Comparison of solid-state anaerobic digestion and composting of yard trimmings with effluent from liquid anaerobic digestion

Bioresource Technology 169 (2014) 439–446

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Comparison of solid-state anaerobic digestion and composting of yard trimmings with effluent from liquid anaerobic digestion Long Lin a,b, Liangcheng Yang a, Fuqing Xu a,b, Frederick C. Michel Jr. a, Yebo Li a,⇑ a Department of Food, Agricultural and Biological Engineering, The Ohio State University, Ohio Agricultural Research and Development Center, 1680 Madison Ave., Wooster, OH 44691, USA b Environmental Science Graduate Program, The Ohio State University, 3138A Smith Lab, 174 West 18th, Columbus, OH 43210, USA

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Solid-state anaerobic digestion (SS-

Solid-state anaerobic digestion (SS-AD) and composting of yard trimmings with effluent from liquid anaerobic digestion were conducted at TS content of 22–30% and 35–55%, respectively. Carbon loss was compared at feedstock to effluent ratio ranged from 4 to 6. The greatest total carbon loss was observed at 35% TS in composting, which was about 50% higher than that in SS-AD; while, using SSAD, more than half of the degraded carbon was converted to methane as a renewable energy carrier. 30 Total carbon loss, %

AD) and composting were compared.  High total solids content negatively affected performance of SS-AD and composting.  The preferred feedstock/effluent ratio for SS-AD was 4–6.  The total carbon loss during composting was up to 50% greater than that in SS-AD.  Both SS-AD and composting generated nutrient-rich (N, P, K) end products.

CO -C %%CO2-C

25

%%CH4-C CH -C

20 15 10 5 0 2

3

4

5

6

TS = 22%

4

SS-AD

a r t i c l e

i n f o

Article history: Received 13 May 2014 Received in revised form 30 June 2014 Accepted 1 July 2014 Available online 9 July 2014 Keywords: Solid-state anaerobic digestion Composting Thermophilic Biogas Carbon loss

5

6

TS = 25%

4

5

6

TS = 30%

4

5

6

TS = 35%

4

5

6

TS = 45%

4

5

6

TS = 55%

Composting

a b s t r a c t Solid-state anaerobic digestion (SS-AD) and composting of yard trimmings with effluent from liquid AD were compared under thermophilic condition. Total solids (TS) contents of 22%, 25%, and 30% were studied for SS-AD, and 35%, 45%, and 55% for composting. Feedstock/effluent (F/E) ratios of 2, 3, 4, 5, and 6 were tested. In composting, the greatest carbon loss was obtained at 35% TS, which was 2–3 times of that at 55% TS and was up to 50% higher than that in SS-AD. In SS-AD, over half of the degraded carbon was converted to methane with the greatest methane yield of 121 L/kg VSfeedstock. Methane production from SS-AD was low at F/E ratios of 2 and 3, likely due to the inhibitory effect of high concentrations of ammonia nitrogen (up to 5.6 g/kg). The N–P–K values were similar for SS-AD digestate and compost with different dominant nitrogen forms. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction

⇑ Corresponding author. Tel.: +1 330 263 3855. E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.biortech.2014.07.007 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

Municipal solid waste (MSW) has become one of the largest environmental concerns in recent decades due to its increasing quantity. Besides recycling, there are globally four methods used

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for the management of MSW: landfilling, incineration, composting, and anaerobic digestion (AD). In 2012, about 228 million tonnes of MSW were generated in the United States, and yard trimmings were the third largest component comprising 13.5% (USEPA, 2012). However, current policy aims to divert yard trimmings from landfills and incinerators due to potential pollution. More than 50% of yard trimmings were diverted by composting in the U.S. (USEPA, 2012). Furthermore, yard trimmings may also serve as a feedstock of anaerobic digestion for renewable biogas production (Liew et al., 2012). However, AD or composting using yard trimmings may be hindered by high carbon to nitrogen (C/N) ratios (ForsterCarneiro et al., 2008; Li et al., 2011a). This problem can be solved by introducing a nitrogen-rich amendment. AD is a widely used technology that produces biogas, a renewable fuel, through decomposition of organic matter in the absence of oxygen by consortia of microorganisms. Most commercial digesters in the United States are liquid anaerobic digestion (LAD) systems that contain less than 15% total solids (TS) and are fed with manure, sewage sludge, or food waste (USEPA, 2013). The by-product of L-AD, also known as L-AD effluent, usually has a high water content and is expensive to transport long distances. Thus energy intensive dewatering processes are often employed. However, L-AD effluent is rich in nitrogen and active microbial consortia, which could likely improve the rate of yard trimmings conversion in both composting and anaerobic digestion (Xu et al., 2013). With respect to digesting yard trimmings, solid-state anaerobic digestion (SS-AD), which operates with TS content higher than 15%, is a better option than L-AD, because problems of floating and stratification of fibrous materials in L-AD can be addressed in SSAD (Chanakya et al., 1999). Furthermore, due to the lower water content, the by-product of SS-AD, also known as digestate, is much easier to transport than the L-AD effluent (Li et al., 2011a). Recently, SS-AD has been tested as a method to use L-AD effluent as an inoculum and nitrogen source for the production of methane from yard trimmings (Liew et al., 2012). L-AD effluent was found to be a better inoculum source for SS-AD than aerobic waste activated sludge, rumen fluid, or manure, as it provided a balanced microbial consortium with greater methanogenic activity (Forster-Carneiro et al., 2007; Kim and Speece, 2002). In addition, digestion at thermophilic temperatures (55 °C) has been reported to be more efficient in decomposing organic wastes and destroying pathogens than at mesophilic temperatures (37 °C) (Shi et al., 2013). One concern with thermophilic SS-AD is its high energy demand to maintain process temperature and sensitivity of thermophilic AD microbial communities to environmental disturbances, such as pH (Shi et al., 2013). In contrast, composting is an aerobic biological process to degrade organic matters by consortia of microbes. It has also been used to treat L-AD effluent by adding bulking agents such as sawdust and produces a solid saleable end product (Bustamante et al., 2013). Composting generally proceeds through two phases: initial and thermophilic, followed by a mesophilic maturation or curing (Fogarty and Tuovinen, 1991; Liang et al., 2003). Upon completion of these phases, most pathogens have been destroyed (Grewal et al., 2006), thereby converting L-AD effluent and bulking agents to a solid soil amendment (Bustamante et al., 2013). The key factors affecting the performance of composting process are aeration, TS content, and C/N ratio (Fogarty and Tuovinen, 1991). TS contents in the range of 30–40% (60–70% moisture content) have been reported to provide maximum microbial activities (Liang et al., 2003). When L-AD effluent is used to provide nitrogen for composting without additional buffers or nutrient supplements, the feedstock to effluent (F/E) ratio is the sole parameter that determines the pH, alkalinity, and C/N ratio of the mixture to be composted. The optimal C/N ratio for composting has been reported to be in the range of 26–35 (Fogarty and Tuovinen, 1991).

SS-AD and composting have different advantages and disadvantages in treating solid wastes. SS-AD is more complicated and requires a larger investment compared to composting (Li et al., 2011a). However, SS-AD produces renewable biogas as a fuel, while composting does not (Walker et al., 2009).The composting process usually requires a larger area and can emit odor, while SS-AD usually operates under controlled systems with a relatively smaller area (Bustamante et al., 2013). Both SS-AD and composting of yard trimmings have been reported in the literature (Chanakya et al., 1999; Fogarty and Tuovinen, 1991; Liew et al., 2012); however, no side-by-side comparison of thermophilic SS-AD and composting of yard trimmings with L-AD effluent has been reported. The objectives of the present study were to: (1) compare the rate of biogas/CO2 production from thermophilic SS-AD/composting of yard trimmings amended with L-AD effluent; (2) evaluate the effects of TS content and F/E ratio on the performance of SS-AD and composting; and (3) compare carbon loss, degradation of organic compounds, and the fertilizer values of the end products generated from SS-AD and composting. 2. Methods 2.1. Yard trimmings and L-AD effluent Yard trimmings consisting of wood chips (30% w/w), lawn grass (20% w/w), and maple leaves (50% w/w) were used as the feedstock for SS-AD and composting tests. Yard trimmings have a more balanced C/N ratio of around 30 than that of the individual component (Liew et al., 2012). Wood chips, lawn grass, and maple leaves were obtained in June, 2011 from the Ohio Agricultural Research and Development Center (OARDC) campus in Wooster, OH, USA. These feedstocks were dried at 40 °C for 48 h in a convection oven (Precision Thelco Model 18, Waltham, MA, USA) to a moisture content of less than 10%, then ground with a hammer mill to pass through a 9 mm screen sieve (Mighty Mac, MacKissic Inc., Parker Ford, PA, USA), and stored in air-tight containers. Effluent and centrifuged effluent were collected from a mesophilic liquid anaerobic digester that processed municipal sewage sludge (KB BioEnergy, Inc., Akron, OH, USA). Centrifuged effluent was produced with a D5LL solid bowl decanter centrifuge (ANDRITZ AG, Graz, Austria) at the facility and was used to achieve the designed TS contents for composting tests. Both effluents were stored in air-tight buckets at 4 °C. Prior to use, they were acclimated at 55 °C for 1 week. 2.2. SS-AD A full factorial design with three TS contents (22%, 25%, and 30%) and three F/E ratios (4, 5, and 6) was used for the SS-AD experiments. Two additional F/E ratios of 2 and 3 were included at the TS content of 22%. The yard trimmings, deionized (DI) water, and effluent were mixed using a hand-mixer (Black & Decker, 250watt mixer, Towson, MD, USA), and then loaded into 1 L glass reactors and incubated for up to 45 days in a 55 ± 0.3 °C incubator (BioCold Environmental, Inc., Fenton, MO, USA). Triplicate reactors were tested for each condition. Effluent without any feedstock addition was used as a control. Biogas was collected in 5 L Tedlar gas bags (CEL Scientific, Santa Fe Springs, CA, USA) connected to the outlets of each reactor. Biogas composition and volume were measured every 2–3 days. 2.3. Composting For the composting experiments, a similar full factorial design with three TS contents (35%, 45%, and 55%) and three F/E ratios (4, 5, and 6) was used. The yard trimmings, effluent, and/or

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centrifuged effluent were mixed and loaded into 4 L PVC reactors and incubated for up to 45 days at 55 ± 0.3 °C as described by Grewal et al. (2006). To simulate the aerobic composting process, pre-heated and pre-humidified air was conveyed through flow restrictors and a manifold to the bottom of the reactors at a rate of 100 mL/min. Off-gas from the top of each reactor passed through a water bath at 9 °C to condense moisture, and the de-watered offgas was then analyzed for CO2 content (Vaisala, GMT 220, South Burlington, VT, USA). Each reactor was also equipped with a K-type thermocouple that measured temperatures at the center of each reactor. Room temperature and humidity were also measured (Vaisala, HMP 235, South Burlington, VT, USA). Data were automatically recorded on a data logger (Campbell Scientific, CR23X, Logan, UT, USA) every hour. Triplicate reactors were tested for each condition, and effluent without any feedstock addition was used as a control. 2.4. Analytical methods Samples from the reactors were collected at the beginning and the end of the 45-day incubation. The TS, VS, pH, and alkalinity were measured according to Standard Methods Examination of Water and Wastewater (APHA, 2005). Samples for the pH and alkalinity measurements were prepared by diluting 5 g of samples with 50 mL of DI water and then measured using an auto-titrator connected with a pH meter (Mettler Toledo, DL22 Food & Beverage Analyzer, Columbus, OH, USA). Total carbon (TC), total nitrogen (TN) and sulfur (S) contents were determined by an elemental analyzer (Elementar Vario Max CNS, Elementar Americas, Mt. Laurel, NJ, USA). Total ammonia nitrogen (TAN), including NH3 and NH+,4 was measured based on a modified distillation and titration method (ISO 5664, 1984) using 4% boric acid instead of 2% boric acid with a Kjeldahl Distillation Unit B-324 (Buchi Labortechnik AG, Switzerland) and the auto-titrator mentioned above. Total content of volatile fatty acids (VFAs) (acetic, propionic, isobutyric, butyric, isovaleric, and valeric acid) was measured using a gas chromatograph (GC) system (Shimadzu, 2010PLUS, Columbia, MD, USA) equipped with a 30 m  0.32 mm  0.5 lm Stabilwax polar phase column and a flame ionization detector according to methods described by Shi et al. (2013). Extractives of raw materials and digested materials were measured based on the NREL Laboratory Analytical Procedure (Sluiter et al., 2008). Extractive-free samples were used to determine the structural carbohydrates with a two-step acid hydrolysis method (Sluiter et al., 2012). Monomeric sugars (glucose, xylose, galactose, arabinose, and mannose) and cellobiose were analyzed using a high-performance liquid chromatography (HPLC) system

(Shimadzu, LC-20AB, Columbia, MD, USA) equipped with a Biorad Aminex HPX-87P column and a refractive index detector. Water was used as the mobile phase at a flow rate of 0.3 mL/min. The temperatures of the column and detector were kept at 60 °C and 55 °C, respectively. Extractive-free samples were also used to determine total Kjeldahl nitrogen (TKN) with the modified Kjeldahl nitrogen method (ISO 5663, 1984) using a Tecator™ Digester (FOSS, Eden Prairie, MN, USA) with the distillation unit and autotitrator mentioned above. Crude protein content was obtained by multiplying total organic nitrogen (TON, TKN minus TAN) by a factor of 6.25 (Hattingh et al., 1967). The concentrations of 16 elements (P, K, Na, Mg, Al, Ca, Fe, Si, Mn, Ni, Co, Cu, Zn, As, Se, Mo, Cd, Hg, Pb) in digestate and compost were measured using an inductively coupled plasma-mass spectrometer (ICP-MS) (Agilent,7500cx, Santa Clara, CA, USA) (USEPA, 2007). Fecal coliforms were determined by the most probable number (MPN) multiple tube procedure (USEPA, 2010). Concentrations were calculated from a standard MPN table according to the number of tubes that indicated growth of fecal coliforms. The volume of biogas was measured with a drum-type gas meter (Ritter, TG 5, Bochum, Germany) and the composition (CO2, CH4, N2, and O2) was analyzed with a GC system (Agilent, HP 6890, Wilmington, DE, USA) equipped with a 30 m  0.53 mm  10 lm Rt-Alumina BondKCl deactivation column and a thermal conductivity detector. Helium gas was used as a carrier gas at a flow rate of 5.2 mL/min. The temperature of the detector was kept at 200 °C. Carbon loss for SS-AD was determined as the produced gasphase carbon (CO2-C and CH4-C) divided by the initial total carbon. Carbon loss for composting was determined as the CO2-C divided by the initial total carbon. 2.5. Statistical analysis Average results and standard errors were reported based on triplicates for each treatment, with two exceptions. For SS-AD, two reactors (at F/E ratios of 5 and 6) with TS content of 30% were upset and their results were not included. Statistical significance was tested by analysis of variance (ANOVA) using R-project software 3.0.2 version with a threshold value of 0.05. 3. Results and discussion 3.1. Chemical composition of yard trimmings and L-AD effluent Wood chips, lawn grass, and maple leaves were determined to have different C/N ratios (75.4, 14.2 and 35.8, respectively) but

Table 1 Characteristics of yard trimmings and L-AD effluent.

a b

Parameters

Wood chips

Lawn grass

Maple leaves

Yard trimmingsb

Effluent

Centrifuged effluent

TS (%) VS (%) TC (%) TN (%) C/N ratio pH Alkalinity (g CaCO3/kg) VFAs (g/kg) TAN (g N/kg) Extractives (%)a Cellulose (%)a Hemicellulose (%)a Lignin (%)a Crude protein (%)a

89.6 ± 0.1 86.9 ± 0.1 46.4 ± 0.3 0.6 ± 0.0 75.4 ± 2.4 ND ND ND ND 15.5 ± 0.1 27.5 ± 0.3 15.6 ± 0.1 26.9 ± 0.4 5.4 ± 1.2

90.0 ± 0.1 83.3 ± 0.1 43.8 ± 0.7 3.1 ± 0.1 14.2 ± 0.2 ND ND ND ND 30.5 ± 1.2 23.8 ± 0.3 15.8 ± 0.2 12.7 ± 0.0 8.8 ± 1.2

83.4 ± 0.1 73.1 ± 0.2 41.5 ± 0.1 1.2 ± 0.0 35.8 ± 0.8 ND ND ND ND 15.7 ± 0.6 12.2 ± 0.1 9.9 ± 0.2 36.1 ± 0.2 5.0 ± 3.7

86.6 ± 0.1 79.3 ± 0.1 43.4 ± 0.3 1.38 ± 0.0 31.5 ± 1.1 ND ND ND ND 18.6 ± 0.5 19.1 ± 0.2 12.8 ± 0.2 28.7 ± 0.2 5.9 ± 2.0

12.2 ± 0.0 6.6 ± 0.0 4.4 ± 0.0 0.6 ± 0.0 7.6 ± 0.1 8.4 ± 0.0 16.16 ± 0.8 3.6 ± 0.0 5.2 ± 0.1 13.4 ± 0.0 1.4 ± 0.0 0.6 ± 0.1 ND 11.6 ± 0.2

30.1 ± 0.3 15.9 ± 0.1 10.2 ± 0.3 1.0 ± 0.0 10.4 ± 0.2 9.0 ± 0.1 19.2 ± 0.1 0.5 ± 0.0 6.3 ± 0.1 11.2 ± 0.8 1.4 ± 0.1 0.2 ± 0.1 ND 13.8 ± 1.3

Based on TS of sample; the others are based on total weight of sample; ND = not determined. Yard trimmings included wood chips (30% w/w), lawn grass (20% w/w), and maple leaves (50% w/w).

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140 Total CH4 yield (L/kg VSinitial)

(a)

F/E = 2

F/E = 3

F/E = 4

F/E = 5

F/E = 6

120 100 80 60 40 20 0 TS = 22%

TS = 25%

TS = 30% 80

20

(b) 70

16 60 14 50

12

F/E = 2 F/E = 3 F/E = 4 F/E = 5 F/E = 6

10 8 6

40 30

CH4 content (%)

Daily CH4 yield (L/kg VSinitial/d)

18

20 4 10

2 0

0 0

5

10

15

20 25 30 Time (day)

35

40

45

Fig. 2. (a) Effect of TS content and F/E ratio on total CH4 yield, and (b) effect of F/E ratio on daily CH4 yield and CH4 content at 22%TS.

fuged effluent was needed to adjust the TS content to 35% and above in composting. 3.2. Effect of TS content and F/E ratio on the performance of SS-AD

Fig. 1. Effect of TS content and F/E ratio on daily CH4 and CO2 yields during SS-AD of yard trimmings with L-AD effluent. (a) F/E = 4, (b) F/E = 5, and (c) F/E = 6.

similar TC contents (41.5–46.4%) as shown in Table 1. The mixture of these three components resulted in yard trimmings with a balanced C/N ratio of 31.5. Wood chips had the highest holocellulose content (43.1%, cellulose and hemicellulose combined); while lawn grass had the highest extractives content (30.5%, water and ethanol solutes combined) and crude protein content (8.8%). Extractives include compounds such as free sugars, oligosaccharides, and organic acids (Chen et al., 2007), which are easily degradable for biogas generation (Liew et al., 2012). Leaves had the highest lignin content (36.1%). The presence of lignin usually reduces the biodegradation rate of organic feedstocks (Liew et al., 2012). Yard trimmings containing all three components have not only a balanced C/N ratio, but also extractives, holocellulose, and crude protein. Centrifuged effluent increased the TS content of effluent from 12.2% to 30.1%. Due to the low TS content of the effluent, centri-

During SS-AD, the methane production rate showed a bell shape with a short lag phase (Fig. 1), which was consistent with the typical dynamics of anaerobic batch reactors (Li et al., 2011b; Xu et al., 2013). The methane production rate had a single peak, while the carbon dioxide production rate had two peaks. The first peak of carbon dioxide production rate was during the initial 2 days and the second one was synchronous with the peak of methane production rate between days 8 and 16. This profile indicated that during SS-AD there were three distinct stages dominated by different consortia of microorganisms (Appels et al., 2011; Fernández et al., 2010). Hydrolytic, acidogenic, and acetogenic microbes dominated during the initial 2 days when carbon dioxide was dominant in the biogas (80% CO2), indicating the fast hydrolysis of easily degradable organics in the beginning. Methanogenic activity increased after the initial 2 days, when both methane and carbon dioxide were produced and the ratio of these gases was in agreement with the typical composition of biogas (65–75% CH4) (Appels et al., 2011; Forster-Carneiro et al., 2008). Reactors with higher TS contents (lower moisture contents) had prolonged lag phases, delayed peaks in methane production, and

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L. Lin et al. / Bioresource Technology 169 (2014) 439–446 Table 2 pH, total VFAs, alkalinity, and TAN from SS-AD initial and final materials. pH

VFAs (g/kg)

Alkalinity

TAN (g N/kg)

Initial

Final

Initial

Final

Initial

Final

Initial

Final

TS = 22%

F/E = 2 F/E = 3 F/E = 4 F/E = 5 F/E = 6

8.2 ± 0.0 8.2 ± 0.0 8.1 ± 0.1 8.0 ± 0.0 7.9 ± 0.1

8.5 ± 0.1 8.4 ± 0.1 8.4 ± 0.0 8.4 ± 0.0 8.5 ± 0.0

3.0 ± 0.3 2.4 ± 0.0 2.0 ± 0.0 1.7 ± 0.1 1.4 ± 0.0

4.6 ± 1.2 2.9 ± 0.3 2.0 ± 0.3 0.8 ± 0.3 0.4 ± 0.1

13.2 ± 1.0 9.0 ± 0.6 7.1 ± 0.2 5.6 ± 0.3 4.6 ± 0.1

16.1 ± 0.5 14.1 ± 1.0 13.3 ± 1.5 10.8 ± 0.6 9.5 ± 0.0

4.3 ± 0.0 3.4 ± 0.2 2.8 ± 0.0 2.4 ± 0.1 2.1 ± 0.0

5.6 ± 0.0 4.6 ± 0.2 3.8 ± 0.1 3.6 ± 0.0 2.8 ± 0.3

TS = 25%

F/E = 4 F/E = 5 F/E = 6

8.1 ± 0.1 8.2 ± 0.0 7.9 ± 0.0

8.5 ± 0.0 8.5 ± 0.0 8.5 ± 0.0

2.2 ± 0.0 1.9 ± 0.2 1.6 ± 0.0

3.2 ± 0.4 2.5 ± 0.1 1.5 ± 0.3

7.0 ± 0.3 6.5 ± 0.1 4.8 ± 0.3

12.6 ± 0.8 10.9 ± 0.2 10.1 ± 0.5

3.2 ± 0.0 2.7 ± 0.0 2.4 ± 0.0

4.3 ± 0.0 3.8 ± 0.0 3.2 ± 0.0

TS = 30%

F/E = 4 F/E = 5 F/E = 6

8.1 ± 0.0 7.9 ± 0.0 8.0 ± 0.0

8.5 ± 0.0 8.6 ± 0.0 8.3 ± 0.0

2.7 ± 0.0 2.3 ± 0.0 2.0 ± 0.0

5.4 ± 0.6 4.9 ± 0.1 5.3 ± 0.3

7.7 ± 0.2 5.7 ± 0.2 6.6 ± 0.0

14.1 ± 1.7 11.9 ± 0.4 7.8 ± 0.0

3.8 ± 0.0 3.3 ± 0.1 2.8 ± 0.1

4.8 ± 0.0 4.7 ± 0.1 4.0 ± 0.3

All the data are based on total weight of sample.

Average temperature (℃)

65

TS=35% TS=55%

TS=45% Room temperature

60

55

≈ 30

20 0

5

10

15

20 25 30 Time (day)

35

40

45

Fig. 3. Changes of temperature in the center of composting reactors during 45-day composting and room temperature at an F/E ratio of 6.

decreased peak values of biogas production (Fig. 1). For example, at an F/E of 4, no obvious lag phase was observed for 22% TS and the methane production rate was high on day 4 (Fig. 1a). In contrast, at 30% TS, the lag phase was prolonged to day 8. The peak values of both methane and carbon dioxide decreased with increasing TS content. Similar trends were observed at F/E ratios of 5 and 6 (Fig. 1b and c). Mass transfer or diffusion coefficients have been found to decrease drastically with increases of TS content (Abbassi-Guendouz et al., 2012; Xu et al., 2014). On the other hand, water content is important for microbial activity, the lower water content at a higher TS content might have inhibited microbial activity (Abbassi-Guendouz et al., 2012; Fernández et al., 2010). The total methane yield decreased significantly (p < 0.05) by 25–38% when the TS content increased from 22% to 30% at F/E ratios of 4, 5, and 6 (Fig. 2a). This result is consistent with previous studies (Forster-Carneiro et al., 2008; Li et al., 2011b). In addition to mass transfer limitations, the lower methane yield at a higher TS content could be attributed to the inhibitory effects of by-products, e.g. VFAs, which can result from imbalances among hydrolysis, acidogenesis, acetogenesis, and methanogenesis reactions, and inhibit methanogenesis (Ahring et al., 1995). An accumulation of VFAs was observed in SS-AD at 30% TS (Table 2). One of the triplicates at 30% TS and F/E ratios of 5 and 6 failed due to a low pH (5), suggesting that 30% TS is a threshold for SS-AD inhibition. ANOVA test showed that F/E ratios ranging from 4 to 6 had no significant effect on the total methane yield (p > 0.05), nor the interaction between TS content and F/E ratio. However, when lower F/E ratios of 2 and 3 were included at 22% TS, the F/E ratio

Fig. 4. Effect of TS content on daily CO2 yield during composting of yard trimmings with L-AD effluent. (a) F/E = 4, (b) F/E = 5, and (c) F/E = 6.

was a significant factor (Fig. 2a). The methane yield increased by 50% when the F/E ratio increased from 2 to 4. These results were in agreement with a previous study, which showed that an F/E

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30 %% CO2-C CO -C

Total carbon loss, %

25

%%CH4-C CH -C

20 15 10 5 0 4

5

6

TS = 22%

4

5

6

TS = 25%

4

5

6

TS = 30%

4

5

6

TS = 35%

SS-AD

4

5

6

TS = 45%

4

5

6

TS = 55%

Composting

Fig. 5. Comparison of total carbon loss in SS-AD and composting (4, 5, and 6 on x axis represent F/E ratios).

60

(a)

TS = 22%

TS = 25%

TS = 30%

Degradation (%)

50 40 30 20 10

*

0 Extractives

Cellulose

* Hemicellulose

Crude protein

60

(b)

TS = 35%

TS = 45%

TS = 55%

Degradation (%)

50 40 30 20 10 0 Extractives

Cellulose

Hemicellulose

Crude protein

Fig. 6. Degradation of extractives, cellulose, hemicellulose, and crude protein in (a) SS-AD, and (b) composting reactors at an F/E ratio of 4. (⁄Note: no detectable cellulose and hemicellulose degradation at 30% TS in SS-AD).

ratio of 4.58 resulted in more rapid biogas production as compared to 2.43 for thermophilic digestion (Li et al., 2011b). The low daily methane yield at F/E ratios of 2 and 3 could be attributed to the inhibition of high concentrations of TAN that was originated from

the effluent (5.2 g N/kg) and/or degradation of crude protein due to the relatively low C/N ratios (14–16) of the mixture in the reactors. Although the inhibitory level of TAN varies, studies demonstrated that 5.6 g N/L can lead to a 50% decrease in biogas for thermophilic

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digestion (Chen et al., 2008). In this study, TAN concentrations were 4.6–5.6 g N/kg at F/E ratios of 2 and 3 (Table 2), which were higher than their initial values due to degradation of proteins, and may have caused the low methane yields in these treatments. While at F/E ratios of 4–6, the TAN levels were not greater than 3.8 g N/kg. The highest methane yield of 121 L/kg VSfeedstock (104 L/kg VSinitial) was observed at a TS content of 22% and an F/E ratio of 6. This value was close to the previously obtained methane yield of 143 L/kg VSfeedstock for mesophilic digestion of yard trimmings at a low F/E ratio of 0.5 and a substrate concentration of 2 g VSfeedstock/L (Owens and Chynoweth, 1993). 3.3. Effect of TS content and F/E ratio on the performance of composting A typical composting process consists of a thermophilic phase during which most of the organic matter conversion occurs, followed by a mesophilic or curing phase (Fig. 3). In this study only the thermophilic phase was simulated by incubating the reactors at 55 °C. The composting temperatures increased above the set point initially due to microbial activity, and then gradually stabilized. The maximum temperatures were 62.4 °C, 60.8 °C, and 56.9 °C for TS contents of 35%, 45%, and 55%, respectively, at an F/E ratio of 6. Similar temperature profiles were observed for F/E ratios of 4 and 5 (data not shown). The carbon dioxide production rate during composting increased rapidly and reached a peak during days 2–4, and then declined (Fig. 4). The carbon dioxide peaks coincided with the temperature increases in the reactors (Fig. 3), and occurred earlier than the peaks of biogas production in SS-AD reactors (Fig. 1). Most of the carbon dioxide (80–95%) during composting was produced in the first 16 days, indicating that microbial degradation mainly occurred during this period. There was still oxygen in the off-gas, indicating that the composting process was aerobic (Grewal et al., 2006). Similar to SS-AD, the maximum carbon dioxide production rate of composting decreased as TS content increased (Fig. 4). The greatest peak value of 36.52 L/kg VSinitial/d was observed at a TS content of 35% and an F/E ratio of 4 (Fig. 4a), which was consistent with composting literature that maximum microbial activities were observed in the range of 60–70% moisture content (Liang et al., 2003). Unlike SS-AD, a shift in the peak of carbon dioxide production was not observed during composting at high TS contents. When the F/E ratio increased from 4 to 6, the carbon dioxide production rate showed a lower peak value with a slightly wider bell shape at the TS content of 35%, but similar peak values in reactors with TS contents of 45% and 55%. The total carbon dioxide yields at the three F/E ratios were 241– 298, 162–184, and 90–109 L/kg VSinitial for TS contents of 35%, 45%, and 55%, respectively. The total carbon dioxide yield decreased significantly with an increase of TS content (p < 0.05), but the effect of F/E ratio was not significant (p > 0.05). Due to high mass transfer limitations associated with low water content in high TS treatments, as discussed in the previous section, the activity of aerobic microbes might be limited. The minor effect of F/E ratio on the carbon dioxide production rate may be attributed to the fact that C/N ratio was a more important factor during composting than inoculum size (Fogarty and Tuovinen, 1991). Thus, in this study, for all composting reactors with a narrow range of C/N ratios from 17 to 20, the F/E ratio might not be an important factor affecting the composting rate.

ratio did not (p > 0.05) (Fig. 5). For SS-AD, the greatest carbon loss of 15.2% occurred at a TS content of 22%, followed by TS content of 25% TS (12.7% loss), and 30% TS (9.2% loss). A similar phenomenon was observed during composting and the total carbon loss decreased from 23.3% to 9.1% as the TS content increased from 35% to 55%. Overall, the greatest carbon loss during composting was observed at 35% TS content, versus at 22% for SS-AD. This result may be due to the effective oxygen mass transfer during composting, and was consistent with previous research that found composting was a faster degradation process compared to AD (Bustamante et al., 2013; Walker et al., 2009). However, 60% of the total carbon loss during SS-AD was converted to methane, while no methane was detected in composting. 3.4.2. Degradation of organic components Cellulose, hemicellulose, extractives, crude protein, and lignin are the major components in yard trimmings, and consequently, their biodegradation is essential to both SS-AD and composting processes. Except for negligible degradation of lignin (data not shown), both SS-AD and composting showed considerable degradation of the other four components, although the degradation of cellulose and hemicellulose were negligible at 30% TS in SS-AD (Fig. 6). As expected, extractives had the greatest extent of degradation in both SS-AD and composting (Chen et al., 2007; Liew et al., 2012). Composting had slightly higher extents of cellulose and hemicellulose conversion than those in SS-AD, which was probably due to its stronger microbial activities that decompose cellulose and hemicellulose (Walker et al., 2009). In contrast, SSAD showed a greater removal of crude protein. A possible reason could be that crude protein was degraded by microbes to small molecules, such as ammonium, which could be further utilized by microbes for protein synthesis through cell growth. Aerobic microbes generally have higher growth rates than anaerobic microbes, thus more cell protein was synthesized in the compostTable 3 Elementary analysis of residues from SS-AD and composting at an F/E ratio of 4. SS-AD (TS = 22%) Percentage (%) Ash content TC TN C/N ratio TAN TON NO3-N NO2-N S P K Na Mg Al Ca Fe

22.4 ± 2.5 37.1 ± 1.4 3.3 ± 0.1 11.1 ± 0.8 1.9 ± 0.1 1.4 ± 0.2 0.03 ± 0.0 0.002 ± 0.0 1.8 ± 0.4 0.6 ± 0.0 0.5 ± 0.0 0.1 ± 0.0 0.3 ± 0.0 0.03 ± 0.0 5.5 ± 0.2 1.1 ± 0.0

28.2 ± 1.0 38.0 ± 1.7 3.4 ± 0.2 11.3 ± 1.2 0.9 ± 0.0 2.5 ± 0.2 0.1 ± 0.0 0.01 ± 0.0 1.2 ± 0.1 0.6 ± 0.0 0.6 ± 0.0 0.6 ± 0.1 0.3 ± 0.0 0.04 ± 0.0 5.0 ± 0.0 1.1 ± 0.1

ppm Si Mn Ni Co Cu Zn As Se Mo Cd Hg Pb

3.4. Comparison of SS-AD and composting 3.4.1. Carbon loss Statistical analysis showed that TS content significantly affected carbon loss in both SS-AD and composting (p < 0.05), while the F/E

Composting (TS = 35%)

a

146 ± 3 1361 ± 43 92.1 ± 2.9 1.1 ± 0.0 222.6 ± 1.1 678.0 ± 14.9 3.9 ± 0.0 <0.0 2.3 ± 0.0 1.4 ± 0.0 <0.0 5.0 ± 0.0

122 ± 14 1227 ± 55 89.3 ± 0.5 1.0 ± 0.0 182.6 ± 0.0 1338.0 ± 92.9 4.0 ± 0.0 <0.0 2.7 ± 1.2 1.4 ± 0.0 <0.0 3.3 ± 1.8

Note: based on total weight of samples; the others are based on TS of samples.

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ing process and the measured net removal of crude protein (Kjeldahl nitrogen based) for composting was lower. Furthermore, degradations of holocellulose were approximately 30% and 20% at TS of 22% and 25% in SS-AD, respectively, which were much higher than the results from a previous mesophilic SS-AD study in which only 5% degradation was obtained at a TS content of 18–19% and F/E ratio of 3 (Liew et al., 2012). This result could be because thermophilic digestion was more favorable for cellulolytic and xylanolytic microbes than was mesophilic digestion (Shi et al., 2013). 3.4.3. Analysis of digestate and compost Both SS-AD digestate and compost have the potential to be used as fertilizers. Table 3 shows that the TC and TN contents were similar between the SS-AD digestate and compost, while the forms of nitrogen differed. TAN (57%, including NH3-N and NH+4-N) dominated in SS-AD digestate, while TON (74%) was the predominant form in compost. Although both of these forms of nitrogen are available forms for plant uptake, TON may contribute to long-term nitrogen turnover in the soil, while TAN may be rapidly lost because of plant uptake, ammonia volatilization, or nitrification and nitrate leaching (Tambone et al., 2010). Thus SS-AD digestate and compost might require different land application methods. Furthermore, the concentrations of P, K, and S as well as heavy metals were comparable in the SS-AD digestate and compost (Table 3). The concentrations of heavy metals were one to two orders of magnitude lower than the regulated levels based on the US EPA 40 CFR Part 530 Class A Biosolids. No fecal coliforms were detected in either the SS-AD digestate or the finished compost samples (data not shown). Therefore, while SS-AD digestate and composting have different forms of nitrogen, they were comparable in terms of other fertilizer value, did not harbor indicator pathogens, had low heavy metal concentrations, and could be used as a substitute for inorganic fertilizer. 4. Conclusions Both SS-AD and composting successfully degraded organics in yard trimmings and L-AD effluent. Increased TS content negatively impacted the performance of SS-AD and composting, while F/E ratio had minor effects. Daily CO2 production peaked within the first 2 days for both composting and SS-AD (first peak), while the second CO2 peak in SS-AD was synchronous with the peak of methane. Composting had higher total carbon loss than SS-AD, while over half of the degraded carbon was converted to methane in SS-AD. Nutrient-rich (N, P, K) end products with non-detectable coliform were generated from both SS-AD and composting. Acknowledgements This project was funded by the USDA NIFA Biomass Research and Development Initiative Program (Award No. 2012-1000820302) and USDA NIFA Hatch Program. The authors would like to thank Mrs. Mary Wicks (Department of Food, Agricultural and Biological Engineering, OSU) for reading through the manuscript and providing useful suggestions. References Abbassi-Guendouz, A., Brockmann, D., Trably, E., Dumas, C., Delgenès, J.-P., Steyer, J.-P., Escudié, R., 2012. Total solids content drives high solid anaerobic digestion via mass transfer limitation. Bioresour. Technol. 111, 55–61. Ahring, B.K., Sandberg, M., Angelidaki, I., 1995. Volatile fatty acids as indicators of process imbalance in anaerobic digestors. Appl. Microbiol. Biotechnol. 43, 559– 565.

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