Waste Management 80 (2018) 73–80

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Waste Management

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Impacts of different biochar types on hydrogen production promotionduring fermentative co-digestion of food wastes and dewatered sewagesludge

https://doi.org/10.1016/j.wasman.2018.08.0420956-053X/� 2018 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors at: International Science and Technology CooperationCenter for Urban Alternative Water Resources Development, Xi’an University ofArchitecture and Technology, No. 13 Yanta Road, Xi’an 710055, China.

E-mail addresses: saiycain56@aliyun.com (Q. Li), xcwang@xauat.edu.cn(X.C. Wang).

Gaojun Wang, Qian Li ⇑, Mawuli Dzakpasu, Xin Gao, Chaosui Yuwen, Xiaochang C. Wang ⇑International Science and Technology Cooperation Center for Urban Alternative Water Resources Development, Xi’an University of Architecture and Technology, No. 13 YantaRoad, Xi’an 710055, ChinaKey Laboratory of Northwest Water Resource, Environment and Ecology, MOE, Xi’an University of Architecture and Technology, No. 13 Yanta Road, Xi’an 710055, ChinaEngineering Technology Research Center for Wastewater Treatment and Reuse, Shaanxi, Xi’an University of Architecture and Technology, No. 13 Yanta Road, Xi’an 710055, ChinaKey Laboratory of Environmental Engineering, Shaanxi, Xi’an University of Architecture and Technology, No. 13 Yanta Road, Xi’an 710055, China

a r t i c l e i n f o

Article history:Received 23 December 2017Revised 19 August 2018Accepted 25 August 2018

Keywords:PyrolysisBiochar propertyBuffering capacityFermentative hydrogen productionBio-wastes management

a b s t r a c t

Pyrolysis and anaerobic digestion are two important strategies for waste management that may be com-bined for clean energy production. This article investigates the effects of 12 types of biochars derived fromfour feedstocks at three pyrolysis temperatures on H2 production via fermentative co-digestion of foodwastes and dewatered sewage sludge. The results show that feedstock type and pyrolysis temperature sig-nificantly influence biochar properties such as pH, specific surface area and ash contents. Despite the widerange of BET specific surface areas (1.2–511.3 m2/g) and ash contents (5.3–73.7(wt%)) of biochars produced,most biochars promoted theVFAs productionprocess and altered the fermentative type from that of acetatetype to butyrate type, which seemed to have a higher efficiency for H2 production. Moreover, fitting of theresults to themodified Gompertzmodel shows that biochar addition shortens the lag time by circa 18–62%and increases the maximum H2 production rate by circa 18–110%. Furthermore, the biochar derived athigher pyrolysis temperatures enhances H2 production dramatically over those derived at low tempera-tures. Principal components analysis demonstrated that the pH buffering capacity of biochar was criticalto the promotion of fermentative H2 production by mitigating the pH decrease caused by VFAs accumula-tion. Consequently, a sustainable integrated wastemanagement strategy combining pyrolysis and anaero-bic digestion is proposed for the efficient treatment of various bio-wastes.

� 2018 Elsevier Ltd. All rights reserved.

1. Introduction

Hydrogen gas (H2) can be a sustainable and renewable fuel thatproduces no greenhouse gas (GHG). This could help to relieve thereliance on traditional fossil fuels and mitigate climate change(Turner, 2004). Currently, most H2 is generated from non-renewable sources, which is counter to the aim of GHG emissionreduction. Thus, in recent years, more sustainable andenvironmentally-friendly technologies have been developed forproducing H2 production, including photolysis, photo-fermentation and dark fermentation (Kalinci et al., 2009). Of these,

the dark fermentation, an H2 production process via anaerobicmicroorganisms, requires not only minimal energy consumptionbut also has stronger operability for engineering applications.Additionally, dark fermentative H2 production can be achievedusing bio-wastes as sources to which could efficiently integrateorganic waste management and cleaner energy production(Gioannis et al., 2013). In this way, many kinds of bio-wastes, suchas food wastes, sewage sludge, and animal manures, could bedegraded into volatile fatty acids (VFAs) and H2 in anaerobic sys-tems (Gilroyed et al., 2008; Han et al., 2016; Slezak et al., 2017).Therefore, the dark fermentative H2 production strategy, whichaims to reduce bio-wastes, produce valuable products and cleanerenergy shows potential economic and ecological benefits.

Although fermentative H2 production is a promisingtechnology, several issues have limited its engineering application.First, fermentative H2 production occurs within the acidificationprocess of the anaerobic systems. Consequently, continuous VFAs

74 G. Wang et al. /Waste Management 80 (2018) 73–80

production could cause a decrease in the pH, which is crucial forthe metabolism of microbes and the metabolic pathways of H2

production (Slezak et al., 2017). To overcome this problem, somebuffering agents, such as lime mud and NaHCO3, were added tostabilize pH in H2 production systems (Lin & Lay, 2004). Addition-ally, to promote the H2 yield and increase the biomass retentiontime, different types of carriers such as activated carbon, glassbeads and expanded clay, could be added into the H2 productionsystem to enrich H2 production bacteria (HPB) and to increase sta-bility of the system (Barca et al., 2015).

As carbohydrate-rich and readily biodegradable materials,food wastes and other industrial bio-wastes from food produc-tion factories may be ideal substrates for fermentative H2 pro-duction (Lin et al., 2012). However, for some kinds oflignocellulosic wastes that are recalcitrant to anaerobic degrada-tion, other treatment strategies should be considered. Recently,studies on the pyrolysis of lignocellulosic wastes underoxygen-limited or oxygen-absent conditions are burgeoning,whereby these wastes are converted to biochar (Lehmann &Joseph, 2015). As a porous eco-compatible carbon-rich material,biochar has significant potential for environmental management,allowing carbon sequestration, GHG emission reduction, pollu-tant remediation, and soil amendment (Lehmann et al., 2011;Manyà, 2012; Woolf et al., 2010). Moreover, it is reported thatbiochar addition could promote anaerobic biogas (H2/CH4) pro-duction due to its unique properties, such as alkalinity character-istics for pH buffering (Wang et al., 2017) and high specificsurface area (SSA) for biomass attachment (Luo et al., 2015). Ina two-phase anaerobic digestion system for H2 and CH4 produc-tion, the maximum production rates of both H2 and CH4

increased as biochar was added (Sunyoto et al., 2016). In asingle-phase methane production system for cattle manureanaerobic digestion, 1% biochar addition (dry substrate weightbasis) increased biogas production by 31%, but a higher biochardose addition did not result in any additional increase(Inthapanya et al., 2012). Biochar addition also mitigated mildammonia inhibition and supported methanogen growth andmetabolic functions (Lü et al., 2016). It is also reported that10 g/L biochar addition both shortened the methanogenic lagphase by 11.4–30.3% and increased the maximum methane pro-duction rate by 5.2–86.6% (Luo et al., 2015). Furthermore,Meyerkohlstock et al. (2016) investigated the effects of biocharon the solid-state fermentation of bio-waste and reported thatboth biogas and methane yield increased by approximately 5%with 5% biochar addition.

To the best of the authors’ knowledge, although the promotionof anaerobic biogas production by biochar addition is widelyreported, studies on the effects of biochar’ properties on the H2

production process is rare. Thus, the intrinsic mechanism of fer-mentative H2 production enhancement by biochar addition, whichmay be closely related to the properties of biochar, remainsunclear. Generally, the feedstocks and highest treatment tempera-tures (HTTs) of biochar production are two important factors thatsignificantly influence the properties of the derived biochar(Manyà, 2012). Nevertheless, the effects of biochar derived fromdifferent feedstock types at variable pyrolysis HTTs on fermenta-tive H2 production have never been studied. This lack of knowledgeis addressed by examining the physiochemical properties of bio-char produced from four feedstocks under three HTTs in the pre-sent study. Meanwhile, the effects of biochar addition onfermentative H2 production are also elucidated, and the mainderived property of biochar affecting the fermentative H2 produc-tion process is determined. Finally, an integrated strategy for effi-cient and sustainable management of various bio-wastes isproposed.

2. Materials and methods

2.1. Biochar preparation

Four different types of biomass were used as feedstock materi-als. Sawdust (SD) was purchased from a local furniture factory,wheat bran (WB) and peanut shell (PS) were collected from a crop-land in the suburb of Xi’an, Shaanxi Province, China, and sewagesludge (SS) was acquired from the Xi’an No.5 Wastewater treat-ment plant (WWTP). The feedstocks were air-dried at room tem-perature for five days. Subsequently, the feedstocks were placedinto ceramic crucibles, covered with fitting lids, and pyrolyzedunder oxygen-limited conditions in a muffle furnace (Shanghai JingHong Laboratory Instrument Co., Shanghai, China). Pyrolysis wascarried out at HTTs of 300, 500, and 700 �C at a heating rate ofapproximately 15 �C min-1 and then held at the given HTTs for1 h. After the pyrolysis process was complete, the biochar sampleswere allowed to cool to room temperature and then pulverized tosieve to uniform size fractions of 0.25–1 mm. The biochar sampleswere then stored in plastic sealing bags until further analysis. Thebiochar samples were designated as SD3, SD5, SD7, WB3, WB5,WB7, PS3, PS5, PS7, SS3, SS5 and SS7, where the letters representthe feedstocks (SD, WB, PS and SS) and the numbers 3, 5 and 7 rep-resent the HTT of 300 �C, 500 �C, and 700 �C, respectively.

2.2. Biochar property analysis

The biochar yields and proximate analysis were carried outaccording to ASTM D1762-84 (2013). The pH values of the biocharswere measured in a 5% (w v-1) suspension of deionized water pre-pared by shaking at 100 rpm under ambient temperature for 24 husing a pH meter (PHS-3C, Dapu Instrument Co., Shanghai, China).The BET (Brunauer-Emmett-Teller) SSA was measured via N2

adsorption multilayer theory by a V-Sorb X800 surface area ana-lyzer (Gold APP Instrument Co., Beijing, China). The elemental(CN/OH) analysis was carried out using an isotope ratio mass spec-trometer (IRMS, IsoPrime100, Elementar, Germany). The surfacemorphology and textural properties of biochar were characterizedvia scanning electron microscope (SEM; JEOL, JSM-6510LV, Japan)with a tungsten filament. The surface functional group analysisof feedstocks and biochars were determined by Fourier transforminfrared spectroscopy (FT-IR, ThermoFisher, USA) under an attenu-ated total reflectance (ATR) model. The spectrum was recorded inthe wave number range of 500–4000 cm-1. The feedstocks and bio-char samples were pulverized without additional pretreatment. Allthe physiochemical analyses were carried out in duplicate.

2.3. Substrates and inoculum sources

Dewatered activated sludge (DAS) and food waste (FW) wereused as substrates for co-digestion in the batch experiments. TheDAS was collected from a sludge dewatering unit in Xi’an No.5WWTP, Shaanxi Province, China. The FW sample was a syntheticpreparation based on the characteristics of food waste in China,and the components are shown in supplementary materials. Theratio of FW to DAS based on wet weight was 4:1. The raw sub-strates were preserved at 4 �C before use.

The inoculum for the H2 production experiments was collectedfrom the biogas plant of a local brewery, which was steadily oper-ated under mesophilic conditions. The inoculum was stored at 4 �Canaerobically for several weeks before use. The physio-chemicalproperties of the substrates and inocula were measured induplicate and are listed in Table 1.

Table 1Physical and chemical properties of substrates and inoculum.

Properties Substrates Inoculum

Total solid (%) 8.80 ± 0.24 6.82 ± 0.18Volatile solid (%) 7.91 ± 0.28 4.65 ± 0.14C/N ratio 9.1 ± 0.1 –pH 5.2 ± 0.1 7.32 ± 0.1COD (g/L) 123 ± 5 4.82 ± 0.3TVFAs (g COD/L) 2.02 ± 0.04 0.07 ± 0.01Carbohydrate (g/L) 32.7 ± 2.9 –Protein (g/L) 11.7 ± 2.7 –

G. Wang et al. /Waste Management 80 (2018) 73–80 75

2.4. Batch experiments design for H2 production

The batch anaerobic digestion experiments for H2 were carriedout in 120-mL glass bottles each containing a 15 mL substrate, and5 mL inocula, and tap water at a total working volume of 90 mL.The initial pH was adjusted to about 5.5 with 6 M NaOH and 6 MHCl solutions to inhibit the activity of methanogenic archaea. Sub-sequently, 10 g/L of each type of biochar was separately added intoa reactor. Together, there were 12 biochar-amended bottles andone control bottle without biochar addition. The bottles werepurged with high purity nitrogen gas for 3 min to make the reac-tors anaerobic and then were sealed with rubber stoppers and alu-minum caps. The bottles were then placed in a water bath (35 �C)and stirred at 150 rpm for reaction. The mesophilic condition wasused in this study because compared with the stable mesophilic ADprocess, thermophilic AD process often was more sensitive forenvironmental pressure and also caused higher energy consump-tion. On reaching the set temperature, the headspace of each bottlewas vented using a syringe to release the pressure caused by ther-mal expansion. Biogas was collected for volume analysis using aglass syringe. The composition of the biogas was collected usinga 400 ll micro syringe. Fermentative liquid samples were takenfor VFA analysis. The experiments were carried out in duplicateover 7 days (day 0 in the figures indicates the starting point) untilH2 production was complete.

2.5. Analytical methods

The composition of the biogas was measured with a gas chro-matograph (GC) (GC7900, Tianmei, China) equippedwith amolecu-lar sieve packed stainless-steel column (TDX-01, length � diameterof 2.0 m � 3.0 mm, Shanghai Xingyi Chrome, China) and a thermalconductivity detector (TCD). The VFAs were measured with aGC (PANNO, China) equipped with DB-FFAP column (u0.32 mm � 50 m; Agilent, USA) and flame ionization detector (FID).

Table 2Physicochemical characteristics analysis of different types of biochar produced.

Parameter (unit) SD3 SD5 SD7 SS3 SS5

Biochar yield (wt%) 32.7 22.6 19.6 64.5 54.0Fixed carbon (wt%) 57.5 68.5 65.4 19.3 19.9Volatile carbon (wt%) 37.2 23.7 19.6 25.3 12.9Ash content (wt%) 5.3 7.8 15.0 55.4 67.2Organic content (wt%) 94.7 92.2 85.0 44.6 32.8Ammonia content (mg/g) 0.21 0.28 0.24 0.22 0.21Fixed carbon yields (wt%) 18.8 15.5 12.8 12.4 10.7pH 7.27 9.15 10.07 7.42 7.80BET surface area (m2/g) 15.3 248.6 511.3 1.2 11.3C (wt%) 66.55 74.45 78.84 28.04 20.44N (wt%) 0.43 0.44 0.43 4.62 3.31H (wt%) 6.85 4.80 3.94 5.27 3.89O (wt%) 18.54 12.19 10.86 21.63 21.38C/N 154.8 169.2 183.3 6.1 6.7H/C 0.10 0.06 0.05 0.19 0.19O/C 0.28 0.16 0.14 0.77 1.05

The batch experimental data was simulated by using the mod-ified Gompertz equation:

P ¼ P0 � exp �expRmax � e

P0� t0 � tð Þ þ 1

� �� �

where P is the accumulated H2 production (mL), P0 is the H2 produc-tion potential (mL), Rmax is the maximum H2 production rate (mL/d), t0 is the lag time (days) and e = 2.718281828. The Origin 8.0 soft-ware (OriginLab Corporation, MA, USA) was used to fit the H2 pro-duction curves.

Principal Components Analysis (PCA) was carried out usingSPSS 20.0 (IBM, NY, USA) to explore the relationships between bio-char properties and the main parameters of H2 production.

3. Results and discussion

3.1. Effects of HTTs and feedstock types on biochar characteristics

3.1.1. Effects of HTTs on biochar characteristicsTable 2 shows the physiochemical properties of biochar pro-

duced with the four types of feedstocks at three HTTs. The criticalproperties of biochar that are likely to influence the fermentativeH2 production process, namely pH, SSA, and ash contents areshown in Fig. 1. The pH of the biochar samples correlated stronglywith HTTs. There are two potential explanations for this correla-tion. First, the higher HTTs caused the consumption and/or depro-tonation of more oxygenated functional groups such as acidiccarboxyl groups to the conjugate bases. At the same time, theash content of the biochar samples showed a significant increasewith increased HTTs, and the relatively higher ash contents in bio-char produced at higher HTT increased the amounts of some inor-ganic salts or metallic oxides (carbonates, MgO, CaO, and Fe2O3)(Ronsse et al., 2013; Yuan et al., 2011). These two processes causedthe pH of biochar to remain within the alkaline range. For the bio-char produced from the same type of feedstock, the SSA increasedwith increasing HTTs because higher HTTs expanded the structuralpores from ‘‘micro-pores” to ‘‘macro-pores” by breaking down thewalls between adjacent pores. This was shown previously toincrease the total pore volume and surface area (Liu et al., 2015).

3.1.2. Effects of feedstock types on biochar characteristicsDifferent feedstock types treated at the same HTT showed dif-

ferent physiochemical properties. As a physical indicator, SSA isimportant to evaluate the structure of biochar for microbialattachment. The SSA of biochar samples produced from SD(15.3 m2/g–511.3 m2/g) and PS (7.5 m2/g–320.9 m2/g) were higherthan those produced from WB (4.2 m2/g–45.9 m2/g) and SS

SS7 PS3 PS5 PS7 WB3 WB5 WB7

48.2 37.9 29.8 25.1 33.9 25 20.917.1 59.2 64.2 64.2 64.9 65.5 69.49.2 32.7 26.8 13.8 24.1 18.6 13.173.7 8.1 9.0 22.1 11.0 15.9 17.526.3 91.9 91.0 77.9 89.0 84.1 82.50.28 0.19 0.21 0.17 0.20 0.18 0.148.2 22.4 19.1 16.1 22.0 16.4 14.59.23 8.19 9.29 9.74 7.37 10.11 10.3338.1 7.5 33.3 320.9 4.2 12.4 45.916.10 62.72 65.84 68.79 58.72 60.91 61.932.0 1.32 1.40 1.11 4.74 4.58 3.762.77 6.43 5.58 3.55 7.44 5.89 4.5520.95 19.21 14.86 12.70 17.67 13.91 12.888.1 47.5 49.1 53.0 12.4 13.3 16.50.17 0.10 0.08 0.06 0.13 0.10 0.071.30 0.31 0.23 0.22 0.30 0.23 0.21

Fig. 1. Relationship between HTT of biochar production and (a) pH of biochar solution, (b) ash content and (c) BET specific surface area.

76 G. Wang et al. /Waste Management 80 (2018) 73–80

(1.2 m2/g–38.1 m2/g) (Fig. 1(c)). This finding suggested that thelignin-enriched precursor biomass were more suitable feedstocksfor producing biochar with a high SSA. Although the ash contentsof the SS biochars were considerably higher than the other bio-chars, their pH was mostly lower, which demonstrated that theeffects of organic functional groups had a more significant influ-ence on the variability of pH than the inorganic ash contents. Theash contents of SS-derived were very high because sewage sludgecontains relatively high amounts of inorganic matter, including sil-ica (grits) and phosphate. For the other three feedstocks, organiclignocellulosic substances were the main components.

The ratios of H/C and O/C are two important indices for estimat-ing the degree of aromaticity and maturation of charcoal (Kookanaet al., 2011). In this study, the H/C ratio of biochar samplesdecreased with increasing HTTs, indicating a higher degree of aro-maticity and chemical stability after pyrolysis. Elemental analysisresults of the three lignocellulosic feedstock-derived biochar sam-ples revealed that C contents were higher with increased HTT, con-sistent with the fixed carbon content variation trends describedabove. The C contents of the SS biochar sample negatively corre-lated with HTTs because the ash content accounted for more thanhalf of the total biochar weight. SEM images of all twelve types ofbiochars are shown in the supplementary materials. SEM of the SD

and PS biochars exhibited many macropores and porous-holestructures, which are attributable to the high content of ligninand coarse cellulose. For the WB biochars, the micropore sizes ran-ged from nanometers to micrometers, with a narrow channelformed during the pyrolysis process. This size distribution likelyrelated to the structure and components of the WB feedstock. Onthe other hand, the SS biochar showed different apparent charac-teristics. The rough surface and irregular nanometer-sized microp-ores of the SS biochar resulted in a relatively low SSA (1.2–38.1 m2/g). This finding is consistent with that of previous studies (Jin et al.,2016b; Yuan et al., 2015).

3.1.3. Characteristics of FT-IR analysisThe FT-IR spectra for biochars derived from the various feed-

stocks at 300 �C, 500 �C and 700 �C are shown in the supplemen-tary materials. Descriptions of the different functional groups arealso shown in the supplementary materials. For the SD, PS, andWB feedstocks, as the HTTs increased from 0 �C (feedstock) to500 �C, the carbon contents transformed from aliphatic to aromaticforms (Keiluweit et al., 2010). Additionally, a high N content in SSwas evident by three sharp bands between 1600 cm-1 and1400 cm-1 due to the presence of protein-like substances such asamide, amine, and pyridine rings (Cantrell et al., 2012). At 700 �C,

Table 3Fitting results of hydrogen production process as influenced by different types ofbiochar to the modified Gompertz equation.

Biochar addition type t0 (day) Rmax (mL/day) P0 (mL/g VS) R2

SD3 3.9 ± 0.1 50.9 ± 2.7 66 ± 5 0.99

G. Wang et al. /Waste Management 80 (2018) 73–80 77

most C-derived functional groups of the three lignocelluloses-derived biochars disappeared and the spectra began to resemblethat of pure graphite. Overall, as the HTTs increased, part of the ali-phatic carbon in the feedstock was first transferred to aromaticcarbon and then formed a graphite-like structure.

SD5 3.5 ± 0.1 52.8 ± 2.3 66 ± 3 0.99SD7 2.9 ± 0.2 83.6 ± 4.5 81 ± 3 0.97SS3 3.0 ± 0.2 83.5 ± 3.2 65 ± 4 0.99SS5 2.4 ± 0.1 64.0 ± 5.4 76 ± 2 0.99SS7 2.4 ± 0.1 65.2 ± 4.3 73 ± 3 0.99PS3 2.8 ± 0.1 62.6 ± 4.5 68 ± 4 0.95PS5 2.6 ± 0.2 47.2 ± 5.8 65 ± 3 0.98PS7 1.8 ± 0.1 67.8 ± 2.7 40 ± 2 0.98WB3 3.4 ± 0.1 52.3 ± 3.9 76 ± 3 0.99WB5 2.8 ± 0.1 65.4 ± 3.3 76 ± 2 0.99WB7 2.8 ± 0.1 64.4 ± 5.3 81 ± 2 0.99Control 4.7 ± 0.1 39.8 ± 3.4 72 ± 3 0.99

3.2. Effects of different biochars on H2 production

The effects of different biochars on H2 production processeswere determined and are shown in Fig. 2. Compared with the con-trol group, biochar addition speeds up the startup of the H2 pro-duction process significantly. The modified Gompertz equationfitting results demonstrated (Table 3) that with biochar addition,the lag time decreased from 4.7 days to 1.8–3.9 days and the max-imum H2 production rate (Rmax) increased from 39.8 mL/day to47.2–83.6 mL/day. Furthermore, the addition of SD-, SS- and WB-derived biochars, at increasing HTTs, decreased the lag time weredecreased by 17.0–48.9% and accelerated Rmax by 27.9–109.8%.Moreover, the addition of biochars produced at high HTTs exhib-ited stronger promotion of H2 production. For the PS biochars,although the lag time was also somewhat decreased, the Rmax

decreased from 62.6 mL/day with the addition of PS3 to 48.2 mL/-day with the addition of PS5. Also, the addition of PS7 stronglyinhibited the H2 yield, to less than that of the control group.Nonetheless, apart from the group with PS7 added, the H2 produc-tion potential of the biochar-amended groups (65.3 mL/g VS-81.0 mL/g VS) showed no significant difference to that of the con-trol group (72.4 mL/g VS).

The pH of each group was measured after H2 production (Fig. 3).The 12 types of biochars showed pH buffering capacities in the H2

production system. In the control group, pH was 4.9 at the end of

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Fig. 2. Cumulative volume of hydrogen produced by the addition of different types of biPeanut shell-derived biochar; (d) Wheat bran-derived biochar.

the H2 production process, which likely caused by the rapid acidifi-cation of the main component of mixed substrate, the food waste.However, the pH of the biochar amended groups ranged from 5.13and 5.65, a more suitable range for H2 production (Chu et al.,2013; Lin et al., 2011). Higher pH was also beneficial for the stimu-lation of hydrogenase activity of H2-producing bacteria (Wonget al., 2014). Overall, these results show that the pH buffering capac-ity of biochar relieved the decrease in pH caused by VFAsaccumulation.

VFA types and yields are two critical parameters affecting fer-mentative H2 production (Barca et al., 2015). The background con-centration of acetate (4.02 mmol/L) in the substrate was the mainsource of VFAs before the start of the experiment. On day 2, most ofthe biochar-amended groups began to produce VFAs, whereas thecontrol group did not. Acetate was the main product for all groups

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ochar addition (a) Sawdust-derived biochar; (b) Sewage sludge-derived biochar; (c)

Fig. 3. pH after the addition of different types of biochar in hydrogen production.

78 G. Wang et al. /Waste Management 80 (2018) 73–80

except the SS7 group, which produced 0.86 mmol/L butyrate. Thus,it is believed that biochar addition stimulated VFAs production inthe H2 process.

For each feedstock, the biochar produced at the HTTs of 500 �Cand 700 �C exhibited greater effects on VFAs production. For exam-ple, on day 3.5, VFAs production was significantly higher in the SS5,SS7, and PS7 groups than the other groups. In addition, the SS5 andSS7 groups produced significant amounts of acetate and butyrateconcurrently with H2 production, whereas in the PS7 groups, propi-onate and valerate constituted 66.9% of the VFAs produced. Gener-ally, H2 production occurs via either butyrate-type fermentationpathway (Eq. (1)) or the acetate-type fermentation pathway (Eq.(2)), but the propionate-type fermentation pathway is an H2-consuming process (Eq. (3)). Therefore, the addition PS7 promotedVFAs production but suppressed H2 production. Moreover, acetateand butyrate productionwere considerably less in the control groupthan the biochar-amended groups on day 3.5. Nonetheless, whenH2

production was completed, the total VFAs yields of the control andthe biochar-amended groups were not significantly different, butthere were differences in the types of VFAs formed (Fig. 4). ForPS5, PS7, and control groups, acetate was the main VFA produced,and themolar ratio of acetate tobutyratewashigher than2, suggest-ing that the fermentative H2 production type in these three groupstended to occur via the acetate pathway. In the other groups, themolar ratio of acetate to butyrate was less than 2, which suggestedthat the butyrate pathwaywas the predominant fermentative path-way (Barca et al., 2015). Theoretically, the efficiency of the acetate-type fermentative pathway for H2 production is higher than that ofthe butyrate-type. However, homoacetogenesis also could consumeH2 and generate acetate (Eq. (4)) (Hawkes et al., 2007). Overall, theresults of this study indicate that the addition of some biochars pro-motes H2 production by shifting the fermentative H2 productionpathway from that of acetate to butyrate.

C6H12O6 + 2H2O ! 2CH3COOH + 2CO2 + 4H2 ð1Þ

C6H12O6 ! CH3CH2CH2COOH + 2CO2 + 2H2 ð2Þ

C6H12O6 + 2H2 ! 2CH3CH2COOH + 2H2O ð3Þ

4H2 + 2CO2 ! CH3COOH + 2H2O ð4Þ

3.3. Relationships between biochar properties and H2 production

Fig. 5(a) evaluated the relationship of the pyrolyzed tempera-ture of various biochar, the lag time and Rmax of H2 production.

Compared with the control, all the biochars produced under700 �C showed lower lag time and higher Rmax whatever the feed-stocks used, but part of biochars produced under lower pyrolytictemperature had a less promotion efficiency, which suggested thatcompared with the feedstock used for biochar preparation, the pyr-olytic temperature seemed to be a more dominant factor to influ-ence the promotion efficiency of fermentative H2 production.

To analyze the intrinsic driving factors of biochar addition forH2 production enhancement further, the PCA method was usedand the results are shown in Fig. 5(b). Three principal componentswere confirmed to explain over 75% of the total variance in H2 pro-duction process. The first principal component was explained byHTTs. Also, and there was a strong positive correlation betweenthe Rmax of H2 production and the pH of biochar. Conversely, thelag time of H2 production negatively correlated with the Rmax,which indicated that pH is as a crucial indicator of biochar to pro-mote fermentative H2 production, for all feedstocks tested. Thus,maintaining the pH buffering capacity was very important for thesteady operation of the anaerobic digestion systems. In this study,acidification occurred in the H2 production phase. The considerablyhigher pH maintained in all the biochar-amended groups than thecontrol group demonstrates the strong pH buffering capacities ofthe biochars.

Alkalis of biochars are both inorganic alkalis and organic alkalis.Carbonates are considered the major alkalis in the biochar pro-duced at higher HTTs (500 �C and 700 �C). By contrast, someorganic functional groups (ACOO– and AO–) contribute signifi-cantly to the alkalinity in biochar produced at lower HTTs(300 �C) (Yuan et al., 2011). The ash content of the SS-derived bio-chars (55.4–73.7%) were much higher than those of the other bio-chars derived from lignocellulosic feedstocks (5.3–22.0%) due tohigh contents of phosphorus salts, alkali metal salts, alkali earthsalts and even some heavy metals in SS (Zhang et al., 2015).Although these trace elements supplied adequate nutrients forthe microbes, the presence of some heavy metals also likely causedeco-toxicity to microorganisms. However, the addition of SS bio-chars did not exhibit inhibitory effects on biogas production, prob-ably due to the thermal pyrolysis treatment process (Jin et al.,2016a).

In this study, different HTTs and feedstock types resulted in bio-char with a wide range of SSA from 1.2 to 511.3 m2/g. The PCAresults showed no correlation between the SSA of biochar and Rmax,which suggested that SSA was not a critical parameter to influenceH2 production. In previous studies, the high SSA of biochar wasconsidered significant for the attachment of microbes, which isbeneficial to increase the biomass (Luo et al., 2015; Sunyotoet al., 2016). In this study, attachment of some microorganismsto the biochar surface was observed after H2 production (supple-mentary materials). However, although the SSA of the SD7 biocharwas notably higher than that of the SS biochar, its effect on the pro-motion of Rmax was insignificant. Thus, SSA should not be con-firmed as the primary parameter of biochar to promote anaerobicH2 production. Considering that a batch mode used in this study,the possibility remains that the high SSA of biochar could assistthe increase in biomass and enhance the H2 production efficiencyin a long-term or continuous operation condition.

Besides the factors analyzed above, the redox-active organicfunctional groups of biochar, such as the electron-shuttle of qui-none and phenol, was considered as an important property of bio-char likely to influence the anaerobic metabolic activity, eventhough it was difficult to quantify. The FT-IR results showed thatquinone and phenol (present at 1557–1567 cm-1 and 1416 cm-1)(Klüpfel et al., 2014) were enriched in the biochar produced at300 �C and 500 �C, whereas those produced at 700 �C showed feworganic functional groups (supplementary materials). Thus, theredox organic functional groups seemed not to be the main factor

Fig. 4. Cumulative VFAs concentration after the addition of different types of biochar addition at (a) Day 2, (b) Day 3.5, and (c) Day 7.

35

40

45

50

55

60

65

70

1 1.5 2 2.5 3 3.5 4 4.5 5

Max

imum

hyd

roge

n pr

oduc

tion

rate

(ml/d

ay)

Lag time of hydrogen production (days)

300ºc biochar addition

500ºc biochar addition

700ºc biochar addition

Control

(a) (b)

Fig. 5. (a) Effects of different biochar addition on the maximum hydrogen production rate and lag time; (b) Principal Components Analysis results of hydrogen production.

G. Wang et al. /Waste Management 80 (2018) 73–80 79

influencing fermentative H2 production. One possible reason isthat the common electro-active bacteria such as Geobacter andShewanella were not the dominant microbes in the fermentativesystem with complex bio-wastes as substrate.

Based on findings from the present study, a sustainable andintegrated bio-wastes management strategy is proposed. In thisstrategy, forest or farm wastes and municipal sewage sludge couldbe subjected to pyrolysis for biochar production. The derived

80 G. Wang et al. /Waste Management 80 (2018) 73–80

biochar could be added into an anaerobic digestion system to pro-mote biogas production. The digestate of the anaerobic digestioncontaining biochar can then be returned to the farmland as aneco-compatible fertilizer to enhance crop growth. This strategyallows the integration of cleaner energy production, reduction ofGHG emissions, and positive soil amendment.

4. Conclusions

The effects of the addition of different types of biochar on fer-mentative H2 production were elucidated. The type of feedstockand HTTs were the two crucial parameters affecting the yields,pH, ash content, and SSA of biochar. Furthermore, biochar additionshortened the lag time and increased the maximum H2 productionrate considerably. Despite the low specific surface area and highash contents of sewage sludge-derived biochar, it was alsoconfirmed as an effective additive for promoting H2 productionpromotion. Principal component analysis found positive correla-tion between the pH of biochar and the maximum H2 productionrate, which suggested that the strong pH buffering capacity of bio-char was critical to keep the stable pH during the VFAs accumulat-ing and H2 producing process. An integrated waste treatmentstrategy that combines pyrolysis and anaerobic digestion is pro-posed for sustainable bio-wastes recycling and management,although its feasibility in engineering-scale applications shouldbe evaluated in the future.

Acknowledgements:

This work was supported by National Natural Science Founda-tion of China (Grant No. 51608430), the Natural Science Founda-tion for Young Scientists of Xi’an University of Architecture andTechnology, China (Grant No. QN1615), and the Scientific ResearchProgram Funded by Shaanxi Provincial Education Department(Grant No. 17JS077). The authors have no conflict of interest todeclare.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at https://doi.org/10.1016/j.wasman.2018.08.042.

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