Process Biochemistry 41 (2006) 1235–1238
www.elsevier.com/locate/procbio
A pH-based control of ammonia in biogas during anaerobic
digestion of artificial pig manure and maize silage
D.P.B.T.B. Strik, A.M. Domnanovich, P. Holubar *
Institute of Applied Microbiology, Department of Biotechnology, University of Natural Resources and Applied Life Sciences,
Muthgasse 18, 1190 Vienna, Austria
Received 7 June 2005; accepted 6 December 2005
Abstract
The purposes of this study were to prove that ammonia can be present in biogas from anaerobic digestion and to control this ammonia by
reducing the reactor pH. Ammonia containing biogas was produced for a period of more than 100 days, with a maximum of 332 ppm. Especially
during periods of high free ammonia concentrations in the reactor was ammonia present in biogas. The free ammonia was effectively reduced to
less than the inhibition level by pH-based control and the ammonia in biogas concentration was reduced to 9 ppm. Simultaneously the CH4-yield
was severely reduced. A pH-based control of ammonia in biogas with a satisfactory biogas production was thus so far proven not to be achievable.
In the carrying out this study it was shown that high ammonia concentrations lead to a range of problems: process inhibition, decreased COD
removal efficiency, reduced biogas production, malodour and a poor biogas quality that requires treatment.
# 2005 Elsevier Ltd. All rights reserved.
Keywords: Ammonia; Biogas; pH-based control; Inhibition; Anaerobic digestion
1. Introduction
Nitrogen is an essential nutrient for anaerobic organisms [1].
The ammonia concentration must be maintained in excess of at
least 40–70 mg N l 1 to prevent reduction of biomass activity
[2]. On the other hand high ammonia concentrations, from e.g.
the anaerobic digestion of livestock waste [3], inhibit the
anaerobic digestion process [4–6]. From an overview of
ammonia inhibition [5] it was shown that free ammonia caused
the inhibition rather than the total ammonia. From these studies
tolerable free ammonia concentrations have been suggested.
These concentrations varied from 55 11 to 150 mg NH3 l 1.
Depending on the actual feed material to the digester it was
observed that ammonia can also be present in biogas up to
450 ppm [7]. If this biogas was used in gas engines it may lead
to increased environment polluting NOx emissions. A currently
researched [8–10] application of biogas into fuel cells, which
will result in a significantly higher electrical efficiency [11] and
can contribute to an increase of renewable energy production,
will also be problematical. Ammonia will actually decrease the
fuel cells life time [11]. Thus, if biogas from high nitrogen
* Corresponding author. Tel.: +43 1 360 06 6212; fax: +43 1 36 97 615.
E-mail address: [email protected] (P. Holubar).
1359-5113/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.procbio.2005.12.008
containing feeds is to be used to produce electricity, ammonia
in biogas should be monitored and controlled.
In the past some studies have focussed on the long term
exposure of methanogenic sludge to high ammonium concentrations [3,12]. In the first study it was demonstrated that a
stable digestion process with manure led to ammonium
concentrations exceeding 4 g N l 1 and approximately
900 mg free ammonia l 1. The methane yield was about
25% lower and the VFA level was higher (about 3 g l 1)
compared to a lower ammonia load. Some authors interpreted
this stable process as adapted [13], however, it is debatable
whether a 25% lower methane yield can be interpreted as
adapted. Velsen [12] also concluded that the methane producing
biomass was able to acclimate to ammonium concentrations
as high as 5 g l 1. However, from the presented graphs it could
clearly be seen that the methane production was severely
delayed. Thus, so far, it has not been proven that anaerobic
digestion can completely adapt to high free ammonia
concentrations. From this point of view it would be interesting
to control ammonia in anaerobic digestion. Of the few methods
reported in literature two were from Kayhanian [5]. The
first method was the dilution of the reactor contents, however
this had a significant negative effect on biogas production.
The second method was adjustment of the feedstock C/N ratio,
however, in a practical situation this ratio is substrate
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D.P.B.T.B. Strik et al. / Process Biochemistry 41 (2006) 1235–1238
Table 1
Composition and analysis of the artificial feed
Type of artificial
feed
Experimental
time (days)
COD
(g l 1)
Total N
(g l 1)
Starch
(g l 1)
Meat extract
(g l 1)
Flour (g l 1)
NH4Cl
(g l 1)
Maize silage
Pig manure
4–25; 60–91
26–47; 92–160
132.7
111.7
1.17
6.16
0
20
6
20
150
80
0
10.3
Note: on the days 47–59 the OLR was reduced to zero.
determined and not easily adjustable. Thus both methods were
not practically applicable. Another approach was published by
Krylova et al. [14]. Ammonium inhibition was studied in this
work and the biogas production was enhanced under high
NH4Cl concentrations with addition of phosphorite ore. Some
ambiguity whether phosphorite indeed affected the ammonium/ammonia concentration was noted. One more potential
technique was ammonium precipitation to struvite. Some
studies have investigated this possibility in anaerobic
digestion and industrial wastewater and demonstrated that
ammonium concentrations could be reduced significantly [15–
17]. However, to the best of the author’s knowledge non of
these control applications have been successfully applied in
practice.
The first objective in this study was to prove unambiguously that ammonia can be present in biogas. For this reason
a high total nitrogen containing synthetic feed was digested
under thermophilic conditions. At this higher operating
temperature the free ammonia fraction of the total ammonia
nitrogen will be higher due to the temperature dependent
dissociation constant [5]. Also the chance to produce
ammonia in the biogas will increase, because the temperature
dependent Henry’s law coefficient [18] pushes the fraction of
free ammonia more into the gas phase. After accomplishing
the first objective, the second objective was to control the
ammonia in biogas. Therefore a pH-based control strategy
was developed and applied. The consequence of this control
was that the ammonia concentration in biogas was
reduced, but unfortunately the methane production was also
affected.
2. Materials and methods
2.1. Reactor set-up
A 20 l lab-scale anaerobic CSTR (continuous-flow stirred tank reactor)
was operated at 60 8C. Thermophilic digesting sludge was obtained from the
waste water treatment plant at Altenmarkt (Germany). A detailed description
of the reactor set-up and references of several measurements (carbon oxygen
demand (COD), biogas-production, methane-concentration) have already
been published [19]. For pH-control, which was introduced towards the
end of the experiment, a membrane pump (ProMinent, Germany) was used
to dose alkali (5 M NaOH) or acid (14% HCl). The total nitrogen of the feed
was analysed using the LCK338 test-kit (Dr. Lange, Austria). Ammonium and
ammonia in the reactor were determined with a gas-sensitive electrode
NH500/2 (WTW, Germany) and pH measurement with a Sensolyt-SE electrode (WTW, Germany). The ammonia in biogas was measured daily with
Dräger tubes (Dräger, Germany). Total volatile fatty acids (VFA) were
measured with a Fourier Transform Infrared spectrometer (Perkin-Elmer,
Austria).
2.2. Feed
The reactor was fed with a pH adjusted (pH 7.5) synthetic medium
containing (mg l 1): KH2PO4 (6000), MgSO47H2O (100), CaCl2 (10),
H3BO4 (0.2), FeCl24H20 (8), ZnCl (0.2), MnCl24H2O (2), CuCl22H2O
(0.152), (NH4)6Mo7O247H2O (0.2), CoCl26H2O (8), NiCl26H2O (0.568),
Na2SeO35H2O (0.656), EDTA (4), AlCl36H2O (0.36), Resazurine (2),
FeCl36H20 (3000) and HCl 37% (0.008 ml l 1). During the experiment the
medium was supplemented with extra starch, meat extract, flour Type W480 and
NH4Cl according to Table 1. The feed was continuously mixed and cooled in a
refrigerator at 4 8C and batch fed over the day. The actual daily amount of feed
fluctuated during the experiment.
3. Results and discussion
3.1. Production of ammonia in biogas
The results of the experiment are shown in Figs. 1–3. During
the first 46 days the OLR (organic loading rate) was increased
to 2 kg COD m 3 d 1. In the meantime the total-VFA
increased with a similar trend to 2.2 g l 1. The biogas
production seen to fluctuate. In the same period the NH3 in
biogas increased from <5 to 8 ppm. The reported tolerable free
ammonia concentration of 150 mg l 1 [5] was exceeded and
indicated that the anaerobic process was maybe inhibited. To let
the system adapt to the higher free ammonia concentrations and
to examine if the VFA could be reduced, the OLR was reduced
to zero (days 47–59). However, the VFAs did not reduce and the
pH increased in this period from 7.43 to 7.74. Due to the pH
dependent ammonia dissociation the free ammonia increased to
about 600 mg l 1, which could be even more inhibiting to the
anaerobic processes. The NH3 in the biogas showed a similar
increase with a maximum value of 95 ppm on day 59. Next it
was decided to load the reactor with a low OLR until day 94.
The biogas production increased and VFA decreased, which
indicated that the system was adapting while the free ammonia
Fig. 1. Experimental results for ammonium and ammonia concentration in the
reactor and the NH3 concentration in the biogas in relation to hydraulic retention
time (HRT).
D.P.B.T.B. Strik et al. / Process Biochemistry 41 (2006) 1235–1238
Fig. 2. Experimental data for total-VFA and biogas production in relation to the
organic loading rate (OLR).
concentration remained high (average 649 mg l 1). The
average NH3 in the produced biogas was at this time
113 ppm. In the next period the OLR was increased from
around 1.2 to 2 to demonstrate that ammonia can also be present
in biogas under a higher and more practical applied OLR. From
day 95 the OLR, the biogas production, the biogas-yield and
total-VFA increased. The system was operating quite well
under the high free ammonia concentration (average
693 mg l 1) and NH3 (280 ppm on day 11) in the biogas until
day 113. Then after a short feed reduction the total-VFA
doubled in 2 days (days 115–117). As the result of this the pH
dropped (from 7.74 on day 115 to 7.29 on day 117) and in the
same time the biogas production and yield decreased. It was
apparent that the system was under stress. Thereafter the system
was fed with an OLR of 2 kg COD m 3 d 1. The biogas
production and VFA fluctuated somewhat. From day 137 on the
VFA increased rapidly and the already decreasing biogas
production continued to decrease. The most likely explanation
for these effects was the increase of inorganic nitrogen
concentration in the reactor. From day 135 the pH dropped due
to the increase in VFA.
1237
the free ammonia concentration and is likely to reduce the
ammonia inhibition and simultaneously the ammonia in biogas
concentration. This control strategy was applied to the reactor.
The pH was regulated with alkali and acid pump to a lower value
so that the free ammonia concentration remained low and the
concentration of ammonia in the biogas would also decrease. As
expected, the free ammonia decreased to 57 mg l 1 on day 145
and the ammonia in biogas concentration to 9 ppm. The free
ammonia inhibition was reduced below the value at which free
ammonia inhibition might be observed [5]. The pH dropped
below 6.5 (absolute minimum 6.0), which is just in the range
where methanogenesis will proceed but the activity will already
be negatively effected [20]. Especially under low pH conditions
the volatile fatty acids exist mostly in the free form, so diffusion
in the cell will be higher and consequently the inhibition will
increase [20]. This apparently happened since the methane yield
was at a very low level (days 139–151). In full scale anaerobic
digestion of livestock wastes [3] the reactor pH is often high
(about 8) and free ammonia is 10 times the tolerable
concentration. In accompanying laboratory studies [3] it was
also seen that the VFA concentration will be higher under these
circumstances. In practice keeping the free ammonia and free
volatile fatty acids concentration low by regulation of the pH will
be impossible since ammonia is mostly ionised at a lower pH and
the VFAs at a higher pH. A pH-based control of ammonia in
biogas with a satisfactory biogas production is thus so far proven
unachievable. Nevertheless, the primary goal was to reduce the
ammonia in the biogas and this happened since ammonia in
biogas was reduced from 150 to 5 ppm (day 158). At the end of
the experiment the pH in the reactor was increased slowly so the
biogas and methane production could be enhanced while keeping
the ammonia in the biogas at a low concentration. Unfortunately
the methane production did not improve. The methane yield
stayed low and the VFA concentration increased to an
exceptional concentration of 22 g l 1 (day 159). The anaerobic
system had collapsed.
3.3. Ammonia partition in the liquid and gas phase
3.2. pH-based control of ammonia in biogas
Since the ammonia/ammonium concentration is pH dependent, it is theoretically possible to control this equilibrium by
dosing acid to the anaerobic reactor. A reduced pH will reduce
Fig. 4 shows a regression analysis of the NH3 in the biogas
and free ammonia in the reactor liquid phase. From a statistical
point of view the correlation is moderate. There is a clear main
trend that an increase of free ammonia leads to an increase of
Fig. 3. Experimental data for pH and methane yield.
Fig. 4. Regression analysis for NH3 in the biogas and the free ammonia in the
reactor liquid phase. The graph also shows the theoretical equilibrium in water
at 60 8C.
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NH3 in the biogas. It was not directly possible to find a free
ammonia concentration in the reactor whereby no ammonia
would be present in the biogas. At the start of the experiment
there was a much higher free ammonia concentration in the
reactor than at the end, but the ammonia in the biogas was the
same. The complexity of the physical and chemical liquid–gas
transfer processes was not simply describable through a
regression analysis. Fig. 4 also shows the equilibrium line of
ammonia at 60 8C in a pure water solution (KH of 51 M atm 1).
However, in anaerobic digesters higher salt concentrations and
increased ion activity rise, which will generally decrease the
solubility [21]. In this work the opposite effect was observed.
The solubility was about six times higher than in a pure water
solution. The estimated KH belonging to the linear fit of the
measurements was 310 M atm 1. Since the ammonia in the
biogas was lower than the theoretical liquid/gas equilibrium,
this could imply that there was a huge liquid/gas transfer
limitation for ammonia. This would however not correspond
with findings from Pauss et al. [22] who stated that highly
soluble gases were not limited in liquid/gas transfer. More
sophisticated models could make this phenomenon clearer.
4. Conclusions
It was shown that ammonia can be present in biogas of
thermophilic anaerobic digestion. A maximum NH3 concentration in biogas of 332 ppm was measured. It could be
expected that in biogas from the full scale digestion of livestock
waste NH3 will be present. To protect the environment against
extra NOx emissions and to make a potential biogas fuel cell
application possible, it is important to monitor the NH3 in
biogas and take the necessary measures to reduce this.
By controlling the pH it was possible to reduce the amount
of ammonia in the biogas, but as a consequence the biogas
production was severely affected. In this study the reduction of
the pH was thus not a successful approach. It may therefore be
more practical to install pre-treatment technology at a biogas
plant to remove ammonia from the biogas. Literature reports
several gaseous ammonia treatment techniques, with for
example a chemical scrubber [23] or a bio-filter [24].
Acknowledgements
Gratefully acknowledged is the European Commission for
funding the AMONCO (Advanced Prediction, Monitoring and
Controlling of Anaerobic Digestion Processes) project (NNE52001-00067).
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