Bioresource Technology 200 (2016) 753–760
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Comparison of digestate from solid anaerobic digesters and dewatered effluent from liquid anaerobic digesters as inocula for solid state anaerobic digestion of yard trimmings Fuqing Xu a,1, Feng Wang a,b,1, Long Lin a,c, 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 Hunan Provincial Key Laboratory of Food Science and Biotechnology, Hunan Agricultural University, Changsha 410128, China c 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 Using effluent from liquid AD as inoculum resulted in faster startup of SS-AD. Comparable methane yields were obtained using SS-AD digestate as inoculum. Substrate-to-inoculum (S/I) ratio should be below 1 for SS-AD of yard trimmings. Reducing TS content enhanced both hydrolysis and methanogenesis.
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Article history: Received 13 September 2015 Received in revised form 30 October 2015 Accepted 31 October 2015 Available online 4 November 2015 Keywords: Biogas Methane Inoculation Total solids content Lignocellulosic biomass
a b s t r a c t To select a proper inoculum for the solid state anaerobic digestion (SS-AD) of yard trimmings, digestate from solid anaerobic digesters and dewatered effluent from liquid anaerobic digesters were compared at substrate-to-inoculum (S/I) ratios from 0.2 to 2 (dry basis), and total solids (TS) contents from 20% to 35%. The highest methane yield of around 244 L/kg VSfeed was obtained at an S/I ratio of 0.2 and TS content of 20% for both types of inoculum. The highest volumetric methane productivity was obtained with dewatered effluent at an S/I ratio of 0.6 and TS content of 24%. The two types of inoculum were found comparable regarding methane yields and volumetric methane productivities at each S/I ratio, while using dewatered effluent as inoculum reduced the startup time. An S/I ratio of 1 was determined to be a critical level and should be set as the upper limit for mesophilic SS-AD of yard trimmings. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Anaerobic digestion (AD) is a biological process that converts organic matter into biogas (about 70% CH4 and 30% CO2) under oxygen-free conditions. AD is believed to be a cost-effective technology that both produces renewable energy and reduces the volume of organic waste (Forster-Carneiro et al., 2008). Depending on the operating total solids (TS) content of the digester substrate, AD systems can be categorized into liquid AD (L-AD) and solid-state AD (SS-AD). L-AD systems typically operate with a TS content of less than 15%, while SS-AD operates with a TS content higher than 15%. In the past decades, L-AD systems have been successfully used ⇑ Corresponding author. Tel.: +1 330 263 3855. 1
E-mail address:
[email protected] (Y. Li). Authors contributed equally to this work.
http://dx.doi.org/10.1016/j.biortech.2015.10.103 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
to convert liquid organic wastes such as animal manure and food processing waste into bioenergy. However, a barrier for the wide application of L-AD technology is the high cost of handling its large quantities of effluent, which can account for more than 30% of the operating costs for L-AD systems (Mata-Alvarez et al., 2000). In contrast, SS-AD systems are suitable to treat solid organic wastes and lignocellulosic biomass without the floating and stratification problems associated with fibrous materials in L-AD (Li et al., 2011a). SS-AD also has advantages of a relatively small digester volume and higher volumetric loading capacity and methane productivity (Brown et al., 2012), which can potentially reduce the cost for construction and heating (Guendouz et al., 2010). Due to the low water content, SS-AD generates a compost-like end product which can be used as a soil amendment (Li et al., 2011b). SS-AD currently represents 70% of the total installed AD capacity in Europe for treating the organic fraction of municipal solid
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waste and other solid organic materials, such as energy crops (De Baere and Mattheeuws, 2012; European Bioplastic, 2015). Compared to L-AD, SS-AD tends to be more difficult to start up and control, thus inoculation is essential for this process (Le Hyaric et al., 2012). It was reported that in commercial SS-AD digesters, about 50–70% (wet basis) of the finished solid materials (digestate) need to be recycled as inoculum, significantly reducing digester volume for feedstocks (Karthikeyan and Visvanathan, 2013). A recent lab scale study on continuous SS-AD used a digestate recycling ratio of around 55% (wet basis), and found that a TS content of about 26% and above would cause volatile fatty acid (VFA) accumulation and a pH drop (Benbelkacem et al., 2015). However, currently very limited information can be found for the optimal substrate-toinoculum (S/I) ratio and the operating TS content using recycled digestate as inoculum. Several researches have reported that the best inoculum for SSAD is the finished material (effluent) from L-AD digesters (ForsterCarneiro et al., 2007; Karthikeyan and Visvanathan, 2013). L-AD effluent has been widely used in scientific research as inoculum for SS-AD, as well as to supplement nitrogen, water, trace elements, and alkalinity to the system (Shi et al., 2014; Yang et al., 2015). However, L-AD effluent has not been widely used as the inoculum in commercial scale SS-AD, due to difficulty of transporting L-AD effluent to the SS-AD site. A pilot scale integrated anaerobic digestion system (iADs) has been built in Zanesville, OH (Li et al., 2011a). In the iADs, L-AD and SS-AD systems are constructed side-by-side and L-AD effluent is used to inoculate solid organic waste (e.g., crop residues, yard waste, and municipal solid waste) for biogas production in the SS-AD reactor. Both digestate and dewatered effluent have advantages and disadvantages as inoculum for SS-AD. Inoculating SS-AD with L-AD effluent, as adopted in the iADs, has the potential to reduce effluent treatment costs of L-AD, and rapidly initiate the SS-AD process (Li et al., 2011a). On the other hand, using SS-AD digestate as inoculum makes the system more self-sustainable, allows for a higher TS content, and provides a microbial community acclimated to the feedstock and operating conditions. Currently, there has been no side-by-side comparison of these two inoculation options, and the optimal TS range and S/I ratio for using SS-AD digestate as inoculum have not been studied scientifically. Thus the objectives of this research were to determine the optimal range of the operating TS content and S/I ratio using recycled digestate as an inoculum for SS-AD of yard trimmings, and to compare the effects of inoculating SS-AD with L-AD effluent or SS-AD digestate on methane yield, volumetric productivity, and startup time, in order
to facilitate the design and operation of large scale continuous SSAD systems. 2. Methods 2.1. Substrate and inoculum The yard trimmings used as substrate were a mixture of three common components of urban yard waste: wood chips, lawn grass, and maple leaves (Table 1). The percentages of these three components have seasonal variations in real yard trimmings, such as more grass in summer and more leaves in fall. For this study, the yard trimmings components were mixed at a ratio of 1:1:1 (wet basis), which had been found to achieve a good nutrient balance and high methane yield in thermophilic SS-AD (Lin et al., 2015). The mixed substrate did not require nitrogen supplementation because its C/N ratio of 24.6 was within the optimal range for microbial growth (Li et al., 2011b). The three yard trimming components were collected in November, 2013 from the Ohio Agricultural Research and Development Center campus in Wooster, OH, USA. Upon receipt, all components were dried at 40 °C for 48 h in a convection oven (Precision Thelco Model 18, Waltham, MA, USA) to reduce the moisture content to less than 10%, ground with a hammer mill to pass through a 9 mm screen sieve (Mighty Mac, MacKissic Inc., Parker Ford, PA, USA), and then stored in air-tight containers. Effluent from a mesophilic liquid anaerobic digester (KB BioEnergy, Inc., Akron, OH, USA) fed with municipal sewage sludge was used to prepare both types of inoculum. The digestate was prepared by mixing the effluent with yard trimmings at a S/I ratio of 4 (based on volatile solids (VS)) and a TS content of around 20%, and anaerobically digesting it at 37 °C for 44 days in a 20 gallon digester. The dewatered effluent was prepared by centrifuging (10,000 rpm for 15 min) the effluent to increase its TS content to be similar with the digestate. Both types of inoculum were stored in air-tight buckets at 4 °C and activated at 37 °C for 1 week prior to use. Characteristics of the substrate and inocula are shown in Table 1. 2.2. Solid-state batch anaerobic digestion Each SS-AD reactor (1 L glass bottle with working volume of 700 ml) was loaded with a mixture of yard trimmings and inoculum mixed at S/I ratios of 0.2–2.0 (based on dry weight) and TS content ranged from 20% to 35% (Table 2). The water added (if any) was counted toward the wet weight of feedstock. The high
Table 1 Characteristics of feedstock and inoculum. Parameters
Yard trimmingsb
Wood chips
Lawn grass
Maple leaves
Dewatered effluent
Digestate
TS (%) VS (%) Density (kg/m3) Total carbon (%)a Total nitrogen (%)a C/N ratioa pH Alkalinity (g CaCO3/kg) VFAs (g/kg) TAN (g N/kg) Extractives (%)a Cellulose (%)a Hemicellulose (%)a Lignin (%)a
87.7 ± 0.1 81.1 ± 0.1 152.6 ± 3.0 46.8 ± 0.2 1.9 ± 0.0 24.6 ± 0.1 5.0 ± 0.0
