Fenton oxidation as pretreatment for biomass gasification condensate: cost and biomass inhibition evaluation

Gasification is an efficient technology to sustainably dispose of solid biomass (Farzad et al. 2016). During the gasification process, the feedstock undergoes a succession of thermal reactions, mainly drying, pyrolysis, combustion, and gasification. The high temperatures in the gasifier (1,100–1,500 °C) guarantee a thorough decomposition of the biomass, such as agriculture wastes, wood waste materials and sewage sludge (Liu et al. 2011; Sansaniwal et al. 2017; Widjaya et al. 2018).

Spent mushroom compost (SMC) is a waste stream from the industrial mushroom production. Considering a worldwide mushroom production of 35 million tons, an estimated 140–175 million tons of SMC is produced every year (Grimm & Wösten 2018; Umor et al. 2021). Due to slow decomposition rates, high content of water and ash as well as a potential pollution to underground and surface water by phosphorus and nitrogen, the SMC is not suitable to be treated by composting, anaerobic digestion, incineration or landfilling (Huang et al. 2018). However, the upper calorific value of dried SMC with a value of 11.95 MJ/kg, is comparable to dried sewage sludge, making it suitable for gasification. (Williams et al. 2001)

The watery phase of the condensate is produced during the drying of the pyrolytic gas, containing 82–93% water and 7–18% organic compounds (Muzyka et al. 2015). Independent of the feed substrate, organic acids, aldehydes, benzenes, ethers, furans, hydrocarbons, ketones and phenols are produced during gasification and pyrolysis processes (Zhang et al. 2019a). The recovery of valuable organic compounds from the condensate is limited by its complex composition and low individual concentrations. Moreover, these organic compounds show a moderate or high acute toxicity to the environment and a tendency for bioaccumulation (Ji et al. 2016; Huang et al. 2019).

Due to the inhibitory effects of the condensate to microorganisms, a conventional treatment of activated sludge processes observed a long lag phase and requires an extended hydraulic retention time, that leads to a higher operation cost (Muzyka et al. 2015; Mishra et al. 2021). Advanced oxidation processes (AOPs) were widely applied to abate the organic pollutants, such as pharmaceutics, phenols, formaldehyde, dye intermediate in wastewater. (Muzyka et al. 2015; Cetinkaya et al. 2018; Guo et al. 2018; Nidheesh et al. 2021). The Fenton oxidation relies on the in-situ production of hydroxyl radical from hydrogen peroxide through catalysis by Fe²⁺ at a low pH value. It is a homogeneous catalytic oxidation process. Owing to the wide availability, low costs and low toxicity, Iron(II) is typically used in the form of iron sulfate as catalyst (Nidheesh et al. 2021). Due to a narrow working pH range, high chemical consumption and excessive iron sludge production, numerous optimized Fenton-based AOPs such as UV-Fenton oxidation, heterogeneous catalytic Fenton oxidation and electro-Fenton oxidation have been developed in recent years. However, the UV-Fenton oxidation is limited by light irradiation, rendering it unsuitable for wastewaters with high turbidity or deep color (Brillas 2020). Owing to complicated synthesis routes and high costs of heterogenous catalysts, the heterogeneous Fenton oxidation was often limited to laboratory-scale applications until now (Zhang et al. 2019b). The chemical consumption is lowered in electro-Fenton oxidation, but their overall costs are often increased due to the costly electrodes and high electricity demand. A relative COD elimination cost of 5.76 $/kg was achieved in textile wastewater treatment by electro-Fenton oxidation (Kaur et al. 2019).

The complete COD degradation by Fenton oxidation observed a high operation cost, the specific operation cost in biomass gasification wastewater treatment reached US$8.7/kg COD elimination (Muzyka et al. 2015). Through coupling of Fenton oxidation with biological degradation, the treatment process is more cost effective. (Tripathi et al. 2013; Zhu et al. 2018; Mishra et al. 2021). However, generated transformation products and intermediates of the Fenton process may increase the toxicity of the feed (Babu et al. 2019). The overall biomass inhibition of wastewater is most efficiently assessed by toxicity assays. Identifying singular components, transformation products and intermediates and evaluating their inhibitory effects individually have been inconclusive (Sharma et al. 2018).

In this paper, Fenton oxidation was investigated as a pretreatment for the watery phase of SMC gasification condensate. The operation conditions such as pH value, the dosage of H₂O₂ and Fe²⁺ were optimized in single-factor experiments with regards to detoxification efficiency through respiration inhibition assays (OECD/OCDE 2010; Babu et al. 2019). The corresponding removal rates of organic phenolic compounds, COD, and ammonium after Fenton oxidation were also investigated. Moreover, biogas production tests were conducted as an additional evaluation tool for the detoxification effect of Fenton oxidation on the wastewater.

The counterflow biomass gasification plant located in Westphalia, Germany is fed with SMC (periodically mixed with manure and forestry residues). The pyrolytic gas produced in the gasification process was dried before its valorization in a combined heat and power unit. In the drying process a condensate was segregated by a cyclone water trap. The condensate was separated into a sinking layer with tars and heavy oils, a floating light oil layer, and the watery phase (80–90% water content). The sinking and floating layers were reintroduced into the gasifier, as both mixtures have high calorific value. The remaining condensate of water-soluble components needs to be disposed of before discharge. The overview of the wastewateŕs main composition is given in Table 1. The wastewater was stored at 6 °C, before experiments the wastewater was pretreated by centrifugation at 4,000 rpm for 20 min followed by membrane filtration (0.45 μm).

HCl (37%, 7647-01-0) from Merck & Co., and NaOH (≥98%, 1310-73-2) from Carl Roth GmbH were utilized to adjust of pH values during tests. H₂O₂ (35%, 7722-84-1) from Carl Roth GmbH and FeSO₄·7H₂O (99.5%, 7782-63-0) from Merck & Co. were utilized in the Fenton oxidation tests. The aerobic and anaerobic activated sludge samples were collected from the local municipal wastewater treatment plant in Schönerlinde, Berlin. The aerobic activated sludge, with a dry matter concentration of 6.7 g/L, was collected directly from the aeration pool. It was further used in the respiration inhibition assays. The anaerobic sludge was used for the biogas production tests. Table 2 gives an overview of the physico-chemical properties of the activated sludges.

The Fenton experiments were conducted in batch reactors as shown in Figure 1 in supplemental material. Here, 100 mL of the wastewater sample was treated in quartz stube with or without UV irradiation for 120 min. Single-factor test series were conducted to isolate the effect of the initial pH value, Fe²⁺ and H₂O₂ dosage on their detoxification efficiency and economic viability.

To estimate the most efficient pH value of Fenton oxidation, the initial pH value was set at 3, 5 and unchanged condition of pH 8.9. The H₂O₂ with 30% of the normalized stoichiometric ratio (NSR) to COD (275 mM H₂O₂) and Fe²⁺ ions with a molar ratio of H₂O₂:Fe²⁺ at 15:1 (18.3 mM of Fe²⁺) were dosed as oxidant and homogeneous catalyst in feed. Subsequently, the most cost-efficient Fe²⁺ dosage was determined from tests at initial pH 5 and an initial H₂O₂ dosage of 30% NSR, among molar ratios of H₂O₂:Fe²⁺ between 10 and 30. To estimate the optimal H₂O₂ dosage in range of 15–100% NSR, the tests with conditions of initial pH 5, molar ratio of H₂O₂:Fe²⁺ at 15:1 were investigated.

To estimate the chemical costs of Fenton oxidation, the samples were neutralized to pH 7 ± 0.5 by NaOH. The iron hydroxide was precipitated through coagulating or adsorbing with humic substances and other organic compounds. To estimate the sludge production and organic content, the entire precipitated sludge was filtered through a paper filter. The sludge production was quantified after overnight drying at 105 °C.

To prepare the Fenton oxidation treated samples for further analysis and experiments, 5 g/L MnO₂ were added into feed solution to remove residual oxidants. Due to adsorption effect of phenolic compounds onto MnO₂, the concentration of phenolic compounds was decreased by 34–38% in the blank control by this procedure. However, the concentration reduction of phenolic compounds by adsorption of MnO₂ was not subtracted.

The respiration inhibition assays were conducted based on the modified OECD guideline 209. The experimental setup is depicted in Figure 2 in the supplementary material. They were conducted with a total residence time of 3 days at 25 °C.

The OxiTop^®-C works at absolute pressure of 1,000 ± 250 mbar. As the whole air in reactor was blown out by pure oxygen, the maximal reduction of partial oxygen pressure in this respiration assay is 0.25 bar. The dissolved oxygen in the feed samples is considered within 25% deviation. The 0.25 bar partial oxygen pressure corresponds to a biochemical oxygen consumption of 357 mg. Thus, considering the biological availability, the COD of samples was set at 550 mg.

To show the efficiency of the Fenton oxidation on wastewater detoxification, the original wastewater and the Fenton oxidation-treated wastewater were provided in 20 days’ biogas production tests. Before tests, all samples were adjusted to pH 7 ± 0.3. The composition of samples is described in Table 4. The experimental setup is depicted in Figure 3 in the supplementary material.

The total phenol concentration was detected through a wet chemical color-spectrophotometric analysis method using the Folin–Ciocalteau reagent. The method is described in the supplementary material. The concentration of NH₄⁺ was detected by flow injection analysis (FIA) FOSS FIA Star 5000. The concentrations of dissolved organic carbon and total nitrogen in samples were measured by thermal catalytic oxidation process at 850 °C with an Analytik Jena multi N/C 3100 + CLD. The chemical oxygen demand (COD) was detected through total combustion of sample at 1,200 °C by COD analyzer from company LAR process Analyzers AG. The iron concentrations in wastewater samples were detected by atomic absorption spectrometry (AAS) with model 900AA from Perkin Elmer-AAS-PinAAcle^TM.

The optimal pH was determined among the pH values 3, 5 and 8.9 (original pH). The tests were conducted with a 30% NSR H₂O₂ dosage (275 mM) and a molar ratio of H₂O₂:Fe²⁺ at 15:1 (18.3 mM Fe²⁺). After 2 hours’ reaction, the pH values of the samples decreased from 3 to 2.48, 5 to 2.69 and 8.9 to 7.93 (Figure 1(a)). Similar removal rates of phenolic compounds, COD and NH₄⁺ were observed in tests with initial pH 3 and 5. The sample without pH adjustment (pH 8.9) showed a higher ammonium removal rate (30%). Two effects contributed to this phenomenon: a lower concentration of ammonium was generated from nitrogen-containing organic compounds by Fenton oxidation at pH 8.9; and a higher escape rate of ammonia was achieved at alkaline conditions of pH 8.9 (Gamaralalage et al. 2019).

Due to the pH buffering system (such as humic substances), a further pH lowering from pH 5 to pH 3 was deemed uneconomical in this application scenario. Owing to an obvious higher COD removal rate in comparison with pH 8.9, the optimal pH value of the Fenton oxidation was determined to be pH 5. This contrasts with other research and the theoretical basics of Fenton oxidation, that the most effective pH of Fenton oxidation was found at around 2.5 (Pignatello et al. 2006). The aqueous phase from hydrothermal carbonization process contains high concentrations of humic substances (Usman et al. 2020). Humic substances building complex bonds to ionic iron, act as chelating agents in Fenton oxidation processes. Through applying chelating agents in feed, the Fenton oxidation achieved also high efficiency at pH 5–8 (Zhang & Zhou 2019).

UV irradiation can regenerate Fe²⁺ from Fe³⁺, and improve the COD degradation efficiency in Fenton experiments (Hu et al. 2011; Nidheesh et al. 2021). A 15 W UV lamp was used to irradiate the wastewater in tests of initial pH 5 and 8.9. However, in the comparison in Figure 1(a), the removal rates of COD and ammonium were not obviously improved as shown in Figure 1(b). The COD removal rate was slightly increased from 33% to 38% under UV irradiation in the test with an initial pH of 5. The wastewater in this work had an intense color, likely impeding the transfer of UV light. Compared with the initial Fe²⁺ concentrations in a referenced publication (40 mg/L), it is much higher (1,021 mg/L) in this work. Thus, the COD degradation was not obviously enhanced by regenerated Fe²⁺.

The iron and H₂O₂ dosages are the two main operational parameters in Fenton oxidation. For the treatment of similar industrial wastewater (originating from e.g. petroleum refineries, pharmaceutical and antibiotic fermentation processes) the molar ratios of H₂O₂:Fe²⁺ have been set in range of 8.2–20 (Ribeiro & Nunes 2021). Based on 275 mM H₂O₂ dosage (NSR value of 30%), the molar ratio of H₂O₂:Fe²⁺ was set between 10 and 30 in this work. At these reaction conditions, the Fe²⁺ concentration showed little influence on the removal rates of phenolic compounds and COD. Their removal rates ranged at 67–72% and 34–38%, respectively. The pH value of the feed was more obviously decreased at higher Fe²⁺ dosage. With a molar ratio of H₂O₂:Fe²⁺ of 10, 15, 20, 30, the pH values decreased from 5 to 2.57, 2.69, 2.76 and 2.91, respectively. Among these tests, the highest NH₄⁺ removal (22%) was achieved at a molar ratio of 15:1.

To investigate the correlation between H₂O₂ dosage and the degradation efficiency, H₂O₂ was dosed between 15 and 100% NSR with a molar ratio to Fe²⁺ of 15:1. The drop in pH values after Fenton oxidation was increased by higher H₂O₂ dosage, correspondingly to final pH values between 3.08 and 2.44 (Figure 1(d)). The phenolic compounds and COD were degraded more efficiently by higher H₂O₂ dosage. After increasing the H₂O₂ dosage from 15% to 45% NSR, their removal rates increased from 54% to 82% and 15% to 45%. The removal rate of COD plateaued at 30% NSR and reached a maximum of 51% at H₂O₂ dosage of 60% NSR. The removal efficiency of COD by Fenton reaction was efficiently improved through a multistage neutralization–coagulation process with limestone (Guo et al. 2018). However, the precipitates must be treated as hazardous solid waste. Due to the increased sludge production yield from the lime and its secondary effect to organic compounds such as adsorption or agglomeration, larger amounts of sludge should be managed (Gao et al. 2022). The solid waste management requirements limit the applicability of Fenton oxidation in industrial-scale utilization.

The NH₄⁺ removal rates were comparably lower, ranging between 12 and 20% with H₂O₂ dosages of higher than 30% NSR. The nitrogen-containing organic compounds shared around one-third of the nitrogen-source in the condensate, this is connected to the formation of ammonium as a by-product from the decomposition of nitrogen-containing organic compounds. The same phenomenon was observed in palm oil mill effluent treatment, a removal rate of NH₄⁺ was achieved by 97% within 15 min, and subsequently decreased to 35% after 90 min reaction time (Gamaralalage et al. 2019).

This work focused on the detoxification of wastewater by Fenton oxidation, in comparison with the mentioned paper, the dosages of H₂O₂ and Fe²⁺ were reduced by 30 and 93%. (Gamaralalage et al. 2019). For a higher removal rate of NH₄⁺, the electro-Fenton oxidation could be performed. As announced by Menon and coworkers, the NH₄⁺ removal rate reached 59% in textile wastewater treatment (Menon et al. 2021).

To make a complete economic evaluation of pretreatment by Fenton oxidation, the chemical consumption and sludge production yield after neutralization of NaOH are summarized in Table 5. To further provide a sustainable reuse application of precipitates, the iron content in sludge was also determined.

With an initial dosage of H₂O₂ and Fe²⁺ at 30% NSR and 1,021 mg/L, the amount of precipitated sludge after neutralization reached 7.5–9.5 kg/ton wastewater in Figure 2. The iron content reached 9.9% in the sludge sample of pH 8.9, which was much higher than the content in the samples of pH 3 or pH 5. This is a consequence of a lower solubility of iron hydroxide at neutral pH value, transferring most iron into the solid sludge. Su and coworkers researched the influence of the final pH on the sludge production. With final pH of 3, the sludge production yield was higher than that at pH 5 (Su et al. 2019). However, the sludge in this work was collected after neutralization, the lower sludge production from test with initial pH 5 might be owing to a slight alkalization in neutralization step.

The sludge production yield and iron content of the sludge augmented along with the H₂O₂ and Fe²⁺ concentration in the sample. With a molar ratio of H₂O₂:Fe²⁺ at 15:1, the H₂O₂ was dosed in the range of 15–100% NSR into the feed, increasing the sludge yield from 6.4 to 18.3 kg sludge/ton wastewater. The content of iron grew correspondingly from 3 to 16.8%. The highest sludge production yield reached 18.3 kg/m³ with an iron content of 16.8%, in tests with H₂O₂ dosage of 100% NSR. Under the operational condition of pH 5, the molar ratio of H₂O₂:Fe²⁺ of 15 and H₂O₂ dosage of 30% NSR, the sludge production reached 7.5 kg/ton with an iron content of 4.5%.

In comparison to initial wastewater, the content of toxic compounds such as polycyclic aromatic hydrocarbons is lower in Fenton sludge (Wang et al. 2019). The Fenton sludge could be processed in anaerobic digestion trials for biogas production. An enhancement on hydrolysis of macromolecular organic compounds was also found in the hydrolysis or acidification process of wastewater treatment (Wang et al. 2022). Moreover, it was applied as source material for the production of magnetic adsorbent or catalysts (Liu et al. 2017; Gan et al. 2020; Tong et al. 2021). In this work, the Fenton sludge could be introduced as feedstock into a gasifier. Compared with other publications, the Fe²⁺ dosage is obviously lower in this work, the ash production of the gasifier will not be seriously impacted by the introduced Fenton sludge. (Gamaralalage et al. 2019; Wu et al. 2021).

In the experiment with 100% NSR H₂O₂ dosage, the Fe²⁺ concentration was set at 3,404 mg/L. The residual iron concentration in this sample was low at 83 mg/L. This corresponds to a 98 wt.-% transfer of iron into the sludge. One reason for the near complete precipitation of the iron is the overdosed H₂O₂ at 100% NSR. The generated hydroxyl radicals oxidized the Fe²⁺ to Fe³⁺, wasting their oxidation potential on the iron. (Nidheesh et al. 2021). Due to the low solubility of Fe³⁺ hydroxides, the iron was more efficiently filtrated from feed, resulting in a low iron ion concentration after neutralization (Cravotta 2008).

In the other samples, the iron ion concentrations kept between 187 and 713 mg/L. As shown in previous publications, the residual iron ions negatively impacted the nitrogen removal in subsequent biological treatments (Philips 2003). With an iron ion concentration of 112 mg/L (Fe²⁺ or Fe³⁺), the nitrogen removal was 40% or 60% lower than the iron free blank sample. After adaptation for 12 days, they were still 9% or 30% lower. Thus, the wastewater after Fenton oxidation may inhibit on nitrification process.

The total oxygen consumption of samples are shown in Figure 3. The blank sample containing only activated sludge and synthetic sewage, showed a sharp increase in oxygen consumption within the first 8 hours and a flat curve that was moderately constant over the remaining 64 hours in Figure 3(a). The highest oxygen consumption rate reached 7.2 mL/h in 5 hours. The total oxygen consumption was accounted for at 100 mL. Compared with the blank sample, the original sample showed a slightly higher oxygen consumption rate of 7.5 mL/h at 7 h. Owing to toxic compounds in the wastewater, the highest oxygen consumption rate was 2 hours delayed, the total oxygen consumption reached as low as 56.8 mL in 72 hours.

After Fenton oxidation with an initial pH of 3 or 5, the toxic transformed organic compounds might be built in feed. At the beginning of the respiration assays, the oxygen consumption of samples at pH 3 and pH 5 had underperformed compared with the original sample (Figure 3(a)). Their oxygen consumption rates reached the highest value of 5.6 and 10.4 mL/h at residence times of 20 and 21 h. Compared with the sample at pH 3 (lag phase with 18 h), less amounts of toxic compounds were generated at pH 5 (lag phase with 14 h).

The toxic organic compounds were efficiently removed after the Fenton reaction with an initial pH of 8.9, the lag phase of the sample at pH 8.9 was comparable to the blank sample, its highest oxygen consumption rate reached 11.2 mL/h in 5 hours. The total oxygen consumption reached among tests a highest value of 180 mL, in the sample at pH 5. Considering the COD removal rate and total oxygen consumption, the optimal pH value was indicated at pH 5.

Figure 3(b) shows a similar oxygen (Table 6) for the wastewater samples at pH 8.9 with and without UV irradiation. However, the oxygen consumption rate of the sample UV-5 is much lower than for the sample without UV at pH 5. Although a slightly higher COD removal rate was achieved by UV-Fenton oxidation, larger amounts of transformed toxic compounds might be generated at pH 5. Thus, considering the increased toxicity, UV irradiation is not recommended.

The influence of initial Fe²⁺ concentration on the toxicity of treated wastewater was also investigated. The Fe²⁺ was dosed into the wastewater with a molar ratio of H₂O₂:Fe²⁺ in the range of 10–30 that corresponds to an initial Fe²⁺ concentration of 1,532–511 mg/L. As shown in Figure 3(c), the wastewater sample of 15:1 and 20:1 showed a higher total oxygen consumption than the original sample for the residence time of 16 h. With the 72 h residence time, the total oxygen consumption reached 173.6 (20:1), 176.6 (15:1), 79.2 (10:1) and 84 mL (30:1). This indicates an optimal molar ratio of H₂O₂:Fe²⁺ at 15:1 in Fenton oxidation.

To determine the influence of the hydrogen peroxide dosage on the toxicity of Fenton oxidation-treated wastewater, the tests were conducted with a fixed molar ratio of H₂O₂:Fe²⁺ at 15:1 and a varied H₂O₂ dosage of 15–100% NSR. The sample F-15% shows the lowest oxygen consumption rate in Figure 3(d). With the residence time of 72 h, the total oxygen consumption reached 20 mL. The toxicity of wastewater was decreased with a higher H₂O₂ dosage. With the initial H₂O₂ dosage of 30 and 45%, the oxygen consumption rates reached their highest values of 10.4 and 11.2 mL/h at the residence time of 21 h. However, the biological available organic compounds were degraded by highly dosed H₂O₂, the highest oxygen consumption rates of samples F-60% and F-100% reached lower values of 7.2 and 5.6 mL/h.

As identical results, the samples of F-30% and F-45% observed total oxygen consumption of 180 and 173 mL at the residence time of 72 h. Owing to lower chemical consumption, the favorite H₂O₂ dosage was estimated at 30% NSR. Thus, through the respiration assays, the optimal condition of the Fenton oxidation was determined as pH 5, with the H₂O₂ dosage of 30% NSR and a molar ratio of H₂O₂:Fe²⁺ at 15:1.

The biogas production of Fenton sample (F515), original wastewater and anaerobic sludge over a period of 20 days is shown in Figure 4. The original wastewater displayed a noteworthy inhibition on biogas production. Compared with the blank or the treated sample, only roughly one-third biogas was produced in first 3 days. The anaerobic sludge had the lowest absolute gas production, oppositely a highest biogas production relating to the COD removal. This implies the decay of organisms at nutrient lack conditions, the released nutrition from organisms were utilized as feedstock of biogas production.

The sample of Fenton produced 84% more biogas than the original sample in around 20 days. That confirms the 66% higher gas production relating to COD removal. A higher concentration of biodegradable organic compounds, especial the volatile fatty acids was generated after the Fenton reaction (Feki et al. 2020). Through detection by Hach-Lang cuvette LCK365, the organic acid concentration in the wastewater was increased from 1.02 to 2.03 g butyric acid equivalents (BAE)/L after Fenton oxidation. After anaerobic digestion for around 20 days, the concentration of organic acid was decreased to 0.28 g BAE/L.

The LCK 345 cuvette from the Hach-Lang company was developed based on the 4-aminoantipyrine (AAP) method to measure the concentration of small molecular phenolic compounds. It was utilized to track the small phenolic compound concentrations over the wastewater treatment processes. After pretreatment by Fenton oxidation, the concentration of small molecular phenolic compounds was decreased by 70%, from 184 to 55 mg/L. It was further decreased to 24 mg/L after 20 days’ anaerobic treatment. In contrast, 84 mg/L of small molecular phenolic compounds were found in the original wastewater after anaerobic treatment. The organisms had an inhibitory effect from phenol with an aqueous concentration of 90 mg/L (Zhao et al. 2015). Therefore, the lower biogas production yield in the original sample might be owing to a continuous inhibition of small molecular phenolic compounds to organisms (Table 7).

Figure 5 shows the mass flow of COD over the course of the Fenton oxidation and anaerobic treatment. Through Fenton oxidation, the COD concentration was decreased from 16.4 to 8.7 g/L, equivalent to a 53% COD removal. During anaerobic treatment, the COD concentration was further diluted by the mixed anaerobic sludge. Over the digestion for 20 days, the COD concentration was further decreased from 4.43 to 2.41 g/L. The COD concentration could be further decreased with a longer digestion time. For future work, the adsorption effects of anaerobic sludge, biomass accumulation and the composition of produced biogas could be investigated.

The cost of Fenton oxidation was calculated from chemical consumption, sludge disposal, electricity cost and the operation costs. The energy cost of drying or transport were not focused on. To be informed by statistics from the Independent Commodity Intelligence Services (ICIS), the market price of technical grade HCl in Germany was assessed at 35–70 €/ton in 2019, thus the price of HCl at 60 €/ton was used in this work (Barker 2019). The sale price of H₂O₂ varied in the range 700–1,200 $/ton, in this work it was fixed at 800 €/ton (Ciriminna et al. 2016). The price of ferrous sulfate heptahydrate was estimated at 115 $/ton (97 €/ton) (Fac. MR 2018) (Table 8).

The NaOH was utilized to neutralize the pH value of treated wastewater to around 7. The price of NaOH was indicated with 300–500 €/ton (400 €/ton in this work) (Barker 2018). The sludge disposal costs varied strongly based on geographical region and composition. In the region of Lahn-Dill in Germany, the disposal price of the industrial sludge, which may have a high iron content was set at 220 €/ton (Lahn-Dill 2018). In this work, the disposal price of sludge was estimated at 220 €/ton. The electric consumption of the Fenton process was largely independent of the operational parameters. The energy consumption of UV irradiation was defined as additional electricity in Table 9, the price of electricity for industrial utilization was set at 0.11 €/kwh (Hein et al. 2021).

The operational costs of Fenton oxidation with different setting conditions are shown in Figure 6. The absolute costs of Fenton oxidation were between 7.3 and 33.5 euro per ton wastewater, while the specific COD elimination costs varied between 2.2 and 18.8 €/kg COD_eli. With an initial pH of 3 or 5, the main costs were the hydrogen peroxide with a share of 56–59%, this was used for neutralization (18%) and the sludge disposal observed a share of 13–16%. The sample at pH 8.9 showed the least absolute cost (no chemical costs for the adjustment of the pH value before and after the process), however owing to an inefficient reaction, it performed the high relative cost of 12.2 €/kg COD_eli.

The electricity cost of 2 hours’ UV irradiation was at around 8.3 €/ton wastewater. Although the electricity cost shared around 40–46% of total costs in UV-Fenton oxidation, the organic compounds were not obviously better degraded. The cost of COD degradation reached 3.46 and 18.83 €/kg COD_eli in tests with an initial pH of 5 and 8.9. Considering the high energy cost, the UV-Fenton oxidation is not recommended in the treatment of the condensate from gasification processes.

The influence of the initial Fe²⁺ on the costs was also investigated. As shown in Figure 6(c), the three highest costs of these tests were H₂O₂ consumption (55–64%), neutralization (15–18%) and sludge disposal (13–27%). Owing to a higher Fe²⁺dosage, the costs of neutralization, sludge disposal in the test with a molar ratio of H₂O₂ : Fe²⁺ of 10 : 1, was slightly higher than others. However, owing to a higher COD removal efficiency, the relative costs of COD elimination were similar in the range 2.26–2.19 €/kg COD_eli.

Figure 6(d) shows the cost of Fenton oxidation with the H₂O₂ dosage of 15–100% NSR at molar ratio for H₂O₂:Fe²⁺ of 15:1. The largest share of expenses in these tests is the H₂O₂. With rising H₂O₂ dosages from 15% to 100% NSR, the cost share of H₂O₂ increases from 51% to 74%. The COD removal increased correspondingly from 20% to 51%. That conforms to the increasing relative cost of COD elimination from 2.2 to 4.2 €/kg COD_eli. Moreover, the costs of sludge disposal were increased correspondingly from 1.4 to 4.0 €/ton. The lowest cost of COD degradation was 2.2 €/kg at the H₂O₂ dosage of 30% NSR.

The detoxifying effect of Fenton oxidation on a biomass gasification condensate was assessed using respiration inhibition assays. Through the thorough analysis of the samples after Fenton oxidation, the most cost-efficient operational parameters were determined with a total operational cost of 2.2 €/kg COD_eli. For the test with an initial pH 5, 30% NSR dosage of hydrogen peroxide and a molar ratio of H₂O₂:Fe²⁺ of 15:1 produced the most efficient pretreatment results. Compared with the original wastewater, the total oxygen consumption of the Fenton oxidation-treated sample was increased by 316% in a 3 days’ respiration inhibition assay. The biogas production yield relating to COD removal was increased by 81% from 0.97 to 1.61 L_biogas/g _CODeli. Compared with the conventional Fenton reaction, which aimed at the complete COD removal, the dosage of H₂O₂ and Fe²⁺ was decreased by 30 and 93% in this work. Moreover, to sustainably reuse the Fenton sludge in the future, the sludge production yield and iron content in produced precipitates were found to be between 6.2–18.1 kg/ton wastewater and 0.4–16.8%.

However, two major issues of Fenton oxidation should be overcome: the low removal efficiency of ammonium and high residual iron concentration in the effluent. The electro-Fenton oxidation approaches aim to overcome these challenges. It requires lower consumption of oxidant and Fe²⁺, leading to lower residual iron concentrations in effluent, higher removal rates of nitrogen and lower sludge yields.

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