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Anaerobic digestion for methane generation andammonia reforming for hydrogen production:A thermodynamic energy balance of a modelsystem to demonstrate net energy feasibility
David M. Babson a, Karen Bellman b, Shaurya Prakash b,*,Donna E. Fennell a,**aDepartment of Environmental Sciences, Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901,
United StatesbDepartment of Mechanical and Aerospace Engineering, The Ohio State University, 201 W. 19th Ave., Columbus,
OH 43210, United States
a r t i c l e i n f o
Article history:
Received 6 October 2012
Received in revised form
6 May 2013
Accepted 24 May 2013
Available online
Keywords:
Anaerobic digestion
Ammonia
Bioenergy
Bioammonia
Hydrogen
Anaerobic digestion-bioammonia
to hydrogen (ADBH)
* Corresponding author. Tel.: þ1 614 688 404** Corresponding author. Tel.: þ1 848 932 574
E-mail addresses: [email protected] (S0961-9534/$ e see front matter ª 2013 Elsevhttp://dx.doi.org/10.1016/j.biombioe.2013.05.
a b s t r a c t
During anaerobic digestion, organic matter is converted to carbon dioxide and methane,
and organic nitrogen is converted to ammonia. Generally, ammonia is recycled as a fer-
tilizer or removed via nitrificationedenitrification in treatment systems; alternatively it
could be recovered and catalytically converted to hydrogen, thus supplying additional fuel.
To provide a basis for further investigation, a theoretical energy balance for a model sys-
tem that incorporates anaerobic digestion, ammonia separation and recovery, and con-
version of the ammonia to hydrogen is reported. The model Anaerobic Digestion-
Bioammonia to Hydrogen (ADBH) system energy demands including heating, pumping,
mixing, and ammonia reforming were subtracted from the total energy output from
methane and hydrogen to create an overall energy balance. The energy balance was
examined for the ADBH system operating with a fixed feedstock loading rate with C:N
ratios (gC/gN) ranging from 136 to 3 which imposed corresponding total ammonia nitrogen
(TAN) concentrations of 20e10,000 mg/L. Normalizing total energy potential to the
methane potential alone indicated that at a C:N ratio of 17, the energy output was greater
for the ADBH system than from anaerobic digestion generating only methane. Decreasing
the C:N ratio increased themethane content of the biogas comprising primarily methane to
>80% and increased the ammonia stripping energy demand. The system required 23e34%
of the total energy generated as parasitic losses with no energy integration, but when
internally produced heat and pressure differentials were recovered, parasitic losses were
reduced to between 8 and 17%.
ª 2013 Elsevier Ltd. All rights reserved.
5; fax: þ1 614 292 3163.8; fax: þ1 732 932 8644.. Prakash), [email protected] (D.E. Fennell).ier Ltd. All rights reserved.024
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5494
1. Introduction
1.1. Anaerobic digestion processes and applications
In recent years, major attitudinal shifts have occurred in
modern society to allow wastes to be considered resources
rather than just materials requiring disposal [1e3]. Waste as a
resource can be envisioned as a means of reducing energy
consumption by minimizing the need for producing raw ma-
terials, reducing pre-disposal processing, and extracting us-
able energy from the waste as a feedstock [4e6]. One
established technology for extracting energy from waste is
anaerobic digestion (for recent reviews see Refs. [7e11]).
Anaerobic digestion of crop biomass, agricultural wastes
and residuals, and source-separated mixed organic wastes
have been employed at full scale for decades in Europe (nearly
200 digesters, 2010 [12]), China (10,000 digesters, 1986 [13]) and
India (2,000,000 digesters, 2000 [14]), among other places [15,16].
These digesters operate to generate biogas, comprising pri-
marily of methane, as a fuel source. In the US, anaerobic
digestion is primarily used for wastewater treatment plant
(WWTP) sludges [17], animal manures [18], andmunicipal solid
waste (MSW) in landfills [19]. There aremore than 500 large (>5
million gallons per day) municipal wastewater treatment fa-
cilities [20] and 176 animalmanure digesters in the US [18]. One
of the most prevalent large-scale applications of anaerobic
digestion in the US is in landfills where anaerobic conditions
dominate the operational timeline [21]. However, as of June
2012, 594 of more than 1700 US landfills utilized biogas for en-
ergy production while the remainder flared biogas without
recovering biogas energy [22]. The active landfill projects lead to
generation of over 1800 MW of equivalent energy [15].
Compared to several other countries around the world,
anaerobic digestion has been relatively under-utilized in the
US for a variety of economic and technical reasons. These
include traditionally low energy and/or fuel prices, lack of
governmental incentives for implementing new anaerobic
power plants, the need for abundant and suitable land for site
development facilities and disposal of residuals, the need to
provide high quality reliable heat for the process to achieve
acceptable or commercially viable conversion efficiencies,
and the reputation of anaerobic processes as odor-generating
and difficult to operate [23e26]. The purpose of this paper is to
show through amodel system that when part of an integrated
system, anaerobic digestion can be a powerful resource for
wastemanagement and energy extraction. Specifically, in this
paper, a thermodynamic energy balance for amodel system is
presented demonstrating that anaerobic processing of waste
for harvesting both methane and ammonia as multiple fuel
sources in contrast to methane alone can provide an addi-
tional avenue to a net increase in extracted usable energy
from waste processing.
1.2. Inorganic nitrogen mitigation and removal
The environmental advantages of in-vessel anaerobic di-
gesters include stabilization of biochemical oxygen demand,
generation of biogas, production of digestate as a soil
amendment, and reduction of the environmental footprint
associated with land-filling [15,27,28]. However, several envi-
ronmental concerns as discussed below dictate post-
treatment steps needed for the digestate produced during
anaerobic digestion of organic feedstocks [29,30]. Of particular
concern is ammonia which is toxic to aquatic organisms,
causes eutrophication, and exerts oxygen demand in surface
waters [31]. Further, processes to remove ammoniaenitrogen
from aqueous effluent can require energy-intensive treatment
with large reaction vessels and long holding times [32e34].
Theammonia that accumulates inanaerobicdigesters exists
in two forms, ammonium ion ðNHþ4 Þ and free ammonia (NH3),
and is in equilibrium in aqueous systems (Equation (1)) [17].
NHþ44NH3 þHþ (1)
Total ammonia nitrogen (TAN) is the sum of NHþ4 and NH3
expressed as total N on a mass basis. The ratio of NH3-N to
NHþ4 �N in an aqueous system is governed by pH and tem-
perature (Equation (2)) [17].
½NH3 �N� ¼ ½TAN��1þ ½Hþ�
Ka
� (2)
where NH3-N is the free ammonia nitrogen concentration and
Ka is the temperature dependent dissociation coefficient for
Equation (2). TAN accumulates in digesters when proteins,
urea, nucleic acids, and other nitrogen-containing com-
pounds degrade, and its concentration must be controlled by
removal or by altering feedstock carbon to nitrogen (C:N) ra-
tios to prevent inhibition of the microbial process by higher
concentrations of free ammonia [35,36].
Anaerobic digestate is frequently used as a soil amend-
ment. However, application of anaerobic digestate to land as a
soil amendment must be carefully managed to avoid release
of excess nitrogen to surface waters, infiltration to ground
water, and the atmosphere. Particularly affected by these
problems are swine, poultry, and dairy operations, where land
application of digestate is an important disposal route [37e43].
Ma et al. (2005), for example, estimated that for Tompkins
County, NY, USAwith a total dairy herd of 9500 approximately
20,000 acres of suitable land would be needed to house di-
gesters and solids/liquids handling systems and to provide a
land sink for the resulting digestate [24]. With increasing
population pressures throughout the world, demand for such
large land resources can pose a significant problem for post-
processing of N or ammonia-rich waste feedstock. In addi-
tion, large domestic WWTP digesters located in metropolitan
environments are often at considerable distances from suit-
able land (>10 km). Consequently, these WWTPs must treat
ammonia onsite via nitrification/denitrification or haul
nitrogen-rich digestate to distant land sinks for disposal [24]
causing challenges for energy efficient waste management.
Anaerobic digester supernatant currently recycled to the
influent of some WWTPs may account for as much as 30% of
the incoming nitrogen loading to the facility [44] and also
constitutes a substantial regulatory concern and energy sink
[45]. Ammonia contained in leachate is also an important
factor controlling the long-term monitoring and post-closure
concerns of MSW landfills [46].
Conventional biological nutrient removal that combines
nitrification and denitrification requires long solids retention
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5 495
times (on the order of several hours to days) and energy-
intensive aeration to accommodate nitrifying bacteria
[32e34]. Further, denitrification mediated by heterotrophic
bacteria may divert carbonaceous substrates from methane
generation in digesters or require external electron donor
addition in wastewater applications [47]. Combined processes
of partial nitrification of ammonium to nitrite followed by
denitrification of nitrite (e.g., Canon/Sharon Anammox pro-
cesses [47e49]) have been developed to reduce energy and
oxygen demands and thus eliminate the need for external
electron donor addition [34]. However, these processes do not
eliminate the treatment energy demands completely nor do
they allow for the extraction of ammonia as a potential fuel.
Ammonia is a valuable industrial and agricultural chemical
used to produce fertilizer, solvents, cleaning agents, and re-
frigerants [50]. Furthermore, ammonia has been proposed as a
potential feedstock for hydrogen [51] due to the high density of
hydrogen per unitmass or volume of ammonia, and it can also
be directly harvested for energy conversion in a direct-
ammonia fuel cell [52e55]. Chemical routes for synthesis of
ammonia tend to be energy and material intensive [56] and
bio-ammonia as a sustainable fuel source is therefore
receiving increased interest for a variety of applications [57].
As discussed above, ammonia has a high-density of hydrogen
per unit volume on a weight basis of source material
(w0.18 g H2/g NH3), and compares favorably to other materials
used for hydrogen storage [58]. However, high temperature
(w800e900 �C) is required for thermal reforming to generate
hydrogen from ammonia i.e. energy input is needed to harvest
hydrogen as a fuel.
This paper evaluates whether ammonia liberated biologi-
cally (bio-ammonia) during anaerobic digestion could be har-
vested via stripping [59,60] and utilized as a source of hydrogen
as part of a coupled Anaerobic Digester-Bioammonia to
Hydrogen (ADBH) system (Fig. 1), and be a beneficial operational
approach in addition to harvesting methane from biogas.
Fig. 1 e Model anaerobic digester used for developing the
theoretical model for analysis. This system is referred to as
the anaerobic digester for bioammonia to hydrogen
(ADBH). The schematic shows flows of different streams
with details on each stream tabulated in Table 1. The
dotted line around the system represents the control
surface for thermodynamic analyses.
Ammonia-stripping has been used for treating animal waste
slurries [61e63], landfill leachate [64] and fertilizer plant wastes
[65]; however, it has not been extensively studied as a means
of ammonia removal from digester effluents [66]. Stripping
has been shown to reduce ammonia in effluents to less than
10 mg NH3-N/L [67]. Recovered ammonia gas could therefore
become the reforming fuel for catalytic reforming as shown in
Equation (3).
2NH3(g) / N2(g) þ 3H2(g) (3)
Thus, for an anaerobic digester producing biogas contain-
ing methane and discharging digestate rich in TAN, the in-
clusion of an ammonia recovery and reforming system to
generate hydrogen could allow additional biofuel or provide
an alternate route to harvesting an important industrial
chemical in itself. Consequently, the specific purpose of this
paper is to develop a conceptual model of an ADBH system
generating usable energy by harvesting multiple fuel source
streams in biogas and validate this concept model through a
thermodynamic energy balance based on the first law for
feedstocks of varying C:N ratios. Therefore, this paper (1) es-
tablishes a theoretical design scheme for an integrated system
to carry out anaerobic digestion and ammonia recovery to
demonstrate a quantifiable increase in overall energy gener-
ation from waste, (2) characterizes the energy demands and
energy production by focusing on two fuel sources in the
forms of methane and hydrogen, (3) considers different cases
of energy recovery within the integrated system to improve
the overall operation efficiency of the system from a net en-
ergy output perspective, and (4) identifies areas for further
scientific and engineering research needed to produce a net-
positive energy ADBH system. As discussed above, the en-
ergy balance estimates theoretical energy inputs and outputs
based on the first law of thermodynamics analyses and does
not account for process entropy changes.
2. Theoretical model framework
2.1. Model ADBH system description
The model ADBH system shown in Fig. 1 includes an anaer-
obic digester that stabilizes waste, produces biogas containing
methane and carbon dioxide as the main constituents, and
discharges digestate containing TAN. Further, the system
utilizes a solideliquid separator to concentrate the solid con-
tent in the digestate and produces liquid leachate containing
TAN. In addition, two pH shift reactors were included, to first
increase the pH of the leachate for converting TAN to NH3,
then later to neutralize the pH for recycle back to the digester.
After the leachate pH has been increased, gaseous ammonia is
recovered in a stripper. Finally, a combustion-based heat
source uses a fraction of the methane generated by the
digester as an energy source for ammonia reforming to pro-
duce hydrogen as an additional fuel. Descriptions of the con-
ceptual ADBH system flows indicated by arrows in Fig. 1 are
summarized in Table 1.
Stream 1, the solid organic waste flow, and Stream 2, the
influent additional liquid flow are the input streams to the
Table 1 e Theoretical anaerobic digester-bioammonia tohydrogen (ADBH) system flow descriptions andconstraints.
Number Type Phase Components
1 Influent Solid Organic waste feedstock
2 Influent Liquid
<1% TS
Aqueous stream
(TAN ¼ 200 mg/L)
3 Internal Slurry
10% TS
Digestate
4 Effluent Semi-solid
99% TS
Digestate solids with
CaCO3 precipitate
5 Internal Liquid
<1% TS
Digestate leachate
ðTAN ¼ NHþ4ðaqÞ þNH3ðaqÞÞ
6 Internal Liquid
<1% TS
Digestate leachate
(TAN ¼ NH3 (aq))
7 Recycle Liquid Digestate leachate with
N removed
8 Internal Liquid
<1% TS
Digestate leachate with
N removed (alkaline)
9 Recycle Liquid Digestate leachate with
N removed
10 Internal Gas NH3 (g)
11 Internal Gas Digester biogas (CH4, CO2)
12 Recycle Liquid
<1% TS
Digestate leachate with N
removed (neutral)
13 Internal Gas Concentrated biogas (CH4, CO2)
14 Effluent Liquid
<1% TS
Digestate leachate with N
removed (neutral)
15 Internal Gas Concentrated biogas (CH4, CO2)
16 Effluent Gas Effluent concentrated biogas
(CH4, CO2)
17 Influent Gas Air
18 Effluent Gas H2/N2
19 Effluent Gas Exhaust (CO2, H2O)
Table 2 e Variable input values for analysis of C:N ratioimpact on system energy accumulation.
Variable description Input value
C:N ratio Variable from 3
to 136 (g C/g N)
Dry-solids mass flow 1000 kg TS/h
Fraction of feedstock degraded 0.80
Moisture content (wt.% water) 90.0%
Aqueous TAN loading (in Stream 2) 200 mg NHþ4 �N=L
Percent recycle of Stream 8 65.0%
Ambient-digester temperature difference 40.0 �CInternal efficiency 0.35
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5496
anaerobic digester and control all individual system compo-
nents and reactor sizes, aswell as the internalmass flows, and
effluent mass flows. Stream 2 is controlled by the specified
operating moisture content in the digester, and the internal
liquid recycle flow (Stream 12). The inclusion of Stream 2 also
allows for treatment of aqueous flows containing TAN. The
volumetric or mass flow and composition of the influent
streamswas systematically varied to allow different scenarios
to be evaluated, as discussed later. In the following sections,
each system component is quantitatively described with a
discussion of net incoming or outgoing energy flows.
2.2. Anaerobic digester component
The anaerobic digester component consists of the anaerobic
reactor; incoming organic solid waste as Stream 1, a liquid
stream for achieving the desired operating moisture content
as Stream 2, liquid recycle Stream 12 formaximumusable fuel
recovery, outgoing biogas Stream 11, and the outgoing diges-
tate slurry from the anaerobic digester to the solideliquid
separator as Stream 3. In addition, heat and mechanical and/
or electrical power inputs to maintain digester temperature
and facilitate mixing were also included.
Influent Stream 1 for the digester is characterized by the
organic waste input quantified as a dry solids feedstock mass
flow rate, and is assigned both a specific carbon to nitrogen
(C:N) ratio (g C/g N) and a degradable organic fraction assumed
to be 80%, a commonly used fraction [17]. Additionally, the
reactor moisture content, the [TAN] concentration of the
additional liquid stream, and the internal leachate recycle
flow following ammonia stripping (Stream 12) were also
specified (Table 2).
Amodel feedstockmolecularmake-up based on a survey of
existing literature was designated to facilitate computation of
the methane, carbon dioxide, and TAN produced during
anaerobic digestion. The TAN of the digestate at pH 7 would
consist of 3.8% NH3 and 96.2% NHþ4 at an operational tem-
perature of 55 �C (Equation (2)). The digester is assumed to
operate under thermophilic conditions (55 �C) rather than
mesophilic conditions (37 �C) because a separate analysis
found that even minimal increases in methane generation
under thermophilic conditions were sufficient to offset any
additional heating demands, and could potentially net more
total fuel [67a]. Although no such methane production en-
hancements were assumed for analyses presented here,
maintaining conservative estimates of our system’s perfor-
mance, additional heating requirements were assessed for
operation at 55 �C. An additional advantage of thermophilic
operation over mesophilic operation is that the liberation of
ammonia from biomass during hydrolysis may proceed to a
greater extent and themicrobial communitymay have greater
tolerance to ammonia [68]. Two separate molecular feedstock
formulas were considered. First, the conversion of the
degradable fraction of the organic matter to methane and
carbon dioxide was via Equation (4) [19].
C6H10O4 þ 1.5H2O / 3.25CH4 þ 2.75CO2 (4)
Organic compounds containing nitrogen were modeled as
C2nH5nO2nNn to allow themolar amounts of nitrogen to vary in
response to the C:N ratio of the feedstock. A simple protei-
nogenic amino acid (glycine) was used as the empirical
structural basis for nitrogen containing species as it had the
greatest C:N ratio, and based on reported stoichiometry it was
taken that conversion of the organic nitrogen fraction to
methane proceeded via Equation (5) [69].
C2nH5nO2nNnþ0:5nH2OþHþ/0:75nCH4þ1:25nCO2þnNHþ4 (5)
The overall stoichiometry of the methanogenic breakdown of
the feedstock was therefore the sum of Equations (4) and (5) to
yield Equation (6).
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5 497
Cð6þ2nÞHð10þ5nÞOð4þ2nÞNn þ�1:5þ 0:5n
�H2Oþ nHþ/
�3:25þ 0:75n
�� CH4 þ
�2:75þ 1:25n
�CO2 þ nNHþ
4
(6)
where n, the stoichiometric amount of nitrogen contained in
the feedstock formula (mol N/mol feedstock), was computed
from the C:N ratio of the influent feedstock by Equation (7).
n ¼ 6$ðAWCarbonÞðC : NÞratio$
�AWNitrogen
�� 2$ðAWCarbonÞ(7)
From Equations (6) and (7) it can be seen that there are 6moles
C per mol of the organic portion of the feedstock and 2 addi-
tional moles of C per mol of feedstock for each mole N added,
AWCarbon is the atomic weight of C (12 g C/mol C), AWNitrogen is
the atomic weight of nitrogen (14 g N/mol N) and (C:N)ratio is
the carbon to nitrogen ratio (g C/g N) contained in the desig-
nated feedstock.
To fully account for carbon and nitrogen flows in the
model, microbial growth was also included [70] in a manner
similar to other waste treatment mass balance models [71].
The net fraction of the feedstock (C(6þ2n)H(10þ5n)O(4þ2n)Nn)
electron equivalents incorporated into new microbial
biomass, fs, was calculated according to Equation (8) [17,70,72].
fs ¼ f 0s $
�1þ �
1� fd�$b$T
1þ b$T
�(8)
where f 0s is the theoretical fraction of feedstock electrons used
for synthesis and was taken to be 0.05 [72]; fd is the fraction of
the microbial biomass that is biodegradable and was taken as
0.80; b, the decay rate was 0.03 d�1 [17]; and T, the retention
time was 20 days. The net fraction of electrons used for en-
ergy, fe, was fe¼ 1� fs [17,70,72]. Incorporating fs and fe and cell
synthesis modified the overall digester reaction stoichiometry
(Equation (9)) [17] to
CaHbOcNd þ�2aþ d� c� 0:45efs � 0:25efe
�H2O/
�0:125efe
�CH4
þ �a� c� 0:2efs � 0:125efe
�CO2 þ
�0:05efs
�C5H7O2N
þ �d� 0:05efs
�NHþ
4 þ �d� 0:05efs
�HCO�
3
(9)
where a (6þ 2n), b (10þ 5n), c (4þ 2n) and d (n) correspond to the
feedstockmaterial (C(6þ2n)H(10þ5n)O(4þ2n)Nn), e¼ 4aþ b� 2c� 3d,
and themicrobial biomass was assumed to have themolecular
formula C5H7O2N, which controlled the amount of N incorpo-
rated during synthesis of new cells [17]. The TAN concentration
in the digestate under differing C:N ratios was equal to the
NHþ4 �N component of Equation (9). Further, the methane
generated from each test condition was determined from
Equation (9).
Calculations for external heating required to raise the
temperature of the influent material to 55 �C and also to
maintain the digester at 55 �C are described in Section S1 of
the Supplementary Materials. The power required for me-
chanically mixing the digester, which was assumed to be a
continuously stirred tank reactor (CSTR), is described in Sec-
tion S2 of the Supplementary Materials. The solidseliquid
separation unit, handling digestate flow from the digester,
was assumed to be a gravity separation unit operating at 95%
efficiency [73]. The unit size was a function of the influent
digestate total solids content (%TS), the volumetric flow rate,
the average assumed particle size (0.5 mm) [73], and the
specified solids removal efficiency (95%) [73]. Flow through the
unit was assumed to be driven by the hydrostatic pressure
drop between the digester and the first pH-shift reactor.
2.3. Ammonia gas recovery component
The ammonia gas recovery system consisted of two pH-shift
reactors, an aqueous ammonia stripper, and Streams 5e14.
The NHþ4 in the digestate leachate (Stream 5) was converted to
NH3 in the first pH shift reactor where calcium hydroxide
(lime) was added to increase the pH from 7 to 11 (Equations
(10) and (11)).
CaðOHÞ2/Ca2þðaqÞ þ 2OH�
ðaqÞ�DH ¼ �16:7 kJ=mol
�(10)
NHþ4ðaqÞ þOH�
ðaqÞ/NH3ðaqÞ þH2O�DH ¼ �366:6 kJ=mol
�(11)
Heat generated from reactions described by Equations (10) and
(11) is sufficient to increase the temperature of the aqueous
stream entering the stripper to the solution boiling point from
the digester discharge temperature of 55 �C. NH3(aq) contained
in Stream 6 is then stripped to the gas phase in the ammonia
stripper using steam, produced from Stream 7, as the strip-
ping gas. The alkaline, TAN reduced digestate leachate
(Stream 8) flows to the second pH shift reactor and the pH is
neutralized using the carbon dioxide in the biogas, Stream 11
(Equation (12)).
Ca2þ þ 2OH� þCO2/CaCO3 þH2O�DH¼�97:10 kJ=mol
�(12)
This neutralization process removes carbon dioxide from the
biogas stream and converts it to a dissolved form in the
aqueous phase, subsequently producing a concentrated
biogas stream with higher methane content than coming
directly from the digester (Stream 13). A finite energy input is
required to operate the ammonia stripper and to homogenize
the reactants in the pH-shift reactors; however, excess pro-
cess heat could be recovered from the reactions described by
Equations (10) and (12). This is included in themodel based on
the enthalpies of each reaction as discussed below, which
assume that these reactions go forward to completion. The
reactions are assumed to reach completion based on available
data (the concentrations of CaCO3 at pH 11 andOH� at pH 7 are
each less than 10�7 M respectively), and this assumption is
provided to validate the reaction enthalpy values utilized.
However, the model accounts for reaction equilibriums to
rectify stoichiometry and quantify mass balances. Thus, the
calculated heat values correspond to the predicted equilib-
rium of each reaction for which kinetic and equilibrium data
has been previously reported.
Elston and Karmarkar (2003) considered aqueous ammonia
stripping technologies for selective catalytic reduction appli-
cations (SCR) [67]. Comparing air and steam as stripping
media they identified a linear relationship for power re-
quirements for aqueous ammonia stripping (19 wt.% NH3(aq))
for a range of aqueousmass flows [67]. Based on their findings,
the power requirements for stripping ammonia from the
digestate leachate using either air or steam were estimated to
be 0.265 kJ/kg-aqueous mass treated (kJ/kg-aq.) or 1.574 kJ/kg-
aq., respectively [67].
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5498
Heat produced during each of the pH shifts was determined
using the standard enthalpy of formation (DHf) for the reactants
and products [74,75]. Heat from these exothermic reactionswas
assumed to be captured and re-used with an internal heat
transfer efficiency of 35% [76]. In this work, the energy input for
operating the mechanical components required optimizing
this internal heat utilization, for example, through the use of
additional pumping to and from a heat exchange unit, was
not estimated. The sensitivity of this latter assumption for
influencing the overall energy balance was assessed as part of
the energy integration cases (see Section 2.7).
Mixing of pH-shift reactors was assumed to occur via ve-
locity gradient mixing with retention times of 30 s [77] and the
power requirementwas determined as described in Section S3
of the Supplementary Materials.
2.4. Ammonia reforming component
The ammonia reforming component consisted of two sepa-
rate processes. The first was a combustion-based heat source
to heat a catalyst within a reformer where the incoming NH3(g)
(Stream 10) is catalytically pyrolyzed to yield N2 and H2
(Stream 18). A fraction of the concentrated methane biogas
(Stream 13) is diverted from the splitter to the combustor
(Stream 15), combined with air (Stream 17), and burned, pro-
ducing combustion exhaust (Stream 19). The ammonia
reforming reaction shown in Equation (13) is endothermic,
requiring heat to proceed.
2NH3ðgÞ/N2ðgÞ þ 3H2ðgÞ�DH ¼ 46 kJ=mol
�(13)
The reaction at 850 �C is approximately 90% efficient [78], and
the air stream was assumed to be at 15 �C, thus requiring
heating of 13.4 kJ/mol-NH3 converted for Equation (13). The
amount of biogas diverted to the combustor is dependent on
themethane content of the biogas, the energy demand for the
system, NH3(g) flow rate (Stream 10), and the heating value of
methane, assumed to be 50 MJ/kg (lower heating value of
methane for combustion) [79].
For all reactions listed, a single-step reaction mechanism
without reaction intermediates or kinetic considerations is
assumed. While the analytical model presented in this paper
could be optimized further, it nevertheless provides several
valuable insights into developing anaerobic digesters as a
potential waste processing system to generate multiple fuel
streams.
2.5. Stream transfer pressure-drop and pumpingrequirements
Power demands resulting from movement of internal gas and
aqueous flows were estimated using mass and energy bal-
ances for individual streams as described in Section S4 of the
Supplementary Materials. The associated power as a function
of the required pressure drop and flow for each internal
stream was assessed based on specific process configurations
as case-studies. Values for a specific process configuration or
stream were input variables in the model allowing different
configurations and operating conditions to be tested analyti-
cally. Although not a fluid stream, the energy of transporting
the feedstock (Stream 1) into the digester was calculated by
determining the work required to raise its mass to the height
of the digester assuming the energy utilization was 35% effi-
cient. The dimensions of the model system components were
determined as described in Section S4 of the Supplementary
Materials.
2.6. Mass and energy balance model
Material and energy flows for the conceptual system were a
function of the input flows (Table 2), which were systemati-
cally varied to estimate a system-wide energy balance for a
range of input values as discussed later. Further, mass flows of
C and N were tracked through the system, and C and N as a
percentage of the total C and N entering the system were
computed for each component flow. The modeled outputs
were a function of the system inputs (e.g. input flow rate, size,
geometry, retention times, etc.), which were held constant
while the feedstock C:N ratio was varied (Table 2). Results of
the energy balance were expressed either as a function of the
C:N ratio or as a function of the corresponding TAN as
described both in the text and accompanying figures. To
further evaluate the system, the methane plus hydrogen from
ammonia reforming was classified as ‘net usable energy’ as
these two materials would act as fuel sources. The energy
consumed by the ADBH system was normalized to the
methane production potential for the feedstock at each C:N
ratio as determined from Equation (9). It should be noted that
while the harvesting of bioammonia presents other avenues
for useful resources either as direct fuel or an important in-
dustrial chemical, those benefits are not accounted for in the
‘net usable energy’ estimates discussed in this work. The
normalization to the methane production potential was cho-
sen since methane is the most commonly and widely har-
vested fuel source from anaerobic biogas and provides a ready
reference for comparison. Therefore, the equivalent usable
energy was calculated as the heat generated during the
complete burn of the total fuel (methane and hydrogen) dur-
ing complete, single-step oxidation.
2.7. Model application to different case scenarios
Three test operating cases were examined for comparison
(Table S1). For all cases, identical inputs (Table 2) were used,
but the modeled outputs were calculated based on different
assumptions as discussed next. For Case 1, the heat generated
from chemical reactions and the hydrostatic pressure avail-
able from each reactor was assumed to be recaptured at 35%
efficiency to offset heating and pumping requirements else-
where in the process. For Case 2, the same assumptions were
made as for Case 1 with an additional assumption that the
system utilized the N2/H2 gas stream (Stream 18) as the
stripping gas, reducing the stripping power requirements
from 1.574 to 0.265 kJ/kg-aq. In Case 3, no internal energy re-
covery/integration was included.
The thermodynamic energy balance model was then used
to identify the effect of the feedstock C:N ratio on the overall
energy balance for Cases 1, 2, and 3 (Table S1) under steady-
state operation. A first law thermodynamic analysis was
conducted to estimate the theoretical maxima for the energy
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5 499
output of the envisioned ADBH system to identify conditions
where net energy output exceeds energy input and potentially
presents a viable operating case. Other aspects of process
viability, such as economics and feasibility of the design to
account for irreversible (entropy) losses were not included in
the present analysis, but the currentmodel can be extended to
enable further work to include a second law analysis and an
economic analysis to demonstrate the viability of developing
such systems for fuel generation from anaerobic waste
processing.
Fig. 3 e ADBH mass percent flux of carbon with feedstock
C:N ratio of 3.0. Thicker arrows represent streams with
greater mass flows while the non-bold text displays the
breakdown percentages of the bold category directly above
it. Numbers in parentheses correspond to stream numbers
presented in Fig. 1.
3. Results and discussion
The energy balance of the ADBH system was determined for
feedstock C:N ratios (g C/g N) between 3.0 and 136, presenting
the lower and upper bounds for reasonable waste feedstock as
a starting point. The relationship between the feedstock C:N
ratio and corresponding digester TAN concentration is pre-
sented in Fig. 2. Higher C:N ratioswere not considered because
above 136, the system nitrogen, even combined with the an
aqueous nitrogen loading of 100 mg TAN/L, was too low to
support microbial cell growth as predicted by Reaction (5).
Similarly C:N ratios below 3 caused digester TAN to rise above
10,000 mg/L, which would severely limit the viability of
anaerobic microorganisms [35,36]. Next a discussion of the
three specific test cases is presented.
3.1. Mass balances on C and N
The effect of system energy integration as tested by comparing
Cases 1, 2, and 3 (see discussion below) did not change overall
system mass fluxes of C and N. Thus, Figs. 3 and 4 show the
mass fluxes for C and N, respectively, for a C:N ratio of 3 as a
representative example. The greatest mass percent C in the
effluent from the ADBH system was associated with carbonate
(32.4%) and methane (31.1%). The remainder was associated
with digestate (residual feedstock and biomass, 22.3%) and
Fig. 2 e Comparison of resulting digester TAN
concentration and feedstock carbon to nitrogen (C:N) ratio.
The limits of the C:N ratios were chosen on ability to
support microbial growth and viability.
carbon dioxide gas (14.2%). Note that the calcium carbonate
precipitate generated in the pH shift reactors accounted for
9.9% of the carbon flow. The greatest mass percent N in the
effluent was associated with nitrogen gas (61.1%), and the
remainder was associated with digestate (residual feedstock
and biomass, 38.9%).
3.2. Case 1 assessment (internal heat and pressuredifferentials recovered)
3.2.1. Fuel, heat, and energy analysisAs the C:N ratio decreases from 136 to 3 (i.e., the organic ni-
trogen content increases with fixed carbon content), the
operating TAN concentration in the digester increases from
Fig. 4 e ADBH mass percent flux of nitrogen with
feedstock C:N ratio of 3.0. Thicker arrows correspond to
streams with greater mass flows. Numbers in parentheses
correspond to stream numbers presented in Fig. 1.
Fig. 5 e Net system energy generation attributable to
methane and hydrogen respectively as a function of
steady-state digester TAN concentration (TAN is a function
of feedstock C:N ratio, as shown in Fig. 2).
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5500
20 mg/L to 10,000 mg/L. The model predicts that as the oper-
ating TAN concentration in the digester increases, more
hydrogen is recovered from the ammonia reformer and more
heat is recovered from the pH-shift reactors. Biofuel energy
fromhydrogen increases from 0 to 2.5MJ-biofuel/kg-feedstock
and heat output from the pH shift reactors increases from 0.64
to 1.02 MJ-heat/kg-feedstock.
The heat energy required to operate the digester (to in-
crease inlet feed flow temperature from 15 �C and to maintain
the digester at 55 �C) was not offset by the heat generated in
the pH-shift reactors for TAN concentrations below 7500mg/L
(i.e., C:N ratios > 4.3). Thus, the system requires external heat
for operation up to a TAN concentration of 7500 mg/L. This
external heat input decreases as the operating TAN concen-
tration in the digester increases, and the need for external
heating becomesminimal at TAN concentrations of 7500mg/L
because the heat produced in the pH-shift reactors is a func-
tion of the TAN concentration.
The mass flow rate of ammonia (kg/h) recovered is pro-
portional to TAN concentration increases and the power input
required to strip the dissolved ammonia from the aqueous
phase. However, since the stripping power required is a
function of the total aqueous mass flow (10,047 kg/h), the
marginal increase in the ammonia to be recovered has a
minimal impact (<1%) on the stripping power demand.
The model shows that the methane diverted to facilitate
heating and reforming in the combustor also increases as the
amount of ammonia gas treated increases. In return for the
potentially usable methane energy that is diverted to the
combustor to drive the ammonia reformer, an increasing
amount of hydrogen energy is recovered. The equivalent en-
ergy from hydrogen increases as the operating TAN concen-
tration in the digester increases between 20 and 10,000 mg/L
(between C:N ratios of 136 and 3.0). As a result, the net system
bio-fuel energy generation (i.e., the sum of the equivalent
energy associated with the biogas comprising primarily
methane and hydrogen through reforming of ammonia) de-
creases as a function of the operating TAN concentration
(Fig. 5). The relative contribution of methane and hydrogen to
the net energy shows that methane is decreasing and
hydrogen is increasing as the C:N ratio decreases (Fig. 5). Note
that the sum of equivalent methane and hydrogen energy at
10,000 mg/L TAN (C:N ratio 136:1) is less than the equivalent
methane energy at 20 mg/L TAN (C:N ratio 3:1) (Fig. 5). Thus
overall, as the C:N ratio decreases, net usable energy pro-
duction decreases. This result is a product of the reaction
stoichiometry shown in Reaction (5), where it is observed that
the total amount of methane that can be produced from a
fixed mass of organic substrate decreases as the C:N ratio
decreases. This occurs because as the C:N ratio decreases,
more of the model substrate described in Equation (6) is
incorporated. As a result of the reaction stoichiometry
(Equation (9)), the moles of C available for producing methane
decrease. Therefore, it would appear that the ADBH system is
not a viable technology for harvesting fuel streams other than
methane, which in itself would be a major finding. However,
as shown next this is not the complete picture.
To further evaluate the ADBH system, the energy gener-
ated and consumed by the process was normalized to the
methane production alone from the feedstock at each C:N
ratio (Fig. 6). The normalization permits a direct comparison
in the net usable system energy in contrast to the energy that
can be potentially generated by extracting and consuming
methane only as a fuel. For higher C:N ratios, the energy
produced approaches the total methane potential of the sub-
strate (100% in Fig. 6b). This is based on the stoichiometry
from Equation (9) and the defined fraction of the degradable
portion defined in Table 2.
As the C:N ratio decreases (TAN concentration in the
digester increases), and ammonia stripping with hydrogen
recovery is utilized, some methane is diverted to the
combustor to power the ammonia reforming process. Thus,
themethane recovered for external fuel use decreases relative
to the total available methane. However, the normalized
result shows that the reduction in methane via diversion to
heat the combustor is compensated by the hydrogen energy
produced (Fig. 6b). Results above 100% indicate that the total
recoverable energy from the ADBH process recovering both
hydrogen and methane exceeds the potential recoverable
energy from methane alone.
The thermodynamic analysis shows that the recovered
equivalent hydrogen energy is not always sufficient to offset
the system operation energy requirements. The amount of
energy consumed by the system to generate (via the digester),
recover, and reform ammonia is greater than the equivalent
hydrogen energy generated for TAN concentrations below
approximately 8500 mg/L (C:N < 3.7). However, the amount of
energy consumed by the system to simply recover and reform
ammonia, excluding energy needed to operate the digester, is
greater than the equivalent hydrogen energy generated for
TAN concentrations below approximately 1000 mg/L
(C:N < 31). Thus, operation of the ADBH system may not be
Fig. 6 e a) Potential methane production rate as a function
of feedstock stoichiometry controlled by feedstock C:N
ratio; b) Energy from methane and hydrogen as a
percentage of the potential methane production versus
operating digester TAN concentration (TAN is a function of
feedstock C:N ratio).
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5 501
favorable for feedstock C:N ratios greater than 31 (corresponds
to digester TAN less than 1000mg/L). In general, operating the
ADBH system at TAN concentrations above 1000 mg/L (C:N
less than 31) improves the overall energy available by asmuch
as 1.8% at 1000 mg/L and 26.3% at 10,000 mg/L compared to
anaerobic digestion with methane recovery alone.
3.2.2. Influent feedstock effects on digester TAN and biogasqualityThe effect of the influent feedstock C:N ratio on digester TAN
and effluent biogas quality was analyzed by assuming and
holding constant one set of specific operating conditions
(Table 2). Although the operating TAN concentration in the
digester is used as a reference for the energy outputs in this
analysis, it is the C:N ratio of the dry solids flow that controls
TAN since other inputs are held constant. The designated
waste stream inputs and operating parameters were based on
typical design values [73] that contain a solid fraction of 10% or
less of the total incoming volume. Hence the specific heating
properties for the waste stream were approximated to be that
of liquid water in agreement with previously established
methods [73]. The operating TAN concentration was used as a
reference because high TAN concentrations (above
5000e6000 mg/L) have been reported to decrease methano-
genesis as a result of toxicity and inhibition of the microbial
community [80]. A C:N ratio between 6.8 and 5.6 would
correspond to digester TAN concentration greater than
5000e6000 mg/L based on the input parameters to the model
used in this paper. One caveat should be noted with respect to
the energy balance reported here. The presentmodel does not
explicitly evaluate process trade-offs to account for the rela-
tionship between the feedstock C:N ratio and the operating
TAN concentration in the digester which are a function of the
solids flow rate, recycle flow rate, set moisture content in the
digester, retention time, and digester aspect ratio. While such
a detailed system optimization will be no doubt useful it is
beyond the scope of the current effort. Specifically, for an in-
dividual C:N ratio (even for low ratios, C:N ¼ 1e3), the design
characteristics of the system could be adjusted to maintain a
viable operating TAN concentration (and corresponding free
ammonia concentration, the form of TAN thought to impose
toxicity [81,82]) in the digester such that TAN levels would not
become inhibitory. It is expected that this work will spur
further debate in the scientific community to design and build
ADBH systems accounting for complete system operation and
optimization.
The C:N ratio of the feedstock also affects the system
biogas composition by controlling the amount of carbon di-
oxide removed from Stream 11 in the second pH-shift reactor.
As the C:N ratio increases over the considered range, the
molar percent of carbon dioxide removed in the second pH
shift reactor rose from 22.0% to 48.2%. The biogas composition
in Stream 11 thus varied from 54.3% to 62.8% methane as the
C:N ratio was decreased. For feedstock C:N ratios of 136 and 3,
the methane content leaving the second pH-shift (Stream 13)
was thus predicted to be 64.3% and 80.7% methane, respec-
tively. This result presents another tradeoff to be considered.
If purified methane were the desired downstream product,
decreasing the C:N ratio of the influent solids material stream
can be seen as a means of improving downstream biogas
methane enrichment. Such an approach would impact the
energy balance for a system seeking to produce a nearly pure
methane stream with no carbon dioxide.
3.2.3. Power and pressure drop requirementsRequired pressure drops to power fluid flow, compensating for
frictional energy losses along the length of the pipe, were
included in the model (see Section S4 and Table S1 of the
Supplementary Materials) based on well-established pipe
flow models [83]. The magnitude of the frictional losses and
corresponding power required to maintain the calculated
pressure drop was almost exclusively a function of the spec-
ified influent feedstock mass flow rate, which was held con-
stant throughout the analysis. Thus, it was not unexpected
that the total power requirements for pumping (sum of the
hydrostatic and frictional power requirements), mixing, and
Fig. 7 e Normalized total equivalent energy generated by
ADBH system as a function of the operating TAN
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5502
ammonia stripping increased by less than 2% (from 86.4 to
87.5 kW) as the C:N ratio decreased from 136 to 3. A compar-
ison of the relative power demands for pumping, mixing, and
ammonia stripping showed that the mixing in the digester
and pH-shift reactors required the most power (Table 3). The
sum of the power to sustain mixing in the digester and pH-
shift reactors, maintain the pressure drop and facilitate
ammonia stripping from the aqueous stream was two orders
ofmagnitude smaller than the equivalent energy generated as
fuel (methane rich biogas and ammonia reformed to
hydrogen) and heat. Subtracting the total power required for
these demands from the net energy equivalent generated
consequently had a relatively minor impact on the ADBH
system energy generation potential.
The overall process fuel and heat energy generation as a
function of the influent solids C:N ratio is shown in Fig. 7. The
normalized output indicates that above an operating digester
TAN concentration of approximately 2000 mg/L, the total
biofuel energy (CH4 and H2) output is greater than could be
expected to be produced from anaerobic digestion (CH4) alone
providing theoretical evidence for an increase in net usable
energy for implementing an integrated ADBH system.
concentration in the digester (TAN is a function of
feedstock C:N ratio). The solid line represents the expected
externally usable energy produced by anaerobic digestion
alone.
3.3. Effects of system energy integration: comparison ofcases 1, 2, and 3
The effects of various energy integration scenarios (Table S3)
were assessed by comparing the fraction of the energy
generated diverted to power the integrated process as a
function of the operating digester TAN concentration. The
percent of the energy required increased as the influent
feedstock C:N ratio decreased (corresponding TAN concen-
trations in the digester increased) for all cases because
pumping and mixing requirements downstream from the
digester were a function of the mass flow of ammonia. Addi-
tionally, the energy content of the feedstock decreased as the
C:N ratio decreased, allowing less methane to be generated
(Fig. 6a) from approximately 18 kmol/h for a C:N ratio of 136
decreasing to nearly 7 kmol/h for a C:N ratio of 3. The total
percentage of energy diverted remained under 35% for all
cases and feedstock C:N ratios. The ADBH system operated
under the Case 1 and Case 2 scenarios did not generate greater
amounts of methane or hydrogen than when operated under
the Case 3 scenario, but as expected, less biofuel (methane
Table 3 eADBH fractional power requirements: demandsfor pumping, mixing and ammonia stripping, as well asrelative requirements of mixing in the digester and pH-shift reactors.
Process Fractional power requirement
Mixing 83%
Digester 42%
pH-Shift 1 29%
pH-Shift 2 29%
Stripping 15%
Pumping 2%
and/or hydrogen) was diverted to provide energy needed by
the ADBH system.
Case 3 had the highest fraction (23e35%) of its biofuel being
diverted to power various systemdemands for all feedstock C:N
ratios because available energy integration was not considered
in this case, i.e., heat generated and available hydrostatic
pressures to facilitate stream flow were not utilized. With no
energy recovery, the system losses were 23 to 35% of the total
energy generated (fraction of methane and hydrogen).
Recovery of heat and available hydrostatic pressure were
considered for integration in Case 1 and Case 2. Case 2 differed
from Case 1 in that it also utilized a fraction of the available
hydrogen and nitrogen stream (Stream 18) as the stripping gas
as opposed to steam alone. For Case 1, liquid vaporization and
stripping required 1.574 kJ/kg-aq., whereas for Case 2, using
the available gas stream eliminated the need for vaporization
to produce steam, and ammonia stripping thus required only
0.265 kJ/kg-aq, improving the amount of energy recovered
nearly 7 times. When internally produced heat and pressure
differentials were recovered (Case 1), system losses were
reduced to between 8 and 17% in contrast to a high of 35% for
no energy recovery. The energy required for ammonia strip-
ping in Case 2 assumed that the hydrogen and nitrogen
stream (Stream 18) could strip ammonia from the liquid
stream as effectively as air. Using steam as the stripping gas
allows ammonia to be concentrated by allowing steam to be
selectively removed via condensation. It is likely that the
minimal (<1%) reduction in energy use observed by utilizing a
fraction of the nitrogen and hydrogen stream for stripping
would be offset by the need to re-concentrate the diluted
reforming gas stream.
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5 503
3.4. Summary, implications and areas for furtherinvestigation
The ADBH system energy balance remained positive (gener-
ating net usable energy in contrast to energy consumed) while
digesting biomass, removing aqueous ammonia, and reform-
ing ammonia gas to produce hydrogen for all feedstock C:N
ratios considered, but the energy balance favored C:N ratios
lower than 31. The model did not account for microbial
sensitivity to aqueous TAN concentrations, entropic losses,
and, unless otherwise specified, assumed 1-step reactions and
Arrhenius kinetics. Feedstocks C:N ratios (below 4.0) might be
better managed by employing a separate hydrolysis-
fermentor reactor upstream from the digester. Ammonia
release during hydrolysis occurs more rapidly than other
anaerobic processes in the digester [68], and operating a
separate hydrolysis reactor could potentially reduce nitrogen
loading in the downstream digester. This could prevent
ammonia toxicity and improve digester stability andmethane
generation, while still allowing bioammonia to hydrogen
conversion.
As the model and discussion above illustrates, more
research needs to be conducted before full-scale ADBH sys-
tems could be pursued. The cost for constructing and oper-
ating ADBH systems versus the relative benefits of energy
generation and nitrogen removal need to be assessed and
evaluated in the context of alternative methods of disposing
of TAN. Since aqueous nitrogen species are typically a concern
for digester effluents and may require significant subsequent
treatment, the energy and economic savings of using ADBH
systems versus conventional digestion and downstream ni-
trogen mitigation processes can be important. Biological
mediation of aqueous nitrogen species are typically aerobic
processes that require organic substrate, and do not offer the
prospect of energy recovery. In fact, aeration typically is the
greatest energy sink at wastewater treatment plants and the
organic substrate required could be viewed as substrate that is
diverted from methane generating anaerobic processes.
Finally, though not considered explicitly in this work, har-
vesting of ammonia as a direct fuel source or chemical source
can also be pursued, providing additional avenues to manage
waste streams and eliminating the need to divert methane for
internal processes as in the present ADBH system.
4. Conclusions
The total equivalent energy generated by the ADBH system is
greater than the energy consumed to operate the system for
the conditions considered over the entire range of C:N ratios
tested in contrast to extracting methane only as an energy
source. When the ammonia recovery and combustor systems
are coupled to the digester system (becoming the single
ADBH system), the equivalent energy from hydrogen alone is
not sufficient to offset system energy requirements with
digester operation when the feedstock C:N ratio is above 3.7
(<8500 mg/L TAN). Operating the ADBH system increased the
total energy recovery potential compared to the methane
potential for feedstock C:N ratios less than 31. Without
integrating the digester process energy, the ADBH system
only requires 23e34% of the total energy generated (fraction
of methane and hydrogen), and process integration can
reduce the system loss to between 8 and 17%. For a C:N ratio
of 3, the lowest value evaluated, the integrated ADBH process
produced 25.5% more energy than anaerobic digestion
generating methane alone. Finally, it is expected that this
analysis and design scheme will spur debate within the sci-
entific community, and hopefully lead to more detailed
optimization and future construction of integrated ADBH
systems for harvesting usable fuel feedstocks from waste
streams.
Acknowledgments
This project was funded in part by the Rutgers University
Academic Excellence Fund. The authors thank the Depart-
ment of Environmental Sciences and the New Jersey Water
Resources Research Institute at Rutgers University for partial
support as well as Defense Advanced Research Projects
Agency (DARPA) and the Samsung Group for personnel sup-
port. The authors also acknowledge support from the
Department of Mechanical Engineering at The Ohio State
University for support during preparation of the manuscript.
Shaurya Prakash would also like to thank Prof. Mark A.
Shannon at the University of Illinois for several insightful
discussions.
Appendix A. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
biombioe.2013.05.024.
r e f e r e n c e s
[1] Guest JS, Skerlos SJ, Barnard JL, Beck MB, Daigger GT,Hilger H, et al. A new planning and design paradigm toachieve sustainable resource recovery from wastewater.Environ Sci Technol 2009;43:6126e30.
[2] Grant SB, Saphores J-D, Feldman D, Hamilton A, Fletcher T,Cook P, et al. Taking the “waste” out of “wastewater” forhuman water security and ecosystem sustainability. Science2012;337:681e5.
[3] Augedlo-Vera C, Leduc W, Mels A, Rijnaarts H. Harvestingurban resources towards more resilient cities. ResourConserv Recycl 2012;64:3e12.
[4] Iranpour R, Stenstrom M, Tchobanoglous G, Miller D,Wright J, Vossoughi M. Environmental engineering: energyvalue of replacing waste disposal with resource recovery.Science 1999;285:706e11.
[5] Wang X, Junxing L, Ren N-Q, Yu H-Q, Lee D-J, Guo X.Assessment of multiple sustainability demands forwastewater treatment alternatives: a refined evaluationscheme and case study. Environ Sci Technol2012;46:5542e9.
[6] McCarty PL, Bae J, Kim J. Domestic wastewater treatment as anet energy producer e can this be achieved? Environ SciTechnol 2011;45:7100e6.
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5504
[7] Nallathambi-Gunaseelan V. Anaerobic digestion of biomassfor methane production: a review. Biomass Bioenergy1997;1:83e114.
[8] Schink B. Anaerobic digestion: concepts, limits andperspectives. Water Sci Technol 2002;45:1e8.
[9] Nasir IM, Ghazi TM, Omar R. Production of biogas fromorganic solid wastes through anaerobic digestion: a review.Appl Microbiol Biotechnol 2012;95:321e9.
[10] Weiland P. Biogas production: current state andperspectives. Appl Microbiol Biotechnol 2010;85:849e60.
[11] Demirel B, Scherer P, Yenigun O, Onay T. Production ofmethane and hydrogen from biomass through conventionaland high-rate anaerobic digestion processes. Crit RevEnviron Sci Technol 2010;40:116e46.
[12] De Baere L, Mattheeuws B. Anaerobic digestion of MSW inEurope. BioCycle 2010;51:24.
[13] Hawkes DL. Review of full-scale anaerobic digestion inChina. Agric Wastes 1986;18:197e205.
[14] Ho M. Biogas bonanza for third world development. London,UK: Institute of Science in Society; 2005.
[15] Kellaher M. Anaerobic digestion outlook for MSW streams.Biocycle Energy 2007;48:51e5.
[16] Ortenblad H. Development of anaerobic digestion plants andprogress in selected countries. Herning, Denmark:EnergiGruppen Jylland; 2002.
[17] Rittmann BE, McCarty PL. Anaerobic treatment bymethanogenesis. Environmental biotechnology: principlesand applications. Boston, MA: McGraw Hill; 2001. p. 570e629.
[18] AgSTAR U, editor. Anaerobic digesters continue growth inU.S. livestock market. Washington D.C.: USEPA; 2011.
[19] Themelis NJ, Ulloa PA. Methane generation in landfills.Renew Energy 2007;32:1243e57.
[20] USEPA. Municipal wastewater treatment facilities. In:Combined heat and power partnership. Washington D.C.:USEPA; 2010.
[21] Reinhart DR, Townsend TG. Landfill bioreactor design andoperation. Boca Raton, FL (USA): CRC Press LLC; 1998.
[22] USEPA. Energy projects and candidate landfills. Landfillmethane outreach program. Washington DC: USEPA; 2012.
[23] Chen Y, Cheng JJ, Creamer KS. Inhibition of anaerobicdigestion processes: a review. Bioresour Technol2008;99:4044e64.
[24] Ma J, Scott NR, DeGloria SD, Lembo AJ. Siting analysis offarm-based centralized anaerobic digester systems fordistributed generation using GIS. Biomass Bioenergy2005;28:591e600.
[25] Matteson GC, Jenkins BM. Food and processing residues inCalifornia: resource assessment and potential for powergeneration. Bioresour Technol 2007;98:3098e105.
[26] Brown BB, Yiridoe EK, Gordon R. Impact of single versusmultiple policy options on the economic feasibility of biogasenergy production: swine and dairy operations in NovaScotia. Energy Policy 2007;35:4597e610.
[27] Chung SS, Poon CS. Evaluating waste managementalternatives by multi criteria approach. Resour ConservRecycl 1996;17:189e210.
[28] Murphy JD, Power N. A technical, economic, andenvironmental analysis of energy production fromnewspaper in Ireland. Waste Manag 2007;27:177e92.
[29] Van Horn HH, Hall MB. Agricultural and environmental issuesin the management of cattle manure. Agricultural uses of by-products and wastes. American Chemical Society; 1997.
[30] Chernicharo CA. Post-treatment options for the anaerobictreatment of domestic wastewater. Rev Environ SciBiotechnol 2006;5:73e92.
[31] Metcalf, Eddy. Constituents in wastewater. In:Tchobanoglous G, Burton F, Stensel HD, editors. Wastewaterengineering. Boston: McGraw Hill; 2003. p. 27e137.
[32] Metcalf, Eddy. Biological denitrification. In:Tchobanoglous G, Burton F, Stensel HD, editors. Wastewaterengineering: treatment and reuse. Boston: McGraw Hill; 2003.p. 619.
[33] Metcalf, Eddy. Aggregate organic constituents. In:Tchobanoglous G, Burton F, Stensel HD, editors. Wastewaterengineering: treatment and reuse. Boston: McGraw Hill; 2003.p. 87.
[34] Ahn YH. Sustainable nitrogen elimination biotechnologies: areview. Process Biochem 2006;41:1709e21.
[35] Chen Y, Cheng JJ, Creamer KS. Inhibition of anaerobicdigestion process: a review. Bioresour Technol2008;99:4044e64.
[36] Kayhanian M. Ammonia inhibition in high-solidsbiogasification: an overview of practical solutions. EnvironTechnol 1999;20:255e365.
[37] Clemens J, Trimborn M, Weiland P, Amon B. Mitigation ofgreenhouse gas emissions by anaerobic digestion of cattleslurry. Agric Ecosyst Environ 2006;112:171e7.
[38] Hansen KH, Angelidaki I, Ahring BK. Anaerobic digestion ofswine manure: inhibition by ammonia. Water Res1998;32:5e12.
[39] Freibauer A. Biogenic emissions of greenhouse gases fromEuropean agriculture. Eur J Agron 2003;19:135e60.
[40] Pechan Z, Knappova O, Petrovicova B, Adamec O. Anaerobicdigestion of poultry manure at high ammonium nitrogenconcentrations. Biol Wastes 1987;20:117e31.
[41] Rumburg B, Mount G. Atmospheric flux of ammonia fromsprinkler application of dairy waste. Atmos Environ2006;40:7246e58.
[42] Vanotti MB, Szogi AA, Hunt PG, Millner PD, Humenik FJ.Development of environmentally superior treatment systemto replace anaerobic swine lagoons in the USA. BioresourTechnol 2007;98:3184e94.
[43] Whitehead TR, Cotta MA. Isolation and identification ofhyper-ammonia producing bacteria from swine manurestorage pits. Curr Microbiol 2004;48:20e6.
[44] Bowden G. Fennell DE, editor. Nitrogen reflux at wastewatertreatment facilities. New York, NY: AECOM; 2008.
[45] Gander M, Jefferson B, Judd S. Aerobic MBRs for domesticwastewater treatment: a review with cost considerations.Sep Purif Technol 2000;18:119e30.
[46] Berge ND, Reinhart DR. The fate of nitrogen in bioreactorlandfills. Crit Rev Environ Sci Technol 2005;35:365e99.
[47] Jetten M, Schmid M, Schmidt I, Wubben MJGK. Improvednitrogen removal by application of new nitrogen-cyclebacteria. Rev Environ Sci Biotechnol 2002;1:51e63.
[48] Strous M, Fuerst J, Kramer E, Logemann S, Muyzer G, Pas-Schoonen KVD, et al. Missing lithotroph identified as newplanctomycete. Nature 1999;400:446e9.
[49] Hellinga C, Schellen AAJC, Mulder JW, Loosdrecht MV.Sharon process: an innovative method for nitrogen removalfrom ammonium-rich wastewater. Water Sci Technol1998;37:135e42.
[50] Ammonia. In: Bohnet M, Bellussi G, Bus J, Cornils B, Drauz K,Greim H, et al., editors. Ullmann’s encyclopedia of industrialchemistry. John Wiley & Sons, Inc.; 2010.
[51] Dunn-Rankin D, Leal EM, Walther DC. Personal powersystems. Prog Energy Combust Sci 2005;31:422e65.
[52] Maffei N, Pelletier L, Charland JP, McFarlan A. An intermediatetemperature direct ammonia fuel cell using a protonconducting electrolyte. J Power Sources 2005;140:264e7.
[53] Ma Q, Peng R, Lin Y, Gao J, Meng G. A high-performanceammonia-fueled solid oxide fuel cell. J Power Sources2006;161:95e8.
[54] Fournier GGM, Cumming IW, Hellgardt K. High performancedirect ammonia solid oxide fuel cell. J Power Sources2006;162:198e206.
b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 9 3e5 0 5 505
[55] Meng G, Ma G, Ma Q, Peng R, Liu X. Ceramic membrane fuelcells based on solid proton electrolytes. Solid State Ionics2007;178:697e703.
[56] USDOE. Energy and environmental profile of the U.S.chemical industry. Agricultural chemicals: fertilisers.Washington DC: US Department of Energy; 2000 [chapter 5].
[57] Zamfirescu C, Dincer I. Using ammonia as a sustainable fuel.J Power Sources 2008;185:459e65.
[58] Klerke A, Christensen CH, Norskov JK, Vegge T. Ammonia forhydrogen storage: challenges and opportunities. J MaterChem 2008;18:2285e392.
[59] Air stripping for ammonia removal. In: WEF, editor. Design ofmunicipal wastewater treatment plants. Alexandria, VA:WEF; 1998.
[60] Sedlak R. Air stripping of ammonia. In: Sedlak R, editor.Phosphorus and nitrogen removal from municipalwastewater: principles and practice. Boca Raton, FL: CRCPress LLC; 1991.
[61] Gangagni Rao A, Sasi Kanth Reddy T, Surya Prakash S,Vanajakshi J, Joseph J, Jetty A, et al. Biomethanation ofpoultry litter leachate in UASB reactor coupled withammonia stripper for enhancement of overall performance.Bioresour Technol 2008;99:8679e84.
[62] Liao PH, Chen A, Lo KV. Removal of nitrogen from swinemanure wastewaters by ammonia stripping. BioresourTechnol 1995;54:17e20.
[63] Bonmatı A, Flotats X. Air stripping of ammonia frompig slurry:characterisation and feasibility as a pre- or post-treatment tomesophilic anaerobic digestion. Waste Manag 2003;23:261e72.
[64] Cheung KC, Chu LM, Wong MH. Ammonia stripping as apretreatment for landfill leachate. Water Air Soil Pollut1997;94:209e20.
[65] Minocha VK, Prabhakar AVS. Ammonia removal andrecovery from urea fertilizer plant waste. Environ TechnolLett 1988;9:655e64.
[66] Lei X, Sugiura N, Feng C, Maekawa T. Pretreatment ofanaerobic digestion effluent with ammonia stripping andbiogas purification. J Hazard Mater 2007;145:391e7.
[67] Elston JT, Karmarkar D. Aqueous ammonia strippingtechnology for SCR applications. Houston, TX: Electric Power;2003;[67a]Babson DM. Enhancing energy recovery from biomasswaste streams e from mega-landfills and biorefineries tomicrobial communities. Ph.D. dissertation, RutgersUniversity; 2010.
[68] Gallert C, Bauer S, Winter J. Effect of ammonia on theanaerobic degradation of protein by mesophilic andthermophilic biowaste population. Appl Microbiol Biotechnol1998;50:495e501.
[69] Ramsay IR, Pullammanappallil PC. Protein degradationduring anaerobic wastewater treatment: derivation ofstoichiometry. Biodegradation 2001;12:247e57.
[70] Rittmann BE, McCarty PL. Stoichiometry and bacterialenergetics. Environmental biotechnology: principles andapplications. Boston, MA: McGraw Hill; 2001. p. 126e61.
[71] Ekama GA. Using bioprocess stoichiometry to build a plant-wide mass balance based steady-state WWTP model. WaterRes 2009;43:2101e20.
[72] Rittmann BE, McCarty PL. Microbial kinetics. Environmentalbiotechnology: principles and applications. Boston, MA:McGraw Hill; 2001. p. 165e98.
[73] Metcalf, Eddy. Treatment, reuse, and disposal of solids andbiosolids (anaerobic digestion). In: Tchobanoglous G,Burton F, Stensel HD, editors. Wastewater engineering.Boston: McGraw Hill; 2003. p. 1505e46.
[74] Tester JW, Modell M. Standard enthalpy and Gibbs freeenergy of formation. In: Amundson NR, editor.Thermodynamics and its applications. 3rd ed. Upper SaddleRiver, NJ: Prentice Hall PTR; 1996. p. 291.
[75] Tester JW, Modell M. Estimating physical properties: groupcontribution methods for estimating pure componentproperties. In: Amundson NR, editor. Thermodynamics andits applications. 3rd ed. Upper Saddle River, NJ: Prentice HallPTR; 1996. p. 555e83.
[76] Neveu P, Castaing J. Solidegas chemical heat pumps: field ofapplication and performance of the internal heat of reactionrecovery process. Heat Recovery Syst CHP 1993;13:233e51.
[77] Metcalf, Eddy. Physical unit operations. In: Tchobanoglous G,Burton F, Stensel HD, editors. Wastewater engineering. 4thed. Boston: McGraw Hill; 2003.
[78] Ganley JC, Seebauer EG, Masel RI. Porous anodic aluminamicroreactors for production of hydrogen from ammonia.AIChE J 2004;50:829e34.
[79] NIST. NIST standard reference database number 69. NIST;2008. 2008 ed.
[80] Bhattacharya SK, Parkin GF. The effect of ammonia onmethane fermentation processes. Water Pollut Control Fed1989;61:55e9.
[81] Sprott GD, Patel GB. Ammonia toxicity in pure culturesof methanogenic bacteria. Syst Appl Microbiol1986;7:358e63.
[82] Gallert C, Winter J. Mesophilic and thermophilic anaerobicdigestion of source-separated organic waste: effect ofammonia on glucose degradation and methane production.Appl Microbiol Biotechnol 1997;48:405e10.
[83] Wilkes JO. Mass, energy, and momentum balances. In:Anzalone J, editor. Fluid mechanics for chemical engineers.Upper Saddle River, NJ: Prentice-Hall, Inc.; 1999.