Thermal hydrolysis treatment of digestates derived from food waste and sewage sludge – effect on residual methane potential Open Access

Anaerobic digestion (AD) is a biological process that degrades organic material in the absence of oxygen to produce biogas. Additionally, a byproduct called digestate is also generated, typically containing 50% of the total solids (TS) added to the reactor, including non-degraded organic content (OC) and most of the nutrients (N, P, and K) (Costa et al. 2015). The presence of residual OC depends on feedstock and operational parameters like organic loading rate (OLR) or hydraulic retention time (HRT) (Garuti et al. 2023). Typically, the RMP of digestates from biogas plants constitutes between 0.6 and 10% of the feedstock potential, but this value can reach up to 50% (Ruile et al. 2015). The RMP remaining within digestate varies from 20 to 240 mL CH₄ per gram volatile solids (VS), which is influenced by substrate composition, pretreatment, and operational parameters like OLR, hydraulic retention time, temperature, or pH (Ekstrand et al. 2022; Uddin & Wright 2023).

In 2023, 18,774 biogas plants were reported to be operating in the EU, with rapid growth expected (Motola et al. 2023). Consequently, digestate production is anticipated to increase. Using as fertilizer is one of the typical ways of utilizing the digestate; however, this brings concerns about phytotoxins or difficulties with transportation and storage (Song et al. 2021). Also, without proper management, digestate could result in significant greenhouse gas emissions (GHGs), mainly in the form of methane (CH₄), which has a global warming potential of 28 times greater than CO₂ over a 100-year period (United States Environmental Protection Agency, n.d.). Therefore, recovering RMP from digestate is essential to minimize adverse environmental impacts and enhance the biogas output from the biogas plant. However, it is challenging to recover CH₄ from digestate as the most readily degradable OC has already been degraded, while recalcitrant substances like lignin tend to accumulate in the digestate (Sambusiti et al. 2015). Thus, post-treatment to enhance biodegradability of digestate might be a solution to recover RMP efficiently.

To date, most post-treatment methods have focused on improving dewaterability, stabilization of the digestate (inactivation of pathogens or COD removal), or removal of toxic metal contaminants to meet the requirement for environmental discharge (Svennevik et al. 2019; Wang et al. 2021). These methods are often energy-intensive and require additional costs, reducing economic benefits (Romio et al. 2021). However, there has been increasing interest in post-treatment methods to enhance biodegradability and maximize the methane yield recovered from digestate. One potential method, thermal hydrolysis (TH), defined as a process involving high pressure, temperature, and often sudden pressure release (so-called stream explosion) (Ngo et al. 2023), has gathered significant attention and found favorable for those goals (Axelsson Bjerg et al. 2024). TH involves sludge treatment at high temperatures (160–180 °C) and pressure (6 bars), followed by sudden pressure release, resulting in cell wall breakdown (Mágrová & Jeníček 2021). This process enhances the organics' solubility, elevating the subsequent biogas production and reducing the anaerobic digester's required HRT (Liu et al. 2020). TH has been widely used as a pretreatment method before AD and applied in wastewater and sludge treatment plants worldwide (Abelleira et al. 2012). Even though most studies report TH as pretreatment, relocation of TH to post-AD also offers advantages, including higher methane yields and further reduction of organic matter (Svensson et al. 2018). For instance, applying TH at 160 °C as a post-treatment process for digestate from wastewater treatment increased methane yield from less than 10 mL CH₄/gCOD_added to 175 mL CH₄/gCOD_added (Cai et al. 2021). The breakdown of extracellular polymeric substances and soluble proteins during TH can be one of the reasons why methane yield is improved (Cai et al. 2021). For the case of digestate from food waste (FW) co-digestion plants, the application of TH led to a 22% increase in methane potential (Nordell et al. 2022). However, implementing TH requires significant energy (Yang et al. 2019). The total volume needed for TH post-treatment is lower than pretreatment due to the reduced total volume (20–50%) following AD (Weiland 2010). Thus, less energy is spent applying TH as a post-treatment. Additionally, more efficient methane production is possible if liquid and solid fractions were processed separately since those phases respond differently to TH. The biodegradability of solid fractions is reported to be unaffected by TH, while the liquid fraction may gain a 30% enhancement in methane production (Pérez-Elvira et al. 2016).

After methane recovery, the remaining residues are commonly used for field application (van Midden et al. 2023). The addition of acids is an effective approach to enhance nutrient availability, particularly phosphorus, by promoting its solubilization in the liquid phase, thereby improving its suitability for agricultural reuse (Ekpo et al. 2016). However, it remains uncertain whether acid addition could also provide additional benefits for methane production from digestate, especially when TH is involved. Therefore, the main objective of this study was to investigate the impact of TH post-treatment on RMP from each fraction. The effects of TH and acid addition on the transfer of organics and the potential for phosphate recovery were also explored. These findings could support the development of comprehensive strategies for digestate post-treatment and utilization.

This study collected raw digestate from two biogas plants in Norway. The first biogas plant, operated by Renovation and Recycling Agency, Oslo (referred to as the first biogas plant), is operated at a mesophilic temperature of 41 °C and mainly processes FW with a retention time of 28 days; the second plant, operated by Bergen Municipality (Rådal, Norway) (referred to as the second biogas plant), is operated at a thermophilic temperature of 55 °C and uses SS as the primary input substrate, with a retention time of 21 days.

The original inoculum used in this work was collected from a 15 L (working volume) lab-scale continuous stirring tank reactor (CSTR). The lab-scale CSTR has been fed with cow manure for one year. The properties of these two digestate sources and the inoculum are given in Table 1.

Digestate samples based on either FW or SS were prepared to test the effect of TH as a post-treatment method. To begin with, 333.3 g of digestate was dried and milled using a knife type of mill with mesh holes of 1 mm size. Then, it was mixed with 1,666.7 g of distilled water in a 5 L beaker. The mixture was then transferred to a 2.7 L autoclave, followed by N₂ flushing to maintain an initial pressure of 10 bars to avoid water evaporation. TH was performed as a single experiment at two temperatures, 160 and 190 °C. These temperatures were selected based on the reviewed literature, where pretreatment temperatures at 160 to 190 °C were reported as optimal for methane production (Mirsoleimani Azizi et al. 2024). The heating was performed gradually, and once the target temperature was reached, the reaction time was set to 60 min to ensure a long enough holding time. The samples were then left to cool overnight and separated the day after. It should be noted that this is different from typical commercial treatments, which include a rapid decompression of the reactor (steam explosion) due to the equipment constraints. Control experiments were performed on samples that did not go through TH. However, those were also mixed with water, left overnight at ambient conditions, and separated the day after. Details regarding the setup are shown in Table 2, while mineral compositions of the untreated samples are shown in Table 3.

No additional water was added to the system during the experiment. After treatment, each sample was filtrated using a Büchner funnel with coffee filter paper to separate the solid and the liquid phases. The obtained solid phase was then dried in a drying cabinet at 105 °C for approximately 24 h, while the liquid phase was stored in plastic bottles (5 °C) prior to the experiment.

In the first setup (before thermal hydrolyzation, BTH), 0.1 M H₂SO₄ solution (95%, technical grade, VWR Chemicals) was used instead of distilled water during sample preparation, followed by TH at 160 °C. In the second setup (after thermal hydrolyzation, ATH), samples were prepared with water and treated at 160 °C in the autoclave, but H₂SO₄ was added right before the filtration step. Specifically, concentrated H₂SO₄ was added dropwise under stirring to the treated slurry after collection from the autoclave so that the final concentration was 0.1 M. The slurry and acid were thoroughly mixed using a magnetic stirrer for an additional 60 min at room temperature, after which the mixture was filtered.

TS and VS were measured according to the standard methods (American Public Health Association 2005). Biogas pressure and composition were measured daily from day 1 to 7, followed by measurements on days 9, 12, 15, 20, and 25 as the production rate declined. Gas pressure was monitored using a GMH 3161 hand-held pressure gauge (Greisinger, 93128 Regenstauf, Germany). Gas chromatography (Agilent 990 Micro GC, Agilent Technologies, CA 95051, USA) with a flame ionization detector and N₂ as the carrier gas was performed to measure the gas composition (CH₄, CO₂, and H₂). The GC was equipped with two capillary columns (10MX0.25MMX0UM ST & MS5A SS, 10MX.025MMX30UM BF) with an injection temperature of 110 °C and column temperature of 80 °C. The pH was measured using a pH meter (Thermo Scientific Orion Dual Star, USA). Phosphate content was measured spectrophotometrically (Hach & Lange, DE3900) using HACH Kits with a range of 2–20 mg/L while chemical oxygen demand (COD) was measured spectrophotometrically using Merck Spectroquant COD Cell Tests with a range of 500–10,000 mg/L. For phosphate content analysis, the term ‘Available Liquid’ refers to the free water not retained within the solid phase after solid-liquid separation. The samples' OC (VS) was calculated based on the weight differences between dry weight and ash (inorganic fraction) weight.

This result was likely due to the formation of recalcitrant compounds, such as melanoidins, which are generated during TH at high temperatures (>140 °C) (Zhang et al. 2020; Yan et al. 2022). Dwyer et al. (2008) observed that when the temperature was raised from 140 to 165 °C, the degree of browning that is associated with the Maillard reaction increased fourfold, yet TH effluent's anaerobic biodegradability did not show a considerable difference. Based on these reports, it can be suggested that while TH with temperatures up to 160 °C is beneficial, increasing temperature beyond this point may lead to the formation of recalcitrant substances that reduce biodegradability.

The impact of acid addition on mass transfer was also evaluated (data not shown). It was revealed that acid addition plays an insignificant role in mass transfer. For instance, mass transfer from FW digestate samples was found to be 15 and 14.5% with acids addition prior to or after TH, respectively, while for SS samples, the transfer was 17% in both cases. These results indicate that acid addition does not positively influence mass transfer.

A similar trend was observed for the SS samples, where the highest methane yield (247 ± 22 NmL/gVS_added) was obtained from the sample treated at 160 °C (SS-160). The sample with acid addition after TH (SS-160-ATH) followed as the second highest, while the lowest methane yield was reported for the non-treated sewage sludge sample (SS) with 82 ± 10 mL/gVS_added. TH treatment led to a threefold increase in methane yield.

This observation is consistent with Balasundaram et al. (2023), which showed that SS treated at 160 °C produced 4.2 times more methane than an untreated sample. When Analysis of Variance (ANOVA) was performed to compare the methane yields, the comparison between FW and FW-160 revealed a significant difference in biomethane yield with p < 0.05. Similarly, the comparison between SS and SS-160 showed a significant difference, with p < 0.05. These results show that the TH treatments significantly enhanced methane production compared to their respective baseline samples (FW and SS). As shown in Figure 3, samples subjected to acid addition (SS-160-ATH & FW-160-ATH) produced less biomethane compared to non-acid treated samples; FW with acid addition yielded 17% less biomethane while acid-added SS sample showed a 26% reduction. When the ANOVA test was performed comparing acid-added samples (FW-160-ATH and SS-160-ATH) and samples treated with only TH (FW-160 and SS-160), FW-160 exhibited a higher mean yield compared to FW-160-ATH, which indicates that the addition of sulfuric acid post-thermal hydrolyzation significantly reduced biomethane production relative to FW-160 (p < 0.05). Similarly, a significant difference was observed between SS-160 and SS-160-ATH (p < 0.05). In contrast, Takashima & Tanaka (2014) reported an increased methane yield of up to 190% when acid was added during TH at different temperatures. However, it should be noted that the processes of these two works are considerably different; while one does the acid treatment along with TH, the other tests samples with acid addition after the process.

This observation indicates a synergistic impact on yield, which can only be expected when acid is added during the TH process. The earlier findings on the effect of TH on OC correlate with the RMP results. TH application increased the transfer of OC to the liquid fraction, resulting in higher CH₄ yields for both digestates (Kim et al. 2024). This decrease in methane production could result from the chemical environment being altered unfavorably by the acid addition post-TH. This alteration could result in disruption of organic material degradation or direct inhibition of methanogenic activity. The addition of sulfuric acid can also stimulate sulfate-reducing bacteria to produce H₂S, which is toxic to methanogens (Hendriks & Zeeman 2009).

In conclusion, TH at 160 °C significantly enhances methane yields (1.5× for FW, 3× for SS) and organic release compared to untreated samples. Moreover, TH at 160 °C resulted in higher methane production than treatment at 190 °C. While acid addition post-TH application reduces CH₄ yield by approximately 17–26%, it effectively releases phosphorus into the liquid phase, with up to 28 times more of total P detected in the liquid fraction compared to non-acid-added sample, improving the nutrient quality of the digestate. These results suggest a potential strategy to optimize biomethane and P recovery by adding acid after TH-enhanced digestion. Further research should focus on investigating the specific inhibitory compounds generated at higher treatment temperatures, as well as the potential carbon loss due to acid addition.

This work was supported by the Research Council of Norway through grants 257622 (Bio4Fuels) and 319723 (BioSynGas).

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

The authors declare there is no conflict.