Membrane bioreactor sludge rheology at different solid retention times

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.

Membrane bioreactor assisted volatile fatty acids production from agro-industrial residues for ruminant feed application

Volatile fatty acids (VFAs) supplementation in ruminants' diet as a source of energy and chemical precursors and their effect on animal's physiology and well-being has long been of scientific interest. Production of VFAs through anaerobic digestion of agro-industrial residues not only creates value but also presents an alternative sustainable approach for ruminant feed supplementation. Therefore, this study aimed to investigate the bioconversion of agro-industrial residues produced in large quantities such as apple pomace (AP), thin stillage (Ts), and potato protein liquor (PPL) to VFAs, fully complying to regulations set for ruminant feed supplement production. In this regard, batch acidogenic fermentation assays (pH 6-10) and semi-continuous immersed membrane bioreactor (iMBR) were applied. In batch assays, at pH 10 the co-digestion of Ts and PPL produced the highest VFAs concentration (14.2 g/L), indicating a yield of 0.85 g CODVFAs/g volatile solids (VS)added. The optimum batch condition was then applied in the iMBR for in situ fermentation and recovery of VFAs at different organic loading rates (OLR). With increasing the OLR to 3.7 gVS/L.day, the highest VFAs concentration of 28.6 g/L (1,2 g CODVFAs /gVSadded) was achieved. Successful long-term (114 days) membrane filtration was conducted in a media with a maximum of 40 g/L of total solids (TS), facing irreversible membrane fouling in the final stages. Acidogenic fermentation using an iMBR has the potential to play an important role in the future of feed additive provision through the biorefining of agro-industrial wastes via the carboxylate platform, given the role of VFAs production from organic residues.

Membrane BioReactor (MBR) Activated Sludge Surrogate Alternatives Carboxymethyl Cellulose and Xanthan Gum: A Statistical Analysis and Review

Membrane Bioreactors (MBR) combine traditional biological treatments such as Activated Sludge (AS) with a membrane-based filtration process to extract suspended and organic solids. MBR operation involves high shear rates near the membrane surface due to the high crossflow velocity, which complicates any simulation process from a hydrodynamic point of view. In this regard, the viscosity as a function of total suspended solids (TSS) plays an essential role in characterizing and modeling the behavior of activated sludge (AS). However, AS has an intransparency property that prevents experimental measurements (i.e., velocity profiles) commonly associated with optical techniques from being peformed. In light of this limitation, two polymeric compounds, carboxymethyl cellulose (CMC) and xanthan gum (XG), are considered here in order to explore the possibility of mimicking the rheological behavior of AS. These compounds are commonly used in the food industry as food thickeners, and their rheological behavior is supposedly well defined in the literature. In this work, we reviewed the viscosity behavior of these compounds through their reported flow behavior and consistency indexes. It was found that the rheological properties of these two polymers differ depending on the chemical manufacturer, rheometers, and measurement protocols involved. Different curves (shear rate vs. viscosity/shear stress) are obtained, as each device and procedure seem to modify the polymer structure. Therefore, a statistical analysis was performed based on the flow and consistency indexes using different concentrations and temperatures reported in experimental data. Several insights regarding CMC, XG, and AS performance were obtained, including a better relationship with concentration than with temperature or certain exponential-based performances, which can support further MBR design and operational decision-making.

Dynamics of microbial community and their effects on membrane fouling in an anoxic-oxic gravity-driven membrane bioreactor under varying solid retention time: A pilot-scale study

Membrane fouling in a membrane bioreactor (MBR) is highly influenced by the characteristics of the influent, the mixed liquor microbial community and the operational parameters, all of which are environment specific. Therefore, we studied the dynamics of microbial community during the treatment of real municipal wastewater in a pilotscale anoxic-oxic (A/O) MBR equipped with a gravity-driven membrane filtration system. The MBR was operated at three different solid retention times (SRTs): 25, 40 and 10 days for a total period of 180 days in Nordic environmental conditions. Analysis of microbial community dynamics revealed a high diversity of microbial species at SRT of 40 days, whereas SRT of 25 days was superior with microbial richness. Production of soluble microbial products (SMP) and extracellular polymeric substances (EPS) was found to be intensely connected with the SRT and food to microorganism (F/M) ratio. Relatively longer operational period with the lowest rate of membrane fouling was observed at SRT of 25 days, which was resulted from the superior microbial community, lowest production of SMP and loosely bound EPS as well as the lower filtration resistance of larger sludge flocs. Abundance of quorum quenching (QQ) bacteria and granular floc forming bacterial genera at SRT of 25 days provided relatively lower membrane fouling tendency and larger floc formation, respectively. On the other hand, substantial amount of various surface colonizing and EPS producing bacteria was found at SRT of 10 days, which promoted more rapid membrane fouling compared with the fouling rate seen at other tested SRTs. To sum up, this research provides a realistic insight into the impact of SRT on microbial community dynamics and resulting characteristics of mixed liquor, floc size distribution and membrane fouling for improved MBR operation.

Direct sludge granulation by applying mycelial pellets in continuous-flow aerobic membrane bioreactor: Performance, granulation process and mechanism

This study provides a sustainable manner for direct cultivation of aerobic granular sludge (AGS) by addition of mycelial pellets (MPs) into continuous-flow aerobic MBR. The results showed that the granulation time in MPs-MBR was shortened by at least 65 days, accounting for enhanced mean size of granules (0.68-0.76 mm), increased mixed liquor suspended solids (MLSS) concentration (12.8 g/L) and improved settling ability (78.1 mL/g), in comparison with that of 0.23-0.28 mm, 9.8 g/L and 102.1 mL/g in control MBR. MPs-MBR demonstrated significant advantages in terms of COD reduction (97.0-99.1%), NH₄⁺-N reduction (100%) and TN reduction (32.27-42.33%). MPs, extracellular polymeric substances (EPS) and filamentous bacteria acted as inducible nucleus, crosslinking matter and supporting skeleton, respectively, in favor of promoting the formation and stabilization of AGS with a four-layered structure. The relevant mechanism was underlined by rheological analysis, indicating that MPs addition enhanced non-Newtonian flow characteristics and network structure of sludge.

Mechanism of zero valent iron and anaerobic mesophilic digestion combined with hydrogen peroxide pretreatment to enhance sludge dewaterability: Relationship between soluble EPS and rheological behavior

This study proposed a novel two-step conditioning strategy to enhance activated sludge (AS) dewatering performance. The method involved a zero valent iron (ZVI), anaerobic mesophilic digestion (AMD) process, and hydrogen peroxide (H₂O₂) oxidation. Response surface methodology (RSM) was applied to achieve optimum dewatering conditions. After the combined conditioning, dewatering was significantly better in the treated sludge compared to the raw AS. The specific resistance of filtration (SRF) of the treated sludge decreased to 2.48 × 10¹¹ m/kg; this SRF level was 93.60% lower compared to the raw AS. The bound water content (BWC) decreased to 1.19 g/g dry solid (DS); this BWC level was 15.2% lower compared to the raw AS. The water content of the treated sludge cake decreased to 44.18 ± 0.46%. An economic analysis shows that ZVI-AMD-H₂O₂ can be used in real-world settings. Investigations of the underlying mechanisms showed that small block structures were formed after conditioning; viscosity and the colloidal forces of the sludge decreased; and organic matter and BWC were released from inner extracellular polymeric substances (EPS) layers to form soluble (SB)-EPS. This study illuminated the relationship between SB-EPS and the rheological behavior of AS. There is a high correlation coefficient between rheological parameter τ_y and N-containing substances in SB-EPS (R = -0.993, p < 0.05). The ZVI-AMD-H₂O₂ process effectively changed the EPS content, especially protein materials. This led to a decrease in AS viscosity and an increase in sludge dewaterability.

Impact of mechanical history on sludge rheological properties: Role of the organic content

The rheological behavior is known to be of great interest in sludge treatment, but the material complexity induces strong measurement difficulties. The literature highlights two characteristics deeply impacting sludge rheological properties and their characterization: Thixotropic behavior and organic matter (Seyssiecq et al., 2003; Eshtiaghi et al., 2013), both being related to each other. Indeed, by looking at how thixotropy affects elastic modulus and yield stress with different mechanical histories at different organic matter ratios, it turns out that the higher the organic matter, the higher the thixotropic behavior.

Assessing activated sludge morphology and oxygen transfer performance using image analysis

The morphology of the microbial communities can have dramatic impacts on not only the treatment performance, but also the energy use performance of an activated sludge process. In this research, we developed and calibrated an image analysis technique to determine key morphological parameters such as the floc diameter and the specific filament length (SFL) and discovered that the SFL has significant impacts on sludge floc size, the specific extracellular polymeric substances production, the settleability, mixed liquor viscosity, and oxygen transfer efficiency. When the SFL increased from 2.5 × 10⁹ μm g^-1 to 6.0 × 10¹⁰ μm g^-1, the apparent viscosity normalized by the mixed liquor suspended solids concentration increased by 67%, and the oxygen transfer efficiency decreased by 29%. A long solids retention time (SRT) of 40 day reduced SFL, improved sludge settling performance, and improved oxygen transfer efficiency as compared to shorter SRTs of 10 and 20 day. The findings underscore the need to assess microbial morphology when quantifying the treatment performance and energy performance of activated sludge processes.

Interplay between physical cleaning, membrane pore size and fluid rheology during the evolution of fouling in membrane bioreactors

Fouling is one of the most pressing limitations during operation of membrane bioreactors, as it increases operating costs and is the cause of short membrane lifespans. Conducting effective physical cleanings is thus essential for keeping membrane operation above viable performance limits. The nature of organic foulants present in the sludge and the membrane properties are among the most influential factors determining fouling development and thus, efficiency of fouling mitigation approaches. The role of other factors like sludge viscosity on fouling is still unclear, given that contradictory effects have been reported in the literature. In the present study we use a new research approach by which the complex interplay between fouling type, levels of permeate flux, membrane material and feed properties is analyzed, and the influence of these factors on critical flux and membrane permeability is evaluated. A variety of systems including activated sludge and model solutions with distinct rheological behavior has been investigated for two membranes differing in pore size distribution. We present a novel method for assessing the efficiency of fouling removal by backwash and compare it with the efficiency achieved by means of relaxation. Results obtained have proven that backwash delays development of critical fouling as compared with relaxation and reduces fouling irreversibility regardless of fluid rheology. It was shown that backwash is especially effective for membranes for which internal fouling is the main cause of loss in permeability. Nonetheless, we found out that for membranes with tight pores, both relaxation and backwash are equally effective. The critical flux decreases significantly for high-viscosity fluids, such as activated sludge. This effect is mainly caused by an intensified concentration polarization at the feed side rather than by internal fouling events. However, membrane permeability has been proven to rely more on the permeate viscosity than on the feed viscosity: poor rejection of organic fractions showcasing high viscosity causes an acute decline in membrane permeability as a consequence of increased shear stress inside the membrane pores.

Assessing the abundance of fungal populations in a full-scale membrane bioreactor (MBR) treating urban wastewater by using quantitative PCR (qPCR)

The abundance of fungi in a full-scale membrane bioreactor (MBR) treating urban wastewater and experiencing seasonal foaming was assessed by quantitative PCR (qPCR), comparing three different sets of widely used universal fungal primers targeting the gene encoding the small ribosomal subunit RNA, 18S-rDNA, (primers NS1-Fung and FungiQuant) or the internal transcribed spacer ITS2 (primers ITS3-ITS4). Fungi were a numerically important fraction of the MBR microbiota (≥10⁶ 18S-rDNA copies/L activated sludge), and occurred both in the aerated and anoxic bioreactors. The numbers of copies of fungal markers/L activated sludge calculated using the NS1-Fung or ITS3-ITS4 primer sets were up to 2 orders of magnitude higher than the quantifications based on the FungiQuant primers. Fungal 18S-rDNA counts derived from the FungiQuant primers decreased significantly during cold seasons, concurring with foaming episodes in the MBR. Redundancy analysis corroborated that temperature was the main factor driving fungi abundance, which was also favored by longer solid retention time (SRT), lower chemical oxygen demand/biochemical oxygen demand at 5 days (COD/BOD₅) of influent water, and lower biomass accumulation in the MBR.

A comprehensive review on rheological studies of sludge from various sections of municipal wastewater treatment plants for enhancement of process performance

Large quantities of sludge is generated from different sections of a wastewater treatment plant operation. Sludge can be a solid, semisolid or liquid muddy residual material. Understanding the flow behaviour and rheological properties of sewage sludge at different sections of a wastewater treatment plant (WWTP) is important for the design of pumping system, mixing, hydrodynamics and mass transfer rates of various sludge treatment units, optimization of conditioning dose and for sustainable sludge management. The current article provides a comprehensive review on up to date literature information on rheological behaviour of raw primary sludge, excess activated sludge, thickened excess activated sludge, mixture of raw primary and thickened excess activated sludge (mixed sludge), digested sludge, and biosolid under the influence of different operating parameters and their impacts on process performance. The influences of various process parameters such as solid concentration, temperature, pH, floc particle size, primary to secondary sludge mixing ratio, aging and conditioning agent doses on the rheological behaviour of sludge from different treatment units of WWTPs are critically analysed here. Yield stress was reported to increase with increasing solid concentration for all types of sludge whereas viscosity showed a decreasing trend with decreasing total solid concentration and percentage of thickened excess activated sludge in the mixture. Temperature showed an inverse relationship with yield stress and viscosity. Viscosity was reported to be decreased with decrease in pH. The effect of various conditioning agents on the rheological behaviour of sludge are also discussed here. The applicability and practical significance of various rheological models such as Bingham, Power Law (Ostwald), Herschel-Bulkley, Casson, Sisko, Careau, and Cross models to experimental rheological characteristics of various sludges were presented here. The reported results on various rheological parameters such as shear stress, yield stress, flow index, infinite, zero-rate viscosity, and flow consistency index of different sludge types obtained from the best fitted model were also compiled here. Conclusions have been drawn from the literature reviewed and few suggestions for future research direction are proposed.

Comparative power demand of mechanical and aeration imposed shear in an immersed membrane bioreactor

The power demanded for the application of mechanically-imposed shear on an immersed flat sheet (iFS) membrane bioreactor (MBR) has been compared to that of conventional membrane air scouring. Literature correlations based on the Ostwald model were used to define the rheological characteristics of an MBR sludge. The correlation of specific power demand (P^'¯, in Watts per m² membrane area) with shear rate γ in s^-1 was developed from first principles through a consideration of the force balance on the system in the case of mechanically-imposed shear. The corresponding aeration imposed shear correlation was interpreted from literature information. The analysis revealed the energy required to impose a shear mechanically through oscillation (or reciprocation) of the membrane to be between 20 and 70% less than that demanded for providing the same shear by conventional aeration of the immersed membrane. The energy saving increases with decreasing shear in accordance with a power demand ratio (aeration:mechanical) of 1400γ^-1.4 for a specific sludge rheology. Whilst the absolute P^'¯ value is dependent on the sludge rheology, the aeration:mechanical power demand ratio is determined by the difference in the two exponents in the respective correlations between P^'¯ and γ. Consequently, aeration-imparted shear becomes energetically favoured beyond some threshold shear rate value (∼180 s^-1, based on the boundary conditions applied in the current study). The outcomes qualitatively corroborate findings from the limited practical measurement of energy demand in MBRs fitted with reciprocating immersed membranes.

Rheology and Microbiology of Sludge from a Thermophilic Aerobic Membrane Reactor

A thermophilic aerobic membrane reactor (TAMR) treating high-strength COD liquid wastes was submitted to an integrated investigation, with the aim of characterizing the biomass and its rheological behaviour. These processes are still scarcely adopted, also because the knowledge of their biology as well as of the physical-chemical properties of the sludge needs to be improved. In this paper, samples of mixed liquor were taken from a TAMR and submitted to fluorescent in situ hybridization for the identification and quantification of main bacterial groups. Measurements were also targeted at flocs features, filamentous bacteria, and microfauna, in order to characterize the sludge. The studied rheological properties were selected as they influence significantly the performances of membrane bioreactors (MBR) and, in particular, of the TAMR systems that operate under thermophilic conditions (i.e., around 50°C) with high MLSS concentrations (up to 200 gTS L−1). The proper description of the rheological behaviour of sludge represents a useful and fundamental aspect that allows characterizing the hydrodynamics of sludge suspension devoted to the optimization of the related processes. Therefore, in this study, the effects on the sludge rheology produced by the biomass concentration, pH, temperature, and aeration were analysed.

Effect of carbon to nitrogen ratio of feed wastewater and sludge retention time on activated sludge in a submerged membrane bioreactor

This paper investigated the effects of extracellular polymeric substances (EPS) on the activated sludge rheology in a submerged membrane bioreactor (sMBR) operated at different sludge retention time (SRT) values and different carbon to nitrogen ratios (C/N) of feed wastewater. The C/N ratios of the feed were adjusted accordingly so that synthetic wastewaters prepared simulated municipal wastewater, non-toxic wastewater with high C/N ratio and non-toxic wastewater with low C/N ratio. A number of important operational parameters such as mixed liquor suspended solid (MLSS), protein fraction of EPS (EPSp), carbohydrate fraction of EPS (EPSc), protein fraction of soluble microbial product (SMPp), carbohydrate fraction of SMP (SMPc), apparent viscosity, critical flux and hydrophobicity in mixed liquor and their correlations were investigated in the sMBR systems operated. The statistical analysis indicated that the C/N ratio of feed, SRT, MLSS and SMPc were found to have positive effects on apparent viscosity at three different shear rates. On the other hand, a negative impact was detected between the apparent viscosities and the critical fluxes. It was also observed that there is a significant positive correlation between hydrophobicity and both EPSp and SMPp.

Rheological behavior of sewage sludge with high solid content

Sludge rheological properties play a fundamental role in determining its performance in pipes, tanks or reactors. However, the relative information on high-solids sludge is very rare. In this study, the rheological properties of high-solids sludge were investigated systematically and a new rheological model was built. The results showed that the low-solids sludge with total solids content (TS) 2-15% was pseudoplastic fluid, and the high-solids sludge with TS 7-15% exhibited thixotropic property. Sludge viscosity increased exponentially with the increasing TS, and decreased in function of power along with the increasing shear rate. The new complex model combining the exponential model and the power model can perfectly describe the relation between TS, shear rate and viscosity of the high-solids sludge. Both sludge organic content and temperature have influence on sludge viscosity, but the influence was not significant for the low-solids sludge. For the high-solids sludge with TS 6%, 8%, 10% and 12%, their viscosities increased by 5.0, 9.1, 25.7 and 24.9 times, respectively, when sludge organic content increased from 28% to 53%, and decreased by 36.5%, 49.5%, 54.0% and 65.4%, respectively, when sludge temperature rose from 9 to 55 °C.

Development of low-cost rotational rheometer

Liquids with non-Newtonian properties are presented in many engineering areas, as for example in membrane bioreactors where active sludge exhibits shear thinning properties. Therefore, the ability to determine the rheology's dependence on shear is important when optimising systems with such liquids. However, rheometers capable of determining the viscosity are often expensive and so a cheaper alternative is constructed with this exact capability. Using the principle of rotating rheometers, a low-cost rheometer was built to determine the rheology of Newtonian and non-Newtonian liquids. The general principles and background assumptions and the physics are described. The rheometer was calibrated by comparison with measurements conducted on a Brookfield viscometer for Newtonian liquids. For validation measurements on non-Newtonian liquids, xanthan gum solutions were made and compared with measurements on the Brookfield viscometer and with values from other sources. Furthermore, the effect of excluding the different shear rates in the system is discussed and good practice hereto is given.

Effect of ultrasound, low-temperature thermal and alkali pre-treatments on waste activated sludge rheology, hygienization and methane potential

Waste activated sludge is slower to biodegrade under anaerobic conditions than is primary sludge due to the glycan strands present in microbial cell walls. The use of pre-treatments may help to disrupt cell membranes and improve waste activated sludge biodegradability. In the present study, the effect of ultrasound, low-temperature thermal and alkali pre-treatments on the rheology, hygienization and biodegradability of waste activated sludge was evaluated. The optimum condition of each pre-treatment was selected based on rheological criteria (reduction of steady state viscosity) and hygienization levels (reduction of Escherichia coli, somatic coliphages and spores of sulfite-reducing clostridia). The three pre-treatments were able to reduce the viscosity of the sludge, and this reduction was greater with increasing treatment intensity. However, only the alkali and thermal conditioning allowed the hygienization of the sludge, whereas the ultrasonication did not exhibit any notorious effect on microbial indicators populations. The selected optimum conditions were as follows: 27,000 kJ/kg TS for the ultrasound, 80 °C during 15 min for the thermal and 157 g NaOH/kg TS for the alkali. Afterward, the specific methane production was evaluated through biomethane potential tests at the specified optimum conditions. The alkali pre-treatment exhibited the greatest methane production increase (34%) followed by the ultrasonication (13%), whereas the thermal pre-treatment presented a methane potential similar to the untreated sludge. Finally, an assessment of the different treatment scenarios was conducted considering the results together with an energy balance, which revealed that the ultrasound and alkali treatments entailed higher costs.

Essence of disposing the excess sludge and optimizing the operation of wastewater treatment: rheological behavior and microbial ecosystem

Proper disposal of excess sludge and steady maintenance of the high bioactivity of activated sludge in bioreactors are essential for the successful operation of wastewater treatment plants (WWTPs). Since sludge is a non-Newtonian fluid, the rheological behavior of sludge can therefore have a significant impact on various processes in a WWTP, such as fluid transportation, mixing, oxygen diffusion, mass transfer, anaerobic digestion, chemical conditioning and mechanical dewatering. These are key factors affecting the operation efficiency and the energy consumption of the entire process. In the past decade-due to the production of large quantities of excess sludge associated with the extensive construction of WWTPs and the emergence of some newly-developed techniques for wastewater purification characterized by high biomass concentrations-investigations into the rheology of sludge are increasingly important and this topic has aroused considerable interests. We reviewed a number of investigations into the rheology of sludge, with the purpose of providing systematic and detailed analyses on the related aspects of the rheological behavior of sludge. It is clear that, even though considerable research has focused on the rheology of sludge over a long time period, there is still a need for further thorough investigation into this field. Due to the complex process of bio-treatment in all WWTPs, biological factors have a major influence on the properties of sludge. These influences are however still poorly understood, particularly with respect to the mechanisms involved and magnitude of such impacts. When taking note of the conspicuous biological characteristics of sludge, it becomes important that biological factors, such as the species composition and relative abundance of various microorganisms, as well as the microbial community characteristics that affect relevant operating processes, should be considered.

Rheological measurements on different types of sewage sludge for pumping design

Sewage sludge pumping could represent an optimal solution to assure adequate treatment of sludge in centralized plants, with a consequent reduction of the environmental impact of sludge disposal (volume, odour, putrescence), because small wastewater treatment plants usually do not provide an adequate treatment due to high costs. An accurate knowledge of rheological parameters is required to compute head loss for pipeline design, but only few data are available. In order to circumvent the problem of the scarcity of sludge rheological data, we have performed tests on different types of sludge, with solids concentration and temperature in the typical range of a conventional wastewater treatment plant. Bingham rheological parameters and sludge thixotropy values have been processed by regression analysis to identify their dependence on solids concentration or temperature. The results of this study allow the definition of guidelines and optimal strategies for designers in order to reduce pumping costs.

Rheological characterisation of municipal sludge: a review

Sustainable sludge management is becoming a major issue for wastewater treatment plants due to increasing urban populations and tightening environmental regulations for conventional sludge disposal methods. To address this problem, a good understanding of sludge behaviour is vital to improve and optimize the current state of wastewater treatment operations. This paper provides a review of the recent experimental works in order for researchers to be able to develop a reliable characterization technique for measuring the important properties of sludge such as viscosity, yield stress, thixotropy, and viscoelasticity and to better understand the impact of solids concentrations, temperature, and water content on these properties. In this context, choosing the appropriate rheological model and rheometer is also important.

Activated sludge rheology: a critical review on data collection and modelling

Rheological behaviour is an important fluid property that severely impacts its flow behaviour and many aspects related to this. In the case of activated sludge, the apparent viscosity has an influence on e.g. pumping, hydrodynamics, mass transfer rates, sludge-water separation (settling and filtration). It therefore is an important property related to process performance, including process economics. To account for this, rheological behaviour is being included in process design, necessitating its measurement. However, measurements and corresponding protocols in literature are quite diverse, leading to varying results and conclusions. In this paper, a vast amount of papers are critically reviewed with respect to this and important flaws are highlighted with respect to rheometer choice, rheometer settings and measurement protocol. The obtained rheograms from experimental efforts have frequently been used to build viscosity models. However, this is not that straightforward and a lot of errors can be detected with respect to good modelling practice, including fair model selection criteria, qualitative parameter estimations and proper model validation. These important steps are however recurrently violated, severely affecting the model reliability and predictive power. This is illustrated with several examples. In conclusion, dedicated research is required to improve the rheological measurements and the models derived from them. At this moment, there is no guidance with respect to proper rheological measurements. Moreover, the rheological models are not very trustworthy and remain very "black box". More insight in the physical background needs to be gained. A model-based approach with dedicated experimental data collection is the key to address this.

Critical review of membrane bioreactor models--part 2: hydrodynamic and integrated models

Membrane bioreactor technology exists for a couple of decades, but has not yet overwhelmed the market due to some serious drawbacks of which operational cost due to fouling is the major contributor. Knowledge buildup and optimisation for such complex systems can heavily benefit from mathematical modelling. In this paper, the vast literature on hydrodynamic and integrated MBR modelling is critically reviewed. Hydrodynamic models are used at different scales and focus mainly on fouling and only little on system design/optimisation. Integrated models also focus on fouling although the ones including costs are leaning towards optimisation. Trends are discussed, knowledge gaps identified and interesting routes for further research suggested.

Effects of chemical additives on filtration and rheological characteristics of MBR sludge

The main goal of this study was to control the fouling phenomena in MBR using chemical additives. In the first phase of the study, SMP removal and bound EPS formation capacity of chemical additives were determined. Highest SMP removal (72%) was achieved by the Poly-2 additive. In the second phase of the study, short term filtration tests were conducted. Poly-1 exhibited highest performance based on membrane resistance, permeability and average TMP. According to the results obtained from constant shear rate tests in fourth phase, no significant change in viscosity with time was observed. Studies for the adaptation of rheograms to common flow models showed that chitosan and starch was not able to fit to Ostwald de Waele and Bingham models. At a shear rate of 73.4 s(-1) viscosities of all samples were close to each other. Chitosan and starch achieved highest viscosity values at the shear rate of 0.6 s(-1).

Clear model fluids to emulate the rheological properties of thickened digested sludge

Optimising flow processes in wastewater treatment plants requires that designers and operators take into account the flow properties of the sludge. Moreover, due to increasingly more stringent conditions on final disposal avenues such as landfill, composting, incineration etc., practitioners need to produce safer sludge in smaller quantities. Anaerobic digestion is a key treatment process for solids treatment and pathogen reduction. Due to the inherent opacity of sludge, it is impossible to visualise the mixing and flow patterns inside an anaerobic digester. Therefore, choosing an appropriate transparent model fluid which can mimic the rheological behaviour of sludge is imperative for visualisation of the hydrodynamic functioning of an anaerobic digester. Digested sludge is a complex material with time dependent non-Newtonian thixotropic characteristics. In steady state, it can be modelled by a basic power-law. However, for short-time processes the Herschel-Bulkley model can be used to model liquid-like properties. The objective of this study was to identify transparent model fluids which will mimic the behaviour of real sludge. A comparison of three model fluids, Carboxymethyl Cellulose (CMC), Carbopol gel and Laponite clay revealed that these fluids could each model certain aspects of sludge behaviour. It is concluded that the rheological behaviour of sludge can be modelled using CMC in steady state flow at high shear rates, Carbopol gel for short-time flow processes and Laponite clay suspension where time dependence is dominant.

Modelling the energy demands of aerobic and anaerobic membrane bioreactors for wastewater treatment

A modelling study has been developed in which the energy requirements of aerobic and anaerobic membrane bioreactors (MBRs) are assessed in order to compare these two wastewater treatment technologies. The model took into consideration the aeration required for biological oxidation in aerobic MBRs (AeMBRs), the energy recovery from methane production in anaerobic MBRs (AnMBRs) and the energy demands of operating submerged and sidestream membrane configurations. Aeration and membrane energy demands were estimated based on previously developed modelling studies populated with operational data from the literature. Given the difference in sludge production between aerobic and anaerobic systems, the model was benchmarked by assuming high sludge retention times or complete retention of solids in both AeMBRs and AnMBRs. Analysis of biogas production in AnMBRs revealed that the heat required to achieve mesophilic temperatures (35 degrees C) in the reactor was only possible with influent wastewater strengths above 4-5 g COD L(-1). The general trend of the submerged configuration, which is less energy intensive than the sidestream configuration in aerobic systems, was not observed in AnMBRs, mainly due to the wide variation in gas demand utilized in anaerobic systems. Compared to AeMBRs, for which the energy requirements were estimated to approach 2 kWh m(-3) (influent up to 1 g COD L(-1)), the energy demands associated with fouling control in AnMBRs were lower (0.80 kWh m(-3) for influent of 1.14 g COD L(-1)), although due to the low fluxes reported in the literature capital costs associated with membrane material would be three times higher than this.

Rheological characterization of biological treatment sludges in a municipal wastewater treatment plant

This study examined the rheological characteristics of biological treatment sludges originating from secondary stage and sludge treatment unit of a municipal wastewater treatment plant (WWTP), using a rotational viscometer equipped with an ultralow adapter or small cylindrical-shaped spindle. A total of four mathematical approaches were used to investigate the fitting of the dependence of the shear stress to the shear rate. A generalized multi-statistical model was proposed to describe the rheological properties of the sludge. The overall results indicated that Ostwald equations have stronger prediction capabilities of the apparent viscosity of treatment sludges than other theological modeling approaches and the proposed model. Although the proposed model structure did not exhibit a desirable performance, it may have an application potential to calculate the apparent viscosity for a particular shear rate and solid content, which can be measured easily in WWTPs.

Effects of the acidogenic biomass on the performance of an anaerobic membrane bioreactor for wastewater treatment

Continuous flow experiments were performed to study the effects of acidogenic biomass development, induced by feeding with non-acidified substrate, on the operation and performance of an anaerobic membrane bioreactor (AnMBR). The AnMBR was operated at cross-flow velocities up to 1.5m/s and fed with a gelatine-starch-ethanol mixture. A significant fraction of acidogenic biomass developed during reactor operation, which fully determined the sludge rheology, and influenced the particle size distribution. As a result, flux levels of only 6.5l/m(2)h were achieved, at a liquid superficial velocity of 1.5m/s. Even though the soluble microbial products levels in the AMBR were as high as 14g COD/l, the observed hydraulic flux was not limited by irreversible pore fouling, but by reversible cake layer formation. Propionate oxidation was the limiting step for the applied organic loading rate. The assessed specific methanogenic activity (SMA) with propionate as substrate was, however, similar to the values found by others during thermophilic treatment of non or partially acidified substrates in granular sludge bed reactors, indicating an appropriate level of the propionate oxidation capacity.

Simulation and assessment of sludge concentration and rheology in the process of waste activated sludge treatment

The process of using flat-sheet membrane for simultaneous sludge thickening and digestion (MSTD) was employed. The variations of sludge concentration and rheology were characterized and simulated. Based on mass balance analysis, mathematical models were developed and successfully used to predict and evaluate the variations of sludge concentration and the digestion efficiency in the MSTD process. The apparent viscosity of sludge could be modeled as functions of mixed liquor suspended solids and shear rates. The sludge in the MSTD process showed both shear-thinning and viscoplastic behaviour, and under various shear rates different rheological models could be chosen to predict their flow behaviour. It was also found that sludge concentration and viscosity had significant correlations with membrane fouling in the MSTD process.