Possibilities of obtaining biogas from manure and amaranth

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.

Том 31, № 3. 2021

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 pyrolysis liquid

F i g. 1. The AD-Py technology

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 Amarantha-ceous 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 Ama-ranthus 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).

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).

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

F i g. 2. Experimental set-up

Composition of samples for experiments

T a b l e 1

Sample	Dairy manure mass, g	Biomass additive	Biomass / Manure ratio	Total volatile solids, g VS∙L–1	Ultrasonic pre-treatment
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

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 ( l ) = W exp I exp I Rw^e ( a - 1 ) + i ll , (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.

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 retrofle-xus L., and AC is biomass of the Ama-ranthus cruentus L.

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.

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

T a b l e 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

T a b l e 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

T a b l e 4

One way ANOVA for biogas yield