Bioresource Technology 102 (2011) 8814–8819

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Bioresource Technology

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

Andrea Schievano ⇑, Giuliana D’Imporzano ⇑, Valentina Orzi, Fabrizio AdaniRicicla 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 2011Received in revised form 26 June 2011Accepted 6 July 2011Available online 14 July 2011

Keywords:Anaerobic digestionBiogasMethane potentialProcess efficiencyRenewable energy

0960-8524/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.biortech.2011.07.012

⇑ Corresponding authors. Tel.: +39 02 503 16543 (A16546 (G. D’Imporzano).

E-mail addresses: andrea.schievano@unimi.it (A. Sno@unimi.it (G. D’Imporzano).

a b s t r a c t

Biogas plants need easy and practical tools for monitoring and evaluating their biological process effi-ciency. As soon as, in many cases, biomass supply present considerable costs, full-scale anaerobic diges-tion (AD) processes must approach, as much as possible, the potential biogas yield of the organic mixturefed to the biodigesters. In this paper, a new indicator is proposed (the bio-methane yield, BMY), for mea-suring the efficiency in full-scale AD processes, based on a balance between the biochemical methanepotential (BMP) of the input biomass and the residual BMP of the output materials (digestate). For thispurpose, a one-year survey was performed on three different full-scale biogas plants, in the Italianagro-industrial context, and the bio-chemical processes were fully described in order to calculate theirefficiencies (BMY = 87–93%) and to validate the new indicator proposed, as useful and easily applicabletool for full-scale AD plants operators.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In Europe, the development of anaerobic digestion (AD) beganin the sector of civil sewage treatment plants and by the recoveryof landfill biogas (EurObserv’ER, 2009). Besides, agricultural biogasplants using livestock effluents were, only, about 400 in 1994,while today, in all countries of the EU, more than 6000 anaerobicdigesters operate in this sector; the highest number of AD plantsis situated in Germany, followed by Denmark, Austria, Swedenand Italy and it is rapidly growing (EurObserv’ER, 2009). In 2007,European production of primary energy from biogas reached5.9 million tons of oil equivalent (Mtoe), i.e. 1 Mtoe more than in2006 (increase of the 20%) (EurObserv’ER, 2009). In a recent survey,EurObserv’ER estimates, for the year 2010, a biogas production ofapproximately 8.6 Mtoe.

This increasing number of biogas facilities and the growingcomplexity of this sector, in terms of biomass utilized (dedicatedcrops, byproducts, waste, farming residues, etc.), plant size andtype (agricultural firm, industrial facility, municipal waste treat-ment plant, etc.), lead to the need of investigation for favoring datacollection and correct operation of the biological processes.

First of all, the economical sustenance of biogas facilitiesrequires that the AD process achieve constantly the highest

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. Schievano), tel.: +39 02 503

chievano), giuliana.dimporza

methane yields permitted by the kind of fed-materials, allowingthe maximum hours/year of electric energy (EE) delivering. Liter-ature proposed different approaches to measure the potentialproduction of methane from a substrate, directly under opti-mized conditions by laboratory-scale tests (Hansen et al., 2004;Gunaseelan, 2007; Schievano et al., 2008, 2009a), i.e. the bio-methane potentials (BMP). It is important to know the BMP ofthe organic mixture used to feed a full scale AD process, espe-cially when this input-biomass has a considerable supply cost(e.g. energy-dedicated crops). In these cases, full-scale AD pro-cesses must exploit its BMP as much as possible, by approachingthe yields obtainable under optimized lab-conditions (Schievanoet al., 2009b, 2010). For these reasons, full scale AD performancesneed to be evaluated in order to understand if the plant is effec-tively able to entirely exploit the potential biogas of the input or-ganic mixture.

In literature, AD process efficiency is normally measured bylooking at the organic matter (OM) degradation and the ratio be-tween volatile solids (VS) input and output is often used as indi-cator of the process yield (Demirer and Chen, 2004; Hartmannand Ahring, 2005). Nevertheless, while this indicator directlywell-explains the OM abattoir, it is not incisive for representingthe effective exploitation of the bio-methane potentially produc-ible. In fact, the VS-analysis provides only a quantitativemeasurement of the OM, while nothing tells about its biodegrad-ability under anaerobic conditions (Schievano et al., 2010). Asconsequence of that, low VS degradation measured may be anindex of AD process inefficiency, but also, may be caused bythe consistent presence of recalcitrant fractions in the substrate,

A. Schievano et al. / Bioresource Technology 102 (2011) 8814–8819 8815

which cannot be bio-degraded, neither in ideal AD conditions(e.g. lignin).

The BMP parameter, instead, considers both quantitative (it de-pends on the quantitative characteristics of the substrate, i.e. TSand VS contents) and qualitative (nature of the organic moleculescontained) aspects of a substrate, because this biological test mea-sures only the effectively biodegradable fractions of the OM. Nor-mally, full scale AD processes produce unstable digestates andconsiderable biogas can be further produced, while in most casesthis happens in uncovered storage tanks, especially in summertime(Kaparaju and Rintala, 2003; Kaparaju et al., 2009). The measure-ment of this residual biogas potential under ideal lab-conditions(BMP), compared to the initial biogas potential (BMP of the feedingmaterials) can help in evaluating full-scale AD process effective-ness. A balance of the BMPIN and the BMPOUT can give a correct in-dex of the AD process efficiency achieved. For this calculation, themass balance of the AD process, i.e. the input–output mass flows,must be known. The present paper is the second part of a widerstudy on three full-scale AD plants located in Northern Italy andthe first paper (Schievano et al., in press) aimed at calculating,through different approaches, the mass, carbon and nutrients bal-ances of the three AD processes observed for a one-year period. Inthis second paper, the same three full-scale AD processes were ob-served, through a more detailed one-year data survey of the mainchemical and bio-chemical parameters involved. The description ofthese three case studies allowed to use them for proposing a newindex (based on the BMP) for the evaluation of full-scale AD pro-cess performances.

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

Datum Frequency

Total weight of inputmaterials

Registered every day by plant operators

Total Electricitygenerated

Continuously accounted by the enginesmonitoring systems

CH4/CO2 ratio in thebiogas

Registered every hour by on-line sensors

Operational Temperature Registered continuously by on-line sensors

Sample type Sampling frequency Characterizationa

Input mixture 6 samples, approx.every 2 months

TS, VS, TOC, TN, ON, TP, TK, pH,VFA, TA, N-NHþ4 , BMP, OD20

Intermediatedigesters

6 samples, approx.every 2 months

pH, VFAtot, AlktotN-NH4+

Outputdigestate

6 samples, approx.every 2 months

TS, VS, TOC, TN, ON, TP, TK, pH,VFA, TA, N-NHþ4 , BMP, OD20

a TS = total solids, VS = volatile solids, TOC = total organic carbon, TN = totalnitrogen, 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.

2. Methods

2.1. Data survey on the characteristics of the three full-scale biogasplants observed

The three full-scale plants, operating in the agro-industrial con-text in northern Italy, were observed for a one-year period (April2008–March 2009). All plants operated by continuously-stirred-tank-reactors (CSTR) under ‘‘wet’’ conditions, i.e. with a total solids(TS) content in the reactors below 100 g kg�1 wet weight (w.w.).The first plant (Plant A) was fed with the organic fraction of themunicipal solid waste (OFMSW) (approx. 26,000–28,000 Mg y�1),collected separately by 5 municipalities, which externalize itstreatment to this private facility. The total digestion volume of5000 m3 is divided into 4 digesters (1000 m3 each, loaded in paral-lel 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-utilizesthe swine manure as liquid substrate in the biogas plant (about23,000–25,000 Mg y�1). The feeding mixture is enriched by co-digesting with pig slurry various energy crops (maize silage, triti-cale and sorghum), agricultural residues (barley thresh from beerindustry) and industrial organic by-products, such as glycerin(from bio-diesel production plants), molasses (from sugar caneproduction), bakery-industry waste and olive mill sludge. The totaldigestion volume of 6000 m3 is divided into 2 digesters (1500 m3

each, loaded in parallel with ½ of the total input flow) and 1post-digester (3000 m3), connected in series.

The third plant (Plant C), similarly to Plant B, is located in a farmand its feeding mixture is composed of swine plus cow manure,maize silage, milk whey and rice culture by-products. The totaldigestion volume of 1600 m3 was not divided into subunits (onlyone digester).

Energy, biogas and methane production data were already re-ported 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 weresampled in the plants: the input mixture, the output digested slur-ry – coming out from the last digester (post-digester) – and theslurry contained into the intermediate digesters. For Plant C (whichhad only one digester), the samples were withdrawn directly fromthe digester. All materials were sampled approximately every2 months (6 input samples, 6 output samples and 6 intermediatesamples per each plant). All samples from the digesters were takenwhile the mixers were operating in both digesters and loadingfacility, to avoid any biomass stratification. The chemical charac-terization and the biological assays performed on the samplesare specified in Table 1 and some of the analyses were already pre-sented in the first part of this work (Schievano et al., in press). Sam-ples were dried and ground to 1 mm and stored for subsequentanalyses.

2.3. Chemical characterization

Chemical characteristics of the input and output materials weredetermined in double for each sample (6 input + 6 output per eachplant). Some chemical characteristics were already reported in thePart I of this work (Schievano et al., in press): total solids (TS), vol-atile solids (VS), total organic carbon (TOC), total nitrogen (TN), or-ganic 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 fattyacids, total alkalinity and ammonia concentrations) were madeon the liquor contained in the digesters. Volatile fatty acids (VFAs)and total alkalinity (TA) in the bulk samples, were performed on a5-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 titrationwith 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 ofthe oxygen uptake rate in a water solution during the microbialrespiration in degrading a suspended solid matrix. The microbialrespiration works out in standardized moisture conditions and inmaximized conditions of both oxygenation and bacteria–substrate

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interaction, amplifying the differences among different samples.The potential oxygen uptake was determined as the cumulativeoxygen demand during the 20-h test (OD20: g O2 kg�1 w.w.20 h�1). 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 re-ported by Schievano et al. (2009a). Moreover, it was successfullyused by Schievano et al. (2010) as an indicator for preventing thepossible inhibiting conditions due to organic overloading of thedigesters. Also in this work, the OD20 was used for calculatingthe short-term-degradable organic loading rates (OLRs) of ob-served AD processes.

2.5. Bio-methane potential and process effectiveness determination

The Bio-Methane Potentials (BMPs) of all samples (both fedmixtures and digestates) were determined by using the method re-ported by Schievano et al. (2008, 2009a). Quantitative biogas pro-duction was estimated by withdrawing extra-pressure gas with a60-ml syringe. This procedure was always performed at controlledtemperature of 37 �C; the residual gas pressure in the batches, afterthe gas extraction, was always detected and the measured volumewere reported to standard temperature (25 �C) and pressure(1 atm). Qualitative analyses of the biogas were performed by agas-chromatograph (Micro GC 3000, Agilent Technology), fordetermining the CH4 concentrations (v/v) in the biogas. All thetests were performed in duplicate.

This test was applied for evaluating both the potential bio-methane productions of the fed mixtures and the residual bio-methane producible from the digestates. BMP detected for both in-put and output material were used in joint with mass balance datato determine the biogas yield (BMY1) of the three plants, by usingthe following equation:

BMY1 ð%Þ ¼ ðBMPin � TSin � BMPout � TSoutÞ=ðBMPin � TSinÞ� 100 ð1Þ

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

Table 2Characterization of the input mixtures and the digestates of the three full-scale biogas plaperiod.

Plant A

Fed Digestate

TS g kg�1 121 ± 13 37 ± 5VS g kg�1 TS 878 ± 37 661 ± 49BMP Nd m3 CH4 kg�1 TS 370 ± 40 168 ± 15

Nd m3 CH4 kg�1 w.w 45 ± 5 6 ± 1OD20 g O2 kg�1 TS 20 h 251 ± 47 132 ± 40pH 4.8 ± 0.9 8.2 ± 0.2Ammonia g l�1 1.01 ± 0.07 3.56 ± 0.05VFA g acetic acid l�1 16.9 ± 3.3 2.4 ± 1.1TA g CaCO3 l�1 19.8 ± 3.0 14.6 ± 2.1VFA/TA kg acetic acid kg�1 CaCO3 0.850 ± 0.165 0.170 ± 0.074TOCa g kg�1 TS 481 ± 24 362 ± 16TNa g kg�1 TS 42.2 ± 10.7 137.3 ± 11.9N-NHþ4

a g kg�1 TS 8.3 ± 5.6 96.2 ± 10.0ONa g kg�1 TS 33.9 ± 23.0 41.1 ± 4.3TKa g kg�1 TS 12.8 ± 3.4 43.2 ± 5.5TPa g kg�1 TS 3.4 ± 0.8 11.4 ± 3.0

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

produced (SMP, as N m3 kg�1 TS-input) in the full-scale plants andcalculated 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) gen-erated during the observation period was already reported in thePart I of this work (Schievano et al., in press), as well as the meth-ane content (% v/v) in the generated biogas. No quantitative mea-surement of the biogas generated was possible, because therewere not flux-meters in the on-line monitoring systems of thefacilities. The total methane generation was calculated from the to-tal EE generation assuming a caloric power of the methane of0.2475 kW h mol�1 CH4 and an EE generation yield (indicated bythe suppliers of the internal combustion engine units) of 35%.The total biogas productions were then calculated from the ob-served average methane content. The total methane and biogasproductions were reported in volume units, referred to the stan-dard temperature and pressure conditions (25 �C, 1 atm).

3. Results and discussion

3.1. Input materials

The chemical and biological characterization of the materialsfed 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 tothe others. This was probably due to the presence of biomassescharacterized by low moisture contents, such as glycerin, molassesand 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 fromthe feeding materials (BMP), and referred to TS unit, resulted al-most the same for all the feeding mixtures. However, when BMPwas referred to the wet weight unit, it resulted higher in Plant B,because the feed was more concentrated (Table 2). The biodegrad-ability of the organic mixtures, measured as OD20, resulted almostequal for all the three plants, although with a slightly loweraverage value for Plant C (Table 2). As the OD20 represents an indi-cator of the short-term biodegradability of the organic matter

nts (A, B, C) observed. Data reported as average of 5 samples during the observation

Plant B Plant C

Fed Digestate Fed Digestate

184 ± 7 58 ± 3 130 ± 10 53 ± 16913 ± 15 698 ± 27 890 ± 11 723 ± 25356 ± 29 85 ± 10 375 ± 18 117 ± 8

66 ± 5 5 ± 1 49 ± 2 6 ± 1249 ± 28 66 ± 19 210 ± 49 64 ± 66.5 ± 0.8 7.9 ± 0.1 4.6 ± 0.5 7.9 ± 0.2

2.25 ± 0.02 3.32 ± 0.02 1.46 ± 0.01 1.92 ± 0.035.7 ± 5.0 2.4 ± 2.5 16.2 ± 2.8 1.8 ± 0.6

17.2 ± 2.2 16.3 ± 0.8 8.1 ± 3.0 9.5 ± 0.80.940 ± 0.833 0.140 ± 0.147 0.710 ± 0.122 0.190 ± 0.062

470 ± 30 361 ± 22 486 ± 11 393 ± 2426.3 ± 5.2 90.5 ± 6.2 26.4 ± 1.5 66.5 ± 31.712.3 ± 2.3 57.2 ± 7.1 11.2 ± 1.2 36.2 ± 1.714.0 ± 2.6 33.3 ± 4.1 15.2 ± 1.7 30.3 ± 1.420.3 ± 6.7 70.7 ± 15.0 18.3 ± 19.2 48.0 ± 1.3

6.1 ± 2.5 20.2 ± 6.4 7.5 ± 1.8 18.8 ± 3.0

press).

Table 3Process parameters measured in the three observed full scale biogas plants (A, B, C), during the observation period (April 2008–March 2009).

Plant A Plant B Plant C

Total annual loaded material (as wet weight) Mg a�1 45,251 38,544 22,745Loading rate (as wet weight) Mg d�1 121 ± 5 100 ± 5 62 ± 4Loading rate (as TS) Mg TS d�1 14.6 ± 0.6 18.4 ± 0.9 8.1 ± 0.5Organic loading rate (OLR as VS) kg VS m�3dig d�1 2.571 ± 0.42 2.8 ± 0.54 4.483 ± 0.35Organic loading rate (OLR as OD20) kg O2 (20 h) m�3dig d�1 0.735 ± 67 0.764 ± 49 1.058 ± 58Hydraulic retention time (HRT) d 40 ± 3 57 ± 5 26 ± 2Methane content in biogas (average in 1-year survey) % v/v CH4 57 ± 3 55 ± 4 63 ± 4Operational temperature �C 54.8 ± 1.8 38.2 ± 1.2 55.2 ± 1.8pH (in all the digesters) �log [H+] 8.06 ± 0.12 7.82 ± 0.13 7.93 ± 0.13Total VFA concentration (in all the intermediate digesters) g kg�1 6.572 ± 1.4 2.014 ± 0.5 2.076 ± 0.36VFA/TA ratio (in all the intermediate digesters) 0.41 ± 0.13 0.128 ± 0.04 0.19 ± 0.08Ammonia concentration (in all the intermediate digesters) g kg�1 3.54 ± 0.58 3.04 ± 0.28 1.92 ± 0.09

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(Schievano et al., 2010), the lower OD20 observed for Plant C couldbe ascribed to probable higher presence of fibers (contained incrops and in cow manure), which are bio-degradable under anaer-obic conditions only by longer-term degradation process (Schieva-no et al., 2010). The initial pH was higher in mixture B, than othermixtures and the low pH of mixtures A and C were probably due tothe fact that in these plants the feeding materials were mixed andstored into big tanks for 2–3 days, during which partial acidogen-esis may take place. In Plant B, instead, the solids materials weremixed with the swine manure every hour in a batch-charge systemand fed. The same reasons explain the VFA concentrations and theVFA/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 pro-cesses 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, organ-ic loading rate (OLR) and the hydraulic retention time (HRT), whilethe biological processes status was evaluated by the average ob-served methane content in the biogas, the average temperatures,the pH in the digestion-body and the concentrations of some rele-vant 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 respectivelyof 45,251, 38,544 and 22,745 Mg (as wet weight), corresponding toa 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 higherthan in Plant B, but this did not correspond to higher TS and VSloading rates (Table 3). Furthermore, the HRT of Plant B was muchhigher (HRT of 57 days) than in Plant A (HRT of 40 days) and, aboveall, than the Plant C (HRT of 26 days). This was probably necessarybecause the loaded TS were higher in Plant B (Table 3) than in theother two plants and because, also, the mesophilic process (PlantB) should determine a slightly lower degradation kinetics com-pared 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 (4.48kg VS m�3 d�1), i.e. nearly the 100% higher than OLRs of plants Aand B (Table 3). However, the OLRs were calculated, also, usingthe 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 frac-tions of the VS, which are, under anaerobic conditions, quickly fer-mented to VFAs, these latter responsible for partial inhibitions ofthe methanogenic activity. Fore these reasons, the OLR, calculatedon the OD20-basis, may result a useful parameter for describingeventual overloading conditions. Plant C showed OLRs (as OD20)only the 20% higher than those applied to Plants A and B, so that

possible overloading stress, caused by the higher VS loading in PlantC (almost double) than in Plants A and B, was mitigated by rela-tively low loading of short-term degradable OM (OLR, as OD20).

This was corroborated by the fact that both values of VFA con-centrations in the intermediate digesters and VFA/TA ratios, forPlants B and C (Table 3), were in an acceptable ranges, while PlantA showed VFA concentration more than three times higher than inthe other two plants (Table 3). This value is close to the limitsindicated for process inhibition (VFA concentration over 6 g l�1

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 themethanogenic activity in the intermediate digesters of Plant A.An explanation to this may be found in the higher ammoniaconcentrations measured in Plant A (Table 3), which may have par-tially inhibited the methanogens and the consequent VFA concen-tration, as reported by Chen et al. (2008), for concentrations above3 g N-NHþ4 l�1. In any case, this problem was partially compensatedby a strong methanogenic activity in the post-digester, where thetotal VFA concentration was as low as in the other two plants(around 2 g l�1) (Table 3). In any case, all the observed plantsshowed a correct and well developed AD process and so they canbe considered as well representative case-studies for successivediscussion.

3.3. Characterization of the output materials and changes in theorganic matter characteristics after the AD process

Table 2 reports the characteristics of the digested materials, asaverage of the observed period. The AD process determined impor-tant reductions of the TS contents in the digested materials and, forall plants, the comparison between the characteristics of the inputand the output materials confirmed that the AD processes inducedeep modifications of the OM, as indicated for example by Tam-bone et al. (2010). In particular, the VS contents (600–700 g kg�1)were lower than in the input materials (around 900 g kg�1 TS) (Ta-ble 2). The bio-degradable fractions of the total OM, detected bythe BMP (long-term biodegradability) and OD20 (short term biode-gradability), were also severely reduced in the digestates, with re-spect to the fed materials (Table 2).

More in depth, comparing Plants A, B and C, the VS contentswere lower in the digestates of Plant A, while the OD20 and BMPwere higher (Table 2). Plant C had higher VS content, higher resid-ual BMP, but, on the other hand, low OD20, similar to those of PlantB, which, instead, was characterized by the lowest VS, OD20 andBMP in the digestates. This means that Plant B was able to degradeboth the short- and long-term biodegradable fractions contained inthe VS, better than Plant A and C (Table 2) and to obtain higher bio-logical stability of the digested OM (low BMP and OD20). The TOCcontents, also, decreased in the digestates, with respect to the

Table 4Bio-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).

Plant A Plant B Plant C

TS degradation yield % 72% 73% 63%VS degradation yield % 79% 79% 70%Bio-methane yield (BMY1) % 87% 93% 88%Bio-methane yield (BMY2) % 88% ± 9% 93% ± 13% 84% ± 8%Total bio-methane production (1 year) N m3 a�1 1788121 2355315 931844Total biogas production (1 year) N m3 a�1 3845808 ± 202411 4565892 ± 332065 1823650 ± 115787Bio-methane production rates N m3 CH4 d�1 4899 ± 352 6453 ± 254 2553 ± 197Biogas production rates N m3 biogas d�1 10536 ± 757 12509 ± 492 4996 ± 386Volumetric bio-methane production rate N m3 m�3dig d�1 0.98 ± 0.07 1.08 ± 0.04 1.60 ± 0.12Specific biogas productions on w.w. basis N m3 Mg�1 85.0 ± 4.5 118.5 ± 8.6 80.2 ± 5.1Specific biogas productions on TS basis N m3 Mg�1 TS 703 ± 68 644 ± 86 619 ± 58Specific bio-methane productions (SMP) on w.w. basis N m3 CH4 Mg�1 39.5 ± 4.4 61.1 ± 8.5 41.0 ± 4.0Specific bio-methane productions (SMP) on TS basis N m3 CH4 Mg�1 TS 327 ± 36 332 ± 46 316 ± 31

8818 A. Schievano et al. / Bioresource Technology 102 (2011) 8814–8819

input materials, while, on the contrary, the TN contents (referred tothe TS) of the digestates increased (Table 2). The organic fraction ofthe total nitrogen (ON) also increased, but to less extent, comparedto the TN. The ammonia contents, in fact, increased more than pro-portionally, compared to the TN contents. This concentration-effectwas particularly evident in Plant A, because of the higher ON con-tent in the input material (33.9 g N kg�1 TS), with respect to theother two plants (14.0 and 15.2 g N kg�1 TS for Plant B and C,respectively (Table 2).

The final pHs were sub-alkaline in all the plants and in an opti-mal range for the methanogenic activity (Pind et al., 2003). Thiswas confirmed by the VFA and TA concentrations and their ratios(Table 2), which showed values compatible with stable methano-genic conditions, i.e. VFA < 6 g l�1 as acetic acid and VFA/TA < 0.4,as suggested by Pind et al. (2003).

3.4. Process performances: degradation yields and methaneproductions

The mass balance of the observed AD processes were object ofstudy in the first part of this work (Schievano et al., in press) andin this second paper those results were used for calculating theTS and VS degradation yields achieved during the AD process.The TS and the VS were significantly degraded in all the plantsby mean of approximately the 60–70% and the 70–80%, respec-tively (Table 4), similarly to what reported in literature for variouslaboratory-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 de-graded and it is important to understand whether the cause wasthe recalcitrance of such fraction of the OM or, on the other hand,the inefficiency of the AD process on potentially biodegradablefractions. However, no clue can be found in quantitative measure-ments of the OM (TS, VS). To understand if the un-degraded VSwere effectively un-degradable (recalcitrant molecules, such asaromatic compounds, lignin, cutin, long chain fatty acids, poly-phenols, etc.) or if the AD process was inefficient on potentiallydegradable fractions of the organic matter, the degradation yieldswere re-calculated, basing on more qualitative measurements ofthe OM, i.e. the BMP.

The degradation yields calculated on the BMP, i.e. the bio-meth-ane yields (BMY1, Eq. (1)) indicated relatively high efficiency of thefull scale processes (87–93% of the BMPin) (Table 4), compared tocontrolled AD processes in optimized lab-scale trials (BMP tests).However, some differences between the three considered plantsmust be noted, as soon as Plant B showed both the best VS degra-dation and BMY1. Plant C, instead, showed slightly lower VS degra-dation, if compared to Plants A and B and, at the same time, Plant Cshowed lower BMY1. This was probably caused by the absence of

any post-digester in Plant C and to the HRT (Table 3), shorter thanin the other two plants, that did not allow sufficient time to de-grade the more recalcitrant organic fractions (e.g. fibers containedin crops) as much as in Plants A and B. On the other hand, also PlantA, even with the same VS-degradation yield of Plant B, showedlower BMY1 (similar to Plant C). In fact, the residual VS of Plant Awere capable of producing further bio-methane (under the BMPtest), more than those of Plant B (Table 2). The cause of this wasprobably the slight inhibition of the methanogenic activity, oc-curred in the intermediate digesters of Plant A, as it was alreadydescribed above.

The total methane (or biogas) productions obtained at the endof the observation period (March 2009) were reported in Table 4.To compare the performances achieved by the three plants, (Table4) the specific methane and biogas productions (N m3 Mg�1 of in-put-biomass) were also reported and the volumetric (i.e. per diges-tion volume unit) methane and biogas production rates(N m3

gas m�3dig d�1). The highest total biogas and methane produc-

tions (N m3 Mg�1w:w:) were found for Plant B, because the input mix-

ture was more concentrated in terms of TS (Table 2). Despite that,as Plant C was fed with shorter HRTs and higher OLRs (Table 3), thevolumetric biogas and methane production rates resulted higherthan in Plants A and B. Furthermore, the specific methane produc-tions (SMP), calculated on TS basis, were almost the same in allplants (Table 4).

The SMPs measured for the three AD processes, re-calculated asa fraction (BMY2, Eq. (2)) of the BMP of the input mixtures, give anidea of how close the full scale processes were to the ideal condi-tions reached in the batch tests and can be compared to theBMY1. The highest BMY2 resulted for Plant B (Table 4), confirmingthe results obtained by the BMY1 parameter (Table 4). Plants A andC gave similarly lower BMY2, also in accordance with the previ-ously calculated BMY1 (Table 4).

3.5. New approach proposed for the evaluation of full-scale AD-processefficiency

The VS degradation yield, based on the annual average concen-trations observed, was calculated as indicators of the process effi-ciency achieved by the three plants (Table 4). This parameter isnormally used in literature for evaluating the process perfor-mances obtained in AD processes (Hartmann and Ahring, 2005;Chen et al., 2008). In the present work, the aimed was proposingthe BMY1 (Eq. (1)) as a new indicator for the same purpose. TheBMY1 (Eq. (1)), which is a comparison between the residual BMPof the digested materials (output digestate) and the initial BMP (in-put mixture), should be considered as more correct than the VS-yield for describing the real capacity of the full-scale process of

A. Schievano et al. / Bioresource Technology 102 (2011) 8814–8819 8819

achieving the maximum producible bio-methane. As it was alreadydiscussed, the BMP is a direct measurement of that fraction of theorganic matter that can be really bio-degraded by anaerobic micro-bial consortia, while the VS measures the overall OM, includingalso those fractions that are not bio-degradable in an AD process,i.e. recalcitrant molecules, such as lignin-cellulose complex, recal-citrant lipids (Tambone et al., 2010) and other aromatic com-pounds. For this reason, low VS degradation yields may becaused by inefficiency of the AD process, but may also be the con-sequence of the presence of un-degradable fractions, so that thisindicator would not be appropriate.

Considering, as example, the three case-studies presented here,all plants showed higher BMY1 (87–93%) compared to the VS-deg-radation efficiencies (70–79%), indicating that part (around half) ofthe un-degraded VS (21–30%) were not possible to be degraded,even in ideal lab-AD conditions (BMP test). At the same time, PlantC resulted in lower (70%) VS-degradation yield compared to PlantsA 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 halfof the un-degraded VS in plant C were actually recalcitrant to bio-degradation, so that they could not be transformed into biogas, nei-ther 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-meth-ane production measured in the full-scale plants, as average yieldobtained during the observation period (Table 4) and the BMPin,measured in the laboratory (Eq. (2)). Both BMY1 and BMY2 gavevery similar results for all plants, demonstrating that they bothcould be used as indicators of the efficiency of a process. TheBMY1 may be more interesting for applications, because it can bemeasured directly by sampling the input and output materials inthe plant and without measuring any production data. This wouldbe important, especially in small-sized agricultural biogas plants,where it is often difficult to obtain certain and correct data fromthe plant instruments and sensors (Walker et al., 2009).

4. Conclusions

This second paper (Part II) aimed at completing the study offull-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 correctlyasses full-scale-AD process efficiency (BMY1), This indicator (basedon the BMP test, applied to the input and output materials) mea-sures how much the bio-methane productions of a full-scale ADprocess approach those obtainable in optimized AD process in lab-oratory conditions (BMP test) and it is an easily applicable tool foron-field efficiency evaluations.

Acknowledgements

This work was performed and took part of a wider applied-re-search project: Biomass to Biogas, Bio.Bi., financed in the period

2008–2009 within ‘‘Programma regionale di ricerca in campo agr-icolo 2007–2009, Domanda di contributo per le attività di ricerca,sperimentazione e dimostrazione’’, by Regione Lombardia, Milan,Italy.

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