Sludge Rheology Research Articles

Rheological properties of thermally treated and digested sludge: Implications for energy requirements of pumps and agitators

Understanding sludge rheology and optimizing equipment performance is crucial for energy efficiency in wastewater treatment plants (WWTPs). This study examined sludge rheology after thermal hydrolysis pre-treatment (THP) at 60, 80, and 120 °C for 2 h, followed by anaerobic digestion (AD) at 37 °C for 20 days, and assessed impacts on pump and agitator performance. Post-treatment, sludge showed reduced viscosity and improved flowability, indicated by changes in Herschel-Bulkley parameters, enhancing pump and agitator efficiency, particularly at 120 °C. These rheological improvements were correlated to the solubilization of sludge components after THP and solids reduction after AD, highlighting the interconnectedness of rheology and treatment outcomes. Despite high heat demands, an energy balance showed that THP scenarios, especially at 120 °C, had lower energy requirements for pumps and agitators, leading to energy savings without increased heat consumption. These findings underscore the influence of rheological changes in improving energy efficiency in WWTPs.

Preliminary analysis: Effect of a rotary generator of hydrodynamic cavitation on rheology and methane yield of wastewater sludge

Slightly acidic (pH 5.1) waste sludge with 4.7 % Total Solids (TS) was treated on a laboratory scale pined disc rotary generator of hydrodynamic cavitation (PD RGHC). Influence of four rotor discs with different number of cavitation generation units (CGUs) was investigated: 8-pins, 12-pins, 16-pins and 8-prism elements. The effect of hydrodynamic cavitation (HC) was investigated by analyzing rheological properties, surface tension, dewaterability, and particle size distribution. After subjecting the sludge to 30 cavitation passes, the dewatering ability of the sludge significantly decreased, resulting in a more than two-fold increase in Capillary Suction Time (CST). All regimes were successful in disintegrating particles to smaller sizes. A slight increase of sludge surface tension was measured post cavitation. Cavitated samples displayed a zero-shear viscosity, in contrast to the untreated sample, where viscosity noticeably increased as shear stress decreased. HC did not improve methane yield. Statistically significant correlations between physio-chemical properties and apparent viscosity at low shear stress were identified. Although there were no discernible statistical differences in sludge characteristics, some trends are visible among investigated CGU designs and warrant further research.

A model-based approach for evaluating the effects of sludge rheology on methane production during high solid anaerobic digestion: Focusing on mass transfer resistance

Mass transfer process is vital for biochemical reactions, which is related to sludge rheology. But this inherent interaction has not yet been established for sewage sludge anaerobic digestion (AD). This work investigated the role of sludge rheology in bio-kinetics of AD. AD of sodium acetate and microcrystalline cellulose under different total solids (TS of 6.29%, 7.63%, 9.02%, 11.06%) of digested sludge was set up to identify the role of sludge rheology. Results showed that rheological properties, e.g., viscosity and viscoelasticity, increased with the increase of TS content. These resulted in negative effect on methane production of sodium acetate with lag phase increased from 0.93 d to 1.57 d, but almost no effect was observed for microcrystalline cellulose. An extended ADM1 model was developed, which revealed that enhanced sludge rheological properties increased mass diffusion resistance and reduced uptake rate of acetate. This effect can be described explicitly by mass transfer coefficient (kla_Sac). However, from the original ADM1, this effect was indistinctly lumped in apparent half-saturation coefficient (ksac). Based on the role of sludge rheology in mass transfer and bio-kinetics, it is suggested that modeling the methane production based on TS contents and sludge rheology is possible, which should be considered in giving an insight into the micro-scale phenomena (e.g. mass transport, bio-kinetics) in high solid AD.

Optimising flow and mixing in a full-scale gas-mixed anaerobic digester by integrating sludge rheological data using computational fluid dynamics

Gas-mixing is commonly applied in anaerobic digesters, yet the resulting flow and hydraulic mixing are difficult to evaluate because of limited full-scale experimental data and uncertainties in integrating sludge rheological data. This study used computational fluid dynamics (CFD) to assess the impact of treated sludge rheology on flow and mixing characterisation in a full-scale biogas-mixed digester. The CFD model, which was firstly validated using a lab-scale setup, showed that flow and mixing predictions depended on the rheological properties, especially at low shear rates. The predicted dominant shear rate was out of the effective shear-rate range of the Ostwald model, leading to flow and mixing performance overestimation. The results indicated that there are limitations in applying the Ostwald model and the conventional approaches for determining dead-zone. The Herschel-Bulkley model was more appropriate for the prevailing low shear rates and predicted large viscosity gradients in the digester, indicating two distinct compartments with different flow and mixing behaviour based on the gas-sparging height: a plug-flow compartment with dominant vertical convection above, and a dead-zone compartment with considerable segregation below. The results showed that the applied gas-sparging induced insufficient flow and mixing, but contributed to the well-functioning of the digester. To correctly assess flow and mixing, the applied rheological data should be in agreement with the type of sludge that is treated in the digester. Our results indicate that the shear rate in the digester must be increased and various options for achieving this are proposed.

Low-temperature drying of waste activated sludge enhanced by agricultural biomass towards self-supporting incineration

A high moisture content of waste activated sludge (WAS) associated with a low calorific value needs to be deeply dried towards self-supporting incineration. On the other hand, thermal energy with low temperature exchanged from treated effluent has great potential for drying sludge. Unfortunately, low-temperature drying of sludge seems to be low in efficiency and long in drying time. For this reason, some agricultural biomass was added into WAS to improve the drying efficiency. The drying performance and sludge properties were analyzed and evaluated with this study. Experimental results demonstrated that wheat straw was the best in enhancing the drying performance. With only 20 % (DS/DS) of crushed wheat straw added, the average drying rate achieved up to 0.20 g water/g DS·min, much higher than 0.13 g water/g DS·min of the raw WAS. The drying time to the targeted moisture content (63 %) (for self-supporting incineration) was shortened to only 12 min, much lower than 21 min of the raw WAS. The analysis revealed that wheat straw could reduce the specific resistance of filtration (SRF) and increase the sludge filterability (X). Also, the sludge rheology, particle size distribution and SEM images could conclude that agricultural biomass played a positive role in skeleton builders, forming a mesh-like structure in sludge flocs. These special channels could obviously improve the transfer capacities of heat and water inside the sludge matrix and thus greatly increase the drying performance of WAS.

Influence of Heavy Metals on the Rheology of a Thermophilic Biological Sludge for nutrients Recovery: Effect of Iron, Copper, and Aluminium on Fluid Consistency

Currently, thermophilic membrane biological reactors (TMBRs) are used to treat industrial wastewaters and biological sewage sludge with the aim of nutrients recovery. The performance of the biological process is strongly influenced by rheological behaviour of the thermophilic biological sludge (TBS) inside the reactor. Considering the high concentration of heavy metals in matrices fed to the reactors, this work aims to evaluate the influence of heavy metal types and concentration on rheological properties of TBS. Sludge has been extracted from a full-scale TMBR and conditioned with Fe3+, Cu2+, and Al3+. Rheological measures have been conducted and constants k and n of Herschel–Bulkley model were used to define the rheological properties of TBS. Rheological properties of high consistency TBS (0.06 ≤ k0 < 0.2 and 0.6 ≤ n0 < 0.8) were not significantly affected by the conditioning with Fe3+, Cu2+, and Al3+. In case of TBS with initial low consistency (0.02 ≤ k0 < 0.06) and behaviour more similar to Newtonian fluids (0.8 ≤ n0 < 1), Fe3+ and Al3+ determined a significant increase in consistency. On the contrary, the addition of Cu2+ reduced k of conditioned TBS with a lower impact on the distance for Newtonian behaviour (n). This work demonstrates the strong influence of Fe3+, Cu2+, and Al3+ on the rheological properties of TBS depending on the initial consistency of the sludge, and the types and dosage of heavy metals.Graphical

Municipal wastewater sludge rheology: Impact of temperature, solid content, and settling solids.

The analysis of the rheological properties of the sludge produced in biological purification plants is paramount for the design of equipment (pumping, mixing, conditioning, aeration, filtration, etc.) and the management of these plants. In particular, solid content in the sludge plays a significant role in the rheological quantities. In this study, an investigation of the sludge rheology produced in different sectors of an activated sludge plant (aerated sludge, recirculation sludge, and thickened sludge) has been performed. Specifically, for each sludge sample, various physical-chemical parameters have been analyzed, and rheological tests have been performed at different temperatures (10°C, 20°C, 30°C, and 40°C ± 0.1). The sludge was considered as a non-Newtonian Bingham fluid. Viscosity and critical stress were determined, and their dependence at different temperatures, on some physical parameters, such as solid content and settling solids, has been studied. Results evidenced that viscosity and critical stress increase with increasing solid concentration. Moreover, at the same concentration, as the temperature increases, the viscosity decreases, whereas the critical stress decreases until temperature values of about 30°C and then increases again. PRACTITIONER POINTS: An investigation of the sludge rheology produced in different sectors of an activated sludge plant has been performed. For each sludge sample, various physical-chemical parameters have been analyzed, and rheological tests have been performed at different temperatures. Viscosity and critical stress were determined, and their dependence at different temperatures, on some physical parameters, has been studied. Results evidenced that viscosity and critical stress increase with increasing solid concentration.

On the compressional rheology of fresh faeces: Evidence for improving community scale sanitation through localised dewatering

Non-sewered sanitation is currently dependant upon pit latrine emptying, the safety of which is compromised by the high costs of faecal sludge transport to centralised treatment facilities. Transport in turn is hindered by the complex rheology of pit latrine sludge. This study therefore characterised the compressional rheology of fresh faeces and modelled the implications for passive (gravity) or mechanical (forced) solid/liquid separation. This informs on the viability of decentralising dewatering for more efficient volume reduction and improving the economics of transportation. The gel point (ϕg) is the solids concentration where the material has a networked structure and signifies the point when mechanical intervention is required for further solid-liquid separation. For fresh faeces, ϕg ranged between 6.3 and 15.6% total solids (TS) concentration. This is significantly higher than the ϕg observed for wastewater sludge at centralised facilities, and it implies that passive gravity driven processes can suffice to improve localised dewatering. The kinetics of passive sedimentation of faecal material were modelled and illustrate thickening from 3 to 10% TS concentration in <0.5 h. This highlights that early intervention to thicken faeces while fresh can improve solid/liquid separation efficiency. Filtration of fresh faeces was characterised by lengthy cake filtration times and comparably short compression times, more similar to mineral slurries than to wastewater sludge. Consequently, fresh faeces presented improved dewatering characteristics, supporting higher final cake solids concentrations and improved dewatering kinetics. By complementing thickening with 300 kPa filtration, a 1.4 cm thick 25% TS product could be achieved in <24 h. Investigation of matrix properties highlighted that increased conductivity (e.g. exposure to urine) negatively influenced dewaterability, an effect which could be mitigated by introducing solid-liquid separation earlier. The thermodynamically favourable compressional rheology of fresh faeces has identified that focussing on localised dewatering could radically improve the economics of faecal sludge management, primarily through reducing transport costs.

A novel anoxic/aerobic process coupled with micro-aerobic/anaerobic side-stream reactor filled with packing carriers for in-situ sludge reduction

Activated sludge process with anaerobic side-stream reactors (SR) in the sludge recirculation line can achieve in-situ sludge reduction. But sludge reduction efficiency (SRE) is limited with low cell lysis and particle organic matter hydrolysis rates under short hydraulic retention time of SR for practical application. This study firstly inserted alternating micro-aerobic/anaerobic conditions (AMAC) and packing carriers into SR with low hydraulic retention time for improving SRE. The effects of AMAC and packing carriers on sludge rheology properties, molecular characteristics of dissolved organic matter (DOM) and microbial communities were investigated. Results showed that AMAC coupled packing carriers achieved SRE by 51.09% with efficient effluent pollutant removal. AMAC specialized inactive particle organic matter hydrolysis and active cell lysis in the micro-aerobic and anaerobic SR, respectively. Packing carriers intensified this function division effect. This effect weakened colloidal entanglement of flocs. It intensified the cell lysis in mainstream aerobic tank, causing the accumulation of refractory lignins in DOM. The higher utilization of released bioavailable DOM decreased microbial activity. Overall, the improved SRE favored the declined DOM bioavailability by inserting AMAC and packing carriers in SR. The sludge reduction-related bacteria like slow growing (Dechloromonas), predatory (Norank_Saprospiraceae), and hydrolytic/fermentative bacteria (Chelatococcus, Pseudomonas, Tolumonas, and Unclassified_Bacteroidia) were involved in decreasing DOM bioavailability. This study advanced our knowledge about the effects of AMAC and packing carriers on in-situ sludge reduction systems. This research also showed how AMAC and packing carriers enhanced the sludge lysis/hydrolysis for sludge reduction.

Theoretical and Experimental Verification of Rheological Characteristics of High Viscosity Municipal Sludge

After upgrading the wastewater treatment standard, the effluent quality reached the first class A standard by strengthening the process optimization of biological dephosphorization and denitrification. Taking the sludge treatment project of Shanghai Bamboo Garden as an example, the sludge composition from the fourth plant is compared with the weighted value of the sludge before the upgrading of the standard, and the sludge element content is obviously increased and the sludge viscosity is increased. By kinexus lab+ series of rotary rheometer to test the viscosity of the sample sludge, it is concluded that the viscosity zone of the sample sludge is between 45% and 55% solid content. The rheological test of sludge characterization-viscosity-thixotropy and stacking test standard shows that the viscous zone of sludge is about 40% or more. Herschel-Bulkley model is basically suitable to describe the rheology of sludge in this sample when the solid content is more than 44%. To avoid the sludge viscous zone is the key to design the sludge transport system and the subsequent drying incineration system process parameters in the sludge treatment project of Shanghai Bamboo Garden.

Impacts of shearing and temperature on sewage sludge: Rheological characterisation and integration to flow assessment

Accurate rheological characterisation of sewage waste activated sludge (WAS) is of high importance for downstream processing related to optimised sludge pumping and mixing, assessment of energy demands and overall process design. However, to elaborate rheological behaviour is often challenging under dynamic operational conditions in practice. In this study, two practical influencing factors were investigated: long-term shearing and temperature. Compared to anaerobic digestate (DGT), concentrated WAS had more complex and stronger thixotropic behaviour. Under the long-term shearing conditions, the sludge thixotropic behaviour was well characterised by two quantified limitation states. Temperature had a striking impact on the rheological properties, which was strongly correlated to solids content and digestion process. The impact discrepancy between the long-term shearing and temperature, implied different mechanisms to shift the equilibrium of hydrodynamic and non-hydrodynamic interactions for structure deformation and recovery. The distinct rheological properties between the two determined states were clearly reflected in pipe flow behaviour, revealing a concrete link between lab-measured sludge rheology and its practical flow performance. The pipe flows were well assessed using the developed Computational Fluid Dynamics model with effective rheological data integration, which is promising for practical design and optimisation of sewage sludge systems.

Rheological studies of municipal sewage sludge slurries for hydrothermal liquefaction biorefinery applications

Hydrothermal liquefaction (HTL) is a thermochemical process that can convert organic materials in sewage sludge into crude-like oil. Knowledge of the rheology of sewage sludge as a feedstock is important for the design of a HTL Biorefinery as it provides essential information for pumping, pipeline design and heat transfer processes. This study aims at elucidating the rheological behaviour of sewage sludge slurries. Rheological parameters of sludge obtained from different stages in Wastewater Treatment Plants were estimated and used to calculate the pumping power required for slurry transportation in pipelines as part of the front-end engineering design stage for a biorefinery. Sludge slurries were determined to be Non-Newtonian with shear thickening characteristics. Statistically, the Herschel-Bulkley rheological model best fitted the experimental data with a yield stress. The apparent viscosity of municipal sewage sludge slurries measured within a shear rate range from 10 s−1 to 300 s−1 were determined to be between 0.008−0.05 Pa.s for stockpiled biosolids, 0.04−0.11 Pa.s for sludge without polymer dosage and 0.1−0.2 Pa.s for dewatered sludge with polymer. Power requirement for pumping sludge slurries were in the order: Stockpiled biosolid < Sludge without polymer dosage < Sludge with polymer dosage.

Constitutive modelling and pipeline flow of thixotropic viscoplastic wastewater sludge

Due to the inherent rheological complexity of wastewater sludges, conventional sludge pipeline design based on simplified rheological models can result in inefficient sludge transportation systems. These inefficiencies are further exacerbated by a global need for the processing of more concentrated wastewater sludges that have a more pronounced non-Newtonian character, and hence require greater energy for transportation. However, the complex rheology of these materials (typically visco-elastic and thixotropic) requires sophisticated methods for constitutive modelling that are impractical to implement for complex materials such as sewerage sludges. We address this challenge by developing a novel viscoplastic thixotropic constitutive model that exploits the separation of timescales between the thixotropic and viscoelastic processes, leading to simpler and more robust experimental methods, parameter estimation and process simulation methods. This constitutive model combines a kinetic model for thixotropic degradation and agglomeration via a classical structural parameter (λ) approach coupled nonlinearly with a Herschel-Bulkley model to yield a thixotropic viscoplastic model of sludge rheology. Experimental data for thickened digested sludge between 3 and 4.9% solids were collected to validate the assumption for the separation of viscoelastic and thixotropic responses. The fitting procedure was found to be robust and efficient, and several rheological parameters were found to be invariant with solids concentration. Simplified energy calculations for a typical sludge pipeline showed that the pumping energy could be significantly under- or overestimated without considering thixotropy. These simple and robust constitutive models and fitting methods can accurately predict (and hence design and optimise) sludge behaviour over a wide range of wastewater processes.

An investigation into the stickiness of faecal sludge: Preliminary investigation of VIP and UDDT sludge with varying moisture content at ambient temperature

The document is preliminary results for the experimental analysis of the stickiness of faecal sludge using the texture analyser. the document is the first set of results obtained from the procedure. Faecal sludge samples from ventilated improved pit Latrines (VIP) and urine diversion dry toilets (UDDT) from eThekwini Municipality were analyzed to quantify their stickiness. The moisture content of the faecal sludge was varied within the experiments from 40 to 80 wt% to find the effects of moisture content on stickiness. The stickiness was determined using a Stable microsystems TA. XT express texture analyzer. Rheology of the faecal sludge was evaluated using MR51 Anton Paar rheometer.

CFD simulation of anaerobic digestier to investigate sludge rheology and biogas production

Nowadays, the use of anaerobic digester is a good option for renewable energy and the replacement of fossil fuels. Mixing speed in digester is one of the most important factors affecting the biogas production and operating costs of biogas plants. Thus, optimization of mixing speed is necessary to investigate sludge behavior and biogas production. In this study, simulation of mechanical mixing in anaerobic digestion of laboratory scale at mixing speed of 50, 100, and 200 rpm was performed to investigate flow patterns, digestion stability, microbiological communities, and gas production. Investigation of the velocity contours, the turbulence kinetic energy, the turbulence intensity, and the velocity gradient showed that increasing the mixing speed leads to higher levels of turbulence and in this digester, increasing the mixing speed reduces the stability of methane production process. Accordingly, it has detrimentally affecting on biogas production. According to the velocity gradient in digester, which is considered as a substitute for turbulence, it is determined by the optimal range 6–8 s−1. Increasing the mixing speed until the velocity gradient is close to this range increases the biogas production, but increasing the mixing speed above this range is detrimentally affecting to both digestibility and biogas production.

A Specious Correlation between Sludge Rheology and Dewaterability.

A Specious Correlation between Sludge Rheology and Dewaterability.

Analysis of anaerobic digester mixing: comparison of long shafted paddle mixing vs gas mixing.

The anaerobic digestion (AD) process is influenced by a variety of operation parameters, such as sludge rheology, mixing, temperature, solid retention time (SRT), hydraulic retention time (HRT) and solids concentration. The optimum in the mixing lies somewhere between no-mixing and continuous mixing, as the lack or excessive mixing can lead to poor AD performance instead. A three-dimensional computational fluid dynamics steady/unsteady model, incorporating the rheological properties of the sludge, was developed and applied to quantify mixing in a full-scale anaerobic digester. Mechanical and gas mixing solutions were taken into account, keeping constant the daily energy consumption. Results, consisting of velocity magnitude and patterns, dead zone formation and turbulence levels were discussed. Compared to the mechanical mixing, gas mixing had lower percentage of dead zones (about 5% against 50%), larger maximum velocity (about 3 m/s against 1 m/s) as well as larger turbulent kinetic energy levels (0.24 m2/s2 against 0.001 m2/s2).

Impact of methodological artifact on digested sludge flow curve measurement

Inconsistent experimental procedures have been used to characterize sludge rheology in literature. This often has resulted in proposing different rheological models for sludge as well as non-comparable data. Any collected rheological data needs to be interpreted considering the methodology used for its collection because otherwise they cannot be used by engineers for design and troubleshooting. This paper intends to shed light on the influential parameters during data collection procedure to produce a reliable and reproducible data.This paper systematically investigates the impact of different geometries, preshear, equilibrium, rest and storage time on flow curve measurement for digested sludge in the range of 2.3 to 6% total solid and recommends a reliable procedure for reproducible sludge flow curve measurements. While the magnitude of impacts is different, we found all factors are significantly dependent on the sludge solid concentration. Besides, the method of the development of the protocol can be utilized to develop appropriate protocols for rheological characterization of any other sludge. The customization highlights are:•Selecting the geometry according to the sludge solid concentration.•Allocating an equilibrium time at each point of flow curve according to the sludge solid concentration.•Flow curve data requires to visually inspected for instabilities.

Rheology improvement in an osmotic membrane bioreactor for waste sludge anaerobic digestion and the implication on agitation energy consumption

Sludge rheology is an essential factor for anaerobic digestion (AD) processes to control the agitation energy consumption. In this study, the sludge rheology was characterized for an osmotic membrane bioreactor and a conventional sludge anaerobic digestion reactor as the solid content being increased from 3.5–3.7% to 7.5–7.7%. The flow curves were fitted using different rheological models and the mechanism was discussed. The sludge from the osmotic membrane bioreactor exhibited obviously better rheological properties than that of the conventional reactor at a solid content of 7.5–7.7%. Larger particles induced by less negative zeta potential and higher extracellular polymeric substances, together with the higher conductivity resulted by reverse salt flux in the osmotic membrane bioreactor, improved the sludge rheology due to reduced interactions between particles. As a result, the agitation energy consumption of the osmotic membrane bioreactor can save up to 34–39% compared with the conventional one at total solid content of 7.5–7.7%.

Unravelling the influences of sewer-dosed iron salts on activated sludge properties with implications on settleability, dewaterability and sludge rheology

Although the beneficial impacts of iron dosing to sewer on activated sludge unit's performance, especially in relation to phosphate removal, have been reported, the extent of impacts on different sludge properties affecting the operation and performance of the activated sludge unit are not fully understood. In this study, we investigated the influences of iron salt dosing to sewer on both settleability and dewaterability of downstream activated sludge unit. We also examined, based on the comparative assessment of different key activated sludge properties, possible underlying factors responsible for the changes in sludge settleability and dewaterability. For this, iron chloride was dosed to a sewer reactor of integrated laboratory sewer-bioreactor system. The activated sludge samples were obtained from two separate reactors, an experimental sequencing batch reactor (SBR-E) downstream of sewer reactor receiving iron dosing and a control SBR (SBR-C) downstream of a sewer reactor without any iron dosing. Iron-conditioned sludge showed improved settleability and dewaterability over the unconditioned activated sludge. Mean differences in settleability and dewaterability between two sludges were 22.5 ± 7.8 mL/g (p < 0.05) and 7.8 ± 1.2% (p < 0.05), respectively. Iron-conditioned sludge showed lower contents of soluble extracellular polymeric substances (EPS) fractions, protein and polysaccharide contents, and monovalent-to-divalent (M+/D++) cations ratio, but higher humification index as compared to the unconditioned sludge. Iron-conditioned sludge exhibited marginal increment in mean particle size (Dv50) and settleable particle size classes (100–400 μm) but reduction in supracolloidal particle size classes (1–100 μm). In terms of sludge rheology, iron-conditioned sludge exhibited relatively lower relative sludge network strength, viscosity, yield stress, elastic/viscous/complex moduli (G′/G′′/G*), and damping factor tan(δ) but increased shear compliance (J) and shear strain (%) with time.The iron-conditioned sludge therefore exhibited relatively weaker deformation resistance and sludge elasticity. Based on the foregoing results, we posit the combined synergistic effect of favourable changes to the key sludge properties, might be responsible for the observed improvement in settleability and dewaterability of iron-conditioned sludge.