Q IWA Publishing 2009 Water Science & Technology—WST | 59.6 | 2009
1153
Ammonia influence in anaerobic digestion of OFMSW T. Benabdallah El Hadj, S. Astals, A. Galı´, S. Mace and J. Mata-A´lvarez
ABSTRACT The anaerobic digestion of the organic fraction of municipal solid wastes is taking increasing importance in the recent years. The main problem of some anaerobic digestion process is the large quantity of ammonia that is released, especially when high solid digestion is implemented. A fraction of the supernatant is treated and the remaining is recirculated to maintain the reactor in the optimum solids concentration. The question arising is if this recirculation stream should also be treated to improve biogas production. However, when doing the latter the quantity of ammonia inside the reactor increases too which could lead to inhibit the reactor
T. Benabdallah El Hadj S. Astals A. Galı´ S. Mace J. Mata-A´lvarez (corresponding author) Department of Chemical Engineering, University of Barcelona, Martı´ i Franque`s, no. 1, 6th floor, Barcelona 08028, Spain E-mail:
[email protected]
operation. In this paper it appears that not only free ammonia affects the methanogenic fermentation but also ammonium ion concentration. Biogas production profiles are estimated using the Gompertz model. On the other hand, inhibition constants are fitted using a non-competitive inhibition model equation Thus, 50% inhibition of biomethane production was observed at level of 215 and 468 mg NH3_N/L under mesophilic and thermophilic conditions. However, the methane generation under mesophilic and thermophilic conditions was reduced by 50% when ammonium ion reach concentrations of 3,860 and 5,600 mg NHþ 4 _N/L respectively. Under mesophilic conditions, pH higher than 7 impacted the methanogens bacteria negatively. This threshold pH limit, is variable under thermophilic conditions, depending on the total ammonia concentration. Key words
| ammonia, ammonium, high solid anaerobic digestion, inhibition, pH
INTRODUCTION Anaerobic digestion (AD) is a suitable process to stabilise
water/1 kg MSW) until the required moisture level is
the municipal solid waste (MSW) especially when it is
achieved. Hence, it is necessary to perform the AD process
characterised by a high organic matter content. In Catalonia
recirculating partially the water content of digestate.
(Spain), MSW is accepted to be disposed in sanitary
However, the supernatant recirculation can be lead to
landfills only if organic matter concentration is below 15%
a high level of some toxic species as volatile fatty acids
as dry weight (dw/dw). Moreover, AD not only reduces the
(VFA) and ammonia. Here two options appear as it can be
organic matter content but also converts it to energy
seen in Figure 1.
resource, accomplishing the sustainability principle. In
Therefore option (a) or option (b) from Figure 1 will be
Catalonia, the main anaerobic digesters of organic fraction
selected depending on the inhibition degree that will modify
of municipal solid waste (OFMSW) operate in wet mode
the amount of biogas production. An accurate study will be
with solid content below 15%. In this way, a good mixing at
necessary in each case to decide which option is the most
short retention time and toxics dilution were ensured
appropriate.
(Mata-Alvarez et al. 2000). However, this operation mode
Ammonia substance can be found into anaerobic
implies the addition of water to fresh MSW (mainly 1 L
digesters in two forms, unionised ammonia or free ammonia
doi: 10.2166/wst.2009.100
T. Benabdallah El Hadj et al. | Ammonia influence in anaerobic digestion of OFMSW
1154
Figure 1
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Water Science & Technology—WST | 59.6 | 2009
Types of recirculation configuration to treat supernatant from OFMSW anaerobic digester.
(NH3) and ionised ammonia or ammonium ðNHþ 4 Þ. The
pH. Inversely, a low temperature (mesophilic conditions)
ammonium ion may inhibit the methane production
digestion shows a large tolerance of the pH effects on
directly (Sprott & Patel 1986; Kayhanian 1999). However,
NH3 released.
it was reported that unionised form is more toxic because of
Since there are not many bibliographic data on the
its capability to penetrate through cell membrane (Sung &
effect of ammonium ion on anaerobic digestion of
Liu 2003). Gallert & Winter (1997) observed a methane
the organic fraction of municipal solid waste (OFMSW),
production inhibition of 50% at 0.22– 0.28 g NH3_N/L
the main aim of this study is to compare the performance of
and 0.68– 0.69 g NH3_N/L during anaerobic digestion of
both mesophilic and thermophilic anaerobic digestion of
glucose under mesophilic and thermophilic conditions
OFMSW under different free ammonia and ammonium ion
respectively. In other words, it was concluded that a
concentrations at several pH values. This data can be
thermophilic flora tolerates ammonia toxicity twice as
used to quantify the inhibition of methanogens by
great as a mesophilic one. Previously, Angelidaki & Ahring
ammonium and/or free ammonia concentration, and to
(1994) observed the inhibition symptoms on thermophilic
obtain conclusions about the feasibility on treating the
digestion of cattle manure from 0.7 g NH3_N/L and pointed
supernatant stream before recirculation (Figure 1b).
out the negative effect of temperature increase in the thermophilic range (40 –648C). Moreover, Sung & Liu (2003) reported 8 –13 g NHþ 4 _N/L as a concentration of 100% inhibition under thermophilic conditions, depending
MATERIALS AND METHODS
on acclimation conditions and system pH. This parameter together with temperature, are the main factors that can influence the anaerobic process by ammonia, especially free ammonia, since the amount of NH3 is given in function of total ammonia nitrogen (TAN) by Anthonisen et al. (1976) equation: NH3 _N ¼
pH NHþ 4 _N £ 10 6344=273þT e þ 10pH
Experimental Set-up In order to quantify biogas production biodegradability tests were performed using synthetic substrate (pet food) to simulate OFMSW. The main characteristics of this substrate were 6.1% as ash, 24% as protein and 15.4% as lipids (Ferna´ndez et al. 2005). The pet food substrate was conditioned previously to the required conditions for batch test assays. These batch
Thus, the higher temperature or the higher pH, the
tests were carried out in digesters of 250 mL which were
higher amount of free ammonia that is released. Conse-
filled with substrate and inoculum at 0.5 of VSsubstrate/
quently, at high temperature digestion (thermophilic
VSinoculum ratio. Nitrogen ammonia concentration and
conditions), it is suitable to operate at the lowest possible
pH was fitted by addition of NH4Cl, HCl or NaOH and
T. Benabdallah El Hadj et al. | Ammonia influence in anaerobic digestion of OFMSW
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Table 1
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Water Science & Technology—WST | 59.6 | 2009
Gompertz equation was employed to describe cumulative
Substrate and inoculums charcteristics
methane production during batch tests. Substrate
Inoculum
Parameter
Unit
Value
Parameter
Unit
Value
TS
G/kg
921.4
TS
g/kg
65.3
VS
G/kg
720.6
VS
g/kg
40.4
VS/TS
%
94.5
VS/TS
%
61.9
NHþ 4 _N
mg/L
0
NHþ 4 _N
mg N/L
3,227
TKN
mg/g
38.15
TKN
mg N/L
5,530
COD
gO2/kg
1008.84
COD
mgO2/L
61,400
M ¼ M0 exp
2 exp
BRe ðl 2 tÞ þ 1 M0
ð1Þ
where M is the cumulative methane production (mL) at incubation time, t (day); l is the lag-phase-time (days); M0 is the methane production potential (mL); B is the total biomass in the vial (g VS); R is the specific methanogenic activity (SMA) (mL/g VS day) and e is a mathematical constant (2.718…).
phosphate buffer with an excess of alkalinity (Sung & Liu 2003) (Table 1).
SMA determined from batch tests can be inhibited by both ammonium and free ammonia species as decribed in
Equal amounts of substrate, inoculum and chemicals
Equation (2).
were added to all bottles and the effective digesters volume was set to 200 mL by deionised water. The methane produced during a test was measured using a displacement
R ¼ R max
S INH3 INH4 Ks þ S
ð2Þ
liquid device equipped with biogas wash vessel (1 N NaOH solution to remove CO2). The test was performed in
where R is SMA (mL/g VS/day) determined with the
triplicate for each ammonia nitrogen concentration at pH
presence of NH3 and/or NHþ 4 , Rmax is the maximum SMA
of 7; 7.5; 8 and 8.5 under mesophilic and thermophilic
without inhibition (mL CH4/g VS/day) at specific pH, S is
temperature conditions (Benabdallah El-Hadj et al. 2007). The inoculum, previously filtered through 1 mm sieve mesh, was obtained from the metropolitan high solid anaerobic digester (Ecoparc-Montcada) situated in Barcelona, working at 30 days as solid retention time and 180 L CH4/kg VS
the substrate concentration (mg VS/L) and Ks a semisaturation constant (mg VS/L). At specific pH, the effect of ammonium or ammonia on methanogens was described by using the following inhibition models (Siegrist et al. 2002):
as a specific biogas production (SBP). INH3 ¼ Analysis Analysis of total chemical oxygen demand (COD), total
INH4 ¼
K2I;NH3 K2I;NH3 þ S2NH3 K2I;NH4 2 KI;NH4 þ S2NH4
ð3Þ
ð4Þ
solids (TS) and volatile solids (VS), pH, bicarbonate and KI;NH4 are the inhibition constants.
alkalinity, total ammonia nitrogen (TAN) and total Kjeldahl
where KI;NH3
nitrogen (TKN) were performed according to the Standard
A 50% inhibition is reached if KI,i ¼ Si. The square
Methods (1995). Individual volatile fatty acids and gas
functions are necessary to describe the strong increase
composition
of the inhibition with increasing ammonia or ammonium
were
analysed
by
gas
chromatography
equipped with flame ionization detector (FID) and thermal
concentration.
conductivity detector (TCD), respectively (Benabdallah El Hadj et al. 2006).
RESULTS AND DISCUSSION Mathematical tools
Around 72 batch tests were performed to study the effect
Based on the study of Lay et al. (1996) who related bacterial
of ammonia on anaerobic efficiency at different pH
growth to metabolic biogas production, the modified
under each digestion temperature, namely mesophilic and
T. Benabdallah El Hadj et al. | Ammonia influence in anaerobic digestion of OFMSW
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Water Science & Technology—WST | 59.6 | 2009
Figure 3
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Relative methanogenic activity at final total ammonia concentrations and pH values under mesophilic. (a) and thremophilic (b) temperature conditions.
Figure 2
|
Specific biogas production at final total ammonia concentrations (TAN) and pH values under mesophilic. (a) and thremophilic (b) temperature conditions.
concentration range, the inhibition of methane production was detected. Hence, from data illustrated in Figures 2 and 3, thermophilic conditions. The results obtained are illustrated
it can be concluded that the inhibition of biogas production
in Figure 2, which shows the plots of specific biogas
is caused by both species free ammonia and ammonium ion
production (SBP) versus final total ammonia (TAN)
present in the medium. This later phenomenon is more
concentration at different pH values under mesophilic and
noticeable at higher ammonium concentration and low
thermophilic temperature conditions.
pH values.
From Figure 2, it is clear the higher amount of biogas
To determine the kinetic constants in Equation (2), a
produced under thermophilic conditions than under meso-
series of batch tests were conducted at different substrate
philic ones. Besides, the higher the pH, the higher the
concentrations using 0.25, 0.4, 0.5 and 0.8 VSsubstarate/
SBP inhibition observed under mesophilic temperature.
VSinoculum ratio (at a background TAN concentration of
However, under thermophilic conditions the highest SBP
1,080 mg N/L and pH of 7.5). The cumulative methane
was registered at pH of 7.5. On the other hand, it was
production at each level was recorded and was fitted using
observed that at low range of free ammonia, the SBP
the Gompertz’s equation by least-square non linear regres-
inhibition was observed indicating that another inhibition
sion. The values of Monod kinetic constants Ks and Rmax
element was present (see Figure 3).
subsequently determined by linear regression were 4,870 mg
In Figure 3, the relative methanogenic activity was plotted versus final total free ammonia. From this figure it can be observed that, at the same free ammonia
COD/L and 2,895 mL/g VS day, respectively. The constants KI;NH3
and KI;NH4 were estimated using
a least-square non linear regression under mesophilic and
T. Benabdallah El Hadj et al. | Ammonia influence in anaerobic digestion of OFMSW
1157
Figure 4
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Water Science & Technology—WST | 59.6 | 2009
Relative methanogenic activity at final ammonium ion concentrations and pH values under mesophilic. (a) and thremophilic (b) temperature conditions. Figure 5
thermophilic temperature conditions. The obtained values of KI;NH3 were 215 and 468 under mesophilic and
|
Specific biogas production at initial total ammonia concentrations (TAN) and pH values under mesophilic. (a) and thremophilic (b) temperature conditions.
CONCLUSIONS
thermophilic temperature conditions. These values are in accordance with the results observed by Gallert & Winter
During high solids anaerobic digestion ammonia and
(1997). However, the ammonium ion (the difference
ammonium can be released in the medium at high
between TAN and free ammonia amounts) reduced the
concentrations. In this paper it appears that not only free
methane production by 50% at 3,860 and 5,600 mg/L
ammonia can inhibit the methanogenic activity but also the
under mesophilic and thermophilic temperature condi-
ammonium ion. 50% inhibition of methane production was
tions respectively (see Figure 4).
observed at 215 and 468 mg NH3_N/L under mesophilic
To study the role of pH in ammonia/ammonium
and thermophilic anaerobic digestion. However, the
inhibition, SBPs versus pH values were plotted at a given
presence of NHþ 4 _N at 3,860 and 5,600 mg N/L can
initial total ammonia nitrogen (TAN).
reduce the methane production by 50%. This last is more
It seems that the optimum pH values are around 7
acute at low pH where free ammonia is less toxic. The pH
under mesophilic temperature condition independently
increase can affect negatively the biogas generation under
of TAN concentration. However under thermophilic
mesophilic conditions. However, the optimum pH can vary
conditions, it can be observed that 7.5 is the optimum pH
under thermophilic conditions, depending on the total
when TAN is higher than 1,331 and it can be present it
ammonia nitrogen level. The data obtained here suggests
the range 7 –8 if the TAN is below or equal to 1,331 mg/L
that the pre-treatment of recirculating supernatant for
(see Figure 5).
nitrogen removal can overcome the ammonia inhibition
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T. Benabdallah El Hadj et al. | Ammonia influence in anaerobic digestion of OFMSW
especially when ammonium ion has a negative impact on anaerobic digestion. In this way, the anaerobic digestion plant can achieve a significant biogas increase depending on pre-treatment efficiency, anaerobic digester operation conditions and substrate composition. However, a detailed economic study should be performed to determine whether this larger WWTP capacity is feasible or not.
ACKNOWLEDGEMENTS The authors wish to thank the Spanish Ministry of Science and Technology (CTM2005-02877/TECNO) for the financial support of this study.
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