Home Effect of pretreatment with alkali on the anaerobic digestion characteristics of kitchen waste and analysis of microbial diversity
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Effect of pretreatment with alkali on the anaerobic digestion characteristics of kitchen waste and analysis of microbial diversity

  • Xiaofei Zhen EMAIL logo , Shange Li , Ruonan Jiao , Wenbing Wu , Ti Dong and Jia Liu
Published/Copyright: October 25, 2023
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Green Processing and Synthesis
From the journal Green Processing and Synthesis Volume 12 Issue 1

Abstract

Kitchen waste contains high contents of organic matter and moisture, and it is prone to biodegrade and decompose to give odors. If not collected and transported promptly or treated improperly, it is highly likely to pollute the environment and spread diseases. Because the lipid content in kitchen waste is high and a portion of organic matter is not subject to hydrolysis, the development of anaerobic digestion technology has been greatly limited. Kitchen waste was pretreated with NaOH, KOH, and Ca(OH)2 with different concentrations, and 50 days sequencing batch mesophilic anaerobic digestion experiments were conducted. This study sheds light on the pollution reduction and energy generation of kitchen waste. The results are as follows: (1) The lipid content of kitchen waste could be reduced, and the concentration of dissolved organic matter could be increased by pretreating with alkali. The degradation rate of kitchen waste lipid reached a maximum of 50.51%, if 3% NaOH was added, and the soluble chemical oxygen demand concentration was increased by 235.3%. (2) The cumulative methane (CH4) output and biogas production efficiency were improved in the anaerobic digestion process with kitchen waste pretreated with alkali. The maximum daily gas output of kitchen waste pretreated with NaOH and KOH took place on the 11th to 12th day, with the biogas production efficiency of 40.4 and 45.2 mL·g·VS−1. The cumulative CH4 output was increased from 370.2 mL·g·VS−1 (untreated) to 393.1 and 434.1 mL·g·VS−1, respectively. In addition, the concentration of CH4 in biogas was increased from 54.8% (untreated) to 59.1% and 61.7%, respectively. (3) The Chao1 and Ace values of bacteria were increased first and then decreased. On the 10th day, the diversity of bacteria reached the highest value, and on the 20th day, the diversity of archaea reached its maximum. Therefore, it was verified that the improvement in the hydrolysis acidification efficiency and degree was crucial for the rapid and complete anaerobic digestion reactions.

Keywords: kitchen waste; pretreatment with alkali; anaerobic digestion; microbial diversity

1 Introduction

With the development of urbanization and level of national consumption, the output of kitchen waste increases continuously, and kitchen waste has become one of the most important components of urban living waste [1]. Kitchen waste contains high contents of organic matter and moisture, and it is prone to biodegrade and decompose to give odors. If not collected and transported promptly or treated improperly, it is highly likely to pollute the environment and to spread diseases. However, the traditional waste treatment methods, such as landfilling, incineration, and composting, have the disadvantages of occupying large area, high operating costs, or secondary pollution [2], so they are not ideal kitchen waste treatment methods. With the continuous improvement in traditional waste treatment technology and promotion of waste classification policy, the research on anaerobic digestion treatment of kitchen waste has drawn attention. However, because the lipid content in kitchen waste is high and a portion of organic matter is not subject to hydrolysis, the development of this technology has been greatly limited [3,4], so how to reduce the concentration of dissolved organic matter in kitchen waste is particularly important. Chemical pretreatment methods mainly include ozone oxidation pretreatment, acid hydrolysis pretreatment, alkali hydrolysis pretreatment, and so on. Chemical pretreatment methods have been adopted, and good performance has been delivered, by adding weak alkaline solution to reduce the concentration of dissolved organic matter in the produced waste, prevent the accumulation of acid in the process of anaerobic effect and inhibit the anaerobic digestion of gas production, and achieve good results. Acid and alkali pretreatment can be done by chemical reaction. It should promote the conversion of part of the insoluble organic matter in the substrate to dissolved organic matter by adjusting pH. The change of pH value makes fine cell wall dissolution, or by changing osmotic pressure, release of intracellular substances, has a certain hydrolysis effect [4]. Xin et al. [5] pretreated rice straws with 6% NaOH and 3% H2SO4, and the anaerobic-digestion biogas output was increased by 28.5% and 12.5%, respectively, compared to the control group. Based on these studies, we pretreated kitchen waste with NaOH, KOH, and Ca(OH)2 with different concentrations, and the waste was treated in 50 days sequencing batch mesophilic anaerobic digestion experiments. Before and after the pretreatment, the contents of lipid and dissolved organic matter in the kitchen waste were detected, and the effects of pretreatment with alkali on CH4 production rate, cumulative CH4 output, pH, microbial diversity, and other parameters were analyzed. This work is beneficial for the energy generation from efficient anaerobic digestion of kitchen waste.

2 Materials and methods

2.1 Apparatus

As shown in Figure 1, a custom-made anaerobic digestion reactor was used in this experiment. The apparatus was composed of two jars (1 L) and a volumetric flask (1 L), which functioned as the raw-material digestion tank, and biogas-collecting and effluent-collecting containers. These three containers were connected with anti-aging rubber tubes, so an intra-connected apparatus was assembled, and the air tightness was ensured.

Figure 1 Physical and schematic illustration of the apparatus: (1) water bath, (2) digestion-reaction bottle, (3) mixture of kitchen waste and acclimated sludge, (4) air duct, (5) gas cylinder, (6) distilled water, and (7) beaker.
Figure 1

Physical and schematic illustration of the apparatus: (1) water bath, (2) digestion-reaction bottle, (3) mixture of kitchen waste and acclimated sludge, (4) air duct, (5) gas cylinder, (6) distilled water, and (7) beaker.

2.2 Materials

2.2.1 Kitchen waste

The kitchen waste experimented was sampled from the cafeteria in the college. The waste was shredded and thoroughly homogenized by a blender. To ensure the consistency of the samples, 5 kg of kitchen waste was prepared at a time, and was stored at −20°C. Before the experiment, the sample was stored at 4°C for 12 h for thawing.

2.2.2 Activated sludge

The inoculum for anaerobic digestion in the experiment was obtained from the residual sludge of a neighboring sewage treatment plant. The sludge was transported to a large, sealed plastic container at about 20°C. After returning to the laboratory, the sludge was cultured and domesticated at 37°C as follows: 5 L of the sludge was placed in a 25 L sealed plastic container for cultivation at 37°C. Three days later, the sludge had undergone its adaptation period, and 2.5 kg of room-temperature kitchen waste was added for 10 days of cultivation. Then, 5 kg of room-temperature kitchen waste was domesticated for 10 days. The sludge could be used. The dry-matter weights of kitchen waste and inoculated sludge were measured after drying in an oven at 105°C for 24 h, and the organic matter content was measured after calcination with a muffle furnace at 550°C for 4 h. The primary parameters of the wet kitchen waste and activated sludge are shown in Table 1, and the contents of nutrients in the kitchen waste are shown in Table 2.

Table 1

Main parameters of wet ground state of kitchen waste and activated sludge

Parameter TS (%) VS (%) pH TC (%) TN (%)
Kitchen waste 23.29 70.55 5.13 46.07 4.73
Activated sludge 9.32 37.73 7.41 — —
Table 2

Nutrient composition analysis of kitchen waste

Kitchen waste TCOD (g·kg−1) SCOD (g·kg−1) Dissolved carbohydrate (g·kg−1) Soluble protein (g·kg−1) VFAs (g·kg−1)
[8] 238.5 ± 3.8 106.0 ± 5.3 81.7 ± 6.2 5.9 ± 1.4 7.3 ± 0.4
[9] 353.3 ± 1.7 162.5 ± 1.4 69.4 ± 1.7 6.8 ± 1.2 8.1 ± 1.6
The present work 311.5 ± 4.7 134.6 ± 2.4 74.4 ± 3.4 6.3 ± 2.1 7.4 ± 1.3

2.3 Experimental procedures

30 sets of digestion reactors (1 L) were used in the sequencing anaerobic digestion experiment of kitchen waste pretreated with alkali. A total of ten groups of experiments were conducted, and each experiment was repeated thrice. The mean value was taken as the real value. The experimental conditions are listed in Table 3. 1,000 g of kitchen waste was placed in a 1,000 mL jar, and 1, 2, and 3 wt% NaOH, KOH, and Ca(OH)2 were separately added with stirring. The mixtures were stored at 4°C for 24 h. Then, the pH was adjusted to 6.2 ± 0.1 with 1 mol·L·HCl−1. 100 g of the treated sample was placed in a 1 L jar in each experiment, and 300 mL of activated sludge was added. The mixture was diluted with water until the volume reached 1 L. These samples prepared were denoted as RNa1, RNa2, RNa3, RK1, RK2, RK3, RCa1, RCa2, and RCa3. And a control group containing kitchen waste without alkali was denoted as R0. After sealing with wax, these reactors were placed in an electric thermostatic water bath, and the incubation was performed at 37°C for 50 days.

Table 3

Experimental conditions for the pretreatment of kitchen waste with alkali (m·m−1, %)

RNa1 RNa2 RNa3 RK1 RK2 RK3 RCa1 RCa2 RCa3 R0
Alkali type NaOH NaOH NaOH KOH KOH KOH Ca(OH)2 Ca(OH)2 Ca(OH)2 —
Alkali concentration 1 2 3 1 2 3 1 2 3 —

2.4 Analysis methods

2.4.1 Analysis of physical and chemical properties

The kitchen waste and anaerobic digestion sludge were taken and homogenized by stirring or shaking manually. The physical and chemical properties of this sample, such as TS, VS, and lipid content, were measured. Additionally, this sample was centrifugated at a speed of 20,000 rpm for 10 min, and the supernatant was filtrated with a 0.45 μm membrane. The relevant physicochemical properties of soluble components, such as pH, total carbon (TC), total nitrogen (TN), total chemical oxygen demand (TCOD), soluble chemical oxygen demand (SCOD), soluble proteins, carbohydrates, lipids, and volatile organic acids (VFAs), in the filtrate were determined [6]. The measurement methods of these physical and chemical properties are shown in Table 4.

Table 4

Primary physical and chemical characteristics and analytical methods

Physical and chemical characteristics Analytical methods
TS Drying [7]
VS Calcination [8]
pH Electrode method [8]
TC Element analyzer [7]
TN Element analyzer [8]
TOCD Potassium-dichromate method [9]
SCOD Potassium-dichromate method [10]
VFAs Colorimetric method [11]
Protein Protein content = (TN – ammonia nitrogen) × 6.25 [12]
Carbohydrate Carbohydrate = VS – lipid – protein [13]
Lipid Soxhlet extraction method [8]

The biogas output was measured once a day by metering the water expelled by biogas. The cumulative composition of biogas was analyzed by gas chromatography under the following conditions: chromatographic column: stainless-steel column (TDX-01 packing, 2 m × 3 mm) produced by the National Chromatographic Research and Analysis Center of Dalian Institute of Chemical Physics, Chinese Academy of Sciences; detector: TCD; carrier gas: 20 mL·min−1 He; current: 100 mA; attenuation: 1; detection temperature: 200°C; column temperature: 180°C; and injection temperature: 200°C.

2.4.2 Microbial 16 S rRNA sequencing

The anaerobic digestive fluid was filtered through a 0.22 μm membrane, and the membrane was collected and stored at −20°C for testing. The high-throughput sequencing of 16 S rRNA was completed by Shanghai Meiji Biomedical Technology Co., Ltd using the Illumina Miseq PE300 sequencing platform. The samples were extracted using the FastDNA® Spin Kit for lipid extraction kit according to the instructions for DNA extraction. The bacterial 16 S rRNA gene V3–V4 region was amplified using primers 338 F (5′-ACTCCTACGGGAGGCAGCAG-3′) and 806R (5′-GGACTACHVGGGTWTCTAAT-3′), while the archaeal 16 S rRNA gene V4–V5 region was amplified using primers 524F10extF (5′-TGYCAGCCGCCGCGGTAA-3′) and Arch958RmodR (5′-YCCGGCGTTGAVTCCAATT-3′).

2.4.3 Data analysis

In the experiment, the physical and chemical indicators were measured in triplicate. The data were analyzed for significance and correlation using single-factor analysis of variance and multiple comparisons in SPSS v.18.0 software, evaluating the significant differences between each experimental treatment. The least significant difference method, (a = 0.05) was used for multiple comparisons of the mean values. All data graphs were plotted using Origin-8.0.

3 Results and discussion

3.1 Effects of pretreatment with alkali on the organic components in kitchen waste

3.1.1 Lipid content

Figure 2 shows the effects of different pretreatments with alkali on the lipid content of kitchen waste. The lipid content in the control group was 8.77%, while those in the RNa1, RNa2, and RNa3 groups were decreased to 6.11%, 5.56%, and 4.34%, respectively. The lipid contents in the RK1, RK2, and RK3 groups were decreased to 5.87%, 5.66%, and 6.01%, respectively, and those in the RCa1, RCa2, and RCa3 groups were decreased to 6.45%, 6.32%, and 6.54%, respectively. The pretreatments with alkali could effectively promote the degradation of lipids in kitchen waste. In the experiment with RNa group, within the concentration range of 1–3%, the lipid contents in kitchen waste decreased with the increase in NaOH concentration in the pretreatments. Within the concentration range of 1–3%, the pretreatments with KOH and Ca(OH)2 could also promote the reduction in lipid contents of kitchen waste, but the decrease in lipid content was not linearly correlated to their dosages. The degradation rate of lipid in kitchen waste pretreated with 3% NaOH reached a maximum of 50.51%. Among the three alkalis, the degradation rate of lipid in kitchen waste pretreated with Ca(OH)2 was the lowest, approximately 25.42–27.91%. In the pretreatments with strong alkalis, NaOH and KOH reacted with lipid via saponification reactions to give salts of fatty acids and glycerol. In contrast, in the pretreatment with Ca(OH)2, a weakly alkaline environment was generated to promote the hydrolysis reactions of lipids in kitchen waste and to reduce the lipid content.

Figure 2 Lipid content of kitchen waste before and after alkali pretreatment.
Figure 2

Lipid content of kitchen waste before and after alkali pretreatment.

3.1.2 Content of dissolved organic matter

The pretreatment with alkali destroyed the chemical bonds of macromolecules in kitchen waste, thereby promoting the hydrolysis of insoluble macromolecular organic matter by breaking the chemical bonds. The pretreatment with alkali obviously promoted the dissolution of sugars and proteins in kitchen waste and generation of VFAs. The effect of pretreatment with alkali on the soluble organic matter in kitchen waste is illustrated in Figure 3. The contents of SCOD, soluble sugars, soluble proteins, and VFAs in the control group were 134.6 ± 2.4, 74.4 ± 3.4, 6.3 ± 2.1, and 7.4 ± 1.3 g·L−1, respectively. After the pretreatment with alkali, the solubility of organic matter in kitchen waste was improved. Among the RNa1, RNa2, and RNa3 groups, the RNa3 group showed better performance with the highest concentration of soluble organic matter. In detail, the SCOD content reached 249.8 ± 3.4 g·L−1 and was increased by 235.3%. The contents of soluble sugars, soluble proteins, and VFAs were increased to 155.3 ± 4.1, 17.4 ± 1.8, and 8.6 ± 1.1 g·L−1, respectively. Meanwhile, the contents of unknown organic compounds (including methanol, amino acids, long-chain fatty acids, etc.) were significantly increased to 66.57 ± 3.7 g·L−1, accounting for 43.16% of the total SCOD.

Figure 3 Dissolved organic matter concentrations before and after alkali pretreatment.
Figure 3

Dissolved organic matter concentrations before and after alkali pretreatment.

The profiles of SCOD concentrations in kitchen waste pretreated with different alkalis showed different trends. The RNa1, RNa2, and RNa3 groups showed better kitchen waste treatment performance. With the increase in NaOH concentration, the content of dissolved organic matter was also increased. The SCOD contents in these three groups were 233.5 ± 6.7, 245.9 ± 5.4, and 249.8 ± 3.4 g·L−1, respectively. After the pretreatment with KOH, the SCOD contents in kitchen waste increased to 241.3–221.3 g·L−1. Similar to the trend of profile regarding NaOH, the concentration of dissolved organic matter was also increased with the increase in KOH concentration. After the pretreatment with Ca(OH)2, the SCOD concentration was slightly increased to 187.4–201.4 g·L−1. The trend was similar to those of profiles regarding NaOH and KOH, with the increase in Ca(OH)2 concentration, the content of dissolved organic matter was also increased. However, the performance of pretreatment with Ca(OH)2 was poor. Therefore, strong alkaline substances, such as NaOH and KOH, could effectively decompose the organic macromolecules in kitchen waste and transform the insoluble organic matter into dissolved organic matter, increasing the concentration of dissolved organic matter. The stronger the alkalinity is, the better the pretreatment effect is. This principle is consistent with the results reported in the study by Elbeshbishy et al. [14] on the fermentation of pretreated kitchen waste for the production of hydrogen. After their pretreatment of kitchen waste with NaOH (pH = 11.0), the SCOD concentration of kitchen waste was also increased, and the concentrations of soluble sugars and proteins were increased by 21% and 26%, respectively. Similarly, after different pretreatments with alkalis in the present work, the concentrations of soluble sugars, soluble proteins, and VFAs in kitchen waste were increased. Compared to NaOH and KOH, the performance of Ca(OH)2 was poorer. The main reason is that slightly soluble salts would be generated from Ca2+ under alkaline conditions, inhibiting the dissolution of a proportion of organic matter. However, the pH and alkalinity of the substrate could be regulated by Ca2+, promoting the generation of VFAs [15]. In summary, the pretreatment with alkali improves the solubility of organic matter in kitchen waste, shortens the liquefaction time of organic matter such as proteins and sugars during the hydrolysis course, and improves the degradation efficiency.

3.2 Effect of pretreatment with alkali on the anaerobic digestion of kitchen waste

3.2.1 Biogas production characteristics

Figure 4 and Table 5 show the variation trends of CH4 production efficiency, cumulative output of biogas and CH4, and biogas concentration after pretreatment with alkali. The cumulative biogas output and cumulative CH4 output of the control group R0 were 675.5 ± 6.5 and 370.2 ± 3.1 mL·g·VS−1, respectively. The concentrations of CH4, carbon dioxide, and other gases in the biogas of group R0 were 54.8% ± 4.7%, 39.6% ± 1.5%, and 5.6% ± 0.4%, respectively.

Figure 4 Accumulative biogas yield and CH4 production rate during anaerobic digestion of kitchen waste pretreated with (a) NaOH, (b) KOH, and (c) Ca(OH)2.
Figure 4

Accumulative biogas yield and CH4 production rate during anaerobic digestion of kitchen waste pretreated with (a) NaOH, (b) KOH, and (c) Ca(OH)2.

Table 5

Production and proportion of biogas, CH4, and CO2 during anaerobic digestion

Group Cumulative biogas yield (mL·g·VS−1) Cumulative CH4 yield (mL·g·VS−1) Concentration of CH4 (%) Concentration of carbon dioxide (%) Concentration of other gases (%)
R0 675.5 ± 6.5 370.2 ± 3.1 54.8 ± 4.7 39.6 ± 1.5 5.6 ± 0.4
RNa1 650.8 ± 7.8 393.1 ± 0.9 60.4 ± 3.6 31.8 ± 1.6 7.8 ± 0.8
RNa2 671.0 ± 4.5 411.3 ± 2.5 61.3 ± 0.9 30.6 ± 1.7 8.1 ± 0.3
RNa3 703.6 ± 5.3 434.1 ± 1.7 61.7 ± 2.1 30.0 ± 2.1 8.3 ± 0.6
RK1 679.4 ± 4.2 401.5 ± 3.3 59.1 ± 3.7 33.2 ± 3.5 7.7 ± 0.2
RK2 680.8 ± 3.1 407.8 ± 3.1 59.9 ± 3.4 32.5 ± 2.7 7.6 ± 0.3
RK3 692.4 ± 4.1 418.9 ± 0.2 60.5 ± 3.3 31.6 ± 2.6 7.9 ± 0.6
RCa1 670.6 ± 0.5 373.5 ± 4.6 55.7 ± 4.5 37.9 ± 1.7 6.4 ± 0.4
RCa2 657.6 ± 3.3 375.5 ± 6.3 57.1 ± 5.1 36.2 ± 1.9 6.7 ± 0.4
RCa3 678.9 ± 2.1 389.7 ± 4.3 57.4 ± 2.9 35.2 ± 2.0 7.4 ± 0.5

Figure 4a shows that the maximum daily biogas output of untreated kitchen waste took place on the 15th day, with biogas production efficiency of 31.35 mL·g·VS−1. The cumulative CH4 output and biogas production efficiency of anaerobic digestion kitchen waste pretreated with NaOH were improved. The maximum daily biogas output of RNa1, RNa2, and RNa3 groups took place on the 12th, 12th, and 11th day, earlier than that of untreated kitchen waste, with biogas production efficiency of 41.4, 42.9, and 45.2 mL·g·VS−1, respectively. The cumulative CH4 output was increased from 370.2 mL·g·VS−1 (untreated) to 393.1, 411.3, and 434.1 mL·g·VS−1, respectively, and the CH4 concentration in biogas was also increased from 54.8% (untreated) to 60.4%, 61.3%, and 61.7%, respectively. Similarly, the kitchen waste pretreated with KOH also showed improved cumulative CH4 output and biogas production efficiency in the anaerobic digestion process. The maximum daily biogas output of RK1, RK2, and RK3 groups took place on the 12th, 12th, and 11th day, with the biogas production efficiency of 40.4, 41.8, and 43.3 mL·g·VS−1 and improved cumulative CH4 output of 401.5, 407.8, and 418.9 mL·g·VS−1, respectively. The CH4 concentrations in biogas were increased to 59.1%, 59.9%, and 60.5%, respectively. Compared to NaOH and KOH, the cumulative CH4 output and biogas production efficiency in the anaerobic digestion of kitchen waste pretreated with Ca(OH)2 were less improved. The maximum daily biogas output of RCa1, RCa2, and RCa3 groups took place on the 13th day, with the improved cumulative CH4 output of 33.3, 34.1, and 34.2 mL·g·VS−1, improved cumulative CH4 output of 373.5, 375.5, and 389.7 mL·g·VS−1, and improved CH4 concentrations of 55.7%, 57.1%, and 57.4%, respectively.

The main reason for the much higher cumulative CH4 output, biogas production efficiency, and CH4 concentration of the kitchen waste pretreated by NaOH and KOH in the anaerobic digestion process is that the strong alkalinity of NaOH and KOH destroyed the structures of macromolecules in kitchen waste, thereby promoting the bond-breaking and hydrolysis of insoluble macromolecular organic matter and increasing the concentrations of dissolved organic matter such as sugars, proteins, and VFAs. The hydrolysis was promoted to provide preferable early-stage conditions for the transformation of hydrolysates into CH4 by archaea in the later stage. The poor performance of waste pretreated with Ca(OH)2 is ascribed to the weak alkalinity of Ca(OH)2, of which the ability to destroy the chemical bonds in organic matter in kitchen waste was weak. Meanwhile, under alkaline conditions, the Ca2+ in Ca(OH)2 would be transformed into slightly soluble salts, which would inhibit the dissolution of a portion of organic matter. Nevertheless, the pH and alkalinity of the substrate could be adjusted with Ca2+, promoting the generation of VFAs and certain hydrolysis reactions. This is beneficial for the methanation in the later stage. This result is consistent with the study of Cui et al. [16], who reported that NaOH could break the ester bonds in the lignin–carbohydrate composite and release lignin-encapsulated cellulose. Thanks to these effects, the kitchen waste could be hydrolyzed and acidified by anaerobic microorganisms, by which the decomposition could be promoted. As a result, the cumulative biogas output and biogas production efficiency in the anaerobic digestion course could be improved.

3.2.2 Variation trend of pH

Figure 5 shows the variation trend of pH during the anaerobic digestion process of kitchen waste pretreated with alkali. The pH of all the groups at their initial stages was in the range of 6.2 ± 0.1. During the 50 days anaerobic digestion process, the pH profiles of the control group R0 and kitchen waste pretreated with alkali exhibited a concave shape. Among them, the pH of the control group R0 reached its minimum of 5.1 on the 15th day, while that of RNa1, RNa2, and RNa3 groups reached their minimums of 4.8, 4.8, and 4.7 on the 12th, 12th, and 9th day, respectively. At the end of the reactions, the final pH was maintained in the range of 6.4–6.6. The pH of RK1, RK2, and RK3 groups reached their minimums of 4.9, 4.9, and 4.8, respectively, on the 12th day. At the end of anaerobic digestion, the final pH was maintained in the range of 6.3–6.4. The pH of RCa1, RCa2, and RCa3 groups reached their minimums of 5.2, 5.2, and 5.3, respectively, on the 12th day, and the final pH was maintained in the range of 6.7–6.8.

Figure 5 The change in pH during anaerobic digestion of kitchen waste with alkali pretreatment by (a) NaOH, (b) KOH, and (c) Ca(OH)2.
Figure 5

The change in pH during anaerobic digestion of kitchen waste with alkali pretreatment by (a) NaOH, (b) KOH, and (c) Ca(OH)2.

The pH of the control group R0 and those pretreated with alkali decreased and then increased during the 50-days anaerobic digestion process. The main reason is that the organic matter in the kitchen waste was decomposed by hydrolytic microorganisms to produce small-molecule organic acids, leading to the decrease in pH in the reaction systems, and then these small-molecule organic acids were converted by archaea into CH4, resulting in the increase in pH. The pH of RNa and RK groups was lower than that of the control group R0, indicating that NaOH and KOH had a positive effect on the hydrolysis of kitchen waste. The chemical bonds in kitchen waste were destroyed by strong alkalis, thereby promoting the hydrolysis and acidification of organic matter, and transforming insoluble organic matter into soluble organic matter. As a result, the hydrolysis efficiency was improved. The hydrolysis process is the limiting factor determining the rate of anaerobic digestion and CH4 production. With a higher hydrolysis rate and better acidification effect, the methanation in the later stage will be carried out to a higher extent. Therefore, the pH of kitchen waste pretreated with NaOH and KOH was low, and the biogas production efficiency was high. The main reason for the relatively high pH of Ca(OH)2-pretreated kitchen waste in the anaerobic digestion process is that the weak alkalinity of Ca(OH)2 endowed it with weak ability to destroy the chemical bonds. In addition, the presence of Ca2+ increased the alkalinity of the digestion system, so the decrease in pH was hindered [17].

3.2.3 Analysis of effects of pretreatment with alkali on microbial abundance and diversity

The alpha diversity reflects the abundance and diversity of species in a single sample. The abundance of species is the number of species, which can be evaluated by the Chao1 and Ace indexes [18]. The larger the Chao1 and Ace indices are, the more abundant the species are. The diversity of species is affected by the abundance and evenness of species in the community of sample. With the same species abundance, the greater the evenness of each species in the community is, the higher the community diversity is. The diversity of species can be assessed by the Shannon and Simpson indexes. The larger a Shannon index is, the smaller the corresponding Simpson index is, and a small Simpson index indicates high microbial species diversity of the sample. A high-throughput sequencing technology was adopted to select and analyze the microbial communities in the kitchen waste group RNa3 pretreated with 3% NaOH on the 1st, 10th, 20th, and 30th day. These communities were labeled as RNa3 1, RNa3 10, RNa3 20, and RNa3 30, respectively. The analysis results of relative abundance and diversity of bacteria and archaea during the anaerobic digestion process are shown in Tables 6 and 7. The Coverage values in both tables denote the OTU coverage rate. The higher the value is, the higher the probability of species being detected is. This parameter can reflect whether the sequencing results represent the real situation of microorganisms in the sample.

Table 6

Abundance and diversity analysis of bacterium

Sample OTU Chao1 Ace Shannon Simpson Coverage
RNa3 1 315 334 337 3.521 0.089 0.9998
RNa3 10 321 347 351 3.645 0.084 0.9996
RNa3 20 294 343 341 3.456 0.086 0.9997
RNa3 30 277 324 326 3.319 0.090 0.9995
Table 7

Abundance and diversity analysis of bacterium

Sample OTU Chao1 Ace Shannon Simpson Coverage
RNa3 1 78 74.9 75.3 1.921 0.371 0.9996
RNa3 10 79 75.0 75.4 1.975 0.365 0.9994
RNa3 20 83 78.9 79.2 2.076 0.312 0.9997
RNa3 30 79 75.4 76.1 1.971 0.354 0.9998

Table 6 shows that the coverage values of bacterial abundance and diversity in all the samples exceed 0.9990, and the sequencing results can reflect the real situation of microorganisms. The Chao1 and Ace values of RNa3 1, RNa3 10, RNa3 20, and RNa3 30 bacteria showed volcanic trends, indicating that the bacterial abundance in the anaerobic digestion system increased in the first place. The maximum value of microbial diversity took place on the 10th day. The large number of microbial species was beneficial to the decomposition of organic matter in kitchen waste and promoted the transformation of insoluble organic matter into soluble organic matter. The organic acids generated reduced the pH of the system, consistent with the lowest pH at this stage. The Shannon index of RNa3 10 was increased by 0.124 and Simpson index was decreased by 0.005, indicating that the bacterial diversity reached a maximum on the 10th day. In this stage, the reaction substrate in the anaerobic digestion system was sufficient, and the chemical bonds in the kitchen waste could be destroyed by alkali in the pretreatment step. Thereby, the hydrolysis and acidification of the substrate were promoted, and the pH of the system was reduced. Later, due to the decomposition of the reaction substrate and lack of nutrients utilized by microorganisms, the number of microbial species was decreased, resulting in the decrease in Shannon index and increase in Simpson index. On the 30th day, the Shannon and Simpson indexes were 3.319 and 0.090, respectively.

Table 7 shows that the coverage values of abundance and diversity of archaea in all the samples also exceed 0.9990, and the sequencing results can represent the real situation of microorganisms. The Chao1 and Ace values of archaea in RNa3 1, RNa3 10, RNa3 20, and RNa3 30 increased to their maximum on the 20th day, and then declined. Different from the bacterial diversity (maximum on the 10th day), the diversity of archaea reached its maximum on the 20th day, indicating that the kitchen waste pretreated with alkali in the system was hydrolyzed and acidified by bacteria to produce organic acids in the first place, providing nutrients for archaea microorganisms for methanation. Compared to RNa3 1, the Shannon index of RNa3 20 was increased by 0.155 and Simpson index was decreased by 0.059, indicating that the diversity of archaea was the most abundant on the 20th day. At this stage, the reaction substrate in the anaerobic digestion system was sufficient. After the pretreatment with alkali, the chemical bonds in the kitchen waste were destroyed to promote the hydrolysis and acidification of the substrate. After the acidification reactions, the organic acids were utilized by archaea microorganisms to produce CH4, proving that the improving of efficiency and degree of hydrolysis and acidification is critical for rapid and thorough anaerobic digestion.

4 Conclusion

  1. The pretreatment with alkali reduced the lipid content in the kitchen waste and increased the content of dissolved organic matter, which can be done by chemical reaction, and promoted the conversion of part of the insoluble organic matter in the substrate to dissolved organic matter. After the pretreatment with 3% NaOH, the degradation rate of kitchen waste lipid reached its maximum of 50.51%. The SCOD content was increased by 235.3%, and the contents of soluble sugars, soluble proteins, and VFAs were increased to 155.3 ± 4.1, 17.4 ± 1.8, and 8.6 ± 1.1 g·L−1, respectively.

  2. The cumulative CH4 output and biogas production efficiency of kitchen waste pretreated with alkali in the anaerobic digestion process were improved. The maximum daily biogas output of kitchen waste pretreated with NaOH and KOH took place on the 11th to 12th day, earlier than the day when the maximum daily biogas output occurred in the control group. The biogas production efficiency was 40.4 and 45.2 mL·g·VS−1, respectively. The maximum daily biogas output of RCa1, RCa2, and RCa3 groups took place on the 13th day, and the cumulative CH4 output was increased to 373.5, 375.5, and 389.7 mL·g·VS−1, respectively.

  3. The coverage values of bacterial and archaea abundance and diversity in all the samples were higher than 0.9990, and the Chao1 and Ace values of bacteria were increased in the first place and then decreased. On the 10th day, the microbial diversity reached its maximum. The result indicates that increasing of efficiency and degree of hydrolysis and acidification is a crucial factor determining the rapid and thorough anaerobic digestion.

  1. Funding information: This work was funded by the National Natural Science Foundation of China (52206255), the Gansu Province College Youth Doctoral Fund Project (2022QB-069), the Youth Science and Technology Talent Lift Program of Gansu Province (GXH20220530-14), Science and Technology Commissioner Special Project of Gansu Province (22CX8GA061), and Tianyou Youth Talent Lift Program of Lanzhou Jiaotong University.

  2. Author contributions: Xiaofei Zhen: writing – original draft, writing – review and editing, methodology, and formal analysis; Shange Li: writing – original draft and formal analysis; Ruonan Jiao: writing – visualization and project administration; Wenbing Wu: resources; Ti Dong: methodology; and Jia Liu: data curation.

  3. Conflict of interest: The authors state no conflict of interest.

  4. Data availability statement: The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2023-04-24
Accepted: 2023-08-07
Published Online: 2023-10-25

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/gps-2023-0072
Keywords for this article
kitchen waste; pretreatment with alkali; anaerobic digestion; microbial diversity
Creative Commons
BY 4.0

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  84. Adsorption synergy electrocatalytic degradation of phenol by active oxygen-containing species generated in Co-coal based cathode and graphite anode
  85. Antibacterial, antifungal, antioxidant, and cytotoxicity activities of the aqueous extract of Syzygium aromaticum-mediated synthesized novel silver nanoparticles
  86. Synthesis of a silica matrix with ZnO nanoparticles for the fabrication of a recyclable photodegradation system to eliminate methylene blue dye
  87. Natural polymer fillers instead of dye and pigments: Pumice and scoria in PDMS fluid and elastomer composites
  88. Study on the preparation of glycerylphosphorylcholine by transesterification under supported sodium methoxide
  89. Wireless network handheld terminal-based green ecological sustainable design evaluation system: Improved data communication and reduced packet loss rate
  90. The optimization of hydrogel strength from cassava starch using oxidized sucrose as a crosslinking agent
  91. Green synthesis of silver nanoparticles using Saccharum officinarum leaf extract for antiviral paint
  92. Study on the reliability of nano-silver-coated tin solder joints for flip chips
  93. Environmentally sustainable analytical quality by design aided RP-HPLC method for the estimation of brilliant blue in commercial food samples employing a green-ultrasound-assisted extraction technique
  94. Anticancer and antimicrobial potential of zinc/sodium alginate/polyethylene glycol/d-pinitol nanocomposites against osteosarcoma MG-63 cells
  95. Nanoporous carbon@CoFe2O4 nanocomposite as a green absorbent for the adsorptive removal of Hg(ii) from aqueous solutions
  96. Characterization of silver sulfide nanoparticles from actinobacterial strain (M10A62) and its toxicity against lepidopteran and dipterans insect species
  97. Phyto-fabrication and characterization of silver nanoparticles using Withania somnifera: Investigating antioxidant potential
  98. Effect of e-waste nanofillers on the mechanical, thermal, and wear properties of epoxy-blend sisal woven fiber-reinforced composites
  99. Magnesium nanohydroxide (2D brucite) as a host matrix for thymol and carvacrol: Synthesis, characterization, and inhibition of foodborne pathogens
  100. Synergistic inhibitive effect of a hybrid zinc oxide-benzalkonium chloride composite on the corrosion of carbon steel in a sulfuric acidic solution
  101. Review Articles
  102. Role and the importance of green approach in biosynthesis of nanopropolis and effectiveness of propolis in the treatment of COVID-19 pandemic
  103. Gum tragacanth-mediated synthesis of metal nanoparticles, characterization, and their applications as a bactericide, catalyst, antioxidant, and peroxidase mimic
  104. Green-processed nano-biocomposite (ZnO–TiO2): Potential candidates for biomedical applications
  105. Reaction mechanisms in microwave-assisted lignin depolymerisation in hydrogen-donating solvents
  106. Recent progress on non-noble metal catalysts for the deoxydehydration of biomass-derived oxygenates
  107. Rapid Communication
  108. Phosphorus removal by iron–carbon microelectrolysis: A new way to achieve phosphorus recovery
  109. Special Issue: Biomolecules-derived synthesis of nanomaterials for environmental and biological applications (Guest Editors: Arpita Roy and Fernanda Maria Policarpo Tonelli)
  110. Biomolecules-derived synthesis of nanomaterials for environmental and biological applications
  111. Nano-encapsulated tanshinone IIA in PLGA-PEG-COOH inhibits apoptosis and inflammation in cerebral ischemia/reperfusion injury
  112. Green fabrication of silver nanoparticles using Melia azedarach ripened fruit extract, their characterization, and biological properties
  113. Green-synthesized nanoparticles and their therapeutic applications: A review
  114. Antioxidant, antibacterial, and cytotoxicity potential of synthesized silver nanoparticles from the Cassia alata leaf aqueous extract
  115. Green synthesis of silver nanoparticles using Callisia fragrans leaf extract and its anticancer activity against MCF-7, HepG2, KB, LU-1, and MKN-7 cell lines
  116. Algae-based green AgNPs, AuNPs, and FeNPs as potential nanoremediators
  117. Green synthesis of Kickxia elatine-induced silver nanoparticles and their role as anti-acetylcholinesterase in the treatment of Alzheimer’s disease
  118. Phytocrystallization of silver nanoparticles using Cassia alata flower extract for effective control of fungal skin pathogens
  119. Antibacterial wound dressing with hydrogel from chitosan and polyvinyl alcohol from the red cabbage extract loaded with silver nanoparticles
  120. Leveraging of mycogenic copper oxide nanostructures for disease management of Alternaria blight of Brassica juncea
  121. Nanoscale molecular reactions in microbiological medicines in modern medical applications
  122. Synthesis and characterization of ZnO/β-cyclodextrin/nicotinic acid nanocomposite and its biological and environmental application
  123. Green synthesis of silver nanoparticles via Taxus wallichiana Zucc. plant-derived Taxol: Novel utilization as anticancer, antioxidation, anti-inflammation, and antiurolithic potential
  124. Recyclability and catalytic characteristics of copper oxide nanoparticles derived from bougainvillea plant flower extract for biomedical application
  125. Phytofabrication, characterization, and evaluation of novel bioinspired selenium–iron (Se–Fe) nanocomposites using Allium sativum extract for bio-potential applications
  126. Erratum
  127. Erratum to “Synthesis, characterization, and evaluation of nanoparticles of clodinofop propargyl and fenoxaprop-P-ethyl on weed control, growth, and yield of wheat (Triticum aestivum L.)”
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