Low-temperature Thermal Pretreatment Research Articles

Optimization of biogas production from thermal-alkali pre-treated sludge using response surface methodology and random forest regressor model

BackgroundUnderstanding the intricate relationships between input variables and methane potential is crucial for optimizing anaerobic digestion process after thermal-alkali pre-treatment. This study aimed to optimize low-temperature thermal alkali pre-treatment, focusing on dosage based on the total solids (TS) content of raw sludge. A comparative analysis was conducted to optimize input parameters related to biogas generation potential as the primary response. MethodsBiomethane potential data were collected following the pre-treatment of sludge within the temperature range of 40–70 °C, using various doses of NaOH, KOH, Mg(OH)2, and Ca(OH)2 ranging from 40 mg alkali/g TS to 120 mg alkali/g TS. The optimization of biogas and methane production was conducted through Response Surface Methodology (RSM) and the Random Forest Regressor (RFR) model considering dosage and temperature as input variables. Additionally, sensitivity analysis was performed using the permutation feature importance function to gain insights into the influence of alkali types on biogas and methane production. FindingsThe results obtained from the RFR model showcased a notably high accuracy in predicting optimal solutions. NaOH addition was the most effective pre-treatment with an optimal dose of 65 mg/g TS and at a temperature of 56 °C. NaOH pre-treatment increased methane yield to 414.40 mL/g VS, compared to 117.08 mL/g VS for the untreated sludge. The sensitivity analysis results indicated that the temperature had a more significant effect on biogas potential than the alkali dosage.

Evaluation of the optimal sewage sludge pre-treatment technology through continuous reactor operation: Process performance and microbial community insights

This study investigated the impact of two low-temperature thermal pre-treatments on continuous anaerobic reactors’ performance, sequentially fed with sludge of different total solids content (∼3 % and ∼6 %) and subjected to progressively increasing Organic Loading Rates (OLR) from 1.0 to 2.5 g volatile solids/(LReactor⋅day). Assessing pre-treatments’ influence on influent sludge characteristics revealed enhanced organic matter hydrolysis, facilitating sludge solubilization and methanogenesis; volatile fatty acids concentration also increased, particularly in pre-treated sludge of ∼6 % total solids, indicating improved heating efficiency under increased solids content. The reactor fed with sludge pre-treated at 45 °C for 48 h and 55 °C for an extra 48 h exhibited the highest methane yield under all applied OLRs, peaking at 240 ± 3.0 mL/g volatile solids at the OLR of 2.5 g volatile solids/(LReactor⋅day). 16S rRNA gene sequencing demonstrated differences in the reactors’ microbiomes as evidence of sludge thickening and the different pre-treatments applied, which promoted the release of organic matter in diverse concentrations and compositions. Finally, the microbial analysis revealed that specific foam-related genera increased in abundance in the foam layer of reactors’ effluent bottles, dictating their association with the sludge foaming incidents that occurred inside the reactors during their operation at 2.0 g volatile solids/(LReactor⋅day).

Increasing the Biomethane Yield of Hazelnut By-Products by Low Temperature Thermal Pretreatment

Biomethane energy, which has the status of renewable energies, has the potential to be produced from all kinds of organic wastes, as well as from lignocellulosic materials, which are the most common in nature. In this study, hazelnut shells (HS), one of the hazelnut by-products, were used for biomethane production. In order to obtain higher yields from HS, thermal pre-treatments were applied at temperatures of 60°C, 80°C and 100°C. Pretreatment effects were controlled by lignocellulosic substance amount determinations. As a result of thermal pretreatment at 100°C for 2 h, cellulose and lignin removals occurred approximately 15% and 30%, respectively. While the cumulative biomethane yield of raw HS was 32.3 mL•g total solids (TS)‒1, the cumulative biomethane yields of 100°C pretreated HS were measured as 132.3 mL•gTS‒1. As a result of different pretreatment temperatures, different cumulative biomethane yield curves were successfully simulated with the Modified Gompertz equation and R2 values were found to be between 0.9962 - 0.9985.

The novel application of polyoxometalates for achieving sludge deep dewatering using low-temperature thermal hydrolysis pretreatment

Traditional thermal hydrolysis pretreatment (THP) for sludge deep dewatering is energy-intensive and poses the risk of generating bio-toxic substances that affect downstream wastewater treatment processes. To address these challenges, this study introduces the novel application of polyoxometalates (POMs) in THP to achieve sludge deep dewatering at lower temperatures. Specifically, under the optimal conditions with a POMs dosage of 1.0 mM/g VSS and a THP temperature of 140 °C, a reduction of 30.36%, 88.31%, and 55.04% in WC, SRF, and CST, respectively, was achieved compared to 20-RS-THP. Further analysis revealed that both the acidity and the biological effect of POMs played the dominant role in enhancing sludge dewatering. Spectroscopic techniques and molecular simulations revealed changes in protein structure during THP induced the POMs-proteins interaction. Results revealed that at lower THP temperatures (<80 °C), POMs induced protonation of functional groups in amino acid residues, strengthening electrostatic interactions. As temperatures increased (80–140 °C), more binding sites in hydrophilic amino acid residues became available for hydrogen bonding with POMs. This interaction led to structural changes in proteins, facilitating water liberation and the formation of sludge colloidal aggregates with improved drainage channels. However, at higher temperatures (140–180 °C), the exposure of hydrophobic sites, the loss of hydrophilic amino acids, and the occurrence of the Maillard reaction in proteins partially offset POMs' binding effects. Moreover, the development of magnetized POMs-based materials may offer sustainability and economic benefits for POMs-based sludge treatment and disposal technologies.

Low-temperature thermal pre-treated sewage sludge for feeding of black soldier fly (Hermetia illucens) larvae: Protein, lipid and biodiesel profile and characterization

Previous work showed that low-temperature thermal pre-treatment of waste activated sludge (WAS) under different temperatures (30, 60, 75, 90 °C) and durations (2, 4, 8, 16 h) prior to black soldier fly larvae (BSFL) rearing could improve the larval growth. In this work, the biochemical profiles of WAS-fed BSFL and the characteristics of biodiesel produced were reported. The BSFL fed with pre-treated WAS had high protein contents (44%–54%) and moderate lipid contents (21%–25%). The WAS pre-treated at 90 °C for 16 h produced BSFL with the highest protein and lipid yields, achieving 69% and 71% of increments, respectively, as compared with the controlled WAS. Subsequently, high conversion of lipid into biodiesel (>90%) could be achieved through the acid-catalyzed transesterification process. The BSFL biodiesel was dominated by saturated fatty acid methyl esters (FAME), with C12:0 being the abundance. This unique FAME profile enabled BSFL biodiesel to have low iodine value, high cetane number and more oxidatively stable as compared with other reported biodiesels like palm oil and rapeseed oil biodiesels. The BSFL biodiesel also satiated the biodiesel standards of EN 14214 and ASTM D6751, except for the flash point, which was still higher and safer than petroleum diesel. In short, the thermal pre-treatment of WAS was effective in promoting BSFL growth and its lipid yield, ultimately leading to higher biodiesel production.

Preparation of the modified sludge-semi-coke water slurry and analysis of its slurring performance and mechanism

The sludge-semi-coke water slurry (S-SCWS) and modified sludge-semi-coke water slurry (MS-SCWS) can be used as liquid fuels for combustion and gasification. These slurries have apparent economic advantages in sludge recovery and utilization. High concentration MS-SCWS can be prepared by low-temperature thermal alkali-modified sludge. In this paper, the modified sludge was characterized by Fourier transform infrared spectrometry (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), and nitrogen adsorption-desorption methods. The adsorption experiments of composite anionic dispersant (NNO/PCE) on semi-coke, sludge and modified sludge demonstrate that low temperature thermal alkali method changes the properties of sludge particles including hydrophilicity, pore structure, surface properties, mineral composition and zeta potential. The apparent viscosity of MS-SCWS prepared from modified sludge was investigated under different modification conditions, such as temperature (25 °C–90 °C), time (60 min–150 min), and the does of sodium hydroxide (15 wt%-30 wt%). Under the optimized low-temperature thermal alkali experimental conditions, the maximum solid concentration of MS-SCWS reached from 66.67% to 68.1%, when the amount of NNO/PCE dispersant dosage was 0.8 wt%, the apparent viscosity decreased from 711.2 mPa·s to 434.2 mPa·s, and the water separation rate (WSR) declined from 6.09% to 0%, while the stability improved significantly. The adsorption kinetics of NNO/PCE dispersant on the surface of semi-coke, sludge, and modified sludge particles showed that the quasi-second-order adsorption kinetic equation can better describe the adsorption process of dispersant on the particle surface. The result of isothermal adsorption experiment showed that the saturated adsorption capacity of NNO/PCE on the surface of semi-coke particles was significantly higher than that of sludge and modified sludge. The adsorption of NNO/PCE on all particle surfaces was monolayer adsorption. The low-temperature thermal alkali pre-treatment of municipal sludge could change its physical and chemical properties. This method is an effective way to develop and utilize sludge on a large scale. • MS-SCWS was prepared by semi-coke powders, modified sludge and dispersant. • The maximum solid concentration of MS-SCWS reached from 66.67% to 68.1% • The electrostatic interaction of particles improved the stability of the MS-SCWS. • The adsorption of dispersants on semi-coke powders was monolayer adsorption. • The interaction mechanism between the dispersants and particles was proposed.

Hydrolysis kinetics for solubilizing waste activated sludge at low temperature thermal treatment derived from multivariate non-linear model

Low temperature thermal pre-treatment is a low-cost method to break down the structure of extracellular polymeric substances in waste activated sludge (WAS) while improving the sludge biodegradability. However, previous models on low temperature thermal pre-treatment did not adequately elucidate the behaviour of sludge hydrolysis process for the duration ranging from 5 to 9 h. Therefore, this work had developed an inclusive functional model to describe the kinetics of sludge hydrolysis for a wide range of treatment conditions (30 °C–90 °C within 0 and 16 h). As compared with treatment duration, the treatment temperature played a greater impact in solubilizing WAS. Accordingly, the 90 °C treatment had consistently produced WAS with the highest degree of solubility. Nonetheless, the mediocre discrepancies between 90 °C and 75 °C may challenge the practicality of increasing the treatment temperatures beyond 75 °C. The effects of treatment duration on soluble chemical oxygen demand, soluble carbohydrate and soluble protein were only significant during the first 4 h, except for humic substances release that continued to increase with treatment duration. Finally, a good fit with R2 > 0.95 was achieved using an inclusive multivariate non-linear model, substantiating the functionality to predict the kinetics of sludge hydrolysis at arbitrary treatment conditions.

Enhanced Deep Utilization of Low-Organic Content Sludge by Processing Time-Extended Low-Temperature Thermal Pretreatment.

Anaerobic digestion is an important way for maintaining sewage sludge stability, reduction, and resource recovery. However, the low organic content generally limits methane production. Recently, thermal hydrolysis has been widely used for sludge pretreatment to improve the anaerobic digestion efficiency. Generally, an increased temperature is preferred to enhance the solubility of organic matters in the sludge. However, high energy requirement comes with increased temperature. Application of low-temperature thermal treatment could overcome this drawback. However, the appropriate low-temperature pretreatment time is still uncertain. In this study, an extended contact time with low thermal pretreatment (90 °C) was chosen to realize a more efficient and economical digestion process of low-organic content sludge. The results demonstrated that the solubilization of proteins and carbohydrates was significantly promoted by the contact time-extended thermal hydrolysis pretreatment. The following anaerobic digestion efficiency of low-organic content sludge was also dramatically improved with the prolonged contact time. The maximum methane production could reach around 294.73 mL/gVS after 36 h of 90 °C treatment, which was 5.56 times that of the untreated groups. Additionally, based on the energy balance calculation, extending the thermal hydrolysis time resulted in a more economically feasible anaerobic digestion than increasing the temperature. The dewatering properties and the stability of the heavy metals were also reinforced, implying the advanced deep utilization of the digested low-organic content sludge. In conclusion, sludge pretreated by low-temperature thermal hydrolysis with a prolonged contact time could be more effective for low-organic content sludge treatment and disposal.

Life cycle assessment of sewage sludge pretreatment for biogas production: From laboratory tests to full-scale applicability

In this work, different pretreatment technologies (low temperature thermal treatment, ultrasonication, alkali treatment, combined alkali-thermal treatment, icing-thawing) were investigated together with biochar addition to increase biogas yield from sewage sludge. The aim of the work was to develop a standardized protocol, including physicochemical characterization, biochemical methane potential (BMP) tests, digestate characterization, life cycle assessment (LCA) and economic analysis, to verify the techno-economic suitability and the related environmental impacts of upgrading conventional full-scale anaerobic digesters. Low-temperature (65–85 °C) thermal pretreatment showed the best performances in enhancing sludge solubilization and methane yield (up to 110%), followed by ultrasonication (up to 53%) and biochar addition (about 16%). The solubilization rate obtained after each pretreatment was similar for municipal and industrial sludge; however, it did not correspond to a proportional increase in methane yield. LCA showed that ultrasonication led to the worst environmental impacts at laboratory scale, due to the huge electricity request; however, at full-scale conditions, green-house gases (GHG) emissions could be reduced by 22%, fossil depletion by 25% and stratospheric ozone depletion by 18%, when compared to the baseline scenario (raw sludge anaerobic digestion). The economic analysis proved that the high ultrasonication capital costs could not be recovered in the considered timeframe (15 yr), while biochar price should be significantly lowered (down to 6.5 €/ton) to give an overall positive outcome. Finally, the thermal scenario would be convenient only when considering waste heat recovery. The proposed protocol, including physicochemical sludge characterization, BMP tests, digestate characterization, LCA and economic analysis, could be extended to other existing digesters to compare alternative pretreatment strategies, enhancing renewable energy generation in biogas form.

Pretreatments for enhanced biomethane production from buckwheat hull: Effects on organic matter degradation and process sustainability

Buckwheat manufacturing produce a large amount of lignocellulosic residue (buckwheat hull), which could be used as substrate in anaerobic digestion, even if hard lignocellulosic structure represents the main obstacle for its degradation. This study presents the results of a laboratory experiment conducted to evaluate the effects of different pretreatments on buckwheat hull anaerobic digestion. To achieve the aim, five pretreatments (alkaline, thermo-alkaline, microwave, ultrasonication and low temperature thermal pretreatment) were studied and the results were compared to non treated buckwheat hull. Cumulative biomethane yields significantly increased after alkaline and thermo-alkaline pretreatments (+61% and +122% with respect to non treated hull, respectively). These results were mainly related to organic matter solubilisation (+772% and +859% of soluble reducing sugars, respectively) and lignin, hemicellulose and cellulose degradation. Overall, process parameters behaviour and digestate quality were not affected by the pretreatments. Alkaline and thermo-alkaline pretreatments were evaluated for their energetic and economic affordability, showing that combination of thermal and alkaline pretreatments ensures significant advantages.

Study on Dissolution Characteristics of Excess Sludge by Low-Temperature Thermal Hydrolysis and Acid Production by Fermentation.

To investigate the dissolution characteristics of low-temperature thermal pretreatment conditions and the process of sludge fermentation to produce acid, the influence of thermal pretreatment temperature on the dissolution of excess sludge organic composition and the mechanism of cell crushing of sludge thermal pretreatment were analyzed by an experimental method, and the performance of acid production was explored by sludge fermentation after pretreatment at different temperatures. The performance of acid production by sludge fermentation after pretreatment at different temperatures was measured. The results proved that the soluble chemical oxygen demand (SCOD) shows the largest increase in dissolution rate (11.92%) at 70 °C and in dissolution quantity (6518.33 mg/L) at 90 °C. However, at 80 °C, the solubility of total organic carbon (TOC) is the highest (3224.47 mg/L), and at 70 °C, the best dissolution conditions for soluble carbohydrate (SC) and soluble protein (SP) reached 340.07 and 80.92 mg/L, respectively. The degree of sludge breaking starts to increase at 70 °C. Correlation analysis shows that dissolved organic matter is mainly derived from the cell wall and intracellular material and SP is mainly derived from intracellular material. Excitation–emission matrix spectra and parallel factor analysis (EEM-PARAFAC) divides the sludge dissolved organic matter (DOM) into five fluorescent components, including C1 (318/366) tyrosine, C2 (418/470) UVA humic acid, C3 (282/334) tryptophan substances, C4 (322/430) UVC humic acids, and C5 (314, 382, 454/526) UVA humic substances. Fermentation acid production experiment shows that the peak concentration is highest at 80 °C, the arrival time is 2 days, and the acid production type is butyric acid fermentation. Thus, it is proved that low-temperature thermal pretreatment promotes the process of acid-producing fermentation and has no effect on the type of fermentation. The optimal condition for hydrolytic dissolution and acid production under low-temperature thermal pretreatment is 80 °C.

Low-temperature thermal pretreatment process for recycling inner core of spent lithium iron phosphate batteries.

Spent lithium iron phosphate (LFP) batteries contain abundant strategic lithium resources and are thus considered attractive secondary lithium sources. However, these batteries may contaminate the environment because they contain hazardous materials. In this work, a novel process involving low-temperature heat treatment is used as an alternative pretreatment method for recycling spent LFP batteries. When the temperature reaches 300°C, the dissociation effect of the anode material gradually improves with heat treatment time. At the heat treatment time of 120 minutes, an electrode material can be dissociated. The extension of heat treatment time has a minimal effect on quality loss. The physicochemical changes in thermally treated solid cathode and anode materials are examined through scanning electron microscopy with energy-dispersive X-ray spectroscopy. The heat treatment results in the complete separation of the materials from aluminium foil without contamination. The change in heat treatment temperature has a small effect on the quality of LFP material shedding. When the heat treatment temperature reaches 300°C and the time reaches 120 minutes, heat treatment time increases, and the yield of each particle size is stable and basically unchanged. The method can be scaled up and may reduce environmental pollution due to waste LFP batteries.

Effects of low temperature thermal pretreatments in solubility and co-digestion of waste activated sludge and microalgae mixtures

A low temperature thermal pretreatment was applied to three different ratios of waste activated sludge (WAS) and microalgae mixtures to increase their solubility. The performance of three organic flocculants was assessed in order to select the best flocculant for previous microalgae harvesting. The effect of the following variables in the solubilisation were evaluated: ratio of the mixture of WAS and (flocculated and non-flocculated) microalgae (50:1, 25:1 and 10:1, in v:v basis), incubation time (24, 48 and 72 h) and temperature (37 and 60 °C). A cationic polymer of diallyl dimethyl ammonium chloride free of acrylamide was selected for microalgae flocculation due to its high harvesting efficiency (95%). In pretreatments at 37 °C, effect of temperature and time in the solubilisation of the mixtures was negligible. However, results showed a high increase in solubilisation pretreatments at 60 °C using both flocculated and non-flocculated microalgae in the mixtures. Initial soluble chemical oxygen demand in the mixtures containing WAS and flocculated microalgae was higher in comparison with that in the mixtures of the same ratio without previous flocculation of the microalgae. According to the results, the optimal pretreatment conditions were incubation of the mixtures during 24 h at 60 °C in a 25:1 (WAS:microalgae, in v:v basis) mixing ratio. Thus, these optimal mixtures using flocculated microalgae and non-flocculated microalgae were chosen to assess the effect of the pretreatment in the biogas yield. The anaerobic co-digestion of the selected mixtures indicated a lack of proportionality between the soluble chemical oxygen demand attained during the pretreatment and the methane yield obtained (51 and 34% lower in comparison to the control, respectively). This behaviour could be explained by the high organic matter consumption in the low temperature thermal pretreatment before the anaerobic co-digestion, which could have been fostered by the enzymatic activity of the sludge.

Stability of Thermophilic Pig Manure Mono-digestion: Effect of Thermal Pre-treatment and Separation

Anaerobic pig manure digestion holds potential to contribute to a bio-based economy. This work assesses the stability of the thermophilic mono-digestion process. Thermophilic mono-digestion experiments with (i) fresh liquid pig manure and (ii) the fresh fecal fraction from source separation by a pig housing construction were conducted in semi pilot-scale continuous stirred tank reactors. The effect of separation on the digestion stability was studied by comparing thermophilic mono-digestion of fresh liquid (unseparated) and fresh source separated pig manure. Influencing factors and inhibitors were identified during the experiments. An unstable thermophilic mono-digestion process was observed for fresh liquid pig manure at a digester retention time of 60 days, due to high levels of ammonia and sulfur-containing components. Thermophilic mono-digestion of the fresh fecal fraction was more promising in terms of stability, provided enough time for digestion. In addition, the effect of low temperature (70 degrees C) thermal pre-treatment of manure on the digestion stability was investigated. In the case of liquid pig manure, no improvement in the digestion stability was noted upon thermal pre-treatment. For the fecal fraction, the stability of the thermophilic mono-digestion process did improve. Moreover, thermal treatment and subsequent thermophilic mono-digestion of the fresh fecal fractions from two different farms with a similar pig housing construction suggested an effect of the (organic) dry matter content on the process stability.

Effects of mixed alkali-thermal pretreatment on anaerobic digestion performance of waste activated sludge

In the present study, the effect of low-temperature thermal and alkali pretreatments on biodegradability of waste activated sludge was evaluated. Combined with low-temperature thermal pretreatment (80 °C), varied proportions of a mixed alkali (i.e., a mixture of NaOH and Ca [OH]2) were added. The pretreatment procedures were conducted in different orders; thermal-alkali pretreatment (TAP), simultaneous thermal and alkali pretreatment (STAP), and alkali-thermal pretreatment (ATP). Following pretreatment, the sludge with the alkali-thermal pretreatment was most considerably dissolved. The SCOD concentration was up to 18,000 mg/L in the WAS, and the dissolution rate reached 112.8%. Therefore, the anaerobic digestion (AD) performance of waste activated sludge (WAS) by mixed alkali-thermal pretreatment was investigated. With an increase in sodium hydroxide content in the mixed alkali, the cumulative methane production (CMP) of the sludge increased accordingly. The maximum CMP (i.e., 430 mL/g VS [volatile solids]), increased by 308.7% compared with untreated sludge and was obtained by the pretreatment of the mixed alkali (NaOH/Ca(OH)2 = 4:1) at 80 °C. The results show that low-temperature thermal treatment combined with the mixed alkali treatment can reduce the inhibitory effect of mixed alkali on AD, thereby shortening the inhibition period (6d) and increasing methane production (308.7%). The preliminary economic feasibility analysis also showed that the ATP was a cost-effective pretreatment method.

Improved bioleaching of copper and zinc from brake pad waste by low-temperature thermal pretreatment and its mechanisms

A considerable amount of brake pad waste which is composed of phenolic resin and a variety of toxic heavy metals is produced both in China and around the world owing to the flourishing automobile industry. The safe, low cost and eco-sound bioleaching was utilized to extract the valuable metals Cu and Zn from the waste. The results showed that although bioleaching is more efficient in the extraction of Cu and Zn than the chemical counterpart, rather low bioleaching yields of 34% for Cu and 72% for Zn were obtained because of the complicated components and refractory nature of the waste. However, a low-temperature thermal pretreatment at 400 °C notably lifted the bioleaching efficiencies of Cu and Zn to 98% and nearly 100%, respectively. The thermal treatment removed the oil substances, transformed the acid insoluble Cu0 into acid soluble CuO and destroyed the chelation/complexation of the phenolic resin to loose Cu and Zn, promoting bioleaching performance of Cu and Zn. The combined processes of low-temperature thermal pretreatment and bioleaching is totally qualified for the extraction of Cu and Zn from the refractory waste.

Evaluation of Low-temperature Thermal Pre-treatment and Biogas Characteristics using Waste Activated Sludge

Evaluation of Low-temperature Thermal Pre-treatment and Biogas Characteristics using Waste Activated Sludge

Effects of thermal conditions on char yield and char reactivity of woody biomass in stepwise pyrolysis

Stepwise pyrolysis with one low-temperature thermal pretreatment (torrefaction or carbonization) and one high-temperature pyrolysis is an important utilization of biomass. This study aims to investigate the effects of thermal conditions on char yield and char reactivity in the stepwise pyrolysis with woody biomass used as feedstock. The accumulated char yield (on original mass basis) and char reactivity were determined. Pyrolysis products were analyzed by Fourier Transform Infrared Spectroscopy (FTIR) and Raman spectrum. The results show that thermal pretreatment increases the aromaticity of feedstock as well as char yield, and reduces the char reactivity and degree of order, which indicate probable intrinsic correlation among them. The effect of thermal pretreatment on char yield is more pronounced at elevated pyrolysis temperature, but indiscernible at low heating rate. The influences of particle size and residence time of thermal pretreatment are relatively small.

A combination of ammonia stripping and low temperature thermal pre-treatment improves anaerobic post-digestion of the supernatant from organic fraction of municipal solid waste treatment

Municipal Solid Waste is mostly composed of organic material which is often treated in anaerobic reactors in waste treatment plants. In most cases, the obtained digestate undergoes a solid/liquid separation step, producing a liquid fraction (known as anaerobic supernatant) rich in ammonium nitrogen that has to be further treated in order to meet discharge standards. The objective of the present work was to evaluate the feasibility of anaerobic post-digestion (37 °C) of a supernatant rich in carbon and nitrogen from a municipal waste treatment plant. In order to increase the efficiency of the process, a combined pre-treatment consisting of a low-temperature thermal process (75 °C) and ammonia stripping (1.3 Lair Lsupernatant−1 min−1) was applied. The effects of pre-treatment contact time (4 and 8 h) and the hydraulic retention time (HRT) in the anaerobic reactor (20–40 d) were studied. Supernatant pre-treatment with 8-h contact time caused 13% organic matter solubilisation, thus improving methanisation by 18% when the HRT was 40 d. At the same time, ammonia stripping allowed to maintain ammonia concentration in the digester below inhibitory values (less than 100 mg N-NH3 L−1) enabling therefore high methanogenic activity (>0.23 g COD g−1 VS d−1). The final effluent characteristics (low total ammonia nitrogen and aerobically biodegradable organic matter levels) would permit implementing subsequent less energy intensive and more environmental-friendly technologies (such as partial nitritation/anammox) to comply with discharge limits.

Modified ADM1 for high-rate anaerobic co-digestion of thermally pre-treated brewery surplus yeast wastewater.

Co-digestion of surplus yeast with brewery wastewater is a potentially economical method for recovering energy, in the form of biogas, from this difficult to dispose of by-product. In this work a modified version of the ADM1 (Anaerobic Digestion Model No. 1) was calibrated for an anaerobic digester fed with thermally pre-treated brewery yeast surplus wastewater. The model could predict changes to reactor methane production and reduction of biodegradable matter when fed with both pre-treated and untreated yeast surplus wastewater at varying loading rates. Model calibration focused on low temperature thermal pre-treatment as experiments into a combined thermal-alkaline pre-treatment did not show any significant improvements. A low temperature pre-treatment of 60 °C for 30 minutes was sufficient for yeast inactivation and allowed for stable and more efficient operation of the high-rate anaerobic digester over a period of 232 days. The low temperature and time for pre-treatment also reduced the evaporation of easily biodegradable residual ethanol present in the influent, while still maintaining a low level of suspended matter. Inline measurements of gas composition, production and effluent chemical oxygen demand were sufficient for reliable model calibration of these same outputs. More detailed characterization of influent and effluent is required if organic acid concentrations for pH control are needed.