Ammonia influence in anaerobic digestion of OFMSW - Water Science ...

Q IWA Publishing 2009 Water Science & Technology—WST | 59.6 | 2009

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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

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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 (Fernańdez 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


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Table 1

|


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


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Figure 3

|

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


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Figure 4

|


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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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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