Impact of rheological properties of substrate on anaerobic digestion and digestate dewaterability: New insights through rheological and physico-chemical interaction

Comprehensive analysis of the relationship between yield stress and dewatering performance in sludge conditioning: Insights from various treatment methods

Understanding the relationship between sludge yield stress (σy) and dewatering performance is essential for optimizing sludge conditioning processes. This study systematically investigates the effects of various conditioning methods-including thermal hydrolysis (TH), freezing/thawing (FT), anaerobic digestion (AD), polyaluminum chloride (PAC), polyacrylamide (PAM), and Fenton treatment (Fenton)-on sludge yield stress and its correlation with dewatering efficiency. Using linear regression, partial least squares regression (PLSR), and correlation heatmap analyses, we reveal significant variations in the correlation between σy and dewatering indexes, including moisture content (Mc), capillary suction time (CST), and bound water proportion (Wb/Wt), depending on the conditioning method and intensity. Under FT and PAM conditioning, σy shows a strong negative linear correlation with dewatering performance, with Pearson's r values exceeding -0.880, indicating that a decrease in σy corresponds to improved dewatering efficiency. Conversely, AD conditioning exhibits a positive linear correlation, with r values up to 0.993, suggesting that an increase in σy correlates with reduced dewatering efficiency. For TH, PAC, and Fenton treatments, the correlation between σy and dewatering metrics is highly sensitive to changes in treatment intensity. In the PLSR analysis, the VIP values, which quantify the importance of each predictor variable, indicate that Wb/Wt in TH conditioning (VIP = 1.649) and CST in PAC (VIP = 1.309) and Fenton (VIP = 1.299) conditioning strongly influence σy. This study highlights the significant impact of conditioning methods and intensities on the correlation between σy and dewatering performance. While σy provides valuable insights as a predictive indicator, its predictive power is limited in more complex conditioning scenarios. Therefore, optimizing conditioning intensity and incorporating multiple rheological parameters are essential for achieving superior sludge dewatering outcomes.

Neglected sludge solid phase in sludge pretreatment process: Physicochemical characterization and mechanism study of its role in anaerobic degradation

The low anaerobic digestion efficiency of the solid phase separated from pre-treated sludge indicates the need to explore other suitable resource utilization pathways for sludge solid phase. However, there is a lack of comprehensive and in-depth research on the physicochemical properties of sludge solid phase. This study comprehensively analyzes the characteristics of sludge solid phase and elucidates the mechanism of sludge solid phase in the anaerobic degradation of toxic wastewater. The results show that the surface free energy of sludge solid phase after different pre-treatments is mainly contributed by Lewis acid-base hydration free energy. The distribution of proteins on the surface of sludge solid phase plays a major role in the adhesion between sludge solid particles. Metal ions in the sludge solid phase are mainly present in the exchange state, followed by the carbonate state and the organics-bound state. The sludge solid phase obtained by sludge pH 12 + 150 °C treatment has the highest conductivity (1.36 mS/m) and capacitance (25.51 μF/g), mainly due to the presence of melanoidins in the sludge solid phase, which has similar semiquinone radicals to humic acids, thus increasing conductivity. The addition of sludge solid phase promotes an increase in cumulative methane production and rate of methane production. The sludge solid phase might play a role of an auxiliary carbon source acting as an adsorbent to buffer against toxicity inhibition and facilitate electron transfer. This study reveals the characteristics of sludge solid phase and its role in anaerobic digestion, providing theoretical guidance for finding suitable resource utilization pathways for sludge solid phase.

Yield stress Measurement of municipal sludge: A comprehensive evaluation of testing methods and concentration effects using a rotational rheometer

Accurate prediction and measurement of yield stress are crucial for optimizing sludge treatment and disposal. However, the differences and applicability of various methods for measuring yield stress are subjects of ongoing debate. Meanwhile, literature on measuring sludge yield stress is limited to low solid concentrations (TS <10%), understanding and studying the yield stress of medium to high solid concentration sludge is crucial due to increasingly stringent standards for sludge treatment and disposal. So, this study employed a rotational rheometer to measure sludge yield stress across a wide range of TS (4-50%) using steady shear, dynamic oscillatory shear, and transient shear. The study derived significant conclusions by comparing and summarizing the applicability and limitations of each testing method: Dynamic oscillatory shear methods, including G'-σ curve method, γ-σ curve method, and G**γc method can measure sludge yield stress ranging from 4% to 40% TS, while other methods are restricted to low or limited solid concentrations; The G' = G″ method, utilizing the intersection of G' and G″ curves, consistently yields the highest value for yield stress when 4%≤ TS ≤ 12%; The rotational rheometer cannot measure sludge yield stress when the solid concentration exceeds 40% TS; The relationship between sludge yield stress and solid concentration is stronger as a power-law for TS ≤ 25%, transitioning to linear for higher concentrations (28%≤ TS <40%). This study systematically explores the applicability and limitations of various measurement methods for characterizing sludge yield stress across a wide range of solid concentrations, providing valuable guidance for scientific measurement and highlighting challenging research issues.

Rethinking the relationships between gel like structure and sludge dewaterability based on a binary gel like structure model: Implications for the online sensing of dewaterability

The digital transformation of sludge treatment processes requires online sensing of dewaterability. This topic has been attempted for many years based on macroscopic shear rheology. However, the relationship between rheological behavior and dewaterability remains noncommittal, and the reason is unclear. Herein, a binary gel-like structure model was proposed including the interactions network at the supra-flocs level and the gel-like structure at the flocs level. Multiple advanced techniques including optical tweezers were employed to precisely understand the binary gel-like structure and to classify the correlation mechanism between this gel-like structure, rheological behavior, and dewaterability. The analysis of sludge from eight wastewater treatment plants showed the binary gel-like structures at both supra-flocs and flocs levels have significant relationships with sludge dewaterability (p < 0.05). Further deconstruction of the sludge viscoelastic behavior illustrated that the gel-like structure at the supra-flocs level dominates the rheological behavior of sludge. Moreover, the direct description of the binary gel-like structure in four typical sludge treatment processes highlighted the importance of the flocs level's structure in determining the dewaterability. Overall, this study revealed that shear rheology may prefer to stress the interactions network at the supra-flocs level but mask the flocs level's structure, although the latter is important. This observation may provide a general guideline for the design of robust sensors for dewaterability.

Physico-chemical and rheological properties datasets related to batch mesophilic anaerobic digestion of waste activated sludge, primary sludge, and mixture of sludge with organic and inorganic matter

The datasets included in this paper provide periodically measured physico-chemical and rheological properties of mesophilic batch anaerobic digesters' content for 30 days biochemical methane potential tests (BMP). Waste activated sludge (WAS) and primary sludge (PS) were the main substrates and digested sludge from a large scale mesophilic anaerobic digester was the inoculum. The substrates (F) and inoculum (I) were fed into the BMP rectors at different ratios of feed to inoculum (F/I = 1:1, 1:2, and 1:3). Experimental data on co-digestion of WAS with inorganic and organic additives were also reported. The reported characteristics such as total solids, volatile solids, total and soluble biochemical oxygen demand, ammonia, pH, as well as rheological properties over the duration of the BMP test could be used for analysing the changes in digestate properties as the anaerobic digestion process proceeds. The discussion and interpretation of the data have been provided in previous publications [[1], [2], [3]].

Using a combination of different conditioners to promote dewatering of digested sludge: Rheological characteristics

The dewatering capacity of sludge is a key factor in sludge disposal and reuse. In this study, the effects of conditioning with three conditioners (polyacrylamide (PAM), poly aluminum sulfate (PAS), and sludge biochar (SAC)) and their combined conditioning effect at different doses on the dewatering performance of digested sludge were systematically investigated. The mechanism of change in dewatering performance was analyzed based on rheological principles. A Box‒Behnken multifactor experiment based on the response surface method (RSM) was also used to establish a quadratic multiple prediction model for the solids content of filter cake to obtain the optimal ratio of coupled treatment. The results showed for individual conditioner use, PAM with a dose of 3‰ had the best effect on sludge dewatering, and the dewatering effect of the combined conditioner sludge treatment was better than that of the sludge treated with individual conditioners, with the solids content of the filter cake exceeding 35%. The Herschel-Bulkley model was used to fit the rheological data, and the results showed that the yield stress decreased with increasing PAM dose and gradually increased with increasing PAS and SAC doses. The thixotropy of sludge after SAC conditioning was evident compared to that after PAM and PAS conditioning. The yield stress of sludge decreased and flowability deteriorated after combined conditioning. There was a linear relationship between the dewatering performance of conditioned sludge and thixotropy and yield stress, which indicated the feasibility of using rheological indices to evaluate changes in sludge dewatering performance.

Under-loaded operation of an anaerobic-anoxic-aerobic system in dry and wet weather dynamics to prevent overflow pollution: Impacts on process performance and microbial community

Effects of low hydraulic loading rate (HLR) in dry weather and high HLR in wet weather on pollutant removal, microbial community, and sludge properties of a full-scale wastewater treatment plant (WWTP) were extensively studied to explore the risk of under-loaded operation for overflow pollution control. Long-term low HLR operation had an insignificant effect on the pollutant removal performance of the full-scale WWTP, and the system could withstand high-load shocks in wet weather. Low HLR resulted in higher oxygen and nitrate uptake rate due to the storage mechanism under the alternating feast/famine condition, and lower nitrifying rate. Low HLR operation enlarged particle size, deteriorated floc aggregation and sludge settleability, and reduced sludge viscosity due to the overgrowth of filamentous bacteria and inhibition of floc-forming bacteria. The remarkable increase in Thuricola and the contract morphology of Vorticella in microfauna observation confirmed the risk of flocs disintegration in low HLR operation.

Rheological properties and dewaterability of anaerobic co-digestion with sewage sludge and food waste: effect of thermal hydrolysis pretreatment and mixing ratios

Anaerobic co-digestion (co-AD) of sewage sludge (SS) and food waste (FW) converts municipal organic waste into renewable energy, which plays an important role in achieving carbon emissions reduction. The existing anaerobic digestion (AD) treatment projects often have problems such as low organic conversion and unstable performance. SS and FW were used as raw materials to explore the effects of thermal hydrolysis pretreatment (THP) and mixing ratios on the dewaterability and rheological properties of the digestate. The results showed that co-digestion of FW and SS in a ratio of 1:1 obtained the highest biogas production (255.14 mL/g VS), which was 1.53 times and 14.5 times higher than that of mono-digestion of FW and thermal hydrolysis pretreatment sewage sludge (THSS), respectively. However, the dewaterability of this ratio deteriorated sharply after co-digestion, with a decrease of 54.92%. The groups containing a higher proportion of THSS had improved dewaterability after AD. The apparent viscosity and shear stress were reduced by co-digestion compared with mono-digestion of THSS and FW, indicating a higher flow property of the co-digestion matrix. After the Herschel-Bulkley model fitting, there were linear correlations between rheological indices and soluble chemical oxygen demand (SCOD), and digestate dewaterability.

Research trends and strategies for the improvement of anaerobic digestion of food waste in psychrophilic temperatures conditions

The organic fraction of municipal solid waste is mainly composed of food waste (FW), and traditional disposal practices for this fraction are generally considered to have negative environmental and economic impacts. However, the organic characteristics of this fraction could also be exploited through the anaerobic digestion of FW (FW-AD), which represents unique advantages, including the reduction of the area required for final disposal and environmental pollution and the same time the generation of renewable energy (mainly methane gas), and a by-product for agricultural use (digestate) due to its high nutrient content. Although approximately 88% of the world's population resides in areas with temperatures below 8 °C, psychrophilic conditions (temperatures below 20 °C) have hardly been studied, while mesophilic (66%) and thermophilic (27%) ranges were found to be more common than psychrophilic FW-AD (7%). The latter condition could decrease microbial activity and organic matter removal, which could affect biogas production and even make AD unfeasible. To improve the efficiency of the psychrophilic FW-AD process, there are strategies such as: measurement of physical properties as particle size, rheological characteristics (viscosity, consistency index and substrate behavior index), density and humidity, bioaugmentation and co-digestion with other substrates, use of inocula with psychrophilic methanogenic communities, reactor heating and modification of reactor configurations. However, these variables have hardly been studied in the context of psychrophilic conditions and future research should focus on evaluating the influence of these variables on FW-AD under psychrophilic conditions. Through a bibliometric analysis, this paper has described and analyzed the FW-AD process, with a focus on the psychrophilic conditions (<20 °C) so as to identify advances and future research trends, as well as determine strategies toward improving the anaerobic process under low temperature conditions.

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Temperature has a great influence on anaerobic digestion of food waste (FW-AD).

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Studies on the psychrophilic condition are limited, warranting further research.

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Physical properties of the substrate and inoculum influence psychrophilic FW-AD.

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The use of inocula adapted to low temperatures could increase biogas production.

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Changes in reactor configurations could improve biogas yield at low temperature.

Formation mechanisms and mechanical properties of anaerobic lagoon scum

The formation of a floating scum layer on the liquid surface of covered anaerobic lagoons prevents optimal and efficient lagoon operation. Scum can reduce hydraulic retention time, inhibit biogas capture and cause damage to lagoon covers. Managing the negative impact of scum requires understanding what scum is, how it forms and how it consolidates. This paper presents measurements of the physical and mechanical properties of scum and sludge samples from two covered anaerobic lagoons that alternatively treat municipal and abattoir waste. Both scum samples consisted of a large proportion of suspended solids that sank once the sample was diluted, degassed and mixed, indicating that sludge flotation and buoyancy due to biogas generation is a major contributor to scum accumulation. Total and soluble chemical oxygen demand and volatile solids in the scum are approximately 90 % higher than in sludge, which indicates that scum has a large proportion of undigested solids. Fourier-transform infrared spectroscopy demonstrates that scum and sludge have similar organic matter, with both including fats, oils, greases, proteins, and polysaccharides. Scum formation due to gas buoyancy implies that scum accumulation is inevitable and controlling fats, oils, and greases at the source of the wastewater is not enough to stop scum formation. Scum accumulation increases due to buoyancy, which drives scum compaction and increases the strength of the scum, as demonstrated by the measurement of scum compressional rheology. Scum management techniques that disturb the scum layer early enough to release the entrapped gas enable the scum to sink and get digested, thus minimising the impact of scum formation.

Shear and solid-liquid separation behaviour of anaerobic digested sludge across a broad range of solids concentrations

Due to the non-homogeneous and multiphase nature of anaerobic lagoon constituents, CFD modelling for process optimisation requires continuous functions for shear and solid-liquid separation properties across a large range of solids concentrations. Unfortunately, measurement of existing material properties of anaerobic sludges is limited to only shear or solid-liquid separation, or to a limited solids concentration. In this work, the shear properties of an anaerobic sludge were measured from 0.4 to 12.5 vol%, which corresponds to the solids concentrations seen in lagoons. The sludge showed Newtonian behaviour at 0.4 vol% and Herschel-Bulkley yield stress fluid behaviour for higher concentrations ranging from 0.5 to 12 vol%. We compared multiple approaches to determine relationships between the model fitting parameters of consistency, k, flow index, n, and shear yield stress, τ_y with solids volume fraction ϕ.The solid-liquid separation properties were measured from sedimentation and filtration experiments to obtain compressibility and permeability properties across all the above-mentioned concentrations, enabling development of hindered velocity sedimentation curves. Comparison to full-scale anaerobic digestate identified that the pilot lagoon sludge had faster sedimentation at a given solids concentration in comparison to the digestate. This is the first study on simultaneous rheological characterisation and solid-liquid separation behaviour of an anaerobic sludge across a wide range of concentrations, thus enabling CFD modelling of the hydrodynamics and performance of anaerobic lagoons.

Effects of mild thermal pre-treatment combined with H2O2 addition on waste activated sludge digestibility

The pre-treatment of waste activated sludge (WAS) has become more common since it often results in improved bioconversion to methane, in both rate and extent. However, thorough insights on the possible effects and mechanisms of mild pre-treatment techniques, such as temperatures <100 °C combined with the addition of H2O2, are still limited. This study reports the effects of the addition of 5-30 mgH2O2/g TS and its interaction with thermal pre-treatment at 70 °C on methane production, using WAS as the substrate. It was found that the addition of H2O2 increased the methane production rate, coinciding with a decrease in apparent viscosity of WAS, which probably improved mass transfer under non-ideal mixing conditions. While H2O2 solubilized proteins and carbohydrates and mineralized a small fraction of the humic substances in WAS, these biochemical transformations did not suffice to explain the observed extent and rate of methane production. A decreased particle size, the presence of Fenton's reagent, and the presence of cationic polymers in the WAS were discarded as the reasons for the observed decrease in apparent viscosity. It was concluded that the pre-treatment conditions applied in the present study might be a strategy to enhance mixing conditions in full-scale anaerobic digesters.

The three-stage effect of hydrothermal treatment on sludge physical-chemical properties: Evolution of polymeric substances and their interaction with physicochemical properties

Hydrothermal treatment (HT) is effective for the deep dewatering of sewage sludge (SS); however, the effective temperature generally exceeds 180 ℃, resulting in the production of refractory compounds in the sludge filtrates. To explore a new process based on HT, achieving ideal dehydration efficiency at lower temperatures, it is essential to identify the key sludge dewatering mechanism under different HT stages. In this study, the relationship between the properties of sludge polymeric substances (components and molecular structures) and the physical-chemical properties of sludge flocs during HT (120-260 ℃) was investigated. The results indicated that the SS surface hydrophilicity/hydrophobicity was mainly responsible for sludge dewaterability in the solubilization (120 ℃) and hydrolyzation stages (140-180 ℃), while the mechanically bound water and capillary force were the main limiting factors of sludge dewaterability during the carbonization stage (200-260 ℃). Moreover, in the solubilization stage (120 ℃), a plenty of high-Mw (Mw > 70 kDa) polymeric substances with numerous hydrophilic functional groups and a compact structure were released from the intracellular region to the outer layer, which improved the hydrophilicity of sludge floc surface and deteriorated the sludge dewaterability. With the hydrolysis of the polymeric substances (140-180 ℃, hydrolyzation stage), the destruction of proteins secondary structures and peptide chains exposed more hydrophobic groups, resulting in the release of bound water and improvement of sludge dewaterability. At HT temperatures of 200-260 ℃ (carbonization stage), dehydration and amine aldehyde condensation occurred, benefiting the formation of fixed carbon and smooth morphology structure of SS, reducing the capillary force-induced water-holding capacity of sludge flocs. The establishment of the three-stage influencing theory and the identification of key influencing factors are conducive to the further regulation and upgrading of HT.

Revealing the variations in physicochemical, morphological, fractal, and rheological properties of digestate during the mesophilic anaerobic digestion of iron-rich waste activated sludge

Dosing of iron (Fe)-salts in sewers to control odour and corrosion problems have proven to be effective on phosphate and sulfide removal in downstream treatment units. However, the interaction of Fe with sludge may impact the sludge properties during wastewater treatment and sludge digestion. Herein, we investigated the downstream impacts of sewer-dosed Fe-salt on key digestate properties including digestate dewaterability. For this, Fe-salt was dosed to a sewer reactor and resultant iron-rich waste activated sludge (Fe-WAS) was digested in an anaerobic digester (AD) in the experimental line of integrated laboratory system running in parallel to a control system. Iron containing and non-iron containing digestates were sourced from the respective AD reactors of experimental and control lines. Results showed improved dewaterability in iron containing digestate than non-iron containing digestate, which was attributed to the variations in key digestate properties. Compared to non-iron containing digestate, iron containing digestate exhibited the decreased contents of bound water, soluble extracellular polymeric substances (S-EPS), protein, polysaccharide, and monovalent-to-divalent (M⁺/D⁺⁺) cations ratio. Likewise, we observed the increased mean particle size (D_v50) for iron containing digestate than the non-iron containing digestate, but fractal dimension (D_f) values were comparable. Besides, iron containing digestate exhibited a reduced degree of thixotropy, relative sludge network strength, viscosity, yield stress, flow stress, and storage/loss/complex (G'/G''/G∗) moduli but increased creep compliance and shear strain (%) than non-iron containing digestate. The combined synergistic effects of such favorable changes amongst the key properties of iron containing digestate, might have been responsible for improving it's dewaterability.

Anaerobic co-digestion of sewage sludge with cellulose, protein, and lipids: Role of rheology and digestibility

Rheology is known to have an impact on the performance of digesters, but the effect of additional substrates (co-digestion) is poorly understood. The main objective of this study was to investigate the effects of the addition of cellulose, protein and lipids to substrates on the rheological behaviour and biogas production of the mixture of primary sludge (PS) and waste-activated sludge (WAS) in a batch system. A mixture of PS and WAS to form the main substrate was anaerobically co-digested with different types of organic matter (cellulose, protein and lipids) as co-substrates at different co-substrate to main substrate ratios of 2-8 (wt%) under mesophilic conditions and below ammonia inhibition levels. Yield stress (τ_y) and the flow consistency index (k) of the combined feed in the case of cellulose and protein were significantly dependent on the amount of co-substrate added, while there was an insignificant impact on these properties when lipids were added. Cellulose significantly increased τ_y and k in the feed, which resulted in poor fluidity and the improper homogenisation of the digester content, and consequently decreased the biogas yield. In contrast, the biogas yield was improved through the addition of 2% to 6% protein despite an increase in τ_y and k of the feed, but the methane yield decreased at 7% and 8% levels of protein concentration. This observation indicates that the threshold for τ_y and k of the digester media depends on the organic nature and digestibility of the substrate. There was no significant impact on the flow properties of the initial mixture when lipids were added, and their addition increased the biogas yield. A first-order kinetic reaction model was used for predicting the yield of methane from these digesters. The rate constant values revealed an increasing trend, with the highest for protein then lipids then cellulose.

Free nitrous acid pre-treatment enhances anaerobic digestion of waste activated sludge and rheological properties of digested sludge: A pilot-scale study

In this study, the effects of free nitrous acid (FNA) pre-treatment on the rheological properties of digested sludge were investigated at a pilot-scale, along with the improvement in volatile solids (VS) destruction and biogas production. Two pilot-scale anaerobic sludge digesters were operated for one year, one receiving thickened waste activated sludge (TWAS) without pre-treatment (control) and one receiving TWAS pre-treated for 24 h at an FNA concentration of 4.9-6.1 mgN/L (nitrite = 250 mgN/L, pH = 5.0, T = 22-30 °C). The results confirmed the enhancing effect of FNA pre-treatment on methane production (37 ± 1%), consistent with previous laboratory studies. Equally importantly, FNA pre-treatment substantially reduced the shear viscosity of TWAS by 51 ± 8% at 100 s^-1 and 49 ± 7% at 250 s^-1, likely due to the solubilization of the TWAS (11.1 ± 0.8%). Similarly, FNA pre-treatment also reduced these viscosity parameters of the digested sludge by 80 ± 4% and 78 ± 4%, respectively, caused by both enhanced VS destruction and disintegration of the digested sludge. The dewaterability of digested sludge, assessed by dewatered solids content, capillary suction time and specific resistance to filtration, was not improved by FNA pre-treatment. The polymer requirement for dewatering was reduced by 24 ± 0.6% due to the lower solids concentration in the digested sludge achieved with FNA pre-treatment. The changes to sludge rheological properties revealed in this study further enhances the business case for the FNA pre-treatment technology.

Liquid and solids separation for target resource recovery from thermal hydrolyzed sludge

This study proposed an integrated process for biogas generation and biochar production from thermal hydrolysis pretreated sludge (THP sludge). In this study, the liquid and solids fractions of THP sludge were separately processed for the first time. The liquid fraction of THP sludge (THP-L) reached the biodegradability (262.6 ± 5.1 mL CH₄/g tCOD_feed) on the 15th day during anaerobic treatment, while the solids fraction of THP sludge (THP-S) only contributed 31.0% to the total methane production and required more than 30 days digestion time. We investigated the feasibility to convert THP-S into biochar to realize the higher value of the solids fraction. The results prove the produced biochar can be used as slow-release fertilizer. Preliminary energy analysis was performed to evaluate the energy efficiency of the integrated approach, namely, methane generation from THP-L coupled with biochar production from THP-S. The process realized energy surplus of 0.81 MWh/tonne dry sludge. In addition, THP-L digested sludge showed better dewaterability, lower yield stress and reduced viscosity during digestion. The proposed new sludge treatment process therefore has lower operating cost and higher value returns.

Methods for the Evaluation of Industrial Mechanical Pretreatments before Anaerobic Digesters

Different methods were tested to evaluate the performance of a pretreatment before anaerobic digestion. Besides conventional biochemical parameters, such as the biochemical methane potential (BMP), the methane production rate, or the extent of solubilization of organic compounds, methods for physical characterization were also developed in the present work. Criteria, such as the particle size distribution, the water retention capacity, and the rheological properties, were thus measured. These methods were tested on samples taken in two full-scale digesters operating with cattle manure as a substrate and using hammer mills. The comparison of samples taken before and after the pretreatment unit showed no significant improvement in the methane potential. However, the methane production rate increased by 15% and 26% for the two hammer mills, respectively. A relevant improvement of the rheological properties was also observed. This feature is likely correlated with the average reduction in particle size during the pretreatment operation, but these results needs confirmation in a wider range of systems.

Digestate management for high-solid anaerobic digestion of organic wastes: A review

Digestate management for anaerobic digestion (AD) is becoming a bottleneck of the sustainability of AD plants when the use of digestate for agricultural application is restricted due to nutrient surplus and low market acceptance. Digestate quality and treatment in high solid anaerobic digestion (HSAD) can be better than conventional low-solid system. The rheological behavior of digestate in high solid anaerobic digestion (HSAD) can have a great impact on the energy consumption of digestate management. After post-conditioning guided by rheological parameters, the solid digestate can be further treated based on the integrated solutions to enhance the energy efficiency or nutrients recovery. The environmental impacts for some core parts of those integrated systems were also evaluated in this study. This article presented a critical review of recent investigations of digestate management for HSAD, especially focusing on the rheology of HSAD digestate, integrated solutions and their environmental performances.

Effects of post anaerobic digestion thermal hydrolysis on dewaterability and moisture distribution in digestates

Organic waste fractions such as sewage sludge, food waste and manure can be stabilized by anaerobic digestion (AD) to produce renewable energy in the form of biogas. Following AD, the digested solid fraction (digestate) is usually dewatered to reduce the volume before transportation. Post-AD treatments such as the Post-AD thermal hydrolysis process (Post-AD THP) have been developed to improve the dewatering, but the mode of action is not well understood. In this study, samples from 32 commercial full-scale plants were used to assess the impact of Post-AD THP on a broad range of raw materials. Maximum dewatered cake solids after Post-AD THP was predicted by thermogravimetric analysis (TGA). Post-AD THP changed the moisture distribution of the samples by increasing the free water fraction. A consistent improvement in predicted dewatered cake solids was achieved across the 32 samples tested, on average increasing the dry solids concentration by 87%. A full-scale trial showed that dewatering Post-AD THP digestate at 80 °C improved dewatered cake solids above the predictions by TGA at 35 °C. In conclusion, dewatered cake solids were significantly improved by Post-AD THP, reducing the volume of dewatered cake for disposal.

CNash - A novel parameter predicting cake solids of dewatered digestates

Efficient digestate dewatering is crucial to reduce the volume and transportation cost of solid residues from anaerobic digestion (AD) plants. Large variations in dewatered cake solids have been reported and predictive models are therefore important in design and operation of such plants. However, current predictive models lack validation across several digestion substrates, pre-treatments and full-scale plants. In this study, we showed that thermogravimetric analysis is a reliable prediction model for dewatered cake solids using digestates from 15 commercial full-scale plants. The tested digestates originated from different substrates, with and without the pre-AD thermal hydrolysis process (THP). Moreover, a novel combined physicochemical parameter (C/N•ash) characterizing different digestate blends was identified by multiplying the C/N ratio with ash content of the dried solids. Using samples from 22 full-scale wastewater, food waste and co-waste plants, a linear relationship was found between C/N•ash and predicted cake solids for digestates with and without pre-AD THP. Pre-AD THP improved predicted cake solids by increasing the amount of free water. However, solids characteristics like C/N ratio and ash content had a more profound influence on the predicted cake solids than pre-AD THP and type of dewatering device. Finally, C/N•ash was shown to have a linear relationship to cake solids and reported polymer dose from eight full-scale pre-AD THP plants. In conclusion, we identified the novel parameter C/N•ash which can be used to predict dewatered cake solids regardless of dewatering device and sludge origin.