The Impact of Sewage Sludge-Sweet Sorghum Blends on the Biogas Production for Energy Purposes

Abstract

The paper presents research on the impact of adding various forms of sorghum to sewage sludge on the anaerobic digestion process. The use of liquid sewage sludge alone in biogas plants at wastewater treatment plants is inefficient due to the low total solid (dry matter) content of this substrate. The tests revealed that the production of methane in biogas is low and amounted to 17.9% (105.4 Nm³∙Mg⁻¹, VS—volatile solid). Therefore, other substrates should be blended with sewage sludge to increase the total solid of the batch. Sorghum silage, sorghum pomace, and sorghum juice were added to the sewage sludge in various proportions during the research. As a result, the improvement of the biogas process, the stabilization of the biogas production curve, as well as the increase in methane yield were observed. The most biogas and methane were obtained from a mixture of sorghum juice (5%) and sewage sludge (664.8 Nm³∙Mg⁻¹ VS and 53.9%, respectively). Biogas production from other substrates based on sorghum and sewage sludge ranged from 457.4 to 588.8 Nm³∙Mg⁻¹ VS. For a mixture of juice (7%) and sewage sludge, the batch was acidified, and biogas production was only 281.5 Nm³∙Mg⁻¹ VS. Studies have shown that intelligent blending of an alternative raw material (compared to traditional maize silage) with sewage sludge allows for similar biogas yields while maintaining a stable anaerobic digestion process.

1. Introduction

Intensive economic development and improvement of living standards are associated with a significant increase in energy demand [1]. In many regions, energy is still produced using fossil fuels, resulting in a large increase in carbon dioxide emission to the atmosphere and, thus, the deepening of the greenhouse effect [2]. To change this situation, it is necessary to look for alternative energy sources whose carbon footprint is less burdensome for the environment.

Agricultural biogas plants are one of the options to produce renewable energy. The use of agricultural biogas plants is important in the context of sustainable rural development, as they contribute both to the increase in energy independence of the region and also to the increase in the use of local resources, especially biomass [3]. Furthermore, in biogas plants, various types of waste (animal waste, such as cattle slurry, manure, and waste from the agro-food industry) can be utilized, which are a serious problem in their management [4]. In addition, the use of waste substrates in the anaerobic digestion process contributes to reducing methane emissions, whose impact on the greenhouse effect is 21 times more unfavorable than carbon dioxide [5]. During the anaerobic digestion process, the digestate is produced, which can be used as fertilizer in the fields [6]. It is beneficial for biogas power plants, especially in the zero-waste strategy promoted in many EU countries [7].

In Europe, the most popular substrates in the anaerobic digestion process are maize silage and cattle slurry [8]. Corn is a very efficient substrate in terms of biogas production. However, the significant increase in the prices of corn caused the need to search for alternative substrates. In addition, there is a need for crop rotation [9]. One of the solutions may be to use sorghum. Sorghum, in terms of cultivated area, is the fifth plant in the world after wheat, rice, corn, and barley [10]. The plant is cultivated widely in Africa, Asia, South America, and the USA. Its popularity in Europe is also growing, which is influenced by the growing number of droughts [11]. Sorghum is an anatomically and physiologically adapted plant to dry conditions, which results from the deep root system [12]. There are about 30,000 species of sorghum. Sorghum is an annual grass, and its origin is in the African savannah. A wild plant cultivated thousands of years ago, it was domesticated in Ethiopia about 5000 years ago. As much as 90% of the world’s grain production, which is sorghum, is in developing countries, mainly African and Asian ones. Sorghum does not contain gluten, so it is also possible to use it to bake bread for people who suffer from Celiac disease. There are four most common sorghum products: grits, unleavened bread, fried products, and cooked products. Most sorghum is used by people living in Africa, and unit consumption in some countries is 90–100 kg·year⁻¹ [13]. However, Sucrosorgo 506 is used in the research, which is characterized by a high yield of green mass, as well as resistance to drought and lodging. It is also suitable for the production of silages due to its high sugar content. The pH of the soil should be between 5.5 and 6.5. It has few requirements for a crop. To obtain a proper crop, it is necessary to provide the soil with the proper amount of nitrogen and potassium. The amount of nutrients delivered to the soil is similar to that of maize. Sorghum has numerous morphological, biochemical, and physiological adaptations (a large number of leaves, deep root system, C4 photosynthesis) thanks to which it is resistant to very high temperatures. Roots are better developed to absorb water during drought compared to maize. When there is a drought, sorghum goes dormant, and, when there is an increase in soil moisture, vegetation begins again. In terms of use, there are five main types of sorghum: grain sorghum, sugar sorghum, Sudanese grass, hybrids of sorghum, and Sudanese grass. In 2016, sorghum production in the world amounted to 63 million Mg [14].

Sorghum can be used as animal feed, addition to concentrated feed, as a substrate for alcohol or paper production, and as an energy crop [15]. For energy purposes, sorghum can be used for biogas production (electricity and heat) [16], as well as bioethanol to power internal combustion engines [17]. In turn, straw or sorghum pomace can be used to produce pellets and biochar [18]. In the case of appropriate agrotechnical treatments, sorghum is characterized by a high yield of fresh mass, even approximating 100 Mg∙ha⁻¹ [14].

Municipal sewage is another waste product related to human activities. Side effects of its treatment is significant amounts of sewage sludge. In Europe, the amount of sewage sludge produced is over 13 million tonnes of total solid [19]. Sewage sludge is a mixture of domestic and industrial sewage. The amount and quality of sludge depend on the industrialization of the city, the quality of the sewage system, water consumption or source of origin, and their composition changes on a daily, monthly, and annual basis. According to Directive 2008/98/WE on waste, municipal sewage sludge requires appropriate management to avoid its storage. In Poland, depending on the form of the sludge (concentrated, digested sludge stabilized in aerobic processes, dehydrated, dried), it can be used directly as fertilizer [20], composted [21], thermally processed (combustion, pyrolysis, gasification) [22], and also used as a substrate for biogas production [23]. However, the total solid of sewage sludge is small 2–2.5% TS (total solid) [24], and, for efficient biogas production, it is necessary to mix them with other substrates that will increase the total solid content of the whole batch. Due to the low biogas production potential, its production based solely on sewage sludge is economically unjustified. Only large biogas plants, for which the unit cost is lower than small biogas plants, can afford reduced biogas production. Therefore, in the case of such substrates, it seems reasonable to mix them with other materials to improve biogas production while enabling the disposal of sewage sludge (even in smaller agricultural biogas plants).

Sucrosorgo 506 is a late-ripening hybrid of sweet sorghum with a triple cross (Sorghum Partners Inc., Longmont, CO, USA). Studies indicate that the plant is characterized by a high yield of biomass in a temperate climate. Therefore, it is a variety that can be successfully grown in Central Europe. In the process of anaerobic digestion, the physicochemical composition of the substrates is important for the effective production of biogas and methane. The tested variety of sorghum was characterized by the appropriate physical–chemical composition to use the plant for the effective production of electricity and heat. From the Sucrosorgo 506 anaerobic digestion process, the appropriate amount of biogas and methane was obtained to use this sorghum in an agricultural biogas plant as a single substrate or as a mixture of several substrates. Sucrosorgo 506 can also be used to thicken the charge in a biogas plant when substrates with a low dry matter content are subjected to the biogas production process, e.g., cattle slurry, swine slurry, and sewage sludge. As a result, the dry weight of the batch in the biogas increases, and the production of biogas and methane can increase. Greater energy production will contribute to increased revenues in the biogas plant. Until now, a lot of research has been conducted on biogas production, separately from sorghum [25] and sewage sludge [26]. There were investigated also biogas yields as a result of mixing sorghum with rye and maize [27], as well as sewage sludge with food waste and yard waste [28]. However, no data were found concerning the impact of blends consisting of sewage sludge together with various sorghum substrates as a biogas plant batch on the effectiveness of the anaerobic digestion process. This type of research may make it possible to use much larger amounts of sewage sludge because the process will be more profitable. As more biogas will be created, more electricity and heat can be produced in the cogeneration engine in the biogas plant.

Therefore, the study aimed to investigate the influence of sludge supplementation with sugar sorghum substrates on (i) the anaerobic digestion process, (ii) biogas production, and (iii) biogas composition. Sorghum silage, pomace, and sorghum juice were used in the research, and the potential for biogas and methane production from the sludge from the sewage treatment plant was determined. Sorghum substrates were also blended with sludge in appropriate proportions, and co-fermentation was carried out, as well.

2. Materials and Methods

2.1. Materials Used in the Experiment

The sorghum used in this study was sugar sorghum (Sucrosorgo 506) for fodder purposes. This sorghum used for biogas energy is not a competitive substrate for food because Sucrosorgo has a very low grain yield [16]. Additionally, the unstable sludge taken from the Janówek (Wrocław) sewage treatment plant (Lower Silesia, Poland) was used, as well. The following products obtained from sugar sorghum (Sucrosorgo 506) were used as co-substrates: silage sorghum, pomace, and juice. In addition, an inoculum (digestate-containing bacteria participating in the anaerobic fermentation process) from an agricultural biogas plant was added.

Table 1. Characteristics of sewage sludge and co-substrates.

Substrate	Total Solid	Volatile Solid	pH
	%	% T.S.	-
Sewage sludge	0.9 ± 0.1	76.28 ± 0.1	6.62 ± 0.1
Sorghum silage	34.6 ± 0.1	96.65 ± 0.1	3.81 ± 0.1
Pomace	90.7 ± 0.2	97.74 ± 0.1	5.50 ± 0.1
Juice	14.6 ± 0.2	92.49 ± 0.1	5.24 ± 0.2
Inoculum	3.4 ± 0.1	70.33 ± 0.2	7.84 ± 0.2

presents the characteristics of the substrates used in the research.

2.2. Anaerobic Digestion Process

The experiment was conducted in laboratory conditions. A set of fermenters (eudiometers) with a capacity of 1 dm³, as well as a water bath, were used with a mesophilic temperature 38 °C (Figure 1).

The mesophilic fermentation process was carried out following the German standard DIN 38 414-S8 [29]. The fermenters were filled with a mixture of substrates in the amount of 400 g. The content of sludge, inoculum, and sorghum co-substrates in the batch was determined based on the total solid content. The proportions of the substrates are given in

Table 2. The amount of batch in the fermenter.

Substrates	Total Solid in Batch	Substrates from Sorghum	Sewage Sludge	Inoculum
	%	g
Sorghum silage	5	26	74	300
	7	50	50	300
Pomace	5	10	90	300
	7	19	81	300
Juice	5	65	35	300
	7	140	45	215
Sewage sludge	3	-	70	330
Inoculum	3.4	-	-	400

.

The production of biogas volume was controlled every 24 h until the daily amount of biogas released was below 1% of the total amount of biogas produced in the experiment. All values were converted to normal conditions (1013 hPa, 0 °C). The research was performed in three repetitions. In addition, the biogas composition, including CH₄, CO₂, O₂, H₂S, and NH₃ content, was determined using a GA 5000 portable biogas analyzer (Geotech, Warwickshire CV31 3JR, England, UK).

The studies were statistically analyzed in the Statistica 13.3 PL program. Prediction accuracy was assessed using the coefficient of determination (R²) and root mean squared error (RMSE). In predictive models, higher R² values combined with lower RMSE values are indicators of good model fit. To describe the kinetics of anaerobic digestion, the process used the kinetic equation of the first-order Tabasarana model [30].

$${\mathrm{G}}_{\mathrm{t}}={\mathrm{G}}_0\times \left(1-\exp \left(-\mathrm{k}\times \mathrm{t}\right)\right),{\mathrm{Nm}}^3\cdot {\mathrm{Mg}}^{-1}\cdot \mathrm{VS}$$

(1)

where G_t is the cumulative volume of biogas at the end of the process, Nm³∙Mg⁻¹ VS, G₀ is a kinetic biogas production Nm³∙Mg⁻¹ VS, k is a rate constant, 1∙day⁻¹, and t is the duration of anaerobic digestion, day.

3. Results and Discussion

In laboratory tests, the sewage sludge, separately as well as as substrates, was derived from sorghum blended in appropriate proportion with sewage sludge, were investigated. The anaerobic digestion process in terms of both biogas and methane production was efficient and stable (Figure 2 and Figure 3). Biogas yield curves were characteristic of the methane fermentation process. The biogas production process was stable and lasted 29 days for mixing sewage sludge with sorghum silages, pomaces, and juice (5%). Unfortunately, for juice (7%) (with sewage sludge), the process was acidified and was discontinued after eight days. For sewage sludge only, the cumulative biogas production curve was more linear, which probably was due to the low total solid of this substrate. The biogas yield itself was very low: 3.99 Nm³∙Mg⁻¹ FM (fresh matter). For comparison, the mixture of sewage sludge with sorghum silage 5% resulted in ten folds more biogas production (42 Nm³∙Mg⁻¹ FM). The highest biogas yield was observed for juice (5%), sewage sludge, and silage (5%), respectively: 664.8; 588.8, and 531.0 Nm³∙Mg⁻¹ VS (volatile solid). The least biogas was produced from pomace (7%) (490.4 Nm³∙Mg⁻¹ VS) and silage (7%) (457.4 Nm³∙Mg⁻¹ VS). For juice (7%), acidification of the batch occurred, therefore only 281.5 Nm³∙Mg⁻¹ VS was obtained. Zhikai Zhang et al. [31] conducted research on the anaerobic fermentation of sorghum with cow manure. The biogas yield was 478 Nm³∙Mg⁻¹ VS. Biogas production from sorghum stalks was also investigated, and, according to Dahunsi et al. [32], biogas yield was from 287.3 to 354.2 Nm³∙Mg⁻¹ VS. In turn, Szlachta et al. [33] compared biogas yields from different fragmented sorghum and maize silages, and the production amounted from 601 to 736 Nm³∙Mg⁻¹ VS for maize silage and from 483 to 650 Nm³∙Mg⁻¹ VS from sorghum silage. While, in other studies [34], it was found that biogas production from liquid sewage sludge ranged from 18 to 60 Nm³∙Mg⁻¹ VS.

In terms of methane production (Figure 3), the highest average percentage of methane content was obtained for juice (5%) and silage (7%) (53.9 and 47.4%, respectively), and the lowest was obtained for sewage sludge (only 17.9%). In other studies on the anaerobic fermentation process from sorghum, the percentage methane yield was 64.4% from sorghum silage [35]. Whereas, only 12% of methane production was detected from sewage sludge [36]. Likewise, in another study [34], the fermentation of the liquid sewage sludge resulted in low methane content in biogas (from 5 to 18% only).

Thus, in the case of sewage sludge, it is characteristic that, next to high biogas yield from this substrate, a very low percentage share of methane was detected. Therefore, it is better to use sewage sludge with other substrates so that the total solid of the batch is higher. Such blending, using the appropriate proportion of substrates, should lead to an increase in anaerobic digestion efficiency. As a result, more electricity and heat can be obtained in a biogas plant.

Moreover, the amount of methane as Nm³∙Mg⁻¹ VS was determined (Figure 4). Most of the methane was obtained from juice (5%) (358.3 Nm³∙Mg⁻¹ VS), and, for sorghum pomace and silage, the amount of methane was at a similar level: 213.4 and 216.7 Nm³∙Mg⁻¹ VS, accordingly (

Table 3. Summary of anaerobic digestion results.

Substrates	Total Solid in Batch	Biogas Yield	Biogas Yield	Methane Yield	Methane Yield
	%	Nm3∙Mg−1 VS	Nm3∙Mg−1 FM	%	Nm3∙Mg−1 VS
Sorghum silage	5	531.0 ± 62.6	42.0 ± 7.01	40.2 ± 2,89	213.4 ± 8,42
	7	457.4 ± 51.3	70.0 ± 11.11	47.4 ± 1.47	216.7 ± 10.4
Pomace	5	519.1 ± 62.8	36.6 ± 6.27	41.7 ± 3.74	216.4 ± 21.4
	7	485.5 ± 30.7	72.4 ± 4.38	43.3 ± 2.65	212.4 ± 15.4
Juice	5	664.8 ± 7.3	56.5 ± 0.87	53.9 ± 2.33	358.3 ± 21.6
	7	281.5 ± 5.5	11.8 ± 0.12	0.0 ± 0.0	0.0 ± 0.0
Sewage sludge	3	588.8 ± 19.5	3.99 ± 0.16	17.9 ± 1.12	105.4 ± 5.9

). Other authors [37] reported a methane yield of 400 Nm³∙Mg⁻¹ VS for sorghum. In other studies, the methane production from maize sorghum reached values from 232 to 387 Nm³∙Mg⁻¹ VS [38], which is similar to the results obtained in this research. Maize silage is the most popular substrate used in agricultural biogas plants. According to research by Herrmann et al. [39], methane production from maize ranged from 319 to 376 Nm³∙Mg⁻¹ VS.

As a result, the investigated blends are also appropriate for the digester. However, from the sewage sludge, only 105.4 Nm³∙Mg⁻¹ VS of methane was obtained. This value is not much lower than determined by others: 156 Nm³∙Mg⁻¹ VS [40] and 260 Nm³∙Mg⁻¹ VS [27]. Therefore, it seems that smart blending of the batch containing sewage sludge and sorghum might be an interesting alternative to traditional substrates for the anaerobic digestion process, and thus this is also the case for the production of electricity and heat. Unfortunately, no methane was produced in the fermentation chamber for juice (7%) as the process was acidified. Acidification was associated with too much acid substrate, which was sorghum juice at 7%, instead of methane, and the production of carbon dioxide increased. After checking the pH of the batch, which was below 5, the acidified 7% juice sample was removed. The correct pH value during the anaerobic digestion process should be between 6.8 and 7.8. In addition, methane production should increase in contrast to the carbon dioxide content. In the case of 7% juice, the batch was acidified [41,42]. It is important to properly select substrates for biogas plants. When introducing a new substrate, it is necessary to examine the proportions in which it should be added so that there is no reduction in biogas production or even acidification of the batch and interruption of the anaerobic digestion process.

The statistical analysis of the coefficient of determination R² and the root mean squared error RSME showed that the applied model of biogas production is well matched (

Table 4. Kinetic biogas parameters in Tabasarana model.

Substrates	The Efficiency of Biogas Gt, Nm3∙Mg−1 VS	Rate Constant k, 1∙Day−1	Degradation Half-Lives T1/2, Day	Coefficient of Determination R2	Root Mean Squared Error, RMSE	Duration of Process t, Day
Sorghum silage 5%	556.3	0.099	3.001	0.997	8.216	23
Sorghum silage 7%	513.4	0.081	3.205	0.998	5.354	23
Pomace 5%	544.7	0.098	3.006	0.995	10.615	23
Pomace 7%	485.9	0.129	2.739	0.991	12.641	23
Juice 5%	675.9	0.132	2.718	0.969	30.225	23
Juice 7%	293.6	0.553	1.284	0.993	7.875	8
Sewage sludge	3,919,626	0.0005	12.890	0.985	21.302	23

). The value of the coefficient of determination R² was in the range from 0.969 to 0.998, and the highest for Sorghum silage was at 7%. Root mean squared error was the highest for juice (5%) (30.225) and lowest for Sorghum silage (7%) (5.354). The kinetic production of biogas Gt was determined from the first-order equation. Cumulative biogas production from sewage sludge and substrates from sorghum in the Tabasarana model was also performed (Figure 5). The drawings show that the Tabasarana models were suitable for the anaerobic digestion process. The rate constant, k, ranged from 0.0005 for sewage sludge to 0.553 for juice (7%). Degradation half-lives were the highest for sewage sludge (12.890 days), and they were the lowest for juice (7%) (1.284 days).

4. Conclusions

The use of sewage sludge, together with various forms of sorghum, is a good solution for sewage treatment plants because the production of biogas and methane from fresh matter increases, which is recommended for overall energy efficiency. The biogas production from sewage sludge was high. However, due to the low total solid and fresh matter (TS and FM) of this substrate, the amount of methane production, which is crucial in terms of heat and power generation in internal combustion engines, was very low. To improve this unfavorable balance sheet, other feedstock (substrates) should be added to increase the total solid of the batch and, thus, increase the methane production, as well as the efficiency of the anaerobic digestion process. The addition of various forms of sorghum to sewage sludge improved the kinetics of the anaerobic digestion process, and biogas and methane production increased. The addition of sorghum pomace, sorghum silage, and sorghum juice with sewage sludge in various proportions to the agricultural biogas plant at the sewage treatment plant would improve the efficiency of the biogas production process. When adding a new substrate to a biogas plant, it is worth checking whether it will not affect the lower biogas production or even acidification of the feedstock, as it was detected for one of the samples in the tests performed, where too much sorghum juice was added to the sludge (juice, 7%) where acidification occurred.

The performed research provides not only new data in the area of the anaerobic digestion process, but it also opens a space for further studies, especially in the aspect of reducing the presence of hydrogen sulfide in biogas by using appropriate substrates together with sewage sludge. In addition to sorghum, it is also possible to add other substrates to the sewage sludge, which will translate into the efficiency of biogas production in the chamber digester, which will be higher and which will affect greater energy production. Another issue to be considered is the problem of high concentration of hydrogen sulfide in the biogas from the decomposition of sewage sludge in the fermenter. It may affect the corrosion of the co-generator. Fossil fuel resources are running out, and their burning also increases the greenhouse effect. Therefore, as much organic waste as possible must be converted into energy. Due to the low dry matter content of sewage sludge, the production of biogas and methane in the process of anaerobic digestion may be insufficient. Using additional substrate (e.g., in the form of sorghum), renewable energy stably and effectively can be produced. In future studies, it is worth expanding the research to include other varieties of sorghum along with waste (pomace, juice). It is also worth looking for other alternatives to thicken the sludge with another substrate so that the production of electricity and heat in the biogas plant is feasible.