Biogas generation from insects breeding post production wastes

Journal Pre-proof Biogas generation from insects breeding post production wastes Piotr Bulak, Kinga Proc, Małgorzata Pawłowska, Agnieszka Kasprzycka, Wojciech Berus, Andrzej Bieganowski PII:

S0959-6526(19)33647-9

DOI:

https://doi.org/10.1016/j.jclepro.2019.118777

Reference:

JCLP 118777

To appear in:

Journal of Cleaner Production

Received Date: 24 July 2019 Revised Date:

10 September 2019

Accepted Date: 6 October 2019

Please cite this article as: Bulak P, Proc K, Pawłowska Mał, Kasprzycka A, Berus W, Bieganowski A, Biogas generation from insects breeding post production wastes, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/j.jclepro.2019.118777. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Biogas generation from insects breeding post production wastes

1 2 3

Piotr Bulaka,*, Kinga Proca, Małgorzata Pawłowskab, Agnieszka Kasprzyckaa, Wojciech Berusa,

4

Andrzej Bieganowskia

5 6

a

Institute of Agrophysics, Polish Academy of Sciences, Doświadczalna 4, 20-290 Lublin, Poland

7

b

Lublin University of Technology, Faculty of Environmental Engineering, Nadbystrzycka 40B,

8

20-618 Lublin, Poland

9 10

*

11

744 50 67

corresponding author: [email protected], phone: +48 81 744 50 61 int. 135, fax: +48 81

12 13

e-mail

14

[email protected], Agnieszka Kasprzycka [email protected], Wojciech Berus

15

[email protected], Andrzej Bieganowski [email protected]

16 17 18 19 20 21 22

addresses:

Kinga

Proc

[email protected],

Małgorzata

Pawłowska

25

Abstract

26

Insect breeding generates waste: insect excrements, often mixed with the remains of the feed .

27

Insect waste is usually sold as a plant fertilizer, however, there is one more method of its use

28

– methane production via the anaerobic digestion . To the best of the authors' knowledge, this

29

topic is very poorly studied. The aim of this work was the evaluation of the suitability of the

30

waste derived frominsects breeding (Hermetia illucens, Tenebrio molitor and Gryllus spp.)

31

for methane production. The mesophilic anaerobic digestion process was performed in 500 ml

32

bioreactors. The temperature of the process was 37°C ± 1°C and initial pH was 7.0 ± 0.2. The

33

substrate loading comprised 3.5 g of total solids and the inoculum-to-substrate ratio was 2:1.

34

The biomethane potential of investigated wastes was ~177 ml·g-1 TS for H. illucens, ~212

35

ml·g-1 TS for Tenebrio molitor to ~225 ml·g-1 TS for Gryllus spp.. The obtained biomethane

36

potentials are similar to more commonly used substrates for anaerobic digestion like: cattle

37

manure, mink manure, poultry manure, fruit and vegetables waste, ryegrass, switchgrass,

38

wheat, and sewage sludge, which points to the reasonability of their use. Anaerobic digestion

39

can be a new method for valorization of insect post-production wastes.

40 41

Keywords: methane; insect; black soldier fly; mealworm; crickets; waste management

42 43

Abbreviations

44

BMP – biomethane potential; BSF – black soldier fly; COD – chemical oxygen demand;

45

GHG – greenhouse gases; TS – total solids; VFA - volatile fatty acid; VS – volatile solids;

46 47

1. Introduction

48

1

49

Methane is one of the greenhouse gases (GHG) and its uncontrolled emission into the

50

atmosphere is highly environmentally harmful (Walkiewicz et al., 2016). Emissions can occur

51

from natural sources such as wetlands, as well as from anthropogenic ones, such as

52

agriculture or waste management systems (Pawłowska et al., 2011; Yusuf et al., 2012;

53

Brzezińska et al., 2014; Szafranek-Nakonieczna et al., 2018). However, methane can also be

54

produced intentionally via the anaerobic digestion process, in which the organic compounds,

55

contained in plant biomass, organic waste or even wastewater are converted under anaerobic

56

conditions to biogas and digestate, which can be a highly effective and profitable way of

57

utilizing organic waste and producing energy (Esen and Yuksel, 2013; Lalak et al., 2016;

58

Oleszek and Matyka, 2017; Win et al., 2018; Kasprzycka and Kuna, 2018).

59

The development of the insect industry in Europe and worldwide has significantly

60

accelerated in recent years.. Large-scale insect-breeding companies are being set up for the

61

production of animal feed (which has also been approved by the EU from 2017 according to

62

Commission Regulation (EU) 2017/893). The recent draft regulation of the EU Commission

63

describes minimum hygiene requirements for insects, which will be bred for human food (EU

64

Commission Regulation Draft Ares(2019)382900). This change will lead to the development

65

of new markets and new human food industry branches, indicating the great potential linked

66

with insect production.

67

In Asia, the human consumption of insects is popular, as they are a good source of

68

energy, protein, fat, vitamins and minerals (Rumpold and Schluter, 2013; Oonincx et al.,

69

2015). In the Malaysian province of Sabah, about 60 species of insects (including Hermetia

70

illucens) are eaten (even raw), as well as in the form of powder, which acts as a food additive

71

for increasing taste and aroma (Wang and Shelomi, 2017). In Europe, the powdered form of

72

whole insects or insect isolates, e.g., protein, fat or chitin, is relatively accepted compared to

73

visibly whole insects (Mariod, 2013; Wang and Shelomi, 2017). Small- and medium-scale

2

74

insect rearing (less than 100 Mg year-1) is an established activity, which supplies feed to zoos

75

and pet shops and auxiliary insects for biological controls in the European market (Azagoh et

76

al., 2015). Insects have considerable potential as feed, due to their nutritional value, low space

77

requirements and marked acceptability; they are especially recommended for poultry and fish

78

(Mariod, 2013). Chitosan, which is a chitin derivative, displays satisfactory resistance and

79

antimicrobial properties and can be used in the bioplastics and the bio-based composite

80

industries (Azagoh et al., 2015).

81

Hermetia illucens (also known as black soldier fly (BSF)), an insect from the Diptera

82

family with highly interesting properties (Müller et al., 2017; Waśko et al., 2016; Zdybicka-

83

Barabas et al., 2017), is being increasingly proposed and used as an ideal organism for the

84

utilization of different types of organic wastes (Gold et al., 2018), biodiesel production (Wang

85

et al., 2017), and even for entomoremediation (Bulak et al., 2018). It was shown that the

86

larvae of H. illucens can reduce the amount of fresh biomass in waste plant tissues, food

87

scraps, catering waste or solid residual fraction of restaurant waste in the range of 46-66.5%

88

(Kalová and Borkovcová, 2013). Mealworm larvae (Tenebrio molitor) and crickets (Gryllus

89

spp.) are used mainly as live feeder for domestic pets and food for humans (entomophagy)

90

(Anankware et al., 2015). Mealworms contain up to 45% proteins and 40% lipids

91

(Ravzanaadii et al., 2012), while crickets may contain even more proteins (58%) but have a

92

lower fat content (10%) (Wang et al., 2004). Moreover, both insect types are characterized by

93

a high amount of unsaturated fatty acids; this can be as much as 66-77% of the total

94

percentage of fatty acids, comparable to rape and olive oil (Wang et al., 2004; Ravzanaadii et

95

al., 2012).

96

The insect production industry is growing rapidly - according to Dossey et al. (2016), at

97

least 61 companies producing and/or selling insect products were opened between 2014 and

98

2015. The economists forecast the global market value of edible insects to grow from about

3

99

406 million USD in 2018 to over 1.18 billion USD by 2023 (Statista, 2019). The development

100

of this industry will implicate an increase in the amount of post-production residues, which

101

should be considered in the planning of waste management. This type of waste is insect

102

excrements, often mixed with the remains of their feed . Insect waste is not harmful to the

103

environment and is usually sold as a plant fertilizer (Müller et al., 2017). However, there is

104

one more, very poorly studied method of its use – methane production via the anaerobic

105

digestion process. To the best of the authors' knowledge, only one study was directly related

106

to the topic of biogas production from insects, and it reffered to the gasification of H. illucens

107

whole larvae or larvae residues after fat extraction (Win et al., 2018). The results presented in

108

the paper indicate that different types of substrate originating from BSF rearing, including

109

farming residues, have a high biomethane potential (BMP). Therefore,the topic has a great

110

scientific and applicatory potential (Win et al., 2018). There are also few publications on the

111

use of invertebrates other than insects in biogas production. For instance, Serrano et al. (2016)

112

used feces of aquatic worm Lumbriculus variegatus fed on waste activated sludge for biogas

113

production. This concept has been under investigation for more than 10 years and has resulted

114

in the development of test bioreactors (Hendrickx et al., 2010). This technology used also

115

others species from Oligochaete, e.g. Aulophorus furcatus (Tamis et al., 2011), as a source of

116

substrate, but the BMP of the worm feeding residue was studied only by a few researchers

117

(see Serrano et al., 2016).

118

Searching for new or alternative and efficient sources of substrates for biogas production

119

helps to protect environment and promotes the use of crops for food production, and not for

120

energy production - as e.g. in Germany, where a large share of the corn is growing

121

specifically for biogas (Winquist et al., 2019). In case of animal (and insect) excrements its

122

use for biogas production helps to reduce GHG emission (as CH4 and N2O) and

123

eutrophication, which would occur if they were used as an organic fertilizer (Bao et al.,

4

124

2019).The research hypothesis is assuming that raw post-production waste from insect

125

breeding could be a good substrate for biogas production. Taking into account the rising

126

interest in the sourcing of insect-derived proteins and lipids, which would lead to growth in

127

the production of waste from insects breeding, and looking at the advantages of waste usage

128

in biofuel production, the aim of this work was to evaluate the suitability of waste derived

129

from the farming of three different insect species: Hermetia illucens, Tenebrio molitor and

130

Gryllus spp, which can be legally used in the EU as animal forage additives, for methane

131

production. Their applicability was assessed indirectly on the basis of selected properties

132

which are significant for ensuring the proper conditions for methanogenic microorganism

133

development, and directly based on the results of biochemical methane potential tests.

134 135

2. Materials and methods

136 137

2.1. Insect breeding

138 139 140

The residues from the breeding of three types of insect larvae – BSF (H waste), mealworm (M waste) and crickets (C waste) – were used in the experiment.

141

The BSF (H. illucens) larvae were reared in the laboratory of the Institute of Agrophysics

142

of the Polish Academy of Science in Lublin, Poland. H. illucens was used in the experiment

143

regarding the utilization of waste from the fruit and vegetable industry in the form of carrot-

144

beetroot marc. Three batches of approximately 300 BSF larvae (in each batch), fed with

145

medium consisting of carrot-beetroot marc mixed at a volumetric ratio of 3:1 (dry weight of

146

15.54% ± 0.02%), were placed in a laboratory incubator at a temperature of 27°C ± 1°C and

147

in darkness. Breeding was continued until the growth of larvae on a given batch of marc was

148

clearly limited and no marc particles could be detected in the substrate.

5

149

Larvae of mealworm beetle (Tenebrio molitor) were fed on substrate consisting of oat

150

flakes and leftovers of vegetables and fruits (i.e., tomatoes, paprika, dill and parsley stems and

151

leaf residues, apple pieces) in the proportion of 4:1. Mealworms have been reared in the

152

following conditions: relative humidity 60 ± 5%, temperature 23°C ± 1ºC and in darkness.

153

The wastes from the breeding of different genera of crickets (Gryllus spp.) were supplied

154

by a regional commercial breeder (CricketsFarm, Lublin, Poland). All bred cricket species

155

were fed with the same mixture of cereals grains, soy and dried alfalfa plants in the ratio of

156

1:1:1.

157 158

2.2. Analytical methods

159 160

All the wastes were air-dried prior to the examinations. Total solids (TS) and moisture

161

content, as well as volatile solids (VS) and ash contents, of investigated materials were

162

determined gravimetrically, firstly, by drying at 105ºC, and then by igniting at 550ºC (EN

163

12880:20004; EN 12879:20004).

164

Total C and total N were determined by elemental analysis using a Thermo Scientific

165

Flash 2000 Organic Elemental Analyzer according to the manufacturer’s instructions. Protein

166

content was calculated on the basis of total N using a 6.25 multiplier (Win et al., 2018).

167

Volatile

fatty

acid

(VFA)

content

was

determined

spectrophotometrically

168

(Spectrophotometer DR3900, Hach Lange, Düsseldorf, Germany) in water extracted from the

169

waste (preparation conditions: 5 ml·g-1, 1 h, 150 rpm), using the Hach LCK 365 cuvette test

170

system. The results were recalculated on the dry mass of the waste.

171

Crude lipids were analyzed with the use of Soxtec Avanti (Foss, Hillerød, Denmark),

172

while raw fibers were determined by sequential acid-base extraction with hot 1.25% H2SO4

173

and 1.25% NaOH on Fibertec 2010 (Foss, Hillerød, Denmark). Carbohydrate (cellulose,

6

174

hemicellulose and chitin) content was calculated by subtracting the content of all other

175

analyzed components (moisture, proteins, fat, fibers, volatile fatty acids and ash) from the

176

percentage of TS.

177

The pH value was measured potentiometrically by HQ 400 multi-purpose machine (Hach

178

Lange, Düsseldorf, Germany) in the supernatant obtained by mixing the waste with distilled

179

water in the ratio of 1:20 v·v-1.

180 181

2.3. Characteristic of the examined wastes

182 183

The wastes from breeding H. illucens, mealworms and crickets can be seen in Fig. 1. The

184

residues from breeding H. illucens (H waste) were brown in color and, in a large part, took the

185

form of granules of about 1-3 mm in diameter or flakes. It was impossible to visually

186

distinguish the remains of carrot-beetroot marc (Fig. 1a). The residues from mealworm

187

breeding (M waste) were a mixture of dry feces in the form of fine, brown powder and fine

188

remains of crushed feed (Fig. 1b). The waste from cricket breeding (C waste) was a mixture

189

mostly consisting of crushed plant remains and, to a much lesser extent, crushed wheat grains.

190

The remains of dead adults and parts of cricket exoskeletons were present (Fig. 1c).

191 192

2.4. Bio-methane potential assays

193 194

The mesophilic anaerobic digestion process was performed according to the VDI 4630

195

(2016) and DIN 38414 protocols (DIN 38414:1985; VDI 4630:2016). It was carried out in a

196

500 ml bioreactor tank (300 ml of working volume). Prior to setting up the experiment, the

197

inoculum was starved until biogas production ceased. The temperature of the process was

198

37°C ± 1°C and initial pH was 7.0 ± 0.2. The substrate loading was 3.5 g of TS for each

7

199

bioreactor, and inoculum-to-substrate ratio (I/S) was established at 2:1 (based on the TS).

200

Digestate from the biogas plant in Siedliszczki (Lubelskie District, Eastern Poland), which

201

was based on corn silage and distillers’ grains, while whey was used as inoculum. The basic

202

properties of inoculum are presented in Table 1.

203

The composition of biogas was determined by means of multigas analyzer (GFM436, Gas

204

Data, UK) once a day. The volume of biogas was determined by the method of liquid

205

displacement. The experiment was ended when daily biogas production in each feedstock was

206

maintained on a level below 10% of maximum daily production during the next three days.

207

Under these conditions, the experiment lasted for 21 days.

208 209

2.5. Statistical analysis

210 211

All physicochemical analyses of wastes were carried out in three replications. Each

212

value represents the mean ± SD (n = 3). The anaerobic digestion experiment was conducted in

213

three independent replications (presented values are given as the mean ± SD). The analysis of

214

variance (ANOVA) and the post-hoc Tukey test (Statistica 10.0) were conducted to assess the

215

significance of the differences between the compared mean values.

216 217

3. Results and Discussion

218 219

3.1. Physicochemical parameters of insect breeding wastes and inoculum

220 221

A multisubstrate digestate from the biogas plant was used as an inoculum for the

222

fermentation process. There were two main reasons for the selection of such an inoculum.

223

Firstly, the value of pH (Table 1) was in the range of the optimum for methanogens, i.e., 6.7-

8

224

7.5 (Deublein and Steinhauser, 2008). The second reason was the low ratio of C:N, equal to

225

7.4 (Table 1), which indicates the high degree of the biodegradation of organic matter. Thus,

226

no methane release, due to the decomposition of the organic matter, remained in the digestate

227

should be expected. Therefore, the time of the inoculum starvation could be shortened.

228

All the used waste samples were characterized by a high value of dry weight (over 84%)

229

with dominating organic compound (Table 1). Due to high content of VS (84-87 %) they were

230

similar to animal manures (72-93 % VS) and food and food-processing wastes (95% VS),

231

both of which are considered the best resources for biomethane generation (Bharathiraja et al.,

232

2018). The highest dry weight was observed in M waste, and the lowest in H waste. The

233

concentration of mineral compounds (expressed as ash content) was similar in M and H

234

wastes, but significantly lower in C wastes. The chemical composition of organic substance in

235

particular wastes was substantially differentiated. M waste contained the highest amount of

236

proteins (about 32% TS) and fats (about 3.5% TS) compared to the other waste. The lowest

237

content of proteins and fats was observed in H waste, in which this parameter was 2.3 and 5.8

238

times lower, respectively. H waste was richest in raw fiber. The content of raw fiber in this

239

material (about 33% TS) was almost twice as high as in C waste, which was poorest in these

240

substances. The only components which occurred in a significantly larger amount in cricket

241

waste, when compared to the other waste, were carbohydrates (Table 1). The examined

242

wastes differed significantly in terms of the content of total nitrogen. The highest content of

243

this parameter was measured in M waste, and the lowest in H waste. The latter waste was also

244

characterized by the lowest total C content (statistically lower than in other wastes). In turn,

245

the ratio of C:N was the highest for H while the lowest was for M waste. The C:N ratio in M

246

waste was about two times lower than in H waste. M waste also had the lowest pH value

247

(6.01), i.e., 1.3 times lower than the pH measured in H waste, which was characterized by the

248

highest pH. The content of VFA in the waste ranged from 1.6 to 2.9% TS. The highest

9

249

concentration of VFA, significantly different when compared to other substrates, was found in

250

M waste. It was about 76% and 13% higher than in C and H waste, respectively. When

251

considering the number of components, the most complex substrate was used for mealworm

252

rearing, and the simplest (two components only) was used for BSF breeding. This may be

253

explain why M waste was characterized by the extreme values of many parameters. The

254

opposite values of more of these parameters were found for H waste.

255

The content of VFA in M and H wastes exceeded the values found in the waste-activated

256

sludge (1.83 ± 0.12% VS), potato peel waste (2.08 ± 0.14% VS) and food waste (2.79 ±

257

0.18% VS), as examined by Ma et al. (2017). Supplying the examined residues into the

258

anaerobic bioreactor to the amount of 50 kg·Mg-1 of feedstock corresponds to a VFA

259

concentration of 950 to 1,700 mg·dm-3. In contrast, Serrano et al. (2016) found VFA on the

260

maximum level of only 30 mg VFA-COD·dm-3 when fermented waste sludge and whole

261

bodies of L. variegatus. It is commonly known that a high concentration of VFA (especially

262

in form of propionic acid) is an important factor which can disrupt the anaerobic digestion

263

process (Wang et al., 2009). Lee et al. (2015) observed the inhibition of the digestion of food

264

waste leachate at VFA concentrations of 4,000 mg·dm-3; but, in the case of propionic acid, the

265

inhibiting effect was observed already at 900 mg·dm-3. Considering that VFA is the main

266

product in organic matter decomposition during the acidification stage of methanogenesis

267

(Nielsen et al., 2007), it should be noted that the concentration of these compounds in

268

bioreactor feedstock will increase over time and may lead to excessive acidification. This

269

phenomenon is one of the most common reasons for the deterioration of anaerobic digestion

270

processes (Akuzawa et al., 2011). Taking into account the pH of the examined residues,

271

acidification poses a real threat in the case of M and C wastes, which were slightly acid (pH

272

6.01 and 6.18, respectively); but it is not likely to be a problem in the case of H waste, which

273

was alkaline (pH 8.19) (Table 1). Generally, none of the examined waste had a pH value

10

274

considered as optimal for methanogens, which falls within the range of 6.7-7.5 (Deublein and

275

Steinhauser, 2008). Therefore, the monosubstrate anaerobic digestion applied for their

276

treatment can cause technological problems.

277

3.2. Biogas and methane potential

278 279

The daily and cumulative biogas production during the experiment is shown in Fig. 2. At

280

the end of the experiment (21st day), the highest value of the total biogas amount, related to

281

the weight of the waste added to the bioreactor, was observed for M waste (451.1 ± 3.4 ml·g-1

282

VS), followed by C waste (447.4 ± 5.7 ml·g-1 VS) and H waste (412.5 ± 5.1 ml·g-1 VS) (Fig.

283

2; Table 2). The statistical analysis showed significantly lower biogas potential obtained for

284

the H waste compared to the other residues (p < 0.05, Tukey’s test). The maximum efficiency

285

of methane production was obtained during the first 30 days of the experiment (Fig. 2).

286

Serrano et al. (2016), who fermented fecal matter from L. variegatus as well as whole bodies

287

of this aquatic worm, made similar observation that under mesophilic conditions, the

288

maximum methane yield was achieved within 30 days from the start of fermentation.

289

The biogas produced from particular feedstock differed in terms of methane

290

concentration (Table 2). The highest mean value of this parameter was observed in the biogas

291

produced from C waste. This biogas was 1.6% richer in methane than biogas produced from

292

M waste, and over 7.6% richer than that released from H waste. The values of the highest

293

methane concentrations in the biogas obtained during the digestion of particular feedstock

294

ranged from 53.2 to 62.3% and differed significantly between M and H waste (Table 2).

295

Analogically to the mean methane concentration (Table 2), the highest maximum biomethane

296

potential (BMP), equal to 258.8 ± 14.0 ml CH4·g-1 VS, was obtained for C waste, but this

297

value was not significantly different from the BMP of the other waste. Although the insect

298

waste did not differ significantly in terms of BMP related to VS, the H waste was

11

299

characterized by a significantly lower value (177.2 ± 18.3 ml·g-1) than in the case of C waste

300

(225.3 ± 12.2 ml·g-1) when related to TS (Table 2). The BMPs values obtained in this

301

experiment were higher than the mean BMP values obtained for silage of Miscanthus

302

giganteus, but lower than for silage of maize, which were 186 ml CH4 g-1 VS and 381 ml CH4

303

g-1 VS, respectively (Whittaker et al., 2016). BMP of insect wastes tested in this study ranged

304

from 207.9 to 258.8 ml CH4·g-1 VS (Tab. 2). Considering this parameter, the tested

305

substances were similar in this parameter to some more commonly used substrates for

306

biomethane production, such as cattle manure (242-399 ml CH4·g-1 VS), mink manure (239-

307

428 ml CH4·g-1 VS), poultry manure (107-438 ml CH4·g-1 VS), fruit and vegetables waste

308

(153-342 ml CH4·g-1 VS), ryegrass (140-360 ml CH4·g-1 VS), switchgrass (122-246 ml

309

CH4·g-1 VS), wheat (245-319 ml CH4·g-1 VS), and sewage sludge (249-274 ml CH4·g-1 VS)

310

(Kougias and Angelidaki, 2018).

311

In the experiment by Serrano et al. (2016), feces of L. variegatus had the lowest methane

312

yield from all the substrates tested (sludge, bodies of worms, and their feces). The yield

313

ranged from 99 to 176 mgCH4-COD g-1 COD for psychrophilic and from 275 to 429 mgCH4-

314

COD g-1 COD for mesophilic fermentation. Tamis et al. (2011) showed that residues of

315

sludges that pass through the gastrointestinal tract of aquatic worms Aulophorus furcatus

316

exhibit higher anaerobic digestibility, which can be explained by the contribution of extra

317

enzymes and/or bacteria (Serrano et al., 2016). However, Serrano et al. (2016) did not

318

observed this positive effect in processing the worm L. variegatus or its feces. This suggests

319

that the mechanism can be more complicated. Nevertheless, a relatively good BMP observed

320

in the present study may be attributed to the specific microorganisms present in the insect

321

wastes. Poveda et al. (2019) identified 14 bacterial and 6 fungal genera (among other

322

unidentified) in the mealworm feces.

12

323

The BMP of BSF larvae breeding waste determined in this study was 2.4 times lower

324

than the value obtained by Win et al. (2018), who reared the same species of insect in food

325

waste at a temperature of 20-23°C for 30 days, reaching a BMP equal to 502 ± 9 mL CH4 g-1

326

VS. The difference could be explained by the disparate type of feedstock used for insect

327

breeding. Taking into account the voracity of the H. illucens larvae, which makes them useful

328

in biowaste management and waste utilization (Diener et al., 2011; Lalander et al., 2013;

329

Bulak et al., 2018; Lalander et al., 2019), it was decided to use the waste from the fruit and

330

vegetable industry, such as a mixture of carrot-beetroot solid leftover after squeezing the

331

juice, which require appropriate management. Post-consumption food waste, such as residues

332

from dining halls, as used by Win et al. (2018), are usually rich in lipids and proteins (Ho and

333

Chu, 2019), while vegetable waste, such as carrot and beetroot marc is poor in these

334

components but rich in crude fiber (Retnani et al., 2010; Bakshi et al., 2016). The

335

unconsumed breeding feedstock was the important element of the materials used in BPM

336

assays, and its properties significantly influenced the chemical composition of the insects’

337

post-breeding waste. The residues examined by Win et al. (2018) consisted of 20.8% TS of

338

lipids and 19.4% TS of proteins, while the waste used in this study consisted of 0.6% TS of

339

lipids only and 13.8% TS of proteins. The higher content of these compounds, especially

340

lipids in the substrate, implicates greater methane production, because, stoichiometrically, 2.4

341

times more methane is generated from lipids (C57H104O6) than carbohydrates (C6H10O5), and

342

two times more than from proteins (C5H7O2N) (Win et al., 2018). The strong correlation

343

between BPM and lipid content in the digested substrates was experimentally confirmed by

344

Edwiges et al. (2018), who examined 12 different batches of fruit and vegetable waste with

345

different chemical compositions. The higher production of methane in the experiment

346

conducted by Win et al. (2018), compared to the results of this study, could also be

347

contributed to the higher (about 1.2 times) content of VS in the residues used by them.

13

348

The course of decomposition for the examined waste, illustrated by daily biogas

349

production, diverged in detail, although there were no statistically significant differences in

350

the maximum value of cumulative biogas production obtained during the test (Fig. 2). The

351

time when the maximum daily biogas production values occurred was different for particular

352

wastes: it was on second day in C (101.9 ± 0.1 ml CH4·g-1 TS) and H waste (59.0 ± 0.1 ml

353

CH4·g-1 TS), and on the fourth day in M (68.6 ± 10.3 ml CH4·g-1 TS) waste. It is interesting

354

that, in the case of M waste, the second, smaller peak of biogas production occurred on the

355

eighth day (34.3 ± 7.6 ml CH4·g-1 TS). It should be also mentioned that the production of

356

biogas in the case of H waste was mostly unstable. Several local maxima of daily biogas

357

production were observed. The last one was noticed on the ninth day.

358

These differences can be explained by the chemical composition of the waste that

359

influenced their biodegrability. The highest content of proteins and lipids, which are more

360

efficient sources of methane than carbohydrates (Li et al., 2017), were found in M waste. The

361

total share of proteins and lipids in this waste, amounting to 39.7% TS, was 1.4 times higher

362

than in C waste and 2.3 times higher than in H waste. Proteins and lipids are totally

363

degradable under anaerobic conditions, but their degradation is controlled by a hydrolysis

364

constant (Lübken et al., 2015), which determines the rate of methane production. On the other

365

hand, H waste contained the most amount of raw fiber: two times higher than M waste and

366

almost four times higher than C waste. The raw fiber includes non-starch polysaccharides,

367

i.e., cellulose, hemicellulose, pectin and lignin. These compounds belong to anaerobically

368

hardly biodegradable fraction (Lübken et al., 2015). H waste also had the least amount of

369

VFA.

370

The differences in the concentration of particular chemical components in the waste clarify

371

the changes in daily biogas production. Carbohydrates, especially monosaccharides and

372

disaccharides, which have a higher hydrolysis rate than proteins and lipids during anaerobic

14

373

digestion (Li et al., 2017), are digested firstly. Thus, the highest peak in daily biogas

374

production was observed the earliest (on the second day) in C waste, which also contained the

375

highest amount of carbohydrates. Conversely, the lowest content of these compounds was

376

found in H waste; thus the low peak was observed at this time. Proteins need more time to be

377

decomposed. These can explain the appearance of the highest peak of biogas production with

378

a two-day delay (on the fourth day) in the case of M waste, which had the highest content of

379

proteins. Biogas production in C and H waste decreased at this time, which can be linked to

380

the low content of proteins in this material. The appearance of the second-highest peak of

381

biogas production in the case of H waste on the fifth day and the third-highest one on the

382

ninth day could be related to the graduated and time-dependent hydrolysis of raw fiber

383

(accounting for 33% TS), composed of substances such as cellulose and hemicellulose and

384

characterized by different hydrolysis rates. These compounds are degraded in the substrates

385

available for methanogens at different times. On the other hand, the presence of the second-

386

highest peak in the case of M waste on the eighth day can be associated with the

387

biomethanization of VFA released during the hydrolysis of lipids. This multistage process,

388

occurring under anaerobic conditions is slow and requires the collaboration of different types

389

of microorganisms (Cirne et al., 2007). Intense biogas production lasted up to the eighth or

390

ninth day of the experiment (depending on the waste type), then significantly decreased in the

391

days that followed, reaching a daily biogas production of less than 20% of the highest noted

392

value of this parameter, while about 90% of the total amount of biogas was obtained before

393

the 12th day. This provides evidence that organic compounds susceptible to biodegradation

394

decompose rapidly. This gives them an advantage over the common but less readily

395

fermentable substrates used in biogas plants, such as animal manure and municipal solid

396

wastes (Bharathiraja et al., 2018).

397

3.3. Physicochemical parameters of digestates

15

398 399

The physicochemical parameters of digestates obtained after the methane fermentation of

400

a particular substrate, did not differ in terms of VS, ash content and total C (p < 0.05) (Table

401

3). The highest TS were observed in the digestate with H waste, and this value was

402

significantly different to that of the other digestates (Table 3). Digestates from H and M waste

403

had a significantly higher content of total N (4.04 ± 0.24% and 3.60 ± 0.22%, respectively)

404

compared to C waste (2.56 ± 0.74). Only the latter residue had a significantly different C:N

405

ratio, which was 1.7 and two times higher than in M and H wastes, respectively. In general,

406

all the postfermentation residues were slightly alkaline. Their pH values were in the range of

407

7.70 ± 0.04 to 7.86 ± 0.06, and the digestate derived from Hermetia waste had a significantly

408

higher pH value than the others (Table 3). Significantly different pH values for the raw waste

409

– mealworm and cricket wastes were slightly acidic while residues of Hermetia were more

410

alkaline (Table 1) – influenced the final pH of the digestates. The pH value of digestate

411

obtained after H waste fermentation was significantly higher than that of the other digestates.

412 413

5. Conclusions

414 415

This research proved the applicability of post-production residues from insect breeding

416

for biogas generation. Such approach allowed for the valorization of this type of waste as a

417

profitable source of renewable energy and confirm an environmentally friendly way for their

418

management. This should represent, beside the application as fertilizer, a new opportunity for

419

the use of these wastes in a cost-efficient and sustainable manner.

420

This study showed that, despite the significant differentiation in the chemical properties

421

of the examined waste, as revealed in the C:N ratio, and the content of lipids, proteins and raw

422

fiber, their BMP was similar to the substrate applied in biogas plants, which highlights the

16

423

reasonability of this approach to their utilization. But, taking into account the value of the C:N

424

ratio, and the high content of lipids and proteins, it could be supposed that mixing the waste

425

with substrates rich in carbohydrates, such as corn silage, will improve the properties of final

426

feedstock, leading to the enhancement of biomethane production. However, this issue should

427

be experimentally confirmed by further studies.

428

From a practical point of view, it should be emphasized that, due to the high content of

429

TS and VS in waste, their dosing in a bioreactor must be established by considering the

430

organic loading rate value which is optimal for the appropriate system.

431

The high rate of decomposition may be deemed to be a convenient feature of the waste.

432

This means that waste does not require a long retention time inside a bioreactor. On the other

433

hand, the high rate of hydrolysis may lead to a rapid increase in VFA concentration, thus

434

more detailed studies on the composition of VFA and the rate of its release during the

435

fermentation process are needed to assess the potential risk of feedstock acidification.

436 437

Acknowledgments

438 439 440

The authors wish to thank CricketsFarm (Lublin, Poland) for delivering wastes from cricket breeding for scientific purposes free of charge.

441 442 443 444

Conflicts of interest None. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

445 446

References

447

17

448

Akuzawa, M., Hori, T., Haruta, S., Ueno, Y., Ishii, M., Igarashi, Y., 2011. Distinctive

449

responses of metabolically active microbiota to acidification in a thermophilic anaerobic

450

digester. Microbial Ecol. 61, 595–605.

451

Anankware, P.J., Fening, K.O., Osekre, E., Obeng-Ofori, D., 2015. Insects as food and

452

feed: a review. Int. J. Agric. Res. Rev. 3, 143-151.

453

Azagoh, C., Hubert, A., Mezdour, S., 2015. Insect biorefinery in Europe: ‘DESigning the

454

Insect bioRefinery to contribute to a more sustainABLE agro-food industry’. J. Insects

455

Food Feed 1, 159-168.

456

Bao, W., Yang, Y., Fu, T., Xie, G.H., 2019. Estimation of livestock excrement and its

457

biogas production potential in China. J Clean. Prod. 229, 1158-1166.

458

Bakshi, M.P.S., Wadhwa, M., Makkar, H.P.S., 2016. Waste to worth: vegetable wastes as

459

animal feed. CAB Rev. 11, 1-26.

460

Bharathiraja, B., Sudharsana, T., Jayamuthunagai, J., Praveenkumar, R., Chozhavendhan,

461

S., Iyyappan, J., 2018. Biogas production – a review on composition, fuel properties, feed

462

stock and principles of anaerobic digestion. Renew. Sustain. Energ. Rev. 90, 570-582.

463

Brzezińska, M., Urbanek, E., Szarlip, P., Włodarczyk, T., Bulak, P., Walkiewicz, A.,

464

Rafalski, P., 2014. Methanogenic potential of archived soils. Carpath. J. Earth Environ.

465

Sci. 9, 79-90.

466

Bulak, P., Polakowski, C., Nowak, K., Waśko, A., Wiącek, D., Bieganowski, A., 2018.

467

Hermetia illucens as a new and promising species for use in entomoremediation. Sci.

468

Total Environ. 633, 912-919.

469

Cirne, D.G., Paloumeta, X., L. Bjornsson, Alves, M.M., Mattiasson, B., 2007. Anaerobic

470

digestion of lipid-rich waste - effects of lipid concentration. Renew. Energy 32, 965–975.

18

471

Commission Regulation (EU) 2017/893 of 24 May 2017 amending Annexes I and IV to

472

Regulation (EC)

473

Annexes X, XIV and XV to Commission Regulation (EU) No 142/2011 as regards the

474

provisions on processed animal protein. Official Journal of the European Union L 138/92

475

25.5.2017.

476

Commission Regulation (EU) Draft Ares(2019)382900 amending Annex III to Regulation

477

(EC) No 853/2004 of the European Parliament and of the Council as regards specific

478

hygiene

479

https://ec.europa.eu/info/law/better-regulation/initiatives/ares-2018-3849989_en (accessed

480

10 July 2019).

481

Deublein, D., Steinhauser, A., 2008. Biogas from waste and renewable resources.

482

Weinheim, Willey-VCH Verlag GmbH & Co. KGaA.

483

Diener, S., Solano, N.M.S., Gutiérrez, F.R., Zurbrügg, C., Tockner, K., 2011. Biological

484

treatment of municipal organic waste using black soldier fly larvae. Waste Biomass

485

Valori. 2, 357–363.

486

DIN 38414, 1985. Determination of digestion behavior of “Sludge and Sediments”. Beuth

487

Verlag, Berlin (in German).

488

Dossey, A.T., Morales-Ramos, J.A., Rojas, M.G., 2016. Insects as sustainable food

489

ingredients: production, processing and food applications. Academic Press, Cambridge,

490

MA, USA.

491

Edwiges, T., Frare, L., Mayer, B., Lins, L., Mi, Triolo, J., Flotats, X., de Mendonça Costa,

492

M.S.S., 2018. Influence of chemical composition on biochemical methane potential of

493

fruit and vegetable waste. Waste Manage. 71, 618-625.

No 999/2001 of the European Parliament and of the Council and

requirements

for

insects

19

intended

for

human

consumption.

494

EN 12879, 20004. Characterization of sludges. Determination of the loss of ignition of dry

495

mass.

496

EN 12880, 20004. Characterization of sludges. Determination of dry residue and water

497

content.Esen, M., Yuksel, T., 2013. Experimental evaluation of using renewable energy

498

sources for heating a greenhouse. Energ. Buildings 65, 340-351.

499 500

Gold, M., Tomberlin, J.K., Diener, S., Zurbrügg, C., Mathys, A., 2018. Decomposition of

501

biowaste macronutrients, microbes, and chemicals in black soldier fly larval treatment: a

502

review. Waste Manage. 82, 302-318.

503

Hendrickx, T.L.G., Temmink, H., Elissen, H.J.H., Buisman, C.J.N., 2010. Design

504

parameters for sludge reduction in an aquatic worm reactor. Water Res. 44, 1017-1023.

505

Ho, K.S., Chu, L.M., 2019. Characterization of food waste from different sources in Hong

506

Kong. J. Air Waste Manage. Assoc. 69, 277-288.

507

Kalová, M., Borkovcová, M., 2013. Voracious larvae Hermetia illucens and treatment of

508

selected types of biodegradable waste. Acta Universitatis Agriculturae et Silviculturae

509

Mendelianae Brunensis, 61, 77-83.

510

Kasprzycka, A., Kuna, J., 2018. Methodical aspects of biogas production in small-volume

511

bioreactors

512

https://doi.org/10.3390/en11061378

513

Kougias, P.G., Angelidaki, I., 2018. Biogas and its opportunities – a review. Front.

514

Environ. Sci. Eng. 12, 14.

in

laboratory

investigations.

20

Energies

11,

1378.

515

Lalak, J., Kasprzycka, A., Martyniak, D., Tys, J., 2016. Effect of biological pretreatment

516

of Agropyron elongatum ‘BAMAR’ on biogas production by anaerobic digestion. Biores.

517

Technol. 200, 194-200.

518

Lalander, C., Diener, S., Magri, M.E., Zurbrügg, C., Lindström, A., Vinnerås, B., 2013.

519

Faecal sludge management with the larvae of the black soldier fly (Hermetia illucens) -

520

from a hygiene aspect. Sci. Total Environ. 458–460, 312–318.

521

Lalander, C., Diener, S., Zurbrügg, C., Vinnerås, B., 2019. Effects of feedstock on larval

522

development and process efficiency in waste treatment with black soldier fly (Hermetia

523

illucens). J. Clean. Prod. 208, 211-219.

524

Lee, D-J., Lee, S-Y., Bae, J-B., Kang, J-G., Kim, K-H., Rhee, S-S., Park, J-H., Cho, J-S.,

525

Chung, J., Seo, D-C., 2015. Effect of volatile fatty acid concentration on anaerobic

526

degradation rate from field anaerobic digestion facilities treating food waste leachate in

527

South Korea. J. Chem. ID 640717, 9. http://dx.doi.org/10.1155/2015/640717

528

Li, Y., Jin, Y., Borrion, A., Li, H., Li, J., 2017. Effects of organic composition on the

529

anaerobic biodegradability of food waste. Biores. Technol. 243, 836-845.

530

Lübken, M. Kosse, P., Koch, K. Gehring, T., Wichern, M., 2015. Influent fractionation for

531

modeling continuous anaerobic digestion process, in: Gübitz, G., Bauer, A., Bochmann,

532

G., Gronauer, A., Weiss, S. (Eds.), Advances in Biochemical Engineering/Biotechnology,

533

Biogas Science and Technology. Springer International Publishing, London 2015.

534

Ma, H., Liu, H. Zhang, L., Yang, M., Fu, B., Liu, H., 2017. Novel insight into the

535

relationship between organic substrate composition and volatile fatty acids distribution in

536

acidogenic co-fermentation. Biotechnol. Biofuels 10, 137. https://doi.org/10.1186/s13068-

537

017-0821-1

21

538

Mariod, A.A., 2013. Insect oil and protein: biochemistry, food and other uses: review.

539

Agric. Sci. 4, 76-80. doi: 10.4236/as.2013.49B013

540

Müller, A., Wolf, D., Gutzeit, H.O., 2017. The black soldier fly, Hermetia illucens - a

541

promising source for sustainable production of proteins, lipids and bioactive substances.

542

Z. Naturforsch. C. 72, 351-363.

543

Nielsen, H.B., Uellendahl, H., Ahring, B.K., 2007. Regulation and optimization of the

544

biogas process: propionate as a key parameter. Biomass Bioenerg. 31, 820-830.

545

Oleszek, M., Matyka, M., 2017. Nitrogen fertilization level and cutting affected

546

lignocellulosic crops properties important for biogas production. Bioresources 12, 8565-

547

8580.

548

Oonincx, D.G.A.B., van Broekhoven, S., van Huis, A., van Loon, J.J.A., 2015. Feed

549

conversion, survival and development, and composition of four insect species on diets

550

composed

551

https://doi.org/10.1371/journal.pone.0144601

552

Pawłowska, M., Rożej, A., Stępniewski, W., 2011. The effect of bed properties on

553

methane removal in an aerated biofilter - model studies. Waste Manage. 31, 903-913.

554

Poveda, J., Jiménez-Gómez, A., Saati-Santamaría, Z., Usategui-Martín, R., Rivas, R.,

555

García-Fraile, P., 2019. Mealworm frass as a potential biofertilizer and abiotic stress

556

tolerance-inductor in plants. App. Soil Ecol. 142, 110-122.

557

Ravzanaadii, N., Kim, S.H., Choi, W.H., Hong, S.J., Kim, N.J., 2012. Nutritional value of

558

mealworm, Tenebrio molitor as food source. Int. J. Indust. Entomol. 25, 93-98.

559

Retnani, Y., Syananta, F.P., Herawati, L., Widiarti, W., Saenab, A., 2010. Physical

560

characteristic and palatability of market vegetable waste wafer for sheep. Anim. Prod. 12,

561

29-33.

of

food

by-products.

22

PLoS

ONE

10,

e0144601.

562

Rumpold, B.A., Schlüter, O.K., 2013a. Nutritional composition and safety aspects of

563

edible insects. Mol. Nutr. Food Res. 57, 802-823.

564

Serrano, A., Hendrickx, T.L.G., Elissen, H.H.J., Laarhoven, B., Buisman, C.J.N.,

565

Temmink, H., 2016. Can aquatic worms enhance methane production from waste

566

activated sludge? Biores. Technol. 211, 51-57.

567

Statista,

568

https://www.statista.com/topics/4806/edible-insects/ (accessed 01 April 2019).

569

Sun, Q., Li, H., Yan, J., Liu, L., Yu, Z., Yu, X., 2015. Selection of appropriate biogas

570

upgrading technology - a review of biogas cleaning, upgrading and utilisation. Renev.

571

Sust. Energ. Rev. 51, 521-532.

572

Szafranek-Nakonieczna, A., Zheng, Y., Słowakiewicz, M., Pytlak, A., Polakowski, C.,

573

Kubaczyński, A., Bieganowski, A., Banach, A., Wolińska, A., Stępniewska, Z., 2018.

574

Methanogenic potential of lignites in Poland. Int. J. Coal Geol. 196, 201-210.

575

VDI-Verein Deutscher Ingenieure 4630, 2016. Fermentation of organic materials.

576

Characterization of the substrates, sampling, collection of material data, fermentation

577

tests. VDI-Handbuch Energietechnik, Düsseldorf.

578

Walkiewicz, A., Bulak, P., Brzezińska, M., Wnuk, E., Bieganowski, A., 2016. Methane

579

oxidation in heavy metal contaminated Mollic Gleysol under oxic and hypoxic conditions.

580

Environ. Pollut. 213, 403-411.

581

Wang, D., Bai, Y.Y., Li, J.H., Zhang, C.X., 2004. Nutritional value of the field cricket

582

(Gryllus testaceus Walker). Insect Sci. 11, 275-283.

583

Wang, H., Rehman, K., Liu, X., Yang, Q., Zheng, L., Li, W., Cai, M., Li, Q., Zhang J.,

584

Yu, Z., 2017. Insect biorefinery: a green approach for conversion of crop residues into

2019.

Edible

Insects

23

-

Statistics

&

Facts.

585

biodiesel and protein. Biotechnol. Biofuels 10, 304. https://doi.org/10.1186/s13068-017-

586

0986-7

587

Wang, Y., Zhang, Y., Wang, J., Meng, L., 2009. Effects of volatile fatty acid

588

concentrations on methane yield and methanogenic bacteria. Biomass Bioenergy 33, 848-

589

853.

590

Wang, Y.S., Shelomi, M., 2017. Review of Black Soldier Fly (Hermetia illucens) as

591

animal feed and human food. Foods 6, 91.

592

Waśko, A., Bulak, P., Polak-Berecka, M., Nowak, K., Polakowski, C., Bieganowski, A.,

593

2016. The first report of the physicochemical structure of chitin isolated from Hermetia

594

illucens. Int. J. Biol. Macromol. 92, 316-320.

595

Win, S.S., Ebner, J.H., Brownell, S.A., Pagano, S.S., Cruz-Diloné, P., Trabold, T.A.,

596

2018. Anaerobic digestion of black solider fly larvae (BSFL) biomass as part of an

597

integrates biorefinery. Renew. Energy 127, 705-712.

598

Winquist, E., Rikkonen, P., Pyysiäinen, J., Varho, V., 2019. Is biogas an energy or a

599

sustainability product? – Business opportunities in the Finnish biogas branch. J Clean.

600

Prod. 233, 1344-1354.

601

Whittaker, C., Hunt, J., Misselbrook, T., Shield, I., 2016. How well does Miscanthus

602

ensile for use in an anaerobic digestion plant? Biomass Bioenerg. 88, 24-34.

603

Yusuf, R.O., Noor, Z.Z., Abba, A.H., Hassan, M.A.A., Din, M.F.M., 2012. Methane

604

emission by sectors: a comprehensive review of emission sources and mitigation methods.

605

Renev. Sust. Energ. Rev. 16, 5059-5070.

606

Zdybicka-Barabas, A., Bulak, P., Polakowski, C., Bieganowski, A., Waśko, A.,

607

Cytryńska, M., 2017. Immune response in the larvae of the black soldier fly Hermetia

608

illucens. Invertebrate Surviv. J. 14, 9-17. 24

609

25

610

Figure captions:

611

Fig. 1. Macroscopic view of used insect waste: a) Hermetia illucens waste, b) mealworms

612

waste, c) crickets waste.

613

Double column (full width), no color in print

614

Fig. 2. Daily and cumulative biogas production (ml·g-1 TS) obtained for the insect rearing

615

waste. Values are given as means (n = 3) ± SD (bars).

616

Double column (full width), no color in print

617

26

618

Table captions:

619

Table 1. Physicochemical properties of inoculum and insect wastes used for anaerobic

620

digestion (mean value and SD, n = 3). Different letters indicated statistically significant

621

differences (Tukey’s test; p < 0.05).

622

Table 2. Biogas and biomethane potential, and methane concentration in the biogas (means

623

and SD, n = 3) of the insect wastes used in the experiment. Different letters indicated

624

statistically significant differences (Tukey’s test; p < 0.05).

625

Table 3. Physicochemical properties of digestates obtained after anaerobic digestion of

626

insects breeding waste (mean values and SD, n = 3). Different letter indicated statistically

627

significant differences (Tukey’s test; p < 0.05).

27

Waste type Parameter

Unit

pH

Inoculum

Mealworm

Crickets

Hermetia

waste (M)

waste (C)

waste (H)

6.82 ± 0.04b

6.01 ± 0.03d

6.18 ± 0.04c

8.19 ± 0.06a

TS

%

5.36 ± 0.05d

89.40 ± 0.04a

87.33 ± 0.06b

84.00 ± 0.10c

VS

% TS

-

84.02 ± 1.90a

87.06 ± 1.71a

85.21± 0.94a

Ash

% TS

-

15.95 ± 1.71a

12.84 ±0.64b

14.77±0.55ab

Total C

% TS

31.76 ± 0.58b

39.72 ± 1.20a

40.17± 1.51a

33.91± 0.10b

% VS

-

47.27 ± 1.43a

46.14 ± 1.73a

39.80 ± 0.12b

% TS

4.30 ± 0.23ab

5.12 ± 0.22a

3.71 ± 0.53b

2.21 ± 0.23c

% VS

-

6.09 ± 0.26a

4.26 ± 0.61b

2.59 ± 0.27c

-

7.39 ± 0.26c

7.77 ± 0.47c

10.96 ± 1.26b

15.48 ± 1.70a

% TS

-

31.98 ± 1.38a

23.16 ± 3.29b

13.80 ± 1.47c

% VS

-

38.05 ± 0.88a

26.59 ± 3.57b

16.19 ± 1.54c

% TS

-

3.47 ± 0.09a

2.33 ± 0.05b

0.60 ± 0.05c

% VS

-

4.13 ± 0.01a

2.67 ± 0.09b

0.70 ±0.06c

% TS

-

35.45

25.49

% TS

-

39.7

29.2

Total N

C:N

Proteins

Crude fat

14.4

Sum of proteins and fat (P+F) Share of P+F

17.1

% TS

-

17.48 ± 0.08b

8.54 ± 0.06c

33.22 ± 0.04a

% VS

-

20.81 ± 0.37b

9.81 ± 0.24c

38.99 ± 0.41a

% TS

-

7.05 ± 0.80b

26.18 ± 3.26a

3.08 ± 0.71b

% VS

-

8.40 ± 0.98b

30.09 ± 3.87a

3.62 ± 0.87b

Volatile fatty

% TS

-

2.86 ± 0.24b

1.60 ± 0.05a

2.53 ± 0.44a

acids

% VS

-

3.40 ± 0.29b

1.84 ± 0.06 a

2.97 ± 0.52 a

Raw fibre

Carbohydrates

Digested waste Parameter

Unit Mealworm (M)

Cricket (C)

Hermetia (H)

ml∙g-1 TS

379.0 ± 2.9a

389.5 ± 5.0a

351.4 ± 4.4b

ml∙g-1 VS

451.1 ± 3.4a

447.4 ± 5.7a

412.5 ± 5.1b

Biomethane

ml∙g-1 TS

212.2 ± 20.4ab

225.3 ± 12.2a

177.2 ± 18.3b

potential

ml∙g-1 VS

252.6 ± 24.3a

258.8 ± 14.0a

207.9 ± 21.5a

% vol.

56.3 ± 1.5a

57.9 ± 4.1a

50.3 ± 2.7b

% vol.

62.3 ± 3.0a

61.2 ±4.7ab

53.2 ± 3.2b

Biogas potential

Mean CH4 content ± SD Maximum CH4 content

pHH2O Waste

TS

VS

Ash

Total C

Total N

(%)

(% TS)

(% TS)

(% TS)

(% TS)

C:N

(1:20 v∙v-1)

Mealworm

7.70

2.51

69.72

30.28

38.13

3.60

10.64

(M)

± 0.04b

± 0.01b

± 1.24a

± 1.24a

± 1.58a

± 0.22ab

± 1.10b

Crickets

7.71

2.23

73.15

26.85

42.81

2.56

18.13

(C)

± 0.07b

± 0.02b

± 2.98a

± 2.98a

± 5.30a

± 0.74b

± 1.94a

H. illucens

7.86

3.12

70.79

29.21

34.88

4.04

8.65

(H)

± 0.06a

± 0.21a

± 1.61a

± 1.61a

±1.59a

± 0.24a

± 0.59b

Highlights: •

Insect production for food and feed is increasing worldwide.

•

Biogas production from insect wastes is poorly investigated.

•

Insect wastes are suitable for biogas production with good efficiency.

•

Biomethane potential was similar to some manures, plant wastes and sewage sludges.