Subscriber access provided by MT SINAI SCH OF MED
Article
Bioenergy generation from municipal solid waste and glycerin waste: Population dynamics Rosario Solera, Diego Sales, Jose Luis Garcia-Morales, and Soraya Zahedi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01526 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
Bioenergy generation from municipal solid waste and glycerin waste:
2
Population dynamics
3
Zahedi, S*; García-Morales, J.L.; Sales, D.; Solera, R*
4 5 6 7 8 9
Department of Environmental Technologies. University of Cadiz. Faculty of Marine and Environmental Sciences (CASEM), Pol. Río San Pedro s/n, 11510 Puerto Real (Cádiz), Spain. (
[email protected]/
[email protected];
[email protected])
10
Abstract
11
This paper studies the microbial community, effluent characteristics and bioenergy
12
generation (hydrogen and methane production) in a two-phase dry-thermophilic (55ºC)
13
anaerobic co-digestion process treating actual municipal solid waste (20 % solid
14
content) and biodiesel waste (glycerin waste, 1 % v/v). Four different hydraulic
15
retention times (from 11.5 d to 4.5 d) and four organic loading rates (from 8.2 g to 21.1
16
g VS/l/d) were studied to identify optimal conditions (maximum values of biogas and
17
microbial activity). Optimal conditions (2.6 ± 0.3 l H2/l/d; 3.4 ± 0.3 l CH4/l/d and 109±
18
9 x 10-13 l CH4/cell/d) were obtained at 5.9 d HRT. Fermentation end products yield
19
propionic acid as the major product in the secondary effluent. The average values of the
20
ratios of Eubacteria:Archaea and Acetogens:hydrolytic-acidogenic bacteria were
21
respectively 83:17 and 30:53 in the first phase (1.5 d HRT) and 76:24 and 39:37 in the
22
second phase (4.4 d HRT).
23 24
Keywords: hydrogen, methane, glycerol, microbial activity, population dynamic.
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 31
25
1. Introduction
26
Anaerobic digestion (AD) is a widely used process for degrading and stabilizing
27
municipal solid waste (MSW) due to its environmental and economic benefits.
28
According to a recent study 1, electricity generation from biogas in OECD countries
29
grew from 3.7 TWh in 1990 to 78.8 TWh in 2015, making it the third fastest-growing
30
renewable electricity source after wind and solar energy. Due to the advantages of AD,
31
many research studies have sought to optimize the AD of MSW, including the
32
interesting option of the co-digestion process, which increases the load of biodegradable
33
organic matter and produces a higher biogas yield 2–5. Studies on AD of MSW have
34
shown that the C:N ratio of this waste presents average values of 10:1, below the
35
optimum for anaerobic digestion (25:1) 6, while hydrogen production (HP) and methane
36
production (MP) is reduced due to the washout of microorganisms, not to overloading 7–
37
10
38
addition of readily biodegradable organic substances, such as glycerol, a major by-
39
product of biodiesel production, could constitute an ideal strategy 4,11–13. Biodiesel
40
manufacturing worldwide has gained in importance due to several factors: (i) the
41
unavailability of fossil fuels due to demographics and political instability; (ii) modern
42
methods of biodiesel production and new catalyst formulations producing higher
43
biodiesel yields; (iii) the breeding and cultivation of new varieties of oil crops with
44
higher lipid yields; (iv) an increase in the cultivation of inedible oilseed plants on waste
45
land; (v) new engine designs that can use biodiesel and its admixtures as fuel; and (vi)
46
stringent regulations to reduce GHG emissions 14–16. Producing 100 kg of biodiesel
47
yields approximately 10 kg of glycerin waste (GW) as a co-product. Numerous sectors,
48
such as the pharmaceutical, cosmetics, and food processing industries, use refined
. Therefore, an increase in the loading rate employed in the AD process via the
1 ACS Paragon Plus Environment
Page 3 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
49
glycerol as a raw input material. However, the GW generated as a co-product of
50
biodiesel production requires purification before being suitable for use in these
51
industries. The problem is that the high costs of purifying glycerol from impure glycerol
52
has reduced its demand in the market, in addition to the reduction in the cost of
53
petroleum-derived glycerol to less than $20 per barrel 14,17. Thus, GW is often
54
considered a waste stream instead of a co-product 18, which makes its disposal a
55
fundamental environmental concern.
56
Several strategies have been reported in the literature to maximize energy recovery and
57
generate value added products from this type of waste. These options include direct
58
combustion and gasification and AD of waste glycerol to produce biogas (hydrogen or
59
methane), as well as fermentation of glycerol to produce methanol, ethanol, citric acid,
60
1,3- propanediol, polyhydroxyalkanoates (PHA), and the like 13,14,17,19–21. This paper
61
focuses on the joint AD of waste glycerol together with MSW to obtain both hydrogen
62
and methane. Recent studies have demonstrated the effectiveness of two-phase dry-
63
thermophilic AD of actual MSW to produce HP and MP 8, as well as that of GW
64
supplementation (1% v/v) to improve the hydrogen production steps in thermophilic-dry
65
dark fermentation of actual MSW in batch mode12. However, no previous studies have
66
been published on the effect of GW addition on two-phase dry-thermophilic anaerobic
67
digestion of MSW under semi-continuous feeding to produce hydrogen and methane or
68
on the effect on the different microbial groups involved in the digestion process.
69
This study aims to: (1) establish the optimal conditions (organic loading rate (OLR) and
70
hydraulic retention time (HRT)) to maximize gas production (GP), including H2 and
71
CH4; and (2) investigate the population dynamics in dry-thermophilic anaerobic co-
72
digestion of MSW and GW (1% v/v). No previous studies have been published in this
2 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 31
73
respect. With these aims in mind, the effect of four different OLRs (from 8.2 g to 21.1 g
74
VS/l/d) and HRTs (from 11.5 d to 4.5 d) were tested. The effect of the varying operating
75
parameters (HRT or OLR) on soluble chemical oxygen demand (SCOD), volatile fatty
76
acids (VFA), volatile solids (VS), hydrogen production (HP), methane production (MP),
77
GP, sulphide production (SP), specific HP (SHP), specific MP (SMP), microbial
78
population and microbial activities was studied at laboratory scale. Fluorescent in situ
79
hybridization (FISH) was used to determine the main groups involved in the anaerobic
80
process.
81
2. Methods
82
2.1 Inoculum, substrate and feeding
83
The seed used as the inoculum for the acidogenic and methanogenic reactors was
84
collected from an H2-producing and a CH4-producing reactor, respectively. The VS in
85
the acidogenic and methanogenic inoculums were 49 g/ kg and 13 g/kg, respectively.
86
The tested substrate in the first phase was a mixture of MSW and GW (1 % v/v). The
87
chosen supplementation in this trial was 1% GW, as this concentration had been shown
88
to be highly effective in improving hydrogen production in a previous study12. The
89
MSW and GW used were collected from an industrial trommel (30 mm) located at Las
90
Calandrias MSW treatment plant (composting plant) (Jerez de la Frontera, Cadiz, Spain)
91
and the Abengoa Bioenergy biofuel company (San Roque, Cádiz-Spain), respectively.
92
The MSW was stored in 25 kg drums at -4ºC to avoid anaerobic degradation by the
93
microorganisms found in the solid waste itself 10. The TS concentration of the feed for
94
the first reactor was adjusted to 20% (which is characteristic of dry AD) by adding tap
95
water. Subsequently, 1% (v/v) GW was added to the feed. The characterization of the
96
substrate used (mixture of MSW and GW) in the tests is shown in Table 1.
3 ACS Paragon Plus Environment
Page 5 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
97
The tested substrate in the second phase was the effluent from the first phase.
98
In the first reactor, NaOH 10 M was added to the substrate when the pH of the effluent
99
fell below 5.3. In the second reactor, the pH was not controlled.
100 101
2.2 Experimental equipment and operating conditions
102
Two laboratory-scale continuously stirred tank reactors (CSTRs) were employed in this
103
study. The first reactor, dedicated to HP (first phase), had a working volume of 5.5 l,
104
while the second reactor (second phase), dedicated to MP, had a working volume of 5 l.
105
Both were heated by recirculating water through a thermostatic jacket. PRECISTERM
106
6000142/6000389 (SELECTA S.A.) baths, with a maximum capacity of 7 litres of
107
water, were used for this purpose. The lids of the stainless steel reactors have a diameter
108
of 200 mm and contain three openings, one for the biogas outlet, a feed inlet and
109
another opening for the stirring system. The bottoms of the reactors have a discharge
110
valve with a 40 mm i.d., used for sampling. The biogas was collected in 40 litre Tedlar
111
(a polyvinyl fluoride plastic polymer) bags measuring 29.8 cm wide and 45.7 cm long.
112
The stirring systems consisted of an IKA EUROSTAR Power Control visc-P4 overhead
113
stirrer coupled to a stainless steel blade with scrapers that allows homogenization of the
114
waste at a speed of 23 rpm. In CSTRs without recycling of solids, the solids retention
115
time (SRT) and HRT were the same.
116
As regards the feeding regime, each reactor was fed once a day (semi-continuous
117
mode).
118
Four different OLRs or runs ranging between 8.2 and 21.1 g VS/l/d were tested. The
119
HRT in the acidogenic phase was 1.5 d (OLR= 63 1 g VS/l/d), as this was the optimal
120
value previously detected in an acidogenic reactor fed once a day 8. Steady-state
4 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 31
121
conditions lasting for at least three consecutive HRTs were clearly observed for all
122
experimental runs (except for Run IV, as destabilization was observed). The operating
123
conditions are shown in Table 2. The entire experiment lasted 131 d overall. The study
124
was considered concluded when increasing the OLR (decreasing the HRT) resulted in a
125
decrease in MP.
126 127
2.3 Analytical methods
128
The analytical determinations performed in this study can be grouped into two
129
categories: physical-chemical analysis and microbiological analysis.
130
2.3.1 Physical-chemical analysis
131
The following control parameters were determined for the reactors: SCOD, alkalinity,
132
ammonium, VFA and volume and composition of the biogas (H2, CH4, CO2 and H2S).
133
These determinations were performed according to APHA22 and Zahedi 8,12.
134
VS, SCOD and VFA percentage removal rates were calculated according to the
135
following equations:
136 137 138 139 140 141 142 143 144 145
(VSsubstrate – VSsecond phase)*100/ VSsubstrate
(1)
(SCODfirst phase – SCOD second phase)*100/SCOD first phase
(2)
(VFA first phase – VFA second phase)*100/ VFA first phase
(3)
The volume of gas produced in the reactor was measured directly using a high-precision Ritter drum-type gas meter TG-01-Series (Wet-Test). Biogas composition was determined by gas chromatography separation (SHIMADZU GC-2010). H2, CH4, CO2, O2 and N2 were analysed by means of a thermal conductivity detector (TCD) employing a Supelco Carboxen 1010 Plot column. A Supelco Supel-Q Plot column and a flame photometric detector (FPD) were used to determine H2S. Samples were taken using a 1 ml Dynatech Gastight gas syringe under the following operating conditions: split = 100;
5 ACS Paragon Plus Environment
Page 7 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
146 147 148 149
constant pressure in the injection port (70 kPa); 2 min at 40 ºC; ramped at 40 ºC/min until 200 ºC; 1.5 min at 200 ºC; detector temperature: 250 ºC; and injector temperature: 200 ºC. Helium was used as carrier gas (266.2 ml/min). Commercial mixtures of H2, CH4, CO2, O2, N2 and H2S (Abelló Linde S.A.) were used to calibrate the system.
150
2.3.2 Microbiological analysis
151
FISH was used to count the microorganisms contained in the substrate and reactors.
152
These determinations were performed according to previous studies by our research
153
group 8,12. The cellular concentration and percentages of Eubacteria, Archaea, butyrate
154
utilizing acetogens (BUA), propionate utilizing acetogens (PUA), hydrogen utilizing
155
methanogens (HUM) and acetate utilizing methanogens (AUM) were obtained by FISH.
156
The total population was calculated as the sum of the relative amounts of Eubacteria
157
and Archaea. Acetogens were calculated as the sum of the relative amounts of PUA and
158
BUA. HAB were calculated as the difference in the relative amounts of Eubacteria and
159
acetogens 8,12.
160
Microbiological analyses were performed in triplicate under steady-state conditions
161
(except at 3 d HRT in the second phase (Run IV); this condition was not analysed
162
because of the observed destabilization).
163
Methanogenic activity was considered to evaluate the effect of biochemical activity on
164
the OLR. This activity was calculated as the ratio of the volume of CH4 generated and
165
the number of Archaea determined inside the reactor by FISH staining 7.
166
3. Results and discussion
167
This section studies the effect of varying the operating parameters (HRT/OLR) on
168
SCOD, VFA, VS, GP and the microbial population in two-phase dry-thermophilic AD
6 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
169
of MSW and GW. All the values correspond to analytical determinations under steady-
170
state conditions.
Page 8 of 31
171 172
3.1 Process stability
173
pH was the parameter chosen to demonstrate the stability 8,23 of the process (Fig.1). In
174
the first phase, the HRT was maintained at 1.5 d, the pH logically remaining
175
approximately constant throughout the entire experiment (from Run I to Run V),
176
presenting average values of 5.5 ± 0.4. These pH values were optimal for enhanced
177
HAB activity 24,25and it committed to the growth of methanogen and acetogenic
178
bacteria. In the second phase, steady-state operation was observed in Runs I, II and III
179
with average values of 7.4 ± 0.2, 7.5 ± 0.1 and 7.6 ± 0.1, respectively. These values
180
ranged between desirable values of methanogen and acetogenic populations. The
181
sudden decrease in pH in Run IV (3 d HRT or 18.7 g VS/l/d OLR) revealed
182
destabilization of the system, as well as a decrease in MP (Fig.2), methanogenic activity
183
and organic matter removal, as well as an increase in VFA.
184 185
3.2 Leachate quality
186
In the first phase, alkalinity and ammonia remained approximately constant in all the
187
tested runs, with average values of 6 ± 2 g CaCO3/l and 0.9 ± 0.1 g NH3-N/Kg,
188
respectively. These values were in line with a former study 8 employing a similar HRT
189
(1.5 d). Surprisingly, VFA values (18 ± 4 g acetic acid/l) were much higher in the
190
present study (double). The higher amount of acid produced was not due to glycerol
191
addition, however, as the same values were detected in this phase before
192
supplementation with glycerol (data not shown). Furthermore, it has been established
7 ACS Paragon Plus Environment
Page 9 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
193
that the effect of adding GW on hydrogen production from industrial municipal solid
194
waste produces an increase in SCOD, though not in VFA 12. These results (higher VFA
195
values) may be explained by the fact that this reactor was maintained for more than 1
196
year at 1.5 days. Therefore, it is likely that the adequate dynamics (“flexibility”) of the
197
microbial community structure had been established and thus very young and activated
198
HAB had been able to increase their acidogenic activity and hence the VFA produced
199
from dark fermentation of MSW. The dominant fermentation products were butyric acid
200
and acetic acid, ranging from 70-80% and 8-16%, in line with previous studies 9,10,12,26.
201
SCOD values were clearly higher (more than 30%) than those measured before GW
202
addition (data not shown) and in previous studies 8–10. This was due to the fact that 1%
203
GW was added to the substrate 12.
204
In the second phase, alkalinity ranged from 6-11 g CaCO3/l for all tested runs. These
205
values were clearly higher than those obtained when GW was not added (around 4 g
206
CaCO3/l) 8, in line with the higher VFA content (between 4 and 6 g/l). As to VFA
207
consumption under steady-state conditions, this was around 70% (Fig. 3). Butyric acid
208
was the main VFA consumed. Butyric and acetic acid removal rates ranged between 90-
209
99% and 60-92%, respectively. These results are in keeping with others studies on two-
210
phase AD of solid waste 8,26–28. The amounts of propionic acid produced in all the tested
211
runs were high (3-4 g/l), being higher than previous values obtained during AD of
212
MSW when GW was not added 8. However, this fact did not produce an inhibitory
213
effect (pH and MP remained stable). Furthermore, numerous researchers have shown no
214
inhibitory effect of high propionic acid levels on AD of organic matter 29,30. The reason
215
that high values of propionic acid accumulate in the system is twofold. The first reason
216
is related to the high and constant input of readily biodegradable substrate (butyrate and
8 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 31
217
acetate). Öztürk 31 has shown that propionate is converted to acetate only after butyrate
218
and acetate has completely degraded. In studies carried out by Zahedi 8 (in which no
219
GW was added and high levels of propionic acid were not detected), the soluble OLR in
220
the second phase, employing 6.6 and 4.4 d HRT were 5.5 and 8.2 g SCOD/l/d,
221
respectively, whereas in the present study (in which GW was added to the MSW and
222
high levels of propionic acid were detected), operating at 6.6 and 4.4 d HRT, these
223
values were 10.0 and 14.9 g SCOD/l/d, respectively. The second reason is that
224
fermentation end products from glycerol yield propionic acid as the major product and
225
this acid accumulates in the system 32,33. The non-toxicity of the propionic acid thus
226
generated and the high consumption of butyric acid could indicate that PUA and BUA
227
levels were sufficient to ensure the performance of this anaerobic process under steady-
228
state conditions (except in Run IV, employing a 3 d HRT in the second phase (OLR =
229
18.7 g VS/l/d)). However, higher PUA activity could be desirable to reduce the
230
accumulation of propionic acid in the effluent of the second phase resulting from the
231
addition of GW. In Run IV, VFA accumulated in the second phase, reaching maximum
232
VFA, acetic, propionic and butyric acid values of 20.5 g acetic acid/l, 3.7 g acetic acid/l,
233
4.3 g propionic acid/l and 5.3 g butyric acid/l, respectively. These results are in line with
234
those obtained in the substrate (acidogenic effluent). The accelerated increase in VFA
235
concentration in this digester and the decrease in CH4 content and pH suggest that
236
methanogen inhibition occurred at 22.4 g SCOD/l/d OLR.
237
As regards ammonium, all results were between 775-930 mg NH3-N/kg. These values
238
are lower than those reported by authors as causing inhibition of the dry anaerobic
239
process when treating bio-wastes such as those used in this study 34–37. Moreover, the
240
source of inocula used in the present study were dry-thermophilic anaerobic reactors
9 ACS Paragon Plus Environment
Page 11 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
241
with a medium-high ammonium content, so the inocula are expected to acclimate to
242
medium-high levels of acid, allowing the digesters to operate at certain concentrations
243
of acid without jeopardizing their safety 13.
244
Organic matter removal rates (%) in each steady-state run are shown are shown in Fig.
245
3.
246
Decomposition of VS was in the 79-90% range. Similar values were found in previous
247
studies in which glycerol was not used 8, in keeping with those obtained in the
248
anaerobic digestion of organic waste 38 and the co-digestion of mixtures of
249
slaughterhouse waste with MSW 39 and vegetable market waste with MSW 40.
250
Decomposition of SCOD was in the 65–73% range. These results are in line with those
251
obtained by Ueno 41 (79%) in two-phase thermophilic AD of organic waste for an HRT
252
between 4.3-6.8 d (OLR between 12.4-16.6 g SCOD/l/d). The highest VS and SCOD
253
removal rates were obtained in Run I, corresponding to an 11.5 d HRT (1.5 d in the first
254
phase and 10 d in the second phase). Destabilization in the second phase, at 3 d HRT,
255
resulted in an accumulation of organic matter (organic removal rate lower than