Advertisement

BioEnergy Research

, Volume 9, Issue 2, pp 624–632 | Cite as

The Possibility of Meeting Greenhouse Energy and CO2 Demands Through Utilisation of Cucumber and Tomato Residues

  • Marta OleszekEmail author
  • Jerzy Tys
  • Dariusz Wiącek
  • Aleksandra Król
  • Jan Kuna
Open Access
Article

Abstract

The article examines the possibility of using residues from greenhouse cucumber and tomato cultivation as biomass for energy and CO2 production in order to meet greenhouse needs. Methane fermentation and combustion were compared. Moreover, the legitimacy of ensiling as a storage method for biogas plant was evaluated. The tested waste was found to be an unsuitable feedstock for the production of silage due to low sugar and high protein content. Fresh waste had a higher biogas yield than silage; however, its fermentation lasted longer. Furthermore, the results showed that, in the case of fresh residues, the methane fermentation proved to be a more energy-efficient process, while air-dry biomass is a more sustainable feedstock for combustion. The energy and CO2 balance showed that, regardless of the method used, the available quantity of waste is too small to meet the greenhouse needs.

Keywords

Biogas Greenhouse production Combustion Organic waste Tomato Cucumber 

Introduction

In Poland, horticulture is a growing sector of exported raw materials. The total production of vegetables is dominated by the cultivation of tomato (810.6 thousand tonnes) and cucumber (532.0 thousand tonnes), of which as much as 538.7 thousand tonnes of tomatoes and 265.1 thousand tonnes of cucumbers were greenhouse-cultivated in 2014 [1]. Such intensive production involves huge amounts of waste in the form of shoots, stems and leaves. Boulard et al. [2] have reported that the number of greenhouse residues in France is estimated at approximately 170 t ha−1 of the area of greenhouses. However, it should be noted that greenhouse cultures are also characterised by a high demand for heat and carbon dioxide [3]. The annual greenhouse energy demand in Poland and other countries of Northern Europe (The Netherlands, Germany) reaches 36 TJ ha−1 [4, 5]. Menardo et al. [4] reported that an average greenhouse CO2 demand is estimated to 2628 t ha−1. For this purpose, greenhouses must be equipped with gas installations metering carbon dioxide in a pure form or obtain it by combustion of liquefied petroleum gas. In Poland, where rockwool is commonly used as a growing medium, enrichment of greenhouses with external CO2 is necessary. These additional costs could be reduced via the use of anaerobic digestion or combustion of vegetable residues.

Biogas production from this type of biomass or combustion thereof could solve the problem with the disposal of waste, as well as satisfy the need for energy and CO2. Biogas production is a multi-step process of biological decomposition of organic matter. It consists of the following phases: hydrolysis, acidogenesis, acetogenesis and methanogenesis. Each involves various species of microorganism. Biogas contains mainly CH4 and CO2 and small amounts of hydrogen, ammonia and hydrogen sulphide [6]. The most common substrate for biogas production is maize silage, especially in central Europe, due to its high biomass yield per hectare (20–30 t TS ha−1) [7]. In Poland, the mean maize yield in 2014 was ca. 47.8 t fresh matter ha−1 [1]. High yield, adequate availability and high methanogenic potential make it the most desirable substrate [8]. However, because of the threat of monoculture and rising prices, it is necessary to search for a worthy alternative to this material. Many various energy crops that could successfully replace corn, such as organic waste, energy crops [6], and even microalgae [9], have been tested. The high potential of waste is increasingly emphasised, which is primarily due to the low cost of obtaining them and the possibility of simultaneous utilisation. In addition to municipal and kitchen waste, a large part of this group is constituted by waste from agricultural and horticultural production [10, 11, 12]. Agricultural residues could also be suitable biomass for combustion. The calorific values, low moisture and a low concentration of ash, nitrogen (N), sulphur (S), potassium (K) and sodium (Na) are the main aspects determining the suitability. The calorific value of residues can be expressed as follows: higher heating value (HHV), lower heating value of dry biomass (LHVd) or lower heating value of wet biomass (LHVw) [13]. HHV is defined as the amount of heat released when fuel is combusted and the products have returned to a temperature of 25 °C; thus, it takes into account heat associated with water condensation. LHVd is determined by subtracting the heat of vaporisation of water generated during combustion of fuel and taking into account mainly the hydrogen content in biomass. LHVw is equal to LHVd reduced by heat of vaporisation of water present in biomass before combustion.

A high concentration of N and S causes pollutant emissions. Alkali and alkaline earth metals, especially K and Na, are involved in slagging processes and fouling of the combustion chamber and the heat exchanger surfaces by reducing the ash melting temperature [14]. More desirable is the presence of calcium (Ca) and magnesium (Mg), because their mobility and the properties of the deposits it forms are both more favourable to sustained furnace operation [15].

The issue of greenhouse residue disposal was a subject of many studies. Generally, the composting process was investigated as a useful alternative for waste management [16, 17]. The vast majority of studies on the cucumber and tomato methane fermentation process concerned fruits unfit for consumption, rather than green parts of plant such as the leaves, stems and stalks [18, 19]. Few studies on the biogas production from greenhouse residues prove that it is worthy of interest substrate [7]. No studies have been conducted on the ensiling this kind of waste and methane fermentation of such silages. Jagadabhi et al. [7] stated that greenhouses could collect and utilise these crop residues for biogas production and in turn meet their own energy demand, but no attempts have been made to compare the energy produced from this raw material and greenhouses energy demand in the country or region scale. Furthermore, carbon dioxide production as a valuable raw material for greenhouses has not been investigated. The comparison of methane fermentation and combustion as the methods of energy and CO2 production is also not carried out in previous studies.

Hence, the aim of this study was to evaluate the possibility of disposal of greenhouse cucumber and tomato residues through heat and CO2 production by methane fermentation or combustion. Moreover, the suitability of tomato and cucumber waste for silage was analysed given a possible need of storage thereof in a biogas plant.

Materials and Methods

Material

The stems, leaves and stalks remaining after the cultivation of cucumber and tomato under cover were the tested material. The material came from the Chair of Plant Horticulture and Fertilisation, University of Life Sciences in Lublin. The seeds of cucumber were sown in February 2013, and then, seedlings were planted in their permanent place in March 2013 at the density of one plant for 1 m2. Completion of cultivation occurred in July 2013. The tomato was grown from February to October 2013 at the density of 2.4 plants for 1 m2. Plants protection treatments and cultivation were performed in accordance with the relevant recommendations. Rockwool was used as a growing medium. Drip fertigation method was applied in closed system without recirculation of nutrient solution that contained essential macrocomponents and microelements. Bombus terrestris was used for plant pollination, and greenhouse whitefly (Frialeurodes vaporariorum) was biologically controlled with Encarsia formosa. Fruit picking was performed 2–3 times a week [20]. Approximately 15 kg of fresh residues of each species were wilted up to moisture of approximately 75 % and were broken down to fractions of approximately 20 mm. Two kilograms of such biomass were directly subjected to analysis, and the rest of material was previously ensiled in 5-L airtight barrels in three independent replications, in ambient temperature of 22 °C, for approximately 60 days. As a silage additive, Biosilac produced by Bio-Gen was used. In accordance with the manufacturer's recommendations, 1 L of solution with a concentration of 50 g L−1 was prepared and applied by atomiser on each layer of residues. Biomass has been thoroughly compacted to get rid of air.

Chemical Analysis

The fresh and ensiled material was physico-chemically tested for the content of total solids (TS), volatile solids (VS) and ash with a weight-drier according to PN-EN 12880 and PN-EN 12779, the total organic carbon (TOC) content with a TOC-V CPN analyser with a Solid Sample Combustion Unit SSM-5000A in accordance with the manufacturer’s protocol, pH with potentiometry, the content of simple sugars with the Luff–Schoorl method and nitrogen with the Kjeldahl method. Crude protein (CP) was calculated by multiplying the nitrogen content by a coefficient of 6.25. Ammonium nitrogen NNH4 was determined by spectrophotometry and macroelements by ICP OES following the procedure described by Oleszek et al. [21]. Based on the results of the macroelements analysis, the alkali index (I a) is calculated as Jenkins et al. [15]:
$$ {I}_{\mathrm{a}}\kern0.5em =\kern0.5em \left(1\kern0.5em /\kern0.5em Q\right)\ {Y}_{\mathrm{a}}\kern0.5em \left({Y}_{\mathrm{Na}2\mathrm{O}}\kern0.5em +\kern0.75em {Y}_{\mathrm{K}2\mathrm{O}}\right) $$
(1)
in which Q is the higher heating value (HHV) [GJ kg−1], Y a is the mass fraction (dimensionless) of ash in the fuel, Y K2O and Y Na2O are the mass fractions (dimensionless) of K2O and Na2O in the ash.

Biogas Production

Methane fermentation was performed in set of six eudiometers with working volume of 1 L, according to VDI 4630 (2006). The parameters of batch assay were as follows: temperature of 37 °C, pH ca. 7, initial loading of 60 g VS L−1, substrate to inoculum ratio (S/I) of 1:1 (based on the VS). Post-fermentation sludge from a mesophilic, agricultural biogas plant was used as the inoculum, utilising corn silage and whey as the substrates. pH, TS and VS of inoculum were 7.6, 3.8 and 2.5 %, respectively. The volume of biogas was determined by the method of displacement of liquid, which was acidified and saturated solution of sodium sulphate [22]. Once a day, the methane concentration was measured by an automated analyser. Daily methane yield was determined through multiplying daily biogas yield and daily methane content. Overall methane content was determined as a ratio of total methane yield and total biogas yield. The process was carried out until the daily yield was lower than 1 % of the previous total biogas yield. The values of measured biogas volume were converted into standard conditions (1013 mbar, 273 K). The study was performed in three independent replications of fresh and ensiled residues and inoculum as a control. Then, biogas yield of inoculum was subtracted from biogas yield of tested samples.

The amount of heat energy that can be obtained through the methane fermentation process (q FM) was calculated according to the formula:
$$ {q}_{\mathrm{FM}}\kern0.5em =\kern0.5em 0.8\kern0.5em \left({Y}_{\mathrm{M}}\kern0.5em \times \kern0.5em {\mathrm{LHV}}_{\mathrm{M}}\right) $$
(2)
where: q FM—heat energy from a unit of mass of cucumber or tomato residues [MJ kg−1], Y M—methane yield of tomato or cucumber residues [m3 kg−1 TS], LHVM—lower heating value of methane (36 MJ m−3), 0.8—result of the subtraction of 20 % of heat from biogas required to meet own demands of the biogas plant [23].

Combustion

The HHV of the residues was tested using a calorimeter LECO model AC600 with ACWin Software in accordance with the manufacturer’s protocol. Before calorimetric analysis, samples of tomato and cucumber residues were dried, milled and formed into tablets using a tablet press. Lower heating value (LHV) was calculated in accordance with PN-80/G-04511 and PN-ISO 1928. LHV was calculated at moisture level of 0, 25 and 75 %. The second value of moisture is characteristic for air-dried waste and the latter for waste which was used for biogas production.

Energy and CO2 Balance

A simple energy and CO2 balance calculation was conducted by comparison of the results of methane production and combustion and greenhouse demands. In order to estimate the annual heat (D Q) and CO2 (D CO2) demands of greenhouses in Poland and the total annual amount of cucumber and tomato residues in Poland (based on TS and fresh matter (FM)) (M W), the following assumptions were made:
  • Annual heat demand per unit of area (D Q)—36.1 TJ ha−1 of greenhouses [4].

  • Annual carbon dioxide demand per unit of area (D CO2)—2628 t ha−1 [4].

  • Average annual amount of fresh waste per unit of area (m w)—170 t ha−1 [2]. The TS content of waste was taken from the results of chemical analyses.

  • Area of cucumber and tomato greenhouse cultivation in Poland (A)—1229.1 and 2170.8 ha [1].

The calculations were conducted in accordance with the formulas:
$$ {D}_{\mathrm{Q}}\kern0.5em =\kern0.5em {d}_{\mathrm{Q}}\kern0.5em \times \kern0.5em A $$
(3)
$$ {D}_{\mathrm{CO}2}\kern0.5em =\kern0.5em {d}_{\mathrm{CO}2}\kern0.5em \times \kern0.5em A $$
(4)
$$ {M}_{\mathrm{W}}\kern0.5em =\kern0.5em {m}_{\mathrm{w}}\kern0.5em \times \kern0.5em A $$
(5)
In the next step, the amount of heat that can be produced in the methane fermentation and combustion process (Q FM and Q C, respectively) for a specified amount of waste in Poland (M w) was determined according to the formulas:
$$ {Q}_{\mathrm{FM}}\kern0.5em =\kern0.5em {q}_{\mathrm{FM}}\kern0.5em \times \kern0.5em {M}_{\mathrm{w}}\kern0.5em \times \kern0.5em {10}^{-6} $$
(6)
where: Q FM—annual heat energy from available amounts of residues [PJ a−1], q FM—heat energy from unit of mass of cucumber or tomato residues [MJ kg−1], M w—annual amount of residues in Poland [t a−1]
$$ {Q}_{\mathrm{C}}\kern0.5em =\kern0.5em \mathrm{L}\mathrm{H}\mathrm{V}\kern0.5em \times \kern0.5em {M}_{\mathrm{W}}\kern0.5em \times \kern0.5em {10}^{-6} $$
(7)
where, Q C—heat from combustion [PJ], LHV—lower heating value of residues [MJ kg−1], M W—annual amount of residues in Poland [t a−1].
To calculate the amount of carbon dioxide formed in the methane fermentation process (CO2FM), the following statements and assumptions have been applied:
  • Biogas is composed of methane and carbon dioxide

  • Methane is subjected to complete combustion

  • According to the stoichiometry of the reaction equation, from complete combustion of 1 dm3 of methane, 1 dm3 of carbon dioxide is produced

This means that the final carbon dioxide volume after biogas combustion is approximately equal to the biogas volume obtained during the fermentation process (Y bio) (disregarding trace amounts of other gases).

The volume established in this manner was converted to mass:
$$ \mathrm{c}\mathrm{o}{2}_{\mathrm{FM}}\kern0.5em =\kern0.5em {Y}_{\mathrm{bio},}\kern0.5em \times \kern0.5em {P}_{\mathrm{CO}2} $$
(8)
$$ \mathrm{C}\mathrm{O}{2}_{\mathrm{FM}}\kern0.5em =\kern0.5em \mathrm{c}\mathrm{o}{2}_{\mathrm{FM}}\kern0.5em \times \kern0.5em {M}_{\mathrm{W}} $$
(9)
where: co2FM—mass of carbon dioxide produced from methane fermentation per unit of mass of residues [t t−1 TS], CO2FM—annual mass of carbon dioxide produced from methane fermentation of cucumber or tomato residues in Poland [t a−1], Y bio—biogas yield of cucumber or tomato residues, respectively [m3 t−1 TS], P CO2—density of carbon dioxide (1013 mbar, 293 K) [t m−3], M w—annual amount of residues in Poland [t a−1]. CO2C was calculated as follows:
$$ \mathrm{c}\mathrm{o}{2}_{\mathrm{C}}\kern0.5em =\kern0.5em C\kern0.5em \times \kern0.5em 3.67 $$
(10)
$$ \mathrm{C}\mathrm{O}{2}_{\mathrm{C}}\kern0.5em =\kern0.5em \mathrm{c}\mathrm{o}{2}_{\mathrm{C}}\kern0.5em \times \kern0.5em {M}_{\mathrm{w}} $$
(11)
where: co2C—mass of carbon dioxide per unit of mass of waste [kg kg−1 TS], CO2C—annual mass of carbon dioxide produced from combustion of cucumber or tomato residues in Poland [t a−1], C—carbon content in residues [%], M W—annual amount of residues in Poland [t a−1], 3.67—stoichiometric factor

Statistical Analysis

Statistical analysis was performed using STATISTICA 10. The results of the chemical analysis, methane fermentation and combustion were expressed as the mean ± standard deviation (SD) of three independent replicates. The effects of species and ensiling were tested by two-way analysis of variance. Tukey’s test was applied as a post-hoc test. Student’s t test was performed to compare content of macroelements in fresh cucumber and tomato residues and energy and CO2 production by methane fermentation and combustion. The level for accepted statistical significance was p < 0.05.

Results and Discussion

Chemical Analysis

The results of physicochemical analyses are presented in Table 1. Cucumber residues yielded much lower TS compared to tomato residues. However, in both cases, they were below the norm for silage fresh matter. Literature data suggest that the optimum dry matter content of silage forage should be approximately 25–40 % [24]. Insufficient content leads to mass loss in the form of juice leakage [25]. On the other hand, Beaulieu et al. [26], who studied influence of dry matter on silage quality using standard methods, reported that excess content prevents adequate compaction and removal of air. For this reason, too wet biomass must be wilted before ensiling. The process of silaging decreased the TS, VS, nitrogen and protein content. Loss of nutrients in silage is an unfavourable phenomenon associated with metabolic processes e.g. respiration of plants or the growth of yeast and Clostridium sp. [27]. As it has been stated by McEniry et al. [28] in their study of Timothy silage, proteolytic clostridial activity caused high pH and high concentration of butyric acid and ammonia-N. The high pH and high concentrations of NNH4 of tested silages may indicate proteolytic clostridial activity, but concentration of butyric acid has not been determined. The content of nitrogen and protein in the examined waste was significantly higher, and the content of sugar was lower than in other typical silage plants. For example, Amon et al. [8], using Kjeldahl method, stated that maize contains 5.9–10.1 % of proteins. The poor rate of the ensiling process was also manifested by an unpleasant odour. A feedstock with such quality does not meet the requirements for silage [29]. The low C/N ratio is a poor prognostic factor for the fermentation process, since the optimum is 25–30 [30]. However, some literature reports provide examples of effective common biogas production, despite the poor quality of silage. This can be explained by the fact that poor quality silage often contains ethanol and butyric acid, which have a higher theoretical biogas yield (693 L CH4 kg−1 and 604 L CH4 kg−1, respectively) compared to acetic acid and lactic acid (each with 355 L CH4 kg−1) [28].
Table 1

Chemical and physical properties

  

Cucumber residues

Tomato residues

Unit

Fresh

Ensiled

Fresh

Ensiled

TS

[% FM]

10.77 ± 1.11a

23.50 ± 2.22b

17.42 ± 0.54c

23.09 ± 0.29b

VS

[% TS]

74.70 ± 7.17a

66.27 ± 2.15b

81.97 ± 0.41c

82.24 ± 0.34c

Ash

[% TS]

25.30 ± 7.17a

33.73 ± 2.15b

18.03 ± 0.41c

17.76 ± 0.34c

C

[% TS]

52.64 ± 0.81a

50.89 ± 1.21a

53.00 ± 1.93b

49.90 ± 0.61a

Ntot.

[% TS]

4.38 ± 0.16a

2.75 ± 0.01b

3.47 ± 0.13c

2.50 ± 0.02d

NNH4

[% TS]

0.54 ± 0.04a

1.58 ± 0.08b

0.33 ± 0.05c

1.17 ± 0.04d

C/N

 

12.01 ± 021a

18.51 ± 0.12b

15.27 ± 0.24c

19.96 ± 0.07d

CP

[% TS]

27.38 ± 1.0a

17.19 ± 0.63b

21.69 ± 0.81a

15.63 ± 1.25c

Total sugars

[% TS]

1.85 ± 0.11a

1.15 ± 0.05b

1.65 ± 0.09a

1.09 ± 0.02c

pH

 

7.20 ± 0.09a

6.63 ± 0.03b

6.29 ± 0.18c

4.77 ± 0.01d

Ca

[% TS]

3.09 ± 0.01a

1.81 ± 0.03b

K

[%TS]

6.39 ± 0.00a

3.96 ± 0.05b

Mg

[%TS]

0.47 ± 0.00a

0.51 ± 0.06b

Na

[%TS]

0.07 ± 0.01a

0.06 ± 0.00b

S

[%TS]

0.61 ± 0.02a

1.02 ± 0.02b

Alkali index

[kg GJ−1]

0.82 ± 0.09a

0.23 ± 0.05b

Mean values with different superscript letters within row differ significantly (p < 0.05)

TS total solids, FM fresh matter, VS volatile solids, N tot. total nitrogen, N NH4 ammonium nitrogen, CP crude protein

Methane Fermentation

The results indicate that the fermentation of fresh tomato residues was most effective (606.9 ml g−1VS), while ensiled cucumber waste was least effective (327.3 ml g−1 VS) (Table 2). The results of methane yield of fresh tomato and cucumber residues are in accordance with Jagadabhi et al. [7] who found similar methane yield of tomato and cucumber shoots in the two-stage anaerobic digestion. In this study, ensiled wastes began to ferment faster than fresh matter, and their fermentation was completed sooner (Figs. 1 and 2). This was confirmed by Kafle and Kim [31] who, using similar methods of batch assay, studied influence of chemical composition and ensiling on biogas yield from various kind of waste such as Chinese cabbage, fish, bread and apple waste. These authors recommended ensiling as a good method for storage of agricultural by-products.
Table 2

Biogas, methane and carbon dioxide production

  

Cucumber residues

Tomato residues

Unit

Fresh

Ensiled

Fresh

Ensiled

Biogas

[m3 t−1 FM]

39 ± 1a

40 ± 2a

90 ± 2b

31 ± 1c

Biogas

[m3 t−1 TS]

363 ± 6a

270 ± 7b

515 ± 11c

300.4 ± 9d

Biogas

[m3 t−1 VS]

474 ± 8a

327 ± 10b

607 ± 13c

453 ± 15a

CH4

[m3 t−1 FM]

23 ± 1a

21 ± 1b

45 ± 1c

17 ± 1d

CH4

[m3 t−1 TS]

215 ± 4a

137 ± 3b

255 ± 4c

158 ± 4d

CH4

[m3 t−1 VS]

280 ± 5a

166 ± 4b

301 ± 5c

238 ± 7d

CH4

[%]

59 ± 2a

51 ± 2b

50 ± 1b

52 ± 2b

CO2

[m3 t−1 FM]

16 ± 1a

20 ± 1b

45 ± 1c

15 ± 1a

CO2

[m3 t−1 TS]

146 ± 7a

131 ± 3b

257 ± 5c

141 ± 3a

CO2

[m3 t−1 VS]

191 ± 5a

159 ± 5b

302 ± 6c

212 ± 9d

CO2

[%]

40 ± 2a

49 ± 1b

50 ± 1b

47 ± 1c

Mean values with different superscript letters within row differ significantly (p < 0.05)

FM fresh matter, TS total solids, VS volatile solids

Fig. 1

Daily biogas yield

Fig. 2

Cumulative biogas yield

As reported by Jagadabhi et al. [7], better hydrolysis and solubilisation rate of ensiled substrates can be explained by their lower pH, as the optimal pH for hydrolysis is 4–6. Rapid beginning of the methane fermentation can be also related to the initial decomposition of biomass occurring during the ensiling process, making it easily accessible to bacteria. The excess concentration of simple compounds already at the outset of the process sometimes makes microorganisms unable to utilise quickly enough volatile fatty acids that accumulate within a short time. This leads to acidification as well as collapse and faster completion of the process, resulting in a lower biogas yield. Similar results was obtained by Kandel et al. [32] who observed that rapid fermentation of non-structural components in young biomass of reed canary grass can cause a small build-up in acid products and partial inhibition of the process. Also, Ward et al. [33] highlighted that rapid hydrolysis of simple compounds may lead to acidification of a digester and consequent inhibition of methanogenesis, which takes place particularly in the case of fruit and vegetable wastes. Acidification in this study was manifested by decrease in biogas yield between the first and second peak of methane fermentation (Fig. 1) and characteristic inflexion of the curve shown in Fig. 2. This decrease is greater for ensiled residues than for fresh ones. Volatile fatty acids (VFA) accumulated at this stage, are then direct reason of the second peak between 15th and 25th day of the batch assay. About two peaks of methane fermentation was reported also by Wagner et al. [34] and Oleszek et al. [22], who gave accumulation of VFA as a reason for this phenomenon.

Despite the rapid beginning if batch assay, both silage matters gave significantly lower (p < 0.05) total biogas and methane yield than fresh substrates. Apart from acidification, this can be explained also by poor quality of silages.

The best quality of biogas was obtained from fresh cucumber waste (59.1 % average methane concentration of biogas), significantly better than ensiled one. No significant difference was observed between fresh and ensiled tomato residues. Methane concentration in biogas depends on chemical composition of substrate and can be predicted based on the C:H:O:N ratio [35]. Jacobi et al. [36] reported that the methane concentration from sugars, proteins and fats is 50, 70–71 and 67–68, respectively. Figure 3 shows methane concentration of the biogas produced throughout the first 40 days of the batch assay. Fresh residues characterised by higher methane concentration at the beginning of the batch assay than ensiled ones. These results are in accordance with data reported by Kandel et al. [33], who pointed out that the first week of methane fermentation of biomass rich in non-structural and soluble carbohydrates was characterised by lower methane concentration than biomass with higher content of neutral detergent fibre (NDF). Additionally, methane content in biogas is also dependent on the pH, which affects the solubility of CO2 and H2S (norm VDI 4630). At low pH, lower solubility of these gases causes their higher concentration in the headspace.
Fig. 3

Methane concentration of the biogas produced throughout the first 40 days of the batch assay

Energy and CO2 Production by Methane Fermentation and Combustion

HHV and LHV of the tested residues are presented in Table 3. HHV are slightly lower than or similar to other agricultural waste. For example, Skoulou and Zabaniotou [37] reported that HHV of rice straw, vineyard prunings, tobacco stems, wheat straw, sugar beet leaves and corn stalks is 12.1, 16.8, 16.1, 17.9, 17.7, and 17.8 MJ kg−1, respectively. Similar result of HHV of 14.1 MJ kg−1 for greenhouse residues was obtained by Us and Perendeci [38] based on the ASTM D5865-10a method.
Table 3

Amount of energy produced per unit of mass of residues by methane fermentation and combustion, at a moisture level of 0, 25 and 75 %

 

Unit

Cucumber residues

Tomato residues

HHV

[MJ kg−1]

12.00 ± 0.53a

13.58 ± 0.47b

LHV0

11.12 ± 0.53a

12.60 ± 0.47b

LHV25

7.73 ± 0.40a

8.84 ± 0.35b

LHV75

0.95 ± 0.13a

1.32 ± 0.12b

q FM. 0

6.18 ± 0.44a

7.35 ± 0.52b

q FM. 25

4.63 ± 0.33a

5.51 ± 0.39b

q FM. 75

1.54 ± 0.11a

1.84 ± 0.13b

Mean values with different superscript letters within row differ significantly (p < 0.05)

In present study, LHV was compared with q FM. LHV depends on moisture, which often limits the suitability of biomass for combustion. The problem of moisture does not occur in the case of methane fermentation. It can be seen that residues with moisture of 75 % are a more suitable feedstock for biogas production than for combustion. Their q FM,75 is higher than LHV75. However, the use of air-drying proved to be sufficient to increase LHV of the tested residues and made combustion a more energy-efficient process than methane fermentation (LHV25 was significantly higher than q FM, 25, α < 0.05).

Furthermore, the analysis of macroelements has shown that cucumber and tomato residues contain a high concentration of ash, K, Na, Ca and S and have a high alkali index (Table 4). According to Miles et al. [39], above 0.17 kg alkali GJ−1 fouling is very possible, and above 0.34 kg GJ−1 fouling is almost certain to occur. This proves that this kind of biomass, particularly cucumber waste, has a poor quality for combustion and involves a risk of damage to power plant installation. This is a major disadvantage of combustion in comparison to methane fermentation.
Table 4

Amount of CO2 produced per unit of mass of residues by methane fermentation and combustion

 

Unit

Cucumber residues

Tomato residues

CO2FM

kg kg−1 TS

0.71 ± 0.02a

1.01 ± 0.02b

CO2C

1.97 ± 0.03a

1.94 ± 0.04a

Mean values with different superscript letters within row differ significantly (p < 0.05)

The most important disadvantage of methane fermentation process is the high investment cost of approximately 5000 EUR/kW for smaller and 3800 EUR/kW for bigger biogas plants. Furthermore, because of the many sources of costs and revenues and amending the law relating to the system of support, the calculation of the profitability of agricultural biogas plant construction, especially on a small scale, is quite complicated [40].

Taking into account the operating costs of biogas production from various substrates, use of waste is widely regarded as the most economical solution, due to the lack of the need for purchase of substrate. Vegetable residues are free feedstocks; thus, their use significantly improves the economic balance of the process. For comparison, the cost of maize silage ranges from 120 to 160 PLN per tonne [40]. Iglinski at al. [41] compared the profitability of biogas production from waste (slurry, slaughter waste, municipal solid waste) and maize silage. Economic balance of biogas production from maize silage turned out to be negative, due to the high cost of this feedstock. Green biomass from natural grasslands, gardens and parks is cheap, and the use of it for energy production would not affect food prices. Despite that, the energy yield of such biomass is variable because the harvested plant material is diverse in terms of both plant species and chemical composition and very often contains a lot of fibre [42]. However, in the case of waste, very important is their sufficient amount in the place of use, lack of spatial dispersion of substrate, which would result in the need for the collection and transport over long distances. For example, due to the scattered nature of bio-waste sources as well as a low degree of waste segregation in Poland, the technical biogas potential from municipal waste can be estimated at the level of 10 % of the theoretical potential 10 million m3 of biogas (1 PJ) [41]. Therefore, the most reasonable is the construction of biogas plant nearby both producer of substrate and energy consumers. Greenhouses meet both conditions simultaneously. Alternatively, greenhouse collaboration with external biogas plant located within a short distance may be considered.

Table 4 shows the calculated amount of CO2 produced by methane fermentation and combustion. The amount of CO2 formed by combustion is much higher than that yielded by methane fermentation, due to the fact that some carbon remains in the digestate as a component of non-degradable compounds. Menardo et al. [4] also evaluated the possibility of producing energy and CO2 for greenhouse needs, but Miscanthus, untreated and pretreated with the stream explosion method, was used as a substrate. The amount of energy and CO2 produced by methane fermentation of the untreated Miscanthus was 2.37 MJ kg−1 TS and 0.25 kg kg−1 TS and that of the pretreated Miscanthus was 9.76 MJ kg−1 TS and 1.15 kg kg−1 TS, respectively. These values include loss of energy needed for the cultivation of this plant, which is not necessary in the case of waste.

Energy and CO2 Balance

The demands of Polish greenhouses for heat and carbon dioxide and the amount of waste that they generate are presented in Table 5. The amount of dry waste per hectare (18.3 t TS of cucumber and 29.6 t TS of tomato residues) can be larger than the dry yield of typical energy crops, such as maize [1, 8]. The total annual amount of cucumber and tomato fresh waste in Poland is estimated at 577.9 thousand tonnes. This huge part of valuable biomass could be utilised.
Table 5

Energy and CO2 balance

 

Unit

Cucumber residues

Tomato residues

M w

tys. tonnes FM a−1

208.9

369.0

tys. tonnes TS a−1

22.5

64.3

D Q

PJ a−1

44.2

78.1

Q FM

PJ a−1

0.139

0.472

Q C. 75 %

PJ a−1

0.086

0.339

Q C. 25 %

PJ a−1

0.232

0.758

Q C. d

PJ a−1

0.250

0.810

D CO2

tys. tonnes a−1

3230.0

5704.9

CO2FM

tys. tonnes a−1

16.0

64.9

CO2C

tys. tonnes a−1

43.4

125.0

Heat energy that can be obtained by methane fermentation of such an amount of residues is calculated at ca. 0.511 PJ and by combustion from 0.425 to 1.060 PJ, depending on the moisture. Unfortunately, all of the above-mentioned values are significantly lower than the domestic greenhouse energy demand. As reported by Shul [5], only in the Lubelskie Voivodeship, where there were 47 ha of crops under cover in 2002, the annual demand for energy for greenhouse heating was approximately 59 thousand tce (ton of coal equivalent), which is equal to 1.7 PJ. For comparison, the total theoretical potential of the primary energy production from biogas in the Lubelskie Voivodeship is estimated at 172 PJ but as many as 167 PJ of this amount belongs to energy crops [40].

Additionally, the amount of the produced carbon dioxide turned out to be much lower than the demand. These results are consistent with previous calculations of Menardo et al. [4]. The balance showed that the use of such waste biomass could be interesting for greenhouse use in Poland, but only in combination with another supplementary energy source.

Conclusions

The results imply that the wastes of cucumber and tomato cultivation provide a suitable feedstock for methane fermentation, but unsuitable material for silage. Both methane fermentation and combustion are good options for greenhouse residue disposal as well as production of heat and carbon dioxide for greenhouse needs. More energy can be obtained in combustion than through the methane fermentation process, but energy-intensive drying is required, and a risk of fouling and slagging can occur. Despite the fact that the annual amount of residues is too small to be able to meet the total greenhouse heat and CO2 demands, potential of such biomass should be taken into account. Additional study in continuous system of methane fermentation and larger scale are recommended for practical application purpose.

Notes

Acknowledgments

The authors would like to express thanks to the employees of the Chair of Plant Horticulture and Fertilization, University of Life Sciences in Lublin, for providing the material for investigations.

References

  1. 1.
    CSO (Central Statistical Office in Poland) (2015) Crops production results in 2014. Statistical Publishing Establishment, Warsaw, p 94Google Scholar
  2. 2.
    Boulard T, Raeppel C, Brun R, Lecompte F, Hayer F, Carmassi G, Gaillard G (2011) Environmental impact of greenhouse tomato production in France. Agron Sustain Dev 31:757–777CrossRefGoogle Scholar
  3. 3.
    Pietzsch M, Meyer J (2008) Use of reject heat from biogas power plants for greenhouse heating. Acta Hort (ISHS) 801:719–724CrossRefGoogle Scholar
  4. 4.
    Menardo S, Bauer A, Theuretzbacher F, Piringer G, Nilsen PJ, Balsari P, Pavliska O, Amon T (2013) Biogas production from steam-exploded miscanthus and utilization of biogas energy and CO2 in greenhouses. BioEnergy Res 6(2):620–630CrossRefGoogle Scholar
  5. 5.
    Szul T (2011) Demand for energy to heat glasshouses and polytunnels in rural area of Lublin province. Technika Rolnicza, Ogrodnicza, Leśna 6:26–27Google Scholar
  6. 6.
    Oleszek M, Tys J (2013) Lab scale measurement of biogas yield. Przemysł Chemiczny 1(92):126–130Google Scholar
  7. 7.
    Jagadabhi PS, Kaparaju P, Rintala J (2011) Two-stage anaerobic digestion of tomato, cucumber, common reed and grass silage in leach-bed reactors and upflow anaerobic sludge blanket reactors. Bioresour Technol 102(7):4726–4733CrossRefPubMedGoogle Scholar
  8. 8.
    Amon T, Amon B, Kryvoruchko V, Zollitsch W, Mayer K, Gruber L (2007) Biogas production from maize and dairy cattle manure—influence of biomass composition on the methane yield. Agric Ecosyst Environ 118(1–4):173–182CrossRefGoogle Scholar
  9. 9.
    Krzemińska I, Nawrocka A, Piasecka A, Jagielski P, Tys J (2015) Cultivation of chlorella protothecoides in photobioreactors: the combined impact of photoperiod and CO2 concentration. Eng Life Sci 15:533–541CrossRefGoogle Scholar
  10. 10.
    Romano RT, Zhang R (2011) Anaerobic digestion of onion residuals using a mesophilic anaerobic phased solids digester. Biomass Bioenergy 35:4174–4179CrossRefGoogle Scholar
  11. 11.
    Durán-Garcia M, Ramirez Y, Bravo R, Rojas-Solórzano L (2012) Biogas home-production assessment using a selective sample of organic vegetable wastes. A preliminary study. Interciencia 37(2):128–132Google Scholar
  12. 12.
    Scano EA, Asquer C, Pistis A, Ortu L, Demontis V, Cocco D (2014) Biogas from anaerobic digestion of fruit and vegetable wastes: experimental results on pilot-scale and preliminary performance evaluation of a full-scale power plant. Energy Convers Manag 77:22–30CrossRefGoogle Scholar
  13. 13.
    Telmo C, Lousada J (2011) Heating values of wood pellets from different species. Biomass Bioenergy 35(7):2634–2639CrossRefGoogle Scholar
  14. 14.
    Lewandowski I, Kauter D (2003) The influence of nitrogen fertilizer on the yield and combustion quality of whole grain crops for solid fuel use. Ind Crop Prod 17:103–117CrossRefGoogle Scholar
  15. 15.
    Jenkins BM, Baxter LL, Miles TR Jr, Miles TR (1998) Combustion properties of biomass. Fuel Process Technol 54:17–46CrossRefGoogle Scholar
  16. 16.
    Cheuk W, Lo KV, Branion RMR, Fraser B (2003) Benefits of sustainable waste management in the vegetable greenhouse industry. J Environ Sci Health B 38(6):855–863CrossRefPubMedGoogle Scholar
  17. 17.
    Suárez-Estrella F, Vargas-García MC, López MJ, Moreno J (2007) Effect of horticultural waste composting on infected plant residues with pathogenic bacteria and fungi: integrated and localized sanitation. Waste Manag 27:886–892CrossRefPubMedGoogle Scholar
  18. 18.
    Belhadj S, Joute Y, Bari HE, Serrano A, Gil A, Siles JA, Chica AF, Martin MA (2014) Evaluation of the co-digestion of sewage sludge and tomato waste at mesophilic temperature. Appl Biochem Biotechnol 172:3862–3874CrossRefPubMedGoogle Scholar
  19. 19.
    Saev M, Koumanova B, Simeonov M (2009) Anaerobic co-digestion of wasted tomatoes and cattle dung for biogas production. J Univ Chem Technol Metall 44(1):55–60Google Scholar
  20. 20.
    Nurzyński J, Jarosz Z, Michałojć Z (2012) Yielding and chemical composition of greenhouse tomato fruit grown on straw or rockwool substrate. Acta Sci Pol Hortorum Cultus 11(3):79–89Google Scholar
  21. 21.
    Oleszek M, Król A, Tys J, Matyka M, Kulik M (2014) Comparison of biogas production from wild and cultivated varieties of reed canary grass. Bioresour Technol 156:303–306CrossRefPubMedGoogle Scholar
  22. 22.
    Oleszek M, Matyka M, Lalak J, Tys J, Paprota E (2013) Characterization of Sida hermaphrodita as a feedstock for anaerobic digestion process. J Food Agric Environ 11(3,4):1839–1841Google Scholar
  23. 23.
    Horttanainen M, Kaikko J, Bergman R, Pasila-Lehtinen M, Nerg J (2010) Performance analysis of power generating sludge combustion plant and comparison against other sludge treatment technologies. Appl Therm Eng 30(2):110–118CrossRefGoogle Scholar
  24. 24.
    Mohd-Setapar SH, Abd-Talib N, Aziz R (2012) Review on crucial parameters of silage quality. APCBEE Procedia 3:99–103CrossRefGoogle Scholar
  25. 25.
    Gebrehanna MM, Gordon RJ, Madani A, VenderZaag AC, Wood DJ (2014) Silage effluent management: a review. J Environ Manag 143(1):113–122CrossRefGoogle Scholar
  26. 26.
    Beaulieu R, Seoane JR, Savoie P, Tremblay D, Tremblay GF, Thériault R (1993) Effect of dry matter content on the nutritive value of individual wrapped round-bale timothy silage fed to sheep. Can J Anim Sci 73(2):343–354CrossRefGoogle Scholar
  27. 27.
    Radkowski A, Kuboń M (2007) Influence of the green fodders harvesting on the quality of producing silages (in Polish). Inżynieria Rolnicza 7(95):177–182Google Scholar
  28. 28.
    McEniry J, Allen E, Murphy JD, O’Kiely P (2014) Grass for biogas production: the impact of silage fermentation characteristics on methane yield in two contrasting biomethane potential test systems. Renew Energy 63:524–530CrossRefGoogle Scholar
  29. 29.
    Muck RE (1988) Factors influencing silage quality and their implication for management. J Dairy Sci 71(11):2992–3002CrossRefGoogle Scholar
  30. 30.
    Puyuelo B, Ponsá S, Gea T, Sánchez A (2011) Determining C/N ratios for typical organic wastes using biodegradable fractions. Chemosphere 85(4):653–659CrossRefPubMedGoogle Scholar
  31. 31.
    Kafle GK, Kim SH (2013) Effects of chemical compositions and ensiling on the biogas productivity and degradation rates of agricultural and food processing by-products. Bioresour Technol 142:553–561CrossRefPubMedGoogle Scholar
  32. 32.
    Kandel TP, Sutaryo S, Møller HB, Jørgensen U, Lærke PE (2013) Chemical composition and methane yield of reed canary grass as influenced by harvesting time and harvest frequency. Bioresour Technol 130:659–666CrossRefPubMedGoogle Scholar
  33. 33.
    Ward AJ, Hobbs PJ, Holliman PJ, Jones DL (2008) Optimisation of the anaerobic digestion of agricultural resources. Bioresour Technol 99(17):7928–7940CrossRefPubMedGoogle Scholar
  34. 34.
    Wagner AO, Lins P, Malin C, Reitschuler C, Illmer P (2013) Impact of protein-, lipid- and cellulose-containing complex substrates on biogas production and microbial communities in batch experiments. Sci Total Environ 458–460:256–266CrossRefPubMedGoogle Scholar
  35. 35.
    Jacobi HF, Ohl S, Thiessen E, Hartung E (2012) NIRS-aided monitoring and prediction of biogas yields from maize silage at a full-scale biogas plant applying lumped kinetics. Bioresour Technol 103(1):162–172CrossRefPubMedGoogle Scholar
  36. 36.
    Appels L, Baeyens J, Degrève J, Dewil R (2008) Principles and potential of the anaerobic digestion of waste-activated sludge. Prog Energy Combust Sci 34(6):755–781CrossRefGoogle Scholar
  37. 37.
    Skoulou V, Zabaniotou A (2007) Investigation of agricultural and animal wastes in Greece and their allocation to potential application for energy production. Renew Sust Energ Rev 11:1698–1719CrossRefGoogle Scholar
  38. 38.
    Us E, Perendeci NA (2012) Improvement of methane production from greenhouse residues: optimization of thermal and H2SO4 pretreatment process by experimental design. Chem Eng J 181–182:120–131CrossRefGoogle Scholar
  39. 39.
    Miles TR, Miles TR Jr, Baxter LL, Bryers RW, Jenkins BM, Oden LL (1995) Alkali deposits found in biomass power plants: a preliminary investigation of their extent and nature. National Renewable Energy Laboratory, GoldenCrossRefGoogle Scholar
  40. 40.
    Oniszk-Popławska A, Matyka M, Dagny-Ryńska E (2014) Evaluation of a long-term potential for the development of agricultural biogas plants: a case study for the Lubelskie Province, Poland. Renew Sustain Energy Rev 36:329–349CrossRefGoogle Scholar
  41. 41.
    Igliński B, Buczkowski R, Iglińska A, Cichosz M, Piechota G, Kujawski W (2012) Agricultural biogas plants in Poland: investment process, economical and environmental aspects, biogas potential. Renew Sust Energ Rev 16(7):4890–4900CrossRefGoogle Scholar
  42. 42.
    Triolo JM, Pedersen L, Qu H, Sommer SG (2012) Biochemical methane potential and anaerobic biodegradability of non-herbaceous and herbaceous phytomass in biogas production. Bioresour Technol 125:226–232CrossRefPubMedGoogle Scholar

Copyright information

© The Author(s) 2015

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Authors and Affiliations

  • Marta Oleszek
    • 1
    Email author
  • Jerzy Tys
    • 1
  • Dariusz Wiącek
    • 1
  • Aleksandra Król
    • 1
  • Jan Kuna
    • 1
  1. 1.Institute of Agrophysics Polish Academy of SciencesLublinPoland

Personalised recommendations