Scholarly article on topic 'Anaerobic Co-digestion of Olive-mill Solid Waste with Cattle Manure and Cattle Slurry: Analysis of Bio-methane Potential'

Anaerobic Co-digestion of Olive-mill Solid Waste with Cattle Manure and Cattle Slurry: Analysis of Bio-methane Potential Academic research paper on "Agriculture, forestry, and fisheries"

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Abstract of research paper on Agriculture, forestry, and fisheries, author of scientific article — Maurizio Carlini, Sonia Castellucci, Marta Moneti

Abstract In the present work, the bio-methane potential from anaerobic digestion of OMSW with inoculum and co-digestion of OMSW with cattle manure (CM) and cattle slurry (CS) has been analysed. The OMSW used for the analysis was a pollutant waste came from olive oil processing through three-phase centrifugation systems. A batch stirred tank reactor has been used, under mesophilic conditions (38°C), in order to study the biogas yield. The whole process has been carried out under wet conditions, and with a Hydraulic Retention Time (HRT) of 55 days. Several tests have been carried out to achieve the best biogas yield.

Academic research paper on topic "Anaerobic Co-digestion of Olive-mill Solid Waste with Cattle Manure and Cattle Slurry: Analysis of Bio-methane Potential"

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Energy Procedía 81 (2015) 354 - 367

69th Conference of the Italian Thermal Machines Engineering Association, ATI2014

Anaerobic co-digestion of olive-mill solid waste with cattle manure and cattle slurry: analysis of bio-methane potential

Maurizio Carlinia, Sonia Castelluccib*, Marta Monetia

aDAFNE,University of Tuscia, Via San Camillo de Lellis snc, 01100 Viterbo, Italy bCIRDER, University of Tuscia, Via Santa Maria in Gradi 4, 01100 Viterbo, Italy

Abstract

In the present work, the bio-methane potential from anaerobic digestion of OMSW with inoculum and co-digestion of OMSW with cattle manure (CM) and cattle slurry (CS) has been analysed. The OMSW used for the analysis was a pollutant waste came from olive oil processing through three-phase centrifugation systems.

A batch stirred tank reactor has been used, under mesophilic conditions (38°C), in order to study the biogas yield. The whole process has been carried out under wet conditions, and with a Hydraulic Retention Time (HRT) of 55 days. Several tests have been carried out to achieve the best biogas yield.

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Scientific Committee of ATI 2014

Keywords: olive mill solid waste; anaerobic digestion; biomethane

* Corresponding author. Tel.: +39-0761-401343; fax:+39-0761-401343. E-mail address: sonia.castellucci@unitus.it

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Scientific Committee of ATI 2014

doi:10.1016/j.egypro.2015.12.105

Nomenclature

OMW Olive Mill Wastes

OMSW Olive Mill Solid Waste

CM Cattle Manure

CS Cattle Slurry

OMWW Olive Mill WasteWater

HHV Higher Heating Value

LHV Lower Heating Value

BMP Bio-Methane Potential

HRT Hydraulic retention Time

COD Chemical Oxygen Demand

VS Volatile Solids

DM Dry Matter

OM Organic Matter

TS Total Solids

1. Introduction

In Mediterranean regions the olive tree cultivation and the olive oil extraction are widespread since thousands of years. Nowadays about 10 million hectares of land in all the world are cultivated with 900 million of olive trees, of which about 98% are in the Mediterranean area [1]. The European Union, from 2007 to 2013, have contributed on average of about 72% of the olive oil world total production. Italy has contributed of about 22% of the European production with about 6600 olive mills which produce an average of 455 thousands of tons of olive oil per year. Italy represents, in this way, the second European producer after Spain, and it is followed by Greece, Turkey, Syria and Tunisia [2]. The olive oil industry produces huge quantities of wastes which represent a significant environmental problem in Mediterranean area. This waste are produced in high quantity in a short time, furthermore they are not biodegradable due to the high concentration in organic and phenolic compounds [3,4]. The waste derived by olive mills can be divided in olive mill wastewater (OMWW) and olive mill solid waste (OMSW). The composition and quantity of this waste depends from the olive oil extraction technology used [3]. In general the OMWW is composed of vegetation water and suspended solid, whereas OMSW is made up by olive pulp, peel, pieces of pit and an oil content [5].

Olive oil extraction can be done through discontinuous (traditional pressing system) or continuous (centrifugation system) processes. The discontinuous pressing system is the oldest and more widespread squeezing method, and it is used in the traditional olive mills. This is a low cost and technically simple method. It produces as by-product OMSW and low quantity of OMWW (40-60 l per 100 kg of olives) [6]. The disadvantages of this system are especially the discontinuity of the process and the high costs of manpower. The OMWW of these kinds of olive mills has however a high COD compared to the others methods. In many countries the traditional pressing system is still used even if it is quite expensive [7]. Continuous extraction processes separate the different phases by centrifugation. They are based on the density difference of the diverse components of the olive paste, and can operate at two or three phases. In the three-phase system hot water at the centrifugation step is added, and this produces a greater quantity of OMWW [8]. The three-phase systems generate three fractions: OMSW, OMWW and oil with a low quantity of water. The advantages of these systems are the full automation, a better oil quality, and the necessity of smaller spaces; instead the disadvantages are a greater water and energy consumption, a greater OMWW fraction at the output and high installation costs [3]. The three-phase systems are the most widely used, especially in countries that produce high olive quantities. During the last years they became popular also in Italy. To minimize the OMWW volume and to reduce the washing of phenols, two-phase continuous extraction processes have been developed. Using this system the olive paste is divided in two phases: oil and wet OMSW. The wet OMSW is a semi-solid by-product, a combination of OMSW and OMWW. The two-phase systems are defined ecological because of their high reduction of water consumption, but the resulting residue, which consists of liquid

and solid phase, is difficult to manage because the pollutant load is highly concentrated. In the last 10 years this system is become the main system in Spain, where it represents about 90% of the total installation [8].

In Italy there are 6.600 olive mills, the three phase system is still the most used with about 46% of the total installation. The pressing system represent 42% of the installation, instead the two-phase continuous system only 2%. However in Puglia, the most important Italian Region for the olives cultivation, 55% of olive mills are continuous cycle [9]. Italy is, therefore, characterized by the presence of many little olive mills that work with traditional process.

In this study the OMSW, obtained by three-phase system, is examined in order to use it for energy production. It is composed of olive pulp, peel and pieces of pit. This OMSW is characterized by a high C/N ratio (>34%) due to the presence of pieces of pit, a Lower Heating Value (LHV) of about 23 MJ/kg and a high moisture content (>46%). OMSW can be used both in thermochemical conversion processes and in anaerobic digestion to produce biogas [1013]. Due to the high moisture content it is not totally convenient to use it in thermochemical processes because it needs to be dried. OMSW cans be used, in a very interesting way, in the anaerobic digestion plants, in particular use in small size biogas production plants, up to 100 kW, is very interesting (D.M. 6 July 2012).

Among the countries that produce primary energy from biogas, first of all there are Germany, United Kingdom and Italy [14]. In Germany the biogas used for the primary energy production is almost totally deriving from digestion plants (87.15% of the total product energy), followed by biogas from sewage sludge (9.95%) and biogas from landfill (2.94%). The situation is totally opposite in United Kingdom and Italy. In the first one the main contribution is given by landfill biogas (84%). In Italy most of the primary energy come from landfill biogas (68.97%) and then from "other biogas" (29.56%).

In Italy the number and the power of bioenergy plants are increasing [15,16], in particular in 2012 has been registered an increase of 81.3% in number, and of 34.6% in power compared to 2011. The greater contribute, in terms of number, is given by the biogas plants, in particular those come from agricultural and forestry activities, an increase of 156% in number and 142% in power has been registered. Instead the dominant contribute in terms of installed power is represented by biomass plants, in particular those of urban wastes. In general bioenergy sector contributes at 8% of the total power of the renewable sector [17].

In the present work, the Bio-Methane Potential (BMP) from anaerobic digestion of OMSW with inoculum and co-digestion of OMSW, Cattle Manure (CM) and Cattle Slurry (CS) has been analysed. The OMSW used for the analysis was a pollutant waste come from olive oil processing through three-phase system, method most used in Italy for oil extraction. Biomass energy characterization has been done and a batch stirred tank reactor has been used, under mesophilic conditions (38°C), in order to study the biogas yield. The whole process has been carried out under wet conditions, and with a Hydraulic Retention Time (HRT) of 55 days.

2. Materials and methods

In order to evaluate which biomass are more suitable for a specific energy conversion process and to optimize the process, biomass characterisation have to be done, identifying the main energy properties.

The first phase of this study has been the energetic analysis of the biomass selected that is OMSW, CM and CS.

The sampling has been done according to "EN 14778:2011. Solid biofuels: sampling" [18]. To prepare the sample "EN 14780:2011. Solid biofuels: sample preparation" [19] standard technique has been followed.

The following analysis have been carried out:

• Ultimate analysis (C, H, N contents);

• Moisture content;

• Ash content;

• High and Low Heating Value (HHV and LHV);

• Volatile Solid (VS);

For the ultimate analysis, an analyzer of carbon, hydrogen and nitrogen contents (Leco CHN-2000) has been used. All the tests have been carried out according to the "EN 15104:2011. Solid biofuels: determination of total content of carbon, hydrogen and nitrogen. Instrumental methods" [20] standard.

The oxygen content has been calculated using some empirical relationships and subtracting all other compounds from 100% [21,22]:

0 = 100 - (C + N + H + S) (1)

Sulphur content data in OMSW, CM and CS have been taken from the literature [23].

In order to determine the moisture content, the procedures provided by the following technical standards have been taken into account:

• EN 14774-1:2009. Solid biofuel. Determination of moisture content, oven dry method. Part 1: total moisture, reference method [24].

• EN 14774-2:2009. Solid biofuel. Determination of moisture content, oven dry method. Part 2: total moisture, simplified method [25]

• EN 14774-3:2009. Solid biofuel. Determination of moisture content, oven dry method. Part 3: moisture in general analysis sample [26].

For ash content determination, the procedure described in the "EN 14775:2009. Solid biofuel: Determination of Ash content" [27] technical standard has been used.

High Heating Value (HHV) of OMSW has been determined using a Parr 6200 calorimeter and according to "EN 14918:2009. Solid biofuels: determination of calorific value' [28].

The Volatile Solids (VS) of OMSW have been determined according to "EN 15148 - Solid biofuels -Determination of the content of volatile matter" [29].

An experimental determination is useful in order to provide the amount of biogas production resulting from anaerobic degradation of organic substance. The BMP is an essential parameter for assessing design, economic and managing issues for the implementation of the anaerobic digestion process [30]. The BMP is defined as the gas production that would be observed during an infinity time of anaerobic degradation. Actually the experimental time is finite and the BMP can be estimated from the trend of the methane production depending on time.

2.1 Experimental setup

In this work a mini-digestor has been used (Fig. 1 and Fig. 2). It is composed of:

• BATCH reactor with a volume of 5 l stainless steel

• Mixing system

• Water column which permits the entrance of the product biogas; in this way, as a consequence of the water movement, is possible to evaluate the gas volume

• pH sensor

• Temperature sensor

• Vacuum pump

• Warmed-up resistance

• Pressure gauge

• Air valve

• Valve and pipe for the gas sample taking. This valve can be manually operated, and it is automatically activated when the pressure into the reactor reaches 0.1 bar.

Fig. 1. Anaerobic digestion mini-plant

Fig. 2. Reactor elements

The gas is then extracted through tedlar bags and sent to gas chromatograph AGILENT TECHNOLOGY. 3. Results and discussion

In a previous work [25] a study on OMSW without olive pit and with a moisture of 69.38% (wet basis) has been done. The maximum quantity of produced biogas had been 25.14 l after 60 days. A cumulative methane production of 9.58 l and a BMP of 0.00147 l/gVS had been reached.

In this study OMSW with pit has been considered to verify the biogas production in a wet anaerobic digestion process. The tests concerned two kind of feed:

• OMSW with pit and inoculum,

• OMSW with pit in co-digestion with CM and CS.

The inoculum is composed of digested coming from a real plant for biogas production from cattle sewage, taken in a farm. InTable 1 and Table 2 inoculum and OMSW properties are reported respectively.

OMSW to digested ratio was 2:1 on weight. In this way the initial content of Dry Matter (DM) was 24%, therefore this value has been modified to obtain wet conditions.

Table 1. Inoculum properties Inoculum - Digested from cattle sewage pH 8.26±0.3 -

Dry matter 12.2±0.13 [%]

Ashes 3.1±0.08 [% dry matter]

Volatile Solids 94.96±0.08 [% dry matter]

Table 2. OMSW properties OMSW

C 51.157±0.041 [%]

H 7.292±0.038 [%]

N 1.478±0.060 [%]

S 0.10 [%]

O 39.973 [%]

C/N ratio 34.61

Moisture (wet basis) 45.89±0.83 [%]

Moisture(dry basis) 84.80 [%]

Dry matter 54.12±0.83 [%]

Ashes 4.381±0.183 [% dry matter]

Volatile Solids 93.11±0.183 [% dry matter]

High Heating Value (HHV) 23.110±0.094 [%]

HHV (dry basis) 24.885 [MJ/kg]

Low Heating Value (LHV) 23.382 [MJ/kg]

pH 5.4±0.2 -

In Table 3 mixture properties are reported.

Table 3. Mixture properties

Biomass properties

Volatile Solids [%DM] 5.07 51.4

Dry matter [%] Weight [kg] Ashes [% DM] Digested 5.15 1 1.5

OMSW 54.12 1.867 2.1

Water_0_1_

Mixture properties

Dry matter [%] Weight [kg] Initial pH Volatile Solids [%DM]

11.88% 3.867 6.51 11.64%

Mixture

VS determination of the mixture, biogas samples taking and analysis of the gas to the gas-chromatograph have been done every 4 days.

In Fig. 3 trend of VS is reported. As it is shown there is a decreasing trend due to the use of the VS by the microorganism during the digestion reaction.

(Л Ш

i 10 IV

1 6 1Л

■M ^

20 Days

Fig. 3. Volatile Solids trend (% wet basis)

The trend of the VS can be compared with cumulative production of biogas during the reaction (Fig.4). The biogas production increases with decreasing of the VS content in the digestion matrix.

Fig. 4. Cumulative volume of biogas

The digestion reaction has been interrupted at Day 32 when the total biogas production had reached 16.05 l. Further the comparison with the cumulative volume of methane into the biogas is congruent with the trend of the VS (Fig. 5).

Fig. 5. Cumulative volume of methane

When VS tend to a constant value also the methane production tends to stop.

Furthermore, as it is possible to see in Fig. Fig. 6, the production percentage of CH4 has had a very alternating trend and the percentages were very low. For this reason the test has been stopped.

Fig. 6. Production percentage of methane

The cumulative volume of methane has reached 0.73 l after 32 days. This percentage is quite low compared to the total biogas production. The BMP is 0.0023 l/gVS.

In Fig. 7 pH trend is reported. At the beginning of the digestion reaction pH was 6.51; however the trend was very fluctuating, showing that had not occurred a good anaerobic digestion reaction.

Fig. 7. Trend of pH

For all these reasons the test has been interrupted and co-digestion test with OMSW and other substrates have been done.

Co-digestion test of OMSW with pit, CM and CS has been done (Table 4).

Table 4. Biomass properties

Biomass Properties

Dry matter Moisture (wet Ashes (dry basis) Organic Weight

[%] basis) [%] [%] matter (dry [kg]

basis) [%]

Cattle slurry 1.04 98.96 35.02 61.73 3.1

Cattle manure 18.46 81.,54 15.96 75.63 0.2

OMSW 54.12 45.89 4.75 94.07 1

Total 4.3

Mixture properties

Total dry matter 14 %

Initial pH 7.07

Total organic matter on dry basis 93.6 %

Total organic matter on wet basis 13.03 %

HRT was 55 days in which the digestor has been maintained in mesophilic conditions with a temperature of 38°C.

Initial pH, thanks to manure and cattle slurry, was 7.07 and the total organic matter on dry basis was 93.6%. In Fig. 8 and Fig. 9 substrate pH trend is reported. In this case pH remains stable during the process, because the process is stable.

Fig. 8. pH trend

Fig. 9. Representative points of pH and trend line

There is an initial decrease followed by an increase, which corresponds to the beginning of the phase of the higher methane production.

Below the methane percentage into the product biogas is showed (Fig. 10).

Fig. 10. Percentage of methane in the biogas

The percentage of methane increases initially, keeping constant from Day 7 to Day 40, and after that it decreases. In Fig. 11 the cumulative methane production is presented. The production has reached 40.5 l at the Day 55.

Fig. 11. Cumulative volume of methane

The cumulative production of biogas has reached 63 l at the end of the test (Fig. 12). Therefore the presence of methane into the biogas was 64%.

Fig. 12. Cumulate volume of biogas

In Fig. 13 the trend of the VS into the mixture is showed. The time trend of the VS shows behaviour consistent with that expected: the reduction of the VS increases with test time.

Fig. 13. VS on wet basis and trend line

BMP has reached in this case a value of 0,103 l/gVS.

Therefore comparing the BMP and the methane average percentage of the two tests (Table 5) can be observe as the co-digestion test of OMSW with pit and manure and cattle slurry has produced better results (0.1034 l/gVS and 64% respectively).

Table 5. BMP and methane average percentage in biogas of the two tests

Test BMP [l/gvS] Methane average percentage in

biogas [%]

Digestion of OMSW without pit and inoculum 0,0023 4,55

Co-digestion of OMSW with pit and manure and cattle slurry 0,1034 64

3.1 Conclusions

Olive-mill wastes represent a significant environmental problem in Mediterranean area where they are produced in huge quantity and seasonally, thus in short periods of time. One of the most promising processes to exploit OMWs for energy production is anaerobic digestion. Especially OMSWs can be used in co-digestion processes with other biomass.

Anaerobic co-digestion technology is increasingly used to simultaneously treat several solid and liquid organic wastes in order to balance the nutrients content, to reduce negative effects of toxic compounds on the process, and, therefore, to increase the biogas yield. Moreover, co-digestion technology contributes to a more efficient use of anaerobic digestion, because multiple streams of wastes can be processed together in a single plant at the same time.

The use of OMSW with pit represents economically a very attractive possibility, in particular for the small olive mills. From the tests carried out with a batch stirred tank reactor results OMSW with pit has better performance if used in co-digestion with other substrates.

The optimal mixture has been found using 4.65% of CM, 72.10% of CS and 23.25% of OMSW with a Total Solid (TS) content of 14%.

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