lab digester

Proceedings Venice 2010, Third International Symposium on Energy from Biomass and Waste Venice, Italy; 8-11 November 2010 2010 by CISA, Environmental Sanitary Engineering Centre, Italy

ANAEROBIC DIGESTION OF SELECTED AGRICULTURAL AND INDUSTRIAL RESIDUES (GRAPE-SEEDS AND LEATHER DUST): LAB-SCALE SYSTEM, COMPARATIVE TESTS AND DIGESTATE CHARACTERISATION

F. ANDREOLA*, L. BARBIERI*, A. BERLONI**, C. CARAMIELLO°, I. LANCELLOTTI*, G. MANGANI**, F. RIGHI°, F. TATÀNO° A ND R. TAURINO*

* Department of Materials and Environmental Engineering, University of Modena and Reggio Emilia, via Vignolese 905, 41100 Modena, Italy ** Laboratory of Environmental Chemistry, Faculty of Sciences and Technologies, University of Urbino “Carlo Bo”, Campus Scientifico – Sogesta, 61029 Urbino, Italy ° DIGeoTeCA – Department of Geological Sciences, Chemical and Environmental Technologies, Faculty of Sciences and Technologies, University of Urbino “Carlo Bo”, Campus Scientifico – Sogesta, 61029 Urbino, Italy

SUMMARY: This papers synthetically presents and discusses the experimental results on comparative anaerobic digestion tests (carried-out at a lab-scale) and consequential digestate characterisation for two peculiar organic substrates, which are consistent at Italian regional and national levels: grape-seeds, having an agricultural origin; and vegetable-tanned leather dust, having differently an industrial origin. Under the assumed experimental conditions of the performed test series, the grape-seed substrate showed a certain attitude to be considered as a possible energy source by means of anaerobic digestion; differently, biomethane production was limited for vegetable-tanned leather dust. The detailed digestate characterisation showed some differences (for instance in terms of nitrogen containing compounds) depending on the specific organic material used as substrate (i.e., grape-seeds or leather dust).

1. INTRODUCTION

The climate change phenomenon is currently the major environmental challenge for the modern human society. There is a general scientific consensus that this phenomenon is due to the increase in concentrations of such greenhouse gases (GHG) produced by human activities, primarily the combustion of fossil fuels.

Venice 2010, Third International Symposium on Energy from Biomass and Waste

In order to define and implement reliable policies and solutions for the reduction of anthropogenic GHG emissions, the overall waste management sector could realistically play a non-negligible role (ISWA, 2009). In the specific waste context, particularly the “bioenergy” option from biomass resources and organic waste has gained increasing importance in the integrated strategies to mitigate climate change (Scheffran et al., 2004; Marshall, 2007). Indeed, this peculiar mitigation potential is officially considered as a possible key option by the Intergovernmental Panel on Climate Change (IPCC). According in fact to the current IPCC’s Fourth Assessment Report (IPCC, 2007), “bioenergy” is expressly mentioned as a possible mitigation opportunity for renewable heat and power within the “energy supply” sector. Interestingly, from the legislative point of view for instance at the EU level the Directive 2009/28/EC on renewable energy sources applies (at its Article 2) a definition of “biomass” which also includes: (1) “the biodegradable fraction of products, waste and residues from biological origin from agriculture”; (2) “the biodegradable fraction of industrial waste”. Therefore, the organic fraction of diversified typologies of agricultural and industrial wastes and residues is in principle considered as a possible renewable energy resource, whose potential can be eventually exploited through an anaerobic digestion process for biogas production with an expected (or desired) significant methane content. In fact, anaerobic digestion can plays a certain role for global warming savings (Møller et al., 2009): (1) primarily from substitution of fossil fuel by the produced biogas; (2) but also from carbon storage in soil and inorganic fertiliser substitution, in the general and predominant circumstance of final digestate use as a fertiliser. Indeed, for a complete GHG accounting of the anaerobic digestion option also the following aspects should be evaluated (Møller et al., 2009): (1) the possible use of fossil energy at the full scale process facility; (2) the possible emissions from the bioreactor and biogas combustion; (3) the possible emissions from the final digestate when applied to soil.

Given the above-synthesised technical-scientific and regulative framework, specific organic residues of both agricultural and industrial origin, which are quantitatively and qualitatively peculiar at given territorial scales (provincial, regional, national), should be in principle attentively evaluated, characterised and tested as potential and alternative energy sources. In this applied research perspective, the Environmental-sanitary Engineering Research Unit (with the support of the Environmental Chemistry Laboratory) at the University of Urbino “Carlo Bo” and the Research Unit on Chemistry of Materials and Wastes at the University of Modena and Reggio Emilia have recently started a research cooperation focused on the feasibility of anaerobic digestion for methane production from selected organic residues. The specific objectives of this research cooperation are: (1) the development and progressive, temporal improvement/adaptation of a complete lab-scale system and procedure for the combined evaluation of (firstly) the methane production potential from anaerobic digestion tests of given organic residues and (subsequently) the detailed chemical-physical characterisation of the resulting digestate products (in view of the individuation of reliable reuse and recovery options); (2) the exclusive study and characterisation as potential (methane) energy source of agricultural and industrial organic wastes which have real importance, consistency and value at given territorial scales. The present paper deals with the research data of the first temporal phase of this university cooperation, which has been focused on the initial development of the lab-scale system and procedure and the consequential application for the comparative evaluation of methane production and digestate characterisation with the following residual types (see Section 2): (1) an agricultural type represented by grape-seeds (Figure 1 A), with a significant territorial relevance in terms of estimated annual residual production in both Emilia Romagna region (where the University of Modena and Reggio Emilia is located) and Marche region (where the University of Urbino is located), and also at the national Italian level; and (2) a particular industrial type represented by vegetable-tanned leather dust (Figure 1 B), which is generated by the world renown high quality shoe industrial district of Marche region (Tatàno et al., 2008).


Figure 1. Views of the selected agricultural and industrial residues for the carried-out anaerobic digestion tests: A) Grape-seeds; B) Vegetable-tanned leather dust.

2. TERRITORIAL CONSISTENCY OF THE CONSIDERED RESIDU ES

With regard to grape-seeds, resulting from the wine production, some literature indications can be usefully considered for the possible estimate of annual quantities at given territorial scales. At first assuming the produced wine as a basic calculation unit, a generation rate of about 18 kg of grape-marc (without stalks) for each hectolitre of wine is reported in ANPA (2002). Thus, by additionally considering that in 100 g of fresh grape-marc (without stalks) the grape-seed weight is 30 g in average (De Rosa and Castagner, 1994), a corresponding generation rate of about 5.4 kg of grape-seeds for each hectolitre of produced wine is obtained.

By assuming instead the wine-grape production as alternative calculation unit, at least the following literature indications can be usefully derived as generation rate of grape-seeds: in average 3.5 kg, or 4.7 kg or finally 2.5 kg of grape-seeds in 100 kg of wine-grapes, respectively according to Odello (2002), Busi (2002) and Shrikhande (2000). As a consequence, an average quantity rate of almost 3.6 kg of grape-seeds in 100 kg of wine-grapes can be assumed.

According to official data recorded by “ISTAT, Italian Statistical Institute”, Table 1 contains the total productions of both wine-grapes and wine related to the last available year (2009) and disaggregated for the mentioned Marche and Emilia Romagna regions and the overall Italian agricultural territory. Consequently, as the result of an average determination procedure derived from both the previous assumptions for generation rate based either on the wine-grape production or on the wine production, the following relevant annual weight availabilities of grape-seeds concern with the considered territorial areas: (1) 5,175 tons in Marche region; (2) about 34,230 in Emilia Romagna region; and (3) in total 248,220 tons at the Italian level.

With reference instead to leather dust, exclusively vegetable-tanned, this industrial residue is produced by carding and cutting operations on vegetable-tanned leather shoes, which are typical of the high-quality shoe manufacturing district of the Provinces of Macerata, Ascoli Piceno and Fermo located in Marche Region (Tatàno et al., 2008). According to the specific elaboration of the compulsory waste declaration data for the year 2003 (Mangani et al., 2006), the annual generation of vegetable-tanned leather dust is estimated to amount at about 5,320 tons in Marche region.

Table 1. Agricultural area and total production of wine-grape and wine for selected Italian territorial scales during the last year 2009 (ISTAT, 2009).

2009 Wine-grape Wine Territorial scale Production area

[ha] Collected

production [q] Production

[hl] Marche region 18,012 1,407,765 977,939

Emilia-Romagna region 55,407 8,924,620 6,726,931 Italy 703,440 67,046,021 47,235,875

A) B)


3. MATERIALS AND METHODS

3.1 Collection and initial characterisation of considered inoculum and substrates

The used inoculum for both substrates (grape-seeds and leather dust) was an anaerobic sludge from the anaerobic digestion treatment stage of a large wastewater treatment plant (WWTP) located in Central Italy, Adriatic Sea side. The grape-seed material was directly available from a wine grower’s cooperative located in Emilia Romagna region, whilst the vegetable-tanned leather dust was directly collected at a recovery firm located in Marche region and involved in the aerobic treatment process for the generation of an organic-nitrogen fertiliser from this leather dust. For these considered materials, moisture content and volatile solids (VS) were determined in accordance with the official Italian “IRSA-CNR” analytical methods (IRSA, 1984), by using the following laboratory devices: drying oven (model 400, Memmert GmbH), muffle furnace (model BE1185, Bicasa SpA), dryer (Duran) with silica gel, analytical balance (model E/50, Gibertini) with capacity 200 g-0.1 mg, and typical laboratory glassware (graduated cylinders, beaker, pipettes and ceramic dishes). In addition, the ultimate analysis (Tchobanoglous et al., 1993; Niessen, 2002), or equivalently definable elemental analysis, was directly performed on a sample of grape-seeds, whilst the same determination representative for vegetable-tanned leather dust was already available from a previous research study (Mangani et al., 2006).

3.2 Lab-scale system

The lab-scale system (Figure 2), specifically developed for carrying-out the anaerobic digestion tests, represents a variant of the Owen et al. serum bottle method (Owen et al., 1979). Precisely, transparent glass vessels Schott Duran GL45 with a capacity of 1,000 ml (working volume of 500 ml) were used as reaction bottles, with screw caps made of polypropylene. The bottle-caps were properly drilled at their centre for the insertion of a PVC connection tube (Euroset 34 - Eurospital) for biogas transfer to the subsequent measurement bottle. In a complete arrangement, three reaction lines - each consisting of three reaction bottles (Figure 3, left-hand side) - were inserted within a containment plastic (PVC) box in a thermostatic water bath. The simplified method used to measure biogas and methane production from the reaction bottles is based on the hydraulic principle of the volumetric displacement of fluid (Caffaz et al., 2008).

B

A

C

D

G

E

F

Figure 2. Representative view of the experimental lab-scale system developed to conduct and control the anaerobic digestion tests. Legend: A) Internal plastic box for containment of reaction glass bottles in a thermostatic water bath; B) External insulation box; C) Reaction glass bottle; D) Inverted measure bottle; E) PVC connection tube; F) Gas sampling point for possible gas-chromatographic analysis; G) Graduated cylinder.


Consequently, an inverted measure bottle was used containing as liquid barrier a water solution of HCl (pH 4) or NaOH (2%), respectively for the biogas or methane volumetric determination. The inlet of the gaseous mixture from the reaction bottle into the measure bottle was obtained by a simple hypodermic needle (0.80 x 40 mm) installed on a connection tube comprehensive of a roller, a no return valve, and even a “Y”-shaped connector as a possible gas sampling point for future, regular instrumental GC-FID analysis. In order to allow the liquid barrier outflow due to the overpressure of the inflow gas, a second needle - with the same size as for the gas input - was properly inserted in the silicone septum (Microglass Srl) sealing the inverted bottle (Figure 3, right-hand side); finally, a graduated cylinder was adopted for the collection of the moved liquid volume from the inverted bottle. Thus, by assuming the volume of moved liquid equal to the volume of produced gas, it is possible to approximately determinate the total normalised volume of produced biogas/methane as the average value among the considered reaction bottles, with the conclusive derivation of a “specific gas production” (namely “SGPbiogas” for biogas or “SGPCH4

”

for methane, as Nml gVS-1) in relation to the initial total VS content in the feeding mixture. The pH of each organic feeding mixture was measured with a portable pHmeter (Tracer

Pocketester, LaMotte) before closing the reaction bottles. According to the initial values detected for each mixture (see Section 3.3), a pH correction to a level above a minimum threshold of 8 was always performed with a Na2CO3 anhydrous solution. The aim of this adjustment, based on the buffer effect of Na2CO3 in solution, is to avoid an excessive lowering of pH in the early stages of the anaerobic digestion, which would probably cause a slowdown or even a total blockade of the biodegradation process. This operative precaution seems particularly important by considering that pH measurement inside the reaction bottles was no longer possible during the temporal evolution of the anaerobic digestion test, in order to ensure the absence of oxygen in the reactors throughout the test period.

For the temperature control under mesophilic conditions, the mentioned containment box was filled with tap water approximately up to the level of organic mixtures within the reaction bottles, and two water heaters (100W Protherm EU, Hydor-ENG) were placed on the bottom. As an additional and passive measure, during the “Preliminary Test (PT1)” (see Section 3.3) the containment box was optimised with the following composite system of thermal insulation: (1) at first, an intermediate stratum of synthetic wool; (2) subsequently, an external encapsulating container made with sheets of insulating polystyrene as properly sealed. During the carried-out digestion tests, the water temperature inside the containment box was periodically measured with a thermocouple (871A digital thermometer, pan-US).

Figure 3. Details of the lab-scale system for anaerobic digestion tests: the complete arrangement with three reaction lines (each with three reaction bottles) in the containment box (left-hand side), and the bottom of inverted bottle in connection with the top of graduated cylinder (right-hand side).


3.3 Test series of anaerobic digestion

In total, three sequential laboratory test series of anaerobic digestion in batch were realised (Table 2). Firstly, a preliminary test (named as “PT1”) of anaerobic digestion was performed exclusively on the mentioned anaerobic sludge (see Section 3.1) as blank, by using three reaction bottles. The specific aim of test “PT1”, which lasted 39 days, was the optimisation of the initial scheme of the conceived lab-scale system, thus permitting the arrangement of the final system as described in previous Section 3.2 and properly adopted for the subsequent tests.

Then, the second test series (named as “T2”), which lasted 38 days, was performed for the determination of the resulting methane production for both considered substrates (grape-seeds and leather dust). Precisely, during test series “T2” the following reaction lines (each with three reaction bottles) were arranged: (1) mixture of grape-seeds + inoculum (anaerobic sludge); (2) mixture of vegetable-tanned leather dust + inoculum (anaerobic sludge); (3) inoculum (anaerobic sludge) as blank. Finally, an additional test series (named as “T3”) was carried-out for the determination of the resulting biogas production for both considered substrates. This conclusive test series “T3”, which lasted 37 days, was based on the following reaction lines (each with three reaction bottles): (1) mixture of grape-seeds + inoculum (anaerobic sludge); (2) mixture of vegetable-tanned leather dust + inoculum (anaerobic sludge).

For all feeding mixtures in test series “T2” and “T3”, the adopted VS substrate:VS inoculum ratio was 2:1. The intentional selection of a non-low value of the substrate/inoculum ratio (in terms of VS) was finalised to the aim of an expected better evidence of biogas and methane productions directly attributable to the investigated organic substrates. Indeed, despite the possible literature indications of an optimal low value for the substrate/inoculum ratio in terms of VS (Caffaz et al., 2008), for instance the experimental study of Trulli and Mancini (2009), on co-digestion tests with a large range of considered VS organic substrate:VS inoculum ratios (i.e., from 0.8 to 2.4), confirms the usefulness of eventually testing different conditions of the food to microorganism ratio. Thus, in accordance with the assumed VS substrate:VS inoculum ratio (2:1), the weight quantities of organic substrate (grape-seeds or leather dust) and incoulum - as mixture feeding in each reaction bottle - were calculated on the basis of the VS and moisture content properties for the considered materials (see Section 4.1).

Before starting the digestion tests, only for grape-seed material a specific grinding pre-treatment was performed by using a laboratory mortar with a pestle. In fact, grape-seeds are naturally covered by an integument mainly composed of lignin. Thus, it was considered appropriate to break the seeds as a resulting organic substrate more easily available for biomass.

Table 2. Conditions for the laboratory test series of anaerobic digestion. Legend: mean = mean value for three reaction bottles; N, D = Nov., Dec. 2009; J, F = Jan., Feb. 2010.

Test series

Feeding mixture No. reaction bottles

Liquid barrier

Initial pH

(mean)

Corrected pH

(mean)

Duration time

[days]

PT1 Anaerobic sludge (blank) 3 HCl 7.3 8.9 39 (N-D)

T2 Grape-seeds + inoculum 3 NaOH 7.4 8.3 38 (D-J)

Leather dust + inoculum 3 NaOH 6.6 8.1 38 (D-J)

Inoculum (anaerobic sludge)

3 NaOH 7.5 8.6 38 (D-J)

T3 Grape-seeds + inoculum 3 HCl 7.2 8.3 37 (J-F)

Leather dust + inoculum 3 HCl 6.4 8.0 37 (J-F)


3.4 Digestate characterisation

In order to evaluate the properties constancy of digestates and their possible valorisation, the chemical and mineralogical composition of digestates, generated from “T2” and “T3” test series with reference to the original mixtures of grape-seeds + inoculum and leather dust + inoculum, has been determined and compared to digestate originated from anaerobic sludge during the specific blank reaction line of “T2” test series (see Table 2).

In particular, moisture and loss of ignition (LOI) were carried out on the digestates to evaluate volatile and fixed solids. Moisture content was performed by oven-drying the samples at 110 °C for 12 hours until the weight remains constant. Then, the moisture content [%] was calculated from the sample weight before and after drying. The determination of the mass loss by heating in air atmosphere at 600 °C and 1,000 °C was performed to estimate organic/inorganic volatile compound content and to prepare chemically stable ashes with different mineralogical composition.

The first chemical identification of digestate solid component was performed by infrared spectroscopy. The measurements were carried out with a Avatar 330 FT-IR ThermoNicolet. 32 scans between 4,000 cm-1 and 700 cm-1 were averaged for each spectrum at intervals of 1 cm-1.

Total C, N and H were measured with an elemental analyser (CE Instruments GA 1110). Scanning electron microscopy ESEM Quanta 200 (FEI, USA) coupled with X-ray

microanalysis was used to study qualitatively elements in the digestates before and after thermal treatment. All specimens were sputter coated with gold. The measures were performed by applying an accelerating voltage of 25 kV. The quantitative chemical analysis was performed by X-ray fluorescence (XRF) spectrometry measurements (ARL ADVANT’XP X-ray fluorescence spectrometer).

In order to qualitatively examine the crystalline phases present on the raw digestate materials and on the calcined powder samples, X-ray diffraction measurements (XRD) were carried out using a conventional Bragg Brentano diffractometer (PW 3710, Philips Research Laboratories) with Ni-foltered-cu K� radiation on grounded samples (< 25 µm in size) in the 5-70 2θ range at room temperature, with a scanning rate of 0.005°/s and a step size of 0.02°.

Finally, calcium carbonate content was measured by Dietrich-Fruhling calcimeter to verify the results obtained by XRF.

4. RESULTS AND DISCUSSION

4.1 Representative properties of the feeding materials

The experimental characterisation data on moisture and VS contents for the considered substrates and inoculum (as feeding materials in the reaction bottles) are reported in Table 3. Both the organic substrates (grape-seeds and leather dust) present significantly lower moisture content and higher VS content (on dry basis) as compared with the biomass inoculum.

Referring to the used anaerobic sludge as inoculum, the resulting experimental moisture content in Table 3 is practically coincident with the indicative value of 97.5% reported in Imhoff (1999) as representative for not-dewatered and anaerobically digested sludge from a WWTP with activated sludge system. Further, the resulting experimental VS content (on dry basis) for this inoculum is internally close to the upper limit of the range 30-60% reported in Acaia and Ragazzi (1991) as representative for digested sludge, and also slightly higher as compared with the upper limit of the specific range 45-55% reported in Londong (2006) as indicative of a well anaerobically digested sludge.

With regard to the ultimate analyses, the predominance (% w/w, on dry basis) of chemical


elements was in the following levels for the considered substrates: (1) C (50.16%), O (37.94%), H (6.62%), N (2.17%), S (0.12%) and P (0.08%), for grape-seeds; (2) C (42.45%), O (38.54%), H (7.10%), N (6.52%), S (1.12%) and Cl (0.0079%), for vegetable-tanned leather dust.

4.2 Preliminary test “PT1”

Referring to the overall duration time of preliminary test “PT1” (see Table 2), the resulting average values of the water bath temperature and the laboratory ambient air temperature were 33.9 °C and 20.2 °C, respectively. As outlined above, during the preliminary test the overall lab-scale system was optimised up to the final configuration of Figures 2 and 3, by means of specific modifications and/or improvements regarding the connection line between the reaction bottles and the measurement bottle, the proper configuration of the outlet for displaced liquid from the measurement bottle, and the overall insulation of the plastic containment box.

As far as the individual reaction bottles filled with anaerobic sludge are concerned (see Table 2), indeed one out of three bottles did not show any biogas generation along the overall test duration time. Consequently, Figure 4 shows the cumulative curve of biogas production (in terms of Nml referred to the initial VS content) as resulting average condition solely for the remaining two reaction bottles. Observing the curve of Figure 4, a first latency phase persisted for almost 12 days, whilst subsequently a certain biogas production was detected, with a resulting final specific biogas production (SGPbiogas) of 63.9 Nml gVS-1.

Table 3. Representative moisture and volatile solid (VS) contents of the considered substrates and inoculum. Legend: mean = mean value of three samples.

Material Moisture content [%, w/w] (mean)

VS [%, w/w] on dry basis (mean)

VS [%, w/w] on wet basis (mean)

Grape-seeds 13.50 66.78 57.77

Leather dust (vegetable tanned) 10.17 76.17 68.42

Anaerobic sludge (inoculum) 97.35 56.39 1.49

Figure 4. Preliminary test “PT1”, anaerobic sludge as blank: resulting cumulative curve of biogas production (referred to the initial VS content), as average condition for two reaction bottles.


4.3 Test series “T2”

Referring to the overall duration time of test series “T2” (see Table 2), the resulting average values of the water bath temperature and the laboratory ambient air temperature were 38.3 °C and 17.4 °C, respectively. Thus, due to the carried-out composite thermal insulation of the plastic containment box (see Sections 3.2 and 4.2), a clear increase of the average process temperature was revealed in comparison with previous test “PT1”, even under the circumstance of a contemporaneous decreasing of average ambient air temperature.

For this complete test series “T2”, the resulting cumulative curves of methane production (referred to the initial VS total content of each mixture) are condensed in Figure 5. Precisely, both diagrams of Figure 5 related to the mixtures of grape-seeds + inocumum and leather dust + inoculum represent the average curves for the corresponding three reaction bottles, whilst the diagram related to anaerobic sludge refers only to the average condition for two reactions bottle. In fact, one out of three reaction bottles filled with the inoculum as blank practically did not show again any methane generation along the overall test duration time.

At first concerning the mixture of leather dust + inoculum, the corresponding diagram of Figure 5 (upper, right-hand side) shows a certain rise of methane production during almost the initial week of digestion time, followed by a certain limitation of further methane production up the end of test duration time. The final specific methane production (SGPCH4), resulting for the overall mixture of leather dust + inoculum as referred to the initial SV total content (substrate + inoculum), was limited to 12.6 Nml gVS-1.

Figure 5. Test series “T2”, resulting cumulative curves of methane production (referred to the initial VS total content): mixture of grape-seeds + inoculum (upper, left-hand side) and mixture of leather dust + inoculum (upper, right-hand side), as average condition for three reaction bottles; anaerobic sludge (inoculum) as blank (lower, central-side), as average condition for two reaction bottles. Note: for diagram clearness, the vertical axes have purposely different upper value scales.


Differently, Figure 5 (upper, left-hand side) shows for the mixture of grape-seeds + inoculum a considerable higher methane production in comparison with the previous mixture (leather dust + inoculum). Specifically, the average cumulative production curve of Figure 5 (upper, left-hand side) is characterised by a higher increasing slope during the initial phase of digestion process lasting almost ten days as compared with the remaining duration time. The final specific methane production (SGPCH4), resulting for the overall mixture of grape seeds + inoculum as referred to the initial SV total content (substrate + inoculum), was equal to 118.6 Nml gVS-1.

With regard to anaerobic sludge, the corresponding diagram of Figure 5 (lower, central-side) shows an initial limited methane production lasting almost five days, followed by a long phase of absence of further production and a conclusive short phase of additional production. Indeed, the resulting final specific methane production (SGPCH4) was limited to only 5.7 Nml gVS-1.

Significantly, the appropriate determination of the “net” SGPCH4 value, as directly and solely attributable to each tested organic substrate (grape-seeds or leather dust), has been carried out with the following procedure: (1) at first, the incidence of inoculum (identified by the product of the resulting SGPCH4 for anaerobic sludge and the initial VSinoculum content in the considered mixture) was subtracted from the resulting final volume of cumulative methane production for the tested mixture; (2) consequently, the volume value of this subtraction was properly divided by the initial VSsubstrate content in the considered mixture. As a result of this adopted procedure, the following experimental values of net specific methane production (SGPCH4) are directly attributable to the considered substrates: a significant level of 175.0 Nml gVS-1 for grape-seeds, and conversely a limited level of 16.1 Nml gVS-1 for vegetable-tanned leather dust.

4.4 Test series “T3”

Referring to the overall duration time of test series “T3” (see Table 2), the resulting average values of the water bath temperature and the laboratory ambient air temperature were 39.5 °C and 19.4 °C, respectively. Thus, the positive effect of the realised thermal insulation of the plastic containment box is confirmed also for this final test series, as already revealed in the previous one.

At first concerning the mixture of leather dust + inoculum, one reaction bottle practically showed a negligible biogas production along the overall test duration time, as compared to the remaining bottles. Consequently, the cumulative biogas curve of Figure 6 (right-hand side) is representative of the average condition solely for two productive reaction bottles.

Figure 6. Test series “T3”, resulting cumulative curves of biogas production (referred to the initial VS total content): mixture of grape-seeds + inoculum (left-hand side), as average condition for three reaction bottles; mixture of leather dust + inoculum (right-hand side), as average condition for two reaction bottles. Note: for diagram clearness, the vertical axes have purposely different upper value scales.


Differently from the qualitative behaviour of methane curve in previous Figure 5 (upper, right-hand side), the resulting cumulative biogas production curve of Figure 6 (right-hand side) does not present any manifest variation in the increasing slope along the test duration time. The final specific biogas production (SGPbiogas), resulting for the overall mixture of leather dust + inoculum as referred to the initial VS total content (substrate + inoculum), was limited to 18.8 Nml gVS-1.

Referring instead to the mixture of grape-seeds + inoculum, the corresponding average cumulative biogas curve of Figure 6 (left-hand side) confirms the qualitative tendency, as already revealed with the methane curve of Figure 5 (upper, left-hand side), for a higher rise during the initial stage (lasting almost one week in Figure 6), followed by a certain reduction of the increasing slope for subsequent biogas production. On the whole, the final specific biogas production (SGPbiogas), resulting for the overall mixture of grape seeds + inoculum as referred to the initial VS total content (substrate + inoculum), was equal to 173.4 Nml gVS-1.

4.5 Digestate properties

The results of water content for the various types of digestates characterised in this experimental study are as follows: (1) referring to digestates generated from test series “T2”, 84.6% (grape-seeds + inoculum), 85.6% (leather dust + inoculum), and 92.4% (anaerobic sludge); (2) referring instead to digestates generated from test series “T3”, 96.7% (grape-seeds + inoculum) and 93.2% (leather dust + inoculum). Thus, the digestate samples of test series “T3” had higher water content as compared with the corresponding digestate samples obtained in test series “T2”. On the whole, the characterised digestate samples contain high amount of water, and consequently these digestate typologies need to be dewatered in view of any hypothetical reuse option.

A representative ESEM image of the digestate samples is shown in Figure 7, as precisely referred to the anaerobic sludge digestate from test series “T2” (see Table 2). In general, the ESEM micrographs evidenced the presence of areas with different microstructure, in particular samples are characterized by an organic fraction with a porous structure and by an inorganic fraction with a more compact structure. EDS analysis of the dry samples indicates carbon as main constituent, followed by Ca, S, Al, Fe, Mg, Si, P and Cl.

Figure 7. Test series “T2”: representative ESEM micrograph of digestate from anaerobic sludge obtained after drying.


Table 4. Chemical analysis of the residues (at 600 °C) of digestates from test series “T2” and “T3” obtained by XRF. Data in %, w/w on dry digestate sample basis.

Element “T2” - anaerobic sludge [%]

“T2” – leather dust + inoculum

[%]

“T2” – grape-seeds + inoculum

[%]

“T3” – leather dust + inoculum

[%]

“T3” – grape-seeds +

inoculum [%]

Ca 21.66 7.21 9.08 8.99 9.48

Si 3.66 5.26 5.59 1.85 2.53

Fe 10.83 3.81 4.25 5.19 4.27

Al 2.41 3.64 3.87 0.80 1.15

K 0.96 0.48 1.82 0.78 2.90

P 2.88 0.59 1.05 1.14 1.57

S 3.02 1.53 0.97 2.87 1.32

Cl 1.88 0.41 0.71 1.35 1.74

Na 2.54 0.78 0.62 1.54 0.77

Mg 0.46 0.45 0.38 0.58 0.34

Sr 0.60 0.25 0.29 0.39 0.33

Zn 0.41 0.17 0.19 0.52 0.42

Ti 0.46 0.17 0.17 0.21 0.22

Others 0.83 0.55 0.31 0.39 0.36

Table 4 reports the results of chemical characterisation of the residues (at 600 °C) of digestates generated from test series “T2” and “T3”. According to the comparative evaluation of characterisation data in Table 6 related to the complete test series “T2” (see Table 2), the inorganic elements in the digestates generated from the original mixtures of grape-seeds + inoculum and leather dust + inoculum are generally lower than the anaerobic sludge digestate, apart indeed for Si, Al, and K (solely concerning the comparison with grape-seeds + inoculum).

The high amount of calcium measured in the digestates, as shown also in Table 4, indicates the presence of a large amount of calcium carbonate as confirmed by the calcimeter results reported in subsequent Table 5. In particular, digestate deriving from anaerobic sludge (test series “T2”) contains higher calcium carbonate in comparison with the remaining digestates.

According to the elemental analysis data reported also in Table 5, high percentages of C and H were measured in digestates from the original mixtures of grape-seeds + inoculum and leather dust + inoculum with respect to anaerobic sludge as blank.

Table 5. Calcium carbonate and elemental analysis of dry digestates generated from test series “T2” and “T3”.

Digestate Sample CaCO3 [g kg-1] N [%, w/w] C [%, w/w] H [%, w/w]

“T2” – anaerobic sludge 203.1 2.98 22.69 2.99

“T2” – leather dust + inoculum 65.2 6.37 39.12 4.86

“T2” – grape-seeds + inoculum 79.7 2.45 35.22 4.03

“T3” – leather dust + inoculum 50.9 5.28 35.82 4.18

“T3” – grape-seeds + inoculum 60.5 2.95 31.05 3.81


Figure 8. Test series “T2”: FTIR spectra of the obtained digestates. Legend: “LD+I” = digestate from leather dust + inoculum; “GS+I” = digestate from grape-seeds + inoculum; “AS” = digestate from anaerobic sludge.

Further, a significant difference among the digestate samples from the specific mixture of leather dust + inoculum and the others is revealed in Table 5 by the higher content of nitrogen; this circumstance is in agreement with both the elemental (ultimate) analysis of the original leather dust substrate reported in Section 4.1 and the FTIR results reported below.

Figure 8 shows the FTIR spectra of the digestates obtained from test series “T2” (see Table 2). The spectra presented a broad absorption band at 3,000-3,400 cm-1 attributed to O-H vibration of carboxylic and alcoholic groups (Won et al., 2006). The spectrum of digestate sample from the original mixture of leather dust + inoculum had higher absorbance intensity at 1,528 cm-1 due to C=N stretching in secondary amides, compared with the remaining digestate samples (from grape-seeds + inoculum and anaerobic sludge) (Castaldi et al., 2005); in fact, the bands between 1,500 and 1,790 cm-1 are related to nitrogen-containing compounds (Réveillé et al., 2003). The experimental result in nitrogen-containing compounds in digestate from the original mixture of leather dust + inoculum is in agreement with the previous considerations on elemental (ultimate) analyses of both digestate and original substrate.

The absorption band between 1,019 and 1,079 cm-1 could be assigned to vibration in carbohydrates, aromatic ethers and polysaccharides, indicating that these compounds have not been used efficiently as a C source (Won et al., 2006). These bands with the intensity at 2,926 and 2,860 cm-1 suggested an increase of initial release of organic molecules from cracked macromolecules whose degradation has not been completed. All digestates also showed absorption bands at 1,419 and 865 cm-1 that can be attributed to the presence of calcium carbonate (Legodi et al., 2001), in agreement with XRF, XRD and calcimeter results.

Finally, the XRD patterns reported in Figures 9 (for dry digestates) and 10 (for calcinated digestates at 1,000 °C) show the thermal evolution of digestates obtained during test series “T2” (see Table 2) from anaerobic sludge and the original mixtures of anaerobic sludge with grape-seeds and leather dust. In particular in all the dry samples (Figure 9), the main phases are calcite and quartz; instead, at 1,000 °C (Figure 10) the main phase is gehlenite (Al2Ca2O7Si), probably formed from the reaction between decomposed CaCO3 and dissolved quartz, and traces of magnetite (Fe3O4). Significative differences in terms of mineralogical composition, between the corresponding digestates of test series “T2” and “T3” (see Table 2), were not observed.

GS+I

LD+I

AS


q c c c c c c LD+I

GS+I

AS

Counts

Figure 9. Test series “T2”: XRD patterns of the dry digestates. Legend: “LD+I” = digestate from leather dust + inoculum; “GS+I” = digestate from grape-seeds + inoculum; “AS” = digestate from anaerobic sludge; q = quartz; c = calcite.

g g g g g

m

LD+I

AS

GS+I

Counts

Figure 10. Test series “T2”: XRD patterns of calcinated digestates (at 1,000 °C). Legend: “LD+I” = digestate from leather dust + inoculum; “GS+I” = digestate from grape-seeds + inoculum; “AS” = anaerobic sludge digestate; g = gehlenite; m = magnetite.


5. CONCLUSIONS

Specific organic residues of both agricultural and industrial origin, which are quantitatively and qualitatively peculiar at given territorial scales, should be attentively evaluated, characterised and tested as potential and alternative energy sources (see Section 1).

In this perspective, the paper synthetically presented the experimental results of the initial phase (see Section 3) of a research cooperation between the Italian University of Urbino “Carlo Bo” (Environmental-sanitary Engineering Research Unit) and University of Modena and Reggio Emilia (Research Unit on Chemistry of Materials and Wastes), which has been focused on comparative anaerobic digestion tests and consequential digestate characterisation for two peculiar organic substrates (see Section 2): (1) grape-seeds, having an agricultural origin; and (2) vegetable-tanned leather dust, having differently an industrial origin.

Under the assumed experimental conditions of the carried-out batch tests, the grape-seed substrate showed a certain attitude to be considered as a possible energy source by means of anaerobic digestion option, due to a calculated “net” biomethane potential – as directly attributable to this substrate – of 175.0 Nml gVS-1. On the contrary, the calculated “net” biomethane potential directly attributable to the vegetable-tanned leather dust substrate was limited to the low value of 16.1 Nml gVS-1. Consequently, despite a previous research study showed at least the potentiality of aerobic biological process option for the production of an organic-nitrogen fertiliser from vegetable-tanned leather dust (Mangani et al., 2006), the alternative biological option of anaerobic digestion seems not particularly successful in terms of energy production.

Possible explanatory hypotheses of the inhibition of anaerobic digestion process for vegetable-tanned leather dust (i.e.: high nitrogen content in the original substrate as well as in the related final digestate, according to the analyses reported in Sections 4.1 and 4.5; effect of the tanning process) should be eventually evaluated with further experimental studies and elaborations.

The digestate characterisation shows that, whilst there are not significant differences between the obtained materials (from the original mixtures) in test series “T2” as compared with test series “T3”, some differences are however observable depending on the specific organic material used as substrate (i.e., grape-seeds or leather dust), for example nitrogen content in the digestate coming from the mixture with vegetable-tanned leather dust.

The digestate characterisation data will be useful, in the future, for the experimental investigation of possible and alternative reuse options for the digestates (which generally are used as fertilisers), eventually as applications in the material sector.

ACKNOWLEDGEMENTS

The Authors wish to thank: the firm “BIOTEC snc” (Fermo, Marche region), for the availability concerning the collection of vegetable tanned leather dust samples; Dr. M. Di Domenico and Dr. S. Pretelli at “Hera Rimini SpA” multi-utility group (Rimini, Emilia Romagna region), for the availability concerning the collection of anaerobic sludge samples used as inoculum. Mr. O. Rusciadelli and Dr. J. Arduini, both at the University of Urbino “Carlo Bo”, are gratefully mentioned for their technical support. Part of this work, regarding the experimental phase of digestate characterisation, was supported by ENVIREN project, of the Hi-Tech Network, funded by Emilia Romagna region.


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