Co-digestion of energy crops and industrial confectionery by-products with cow manure: batch-scale and farm-scale evaluation

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC-ND 4.0 https://creativecommons.org/licenses/by-nc-nd/4.0/ Co-digestion of energy crops and industrial confectionery by-products with cow manure: batch-scale and farm-scale evaluation © 2002 IWA Publishing Published version Kaparaju, P.; Luostarinen, S.; Kalmari, E.; Kalmari, J.; Rintala, J. Kaparaju, P., Luostarinen, S., Kalmari, E., Kalmari, J., & Rintala, J. (2002). Co-digestion of energy crops and industrial confectionery by-products with cow manure: batch-scale and farm-scale evaluation. In Proceedings 9th World Congress, Anaerobic Digestion 2001,Anaerobic digestion for Sustainability, Antwepren, Belgium, Sep 2-5, 2001 (45, pp. 275-280). IWA Publishing. Water Science and Technology. https://doi.org/10.2166/wst.2002.0352 2002 Co-digestion of energy crops and industrial confectionery by-products with cow manure: batch-scale and farm-scale evaluation P. Kaparaju*†, S. Luostarinen*, E. Kalmari**, J. Kalmari** and J. Rintala* * Department of Biological and Environmental Science, University of Jyväskylä, P. O. Box 35, FIN-40351 Jyväskylä, Finland (†E-mail: [email protected]) ** Vaajakoskentie 104, FIN-41310, Leppävesi, Finland Abstract The possible co-digestion of energy crops and industrial confectionery by-products with cow manure was evaluated firstly, through long-term batch experiments and secondly, in a farm-scale digester. In batch assays, digestion with mesophilically digested cow manure as inoculum resulted in specific methane yields (m3kg–1 VS added waste) of 0.35 for grass hay (particle size <1.0 cm); 0.26 for oats (0.5 cm) and 0.21 for clover (2.0 cm) harvested at vegetative stage and 0.14 (2.0 cm) for clover harvested at flowering stage. Specific methane yields (m3kg–1 VS added waste) for confectionery by-products were 0.37 for chocolate, 0.39 for black candy and 0.32 for confectionery raw material. Out the three particle sizes (2.0, 1.0 and 0.5 cm) tested, particle size of 1.0 cm was found ideal for digestion of grass hay and clover while, particle size reduction did not influence methane production from oats. Stage of the crop influenced the methane yields, with clover harvested at vegetative stage yielding 33% higher methane than when harvested at flowering stage. An approximate 60% enhancement in methane yield was noticed with the co-digestion of industrial confectionery wastes with cow manure in a full-scale farm digester. Keywords Anaerobic digestion; biogas; co-digestion; confectionery by-products; cow manure; energy crops; farm-scale Introduction Agricultural biogas plants offer several environmental benefits including production of renewable energy (methane) and a nutrient-rich stabilized liquor, which when applied to soil, improves soil physico-chemical properties. Further, farm digesters may also assist to reduce the greenhouse gas emissions from manure management. However, to make farm-scale digestion more cost effective, co-digestion of cow manure with supplementary materials is of interest to increase methane yields. Such potential materials include the nonrisk industrial organic by-products, which often have a higher biogas potential than manure, varying from 30 to 500 m3ton–1 (Ahring et al., 1992; Angelidaki and Ahring, 1997). These materials could also give additional income through gate fees. Another possibility is the inclusion of energy crop species e.g., grass or clover in cropping systems, specifically grown as feed stock for energy production (Borjesson, 1998). Biomass thus obtained can be converted directly to energy on-site, in the farm digesters and thereby, diversify agricultural activity (Nordberg, 1996). One of the major problems encountered while digesting energy crops is low digestibility due to their ligno-cellulosic composition. In order to increase the methane yields of energy crops, physical pre-treatment by reducing the particle size to enhance hydrolysis (Sharma et al., 1988) and maturity stage to examine the influence of biochemical plant composition on digestion (Pouech et al., 1998) have been studied. The concept of co-digestion (Ahring and Johansen, 1992; Converti et al., 1997), its advantages (Mata-Alvarez et al., 2000) and its application for centralised co-digestion of organic wastes are not new (Callaghan et al., 1999; Lafitte-Trouque and Forster, 2000). Water Science and Technology Vol 45 No 10 pp 275–280 © 2002 IWA Publishing and the authors 275 Downloaded from http://iwaponline.com/wst/article-pdf/45/10/275/424740/275.pdf by UNIVERSITY OF JYVASKYLA user on 07 September 2022 However, industrial application of co-digestion is still scarce (Mata-Alvarez et al., 2000) and co-digestion of cow manure with energy crops and/or industrial wastes in on-farm biogas plants is seldom reported. Recently one report on a study of co-digestion of pure fat with cattle slurry (20/80% on dry matter basis) in a biogas plant showed a 75% increase in methane yield compared to slurry alone (Amon et al., 1998). The objective of the present study was to select proper materials and to establish successful initiation of the use of supplementary materials in a full-scale cow manure digesting farm digester. For that purpose, the methane yields of various energy crops and industrial confectionery by-products were studied in batch assays with digested cow manure as inoculum. In addition, the effects of particle size reduction and crop maturity for energy crops on methane yields were investigated in batch experiments. The performance of semi-continuous co-digestion was evaluated in a full-scale farm biogas plant. Materials and methods Substrates and inoculum Energy crops (EC) and digested cow manure (DCM) originated from the farm (Kalmari Farm, Leppävesi village, Jyväskylä, Finland). Cow manure was produced from 80 cows (40 adults). EC: clover, grass hay, oats were grown on the farm. Confectionery by-products (CBP): chocolate, black candy and confectionery raw material (CRM) were obtained from a nearby confectionery factory (Panda Oy, Jyväskylä, Finland). Field crops were harvested at a maturity stage corresponding to usual harvest for animal feed. For batch experiment, representative samples of ECs were drawn from the farm and stored at 4°C. For clover, methane production was performed at different stages of maturity (vegetative and flowering). All plant samples were chopped to ca. 0.5, 1.0 to 2.0 cm size particles with stainless steel knife. Mesophilically DCM from the farm digester was used as inoculum. Characteristics of the substrates and inoculum are presented in Table 1. In the farm-scale studies, ECs were harvested and stored in the field under plastic sheet. The stock was mechanically ground with meat grinder to particle size of 2.0 cm before feeding. Batch experiments The batch experiments were performed in duplicate two litre (l) glass bottles at 35 ±1°C. First 1 l (17 gl–1 volatile solids (VS)) of inoculum was added, then substrate (volumes adjusted to have 51 gl–1 VS for all ECs and 25.8 gl–1 VS for all CBPs resulting in VS/VSinoculum -ratios of 3.0 and 1.5 for ECs and CBPs, respectively), and finally distilled water was added to make total liquid volume of 1.5 l. Anaerobic conditions were induced by flushing the contents with nitrogen/carbon dioxide mixture (80/20%) and sealed P. Kaparaju et al. 276 Table 1 Characteristics of the energy crops, confectionery by-products and inoculum used in the batch assays Substrates pH TS (%) VS (%) VS/TS N total (gl–1) Energy crops Clover (vegetative) 7.8 18.7 16.9 0.90 3.1 Clover (flowering) 7.8 13.5 11.9 0.88 3.8 Grass hay 7.9 25.9 23.6 0.91 1.7 Oats 7.6 60.2 55.9 0.93 1.6 Confectionery by-products Chocolate 7.2 97.5 93.7 0.96 n.a Black candy 8.2 84.6 78.3 0.93 n.a CRM 6.1 89.1 89.0 1.00 n.a Inoculum 8.1 2.7 1.7 0.63 2.8 n.a. not analysed Downloaded from http://iwaponline.com/wst/article-pdf/45/10/275/424740/275.pdf by UNIVERSITY OF JYVASKYLA user on 07 September 2022 immediately with butyl rubber stoppers. The outlets provided on the stoppers were used for collecting biogas into aluminium foil bags. DCM alone was assayed to subtract its methane production from those of the samples. Farm biogas plant Biogas plant is a vertical steel digester (150 m3 capacity, liquid volume 120 m3), operated at 35–37°C with a central mechanical stirring system. The feed was prepared every week or on alternate weeks in the feed tank by feeding the well mixed cow manure from the prestorage tank (760 m3 capacity) to a feed tank where EC/CBP were mixed periodically. The feed was pumped to the digester from the feed tank 2–3 times per day with a total average amount of ca. 6 m3d–1. Hydraulic retention time was 22 d. Degassed or digested biomass leaves the digester, and enters the slurry storage tank. The specifically designed slurry storage tank has a dome shaped soft top membrane and functions as gas storage, able to hold biogas, amounting to one week’s consumption and collect biogas (annually on average ca. 10% additional methane, methane production varies e.g. according to temperature) produced from the already degassed slurry in the slurry storage tank. The slurry is spread in spring and autumn resulting in retention times varying from 1 week to 9 months in the slurry tank. The biogas thus produced, is led to the biogas combined heat and power generator. Analyses Analyses were performed as described elsewhere (Kaparaju and Rintala, in preparation.). Results and discussion Batch experiments The methane yields for various CBPs and ECs along with the effect of particle size and maturity stage (for clover alone) were studied on long-term (155 days) batch assays (Figure 1). Among the substrates tested, grass hay produced methane at a faster rate followed by clover, black candy, chocolate, CRM and oats. These substrates also exhibited 1–3 day lag phases. The mean specific methane yields (m3kgVS–1) from duplicate assays (variation standard error less than 13%) are presented in Table 2. With all materials methane production accounted for prolonged time; methane production by day 22 accounted from 70 to >95% for ECs and between 83–95% for CBPs of the final methane yields (Table 2). Among the CBPs, highest specific methane yield (per kgVS–1) was produced by black candy (0.39 m3) P. Kaparaju et al. 277 0 10 20 30 0 20 40 60 80 100 120 140 160 (l) Cumulative Methane Time (days) Figure 1 Mean cumulative methane production from energy crops: clover (vegetative, ▲), clover (flowering, ●), grass hay (■), oats (◆) and confectionery by-products: chocolate (◆◆), black candy (▲▲) and confectionery raw material (CRM, ■■) co-digested with digested cow manure, control (●●) under batch assays at 35°C Downloaded from http://iwaponline.com/wst/article-pdf/45/10/275/424740/275.pdf by UNIVERSITY OF JYVASKYLA user on 07 September 2022 followed by chocolate (0.37 m3) and CRM (0.32 m3) corresponding to methane yields of 284–346 m3(ton of by-product)–1. Of the various ECs tested, grass hay produced the highest specific methane yield per kgVS followed by oats and clover whereas, per ton of material oats had the highest yield. The effects of particle size were different for different ECs. For oats no effect was observed, whereas, 1.0 cm size was optimal for clover and least optimal for grass hay. The methane yields were ca. 10–30% higher at optimal particle sizes as compared to the least optimal ones. This response of particle size reduction on methane production in grass hay might be due to lower lignin content in grass hay than compared to clover and oats (Moore, 1958). On the other hand, Sharma et al. (1988) found that methane yields increased with decrease in particle size. Out of the five particle sizes (0.088, 0.40, 1.0, 6.0 and 30.0 mm) which the authors tested, maximum quantity of biogas (4–10% increase) was produced from particle size of 0.088 and 0.4 mm for raw materials like wheat straw, rice straw, and Bermuda grass. As the hydrolysis of ligno-celluloses is dependent on lignin to cellulosic ratio and the digestibility of grasses is highly correlated with the lignin content (Scharer and Moo-Young, 1979), the stage of crop also influences the methane yields as the composition of the plant changes over the growing season. For the same particle size (2.0 cm), higher methane yield (33%) was produced by clover harvested at vegetative stage than harvested at flowering stage. However, studies by Pouech et al. (1998) showed that crop maturity was weakly influential on methane yields for wheat, clover and rye-grass. The batch results show that CBP and hay could produce at least 50% more methane per ton of VS than cow manure alone (methane yield of 0.21 m3 kgVS–1(10 m3(ton slurry)–1in a 108-day batch assay; Luostarinen and Rintala, (unpublished)) while, other studied ECs would have comparative methane production with cow manure. On the other hand, per ton of the material are more drastic; CBP would give up to about 30 fold and EC up to about 10 fold (oats) more methane per ton of material than the studied cow manure alone (Table 2). Thus suggesting that CBPs can also be thought of as an ideal co-digestates that would give an appreciable enhancement in methane production. This could be due to the positive synergism established in the digestion medium and the supply of missing nutrients by the co-substrates (Mata-Alvarez et al., 2000). P. Kaparaju et al. 278 Table 2 Specific methane yields, pH, soluble chemical oxygen demand (SCOD) for various energy crops and confectionery by-products with digested cow manure as inoculum in batch assay at 35°C Substrate Particle pH (final) SCOD Specific methane yield1Methane yield size (final) on day 22 to final yield1 (cm) (gl–1)(m 3kgTS–1)(m 3kgVS–1)(m 3 t–1) (%) Energy crops Clover (vegetative) 2.0 7.61 5.9 0.19 0.21 35.5 98.6 1.0 7.61 5.4 0.13 0.14 23.7 126.8 0.5 7.62 5.6 0.18 0.20 33.8 97.4 Clover (flowering) 2.0 7.69 7.6 0.12 0.14 16.7 90.6 Grass hay 2.0 7.46 8.3 0.25 0.27 63.7 79.3 1.0 7.48 8.4 0.32 0.35 82.6 73.3 0.5 7.48 8.3 0.29 0.32 75.5 83.8 Oats 2.0 7.44 6.7 0.23 0.25 139 70.0 1.0 7.43 7.2 0.23 0.25 139 72.4 0.5 7.47 7.6 0.24 0.26 145 77.0 Confectionery by-products Chocolate – 7.54 6.1 0.36 0.37 346 90.4 Black candy – 7.44 6.6 0.36 0.39 305 94.6 CRM – 7.42 4.6 0.32 0.32 284 83.5 Inoculum – 7.41 6.3 0.11 0.18 175 41.7 1methane yield of inoculum subtracted Downloaded from http://iwaponline.com/wst/article-pdf/45/10/275/424740/275.pdf by UNIVERSITY OF JYVASKYLA user on 07 September 2022 At the end of all the assays, pH values were between 7.4–7.7 for ECs and 7.4–7.5 for CBPs whereas, SCOD values were only slightly higher than in inoculum, suggesting that no significant accumulation of soluble organics occurred. Farm biogas plant The digester has been in operation since 1998. Co-digestion with EC and/or CBP was taken up in spring 2000 (Figure 2). During this period, the digester showed reliable performance with both co-digestates CBPs and ECs. Approximately 40 to 50% of VS were degraded with <100–200 mgl–1 of volatile fatty acids and 0.6–1.7 gl–1of ammonium nitrogen concentration in digestate. Digestion of cow manure alone produced an average specific methane yield of 0.22 m3kgVS–1added waste. Addition of CBP increased the specific methane yield to about 0.28 m3kgVS–1added waste whereas, with EC methane yield was about similar to that obtained from cow manure alone (ca. 0.21 m3kgVS–1added waste). The plant produced about 150 m3d–1biogas (55–58% CH4content) per 6 m3biomass in codigestion of CBP and cow manure. Without CBP, that amount of manure yielded about 85 m3biogas. These figures also include biogas from the post-storage tank. Conclusions Results from both batchand farm-scale studies suggested that energy crops (EC) and confectionery by products (CBPs) are potential co-substrates to be digested with cow manure. Especially, CBPs showed potential for highly enhanced methane yields compared to digestion of cow manure alone. Pre-treatment of ECs by reducing particle size (2.0, 1.0 and 0.5 cm) did not influence methane yields in oats while, 1.0 cm particle size seems to be P. Kaparaju et al. 279 0 2 4 6 8 10 12 (m3 d-1) 0 100 200 300 400 500 (kg d-1) 0 0.2 0.4 0.6 0.8 (m3 kgVS-1 added waste) 0 30 60 90 120 150 180 210 240 270 a) Quantity added b) Quantity added c) Methane production Time (days) energy crops black candy chocolate methane production cow manure Figure 2 The added amounts of cow manure (a), energy crops and confectionery by-product (b) and the methane production (c) during the mesophilic digestion from farm-scale digester (day 0 =6.2.2000) Downloaded from http://iwaponline.com/wst/article-pdf/45/10/275/424740/275.pdf by UNIVERSITY OF JYVASKYLA user on 07 September 2022 optimal for clover and hay. Application of co-digestion technology of safe industrial byproducts in the farm-scale biogas digesters will ensure new economic incentive for farmers and an utilisation of the by-products of industry. Energy crops would also give additional resources to be converted into energy. 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