Waste Management xxx (2014) xxx–xxx

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Waste Management

journal homepage: www.elsevier .com/locate /wasman

Increasing biogas production from sewage sludge anaerobic co-digestionprocess by adding crude glycerol from biodiesel industry

http://dx.doi.org/10.1016/j.wasman.2014.08.0170956-053X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +1 810 249 4041.E-mail addresses: snartker@kettering.edu (S. Nartker), mammerman@kettering.

edu (M. Ammerman), jaurandt@valicor.com (J. Aurandt), mstogsdi@kettering.edu(M. Stogsdil), Olivia.Hayden@swedishbiogas.com (O. Hayden), Chad.Antle@swedishbiogas.com (C. Antle).

Please cite this article in press as: Nartker, S., et al. Increasing biogas production from sewage sludge anaerobic co-digestion process by adding cruderol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.08.017

Steven Nartker a,⇑, Michelle Ammerman a, Jennifer Aurandt a, Michael Stogsdil a, Olivia Hayden b,Chad Antle b

a Kettering University, 1700 University Avenue, Flint, MI 48504, USAb Swedish Biogas International, 1300 Bluff Street, Flint, MI 48504, USA

a r t i c l e i n f o

Article history:Received 3 June 2014Accepted 19 August 2014Available online xxxx

Keywords:GlycerolAnaerobic digestionSewage sludgeBiogasCo-digestion

a b s t r a c t

In an effort to convert waste streams to energy in a green process, glycerol from biodiesel manufacturinghas been used to increase the gas production and methane content of biogas within a mesophilic anaer-obic co-digestion process using primary sewage sludge. Glycerol was systematically added to the primarydigester from 0% to 60% of the organic loading rate (OLR). The optimum glycerol loading range was from25% to 60% OLR. This resulted in an 82–280% improvement in specific gas production. Following the feed-ing schedule described, the digesters remained balanced and healthy until inhibition was achieved at 70%glycerol OLR. This suggests that high glycerol loadings are possible if slow additions are upheld in order toallow the bacterial community to adjust properly. Waste water treatment plant operators with anaerobicdigesters can use the data to increase loadings and boost biogas production to enhance energy conver-sion. This process provides a safe, environmentally friendly method to convert a typical waste streamto an energy stream of biogas.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Anaerobic digestion (AD) is a multistage process in whichmicroorganisms breakdown biodegradable material in the absenceof oxygen. The AD process starts with hydrolysis of the inputmaterials. During this phase, bacteria convert insoluble organicpolymers such as carbohydrates to soluble derivatives. Thenacidogenic bacteria convert the soluble sugars and amino acidsinto carbon dioxide, hydrogen, ammonia and organic acids. Nextacetogenic bacteria create acetic acid, ammonia, carbon dioxideand hydrogen from the fermentation products of the previous step.Finally methanogens convert the products of acidogenesis andacetogenesis to methane (Grady Jr et al., 2011). AD is a practicalmethod for degrading and stabilizing primary sewage sludge priorto its disposal (Fountoulakis et al., 2010). In order for a sewage ADfacility to operate more economically and efficiently, theproduction of biogas must increase, which can be achieved througha co-digestion process. Anaerobic co-digestion (AcoD) consistsof a mixture of two or more substrates with complementary

characteristics, and has proven to be a reliable option for increas-ing methane yield (Holm-Nielsen et al., 2008). When sewagesludge is combined with highly concentrated organic co-sub-strates, such as food waste, agricultural waste or crude glycerolfrom the biodiesel industry, biogas output and organic matterremoval can be improved without sacrificing reactor stability orhealth (Astals et al., 2011). The AD process is a green method thatcan sustainably convert waste to energy.

The main by-product of biodiesel production is crude glycerol,which is about 10% by weight of the starting materials. Crude glyc-erol is a mixture of glycerol, alcohol, water, salts, heavy metals, freefatty acids, unreacted mono-, di- and tri-glycerides and methylesters (Hu et al., 2012). Co-digestion of glycerol with sewage sludgeis a promising solution, since a renewable source of energy isobtained from the treatment. Several successful studies, in batchand continuously stirred reactor experiments, have been publishedwith reference to the benefits of the addition of glycerol to enhancethe AD of agro-wastes (Amon et al., 2006; Kacprzak et al., 2009),cattle manure (Chen et al., 2008; Robra et al., 2010) fruit and vege-table wastes (Astals et al., 2011; Bouallagui, 2003) organic fractionof municipal solid waste – OFMSW (Fountoulakis and Manios,2009), pig manure (Alvarez et al., 2010; Amon et al., 2006; Astalset al., 2011; Galí et al., 2009), sewage sludge (Fountoulakis et al.,2010), mixture of pig manure and OFMSW (Schievano et al.,

e glyc-

2 S. Nartker et al. / Waste Management xxx (2014) xxx–xxx

2009), mixture of olive mill and slaughterhouse wastewaters(Fountoulakis and Manios, 2009) and mixture of manure andorganic industrial wastes (Holm-Nielsen et al., 2009). The amountsare variable and depend on the quality of the feed material and thechemical process used to obtain the biodiesel (Pagliaro et al., 2008;Robra et al., 2010). In certain markets, crude glycerol can be solddepending on purity and availability (Johnson and Taconi, 2007).In other markets, the glycerol has to be disposed of as waste dueto market saturation; excessive treatment or refining costs(Pachauri and He, 2006); and lack of direct uses (Pagliaro et al.,2008). The utilization of crude glycerol in AD could benefit thecrude glycerol producers as well as the AD operators. Using glycerolas the co-digestate in municipal waste digesters has shown signif-icant increases in biogas production (Fountoulakis et al., 2010).However in order to maintain a stable digestion process, theamount of glycerol added needs to be limited to a certain concen-tration level. Recommendations for glycerol loading remain low,and vary from 0.05 wt% to 1 wt% to avoid the risk of organic over-loading (Fountoulakis et al., 2010; Holm-Nielsen et al., 2008).

The purpose of this study was to evaluate the use of soybeanderived crude glycerol as a co-substrate in order to increase biogasproduction of an anaerobic sewage sludge digester. The effect ofglycerol addition on the methane yield was determined in contin-uously stirred tank reactor experiments. Glycerol load was slowlyincreased in the reactors providing the bacterial community inthe AD time to acclimate to the new food source. From these exper-iments, the optimum and maximum concentration of glycerol thatcan be added to the AD for co-digestion was estimated based onreactor stability and health parameters, which are monitored byanalysis of volatile fatty acids (VFAs), pH, and alkalinity. TheTS/VS of the digestate was continuously monitored to ensure thatOLR levels were maintained.

2. Materials

2.1. Substrates

The samples consisted of crude glycerol and primary sewagesludge (primary). The crude glycerol was derived from local biodie-sel manufacturing that used soybean oil as its primary source ofraw materials. This crude glycerol contained glycerol, fatty acids,methanol, salts and water. The primary sewage sludge, which isuntreated municipal waste and digestate (seed), the materialremaining after the anaerobic digestion of the waste, were obtainedfrom the Water Pollution Control Authority in Flint, Michigan.

The substrates were initially characterized by determining totaland volatile solids (TS/VS) as well as pH. Total solids (TS) weredetermined by drying at 105 �C for 24 h, and then volatile solids(VS) were found by placing those samples in a 550 �C for 1 h inaccordance with APHA standard method 2540 (APHA, 2005). ThepH was measured using a Hach SensION3 pH meter. The methanolcontent was determined by thermal gravimetric analysis and theglycerol content by using GC. These results for the BMP samplesare shown in Table 1.

2.2. Biomethane potential

An AMPTS Biomethane Potential (BMP) Test system (BioprocessControl) was used to initially characterize glycerol and sewage

Table 1Substrate testing.

Sample TS (%) VS (% of TS) pH Methanol (wt%) Glycerol (wt%)

Digestate 4.85 61.68 7.1 – –Glycerol 78.24 95.03 10.4 5.05 46.5

Please cite this article in press as: Nartker, S., et al. Increasing biogas productionerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1

sludge co-digestion. A test series of three replicates was carriedout in sealed glass bottles (500 ml) for the crude glycerolco-digested with digestate, which was the liquid material leftoverfrom the anaerobic treatment of sewage sludge at the waste watertreatment plant (WWTP), and was compared to a reference seriesof digestate alone in three replicates. All bottles were loaded witha 2:1 VS (w/w) digestate to substrate ratio to a total of 400 g andplaced in a 37 �C water bath throughout the experiment. A 3MNaOH solution was used for scrubbing out all CO2 from the pro-duced biogas. Once the gas passed through the NaOH scrubbers,the remaining methane entered a calibrated flow cell whichrecorded the gas production in real time by sending data to theAMPTS software. Prior to the start of the analysis, the system isflushed with nitrogen. The BMP reactors were operated for 30 days.From these experiments the methane production from the glycerolwaste and digestate from the WWTP was obtained.

2.3. Continuously Stirred Tank Reactors

Two Continuously Stirred Tank Reactors (CSTRs) were set up; acontrol CSTR was run using primary as the only feed and this wascompared to a co-digestion CSTR that was run using primary andincreasing amounts of glycerol. Reactors were run on a 32 dayhydraulic retention time (HRT) determined by Eq. (1) where V isthe reactor volume in ml and Q is the daily input flow in ml/day.

HRT ¼ V=Q ð1Þ

Reactors were maintained by feeding 125 ml of new substratedaily and removing digestate to keep the operating volume at 4 L.

The test CSTR began with 1 g of glycerol (with 124 g of primary)and was increased to 10 g, with one additional gram being addedevery 3–4 weeks. The feeding schedule shown in Table 2 was fol-lowed. The stepwise increase in glycerol addition was done toallow sufficient time to buffer changes in primary, and to allowthe microorganisms to adjust to new feed stock. Each new batchof primary required TS/VS analysis to determine the OLR. OLRwas determined using Eq. (2), where M is total daily mass inputflow in kilograms, TS and VS are percentages based on substrateand V is reactor volume in cubic meters.

OLR ¼ ðM � TS � VSÞ=V ð2Þ

The glycerol content was increased from 19% to 69% of the totalOLR. The digesters were run until the glycerol loading became toolarge (�70 wt% of the total OLR), and the reactor could no longersustain gas production. This was done to stress test the digesterto determine maximum glycerol loading. The gas production fromthe glycerol waste and primary sludge were compared throughoutthe test period. The reactors were maintained at 37 �C throughoutthe experiment. The reactors were operated for approximately9 months.

2.4. Analyses

Biogas that was produced in the CSTRs flowed throughcalibrated Ritter wet test meters that continuously measured thebiogas production. The amount of gas produced was recorded dailyto understand the impact of crude glycerol on the co-digestion.Weekly gas analysis was performed to ascertain the methane con-tent in the biogas, which was expected to vary with crude glycerolloading and primary composition. Due to variations in primarycomposition, the primary was subtracted from primary andglycerol to understand the contributions of glycerol alone asshown in Fig. 2. Gas analysis was done using a Perkin Elmer Clarus600 Gas Chromatograph equipped with Perkin Elmer Elite-PLOT Qcolumn and a Thermal Conductivity Detector. Reactor health isdefined by key variables that determine stable operating

from sewage sludge anaerobic co-digestion process by adding crude glyc-016/j.wasman.2014.08.017

Table 2Feeding schedule for the CSTRs.

Glycerol load(grams)

Total OLR(kg VS/m3 d)

Glycerol inOLR (%)

Primary inOLR (%)

Days

1 0.98 19 81 362 1.26 30 70 193 1.70 33 67 354 1.67 45 55 225 1.90 49 51 186 1.88 59 41 257 2.09 64 36 278 2.54 61 39 289 2.42 69 31 2610 2.88 65 35 8

Table 3CSTR health parameters during the experiment and the final analysis.

Primary Primary + glycerol

Overall range (excluding final analysis)pH range during experiment 7.24–7.55 7.12–7.53Average pH 7.34 (±0.08) 7.29 (±0.25)Alkalinity range during experiment (g/L) 1.9–4.2 1.7–3.6Alkalinity average (g/L) 3.0 (± 0.5) 2.7 (±1.2)

Final analysispH 7.3 5.98Alkalinity (g/L) 2.6 1.5OLR 1.11 2.88% OLR from glycerol 0 65

S. Nartker et al. / Waste Management xxx (2014) xxx–xxx 3

conditions as shown in Table 3. Reactor health was monitoredthrough weekly analysis of volatile fatty acids (VFAs), pH,alkalinity, and TS/VS on the digestate. The pH was tested using acalibrated Hach SensION 3 pH meter, alkalinity analysis was per-formed using Hach digital titrator, and VFAs were analyzed usingthe Perkin Elmer Clarus 600 Gas Chromatograph equipped with aPerkin Elmer Elite-FFAP column with a 5 m guard column and aflame ionization detector (FID).

3. Results and discussion

3.1. Increased biogas production – BMP

BMP analysis was initially used to determine hydraulic reten-tion time (HRT), potential inhibitory effects, and biogas potential

0

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Fig. 2. Increased specific gas production usi

Fig. 1. Average BMP results for glycerol and primary sludge without glycerol.

Please cite this article in press as: Nartker, S., et al. Increasing biogas productionerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1

of glycerol co-digestion with digestate, as compared to digestatealone. The hydraulic retention time for co-digestion was deter-mined to be 32 days using Eq. (1) and when the glycerol wasloaded at 33% of the total OLR of the reactor, no inhibitory effectswere evident. The maximum Biological Methane Potential (BMP)for glycerol co-digested with digestate samples was 766 ± 42ml/gVS as shown in Fig. 1. The digestate alone reached a maximummethane production of 112 ± 14 ml/gVS. The average differencebetween the two samples was 608 ml/gVS, which is 7 times asmuch as the gas production of digestate alone. This indicates thatglycerol adds significant methane production when co-digestedwith digestate, and does not show short term toxicity effects whenloaded at 33% of the total OLR. Crude glycerol co-digestion withvarious substrates has been reported to increase gas output inBMP and CSTR tests (Astals et al., 2011; Heaven et al., 2011).

3.2. Increased gas output – CSTRs

After the BMP tests were performed; analyses of the glycerolco-digestion systems were moved to 4 L continuously stirred tankreactors (CSTRs). Co-digestion in the CSTRs was done using pri-mary sewage sludge as the substrate material. The CSTRs allowedfor continuous gas output monitoring using wet test meters andgas composition analysis by collecting samples in Tedlar bagsweekly and performing GC analysis of the biogas. The CSTR alsoallows for variation of glycerol and primary sludge loading. Thisdata along with digestate and co-digestate monitoring, determinedthe range of maximum benefit for addition of glycerol to thesystem. The addition of increasing amounts of glycerol resultedin a corresponding increase in gas production (Fig. 2). The specificgas production increased from 0.35 to 0.92 m3/kg VS d. Fig. 2

5 6 7 8 9 10

of Glycerol

c�on (m3/kg VS*d)Primary

Primary + Glycerol

Glycerol Alone (E-C)

ng glycerol as a co-digestion substrate.

from sewage sludge anaerobic co-digestion process by adding crude glyc-016/j.wasman.2014.08.017

Fig. 3. Increased methane content of primary with indicated amounts of glycerol versus primary sludge.

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Gas

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duc�

on (m

l/da

y)

Tota

l OLR

(kg

VS/m

3d)

Glycerol Loading (grams)

Biogas Produc�on Increases with Glycerol Load

Primary in OLR

Glycerol in OLR

average Gas Flow

Fig. 4. Gas production increases as the percentage of glycerol in the total OLR increases.

4 S. Nartker et al. / Waste Management xxx (2014) xxx–xxx

shows that beginning with the addition of 2 g of glycerol, or 30% ofthe OLR as described in Table 2, addition of glycerol to the digesterproduces more biogas than primary alone; which would increasethe operating efficiency of the digester and plant by providing ahigher flow of gas.

The maximum gain in gas production, the difference betweenthe primary and the co-digested glycerol and primary, wasobserved in the 4–9 g of glycerol loading range representing 45–69% of OLR. The overall maximum benefit of glycerol additionwas observed at 7 g of glycerol loading (64% glycerol in OLR asshown in Table 2) due to enhanced methane content as comparedto primary alone. This maximum benefit is found by consideringthe balance between the amounts of gas produced (Fig. 2), themethane content of the produced gas (Fig. 3) and the amount ofglycerol required to obtain this enrichment. The methane contentwas variable, but after the startup instabilities, there was anupward trend in biogas content from 5 g of glycerol loading to10 g as shown in Fig. 4. It is important to note that variations inprimary sludge, a product of weather conditions, TS/VS and otherWWTP issues were significant and showed an effect on the gas pro-duction throughout the experiment. The maximum benefit rangewas determined to be between 25% glycerol OLR to 60%.

Please cite this article in press as: Nartker, S., et al. Increasing biogas productionerol from biodiesel industry. Waste Management (2014), http://dx.doi.org/10.1

Glycerol load was varied from 0% to 70% of the total OLR, but theconcentration was dependent upon the primary sewage sludge,which showed inconsistency. This altered the actual amounts oforganics available for the digestion process. Fig. 4 shows theincreasing glycerol loading and general primary load reducingtrend. It is also evident in this figure that the increased gas produc-tion rate is due to the increased glycerol loading. After loading at10 g of glycerol (70% OLR), the reactor became unstable and didnot produce biogas due to propionic acid accumulation whichbecame inhibitory.

4. Conclusions

Waste glycerol has been used to increase the gas productionand methane content of biogas within an anaerobic co-digestionprocess using primary sewage sludge. The increase in biogas andconsequently energy production is significant, with a 25% OLRglycerol load; the biogas production increased 82% as comparedto a digester operating on primary sewage sludge alone. This datais meant to give operators at a WWTP confidence in co-digestionwith glycerol as long as the optimized range of 25% OLR to 60%OLR is maintained. Without co-digestion of glycerol, a typical

from sewage sludge anaerobic co-digestion process by adding crude glyc-016/j.wasman.2014.08.017

S. Nartker et al. / Waste Management xxx (2014) xxx–xxx 5

WWTP digesting primary sludge could install a modest enginegenerator set. With the addition of glycerol in the optimized rangeof 25% OLR to 50% OLR, the WWTP would be able to produceapproximately twice the electrical power using 25% glycerol load-ing and four times the kW rating using 50% loading.

Acknowledgments

This research was financially supported by the MichiganSoybean Promotion Committee.

Special acknowledgement to Chad Antle at Swedish BiogasInternational LLC for continuous support.

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