Possibilities of Obtaining Biogas from Manure and Amaranth

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Introduction

Sustainable development requires a systematic approach to solving the problem of organic waste recycling. At the moment, many technologies have been developed to reduce environmental pollution. However, in the application of methods for organic waste recycling, Russia has not yet reached the modern world level. Up to 250 million tons of organic waste is accumulated annually, a significant part of which decomposes in an open environment, posing a serious threat to nature and humans.

The use of improved technologies and also the joining of various technologies for organic waste recycling contribute to the development of a “circular economy” and an increase in the efficiency of resource use [1–3].

At present, a combined technology, including anaerobic digestion (AD) and pyrolysis (Py), is of particular interest [4; 5]. It allows implementing a full cycle of organic waste recycling.

Three types of process integration are known [6]:

– AD-Py. Anaerobic digestate is used for pyrolysis as a valuable feedstock material for energy and biochar production;

– Py-AD. Pyrolysis products such as biochar, gas, aqueous phase can be suitable feedstock or effective additives for the AD process;

– AD-Py-AD combines the two previous methods.

Figure 1 shows the AD-Py technology combining biological and thermochemical processes. Livestock waste and plant biomass are sent for anaerobic digestion. The resulting biogas is used for energy production. The effluent is separated and dried, followed by thermochemical processing [3]. Synthesis gas and pyrolysis liquid are used as energy resources. Char residue is a good soil additive used to increase biomass yields [7]. The biomass is then used as feedstuff in livestock, and its waste is again sent for anaerobic digestion.

 

 

 

Fig. 1. The AD-Py technology

 

Such technologies are poorly studied since they include two processes: methanogenesis and thermochemical processing [8; 9]. This paper presents the experimental studies results of the first key process – anaerobic digestion of dairy manure and dry biomass of the weed plant Amaranthus retroflexus L. [10]. The description of the second process – the thermochemical processing of the effluent – is presented in another publication of the authors [10].

Several parameters influence the performance and biogas production for anaerobic digestion, but especially the organic loading rate (OLR) [11–13] and the hydraulic retention time (HRT) [14; 15]. Reactor control is based on OLR and HRT values [16]. The Modified Gompertz model is often used in practical applications to optimize process parameters for improving the design of the methane tank and the entire technology as a whole [15].

Some experimental studies were carried out in the laboratory of energy systems and technologies of the Institute of Power Engineering and Advanced Technologies of the Kazan Scientific Center of the Russian Academy of Sciences to obtain a modified Gompertz model, reactor control indicators (HRT and OLR) for anaerobic digestion of a new substrate.

The aim of this study is the applicability of co-digestion of manure and amaranth biomass for improving methane production at the mesophilic temperature.

Literature Review

Anaerobic digestion of biomass (organic agricultural and domestic wastes) has a special place in energetics. It allows you to obtain biogas containing about 70 % methane, and disinfected organic fertilizers. Biomass utilization is extremely important in agriculture, where a large amount of fuel is spent on various technological needs, and the need for high-quality fertilizers is continuously increasing [17].

The total cow number in Russia is 8.3 million animal units. Thus annually more than 166.7 million tons of manure is accumulated in the region near livestock farms and poses a serious environmental threat. Besides, there is a tendency to increase the size of farms and reduce their total number. For example, in the Republic of Tatarstan (region of Russia), there are 11 megafarms, that are the largest in Europe.

Biogas production from dairy manure is unprofitable because of the low specific biogas yield [18]. Some researchers propose co-digestion to eliminate this problem [18–20]. The most popular co-substrates at biogas plants are maize, wheat straw, and grass [21–24].

Combined treatment of several substrates under AD can increase the efficiency of biogas production. The synergistic effect is achieved by the fact that the necessary microelements and nutrients contained in substrates in different quantities reach their optimal values under the correct combination [25]. At co-digestion, it is also possible to regulate the C/N ratio, which promotes better biological decomposition of organic waste and, accordingly, increases the biogas yield [26; 27].

Biogas is a promising renewable energy source that is why the search for suitable substrates is at the center of attention. During the period from 2009 to 2018, biogas production in the world doubled and continues to grow [28]. In European countries, 70 % of substrates for biogas production come from the agro-industrial complex and include manure and crop waste¹.

Plants of the Amaranth family are promising co-substrates for a significant increase in the rate of methanogenesis and the amount of produced biogas [29]. Thus, in earlier studies, it was proved that the biomass of plants from the Amaranthaceous family is a co-substrate for anaerobic digestion. But since cultivar amaranth is an expensive raw material, it was necessary to continue the search for affordable and cost-effective methanogenesis stimulating agents. In the present work, an experiment with biomass of Amaranthus retroflexus L. the closest wild-growing relative of amaranth was conducted.

Materials and Methods

The substrate (dairy manure, biomass of amaranth) was the object of the study. It was stored for two days in a refrigerator at 4 °C.

Amaranthus retroflexus L. and Amaranthus cruentus L. were gathered in the dissemination phase at the field in the Republic of Tatarstan (Russia).

The co-digestion process was studied in the laboratory experimental setup consisting of an LB-162 water bath, 0.5 l anaerobic digesters, plastic containers, measuring cylinders, a system of rubber hoses, ultrasonic technological apparatus of the Series “Wave” UZTA-0.2/22 Ohm (Fig. 2).

 

 

 

 

Fig. 2. Experimental set-up

 

 

The experiments were carried out in three repetitions; a thermostatic water bath maintained a mesophilic temperature (37 °C).

The volume of produced biogas was determined daily [29]. The composition of the gas was determined every 7 days in two repetitions by gas-liquid chromatography. Biogas was sampled using a 1 000 µl gas-tight syringe. Khrom 5 gas chromatograph (Austria), Porapak Q column (2.4 m long), thermal conductivity detector and gas carrier. He were used for this purpose.

The following substrates compositions were used in the experiments (table 1).

 

Table 1 Composition of samples for experiments

 

Sample	Dairy manure mass, g	Biomass additive	Biomass / Manure ratio	Total volatile solids, g VS∙L–1	Ultrasonic pre-reatment
No. 1	80	–	–	40.4	no
No. 2	48	Amaranthus retroflexus L.	1:8	40.9	no
No. 3	48	Amaranthus retroflexus L.	1:8	40.9	yes
No. 4	48	Amaranthus cruentus L.	1:8	40.9	yes

 

Pre-treatment was carried out for 4 minutes using the ultrasonic device with the power of 80 W at an oscillation frequency of 22 kHz and an exposure intensity of at least 10 W/cm².

The experiments were considered complete “when the daily biogas yield was less than 1 % of the cumulative gas yield for three days” [30]. The experiment lasted 55 days. For sample No. 1 (control), the digestion period was 37 days. For other samples, it was 55 days. The analysis of biogas yield kinetics was normalized by pressure (P = 101.3 kPa) and temperature (T = 0 °С).

Elemental analysis of the studied samples was carried out using the Euro EA 3000 analyzer (analysis conditions: column temperature 115 °С, furnace temperature 850 °С).

The content of macro- and microelements was studied using the EDX-800HS2 energy-dispersive fluorescence X-ray spectrometer manufactured by Shimadzu (Japan) by a semi-quantitative method [10].

A modified Gompertz model was used by many authors to describe the kinetics of gas formation in batch anaerobic digesters from various organic substrates [21]. The modified Gompertz equation has the following form:

$F\left(l\right)=W\text{exp}\left(\text{exp }\left(\frac{R_{\max }\cdot e}{W}\left(\alpha -l\right)+1\right)\right)$ , (1)

where F(l) – cumulative specific gas production at a time l days, liters per kilogram of volatile solids (L/kg VS); W – the gas production potential (L/kg VS); R_max – maximum gas production rate, L/kg VS∙day; α – lag phase period, day [31].

Results

The main characteristics of dairy manure were: volatile solids (VS) (the percentage of VS content from total solids content), 75.03 ± 4.68 %; total solids (TS), 16.82 ± 1.45 %. Its principal characteristics of amaranth dry biomass were: VS = 75.97 ± 0.6 % (the percentage of VS content from TS content); TS = 89.55 ± 0.3 %. Table 2 presents the results.

 

Table 2 Elemental composition of substrates

 

	C, %	H, %	O, %	N, %
Dairy manure	27.13	4.72	41.54	1.64
Amaranthus retroflexus L.	32.61	5.24	40.45	4.22
Amaranthus cruentus L.	32.33	5.49	40.62	2.91

 

 

The C/N ratio for manure was equal to 16.5 ± 0.3, for Amaranthus retroflexus L.: 7.8 ± 0.35 and Amaranthus cruentus L.: 11.1 ± 0.2.

Table 3 presents the results, where AR is biomass of Amaranthus retroflexus L., and AC is biomass of the Amaranthus cruentus L.

 

Table 3 Content of macro- and microelements in the dry biomass, %

 

Name	Ca	K	P	S	Fe	Mn	Sr	Br	Si	Cl	Mg	Zn
AR	54.58	32.32	2.81	3.47	0.44	0.17	0.10	0.08	0.65	0.84	4.48	0.07
AC	58.74	20.82	3.95	3.15	1.22	0.19	0.10	0.05	2.51	3.35	5.83	0.07

 

 

Very high content of calcium and potassium was observed in the dry biomass of plants leaves.

A statistical analysis was performed. The experimental biogas productions were always referred to average values. Analysis of variance (ANOVA) was used to assess the influence of the analyzed factor. The significance threshold was set at 0.05. Table 4 presents the results of the analysis of variance.

 

Table 4 One way ANOVA for biogas yield

 

SUMMARY
Groups		Number of observations		Sum			Average		Variance
Sample No. 1		37		3 943.29			106.58		3 932.01
Sample No. 2		55		6 137.14			111.58		6 069.09
Sample No. 3		55		7 712.55			140.23		7 769.96
Sample No. 4		55		3 666.73			66.67		6 141.98
ANOVA
Source of variation	Sums of squares		Degrees of freedom (df)		Mean square (MS)	Fisher criterion F		p-value		Critical value Fcrit
Between groups	151 238		3		50 412.69	8.18		3.69 ∙ 10–5		2.65
Within groups	1 220 528		198		6 164.28
Total	1 371 766		201

 

There is a significant difference in the average biogas yield for all substrates with a probability of 95 %. Since the value of F is higher than the value of F_crit for a given number of groups, the dispersion between groups makes a greater contribution to any sum of dispersions than that within the groups. In other words, the co-digestion of dairy manure and co-substrates in the anaerobic digesters has a significant effect on the biogas yield. This effect is accounted for an improvement in the nutrient balance in the mixture of manure and biomass of Amaranthus retroflexus L. and Amaranthus cruentus L. for co-digestion, which increases the solubisation, decomposition and production of biomethane.

The amount of daily produced biogas is an important indicator for comparing the various substrates used to produce biogas. The Figure 3 presents the daily specific biogas yield for the four considered substrates.

 

 

 

Fig. 3. Kinetics of gas formation

 

 

In their study Z. Wang and co-authors reported that, “when carrying out experiments on the co-digestion of acorn slag and dairy manure with bio-based carbon, two peaks of biogas production were also observed” [32]. The authors explain this by “the degradation of carbohydrates and crude protein in the anaerobic digesters” [32]. The two peaks observed in biogas production occur because the carbohydrates decomposition can be very fast within a few days, and sometimes it can take several weeks to transform crude proteins.

Figure 4 shows the cumulative specific methane yield for the four considered substrates. Numerous literature data on the production of biogas from cattle manure indicate a specific biogas yield in the range between 140 and 266 L/kg VS [33]. The obtained experimental data (sample No. 1, control) 171 L/kg VS are consistent with the literature data. Specific biogas yield for sample No. 2 was 228 L/kg VS. The use of ultrasonic pre-treatment allowed increasing the specific biogas yield to 264 L/kg VS (sample No. 3). Specific biogas yield for sample No. 4 was 202 L/kg VS. Thus, Amaranthus retroflexus L. is as good as the herbal supplement Amaranthus cruentus L.

 

 

 

Fig. 4. Cumulative biogas yield

 

 

The content of lignin in the diet of animals has a key influence on biogas production at the digestion of dairy manure. In spring and summer, when fresh grass is in the diet of animals, an increase in the specific biogas yield is observed at anaerobic digestion of dairy manure [33]. Because experimental studies were carried out in the autumn when animals fed on hay, the specific production of biogas was lower than it could be in summer.

The computer program developed at the Kazan Scientific Center of the Russian Academy of Sciences (certificate No. 2018662045) was used to obtain modified Gompertz models describing the kinetics of biogas formation². Table 5 presents the obtained kinetic constants. The maximum biogas production rate is higher in the experiment with biomass of Amaranthus cruentus L. According to the biogas production potential, the dry biomass of Amaranthus retroflexus L. treated with ultrasound would be highly competitive with the biomass of Amaranthus cruentus L. The longest lag-phase was observed in the experiment Amaranthus cruentus L.

 

Table 5 Kinetic constants of the modified Gompertz model

 

No.	W, L/kg VS	Rmax, L/kg VS·day	α, day	R2	Pearson’s correlation  coefficient R2	Standard error, %
1	170.91	12.78	6	1.0000	0.9978	5.09
2	270.00	6.00	9	1.0000	0.9941	2.64
3	300.00	7.60	9	1.0000	0.9958	3.59
4	202.49	13.57	29	0.9999	0.9964	9.30

 

 

Figure 5 shows the total biogas production end the methane content in biogas [34]. The maximum methane production was achieved on the second or third week of digestion. The maximum methane yield in biogas was observed for sample No. 3. It was 65.9 %.

 

 

 

Fig. 5. Total biogas production and methane concentration

 

 

Thus, the specific methane yield for sample No. 1 was 92 L/kg VS, for No. 2 – 140 L/kg VS, for No. 3 – 174 L/kg VS, for No. 4 – 139 L/kg VS. When using Amaranthus retroflexus L. biomass, the specific methane yield is 25 % higher than when using the Amaranthus cruentus L. biomass.

The key control parameters for a conditionally continuous-action methane tank (HRT, OLR) were determined using the computer program developed at the Kazan Scientific Center of the Russian Academy of Sciences³. The initial data were the results of experimental studies for the batch methane tank, namely, the function of the cumulative biogas yield and the biogas yield rate as a function of time (see Fig. 6 for an example). The optimal HRT value was 10 days, OLR 4.1 kg VS/m³ of digester per day. Let us consider a biogas plant, processing daily 18 tons of organic waste. When changing the composition of the processed substrate, 2 tons of Amaranthus retroflexus L. and 16 tons of manure should be added every day to the biogas plant. The material balance of anaerobic digestion was compiled based on the experimental data presented in this work. The material balance of the pyrolysis of the dried digestate was compiled based on the experimental data presented by the scientists [10].

 

 

 

Fig. 6. Material balance of the AD-Py technology

 

 

After applying the preliminary ultrasonic treatment (an oscillation frequency of 22 kHz and an exposure intensity of at least 10 W/cm²) and introducing a new herbal additive into the methane tank, the daily biogas yield will increase to 1 014 m³. Daily use of biogas will make it possible to obtain 5 576.7 kW∙h, synthesis gas – 846.7 kW∙h, and pyrolysis liquid 5 611 kW∙h.

Discussion and Conclusion

In this study, a higher biogas yield was due to a synergistic effect caused by technological (ultrasonic pre-treatment) and microbiological (co-digestion) effects. The paper presents the use of a new phytogenous co-substrate. A weed plant cosmopolite amaranth (Amaranthus retroflexus L.) was used for this purpose. The biogas production potential at mono-digestion of dairy manure was 170.91 L/kg VS when using dry amaranth (Amaranthus retroflexus L.) – 270 L/kg VS when using the preliminary ultrasonic treatment and dry amaranth (Amaranthus retroflexus L.) – 300 L/kg VS.

The use of new co-substrate Amaranthus retroflexus L. allowed increasing the cumulative biogas yield from dairy manure by 52.2 % and the ultrasonic pre-treatment in combination with the herbal supplement by 89.1 %.

The biomass of Amaranthus retroflexus L. was found to be comparable with the biomass of Amaranthus cruentus L. Therefore, further studies on the use of the co-substrate Amaranthus retroflexus L. in biogas plants are needed. It is especially important in the production of biogas from difficultly fermented substrates.

The optimal HRT value was 10 days, OLR 4.1 kg VS/m³ of digester per day.

 

 

¹           Calderon C., Colla M., Jossart J.-M., et al. Statistical Report: Biogas. 2019. 23 р. Available at: https://www.europeanbiogas.eu/wp-content/uploads/2020/01/EBA-AR-2019-digital-version.pdf (accessed 22.03.2021). (In Eng.)

²           [Kinetic Analysis of Biogas Production]: Certificate of State Registration of the Program for ECM 2 018 662 045 Russian Federation. Appl. 25.09.2018; publ. 25.09.2018. (In Russ.)

³           [Optimization of Operating Modes of the Apparatus]: Certificate of State Registration of the Program for ECM 2 015 662 618 Russian Federation. Appl. 27.11.2015; publ. 20.12.2015. (In Russ.)

 

About the authors

Julia V. Karaeva

Institute of Power Engineering and Advanced Technologies, FRC Kazan Scientific Center, Russian Academy of Sciences

Author for correspondence.
Email: julieenergy@list.ru
ORCID iD: 0000-0002-9275-332X
ResearcherId: F-6917-2017

Head of the Power Systems and Technologies Laboratory, Cand.Sc. (Engr.)

Russian Federation, 2/31 Lobachevsky St., Kazan 420111

Svetlana S. Timofeeva

Institute of Power Engineering and Advanced Technologies, FRC Kazan Scientific Center, Russian Academy of Sciences

Email: zvezdochka198512@mail.ru
ORCID iD: 0000-0003-4168-2442
ResearcherId: AAZ-5531-2020

Senior Researcher of the Power Systems and Technologies Laboratory, Cand.Sc. (Engr.)

Russian Federation, 2/31 Lobachevsky St., Kazan 420111

References

Supplementary files