Bioresource Technology 140 (2013) 342–348

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Bioresource Technology

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Anaerobic digestion of crude glycerol from biodiesel manufacturingusing a large-scale pilot plant: Methane production and applicationof digested sludge as fertilizer

0960-8524/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.biortech.2013.04.020

⇑ Corresponding authors. Tel.: +81 229 84 7395; fax: +81 229 84 7391.E-mail addresses: tada@bios.tohoku.ac.jp (C. Tada), nakai@bios.tohoku.ac.jp

(Yutaka Nakai).1 Research Fellow of the Japanese Society for the Promotion of Science (JSPS), Japan.

Yasunori Baba a,1, Chika Tada a,⇑, Ryoya Watanabe a, Yasuhiro Fukuda a, Nobuyoshi Chida b,Yutaka Nakai a,⇑a Laboratory of Sustainable Environmental Biology, Graduate School of Agricultural Science, Tohoku University, Yomogida 232-3, Naruko-onsen, Osaki, Miyagi 989-6711, Japanb Chida Seisou Ltd., Nishida 77, Furukawa-kitsunezuka, Osaki, Miyagi 989-6254, Japan

h i g h l i g h t s

�Methane was produced from crude glycerol using a large-scale pilot plant.� Output energy exceeded input energy at a loading rate of 1 ml glycerol/L-day.� It was possible to recover methane productivity after a power failure for 1 month.� Grass yield increased 1.2 times by applying the digested sludge as liquid manure.

a r t i c l e i n f o

Article history:Received 21 February 2013Received in revised form 3 April 2013Accepted 5 April 2013Available online 13 April 2013

Keywords:Crude glycerolMethane productionPilot plantEnergy balanceDigested sludge

a b s t r a c t

This report is the first to consider methane production energy balance from crude glycerol at a practicalrather than a laboratory scale. Crude glycerol was added to the plant progressively at between 5 and 75 Lglycerol/30 m3-day for 1.5 years, and the energy balance was positive at a loading rate of 30 L glycerol/30 m3-day (1 ml/L-day). At this loading rate over one year, an energy output equivalent to 106% of theenergy input was achieved. The surplus energy was equivalent to transport for 1200 km, so the properfeedstock-transportation distance was within a 12.5-km radius of the biogas plant. In addition, thedigested sludge contained fertilizer components (T-N: 0.11%, P2O5: 0.036%, K2O: 0.19%) that increasedgrass yield by 1.2 times when applied to grass fields. Thus, crude glycerol is an attractive bioresource thatcan be used as both a feedstock for methane production and a liquid fertilizer.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Biodiesel is attracting attention as a substitute for fossil fuels(Siles et al., 2010). In Japan, for example, the Great East JapanEarthquake in March 2011 destroyed a number of petrochemicalcomplexes, and fuel shortages for vehicles became a serious prob-lem in the disaster area. Biodiesel fuel can be produced from usedcooking oil, so this fuel can be supplied even under such circum-stances and contribute to recovery efforts. Biodiesel is producedfrom vegetable oils through transesterification with methanol,which is catalyzed by KOH. In general, 10 kg of crude glycerol isgenerated as a by-product for every 100 kg of biodiesel produced

(Chi et al., 2007). In other words, the production of biodiesel gen-erates approximately 10% glycerol by volume (Leoneti et al.,2012). Glycerol is an important platform chemical (Robra et al.,2010) that has more than 2000 different applications (Siles Lópezet al., 2009) owing to its unique properties. However, the majorityof industrial products use only purified glycerol as a raw material,and consequently crude glycerol is often burned as a waste prod-uct. Even at current production levels, crude glycerol createswaste-disposal problems (Johnson and Taconi, 2007), and in thenear future, crude glycerol is expected to be produced in largeamounts, with negative impacts on the environment. Thus, thereis an urgent need to convert crude glycerol into more valuableproducts. Several methods have already been evaluated (Fanet al., 2010; Leoneti et al., 2012). For example, products such as1,3-propandiol (Nemeth and Sevella, 2008), hydrogen (Sabourin-Provost and Hallenbeck, 2009), ethanol (Oh et al., 2011), docosa-hexaenoic acid (DHA) (Chi et al., 2007), polyesters (Ashby et al.,

Y. Baba et al. / Bioresource Technology 140 (2013) 342–348 343

2004), and lipid (oleic and linoleic acids) (André et al., 2010) havebeen obtained from crude glycerol. Studies have also reported thatcrude glycerol can be used as feed for animals such as pigs and cat-fish (Li et al., 2010; Schieck et al., 2010).

Anaerobic digestion is one of the most useful methods to con-vert crude glycerol into flammable gas (methane gas). Severalstudies have examined methane production from crude glycerol(Astals et al., 2012; Castrillón et al., 2011; Siles et al., 2010; SilesLópez et al., 2009). However, most experiments have been of thebatch type and at a laboratory scale. Although previous studieshave reported continuous methane production, the experimentswere performed only with small-scale reactors of 3–5 L over lessthan 1 year (Nakamura et al., 2008; Robra et al., 2010). To designa practical biogas plant, its operation must be evaluated through-out the year using a pilot plant. It is important that the energy bal-ance is calculated based on actual input/output energy values. Inaddition, consideration of the uses of the digested sludge isimportant.

In the present study, methane was produced from crude glycerolderived from biodiesel manufacturing using a 50 m3 pilot plant forover 1 year. The energy balance of the methane production systemwas calculated by monitoring input and output energy values. Dis-posal of digested sludge after methane production is a major obsta-cle in implementing a biogas plant. Therefore, it is important toexamine both methane production and digested sludge applica-tions, but few studies have evaluated these two processes together.In this study, the effect of digested sludge manure derived frommethane production was verified by applying it to grassland. Themaximum daily loading amount of crude glycerol was determinedand background information on the energy balance and applicationof digested sludge was obtained. This is the first report on the valid-ity of methane production from crude glycerol combined withapplication of digested sludge as liquid manure based on calculat-ing actual energy values using a large-scale pilot plant.

2. Methods

2.1. Substrate

Crude glycerol and excess sludge were used as substrates formethane production. The crude glycerol was derived from biodie-sel manufacturing based on used cooking oil and was provided byChida Seisou Ltd. (Miyagi, Japan). The glycerol contained 47 ± 8.6%pure glycerol, 11.3 mg/L P2O5, and 20.0 g/L K2O. The chemical oxy-gen demand (COD) of the crude glycerol was 1477 ± 235 g/L. Thetotal solids (TS), volatile total solids (VS), and density were 79.5%,72.1%, and 1.02 g/cm3, respectively.

As nitrogen was not included in the crude glycerol, excesssludge derived from the wastewater processing plant of a noodlefactory and provided by Shimadaya Corporation (Miyagi, Japan)was used as a co-fermentation substrate. The sludge contained540 mg/L total nitrogen (T-N), 99.8 mg/L NH4-N, 150 mg/L P2O5,and 69 mg/L K2O. The COD of the sludge was 17.4 ± 3.6 g/L.

2.2. Methane production

Methane was produced in a 50-m3 pilot plant (30-m3 workingvolume) at the Field Science Center, Graduate School of Agricul-tural Science, Tohoku University (38�450N, 140�450E) located inOsaki, Miyagi, Japan. In total, 1000 kg of fresh cattle manure,20 kg of CH3COONa, 20 kg of glucose, 20 kg of NH4Cl, 160 g of KH2-

PO4, 250 g of CaCl2�2H2O, 250 g of MgCl2�6H2O, 300 g of Fe-EDTA,50 g of CoCl2�6H2O, 50 g of NiCl2�6H2O, 50 g of MnCl2�4H2O, and500 g of yeast extract were added to 10 m3 of water (Yang et al.,2004); this mixture was then used as seed sludge to initiate the

experiments. Next, 20 m3 of excess sludge were added to the seedsludge, resulting in a 30-m3 working volume. Semicontinuousmethane production was performed at 35 �C for 1.5 years. Once aweek, 3.5 m3 of excess sludge were added to the plant and a corre-sponding quantity of effluent was removed. Therefore, methaneyield and biodegradability were calculated weekly. After 120 days,crude glycerol loading was initiated and was increased progres-sively from 5 L/30 m3-day to 10 L/30 m3-day, 20 L/30 m3-day,30 L/30 m3-day, and 75 L/30 m3-day. Although the experimentwas temporarily halted from days 250 to 294 because of the GreatEast Japan Earthquake, it was resumed afterwards. After approxi-mately 1 year (day 372), one-third amounts of the above nutrients(excluding CH3COONa and glucose) were added to the plant tomaintain gas productivity. Input energy (such as electric powerand heating oil), output energy (biogas), and atmospheric temper-ature were continuously monitored during the test period. Crudeglycerol was loaded on weekdays and was not loaded on holidays(Saturday, Sunday). Theoretical methane yields were calculatedbased on the COD; typically, 350 ml of methane is produced from1 g of COD (Fox and Noike, 2004; McCarty, 1964).

2.3. Field experiment: fertilizing grass fields with digested sludge

The field experiment was performed from August to October2011 at the Field Science Center of Tohoku University, using a grassfield in third flush. Six plots (10 m � 10 m) were delineated, andfertilized and non-fertilized plots were established in triplicate.Orchard grass, reed canary grass, and various weeds grew in thefield. The digested sludge was collected from the storage tankand applied as fertilizer to the field by a sanitation truck equippedwith tanks and a vacuum hose.

To apply the recommended amount of fertilizer (1.67 kg of T-N,0.62 kg of P2O5, 1.24 kg of K2O), sludge with an equivalent of 2.2 kgof T-N, 0.72 kg of P2O5, and 3.8 kg of K2O was applied. After3 months, three 4-m2 grass samples were harvested in each plotand oven-dried at 70 �C for 7 days to determine the total dry mat-ter yield (biomass) and the components.

2.4. Analysis

Biogas (CH4, CO2, H2) was measured using a gas chromatograph(GC-8A; Shimadzu, Kyoto, Japan) with an inject/detect tempera-ture of 100 �C. A packed column (Shincarbon-ST) was used withnitrogen as the carrier gas, and the unit was equipped with a ther-mal conductivity detector connected to an integrator (C-R8A; Shi-madzu). Volatile fatty acid (VFA) concentrations were analyzedusing a high-performance liquid chromatograph (JASCO, Tokyo, Ja-pan) equipped with an ion-exchange column (RSpak KC-811; Sho-dex, Tokyo, Japan) working at 60 �C using 3 mM HClO4 as eluent ata flow rate of 0.8 ml/min and a UV detector (870-UV; JASCO). CODwas measured using a colorimetric method (Jirka and Carter, 1975)with Hach 0–1500-mg/L vials. Measurements of T-N, P2O5, and K2Owere conducted using the Kjeldahl (Eaton, 1992), ammoniumvanadomolybdate (Williams, 1984), and flame photometric (Wil-liams, 1984) methods, respectively.

3. Results and discussion

3.1. Methane yield and biodegradability of organic matter

As shown in Table 1, crude glycerol was added to 30 m3 of seedsludge progressively from 5 L glycerol/30 m3-day (0.17 ml/L-day)to 75 L glycerol/30 m3-day (2.5 ml/L-day). Excess organic sludgewas added to the seed sludge as a co-fermentation substrate at aratio of 3.5 m3 excess sludge/30 m3-week during the test period.

Table 1Methane yields and removal efficiencies under different glycerol load conditions.

Crude glycerol load (L/30 m3 sludge in fermenter-day) 5 10 20 30 75Organic sludge load (m3/30 m3 sludge in fermenter-week) 3.5 3.5 3.5 3.5 3.5Methane production (m3/30 m3 sludge in fermenter-week) 16.2 ± 2.2 47.0 ± 8.1 83.1 ± 8.4 141.3 ± 9.7 134.7Methane yield (%)a

CH4/TCOD removed 42.6 ± 12.3 67.3 ± 19.7 81.3 ± 17.6 102.4 ± 17.9 81.6CH4/TCOD added 5.4 ± 1.8 19.2 ± 7.3 28.7 ± 6.5 42.6 ± 5.9 27.5

Removal efficiency (%)Total COD 5.9 ± 2.4 28.5 ± 9.7 35.7 ± 6.9 39.1 ± 3.3 33.7Dissolved COD 84.4 ± 7.4 85.9 ± 6.5 84.7 ± 4.1 90.4 ± 3.1 80.9

a Theoretical methane yields were calculated based on the COD; 350 ml of methane is produced from 1 g of COD.

344 Y. Baba et al. / Bioresource Technology 140 (2013) 342–348

As a result, methane production increased with increasing quanti-ties of crude glycerol load; the highest methane production(141.3 m3 CH4/30 m3-week) was obtained at 30 L glycerol/30 m3-day (1.0 ml/L-day). At this loading ratio, the methane yield basedon the COD was 102.4% for total COD (TCOD) removed and 42.6%for TCOD added. On the other hand, the methane yield of 75 L glyc-erol/30 m3-day (2.5 ml/L-day) decreased compared to 30 L glyc-erol/30 m3-day (1.0 ml/L-day). In addition, the crude glycerolloading rate of 75 L glycerol/30 m3-day ended after 1 week becausethe pH decreased significantly. In a previous study (Robra et al.,2010), a glycerol loading rate of 1.3 g VS/L-day was achieved,which corresponds to approximately 1.8 ml/L-day when convertedto the crude glycerol loading rate based on VS. The crude glycerolloading rate did not surpass 1 ml/L-day (30 L glycerol/30 m3-day)in this study, possibly because the amount of glycerol loading in-creased suddenly. The potential for a higher crude glycerol loadingrate remains if the crude glycerol load is increased more gradually.TCOD and dissolved COD (dCOD) removal efficiencies increasedwith increasing glycerol loading rates, particularly the latter. Sincethe dCOD of the excess sludge used in this study was low (0.3–0.4 g/L), while that of the crude glycerol was high (1400–1600 g/L), the dCOD in the fermenter reflected the amount of crude glyc-erol. This suggests that the excess sludge was a persistent sub-stance whereas the crude glycerol was a degradable substrate.

Previous studies of methane production from crude glycerol aredescribed in Table 2. All of these studies, including batch and

Table 2References for methane production from crude glycerol.

Author Batch/continuous

Workingvolume

Time

Siles et al. (2010) Batch 1 L 48 h

Siles López et al. (2009) Batch 1 L 50–60 h

Nakamura et al. (2008) Batch 1 L 22 daysCastrillón et al. (2011) Batch 1.75 L 40 daysCastrillón et al. (2011) Batch 1.75 L 40 daysRobra et al. (2010) Continuous 3 L 10 weeksAstals et al. (2012) Continuous 4 L 196 daysNakamura et al. (2008) Continuous 5 L 60 daysThis study Continuous 30 m3 1.5 years

Author Glycerol concentration (%) COD

Siles et al. (2010) No data 1054 g COD/L

Siles López et al. (2009) No data 1010 g COD/kg

Nakamura et al. (2008) 58 1217 g COD/kgCastrillón et al. (2011) 49.4 837 g COD/kgCastrillón et al. (2011) 49.4 837 g COD/kgRobra et al. (2010) 56–60 No dataAstals et al. (2012) No data No dataNakamura et al. (2008) 58 1217 g COD/kgThis study 47 1477 g COD/L

continuous tests, were at a laboratory scale (1–5 L) and the testingperiod was less than 1 year. To design a practical biogas plant, pi-lot-scale testing at the site of the planned biogas plant throughoutat least 1 year is necessary to examine the project’s validity. This isthe first report on methane production from crude glycerol using alarge-scale pilot plant for more than 1 year. In addition, this studycovered all upstream (methane production) and downstream(application of digested sludge as liquid manure) disposal pro-cesses of the crude glycerol. The maximum daily crude glycerolloading rate is an important factor in methane production effi-ciency. Nakamura et al. (2008) performed continuous methaneproduction from crude glycerol and obtained the maximum meth-ane yield at a loading rate of 1.2 g COD glycerol/L/day. The opti-mum loading rate in the present study was 30 L glycerol/30 m3-day (1.0 ml/L-day), which corresponds to 1.48 g COD/L-day and isroughly in accordance with the results of Nakamura et al. (2008).Previous kinetics studies reported that the methane yield coeffi-cient of crude glycerol was 310 ml CH4/g COD-removed (co-fer-mentation substrate was wastewater derived from biodieselmanufacturing) or 356 ml CH4/g COD-removed (co-fermentationsubstrate was sludge from brewery wastewater) (Siles et al.,2010; Siles López et al., 2009). Castrillón et al. (2011) producedmethane from crude glycerol with cattle manure and obtained211 ml-CH4/g COD-removed (mesophilic fermentation) and348 ml CH4/g COD-removed (thermophilic fermentation). In thepresent study, 358 ml CH4/g COD-removed was achieved at a

Optimum crude glycerol loading Maximum CH4 (or biogas) yield

Kinetics study 310 ml CH4/g COD removed (methane yieldcoefficient)

Kinetics study 356 ml CH4/g COD removed (methane yieldcoefficient)

10 g COD/L 560 ml biogas/g COD removed40 ml/L 211 ml CH4/g COD removed60 ml/L 348 ml CH4/g COD removed1.3 g VS/L-day 825.7 ml biogas/g VS added1.275 g VS/L-day 780 ml biogas/g VS added1.2 g COD/L-day 276 ml CH4 (520 ml biogas)/g COD added30 L-day (1.48 g COD/L-day) 358 ml CH4/g COD removed, 149.1 ml CH4/g

COD added

Co-fermentation feedstock Temperature (�C)

Waste water derived from BDFmanufacturing

35

Granular sludge from brewerywastewater

35

Food waste 55Cattle manure 35Cattle manure 55Cattle manure 35Pig manure 35Food waste 55Excess activated sludge 35

Y. Baba et al. / Bioresource Technology 140 (2013) 342–348 345

loading rate of 30 L glycerol/30 m3-day (1.0 ml/L-day), whichequals or surpasses the results of previous studies.

3.2. pH and VFA

Anaerobic digestion consists of two steps, acid fermentationand methane production, which must be balanced. Excessive acidfermentation causes the accumulation of VFA and inhibits methaneproduction. Therefore, VFA and pH were measured to assess thestate of the pilot plant during the testing period (Fig. 1). An inocu-lum acclimatization period (period I) was performed using excesssludge without crude glycerol. During this period, a pH of 7 orhigher was maintained, and VFA accumulation was not observed.However, after glycerol loading (day 127), pH decreased and aceticacid, propionic acid, and butyric acid were detected. During thisperiod, the heat regulator did not function properly. As a result,the actual temperature in the pilot plant was less than 35 �C (Thiswas corrected at day 200). This is likely to be why the pH decreasedin spite of a small amount of glycerol loading (period II: 5 L glyc-erol/30 m3-day). Although plant operation was suspended fromdays 250 to 294 because of the Great East Japan Earthquake, load-ing of crude glycerol was resumed after day 295. Subsequently,acetic acid, propionic acid, and butyric acid accumulated, but theiraccumulation decreased 2–3 weeks afterwards. Thus, even thoughheating and feedstock loading were not performed for approxi-mately 1 month, methanogenesis could be resumed. This is oneof the advantages of anaerobic digestion. In general, it is difficultto recover methane productivity when methanogenic reactors arehalted because of VFA accumulation (McMahon et al., 2001). How-ever, methanogenic reactors that include no excess VFA may

Fig. 1. pH (A) and VFA production (B) over 1.5 years of methane production.Periods: (I) 0 L glycerol/30 m3-day (0 ml/L-day), (II) 5 L glycerol/30 m3-day(0.17 ml/L-day), (III) 10 L glycerol/30 m3-day (0.33 ml/L-day), (IV) 20 L glycerol/30 m3-day (0.67 ml/L-day), (V) 30 L glycerol/30 m3-day (1 ml/L-day), and (VI) 75 Lglycerol/30 m3-day (2.5 ml/L-day). Asterisk (⁄): Nutrients were added to the plant.

remain stable if halted for a certain period of time. Romano andZhang (2008) examined the effect of power failure on methanogen-ic reactors and found that halted reactors could resume biogas pro-duction at the same level as before the power failure, even whenthe failure caused the pumps and water-jacket heater to be inoper-able for 3 days. Furthermore, the present study shows that it ispossible to recover methane productivity, even if heating and feed-stock loading are not performed for 1 month.

After the addition of nutrients (day 372), VFA accumulation de-creased and the pH increased. During the test period, although thepH dropped significantly after changes in the conditions (e.g., afterchanging the crude glycerol loading rate and after resuming follow-ing the earthquake), it tended to recover and stabilize after a fewdays. However, during period VI (75 L glycerol/30 m3-day), 2000–2500 mg/L of propionic acid accumulated and the pH value de-creased to 6.50. As a result, methane production decreased. Althoughthe pH increased with subsequent suspension of glycerol loading,methane productivity was not restored. In addition, isovaleric acidwas detected only during period VI. This study clarified that methaneproductivity is decreased by VFA (propionic acid in particular) accu-mulation at loading rates greater than 30 L glycerol/30 m3-day (i.e.,1 ml glycerol/L-day; 1.48 g COD glycerol/L-day).

3.3. Energy balance

During the test period, the required input energy (stirring,circulation pump, boiler, heating oil) and output energy (biogasproduction) were monitored automatically. To calculate the energybalance, power consumption, heating oil, and methane were

Fig. 2. Energy balance (A) and average temperature (B) per month. Electric power,heating oil, and methane gas were converted to thermal units (electric power,3.6 MJ/kwh; heating oil, 36.7 MJ/L; methane gas, 37.2 MJ/m3). Asterisk (⁄): Energy ofthe methane produced at a loading rate of 30 L glycerol/30 m3-day.

346 Y. Baba et al. / Bioresource Technology 140 (2013) 342–348

converted into thermal units (power consumption: 3.6 MJ/kWh,heating oil: 36.7 MJ/L, methane: 37.2 MJ/m3). Monthly inputenergy and output energy values (thermal units of methane gener-ated at a loading rate of 30 L glycerol/30 m3-day) are shown inFig. 2A. Heating energy accounted for most of the input energy.The consumption of energy for heating was associated with theaverage temperature and was higher during the winter when tem-peratures were low. As a result, input energy surpassed output en-ergy in December, January, February, and March. The averagetemperature for these months was less than 0 �C (Fig. 2B). Nakay-ama et al. (2006) investigated the correlation between energybalance and temperature based on 16 years of data for a biogasplant in a cold district (Hokkaido) of Japan, similar to the site inthis study. The annual energy balance correlated closely with theaverage temperature for autumn through winter (October throughMarch), which was less than 0 �C, similar to the temperature in thisstudy. Thus, an average temperature of 0 �C can stress the energybalance of a biogas plant. The energy balance will be significantlyimproved if a plant is well insulated or perhaps buried in theground.

Fig. 3 shows annual energy balances. Although input energy ex-ceeded output energy at loading rates of 5, 10, and 20 L glycerol/30 m3-day, output energy exceeded input energy at a loading rateof 30 L glycerol/30 m3-day (1 ml/L-day). The surplus energy was13.3 GJ/year, sufficient for approximately 1200 km based on thefuel consumption of the sanitation truck that carried the feed-stock/digested sludge in this study (fuel consumption = 3.5 km/Llight diesel oil; thermal unit = 38.2 MJ/L light diesel oil). Therefore,at this site, if methane production is performed at a loading rate of30 L glycerol/30 m3-day (1 ml/L-day) and feedstock/sludge is car-ried less than 1200 km/year, the energy balance will be positive.In other words, if feedstock transportation is performed weekly,feedstock must be collected from less than a 12.5-km {1200 km/[4 week � 12 month � 2 (round trip)]} radius around the biogasplant. Berglund and Börjesson (2006) also considered the output/input energy ratio for methane production from domestic animalmanure and reported that the manure should be transported nomore than 10 km to the biogas plant. Similarly, for methane pro-duction from industrial hemp, Prade et al. (2012) reported thatthe proper transportation distance of ‘‘field (hemp) ? biogasplant ? field (digested sludge)’’ was 15 km. From Danish and UKinvestigations regarding the transport of farm livestock manureto a biogas plant, Dagnall et al. (2000) reported that high-mois-ture-content feedstocks (dry matter <10%) are currently collected

Fig. 3. Energy balance pe

from no more than a 10-km radius around a plant, whereas low-moisture-content (dry matter >70%) feedstocks are gathered fromlocations roughly within a 40-km radius. Based on these previousstudies, the pilot test data (less than 12.5-km radius) appear tobe accurate.

In this study, recovery of heat from the sludge was not per-formed. With the implementation of heat recovery and additionof effective insulation, the energy balance has the potential to bepositive even at a rate of less than 30 L glycerol/30 m3-day.

3.4. Application of digested sludge as liquid manure

The disposal of digested sludge after methane production is oneof the obstacles to implementing a biogas plant. If digested sludgeis discharged into rivers, wastewater treatment processes that re-quire large amounts of energy and expense are required to reducethe negative effects of the sludge on the river environments. On theother hand, digested sludge contains nitrogen, phosphorus, andpotassium and can thus be used as liquid manure. Although severalstudies have explored the application of digested sludge as liquidmanure after methane production from domestic animal manure(Jia et al., 2012; Li et al., 2003; Matsunaka et al., 2003; Nakamura,2011), no studies have examined the application of digested sludgeas liquid manure after methane production from crude glycerol. Inthis study, digested sludge was applied as fertilizer to grassland,and the effects were evaluated. As shown in Table 3, althoughthe T-N, P2O5, and K2O levels in the digested sludge were equalto those in digested sludge derived from domestic animal manure,NH4-N levels were slightly lower, because the NH4-N levels in theexcess sludge were low. In addition, no NH4-N was included in thecrude glycerol used in this study.

To follow the recommended rates of fertilizer application(1.67 kg T-N, 0.62 kg P2O5, 1.24 kg K2O), sludge with the equivalentof 2.2 kg T-N, 0.72 kg P2O5, and 3.8 kg K2O was applied. The P2O5

concentrations in this digested sludge were lower than those ofT-N and K2O (Table 3). Based on the levels of fertilization used tomeet the recommended rates of P2O5, T-N and K2O exceeded rec-ommended rates. The grass yield of the fertilized plots was 1.2times higher than that of the non-fertilized plots (Table 4). TheT-N and P2O5 contents of the grass in the fertilized plots increasedslightly compared to the non-fertilized plots (control), whereas theK2O content of the grass in the fertilized plots increased signifi-cantly compared to the control (1.9 times). Typically, grass can ab-sorb only inorganic ions. In general, whereas N and P are organic in

r crude glycerol load.

Table 3Comparison of digested sludge components after methane production under different conditions.

Feedstock pH T-N (%) NH4-N (%) P2O5 (%) K2O (%) References

Chickena 7.5 0.19 -h 0.41 0.16 Jia et al. (2012)Cattleb 7.8 0.35 0.15 0.08 0.36 Matsunaka et al. (2003)Cattleb, vegetablec 7.7 0.35 0.177 0.122 0.39 Nakamura (2011)Cattleb, pigd, Foode 8.6 -h 0.2 0.11 0.21 Li et al. (2003)Pigd 7.5–8.5 0.07–0.39 0.06–0.37 0.008–0.062 -h Chen et al. (2012)Pigd, glycerolf 7.8–8.1 0.14–0.19 0.10–0.14 N.D.i 0.34 Astals et al. (2012)Glycerolf, sludgeg 6.8–7.0 0.11 0.027 0.036 0.19 This study

a Chicken dung.b Cattle manure.c Vegetable waste.d Pig manure.e Food waste.f Crude glycerol.g Excess sludge.h Unrecorded.i Not detected.

Y. Baba et al. / Bioresource Technology 140 (2013) 342–348 347

biosolids such as sludge and compost, K is an inorganic ion. Thus, Kis absorbed more rapidly than organic N and P, which require min-eralization reactions prior to uptake. Organic N and P show fertil-izer effects by mineralization in the following year (Warman andTermeer, 2005). The uptake efficiencies of the digested sludge com-ponents by the grass were calculated according to the followingequation and corresponded to 7.6% (T-N), 9.3% (P2O5), and 29.9%(K2O):

Uptake efficiency ð%Þ ¼ fertilization plot uptake�control uptaketotal amount of nutrient applied

ð1Þ

Warman and Termeer (2005) applied anaerobic septic sludge to Zeamays L. and determined that T-N and P2O5 uptake efficiencies were10% and 8%, respectively, similar to the results of this study. Matsu-naka et al. (2003) applied digested sludge from cattle manure toorchard grass (Dactylis glomerata L.) and determined that T-N up-take efficiency was 20.3%. In Matsunaka et al.’s (2003) study, inor-ganic N (NH4-N) accounted for 42.9% of T-N, which explains whyT-N uptake was more efficient in the grass in their study than inthe present study (rate of NH4-N uptake: 24.5%). Therefore, formethane production from crude glycerol, it may be preferable toco-ferment with a feedstock containing inorganic N when crudeglycerol digested sludge is applied as liquid manure. In addition,heavy metals were not detected (or were within accepted limits)in the digested sludge in this study and it was thus considered safeto apply to grassland.

4. Conclusions

Methane was produced from crude glycerol by a pilot plant formore than 1 year. The optimal methane yield was achieved at aloading rate of 1 ml glycerol/L/day, which resulted in 358 mlCH4/g COD removed and 149.1 ml CH4/g COD added; thus, outputenergy (methane) exceeded input energy. The surplus energy wassufficient to power a sanitation truck for approximately 1200 km/year. The energy balance of the system was positive provided thatfeedstock/sludge was transported only within this range. By

Table 4Grass yields and components with and without fertilization.

Without fertilization With fertilization

Grass yield (kg-dry/a) 17.7 ± 0.7 21.0 ± 1.3

Grass componentT-N (%) 3.2 ± 0.3 3.5 ± 0.7P2O5 (%) 1.4 ± 0.1 1.5 ± 0.1K2O (%) 5.2 ± 0.2 9.8 ± 2.0

applying the digested sludge as liquid manure, grass yield im-proved by 1.2-fold.

Acknowledgements

This work was financially supported by the Japan Society for thePromotion of Science (JSPS), Miyagi Prefectural Government Recy-cling Promotion Division, and the Miyagi Organization for IndustryPromotion. The authors thank Chida Seisou Ltd. (Miyagi, Japan) forsupplying the crude glycerol and transporting the sludge. Theauthors also thank Shimadaya Corporation (Miyagi, Japan) for sup-plying the excess sludge. The authors wish to express their pro-found thanks to Mr. Hiroshi Chubachi, Field Science Center ofTohoku University, for the technical support they offered duringthe collection of the grass.

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