enhancement in hydrogen production by thermophilic anaerobic co-digestion of organic fraction of...
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Enhancement in hydrogen production by thermophilic anaerobic co-digestionof organic fraction of municipal solid waste and sewage sludge- Optimizationof treatment conditions
Vinay Kumar Tyagi, Ruben Angeriz Campoy, C.J. Álvarez-Gallego, L.I.Romero García
PII: S0960-8524(14)00672-5DOI: http://dx.doi.org/10.1016/j.biortech.2014.05.013Reference: BITE 13421
To appear in: Bioresource Technology
Received Date: 3 April 2014Revised Date: 30 April 2014Accepted Date: 3 May 2014
Please cite this article as: Tyagi, V.K., Campoy, R.A., Álvarez-Gallego, C.J., Romero García, L.I., Enhancementin hydrogen production by thermophilic anaerobic co-digestion of organic fraction of municipal solid waste andsewage sludge- Optimization of treatment conditions, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.05.013
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Enhancement in hydrogen production by thermophilic anaerobic co-digestion of
organic fraction of municipal solid waste and sewage sludge- Optimization of
treatment conditions
Vinay Kumar Tyagi*, Ruben Angeriz Campoy, C.J. Álvarez-Gallego, L.I. Romero
García
Department of Chemical Engineering and Food Technology, Faculty of Science,
University of Cádiz-International Campus of Excellence (ceiA3), 11510 Puerto Real,
Cádiz, Spain
([email protected]; [email protected]; [email protected];
* Corresponding Author:
Vinay Kumar Tyagi
Post-Doctoral Researcher
Department of Chemical Engineering and Food Technology,
University of Cadiz, Spain
Email: [email protected]
Mobile: +91-9997112832
Abstract
Batch dry-thermophilic anaerobic co-digestion (55°C) of organic fraction of municipal
solid waste (OFMSW) and sewage sludge (SS) for hydrogen production was studied
under several sludge combinations (primary sludge, PS; waste activated sludge, WAS;
and mixed sludge, MS), TS concentrations (10-25%) and mixing ratios of OFMSW and
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SS (1:1, 2.5:1, 5:1, 10:1). The co-digestion of OFMSW and SS showed a 70%
improvement in hydrogen production rate over the OFMSW fermentation only. The co-
digestion of OFMSW with MS showed 47% and 115% higher hydrogen production
potential as compared with OFMSW+PS and OFMSW+WAS, respectively. The
maximum hydrogen yield of 51 mL H2/g VSconsumed were observed at TS concentration
of 20% and OFMSW to MS mixing ratio of 5:1, respectively. The acetic and butyric
acids were the main acids in VFAs evolution; however, the higher butyric acid
evolution indicated that the H2 fermentation was butyrate type fermentation.
Keywords: Hydrogen production; Organic fraction of municipal solid waste; Sewage
sludge; Dry thermophilic dark fermentation
1. Introduction
Undoubtedly, one of the most important commitments on energy planning in order to
satisfy future energy requirements from our society is promoting hydrogen production
(European Commission, 2003). As a sustainable energy source with minimum or zero
use of hydrocarbons and high-energy yield (122 kJ/g), hydrogen is a promising
alternative to fossil fuels (Rifkin, 2002). Since, conventional physico-chemical
hydrogen production methods (water electrolysis or chemical cracking of hydrocarbons)
are cost-intensive due to high-energy requirements, bio-hydrogen production from
anaerobic digestion (AD) of organic wastes has gained increasing attention due to
energy recovery while reducing the waste (Ming et al., 2008). The main advantages of
anaerobic process are the low level of solids generation, lower energy consumption and
increased level of biogas production; the main drawback is the slow rate of the process.
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However, AD under thermophilic (55°C) and dry conditions (20-30% Total Solids
(TS)) has become increasingly important in recent years mainly due to much rapid
process, the product achieved is much stabilized, the hydrolysis stage on the complex
organic/biological material is better and, moreover, the biogas production is greater with
respect to mesophilic (35°C) and wet (5–10% Total Solids) conditions (Fdéz.-Güelfo et
al., 2010).
Carbohydrates are required as the main substrate for bio-hydrogen production
from the dark fermentation process. One of possible substrates for dark fermentation is
food waste (FW) which consists mainly of starch, protein and fat, with a small amount
of cellulose and hemicellulose, is offered to be the plentiful source of inexpensive
fermentable carbohydrate rich feedstock for feasible hydrogen production (Hawkes et
al., 2002; Yuan et al., 2006). In this sense, the organic fraction of municipal solid waste
(OFMSW) including FW and irrecoverable paper waste would seems to be an ideal
substrate for hydrogen production (Lay et al., 1999; Vazquez et al., 2005; Zahedi et al.,
2013). OFMSW with its high carbon to nitrogen (C/N) ratio and rich organic matter,
and its easily hydrolysable nature has a high hydrogen generation potential (Zhou et al.,
2013). Earlier studies reported that the fermentative hydrogen-producing bacteria such
as Clostridium sp. require stricter pH conditions of approximately 5.5. Without pH
control, the pH of the acidogenesis process (hydrogen producing step in dark
fermentation) can readily decrease to below 4, because, the production of hydrogen is
always accompanied by volatile fatty acids (VFAs). This could significantly suppress
the hydrogen production (Fang and Liu, 2002). Moreover, the OFMSW is low in
nitrogen content, which is a necessary nutrient for hydrogen production (Kim et al.,
2004). Therefore, in order to achieve efficient hydrogen production, supplementation of
4
pH buffer and minerals is essential to optimize the pH conditions and nutrient balance.
However, the cost of operation could be considerably increased (Zhu et al., 2008).
Previous studies testified that the co-digestion of FW and sewage sludge (SS)
could enhance the hydrogen production by providing a more balanced C/N ratio and
better control on the pH. Since, SS contains considerable alkalinity, which could be an
ideal source of base for balancing pH. As well as, SS is rich in proteinaceous
substances, which can provide essential nutrients for metabolic activities and growth of
bacterial cells. A batch study showed that waste activated sludge addition enhanced H2
production due to contribution of proteins, however, the H2 production yield decreased
as WAS addition increased due to the presence of methanogens in the sludge and the
low carbohydrate concentration (Shin et al., 2004). In another attempt, different
mixtures of food waste (FW), PS and WAS were studied to promote H2 production. The
enhancement in H2 production was found due to the improved balance of carbohydrates,
nitrogen, phosphorus and trace metals; moreover, PS and WAS showed a higher
buffering capacity at low pHs in comparison to FW (Zhu et al., 2008). Moreover, it was
proved that some metal contents in sewage sludge are the main reason for synergistic
enhancement in H2 production (Kim et al., 2011). The idea of co-digestion of FW and
SS for hydrogen production is mainly significant as it could incorporate the
management i.e. reduction and stabilization of two most abundant and problematic
municipal organic solid wastes (Kim et al., 2004; Ming et al., 2008; Zhu et al., 2008).
Kim et al (2004) carried out the first study on the feasibility of hydrogen
production from co-digestion of FW and SS at mesophilic conditions. However, after 10
years, only few studies are available on the co-digestion of FW and SS for hydrogen
production (Zhu et al., 2008; Sreela-or et al., 2011; Kim et al., 2011; Zhou et al., 2013).
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Most of the studies were conducted on the unadulterated FW collected directly from
dinning hall or cafeterias that are not the representative FW received at full-scale
municipal solid waste treatment facilities. Therefore, from a practical viewpoint in order
to scaling up the process, the organic fraction of municipal solid waste collected from
the various sources is certainly more representative than individual food sources (Zhou
et al., 2013). Moreover, the previous studies were focused mainly on mesophilic
anaerobic digestion, together with less emphasis was given to optimize the total solids
concentration and best OFMSW to sludge mixing ratio in the system. Therefore, this
study was attempted to optimize the key operational parameters like sludge type
(primary, waste activated sludge and mixed), solids concentration and OFMSW to
sludge mixing ratio, for efficient hydrogen production from co-digestion of OFMSW
and SS by means of dark fermentation at thermophilic conditions. The finding of this
study will establish some fundamentals, which will help the further advancement in
scale up of the hydrogen production process from co-digestion of OFMSW and SS.
2. Material and methods
2.1. Inoculum
The inoculum used as seed for the batch assays was collected from the laboratory scale
semi-continuous thermophilic anaerobic digester treating the organic fraction of
municipal solid waste for hydrogen production. The reactor was operating at a pH of
5.5, temperature of 55°C and a HRT of 1.9 days with hydrogen yield of 24.3 mL H2/g
VSadded (Romero et al., 2013).
2.2. Substrate
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The MSW used as substrate was collected from a full-scale municipal solid waste
(MSW) treatment facility at Las Calandrias, Jerez de la Frontera, Cádiz, Spain. Waste
was collected from a 30-mm trommel after mechanical and manual segregation lines for
recycling and rejecting purposes. The MSW sample was stored at −20°C to avoid its
degradation at room temperature and to ensure that the composition of the samples
along the assays was approximately constant. In order to obtain the finest OFMSW, the
MSW was manually segregated to remove the metals, grit, glass, rags, and plastic
wastes. In order to study the effect of high TS concentration, the moisture content of
OFMSW was reduced by drying the waste at room temperature.
The primary sludge (PS), waste activated sludge (WAS) and mixed sludge (MS)
samples were collected from Cadiz-San Fernando municipal wastewater treatment
plant, Cádiz, Spain. The primary to waste activated sludge ratio in mixed sludge was
30:70 (v/v). The sludge samples were collected prior to the tests and stored in the lab at
4°C.
Sludge and OFMSW samples were brought to room temperature every time
before their use as substrate for batch assays and for sample characterization. The
physico-chemical characteristics of the inoculum and substrates are summarized in
Table 1.
2.3. Batch thermophilic anaerobic digestion assay
The batch assays were conducted using 250 mL serum bottles with a working volume of
120 mL and headspace volume of 130 mL. The bottles were filled with 1:1 ratio of
inoculum (60 mL) and substrate (60 mL). The pH was adjusted by adding few drops of
1N HCl or NaOH solution to the desired initial levels of 5.5 at which methanogenic
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archaeas are inhibited. Moreover, 50 mL of the mixture was reserved for each initial
sample characterization after thorough mixing. All the bottles were purged with N2 gas
for 5 min to displace air. The serum bottles were capped tightly with silicone septum
and placed in a water bath at 55°C. All the tests were triplicated and three inoculum
control bottles (blank) were also prepared without addition of substrate. All the bottles
were shaken manually twice a day to provide better contact between inoculum and
substrate. The volume (calculated by pressure increase) and composition of biogas were
measured daily. The experiments were considered complete when the biogas production
rates began to approach zero i.e. <2 mL/day. The hydrogen gas produced in control
bottle containing inoculum only was excluded from the final cumulative H2 gas
production of all the assays
The volume of inoculum was maintained in 50% with respect to the total working
volume of the assay (for all the cases)
Assay 1: Effect of co-digestion on hydrogen production
In the first assay, the effect of co-digestion of SS and OFMSW on hydrogen production
efficiency was investigated. Four different sets of experiments i.e. control, OFMSW
only, and OFMSW+MS (at a sludge to OFMSW mixing ratio of 1:1) were run for 7
days under thermophilic conditions (Table 2).
Assay 2: Optimization of best sludge combination with OFMSW
In the second assay, the best sludge combination as co-substrate with OFMSW for
maximizes the hydrogen production rate was investigated. Three different sludge to
OFMSW combinations i.e. OFMSW+PS, OFMSW+WAS and OFMSW+MS were
examined at a fixed mixing ratio of 1:1 of sludge to OFMSW (Table 2).
Assay 3: Optimization of best total solids percentage
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In the third assay, four different total solids concentrations of 10%, 15%, 20% and 25%
were studied. In all the cases, the proportion of both substrates (OFMSW and MS) was
maintained 1:1. Water content has been calculated to obtain the required TS percentage
in the final mixture (Table 2).
Assay 4: Optimization of best OFMSW: sludge mixing ratio
In the fourth assay, the mixing ratios of OFMSW to MS were designed to be 10:1, 5:1,
2.5:1 and 1:1 on TS basis (Table 2). In Assay 4, inoculum to substrate ratio (1:1) and TS
percentage (20%) were the fixed variables (Table 2).
2.4. Analytical methods
The following parameters were analyzed for waste characterization and process
monitoring: total solids (TS), volatile solids (VS), alkalinity, ammonical nitrogen (NH4-
N), soluble chemical oxygen demand (sCOD), total kjeldahl nitrogen (TKN), dissolved
organic carbon (DOC) and volatile fatty acids (VFAs). All the analyses were performed
according to Standard Methods (APHA, 2005). TS, VS and pH were analyzed directly
from the feed and digested samples. TS samples were dried in an oven at 105–110°C,
and for VS to the dried ash waste in a furnace at 550±5°C. pH was measured using a
Crison Basic20 pH meter (CRISON Instrument, Spain). The alkalinity, NH4-N, sCOD,
DOC and VFAs were analyzed from the filtrate obtained by means of a lixiviation
procedure (10 g of sample in 100 mL of Milli-Q water during 20 min) of the substrate
and digested samples (Álvarez-Gallego, 2005). However, the PS, WAS and MS
samples were analyzed directly without any lixiviation. Samples for the sCOD and
DOC analysis were filtered through a 0.47 µm glass fiber filter. The DOC analysis was
carried out in an Analytic-Jena multi N/C 3100 carbon analyzer (Measurement range: 0-
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30,000 mg/l C; limit of detection: 4 µg/l C) with Chemiluminescence detector (CLD) by
combustion-infrared method (5310B) of ‘‘Standard Methods”. The oxidizing was
oxygen 5.0 at pressure of 4-6 bars. For VFA analysis (acetic, propionic, iso-butyric,
butyric, isovaleric, valeric, iso-caproic, caproic and heptanoic) samples from the
previous lixiviation and 0.47 µm filtration were filtered again by a Teflon filter of 0.22
µm and analyzed by using a gas chromatograph (Shimadzu GC-2010) equipped with a
flame ionisation detector (FID) and capillary column filled with Nukol (polyethylene
glycol modified by nitro-terephthalic acid). The detection limit for acetic, propionic,
isobutyric, butyric, Isovaleric, valeric, isocaproic, caproic and heptanoic were 31-
489ppm, 39- 622 ppm, 45 -711 ppm, 45-711 ppm, 53-839 ppm, 53-839 ppm, 59-
933 ppm, 59-933 ppm and 67-1056 ppm, respectively. The temperature of the
injection port and detector were 200°C and 250°C, respectively. Helium was the carrier
gas at 50-mL/ min. In addition, nitrogen gas was used at 30-mL/ min flow rate. Total
VFA were calculated as the addition of individual VFA levels (Fernandez et al. 2008).
The biogas composition was determined by using a gas chromatograph
(Shimadzu GC-2014) with a stainless steel column packed with Carbosieve SII
(diameter of 3.2 mm and 3.0 m length) and thermal conductivity detector (TCD). The
injected sample volume was 1 mL and the operational conditions were as follows: 7 min
at 55°C; ramped at 27°C/min until 150°C; detector temperature: 255°C; injector
temperature: 100°C. The carrier was helium and the flow rate used was 30 mL/min
(Fernandez et al. 2008). The instrument detection limit for H2, CO2, N2, O2 and CH4
was 50 ppm. A commercial mixture of H2, CH4, CO2, O2 and N2 (Abelló Linde, Spain)
was used to calibrate the system.
All the analyses were carried out in triplicate, results were averaged and
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presented on dry basis.
3. Results and discussion
3.1. Characterization of OFMSW and mixed sludge
The physico-chemical characteristics of OFMSW, PS, WAS and MS are summarized in
Table 1. The average TS and VS concentration of OFMSW were 526 g/kg and 230
g/kg, respectively, indicating that only 44% fraction of solids in OFMSW was
potentially digestible. However, the volatile fraction of PS, WAS and MS solids was
77%, 83% and 82%, respectively, indicating a high percentage of solids in PS, WAS
and MS were organic in nature. The majority of the volatile fraction of sewage sludge is
proteinaceous materials and a proteinaceous COD is more than 50% of the total COD in
sewage sludge (Kim et al., 2004). VFAs including acetic, propionic and butyric acids
were detected in the OFMSW and MS indicating that fermentation of both wastes had
began. Though, the concentration of VFAs in OFMSW, PS and MS were only 4.1 g/kg,
31 g/kg and 5.4 g/kg, respectively, it was expected that much of the organic matter was
in a fermentable form, which could support the H2 generation. However, no VFAs were
detected in WAS. Although, the concentration of the VFAs (mainly acetic, propionic
and butyric acid) were different in the OFMSW, PS and MS, the presence of VFAs in
these substrates showing that acidogenesis of these wastes had began (Zhu et al., 2008).
The TKN concentration in OFMSW, PS, WAS and MS were observed 11.6 g/kg, 21
g/kg, 70 g/kg and 32.8 g/kg, respectively. A higher TKN concentration in WAS and MS
indicated the presence of high protein content in WAS and MS samples. However,
OFMSW was low in nitrogenous content, which is an essential nutrient for hydrogen
producing bacteria. The low nitrogen content in OFMSW gives it a high C/N ratio of
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21 over PS (C/N 20.4), WAS (C/N 6.6) and MS (C/N 11). Therefore, the hypothesis of
co-digestion of OFMSW and MS was to provide sufficient nutrient balance to enhance
the hydrogen production. The alkalinity of OFMSW was relatively low (10.14 g/kg),
which shows its low pH buffering capacity. The alkalinity of PS, WAS and MS were
more than double i.e. 26 g/kg, 22 g/kg and 21.8 g/kg, respectively, than that of
OFMSW, which indicates the higher pH buffering capacity of PS, WAS and MS.
Therefore, sludge addition as a co-substrate will help to reduce the flux of pH and to
balance the pH of process during hydrogen fermentation.
3.2. Effect of co-digestion on hydrogen production
The effect of SS co-digestion with OFMSW on H2 production efficiency was studied in
first assay. The findings revealed that the co-digestion of OFMSW with SS enhanced
the cumulative H2 production and H2 yield by 20% and 70%, respectively, over the
OFMSW fermentation alone. OFMSW showed lower H2 yield (20 mL H2/ gVSconsumed)
than OFMSW+MS (34 mL H2/ gVSconsumed). However, the H2 concentration in biogas
composition was found higher (43%) in OFMSW assay in comparison with
OFMSW+MS assay (38%). The enhanced H2 production from co-digestion may have
been due to the provision of nutrients and micronutrients that stimulated H2 producing
bacteria. The provision of protein from sludge streams may have provided an improved
C/N ratio, which has been reported as an important parameter for growth of hydrogen
producers (Zhu et al., 2008). No methane production was observed in both the assays.
The VFAs evolution results shows the 400% increment in VFAs concentration for
OFMSW+MS assay, which is 6 times higher than those observed in OFMSW
fermentation alone (70%). The degradation of protein would also produce VFAs (Tang
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et al., 2005). VFAs included mainly acetic, butyric, propionic, valeric and caproic. In
both assays, the major fractions of VFAs in digestate were composed of acetic acid and
butyric acid (73-79%), however, the dominant fermentation products was butyric acid in
both assays (HBu/HAc = 3.1). Butyric acid type fermentation is considered to be one of
the most effective route in H2 production, specifically for clostridium with endogenesis
spores (Ewyernie et al., 2001). As VFA produced along with H2, the pH of the system
dropped and alkaline addition is required. Although, the initial pH in each assay was
adjusted to 5.5, the final pH in the digestate of OFMSW assay was dropped to 5.0, with
a minor increment of 10% in alkalinity. When OFMSW was co-digested with SS, the
final pH was increased to 5.7 alongwith 64% increment in alkalinity. It is evident from
the results that SS had a higher buffering capacity at low pH in comparison with
OFMSW. Previous study also reported the increment in pH values after co-digestion of
food waste+ sewage sludge under initial pH of 5.5 (Liu et al., 2013). Moreover, the
enhanced alkalinity in co-digestion assay could minimize the requirement of base
supplementation to regulate the pH. In co-digestion assay, the ammonia concentration
was increased by 336%, which is almost double than those observed for OFMSW
fermentation alone (177%). The degradation of protein rich sludge would enhance the
ammonia concentration, and the increase in ammonia concentration will contribute to
large amount of alkalinity into the system. For co-digestion assay, the ammonia
concentration in digestate was 4.8 g/kg (55 mg/L), which was lower than the testified
ammonia inhibition concentration for H2 production yield (Salerno et al., 2006). Sewage
sludge co-digestion not only enhanced the H2 production, but also accelerated the
fermentation reaction. The sCOD and DOC concentrations in digestate of OFMSW+MS
were increased by 106% and 81%, respectively, which were 4.6 times and 3 times
13
higher, respectively, than OFMSW only (sCOD= 23% and DOC= 27%). This indicates
that the addition of SS with OFMSW accelerated the organic matter solubilisation
efficiency of the process, thus employed a positive effect on H2 yield. Thus, the findings
revealed that the OFMSW was lacking in some important elements namely nitrogen and
buffering capacity, which were supplied by sewage sludge addition into the system.
3.3. Effect of type of sludge combination with OFMSW
Three types of sludges namely, PS, WAS and MS were co-digested with OFMSW in
individual assays, and the effect on H2 production efficiency was studied. The highest
H2 yield was observed in OFMSW+MS assay (56 mL H2/g VSconsumed) in comparison
with OFMSW+PS (38 mL H2/g VSconsumed) and OFMSW+WAS assay (26 mL H2/g
VSconsumed), indicating that the MS addition supplied more balanced ratio of
carbohydrate, nutrient and trace metals over the WAS and PS. Zhu et al. (2008) also
reported that the ternary mixtures of FW+PS+WAS produced substantially more H2
than the binary mixtures (FW+PS or FW+WAS). The maximum H2 content in biogas
was found for OFMSW+MS (42%), if compare with OFMSW+PS (41%) and
OFMSW+WAS (39%). Only H2 and CO2 were present in the biogas composition;
methane was not generated, since the methanogenic activities were suppressed by
employing the acidic pH of 5.5, which considered optimum for H2 producers
(Zoetemeyer et al., 1982). The higher increment of 96%, 126% and 121% in alkalinity
evolution were observed for OFMSW+PS, OFMSW+WAS and OFMSW+MS,
respectively. pH remained almost stable (5.2±0.1) in all the assays, which was the ideal
pH for optimum hydrolytic and acidogenic bacteria activity (De la Rubia et al., 2009).
Moreover, the findings revealed that despite the similar pH buffering efficiency,
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different types of sludges had different effect on H2 yield. The VFA evolution was
slightly higher in OFMSW+WAS (119%) and OFMSW+MS (119%) in comparison
with OFMSW+PS assay (117%) (Figure 1). The dominant fermentation products were
butyric acid and acetic acid, which contributed to approximately 60% of total VFAs in
digestate. The butyric acid evolution was the highest than other VFAs for all the studied
combinations. Caproic acid was the third major VFA, which constitute 12% of TVFA
concentration in digestate of all the assays. The high concentrations of caproic acid can
be harmful to acidogens (Rinzema et al., 1994), but this kind of effects was not
observed during the present study, since, the microorganisms were well acclimatized to
this kind of substrate. Other organic acids detected in digestate in lower concentrations
were propionic, valeric and isovaleric acids. Figure 1 shows that the highest organics
solubilisation (sCOD and DOC) were observed for OFMSW+WAS and OFMSW+MS
and lowest for OFMSW+PS assay. Despite the highest VS removal of 24% and highest
cumulative H2 production of 55 mL, the H2 yield was found lower in OFMSW+WAS
assay if compared with OFMSW+PS and OFMSW+MS assays. Nevertheless, all types
of sludges addition were observed to significantly enhanced the H2 production during
dark fermentation of OFMSW. On the basis of findings, the mixed sludge was
considered the best co-substrate for dark fermentation of OFMSW for hydrogen
production.
3.4. Effect of Total solids percentage
The effect of different TS concentrations of 10, 15, 20 and 25% on H2 production
efficiency was studied at OFMSW to MS mixing ratio of 1:1. Figure 2a showing that
despite the highest cumulative H2 production of 108 mL at TS concentration of 25%,
15
the highest H2 yield of 69 mL/ g VSconsumed was found at TS concentration of 20%. High
TS concentration adds higher organic matter thus higher carbohydrate concentration,
and it is known that the inhibition of high carbohydrate concentration on H2 production
rate was less severe than that on H2 production potential (Lay, 2001). The H2 yield was
decreased as TS concentration increased further (>20%), which may be due to the
product inhibition by H2 and VFAs. (Van Ginkel et al., 2001). In assays 10% and 15%,
cumulative H2 production reached the maximum value (74%) within 3 days, however,
assays 20% and 25% took 9 days to achieve the similar H2 production levels. Thus, the
lag phase duration for H2 production was 6 days in both 20% and 25% assays (Figure
2a). This lag phase may be the result of slow hydrolysis and subsequent acidogenesis at
higher TS concentration. Similarly, a lag phase was experienced initially, followed by a
rapid H2 production phase, and minimal H2 production at the end during food waste and
sludge co-digestion assays (Zhou et al., 2013). The highest H2 concentration of 41% in
biogas composition was observed at TS concentration of 15%, however, a slight
decrease in H2 concentration was noted as TS concentration increased (Figure 2b).
Negligible or residual quantity of methane in biogas composition (N.D.- 0.68%) was
observed, because, the assays were conducted under acidic conditions (pH 5.5), which
was unfavorable for methanogenic activities. According to Figure 2c the highest VFA
production of 132% was observed at 20% TS concentration, however, the VFAs
production was decreased by 42% at higher TS concentration of 25%. In anaerobic
digestion of organic solid wastes, the production of liquid by-products (VFAs, sCOD,
DOC) is important to assess the process propriety. High proportion of soluble organics
in the digestate enhance the volume reduction of waste and biogas production (Fox and
Pohland, 1994). In all the assays, butyric and acetic acid were the predominant residual
16
organic acids, which together contributed 66% of total VFAs. Caproic acid contributed
the third major portion (6%) of VFAs. The propionic, valeric, iso-valeric, and iso-
butyric acids were also observed in residual concentrations in the digestate of all
assays. The pH values and alkalinity evolution (8%-38%) were decreased as the TS
concentration increased, which might be contributed by the high VFA production (Van
Ginkel et al., 2001). Nevertheless, the observed pH values (5.2 ±0.2) were not inhibitory
for H2 producers (Figure 2d), as it was reported that pH below 5.0 inhibited H2
production (Fang and Liu, 2002). The highest evolution in soluble organics
concentration of 132% (sCOD) and 86% (DOC) were observed at TS concentration of
20%, however, the 42% and 15% fall in increment in sCOD and DOC evolution,
respectively, were observed at higher TS concentration of 25% (Figure 3c). Despite a
lower cumulative H2 production and H2 yield, the highest VS removal of 23% was
observed at TS concentration of 10% and 15% (Figure 2e). On the basis of the findings,
the TS concentration of 20% was considered best for the following assays.
3.5. Effect of OFMSW: sludge mixing ratios
Four different OFMSW to MS mixing ratios of 10:1, 5:1, 2.5:1 and 1:1 were examined
at fixed TS concentration (20%) and inoculum to substrate ratio (1:1). The highest H2
yield of 51 mL/gVSconsumed was observed for OFMSW to MS mixing ratio of 5:1
(Figure 3a). Sludge addition enhanced the H2 production due to contribution of proteins
as a nitrogen source thus better nutrient balance. The H2 yield was decreased as sludge
fraction increased further, as the lowest H2 yield of 26 mL/gVSconsumed was observed for
OFMSW: MS ratio of 1:1, which could be due to the low carbohydrate concentration in
the sludge, which is the primary substrate for H2 production (Kim et al., 2011; Zhou et
17
al., 2013). Zhu et al. (2008) observed that the H2 production was not proportional to the
amount of the sludge added with FW, which may be due to the amount of useful
nutrients added with the sludge was more than required. Nevertheless, contradictory
results were also reported with highest H2 yield at food waste to SS ratio of 1:1 (Zhu et
al., 2008). The biogas produced in all the assays mainly composed of H2 and CO2 with
no methane generation. The H2 concentration in biogas composition was observed
almost similar (36-38%) in all the assays (Figure 3a). Figure 3b shows that the pH
remains almost unchanged (5.5) in all the assays. It shows that the sludge proportion in
feedstock was enough to maintain the pH buffering in medium, which allowed the
fermentation to maintain the pH (5.5), which was optimum for H2 producers. The
alkalinity was increased as the sludge proportion in substrate was increased. The highest
alkalinity increment of 124% was observed for OFMSW: MS ratio of 1:1. The
enhanced alkalinity could diminish the requirement of alkali supplement during dark
fermentation. The VFA evolution results revealed that the VFAs production was
increased with increasing the sludge proportion in substrate (Figure 3c). The main acids
in digestate were acetic and butyric acid i.e. ≈65% of total VFAs. The butyric acid
production was higher than the acetic acid production for all the tested OFMSW to MS
ratios of 10:1, 5:1, 2.5:1 and 1:1 with butyrate to acetate ratio (HBu/HAc) of 1.3, 1.1,
1.2 and 1.1, respectively. The dominant production of butyric and acetic acid is a good
sign that efficient H2 production was achieved (Hawkes et al., 2002). Caproic acid
added 8% of total VFAs, however, propionic, valeric and isovaleric acids were the other
organic acids occurred in minor concentrations. The percentage evolution of propionic
acid was increased with the sludge fraction increased and thus the corresponding H2
yield decreased (Figure 3d). The organics solubilisation (sCOD and DOC) results also
18
follow the same trend as observed for VFAs. The soluble organics concentrations were
increased alongwith increasing the sludge fraction in substrate. The highest increment in
concentration of sCOD (103%) and DOC (158%) were observed for OFMSW:MS ratio
of 1:1. The findings revealed that a significant fraction of organic matter was
hydrolyzed and acidified and dissolved in solution because of VFAs evolution (Zhou et
al., 2013). Despite a higher VFA production at OFMSW:MS ratio of 2.5:1 and 1:1, the
H2 yield was reduced. Which can be understand by the fact that long duration (18 days
in this study) favor the buildup of H2 consumers such as competitors for substrate i.e.
non- H2 -producing acidogens (Wang and Zhao, 2009). The VS removal was increased
with increasing the sludge fraction in substrate with maximum VS removal of 34% was
observed for OFMSW to MS mixing ratio of 1:1 (Figure 3e). The highest VS removal
in OFMSW:MS ratio of 1:1 was mainly due to the fast degradability of proteins and
lipids presented in MS, which ultimately contributed in higher VS removal, but a lower
H2 yield, because H2 is hardly produced from proteins and lipids degradation (Lay et al.,
2003).
4. Conclusions
Sewage sludge proved to be a suitable supplementary co-substrate for enhanced H2
production from dark fermentation of OFMSW. Mixed sludge was found to be the best
co-substrate with OFMSW in comparison with primary sludge and waste activated
sludge. Sewage sludge supplementation could maintain a good buffering capacity at low
pH and enhanced the soluble organics evolution in comparison with OFMSW. At the
optimum OFMSW:MS ratio of 5:1 (C/N 31) and TS concentration of 20%, the
maximum H2 yield of 51 mL H2/g VSconsumed with H2 concentration of 36% were
19
observed. Hydrogen production was chiefly accompanied by acetate and butyrate
production.
Acknowledgement
The authors would like to thank to Agrifood Campus of International Excellence
(ceiA3) by the postdoctoral contract E-11-2013-0076119 granted to Dr. Vinay Kumar
Tyagi in the program of "Attracting Talent to the Agri-food Sector" financed by the
Ministry of Education (Ministerio de Educación, Cultura y Deporte) in the framework
of the CEI Program, 2010.
This work was supported by the project CTM2010-17654 financed by the Spanish
Ministry of Science and Innovation (Ministerio de Ciencia e Innovación) and the
European Regional Development Fund (ERDF).
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Captions for Tables
Table 1. Physico-chemical characteristics of inoculum and substrates
Table 2. Substrate composition of different co-digestion assay
25
Table 1.
Parameters Unit* Inoculum OFMSW PS WAS MS
pH 5.5 (0.5) 6.2 (0.3) 6.0 (0.2) 6.7 (0.2) 6.4 (0.2)
Density kg/L 1.1 (0.05) 1.07 (0.01) 1.03 (0.02) 1.08 (0.05) 0.98 (0.04)
Alkalinity g/kg 74.8 (36.7) 10.14 (2.0) 26 (4.1) 22 (3.5) 21.8 (3.3)
TS % 10.7 (1.1) 52.6 (4.1) 2.2 (0.1) 1.8 (0.2) 4.5 (0.2)
VS % 6.8 (0.5) 23.0 (1.2) 1.7 (0.2) 1.5 (0.3) 3.7 (0.2)
NH4-N g/kg 2.9 (1.2) 0.5 (0.4) 4.4 (0.4) 2.8 (0.6) 1.6 (0.3)
TKN g/kg 13.4 (3.0) 11.6 (2.1) 21 (3.9) 70 (5.8) 33 (4.4)
sCOD g/kg 597 (155) 208 (78.5) 122 (21.3) 36 (14.5) 43 (15.5)
DOC g/kg 150 (24.6) 47 (5.1) 35 (7.5) 4 (4.8) 14 (6.6)
Total VFA g/kg 9 (18.3) 4.1 (1.2) 31 (0.6) ND** 5.4 (0.3)
C/N ratio 35 (2.3) 21 (2.8) 20.4 (0.6) 6.6 (0.5) 11 (0.6)
*all the data on dry basis; **ND= not detected
26
Table 2.
Reactor Inoculum (mL) OFMSW (mL) SS (mL) Water (mL)
Assay 1 Effect of co-digestion on hydrogen production*
Control 120 0 0 0
OFMSW 60 60 0 0
MS 60 0 60 0
OFMSW+MS 60 30 30 0
Assay 2 Optimization of best sludge combination*
Control 120 0 0 0
PS+OFMSW 60 30 30 0
WAS+OFMSW 60 30 30 0
MS+OFMSW 60 30 30 0
Assay 3 Optimization of best total solids (TS) percentage*
Control 120 0 0 0
10% 60 5.8 5.8 48.3
15% 60 12.2 12.2 35.6
20% 60 18.6 18.6 22.8
25% 60 24.9 24.9 10.1
Assay 4 Optimization of best OFMSW: sludge mixing ratio*
Control 120 0 0 0
10:1 60 25.77 2.58 31.65
5:1 60 25.64 5.13 29.23
27
2.5:1 60 25.39 10.16 24.45
1:1 60 24.68 24.68 10.65
* All the tests were carried out in triplicate.
28
Captions for Figure
Figure 1. Optimization of best sludge combination: effect on organic matter
solubilisation
Figure 2. Optimization of total solids percentage (a) H2 production efficiency along
with time (b) effect on H2 yield and percentage composition (c) effect on organic matter
solubilisation (d) effect on pH and Alkalinity (e) effect on volatile solids
Figure 3. Optimization of OFMSW to Sludge mixing ratio (a) effect on H2 yield and
percentage composition (b) effect on pH and Alkalinity (c) effect on organic matter
solubilisation (d) effect on propionic acid production and H2 yield (e) effect on volatile
solids
Figure 1.
(a) (b)
(c) (d)
(e)
Figure 2.
0
20
40
60
80
100
120
140
160
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
H2 P
rod
uc
on
(m
L)
Days
Control 10% 15% 20% 25%
(a) (b)
(c) (d)
(e)
Figure 3.
0
10
20
30
40
50
60
0
10
20
30
40
50
60
70
80
90
10;1 5;1 2.5;1 1;1
H2 y
ield
(m
L/g
VS
co
nsu
me
d)
% i
ncr
ea
se p
rop
ion
ic a
cid
OFMSW:MS ra o
% increase propionic acid H2 Yield
33
Highlights
• H2 was produced from thermophilic anaerobic co-digestion of OFMSW and
sewage sludge
• Enhanced H2 production was achieved for OFMSW+sewage sludge co-
fermentation
• Mixed sludge was the best co-substrate (with OFMSW) among the sludge types
studied
• Best hydrogen yield was achieved at 20% TS concentration
• Highest H2 yield was achieved at OFMSW to mixed sludge ratio of 5:1