane and
hydrogen in the biogas. Biogas production was measured by
an automated displacement gas metering system with
a 100 mL reversible cycle and registration (Angelidaki et al.,
1992). The water used in gas meter was acidified to pH 3 by
HCl added NaCl to prevent CO2 dissolution. Gas production
data was recorded automatically every 6 h. pH was measured
online by a mini CHEM-pH Process Monitor (TPS Pty Ltd.,
Australia). The meter was calibrated against pH 4.00 and pH
6.88 buffers every second week. The pH was recorded auto-
matically every 10 min. Individual VFA concentrations were
measured by an online VFA monitoring system based on ex-
situ VFA extraction (Boe et al., 2007). The reactor had a liquid
circulation loop from which a 40 mL liquid sample was
pumped into an extraction chamber, acidified, added with
salt, and was heated in order to extract the VFA into gas phase
before injecting into a gas chromatograph (GC) for analysis.
The signal output from the GC was then sent to data pro-
cessing system for integration. The VFA concentrations were
analysed and recorded automatically every 6 h.
Method of addition
e Added once, directly into the reactor
e Mixed with feed and fed 4 times
e Added once, directly into the reactor
e Added once, directly into the reactor
e Added once, directly into the reactor
e Added once, directly into the reactor
e Added once, directly into the reactor8 Mixed with feed and fed 4 times
Bjornsson et al. (2001a). The system applied a liquid-to-gas
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
liquid content. Thedissolvedhydrogendiffused throughaTeflon
membrane immersed in the reactor. The diffused hydrogenwas
then oxidized at the surface of a Palladium-Metal Oxide semi-
conductor (Pd-MOS) sensor. The picoammeter converted the
resultingoxidationcurrent toasignal.Thesignaloutput fromthe
picoammeter was then recorded in the data processing system.
The results from hydrogen sensors were presented here as
relative numbers of signal outputs compared to the initial signal,
since it was found unreliable to calibrate absolute concentration
of dissolved hydrogen inmanure against water.
Gas phase methane and carbon dioxide were measured
thermal conductivity detector and a glass column 20m 3mmID packed with Poropack Q (10/80). The temperature of the
injector, the detector and the oven was isothermal at 55 C.Heliumwas used as a carrier gas with the flow rate 40mL/min.
Gas phase hydrogen was measured by a gas chromatograph
(Mikrolab, Arhus) equippedwith thermal conductivity detector
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.
Online data processing was done by a programmable logic
control (PLC) system (Versamax PLC, GE Fanuc Automation
Europe S.A, Luxembourg), with a PC interface. All calculations,
including peak area calculation of the GC were managed
within the PLC. The interface and data logging on the PLCwere
using GE Cimplicity HMI 6.1 (HMI, GE Fanuc Automation
Europe S.A, Luxembourg).
All the measured indicators showed response to the changes
in hydraulic and organic load. During the start-up period (day
0e20), very high VFA concentrations, up to 70 mM, were
observed. Biogas production and VFA levels increased while
pH changed by 0.5e1 unit. Acetate and butyrate were themost
dominant VFA. After day 20, acetate and butyrate decreased
relatively quickly while propionate was the most persistent.
3.1. Response to lipid additionDuring day 122e128, the dissolved hydrogen was measured
by an online hydrogen micro-sensor (Unisense A/S, Aarhus,
Denmark). The sensor principle is based on hydrogen diffusion
from the liquid through a sensor tip silicone membrane, to the
platinum anode which is polarized against an internal refer-
ence. The flow of electrons from the oxidizing anode to the
internal referencereflects linearly thehydrogenpartialpressure
around the sensor tip and is in the pico-amp range. A picoam-
meter converted the resulting oxidation current to a signal. The
signal output from the picoammeter was then recorded in the
data processing system (Unisense A/S, Aarhus, Denmark).
During day 157e200, the dissolved hydrogen was measured
by the online hydrogen measuring system developed byTwo lipid additions were introduced by adding 85 g and 157 g
of rapeseed oil directly into the reactor at day 77 and day 110,respectively (Fig. 1). No increase in biogas production and only
a small increase of VFA were observed after the first addition.
While after the second one a drop of both biogas production
and methane percent, but no clear response in both pH and
VFA were observed. After the second addition, most of the oil
came out undigested with the effluent from the top of reactor
and biogas production returned slowly to normal levels.
3.2. Response to glucose addition
Four glucose additions were introduced by adding 25, 50, 50
and 100 g of glucose directly into the reactor at day 126, 137,
161 and 168, respectively. The results from the first two
additions are shown in Fig. 2, and the results from the last two
additions are shown in Fig. 3.
At approximately 1 day after the 25 g glucose was added,
biogas production increased shortly (Fig. 2a), pH droppedwhile
dissolved hydrogen increased (Fig. 2b), and VFA concentration
increased slightly while methane percent did not show signifi-
cant response (Fig. 2c and d). Hydrogen content in biogas
increased slightly during the same period that dissolved
hydrogen increased. However, the values were very low and
fluctuated. At day 123, dissolved hydrogen showed some
response fewminutesafter the reactorwasopened to repair the
effluent tube.Additionof 50g glucoseat day 137 showedsimilar
response to theaddition of 25 g glucose, however,with stronger
response of VFA, where butyrate, iso-valerate and valerate
increased slightly at both day 137 and 161. At day 161, the dis-
solved hydrogen increased sharply, along with a pH drop,
a slight increase in hydrogen content and a slight decrease in
biogas methane content (Fig. 3b and d).
After the addition of 100 g glucose, biogas production
increased while methane content decreased (Fig. 3a and d).
Acetate and butyrate increased significantly and followed by
an increase of iso-butyrate, iso-valerate, valerate and propio-
nate concentrations (Fig. 3c). pH values dropped and dissolved
hydrogen increased sharply (Fig. 3b). Dissolved hydrogen
dropped back very quickly, while pH slowly increased over
severalhours.VFAtook longer time todecreaseback tonormal.
Also, the gas phase hydrogen content increased slightly.
3.3. Response to protein addition
Proteinwasaddedatday 142byadding25g gelatinedirectly into
the reactor (Fig. 2). Only biogas production and acetate concen-
tration slightly increased while the rest of VFA and pH did not
show significant response. There was no data of dissolved
hydrogen and biogas composition available during this period.
3.4. Response to continuous addition of extraorganic load
The reactor was daily fed with extra organic load, mixed with
the manure in the influent flask, during day 185e196. The
results are shown in Fig. 3. From day 185, the feed (manure)
was supplemented with 8 g bio-fiber and 4 g rapeseed oil per
day. Acetate started to increase and pH began to drop while
small response of dissolved hydrogen was noticed. From day188, the feedwas also supplementedwith 40 g glucose per day.
At this point, the rest of VFA started to increase. A strong
wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 7 3e5 9 8 05976response of dissolved hydrogen was observed, as well as a pH
decrease. Biogas production increased significantly while the
methane content decreased slightly. As the process was
continued to be fed with extra organic load, the dissolved
hydrogen and acetate concentration were the first to decrease
followed by butyrate and iso-butyrate. The rest of VFA started
to decrease shortly after extra organic load was ceased except
for propionate which continued increasing. The pH increased
slightly after removal of extra organic load, but did not reach
the previous level. Dissolved hydrogen decreased back to the
4.1. Reactor response to extra organic load
The reactor responded quickly to the addition of glucose
because glucose is easily degradable. Dissolved hydrogen, VFA
and biogas production levels were all good indicators of
response to this stimulus. After the first lipid addition, no
increase in biogas production and only small increase in VFA
were observed, which could be explained by the slow degra-
dation of lipid. A similar observation was previously reported
by Bjornsson et al. (2001b), where the lipid addition gave low
production of VFA and they suggested that this was due to
hydrolysis being the rate-limiting step for lipid digestion. After
the second addition with doubling the amount of lipid, the
drop in biogas production with small increase of VFA
Fig. 1 e Reactor resultssuggested that the process was probably inhibited by long-
chain fatty acids (LCFA) from the oil. LCFA is known to be
inhibitory to all groups of microorganisms (Angelidaki and
Ahring, 1992; Rinzema et al., 1994). In this case, acidogens
were also inhibited and concentration of VFA alone was not
a suitable indicator for this disturbance. Moreover, the fact
that the reactor slowly recovered after the undigested oil
washed out with the effluent suggested that the reactor was
recovered due to dilution of LCFA by the new feed, rather than
adaptation of microorganisms, agreeing with observations by
Pereira et al. (2003) and Rinzema et al. (1994). The addition of
protein at day 142 did not disturb the process as seen that only
acetate increased slightly and the biogas increased.
4.2. Analysis of indicators
The criteria used for comparing the process indicators in this
paper were their responses during different disturbances, in
relating to their baselines under normal operation. From the
experiment, it was noticed that acetate exhibited faster
dynamics and fluctuated more than propionate. Acetate
increased very fast after the increase of hydraulic or organic
load. However, it decreased again few days later while organic
load was still high. This could be due to fast growth rates of
aceticlastic methanogens compared to propionate degraders,
or a larger population of aceticlasts. Propionate degraders are
having growth rates around 0.49 day1, while aceticlasticmethanogens around 0.6 day1 (Angelidaki et al., 1999). Buty-rate responded also very quickly to an overload. However,
during day 1e120.
wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 7 3e5 9 8 0 5977butyrate accumulation seemed to depend on the substrate
composition and not all disturbances could increase butyrate.
Butyrate accumulatedmostly during the period of adding high
168. However, only a slight increase in butyrate was observed
during additions of 50 g glucose at day 137 and 161, and no
butyrate accumulation was observed during the addition of
25 g glucose at day 125. Valerate showed similar responses
as butyrate, however, with much lower concentration level.
Iso-butyrate and iso-valerate showed similar responses as
butyrate and valerate, respectively. However, the iso-formwas
more persistent in the reactor than the normal-form. During
continuousoverload,propionate remained in thereactormuch
longer than other VFA. Moreover, while acetate and butyrate
started to decrease after the reactor had been exposed to
organicoverload for sometime,propionatekeptaccumulating.
This could be due to the fact that propionate degradation is the
Fig. 2 e Reactor results dmost thermodynamic unfavourable among other VFA degra-
dation,whichmade propionate degraders the slowest growing
and most sensitive compared to acetate and butyrate
degraders which could faster increase their degradation rate
(Ozturk, 1991). In this case, propionate would be a better
parameter to indicate process stress. This observation was
similar to the study of Nielsen et al. (2007) where they sug-
gested that propionate was the best indicator to describe the
normalizing of the process.
Dissolved hydrogen had strong response specially when
adding glucose to the feed. This is consistent with hydrogen
beingamajorproduct fromglucosedegradation (Batstoneetal.,
2002). However, it responded also to other disturbances such as
slight air exposure which had no impact on process perfor-
mance. This ismore likely a semiconductor response to change
in redox. In principle, the increase of both VFA and dissolved
hydrogen in the reactor couldbe linkeddirectly to the increased
uring day 120e147.
wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 7 3e5 9 8 05978activities of acidogens. Thedecrease of acetate could reflect the
increased activity of aceticlastic methanogens. Similarly, the
rapid decrease inhydrogen could be due to increased activity of
hydrogenotrophic methanogens. Fig. 4 shows the dynamics of
dissolved hydrogen in the reactor during continuous overload.
The fast response of hydrogen was clearly observed as oscilla-
tions following the feed interval of four times per day. It was
noticed that the peak of dissolved hydrogen was reached
around 30min after each feed, corresponding to small pH drop.
Moreover, thedissolvedhydrogendecreasedagainwithin a few
hours, while pHwas still increasing.
The decrease of pH corresponded to the VFA accumulation
during overload. However, the level of pH change was not
significant enough to indicate the state of the process in this
case due to the high buffer capacity in manure digester. The
response of pH also corresponded to the dissolved hydrogen
during sudden overload but not during gradual overload. This
Fig. 3 e Reactor results dcould be explained by the pH response as being the result of
overall ion interactions in the solution, while dissolved
hydrogen measurements are not compensated by these
interactions. Thus, during sudden overload of particular
substrate such as glucose, where hydrogen production was
high, the dissolved hydrogen response could be correlated to
the pH. Moreover, it was noticed that after removal of
continuous organic addition, the pH was not back to the
previous level due to VFA accumulation. Dissolved hydrogen
dropped rapidly to normal level.
Biogas production responded very quickly to the change in
organic load. However, it could not indicate the imbalanced
state of the reactor. During continuous overload the biogas
production was increasing along with the VFA concentration.
The overload was then removed due to high VFA concentra-
tion. If only the biogas production was used as indicator of the
process status, the imbalance would not had been discovered
uring day 155e200.
dissolved hydrogen but with very small change whichmade it
wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 5 9 7 3e5 9 8 0 5979and one would had continued with the same loading which
may have led to process failure.
4.3. Suitability of indicators
Several indicators showed interesting responses to the
increase of organic load. The parameters that had the fastest
response were dissolved hydrogen, pH and acetate, followed
by butyrate (in case of glucose), propionate (in case of high
overload), and biogas, respectively. The response time of
biogas composition was quite slow probably due to large
headspace volume of the reactor. However, none of these
indicators showed response to all perturbations. Thus, the
combination of different indicators might be necessary to
cover all imbalance situations. As propionate was the most
persistent in the reactor, it could be an important indicator to
determine the degree of process imbalance. On the other
hand, although biogas production could not indicate process
imbalance, it is main product of interest reflecting overall
process performance. Thus, the combination of acetate/
propionate and biogas production is an effective group of
indicators of both performance, and process balance. This is
in contrast to traditional indicators, which aremainly pH, and
either acetate only, or a combination of all organic acids.
However, it should be remarked that the indicators suggested
here were tested in a manure digester which had very high
buffering capacity. For the processwith low buffering capacity
such as sludge digester or high-rate anaerobic digester, the pH
could still be a useful indicator.
Other important factors to be considered when choosing
Fig. 4 e Dynamics of dissolved hydrthe state indicator for the full-scale application are reliability
and robustness of the online meters. Gas production and pH
are easy to measure and most of the anaerobic wastewater
treatment plants have gas and pH meters as standard
instruments (Spanjers and van Lier, 2006). However, the liquid
phase parameters such as individual VFA are still measured
through manual analysis. The individual VFA online moni-
toring used in this experiment (Boe et al., 2007) is under
further development of the industrial prototype to improve
the robustness for operation in full-scale plants.
Dissolved hydrogen was sensitive to organic overload, espe-
cially when glucose was present in the feed. However, it alsodifficult to indicate the status of the process based on pH
value. Acetate and propionate were very sensitive to organic
overload. Propionate was the most persistent parameter
which was effective indicator of stress status of the process
while acetate decreased faster and was more fluctuated. The
sensitivity of gas phase composition in this study was quite
low, probably due to large headspace volume of the reactor,
and slow gaseliquid dynamics. Biogas productionwas still the
important parameter for indicating overall reactor perfor-
mance although it could not indicate the stress status of the
reactor. A monitoring of both individual VFA such as acetate,
propionate and biogas productionwould allow combination of
process state and performance.
This work was supported by the Ph.D. scholarship from the
Institute of Environment and Resources, Technical University
of Denmark. The work of J.P. Steyer was supported by the
European Communitys Human Potential Programme under
contract MEIF-CT-2005-009500 (CONTROL-AD4H2) and both
are greatly acknowledged.
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State indicators for monitoring the anaerobic digestion processIntroductionMaterial and methodsResultsResponse to lipid additionResponse to glucose additionResponse to protein additionResponse to continuous addition of extra organic load
DiscussionReactor response to extra organic loadAnalysis of indicatorsSuitability of indicators