On-field study of anaerobic digestion full-scale plants (Part II): New approaches in monitoring and evaluating process efficiency

Bioresource Technology 102 (2011) 8814–8819

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On-field study of anaerobic digestion full-scale plants (Part II): New approaches in monitoring and evaluating process efficiency Andrea Schievano ⇑, Giuliana D’Imporzano ⇑, Valentina Orzi, Fabrizio Adani Ricicla Group – Di.Pro.Ve. – Università degli Studi di Milano, Via Celoria 2, 20133 Milano, Italy

a r t i c l e

i n f o

Article history: Received 3 February 2011 Received in revised form 26 June 2011 Accepted 6 July 2011 Available online 14 July 2011 Keywords: Anaerobic digestion Biogas Methane potential Process efficiency Renewable energy

a b s t r a c t Biogas plants need easy and practical tools for monitoring and evaluating their biological process efficiency. As soon as, in many cases, biomass supply present considerable costs, full-scale anaerobic digestion (AD) processes must approach, as much as possible, the potential biogas yield of the organic mixture fed to the biodigesters. In this paper, a new indicator is proposed (the bio-methane yield, BMY), for measuring the efficiency in full-scale AD processes, based on a balance between the biochemical methane potential (BMP) of the input biomass and the residual BMP of the output materials (digestate). For this purpose, a one-year survey was performed on three different full-scale biogas plants, in the Italian agro-industrial context, and the bio-chemical processes were fully described in order to calculate their efficiencies (BMY = 87–93%) and to validate the new indicator proposed, as useful and easily applicable tool for full-scale AD plants operators. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction In Europe, the development of anaerobic digestion (AD) began in the sector of civil sewage treatment plants and by the recovery of landfill biogas (EurObserv’ER, 2009). Besides, agricultural biogas plants using livestock effluents were, only, about 400 in 1994, while today, in all countries of the EU, more than 6000 anaerobic digesters operate in this sector; the highest number of AD plants is situated in Germany, followed by Denmark, Austria, Sweden and Italy and it is rapidly growing (EurObserv’ER, 2009). In 2007, European production of primary energy from biogas reached 5.9 million tons of oil equivalent (Mtoe), i.e. 1 Mtoe more than in 2006 (increase of the 20%) (EurObserv’ER, 2009). In a recent survey, EurObserv’ER estimates, for the year 2010, a biogas production of approximately 8.6 Mtoe. This increasing number of biogas facilities and the growing complexity of this sector, in terms of biomass utilized (dedicated crops, byproducts, waste, farming residues, etc.), plant size and type (agricultural firm, industrial facility, municipal waste treatment plant, etc.), lead to the need of investigation for favoring data collection and correct operation of the biological processes. First of all, the economical sustenance of biogas facilities requires that the AD process achieve constantly the highest

⇑ Corresponding authors. Tel.: +39 02 503 16543 (A. Schievano), tel.: +39 02 503 16546 (G. D’Imporzano). E-mail addresses: [email protected] (A. Schievano), giuliana.dimporza [email protected] (G. D’Imporzano). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.07.012

methane yields permitted by the kind of fed-materials, allowing the maximum hours/year of electric energy (EE) delivering. Literature proposed different approaches to measure the potential production of methane from a substrate, directly under optimized conditions by laboratory-scale tests (Hansen et al., 2004; Gunaseelan, 2007; Schievano et al., 2008, 2009a), i.e. the biomethane potentials (BMP). It is important to know the BMP of the organic mixture used to feed a full scale AD process, especially when this input-biomass has a considerable supply cost (e.g. energy-dedicated crops). In these cases, full-scale AD processes must exploit its BMP as much as possible, by approaching the yields obtainable under optimized lab-conditions (Schievano et al., 2009b, 2010). For these reasons, full scale AD performances need to be evaluated in order to understand if the plant is effectively able to entirely exploit the potential biogas of the input organic mixture. In literature, AD process efficiency is normally measured by looking at the organic matter (OM) degradation and the ratio between volatile solids (VS) input and output is often used as indicator of the process yield (Demirer and Chen, 2004; Hartmann and Ahring, 2005). Nevertheless, while this indicator directly well-explains the OM abattoir, it is not incisive for representing the effective exploitation of the bio-methane potentially producible. In fact, the VS-analysis provides only a quantitative measurement of the OM, while nothing tells about its biodegradability under anaerobic conditions (Schievano et al., 2010). As consequence of that, low VS degradation measured may be an index of AD process inefficiency, but also, may be caused by the consistent presence of recalcitrant fractions in the substrate,

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which cannot be bio-degraded, neither in ideal AD conditions (e.g. lignin). The BMP parameter, instead, considers both quantitative (it depends on the quantitative characteristics of the substrate, i.e. TS and VS contents) and qualitative (nature of the organic molecules contained) aspects of a substrate, because this biological test measures only the effectively biodegradable fractions of the OM. Normally, full scale AD processes produce unstable digestates and considerable biogas can be further produced, while in most cases this happens in uncovered storage tanks, especially in summertime (Kaparaju and Rintala, 2003; Kaparaju et al., 2009). The measurement of this residual biogas potential under ideal lab-conditions (BMP), compared to the initial biogas potential (BMP of the feeding materials) can help in evaluating full-scale AD process effectiveness. A balance of the BMPIN and the BMPOUT can give a correct index of the AD process efficiency achieved. For this calculation, the mass balance of the AD process, i.e. the input–output mass flows, must be known. The present paper is the second part of a wider study on three full-scale AD plants located in Northern Italy and the first paper (Schievano et al., in press) aimed at calculating, through different approaches, the mass, carbon and nutrients balances of the three AD processes observed for a one-year period. In this second paper, the same three full-scale AD processes were observed, through a more detailed one-year data survey of the main chemical and bio-chemical parameters involved. The description of these three case studies allowed to use them for proposing a new index (based on the BMP) for the evaluation of full-scale AD process performances.

2. Methods 2.1. Data survey on the characteristics of the three full-scale biogas plants observed The three full-scale plants, operating in the agro-industrial context in northern Italy, were observed for a one-year period (April 2008–March 2009). All plants operated by continuously-stirredtank-reactors (CSTR) under ‘‘wet’’ conditions, i.e. with a total solids (TS) content in the reactors below 100 g kg1 wet weight (w.w.). The first plant (Plant A) was fed with the organic fraction of the municipal solid waste (OFMSW) (approx. 26,000–28,000 Mg y1), collected separately by 5 municipalities, which externalize its treatment to this private facility. The total digestion volume of 5000 m3 is divided into 4 digesters (1000 m3 each, loaded in parallel with 1=4 of the total input flow) and 1 post-digester (1000 m3), connected in series. The second plant (Plant B) is located in a farm that re-utilizes the swine manure as liquid substrate in the biogas plant (about 23,000–25,000 Mg y1). The feeding mixture is enriched by codigesting with pig slurry various energy crops (maize silage, triticale and sorghum), agricultural residues (barley thresh from beer industry) and industrial organic by-products, such as glycerin (from bio-diesel production plants), molasses (from sugar cane production), bakery-industry waste and olive mill sludge. The total digestion volume of 6000 m3 is divided into 2 digesters (1500 m3 each, loaded in parallel with ½ of the total input flow) and 1 post-digester (3000 m3), connected in series. The third plant (Plant C), similarly to Plant B, is located in a farm and its feeding mixture is composed of swine plus cow manure, maize silage, milk whey and rice culture by-products. The total digestion volume of 1600 m3 was not divided into subunits (only one digester). Energy, biogas and methane production data were already reported in the first part of this work (Schievano et al., in press) and in this second paper they were used for further discussion.

2.2. Sample collection campaign During the observation period, three types of materials were sampled in the plants: the input mixture, the output digested slurry – coming out from the last digester (post-digester) – and the slurry contained into the intermediate digesters. For Plant C (which had only one digester), the samples were withdrawn directly from the digester. All materials were sampled approximately every 2 months (6 input samples, 6 output samples and 6 intermediate samples per each plant). All samples from the digesters were taken while the mixers were operating in both digesters and loading facility, to avoid any biomass stratification. The chemical characterization and the biological assays performed on the samples are specified in Table 1 and some of the analyses were already presented in the first part of this work (Schievano et al., in press). Samples were dried and ground to 1 mm and stored for subsequent analyses. 2.3. Chemical characterization Chemical characteristics of the input and output materials were determined in double for each sample (6 input + 6 output per each plant). Some chemical characteristics were already reported in the Part I of this work (Schievano et al., in press): total solids (TS), volatile solids (VS), total organic carbon (TOC), total nitrogen (TN), organic nitrogen (ON), ammonia (N-NH4+), total phosphorous (TP) and total potassium (TK). For deeper analyses on the AD-processes, some additional chemical determinations (pH, total volatile fatty acids, total alkalinity and ammonia concentrations) were made on the liquor contained in the digesters. Volatile fatty acids (VFAs) and total alkalinity (TA) in the bulk samples, were performed on a 5-times-diluted solution of 2.5 g of wet sample filtered to 0.45 lm. VFAs were determined according to the acid titration method (Lahav et al., 2002). TA was determined in liquid phase by titration with HCl to a pH endpoint of 4.3, as suggested by APHA (1998). 2.4. Specific oxygen uptake rate (SOUR) assay The SOUR test is an aerobic biological assay. It is a measure of the oxygen uptake rate in a water solution during the microbial respiration in degrading a suspended solid matrix. The microbial respiration works out in standardized moisture conditions and in maximized conditions of both oxygenation and bacteria–substrate

Table 1 Sampling campaign and data survey performed in the three full-scale plants monitored in this study. Datum

Frequency

Total weight of input materials Total Electricity generated CH4/CO2 ratio in the biogas Operational Temperature

Registered every day by plant operators Continuously accounted by the engines monitoring systems Registered every hour by on-line sensors Registered continuously by on-line sensors

Sample type

Sampling frequency

Characterizationa

Input mixture

6 samples, approx. every 2 months 6 samples, approx. every 2 months 6 samples, approx. every 2 months

TS, VS, TOC, TN, ON, TP, TK, pH, VFA, TA, N-NHþ 4 , BMP, OD20 pH, VFAtot, AlktotN-NH4+

Intermediate digesters Output digestate

TS, VS, TOC, TN, ON, TP, TK, pH, VFA, TA, N-NHþ 4 , BMP, OD20

a TS = total solids, VS = volatile solids, TOC = total organic carbon, TN = total nitrogen, ON = organic nitrogen, TP = total phosphorous, TK = total potassium, VFA = volatile fatty acids, TA = total alkalinity, N-NH4+ = ammonia nitrogen, BMP = biochemical methane potential, OD20 = oxygen demand in 20 h respiration.

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interaction, amplifying the differences among different samples. The potential oxygen uptake was determined as the cumulative oxygen demand during the 20-h test (OD20: g O2 kg1 w.w. 20 h1). All the tests were performed in duplicate. This test provides a measure of the short-term biodegradability (putrescibility) of the organic matter, and it was performed as reported by Schievano et al. (2009a). Moreover, it was successfully used by Schievano et al. (2010) as an indicator for preventing the possible inhibiting conditions due to organic overloading of the digesters. Also in this work, the OD20 was used for calculating the short-term-degradable organic loading rates (OLRs) of observed AD processes. 2.5. Bio-methane potential and process effectiveness determination The Bio-Methane Potentials (BMPs) of all samples (both fed mixtures and digestates) were determined by using the method reported by Schievano et al. (2008, 2009a). Quantitative biogas production was estimated by withdrawing extra-pressure gas with a 60-ml syringe. This procedure was always performed at controlled temperature of 37 °C; the residual gas pressure in the batches, after the gas extraction, was always detected and the measured volume were reported to standard temperature (25 °C) and pressure (1 atm). Qualitative analyses of the biogas were performed by a gas-chromatograph (Micro GC 3000, Agilent Technology), for determining the CH4 concentrations (v/v) in the biogas. All the tests were performed in duplicate. This test was applied for evaluating both the potential biomethane productions of the fed mixtures and the residual biomethane producible from the digestates. BMP detected for both input and output material were used in joint with mass balance data to determine the biogas yield (BMY1) of the three plants, by using the following equation:

BMY1 ð%Þ ¼ ðBMPin  TSin  BMPout  TSout Þ=ðBMPin  TSin Þ  100

ð1Þ

in which BMPin is the bio-methane potential in the fed mixture (Nm3 kg1 TS), BMPout is the bio-methane potential in the output digestate (Nm3 kg1 TS), TSin are the total solids fed during the observed period (kg) and TSout are the total solids output with digestate during observed period (kg).The methane yields obtained (BMY1) by Eq. (1), were compared to the effective specific methane

produced (SMP, as N m3 kg1 TS-input) in the full-scale plants and calculated by Eq. (2):

BMY2 ð%Þ ¼ SMP=BMPin  100

ð2Þ

2.6. Feeding and production data survey The total input materials and the total electric energy (EE) generated during the observation period was already reported in the Part I of this work (Schievano et al., in press), as well as the methane content (% v/v) in the generated biogas. No quantitative measurement of the biogas generated was possible, because there were not flux-meters in the on-line monitoring systems of the facilities. The total methane generation was calculated from the total EE generation assuming a caloric power of the methane of 0.2475 kW h mol1 CH4 and an EE generation yield (indicated by the suppliers of the internal combustion engine units) of 35%. The total biogas productions were then calculated from the observed average methane content. The total methane and biogas productions were reported in volume units, referred to the standard temperature and pressure conditions (25 °C, 1 atm). 3. Results and discussion 3.1. Input materials The chemical and biological characterization of the materials fed into the observed full-scale plants were reported in Table 2, as averages of the data collected during the period of observation (1 year). The TS contents were higher for Plant B, compared to the others. This was probably due to the presence of biomasses characterized by low moisture contents, such as glycerin, molasses and olive oil production sludge. These materials influenced, also, the VS content of the feed, which was slightly higher for Plant B (Table 2). The potential bio-methane productions, obtainable from the feeding materials (BMP), and referred to TS unit, resulted almost the same for all the feeding mixtures. However, when BMP was referred to the wet weight unit, it resulted higher in Plant B, because the feed was more concentrated (Table 2). The biodegradability of the organic mixtures, measured as OD20, resulted almost equal for all the three plants, although with a slightly lower average value for Plant C (Table 2). As the OD20 represents an indicator of the short-term biodegradability of the organic matter

Table 2 Characterization of the input mixtures and the digestates of the three full-scale biogas plants (A, B, C) observed. Data reported as average of 5 samples during the observation period. Plant A

TS VS BMP OD20 pH Ammonia VFA TA VFA/TA TOCa TNa a N-NHþ 4 ONa TKa TPa a

g kg1 g kg1 TS Nd m3 CH4 kg1 TS Nd m3 CH4 kg1 w.w g O2 kg1 TS 20 h g l1 g acetic acid l1 g CaCO3 l1 kg acetic acid kg1 CaCO3 g kg1 TS g kg1 TS g kg1 TS g kg1 TS g kg1 TS g kg1 TS

Plant B

Plant C

Fed

Digestate

Fed

Digestate

Fed

Digestate

121 ± 13 878 ± 37 370 ± 40 45 ± 5 251 ± 47 4.8 ± 0.9 1.01 ± 0.07 16.9 ± 3.3 19.8 ± 3.0 0.850 ± 0.165 481 ± 24 42.2 ± 10.7 8.3 ± 5.6 33.9 ± 23.0 12.8 ± 3.4 3.4 ± 0.8

37 ± 5 661 ± 49 168 ± 15 6±1 132 ± 40 8.2 ± 0.2 3.56 ± 0.05 2.4 ± 1.1 14.6 ± 2.1 0.170 ± 0.074 362 ± 16 137.3 ± 11.9 96.2 ± 10.0 41.1 ± 4.3 43.2 ± 5.5 11.4 ± 3.0

184 ± 7 913 ± 15 356 ± 29 66 ± 5 249 ± 28 6.5 ± 0.8 2.25 ± 0.02 5.7 ± 5.0 17.2 ± 2.2 0.940 ± 0.833 470 ± 30 26.3 ± 5.2 12.3 ± 2.3 14.0 ± 2.6 20.3 ± 6.7 6.1 ± 2.5

58 ± 3 698 ± 27 85 ± 10 5±1 66 ± 19 7.9 ± 0.1 3.32 ± 0.02 2.4 ± 2.5 16.3 ± 0.8 0.140 ± 0.147 361 ± 22 90.5 ± 6.2 57.2 ± 7.1 33.3 ± 4.1 70.7 ± 15.0 20.2 ± 6.4

130 ± 10 890 ± 11 375 ± 18 49 ± 2 210 ± 49 4.6 ± 0.5 1.46 ± 0.01 16.2 ± 2.8 8.1 ± 3.0 0.710 ± 0.122 486 ± 11 26.4 ± 1.5 11.2 ± 1.2 15.2 ± 1.7 18.3 ± 19.2 7.5 ± 1.8

53 ± 16 723 ± 25 117 ± 8 6±1 64 ± 6 7.9 ± 0.2 1.92 ± 0.03 1.8 ± 0.6 9.5 ± 0.8 0.190 ± 0.062 393 ± 24 66.5 ± 31.7 36.2 ± 1.7 30.3 ± 1.4 48.0 ± 1.3 18.8 ± 3.0

Data already reported in the previous paper (Part I of this work) (Schievano et al., in press).

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Total annual loaded material (as wet weight) Loading rate (as wet weight) Loading rate (as TS) Organic loading rate (OLR as VS) Organic loading rate (OLR as OD20) Hydraulic retention time (HRT) Methane content in biogas (average in 1-year survey) Operational temperature pH (in all the digesters) Total VFA concentration (in all the intermediate digesters) VFA/TA ratio (in all the intermediate digesters) Ammonia concentration (in all the intermediate digesters)

1

Mg a Mg d1 Mg TS d1 kg VS m3dig d1 kg O2 (20 h) m3dig d1 d % v/v CH4 °C log [H+] g kg1 g kg1

(Schievano et al., 2010), the lower OD20 observed for Plant C could be ascribed to probable higher presence of fibers (contained in crops and in cow manure), which are bio-degradable under anaerobic conditions only by longer-term degradation process (Schievano et al., 2010). The initial pH was higher in mixture B, than other mixtures and the low pH of mixtures A and C were probably due to the fact that in these plants the feeding materials were mixed and stored into big tanks for 2–3 days, during which partial acidogenesis may take place. In Plant B, instead, the solids materials were mixed with the swine manure every hour in a batch-charge system and fed. The same reasons explain the VFA concentrations and the VFA/TA ratios measured for the 3 mixtures (Table 2). 3.2. Conditions and parameters of the AD processes All conditions and parameters describing the biological processes observed in the 3 full-scale plants are resumed in Table 3, as average of the values registered during the observation period. The feeding conditions were described by the mass loading, organic loading rate (OLR) and the hydraulic retention time (HRT), while the biological processes status was evaluated by the average observed methane content in the biogas, the average temperatures, the pH in the digestion-body and the concentrations of some relevant chemical species (VFA, VFA/TA ratio and ammonia). As reported in the Part I of this work (Schievano et al., in press), the total annual biomass fed in Plant A, B and C were respectively of 45,251, 38,544 and 22,745 Mg (as wet weight), corresponding to a daily loading rate of, as average, 121 ± 5, 100 ± 5 and 62 ± 4 Mg (Table 3). The total loading (as wet weight) in Plant A was higher than in Plant B, but this did not correspond to higher TS and VS loading rates (Table 3). Furthermore, the HRT of Plant B was much higher (HRT of 57 days) than in Plant A (HRT of 40 days) and, above all, than the Plant C (HRT of 26 days). This was probably necessary because the loaded TS were higher in Plant B (Table 3) than in the other two plants and because, also, the mesophilic process (Plant B) should determine a slightly lower degradation kinetics compared to the thermophilic process (Plant A and C) (Ali et al., 2004), so that higher HRTs are normally needed. Plant C showed the highest OLR in terms of VS (4.48 kg VS m3 d1), i.e. nearly the 100% higher than OLRs of plants A and B (Table 3). However, the OLRs were calculated, also, using the OD20 as indicator of the quality of the organic matter (OM) loaded. As reported in recent works (Schievano et al., 2009a, 2010), the OD20 is an indicator of the short-term degradable fractions of the VS, which are, under anaerobic conditions, quickly fermented to VFAs, these latter responsible for partial inhibitions of the methanogenic activity. Fore these reasons, the OLR, calculated on the OD20-basis, may result a useful parameter for describing eventual overloading conditions. Plant C showed OLRs (as OD20) only the 20% higher than those applied to Plants A and B, so that

Plant A

Plant B

Plant C

45,251 121 ± 5 14.6 ± 0.6 2.571 ± 0.42 0.735 ± 67 40 ± 3 57 ± 3 54.8 ± 1.8 8.06 ± 0.12 6.572 ± 1.4 0.41 ± 0.13 3.54 ± 0.58

38,544 100 ± 5 18.4 ± 0.9 2.8 ± 0.54 0.764 ± 49 57 ± 5 55 ± 4 38.2 ± 1.2 7.82 ± 0.13 2.014 ± 0.5 0.128 ± 0.04 3.04 ± 0.28

22,745 62 ± 4 8.1 ± 0.5 4.483 ± 0.35 1.058 ± 58 26 ± 2 63 ± 4 55.2 ± 1.8 7.93 ± 0.13 2.076 ± 0.36 0.19 ± 0.08 1.92 ± 0.09

possible overloading stress, caused by the higher VS loading in Plant C (almost double) than in Plants A and B, was mitigated by relatively low loading of short-term degradable OM (OLR, as OD20). This was corroborated by the fact that both values of VFA concentrations in the intermediate digesters and VFA/TA ratios, for Plants B and C (Table 3), were in an acceptable ranges, while Plant A showed VFA concentration more than three times higher than in the other two plants (Table 3). This value is close to the limits indicated for process inhibition (VFA concentration over 6 g l1 and VFA/TA ratio over 0.4) by many authors (Chen et al., 2008; Lindorfer et al., 2008). These results highlighted a probable slight inhibition of the methanogenic activity in the intermediate digesters of Plant A. An explanation to this may be found in the higher ammonia concentrations measured in Plant A (Table 3), which may have partially inhibited the methanogens and the consequent VFA concentration, as reported by Chen et al. (2008), for concentrations above 1 3 g N-NHþ . In any case, this problem was partially compensated 4 l by a strong methanogenic activity in the post-digester, where the total VFA concentration was as low as in the other two plants (around 2 g l1) (Table 3). In any case, all the observed plants showed a correct and well developed AD process and so they can be considered as well representative case-studies for successive discussion. 3.3. Characterization of the output materials and changes in the organic matter characteristics after the AD process Table 2 reports the characteristics of the digested materials, as average of the observed period. The AD process determined important reductions of the TS contents in the digested materials and, for all plants, the comparison between the characteristics of the input and the output materials confirmed that the AD processes induce deep modifications of the OM, as indicated for example by Tambone et al. (2010). In particular, the VS contents (600–700 g kg1) were lower than in the input materials (around 900 g kg1 TS) (Table 2). The bio-degradable fractions of the total OM, detected by the BMP (long-term biodegradability) and OD20 (short term biodegradability), were also severely reduced in the digestates, with respect to the fed materials (Table 2). More in depth, comparing Plants A, B and C, the VS contents were lower in the digestates of Plant A, while the OD20 and BMP were higher (Table 2). Plant C had higher VS content, higher residual BMP, but, on the other hand, low OD20, similar to those of Plant B, which, instead, was characterized by the lowest VS, OD20 and BMP in the digestates. This means that Plant B was able to degrade both the short- and long-term biodegradable fractions contained in the VS, better than Plant A and C (Table 2) and to obtain higher biological stability of the digested OM (low BMP and OD20). The TOC contents, also, decreased in the digestates, with respect to the

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Table 4 Bio-methane and biogas productions, process performances and efficiencies achieved by the three observed full scale biogas plants during the observation period (April 2008– March 2009).

TS degradation yield VS degradation yield Bio-methane yield (BMY1) Bio-methane yield (BMY2) Total bio-methane production (1 year) Total biogas production (1 year) Bio-methane production rates Biogas production rates Volumetric bio-methane production rate Specific biogas productions on w.w. basis Specific biogas productions on TS basis Specific bio-methane productions (SMP) on w.w. basis Specific bio-methane productions (SMP) on TS basis

% % % % N m3 a1 N m3 a1 N m3 CH4 d1 N m3 biogas d1 N m3 m3dig d1 N m3 Mg1 N m3 Mg1 TS N m3 CH4 Mg1 N m3 CH4 Mg1 TS

input materials, while, on the contrary, the TN contents (referred to the TS) of the digestates increased (Table 2). The organic fraction of the total nitrogen (ON) also increased, but to less extent, compared to the TN. The ammonia contents, in fact, increased more than proportionally, compared to the TN contents. This concentration-effect was particularly evident in Plant A, because of the higher ON content in the input material (33.9 g N kg1 TS), with respect to the other two plants (14.0 and 15.2 g N kg1 TS for Plant B and C, respectively (Table 2). The final pHs were sub-alkaline in all the plants and in an optimal range for the methanogenic activity (Pind et al., 2003). This was confirmed by the VFA and TA concentrations and their ratios (Table 2), which showed values compatible with stable methanogenic conditions, i.e. VFA < 6 g l1 as acetic acid and VFA/TA < 0.4, as suggested by Pind et al. (2003). 3.4. Process performances: degradation yields and methane productions The mass balance of the observed AD processes were object of study in the first part of this work (Schievano et al., in press) and in this second paper those results were used for calculating the TS and VS degradation yields achieved during the AD process. The TS and the VS were significantly degraded in all the plants by mean of approximately the 60–70% and the 70–80%, respectively (Table 4), similarly to what reported in literature for various laboratory-scale CSTR-AD processes (Demirer and Chen, 2004; Hartmann and Ahring, 2005). By the way, a consistent part of the VS (20–30%) were not degraded and it is important to understand whether the cause was the recalcitrance of such fraction of the OM or, on the other hand, the inefficiency of the AD process on potentially biodegradable fractions. However, no clue can be found in quantitative measurements of the OM (TS, VS). To understand if the un-degraded VS were effectively un-degradable (recalcitrant molecules, such as aromatic compounds, lignin, cutin, long chain fatty acids, polyphenols, etc.) or if the AD process was inefficient on potentially degradable fractions of the organic matter, the degradation yields were re-calculated, basing on more qualitative measurements of the OM, i.e. the BMP. The degradation yields calculated on the BMP, i.e. the bio-methane yields (BMY1, Eq. (1)) indicated relatively high efficiency of the full scale processes (87–93% of the BMPin) (Table 4), compared to controlled AD processes in optimized lab-scale trials (BMP tests). However, some differences between the three considered plants must be noted, as soon as Plant B showed both the best VS degradation and BMY1. Plant C, instead, showed slightly lower VS degradation, if compared to Plants A and B and, at the same time, Plant C showed lower BMY1. This was probably caused by the absence of

Plant A

Plant B

Plant C

72% 79% 87%

73% 79% 93%

63% 70% 88%

88% ± 9% 1788121 3845808 ± 202411 4899 ± 352 10536 ± 757 0.98 ± 0.07 85.0 ± 4.5 703 ± 68 39.5 ± 4.4 327 ± 36

93% ± 13% 2355315 4565892 ± 332065 6453 ± 254 12509 ± 492 1.08 ± 0.04 118.5 ± 8.6 644 ± 86 61.1 ± 8.5 332 ± 46

84% ± 8% 931844 1823650 ± 115787 2553 ± 197 4996 ± 386 1.60 ± 0.12 80.2 ± 5.1 619 ± 58 41.0 ± 4.0 316 ± 31

any post-digester in Plant C and to the HRT (Table 3), shorter than in the other two plants, that did not allow sufficient time to degrade the more recalcitrant organic fractions (e.g. fibers contained in crops) as much as in Plants A and B. On the other hand, also Plant A, even with the same VS-degradation yield of Plant B, showed lower BMY1 (similar to Plant C). In fact, the residual VS of Plant A were capable of producing further bio-methane (under the BMP test), more than those of Plant B (Table 2). The cause of this was probably the slight inhibition of the methanogenic activity, occurred in the intermediate digesters of Plant A, as it was already described above. The total methane (or biogas) productions obtained at the end of the observation period (March 2009) were reported in Table 4. To compare the performances achieved by the three plants, (Table 4) the specific methane and biogas productions (N m3 Mg1 of input-biomass) were also reported and the volumetric (i.e. per digestion volume unit) methane and biogas production rates 1 (N m3gas m3 ). The highest total biogas and methane producdig d tions (N m3 Mg1 w:w: ) were found for Plant B, because the input mixture was more concentrated in terms of TS (Table 2). Despite that, as Plant C was fed with shorter HRTs and higher OLRs (Table 3), the volumetric biogas and methane production rates resulted higher than in Plants A and B. Furthermore, the specific methane productions (SMP), calculated on TS basis, were almost the same in all plants (Table 4). The SMPs measured for the three AD processes, re-calculated as a fraction (BMY2, Eq. (2)) of the BMP of the input mixtures, give an idea of how close the full scale processes were to the ideal conditions reached in the batch tests and can be compared to the BMY1. The highest BMY2 resulted for Plant B (Table 4), confirming the results obtained by the BMY1 parameter (Table 4). Plants A and C gave similarly lower BMY2, also in accordance with the previously calculated BMY1 (Table 4). 3.5. New approach proposed for the evaluation of full-scale AD-process efficiency The VS degradation yield, based on the annual average concentrations observed, was calculated as indicators of the process efficiency achieved by the three plants (Table 4). This parameter is normally used in literature for evaluating the process performances obtained in AD processes (Hartmann and Ahring, 2005; Chen et al., 2008). In the present work, the aimed was proposing the BMY1 (Eq. (1)) as a new indicator for the same purpose. The BMY1 (Eq. (1)), which is a comparison between the residual BMP of the digested materials (output digestate) and the initial BMP (input mixture), should be considered as more correct than the VSyield for describing the real capacity of the full-scale process of

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achieving the maximum producible bio-methane. As it was already discussed, the BMP is a direct measurement of that fraction of the organic matter that can be really bio-degraded by anaerobic microbial consortia, while the VS measures the overall OM, including also those fractions that are not bio-degradable in an AD process, i.e. recalcitrant molecules, such as lignin-cellulose complex, recalcitrant lipids (Tambone et al., 2010) and other aromatic compounds. For this reason, low VS degradation yields may be caused by inefficiency of the AD process, but may also be the consequence of the presence of un-degradable fractions, so that this indicator would not be appropriate. Considering, as example, the three case-studies presented here, all plants showed higher BMY1 (87–93%) compared to the VS-degradation efficiencies (70–79%), indicating that part (around half) of the un-degraded VS (21–30%) were not possible to be degraded, even in ideal lab-AD conditions (BMP test). At the same time, Plant C resulted in lower (70%) VS-degradation yield compared to Plants A and B (79%), but on the other hand the BMY1 was even higher (88%) than that of Plant A (87%). This means that more than half of the un-degraded VS in plant C were actually recalcitrant to biodegradation, so that they could not be transformed into biogas, neither in an efficient AD process. The new indicator BMY1 can be also compared to the BMY2 (Eq. (2)), which represent the ratio (%) between the specific bio-methane production measured in the full-scale plants, as average yield obtained during the observation period (Table 4) and the BMPin, measured in the laboratory (Eq. (2)). Both BMY1 and BMY2 gave very similar results for all plants, demonstrating that they both could be used as indicators of the efficiency of a process. The BMY1 may be more interesting for applications, because it can be measured directly by sampling the input and output materials in the plant and without measuring any production data. This would be important, especially in small-sized agricultural biogas plants, where it is often difficult to obtain certain and correct data from the plant instruments and sensors (Walker et al., 2009). 4. Conclusions This second paper (Part II) aimed at completing the study of full-scale-AD processes, started in the first part (Schievano et al., in press) with a one-year survey on three biogas plants, in Italy. Here, a new indicator was proposed and validated, to correctly asses full-scale-AD process efficiency (BMY1), This indicator (based on the BMP test, applied to the input and output materials) measures how much the bio-methane productions of a full-scale AD process approach those obtainable in optimized AD process in laboratory conditions (BMP test) and it is an easily applicable tool for on-field efficiency evaluations. Acknowledgements This work was performed and took part of a wider applied-research project: Biomass to Biogas, Bio.Bi., financed in the period

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