In order to solve the bottleneck of low methane production in anaerobic codigestion of excess sludge (ES) and plant waste (PW), a new strategy of enhancing hydrolysis and acidification by rhamnolipid (RL) was proposed under thermophilic condition. The results showed that the optimal dosage of RL was 50 g/kg total suspended solids, and the maximum yield of methane was 198.5 mL/g volatile suspended solids (VSS), which was 2.3 times of that in the control. RL promoted the dissolution of organic matter in the codigestion process of ES and PW, and the higher the dosage of RL, the higher the concentration of soluble chemical oxygen demand (SCOD) in the fermentation broth. When RL was 100 g/kg, the maximum content of SCOD in fermentation broth was 2,451 mg/L, and the contents of soluble protein and polysaccharide were 593 mg/L and 419 mg/L on 10 d, respectively, which were significantly higher than other groups. In addition, the yield of VFA in RL group was also significantly increased, and acetate and propionate were the main components of VFAs. This research work provides data support for the resource utilization of ES and PW, and expands the application field of RL.

  • Excess sludge and plant waste are codigested to produce methane.

  • Biosurfactant rhamnolipid enhanced the codigestion of ES and PW for methane production.

  • RL increased the release of dissolved organic matter in the codigestion system.

  • RL increased the accumulation of VFA during the codigestion of ES and PW.

Excess sludge (ES), as the main by product in the process of sewage treatment, has the characteristics of large output and high pollutant content (Wei et al. 2018). If ES is not disposed of effectively, it will pose a great threat to the ecological environment (Li et al. 2020). Compared with landfill or incineration, anaerobic digestion technology has the advantages of economic, environmental protection and sustainable development (Dohányos et al. 2004; Gao et al. 2021). ES anaerobic digestion can effectively reduce the volume of volatile organic compounds, kill harmful pathogens in ES, and obtain energy materials. Methane is an important energy; its effective utilization can reduce the mining and consumption of oil, coal and other chemical raw materials to a certain extent, which is of great significance for the protection of the ecological environment. In recent years, how to enhance the methane production from ES anaerobic digestion has aroused extensive interest (Feng et al. 2014; Kor-Bicakci & Eskicioglu 2019; Zhang et al. 2021).

Carbon nitrogen ratio (C/N) is a key parameter affecting methane accumulation in sludge anaerobic digestion (Dai et al. 2016; Hassan et al. 2016; Kuang et al. 2020a). It is generally considered that the suitable range for ES anaerobic fermentation is 20 ∼ 30/1, while the C/N ratio in ES is only about 7/1, which is far lower than the ideal value. The codigestion of ES with other carbon rich wastes can regulate C/N ratio, enrich microbial community structure, dilute toxic and harmful substances, and thus is favored in practical engineering (Kangle et al. 2012; Xie et al. 2016; Dong et al. 2019). Kitchen waste is usually used to control the ratio of carbon to nitrogen in anaerobic digestion of sludge. However, the actual composition of kitchen waste is complex, especially as it contains a lot of salt and oil, and high salinity and oil have a serious inhibitory effect on anaerobic microbial activities (Awe et al. 2018). Therefore, many scholars have explored that organic matter rich in carbon sources and less toxic substances are used to regulate ES anaerobic digestion.

Plant waste (PW) is an important component of municipal solid waste, especially in the garden workstation. PW is rich in carbon sources and has few endotoxic substances, so PW is an ideal substrate for codigestion with ES (Huang et al. 2020; Lin et al. 2020). Our previous work also confirmed that codigestion of PW and ES can enhance methane production, and the optimal mixing ratio was 1/1 (based on total suspended solids) (Wang et al. 2021a). The production of methane by ES-PW codigestion is limited by the extracellular polymerization in ES and the refractory lignin and hemicellulose in PW. Therefore, a strategy to enhance the anaerobic cofermentation of ES and PW to produce methane needs to be explored.

Rhamnolipid (RL) is an economical and environmentally friendly anionic biosurfactant, and RL is mainly produced by Pseudomonas aeruginosa (Kiran et al. 2016; Eslami et al. 2020). RL has both hydrophilic and lipophilic properties, and RL can significantly reduce the surface tension of the interface and improve the dispersion of substances. The stability of rhamnolipid is good, and it can maintain the surface activity in a wide range of temperature and pH. Even when the temperature was as high as 120 °C, RL could maintain the surface activity for at least 15 min (Zeng et al. 2018). RL is cheap to obtain, and as a biological agent, it can be quickly biodegraded, so it would not cause secondary harm to the ecological environment. In previous studies, RL was used to remove heavy metals from sludge and enhance anaerobic fermentation of sludge (Mulligan et al. 2001; He et al. 2016; Xu et al. 2018), while RL was rarely used to improve the codigestion of ES and PW for enhanced methane production.

Therefore, in this work, we first evaluated the effect of RL on the methane production of ES and PW anaerobic codigestion. Then the influence mechanism of RL on organic matter reduction rate, hydrolysis and acidification in the anaerobic codigestion process was analyzed. Finally, the practical engineering significance of RL-enhanced anaerobic digestion of organic matter is expounded. This research work has a certain guiding significance for the resource utilization of ES and PW, and expands the application field of surfactant RL.

Experimental materials

The ES used in this study is the waste activated sludge from the secondary sedimentation tank of a sewage treatment plant in Nanjing. The ES is filtered to remove the large particles and then precipitated naturally for 24 hours. Then the supernatant of the precipitated sludge is removed manually and the ES obtained is stored in the refrigerator at 4 °C for standby. The total suspended solids (TSS) and volatile suspended solids (VSS) of ES used in this work were 31,500 ± 420 mg/L and 18,500 ± 380 mg/L, respectively.

PW comes from a flower market in Changzhou. PW is mainly composed of branches and leaves of abandoned flowers (roses, tulips, lilies, etc.). First of all, waste such as glass, slag, sand and stone in PW were selected and removed manually. Then the PW was broken to less than 2.0 mm in diameter by mechanical pulverizer. Thirdly, an appropriate amount of deionized water was added to PW to increase the water content. The moisture content of PW was 8.5%, the content of TSS was 541 g/L, and the content of VSS was 348 g/L. Finally, PW and ES were mixed by 1/1 mass ratio, and stirred fully with a stirrer to ensure the homogeneity of the digestion substrate.

The inoculated sludge is taken from the mesophilic anaerobic digestion tank in the laboratory, which mainly deals with urban excess sludge, and the inoculated sludge has good gas production performance. The main characteristics of the inoculated sludge are shown in Table 1.

Table 1

Main characteristics of inoculated sludge and digested substrate used in the experimenta

ItemUnitInoculated sludgeDigested substrate
pH 6.8 ± 0.1 7.1 ± 0.1 
SCOD mg/L 125 ± 5.6 258 ± 10 
SCFA mg/L 85.6 ± 3.2 106 ± 5.2 
C/N 7.2 ± 0.2 23.6 ± 0.3 
Soluble protein mg/L 62 ± 2.1 102 ± 4.6 
Soluble polysaccharide mg/L 35 ± 2.1 115 ± 5.9 
ItemUnitInoculated sludgeDigested substrate
pH 6.8 ± 0.1 7.1 ± 0.1 
SCOD mg/L 125 ± 5.6 258 ± 10 
SCFA mg/L 85.6 ± 3.2 106 ± 5.2 
C/N 7.2 ± 0.2 23.6 ± 0.3 
Soluble protein mg/L 62 ± 2.1 102 ± 4.6 
Soluble polysaccharide mg/L 35 ± 2.1 115 ± 5.9 

aThe data presented in the table represent the mean value and the deviation of the three determinations.

RL was purchased from a biochemical reagent company in Shanghai, and its purity is as high as 95%. The critical micelle concentration was 5.76 × 10−5 mol/L, the hydrophilic lipophilic equilibrium value was 11.2. RL is an oil in water emulsion with pH 6–7.

RL enhanced anaerobic codigestion of ES and PW

The dosage of magnetite is the key to anaerobic digestion. Therefore, this study first optimized the effect of RL on the codigestion. First, 10.0 L of ES and PW mixture (1/1 by mass) was evenly distributed into five reactors. The anaerobic reactor is made of resin glass, and the upper part is equipped with a gas collecting bag, and the feed and discharge ports are arranged on both sides. 0, 10, 20, 50, and 100 g/kg RL (calculated by total TSS content) were added to each reactor, and the above reactors were named R1-R5. Then, the reactor is filled with high-purity nitrogen to discharge oxygen to ensure anaerobic conditions, and then the reactor is quickly closed and transferred to the water bath constant temperature (50 °C) oscillator for the anaerobic digestion test. The whole experimental process lasted for 30 days, and the parameters related to anaerobic digestion were detected every day to evaluate the effect of RL on the codigestion process. All the tests were repeated three times to ensure the accuracy of the experimental data.

Analysis method

COD, SCOD, and orthophosphate were determined by international standard method. Ammonia nitrogen (NH4+-N) was determined by Nessler's reagent method. TSS and VSS were determined by gravity burning method. The solubility indexes determined in this study need to pass 0.45 μM membrane filtration before determination. VFA was determined by gas chromatograph (Shimadzu GC2010). The carrier gas was nitrogen and the detector was a hydrogen flame detector. The temperatures of the injector and detector were controlled at 250 °C and 300 °C respectively. Methane was determined by gas chromatography. The column was a stainless-steel packed column (2.0 m). The detector was a thermal conductivity detector (TCD). High purity argon was used as the carrier gas. The temperatures of the injection port, column box and detector were controlled at 180 °C, 150 °C and 180 °C, respectively.

Statistical analysis

All assays were performed in triplicate. All data were counted in Excel 2020, and the figures were drawn with Origin 8.5. Values less than 5% were considered statistically significant.

Effect of RL on the methane production from anaerobic codigestion substrate

Methane is considered to be the end product of organic matter anaerobic digestion, and the cumulative methane yield can indicate the anaerobic digestion performance of organic matter (Wang et al. 2020). As shown in Figure 1, in the control group, the methane yield increased first and then stabilized with time, and the maximum cumulative yield was 156.2 mL/g VSS. When RL was added to the codigestion system, methane production increased significantly, and the increase was closely related to RL dosage. When RL was low dose, i.e. 10 g/kg, the cumulative methane production increased to 184.2 mL/g VSS, which was about 1.1 times that of the control group. The results showed that low dose RL had significantly increased methane production. In R3, the cumulative methane production increased to 198.5 mL/g VSS, reaching the maximum in this work. However, when RL further increased to 100 g/kg, the cumulative methane production decreased slightly, but still was significantly higher than that of the control group. The results showed that RL could promote the anaerobic codigestion of ES and PW, and the optimal dose of RL was 50 g/kg, and the optimal cumulative methane production was 198.5 mL/g VSS.

Figure 1

Effect of RL on the cumulative methane production in the codigestion system. The error bar represents the mean value of three determinations plus standard error.

Figure 1

Effect of RL on the cumulative methane production in the codigestion system. The error bar represents the mean value of three determinations plus standard error.

RL also affected the volume ratio of methane in the biogas. As shown in Figure 2, the volume fraction of methane in the control first increased with time, and accounted for about 45–56% in the stable period. However, RL increased the volume fraction of methane in the stable period. For example, in R2, the volume fraction of methane in the stable period increased to 51.6–52.3%, which was slightly better than that in the control group. RL, as a surfactant, can enhance the utilization of organic matter by microorganisms, thus increasing the proportion of methane in the mixture (Huang et al. 2015).

Figure 2

Effect of RL on the methane volume percentage in the codigested biogas. The error bar represents the mean value of three determinations plus standard error.

Figure 2

Effect of RL on the methane volume percentage in the codigested biogas. The error bar represents the mean value of three determinations plus standard error.

Effect of RL on the organic matter reduction in the codigestion system

The reduction rate of organic matter in the codigestion process is a key parameter to evaluate anaerobic efficiency (Duan et al. 2012; Romero-Güiza et al. 2016; Pei et al. 2020; Lin et al. 2021; Wang et al. 2021b). As shown in Figure 3, the reduction rate of VSS in each reactor first increased with time, and the change of VSS was not very significant after about 20 days, which was consistent with the cumulative methane production in Figure 1. The presence of RL had a significant effect on the reduction of VSS in the codigested matrix, and RL increased the reduction rate of VSS. In the first 5 days, there was no significant difference in the reduction rate of VSS in each group (p>0.05), which may be due to the low efficiency of organic matter hydrolysis and acidification. In the subsequent test, RL significantly improved the reduction rate of VSS, and the greater the dosage of RL, the greater the reduction rate of VSS. In the control, the maximum reduction rate of VSS was 25.3%, while when RL was 10 g/kg, the VSS reduction rate increased to 27.3%, slightly higher than that of the control, while when RL was increased to 100 g/kg, the VSS reduction rate was increased to 29.8%, significantly higher than that of the control (p<0.05). RL can promote the release of intracellular organic matter, and then be consumed by hydrolytic acidification enzymes to achieve sludge reduction (Luo et al. 2013; Xu et al. 2018). In addition, RL can promote the degradation of cellulose and lignin in PW, which can also strengthen the VSS reduction of PW. The enhanced reduction of VSS with RL in the codigestion matrix is of great significance to the management and control of organic waste (Zheng et al. 2021a). On the one hand, it can reduce the occupation of limited land resources; on the other hand, it can reduce the cost of subsequent biogas residue treatment.

Figure 3

Effect of RL on the reduction of VSS in the codigestion system. The error bar represents the mean value of three determinations plus standard error.

Figure 3

Effect of RL on the reduction of VSS in the codigestion system. The error bar represents the mean value of three determinations plus standard error.

Effect of RL on the organic matter release during the codigestion process

Figure 4 shows the release of SCOD in each reactor in the presence of different doses of RL. It was found that RL also increased the concentration of SCOD in the reactor, and when RL was 10 g/kg, the maximum value of SCOD was 1,621 mg/L, which was about 1.1 times of the blank group. When RL increased to the maximum value of 100 g/kg in this study, the maximum value of SCOD also increased to 2,451 mg/L, which was about 1.7 times the control. These experiments confirmed that the more RL was added, the more organic matter was released during the ES and PW codigestion process. The release of SCOD can indicate the hydrolysis efficiency to a certain extent (Ebenezer et al. 2015; Zhao et al. 2021). RL can promote the cleavage of EPS in sludge and release intracellular polymer to supply water for enzyme consumption (Yi et al. 2013; Li et al. 2019b). In addition, RL as a biosurfactant can enhance the degradation of lignin, which also increased the concentration of SCOD. Protein and polysaccharide are the typical organic matter in ES. The effect of RL on soluble protein and polysaccharide in fermentation broth was also studied. As shown in Table 2, the content of soluble protein and polysaccharide increased with the increase of RL dosage. For example, on 10 d, the concentrations of soluble protein and polysaccharide in the control group were 452 mg/L and 356 mg/L, respectively. When RL was 10 g/kg, the concentrations of soluble protein and polysaccharide increased significantly to 485 mg/L and 389 mg/L. When RL was further increased to 100 g/kg, the concentrations of soluble protein and polysaccharide increased to 593 mg/L and 419 mg/L, the maximum values of all reactors. These results are consistent with the change of SCOD, indicating that RL promoted the release of dissolved organic matter in ES or PW, and then increases the consumption of available organic matter by methanogenic microorganisms. Table 3 further compared the results of RL-enhanced anaerobic digestion of organic matter in different literatures. It can be clearly found that RL alone or in combination with other pretreatment methods was of positive significance to the anaerobic digestion process and promoted the release of SCOD. The enhancement of the dissolution process played a positive role in the subsequent production of methane. In addition, the application of RL to enhance the codigestion of ES and PW was investigated for the first time in this work.

Table 2

Changes of soluble protein and polysaccharide in the presence of RLa

RL/(g·kg−1)Soluble organic matter/(mg·L−1)
Soluble proteinSoluble polysaccharide
452 ± 12 356 ± 17 
10 485 ± 10 389 ± 10 
20 526 ± 9 401 ± 18 
50 581 ± 15 415 ± 15 
100 593 ± 16 419 ± 12 
RL/(g·kg−1)Soluble organic matter/(mg·L−1)
Soluble proteinSoluble polysaccharide
452 ± 12 356 ± 17 
10 485 ± 10 389 ± 10 
20 526 ± 9 401 ± 18 
50 581 ± 15 415 ± 15 
100 593 ± 16 419 ± 12 

aThe sampling time of the reported data was 10 d.

Table 3

Comparison of RL improving the degree of anaerobic digestion of organic waste in different literature

Digestive substrateRL dosageOperating conditionsMain conclusionsReference
Waste activated sludge 0.04 g/g TSS Laboratory scale batch experiment, pH 6.8–7.2, 30 ± 2 °C Dissolved organic matter was significantly improved, SCOD was increased about 6 times Li et al. (2019a)  
Pulp and paper mill waste biosolid 0.009 g/g TS pH 10, 36 ± 1 °C, combined with disperser RLD dose (0.009 g/g TS) with pH 10 had a significant effect on PPST sludge Sethupathy et al. (2020)  
Waste activated sludge 0.04 g/g TSS Batch laboratory-scale test, 35 ± 1 °C, 100 rpm. RL increased the release of dissolved organic matter, and the maximum yield of VFA was 3840 mg COD/L Zhou et al. (2013)  
Waste activated sludge 0.3 g/g SS Batch experiments in clean flasks, 35 ± 1 °C Lysozyme and RL combined treatment could significantly promote ES hydrolysis and decomposition Liu et al. (2019)  
Waste activated sludge 0.2 g/ g TSS Laboratory scale batch test, 35 ± 1 °C, 100 rpm. RL + initial pH10 caused positive synergies on anaerobic fermentation process He et al. (2016)  
ES and PW 100 g/Kg TSS Laboratory scale batch experiment, 50 °C RL enhanced the release of SCOD and was about 1.7 times that of the blank group This study 
Digestive substrateRL dosageOperating conditionsMain conclusionsReference
Waste activated sludge 0.04 g/g TSS Laboratory scale batch experiment, pH 6.8–7.2, 30 ± 2 °C Dissolved organic matter was significantly improved, SCOD was increased about 6 times Li et al. (2019a)  
Pulp and paper mill waste biosolid 0.009 g/g TS pH 10, 36 ± 1 °C, combined with disperser RLD dose (0.009 g/g TS) with pH 10 had a significant effect on PPST sludge Sethupathy et al. (2020)  
Waste activated sludge 0.04 g/g TSS Batch laboratory-scale test, 35 ± 1 °C, 100 rpm. RL increased the release of dissolved organic matter, and the maximum yield of VFA was 3840 mg COD/L Zhou et al. (2013)  
Waste activated sludge 0.3 g/g SS Batch experiments in clean flasks, 35 ± 1 °C Lysozyme and RL combined treatment could significantly promote ES hydrolysis and decomposition Liu et al. (2019)  
Waste activated sludge 0.2 g/ g TSS Laboratory scale batch test, 35 ± 1 °C, 100 rpm. RL + initial pH10 caused positive synergies on anaerobic fermentation process He et al. (2016)  
ES and PW 100 g/Kg TSS Laboratory scale batch experiment, 50 °C RL enhanced the release of SCOD and was about 1.7 times that of the blank group This study 
Figure 4

Effect of RL on the SCOD release in the codigestion system. The error bar represents the mean value of three determinations plus standard error.

Figure 4

Effect of RL on the SCOD release in the codigestion system. The error bar represents the mean value of three determinations plus standard error.

Effect of RL on VFA production in the codigestion system

VFA, the intermediate value product of the organic anaerobic fermentation process, is the preferred carbon source for biological nitrogen and phosphorus removal, and it is also the raw material for the production of bioplastics (Kuang et al. 2020b; Liu et al. 2020). The effect of RL on VFA production during the codigestion of ES and PW is shown in Figure 5. In all reactors, VFA production increased sharply in the first 8 days, and remained in the fluctuation state in the following time. Different from the change of methane, the more RL was added, the more VFA was accumulated. In R5, the dosage of RL was 100 g/kg, and the maximum yield of VFA was 694 mg/L, which was about 1.8 times of the control group. Zhou et al. (2013) also confirmed that RL could enhance the accumulation of VFA in sludge anaerobic fermentation, and when RL dosage was 0.04 g/g TSS, the maximum yield of VFA was 3,840 mg/L, which was significantly higher than the value in this study. This difference may be due to the presence of PW, which is difficult to degrade in the digestive substrate in this study, and the accumulation of VFA is limited due to the presence of a large number of PW. VFA explored in this work mainly contained C2-C5 carboxylic acids. The influence of RL on various components of carboxylic acids is shown in Figure 5(b). It can be seen clearly that acetate and propionate were the main organic acids in each reactor, and the proportion is about 65.4–72.5%. Further study found that RL can enhance the conversion of high carbon carboxylate to low carbon carboxylate. In particular, RL increased the proportion of acetate in mixed carboxylate. In the control group, the proportion of acetate was about 42.3%, and when RL was 100 g/kg, the proportion of acetate increased to 47.5%. Previous studies have shown that acetate or hydrogen can be consumed in methanogenesis, of which acetate accounts for a large proportion (Zhang et al. 2014; Zheng et al. 2021b). In this study, RL increased the proportion of acetate in the mixed carboxylate, which was beneficial to the subsequent activity of methanogenic microorganisms.

Figure 5

Effects of RL on VFA production and the individual components in the codigestion system. The error bar represents the mean value of three determinations plus standard error.

Figure 5

Effects of RL on VFA production and the individual components in the codigestion system. The error bar represents the mean value of three determinations plus standard error.

Figure 6

Effect of RL on ammonia nitrogen release from ES and PW anaerobic codigestion. The error bar represents the mean value of three determinations plus standard error.

Figure 6

Effect of RL on ammonia nitrogen release from ES and PW anaerobic codigestion. The error bar represents the mean value of three determinations plus standard error.

Effect of RL on ammonia nitrogen release from ES and PW anaerobic codigestion

Ammonia nitrogen is a by-product of anaerobic digestion of nitrogenous organic matter. Previous studies have confirmed that ammonia nitrogen concentration below 200 mg was beneficial to the anaerobic digestion process, and there was no antagonistic effect when the concentration was 200–1,000 mg/L, but more than 3,000 mg/L, ammonia nitrogen would have a serious growth inhibition effect on anaerobic microorganisms (Angelidaki & Ahring 1993; Procházka et al. 2012). As shown in Figure 6, the concentration of ammonia nitrogen in each reactor increased first and then stabilized with the digestion time. RL had a significant effect on the release of ammonia nitrogen and the concentration of ammonia nitrogen in the RL group was higher than that in the control group. In the reactor with 100 g/kg RL, the concentration of ammonia nitrogen in stable period was about 249–261 mg/L, which was significantly higher than that in the other groups. To a certain extent, the release of ammonia nitrogen can regulate the hydrolysis process of organic matter (Morgenroth et al. 2002). The greater the release of ammonia nitrogen, the greater the degree of hydrolysis. The experimental results are consistent with the changes of SCOD in Figure 1, which confirmed that RL can promote the release of dissolved organic matter in the codigestion system of ES and PW, which also provides sufficient material guarantee for the subsequent methanogenesis process.

The significance of RL application to practical engineering

RL, as an environmentally friendly biosurfactant, has been applied to the removal of heavy metals in ES, enhanced sludge dewatering and acid production from anaerobic fermentation. However, it is rarely reported that RL has been used to enhance the anaerobic codigestion of ES and PW to enhance methane production. This study fills the gap in this field, broadens the application field of RL, and enriches the resource utilization strategy of ES and PW.

The results of this work showed that RL significantly increased the release of dissolved organic matter in ES and PW, and the higher the RL dosage, the higher the concentration of soluble COD in fermentation broth. When the dose of RL was 50 g/kg, the maximum methane production was 198.5 mL/g VSS, which was significantly higher than that of the control group, and further increasing RL had no significant effect on the increase of methane. RL dosage is appropriate, so it has a certain economy. Pseudomonas aeruginosa can produce rhamnolipids from renewable and economical substrates, such as catering waste oil, oil factory wastewater and oil cake powder (Makkar & Cameotra 1999; Patowary et al. 2019). Zhou et al. (2013) pointed out that rhamnolipid can be produced in situ in waste activated sludge. The above discussion reveals that the acquisition of RL is relatively cheap, and RL can enhance the anaerobic codigestion of ES and PW to obtain energy material methane. The obtained methane can generate electricity after purification, so it has certain economic value. In addition, RL strengthens the codigestion process of ES and PW, reduces the volume of organic waste, and has certain ecological value. Therefore, the application of RL to enhance the codigestion of ES and PW has certain economic and ecological advantages. However, RL is difficult to produce in large scale, so future research can focus on promoting production and reducing costs of RL.

RL, an environmentally friendly surfactant, was first used to enhance the anaerobic codigestion of ES and PW. When the dosage of RL was 50 g/kg, the maximum methane production in the codigestion system was 198.5 mL/g VSS. Mechanism investigation showed that RL promoted the release of dissolved organic matter in the codigestive system. In addition, RL promoted the reduction of VSS during the codigestion of ES and PW. When RL was 100 g/kg, the maximum value of SCOD was 2,451 mg/L, significantly higher than the other groups. RL also strengthened the accumulation of VFA, promoted the conversion of high-carbon organic acids to low-carbon organic acids, and increased the proportion of acetic acid in VFA.

Yongliang Wang: Conceptualization, Writing- original draft, Writing – review and editing. Xiaohui Zhou: Investigation, Validation, Writing – review and editing. Bin Dai: Writing – review and editing. Xiaoqiang Zhu: Investigation, Validation.

The authors declare that they have no competing interests.

This work was financially supported by the special project of talent introduction of Hebei Agricultural University (YJ2020043).

Not applicable.

Not applicable.

All relevant data are included in the paper or its Supplementary Information.

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