The composition of waste-activated sludge (WAS) is complex, containing a large amount of harmful substances, which pose a threat to the environment and human health. The reduction and resource utilization of sludge has become a development demand in sludge treatment and disposal. Based on the technical bottlenecks in the practical application of direct anaerobic digestion technology, this study adopted two different thermal and thermal–alkali hydrolysis technologies to pretreat sludge. A pilot-scale experiment was conducted to investigate the experimental conditions, parameters, and effects of two hydrolysis technologies. This study showed that the optimal hydrolysis temperature was 70 °C, the hydrolysis effect and pH can reach equilibrium with the hydrolysis retention time was 4–8 h, and the optimal alkali concentration range was 0.0125–0.015 kg NaOH/kg dry-sludge. Thermal–alkali combination treatment greatly improved the performance of methane production, the addition of NaOH increased methane yield by 31.2% than that of 70 °C thermal hydrolysis. The average energy consumption is 75 kWh/m3 80% water-content sludge during the experiment. This study provides a better pretreatment strategy for exploring efficient anaerobic digestion treatment technologies suitable for southern characteristic sewage sludge.

  • Enhanced anaerobic digestion of WAS by thermal–alkali pretreatment was studied at a pilot-scale level.

  • Optimal thermal temperature and retention time were confirmed for a pilot-scale study.

  • Energy demand can be basically met based on biogas production.

In recent years, with the acceleration of urbanization in China, the collection and treatment rates of municipal sewage have been continuously improving. As the byproduct of sewage treatment, the production of waste-activated sludge (WAS) has also increased significantly. It is expected that the scale of urban sewage treatment in China will exceed 2.2 × 108 t/d by 2025, and the corresponding sludge amount will exceed 6.6 × 107 t/year (calculated based on a moisture content of 80%) (Liu et al. 2022; Dai et al. 2023). The cost of treatment and disposal of excess sludge accounts for 60% of the total operating cost of wastewater treatment plants (WWTPs) (Murthy et al. 2006). The composition of sewage sludge is extremely complex, containing a large amount of harmful substances (such as pathogenic microorganisms, toxic organic matter, heavy metals, and lots of refractory substances). It may cause great harm to environment and human health.

On the other hand, sludge contains abundant nutrients, which can be used as fertilizer or produce biogas after anaerobic digestion (AD). The main disposal methods for sludge include landfill, incineration, composting, AD, etc. In the United States and EU member states, anaerobic sludge digestion technology has been widely used, and China also encourages the use of AD as the main treatment method for sewage sludge (Zhang et al. 2015; Li et al. 2020; Liang et al. 2021). However, the direct anaerobic sludge digestion process has many drawbacks, such as large land area needed, low gas production rate, and high investment and operating costs.

In order to improve the performance of AD and cell lysis efficiency of WAS, sludge pretreatment technology has received extensive research and attention (Carrere et al. 2010; Wu et al. 2017; Li et al. 2019). Li et al. used ultrasound as a pretreatment method for AD of sludge and studied the AD performance and microbial population changes of WAS (Li et al. 2018). Yu et al. used hypochlorite as an auxiliary agent and Ti/RuO2 mesh electrode for electrochemical pretreatment of excess sludge (Yu et al. 2014). They examined the performance of electrochemical pretreatment in enhancing AD of WAS. They found that compared with the control group, the electrochemical pretreatment improved the methane production performance per unit of organic matter in AD by 63.4%, and the volatile solids (VS) removal rate reached 45.5% (Yu et al. 2014). Zhao et al. found that the abundance of hydrogen autotrophic methane bacteria significantly decreased after adding Fe3O4 and zero valent iron (ZVI) into the AD reactor. Mononas and methane oxidase were particularly enriched in the digestion reactor after adding Fe3O4, indicating that potential interspecies electron transfer may be a key factor in accelerating anaerobic sludge digestion (Zhao et al. 2018). Wu et al. found that short-time free nitrous acid (FNA) pretreatment can disintegrate the structure of extracellular polymeric substances (EPS) and improve the release of bound water (Wu et al. 2018). It was also found that the sludge dewaterability was enhanced by FNA pretreatment during AD (Zhang et al. 2019).

In addition, many researchers have focused on heat–alkaline pretreatment of WAS. The effect of heat–alkaline treatment (HAT) on volatile fatty acid (VFA) production and protein degradation were investigated at pH 11 and 60 °C in excess sludge, soluble and insoluble proteins, and pure cultures (Tan et al. 2012). The effect of ultrasound, low-temperature thermal, and alkali pretreatment on the rheology, hygienization, and methane production of WAS was comparatively examined, which indicated that ultrasound and alkali treatments resulted in higher costs when considering the results based on an energy balance (Ruiz-Hernando et al. 2014). Oh et al. studied the effects of ultrasonic vs. combined heat/alkali sludge pretreatment methods on electricity generation in microbial fuel cells (MFCs), and found that the electricity generation was linearly proportional to the SCOD removal (Oh et al. 2014). Furthermore, the performance of anaerobic sludge digestion with electrochemical and sodium hypochlorite combination pretreatment was evaluated by VS reduction and methane production at a pilot-scale level (Yuan et al. 2016). Most of the researches have confirmed the validity of sludge heat–alkaline pretreatment in improving cell lysis and methane potential. However, most researches on sludge pretreatment technology are still in laboratory-scale, and a few pilot-scale studies on enhanced AD of sludge by heat or alkali pretreatment are shown in Table 1.

Table 1

Pilot-scale study on enhanced anaerobic digestion of sludge by heat or alkali pretreatment

SubstrateTreatment conditionsAD conditionsResultsReference
Waste-activated sludge Electrolysis time: 45 min, voltage: 200 V, current: 5 A; NaClO reagent: 0.4% (v/v) Batch, 21 days
35 °C 
VS reduction: 31.05%, CH4 production increased by 1.89 times on 20 days Yuan et al. (2016)  
Sewage sludge Two conditions: 180 °C/30 min and 160 °C/60 min, pH = 10.0 (hydrothermal and alkaline hydrothermal pretreatments Batch, 9 days
50 °C 
Solid content: 67 and 69%;
Methane yield: 275 ml/g COD and 290 ml/g COD 
Li et al. (2017)  
Sewage sludge (low organic) Bio-thermophilic reactor: 4 days of SRT, 55 °C CSTR, 16 days
35 °C 
VS degradation rate and CH4 yield increased by 19.93% and 53.33% Wang et al. (2025)  
SubstrateTreatment conditionsAD conditionsResultsReference
Waste-activated sludge Electrolysis time: 45 min, voltage: 200 V, current: 5 A; NaClO reagent: 0.4% (v/v) Batch, 21 days
35 °C 
VS reduction: 31.05%, CH4 production increased by 1.89 times on 20 days Yuan et al. (2016)  
Sewage sludge Two conditions: 180 °C/30 min and 160 °C/60 min, pH = 10.0 (hydrothermal and alkaline hydrothermal pretreatments Batch, 9 days
50 °C 
Solid content: 67 and 69%;
Methane yield: 275 ml/g COD and 290 ml/g COD 
Li et al. (2017)  
Sewage sludge (low organic) Bio-thermophilic reactor: 4 days of SRT, 55 °C CSTR, 16 days
35 °C 
VS degradation rate and CH4 yield increased by 19.93% and 53.33% Wang et al. (2025)  

The aim of this study is to comparatively investigate the validity of heat and heat–alkali pretreatment technologies for improving the performance of subsequent anaerobic sludge digestion in pilot scale, and to study the optimal experimental parameters for AD reactor operation and the corresponding conditions of heat–alkali treatment, and to provide efficient AD pretreatment technologies suitable for southern characteristic sewage sludge.

Experimental sludge and heat source

The WAS was obtained from a local WWTP in Guangzhou, the main physicochemical properties of sludge from sludge thickened tank of WWTP are shown in Table 2. Also, the thickened sludge was dewatered to moisture content of about 80% in the plant for easily transferred to experimental site of Zhongshan Mindong Organic Waste treatment Co., Ltd. The inoculum sludge were taken from the AD tank of the company for sludge digestion. Using 99% NaOH (Macklin) for alkaline treatment, the heat source is provided by municipal electricity.

Table 2

Main physicochemical properties of waste-activated sludge

ParameterspHTS (g/L)VS (g/L)CST (s)Zeta potential (mV)
Value 6.80 ± 0.02 30.31 ± 0.57 15.92 ± 0.35 73.61 ± 1.47 −14.2 
ParameterspHTS (g/L)VS (g/L)CST (s)Zeta potential (mV)
Value 6.80 ± 0.02 30.31 ± 0.57 15.92 ± 0.35 73.61 ± 1.47 −14.2 

Pilot reactor operation and pretreatment experiments

The sludge with a moisture content of 80% was transported from the sewage treatment plant of Guangzhou to the experimental site and stored at 4 °C before use. And the dewatered sludge was diluted to about 88%–90% of moisture content, and then was fed into the sludge pretreatment tank (heat–alkali hydrolysis). The sludge was thermally pyrolyzed using electric heating (50–80 °C), and sodium hydroxide (NaOH) was added for alkaline hydrolysis. The heat–alkali hydrolysis sludge was transported to the intermediate tank for cooling, and then transferred to a mesophilic (35 ± 1 °C) AD tank with a diameter of 4.9 m and height of 5.9 m. The effective volume of AD tank is about 100 m3 for methane production, and the sludge retention time (SRT) is 20 days. The generated biogas was collected and reused after metering.

In the study, sludge thermal hydrolysis experiments and alkaline hydrolysis experiments were carried out separately. The specific experimental process is shown as follows:

  • (1) Optimization experiments of heat pretreatment: Three operating conditions of 60, 70, and 80 °C were set to run for 2 days, and then measure the pH, VFA (calculated as acetic acid), alkalinity (ALK, calculated as CaCO3), soluble chemical oxygen demand (SCOD), and organic matter of the sludge at each temperature state after being kept for 2, 3, and 4 h. Finally, the optimal thermal pyrolysis temperature and residence time were determined based on the experimental results and energy consumption calculation.

  • (2) Alkali treatment tests: Based on the optimal residence time and economic temperature of the pretreatment tank obtained from the thermal hydrolysis optimization experiment, the effect of alkali pretreatment was further tested by designing different dosage. Each dosage was run for 2 days, and the sampling and detection indicators are the same as above (each sample was measured at steady states of all periods in triplicate).

Analytical methods

The pH was measured using conventional glass electrodes. ALK was detected using bromocresol green methyl red indicator titration method, and organic matter was analyzed by calcination method. The samples were centrifuged at 6,000 rpm for 20 min and immediately filtered through a cellulose membrane with a pore size of 0.45 μm for analysis of SCOD, and the SCOD was measured by the potassium dichromate method. VFAs were measured by a gas chromatograph (7890B, Agilent, America) equipped with a flame ionization detector (Liu et al. 2022). The amount of CH4, and CO2 was determined using a gas chromatograph (GC9890, Linghua, China) equipped with a thermal conductivity detector.

Effects of thermal pretreatment temperature

The change rate of ALK over time at different temperatures reflects the effect of thermal hydrolysis temperature on ALK concentrations. As shown in Figure 1(a), the effect of temperature lower than 100 °C on ALK in sludge is not significant. Due to the increase of organic acids and ammonia nitrogen during the thermal hydrolysis of sludge, an acid–base balance is achieved.
Figure 1

Alkalinity (a), SCOD concentration (b), and VFA concentration (c) at different thermal hydrolysis temperatures.

Figure 1

Alkalinity (a), SCOD concentration (b), and VFA concentration (c) at different thermal hydrolysis temperatures.

Close modal

However, as shown in Figures 1(b) and (c), temperature has a significant impact on SCOD and VFA in sludge. As the temperature of thermal hydrolysis increases, the rate of cell wall breaking gradually increases, and the dissolved SCOD and VFA in the sludge also increase. The SCOD concentration increased to about 9,400 mg/L after 70 °C heat treatment for 4 h in this study, however, the SCOD increased to 4,500 mg/L by heat/alkali pretreatment with an autoclave at 120 °C for 60 min and 0.04 N NaOH addition (Oh et al. 2014), which may be attributed to the different sludge sources and treatment time. It was also found that the COD removal increased by 30% after 65 °C heat treatment for 1 day (Dumas et al. 2010).

The change of SCOD/TCOD ratio over time reflects the effect of thermal hydrolysis temperature on the organic matter release from sludge (i.e., from solid phase to liquid phase). As shown in Figure 2(a), there is a positive correlation between the effect of temperature on the organic matter leaching rate. As the temperature increases, the degree of microbial hydrolysis and cell wall breaking increases, and the cell fluid flows out of the cell wall, increasing the biodegradable organic matter content in the liquid phase.
Figure 2

SCOD/TCOD ratio (a) and VFA/ALK ratio (b) at different thermal hydrolysis temperatures.

Figure 2

SCOD/TCOD ratio (a) and VFA/ALK ratio (b) at different thermal hydrolysis temperatures.

Close modal

As shown in Figure 2(b), with the increase of temperature, the degree of hydrolysis of the sludge shows an increasing trend. Based on Figures 1(a) and (c), within the experimental temperature range, the change of alkalinity is relatively small, while the change in VFA is relatively large. This further confirms the positive correlation between temperature and sludge hydrolysis degree.

In summary, the degree of hydrolysis is approximately positively correlated with temperature at three temperatures of 60, 70, and 80 °C. However, as the temperature increases, the VFA/ALK ratio of the hydrolysis system gradually increases. When the treatment temperature is 80 °C and the hydrolysis time is 4 h, this VFA/ALK value exceeds 4, causing a serious acid–base imbalance in the sludge system, which is not conducive to subsequent AD. When the hydrolysis temperature is 70 °C, the variation range of VFA/ALK ratio within the 4-h hydrolysis time is 0.4–2.2, which is within a reasonable range, and the ratio change is greater than 60 °C. Therefore, considering the comprehensive factors, 70 °C is selected as the optimal thermal hydrolysis temperature for this study, and this temperature was also found as a optimal treatment temperature in other researches (Gavala et al. 2003; Climent et al. 2007), the differences of previous researches were treatment time and subsequent AD processes (37 or 55 °C), resulting in the different methane production rate.

Effects of treatment time

As shown in Figure 3(a), when the thermal hydrolysis time is between 0 and 12 h, the slope of change curves of VFA and VFA/ALK over time is relatively large. Between 12 and 24 h, the change rate of VFA and VFA/ALK is not obvious, and the degree of hydrolysis tends to be stable. Between 0 and 24 h, the change rate of ALK over time is small and relatively stable. As shown in Figure 3(b), the electricity consumption for hydrolysis of sludge with 80% water-content per ton is approximately proportional to the residence time of the thermal pretreatment tank.
Figure 3

The influence of residence time of thermal pretreatment tank on hydrolysis efficiency (a) and electricity consumption per ton of sludge for different residence time in thermal pretreatment tank (b).

Figure 3

The influence of residence time of thermal pretreatment tank on hydrolysis efficiency (a) and electricity consumption per ton of sludge for different residence time in thermal pretreatment tank (b).

Close modal

As shown in Figure 3, when considering both thermal hydrolysis efficiency and energy consumption, the optimal comprehensive effect is achieved when the retention time is 12 h, however, when considering the acid–base adaptability of AD bacteria, VFA/ALK exceeding 2 at 12 h cannot meet the optimal acid–base requirements. Therefore, when the retention time of thermal pretreatment is 4–8 h, the hydrolysis effect and acid–base properties can be balanced.

Optimization of alkali treatment parameters

Under the conditions of sludge thermal hydrolysis temperature of 70 °C and retention time of 4 h, thermal–alkali treatment experiments were conducted according to alkali addition gradients of 0.005, 0.0075, 0.01, 0.0125, 0.015, 0.0175, and 0.02 (kg NaOH/kg dry-sludge). The effect of alkali addition on sludge hydrolysis efficiency is shown in Figure 4(a), and the pH changes before and after sludge hydrolysis under different alkali addition dosages are shown in Figure 4(b).
Figure 4

The effect of alkaline addition on the hydrolysis efficiency of sludge (a) and the pH changes before and after sludge hydrolysis under different alkaline addition dosages (b).

Figure 4

The effect of alkaline addition on the hydrolysis efficiency of sludge (a) and the pH changes before and after sludge hydrolysis under different alkaline addition dosages (b).

Close modal

When the dosage of alkaline added is within the range of 0.005–0.0125 kg NaOH/kg dry-sludge (Figure 4(a)), the VFA ratio before and after sludge hydrolysis is directly proportional to the dosage of alkali added, indicating an efficient hydrolysis effect. Moreover, the VFA/ALK after hydrolysis is between 1 and 1.5, which meets the initial conditions for AD of sludge. When the dosage of alkali added is within the range of 0.0125–0.02 kg NaOH/kg dry-sludge, the VFA ratio before and after sludge hydrolysis tends to be stable, and the VFA/ALK gradually tends to stabilize after hydrolysis. As shown in Figure 4(a), when the dosage of alkali added is within the range of 0.005–0.015 kg NaOH/kg dry-sludge, the pH difference before and after sludge hydrolysis increases with the increase of alkali added, and the highest pH after hydrolysis is 7.4. When the dosage of alkali added is within the range of 0.015–0.02 kg NaOH/kg dry-sludge, the pH difference before and after sludge hydrolysis tends to stabilize. In summary, the optimal alkali concentration range for sludge thermal–alkali hydrolysis is 0.0125–0.015 kg NaOH/kg dry-sludge. However, the different methods of NaOH dosage were used in literatures, including gNaOH/kgTS, mgNaOH/gVS, or adjusting pH to 11 with 10 M NaOH, therefore, it is difficult to compare the cost-effectiveness with alkali treatment among different researches (Tan et al. 2012; Ruiz-Hernando et al. 2014; Shu et al. 2022).

Analysis of anaerobic biogas production rate and energy consumption

Based on the pilot-scale AD system, complete anaerobic sludge digestion experiments were carried out under conditions of no pretreatment (AD at 35 °C), thermal pretreatment (50 °C), thermal pretreatment (70 °C), and 70 °C thermal–alkali combination pretreatment (70 °C, alkali tank retention time 4 h and alkali dosage of 0.015 kg NaOH/kg dry-sludge). The retention time in the mesophilic anaerobic AD system was 20 days and the temperature was 35 ± 1 °C. After each group of experiments runs stably for 20 days, the sampling of continuous 5-day test data to measure the anaerobic gas production, as shown in Figure 5.
Figure 5

Gas production (a) and gas production rate (b) under different pretreatment conditions.

Figure 5

Gas production (a) and gas production rate (b) under different pretreatment conditions.

Close modal

Based on Figure 5, the AD system with pretreatment has significantly higher biogas yield and rate than that without pretreatment. The anaerobic biogas production with 70 °C thermal hydrolysis is significantly higher than that with 50 °C thermal hydrolysis, and the anaerobic biogas production after 70 °C alkali hydrolysis pretreatment is also significantly higher than that after 70 °C thermal hydrolysis. The average methane production under four operating conditions is calculated to be 7.9, 11.77, 16.66, and 21.86 m3 CH4/t 80% water-content sludge, which suggested that thermal–alkali combination treatment greatly improved the performance of methane production, the addition of NaOH increased methane yield by 31.2% than that of 70 °C thermal hydrolysis. It was found that VS degradation rate and CH4 yield increased by 19.93% and 53.33% at 16 d of HRT during mesophilic pilot-scale AD of low organic sewage sludge (Wang et al. 2025).

The average electricity consumption for direct mesophilic AD (35 °C) and 50 and 70 °C thermal hydrolysis are calculated to be 30, 39, and 48 kWh/t 80% water-content sludge, respectively. If biogas combustion boilers are directly used to provide heat consumption in practical application, the corresponding biogas consumption is 10, 13, and 16 m3 biogas/t 80% water-content sludge. Table 3 shows the comparison of energy balance of different sludge AD processes, herein, electricity consumption, alkali consumption and methane production are mainly considered.

Table 3

Comparison of energy balance of different sludge AD processes (calculated by biogas price of 2.0 Yuan/m3, NaOH price of 6.0 Yuan/kg)

SystemsElectricity consumption (KWh/t 80% water-content sludge)alkali consumption (kgNaOH/t 80% water-content sludge)Methane production (m3 CH4/t 80% water-content sludge)Energy cost (yuan/t 80% water-content sludge)
Mesophilic AD 30 7.90 4.20 
Heat (50 °C) pretreatment + AD 39 11.77 2.46 
Heat (70 °C) pretreatment + AD 48 16.66 −1.32 
Heat (70 °C)-alkali pretreatment + AD 48 3.0 21.86 −2.72 
SystemsElectricity consumption (KWh/t 80% water-content sludge)alkali consumption (kgNaOH/t 80% water-content sludge)Methane production (m3 CH4/t 80% water-content sludge)Energy cost (yuan/t 80% water-content sludge)
Mesophilic AD 30 7.90 4.20 
Heat (50 °C) pretreatment + AD 39 11.77 2.46 
Heat (70 °C) pretreatment + AD 48 16.66 −1.32 
Heat (70 °C)-alkali pretreatment + AD 48 3.0 21.86 −2.72 

At the same time, the power consumption of the system is shown in Figure 6. According to market research, the price of biogas is about 2.0 yuan/m3, and the price of industrial NaOH is 6.0 yuan/kg. The heating and insulation energy costs for the four operating conditions are calculated.
Figure 6

Electricity consumption at different thermal hydrolysis temperatures.

Figure 6

Electricity consumption at different thermal hydrolysis temperatures.

Close modal

Based on the energy balance and power consumption results, it can be seen that under the conditions of no pretreatment and 50 °C thermal pretreatment, the energy consumption of AD methane production cannot offset the energy consumption of insulation heating (i.e. additional energy consumption is required). Under the pretreatment conditions of 70 °C thermal hydrolysis and 70 °C thermal–alkali hydrolysis, the energy consumption of AD biogas production can meet the energy consumption of insulation heating and there is surplus. In addition, the output ratio of 70 °C thermal–alkali pretreatment is the highest efficiency, which will provide a better pretreatment strategy for AD of WAS. The cost-effectiveness and feasibility of thermal–alkali combination pretreatment were confirmed in pilot-scale study, however, the organic content of sludge used in this experiment is over 50%, while the organic content of WAS is often found to be lower than 50%, which will impact the sludge hydrolysis efficiency by pretreatment, limiting the full-scale application this pretreatment technology.

The optimal alkali dosage is 0.0125–0.015 kg NaOH/kg dry-sludge for sludge thermal–alkali combination hydrolysis according to VFA/ALK ratio. 70 °C is the optimal temperature for heat pretreatment. When the retention time is 4–8 h, the sludge hydrolysis effect and pH can reach equilibrium. Thermal–alkali combination treatment greatly improved the performance of methane production, the addition of NaOH increased methane yield by 31.2% than that of 70 °C thermal hydrolysis. The average energy consumption is 75 kWh/m3 80% water-content sludge during the experiment. This study provides a better pretreatment strategy for AD of WAS.

This research was financially supported by the Key Research and Development Program of Guangdong Province (Contract No. 2019B110209002), the Natural Science Foundation of China (Contract No. 51978290), the Science and Technology Innovation Strategy Special Foundation of Guangdong Province (2021A0505020010).

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

The authors declare there is no conflict.

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Author notes

These authors contributed equally to this work.

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