Yield stress and rheological characteristics of activated sludge in an airlift membrane bioreactor

The effect of temperature on fouling in anaerobic membrane bioreactor: SMP- and EPS-membrane interactions

Biofouling is the main challenge in the operation of anaerobic membrane bioreactors (AnMBRs). Biofouling strongly depends on temperature; therefore, we hypothesize that the interactions and viscoelastic properties of soluble microbial products (SMP) and extracellular polymeric substances (EPS) vary with temperature, consequently influencing membrane permeability. This study compares the performance of an AnMBR operated at a similar permeate flux at two temperatures. The transmembrane pressure (TMP) rose rapidly after 5 ± 2 days at 25 °C but only after 18 ± 2 days at 35 °C, although the reactor's biological performance was similar at both temperatures, in terms of the efficiency of dissolved organic carbon removal and biogas composition, which were obtained by changing the hydraulic retention time. Using confocal laser scanning microscopy (CLSM), a higher biofilm amount was detected at 25 °C than at 35 °C, while quartz crystal microbalance with dissipation (QCM-D) showed a more adhesive, but less viscous and elastic EPS layer. In situ optical coherence tomography (OCT) of an ultra-filtration membrane, fed with the mixed liquor suspended solids (MLSS) at the two temperatures, revealed that while a higher rate of TMP increase was obtained at 25 °C, the attachment of biomass from MLSS was markedly less. Increased EPS adhesion to the membrane can accelerate TMP increase during the operation of both the AnMBR and the OCT filtration cell. EPS's reduced viscoelasticity at 25 °C suggests reduced floc integrity and possible increased EPS penetration into the membrane pores. Analysis of the structures of the microbial communities constituting the AnMBR flocs and membrane biofilms reveals temperature's effects on microbial richness, diversity, and abundance, which likely influence the observed EPS properties and consequent AnMBR fouling.

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.

Rheological characterization of biofouling layers developing in nanofiltration processes：macroscale rheological properties of biofouling layer and micro fluid field

Nanofiltration (NF) membrane promotes the rapid development of reclaimed water treatment technology. Biofouling significantly limits NF membranes' performance in the case of wastewater treatment. The rheological characteristics of NF biofouling layer and the microfluid field on the vicinity of membrane surface determine the strategies for alleviating biofouling. Here, we tested the biofouling layer generated on NF in a parallel plate rheometer operated in oscillation and stable shear mode, along with shear stress calculation using computational fluid dynamic simulations on the membrane surface. Results indicated that the biofouling layer was typical viscoelastic material, and the yield stress exhibited shear thinning behavior. The biofouling layer tended to show elasticity during long-term operation, and with higher COD content formed, showed stronger elasticity, larger viscosity, and yield stress. Compared with the shear stress on the membrane surface obtained using numerical simulation (0.57 Pa~1.22 Pa), the yield stress (0.59 Pa~266.98 Pa) of NF biofouling layer was relatively large. Our results suggest that the shear stress generated by water flow may not effectively cause formed biofouling layer shedding. The research will provide theoretical and technical support for biofouling control in NF membrane systems and can be applied in advanced wastewater treatment and reclamation processes.

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.

A new concept of a rotating hollow fibre membrane module:impact of rotation on fine-bubble aeration

A new concept of a rotating membrane module in a membrane bioreactor (MBR) system was tested for its effect on oxygen transfer in clean water and wastewater. The membrane module consists of horizontally aligned hollow fibres connected to the vertically positioned permeate tube which rotates. The results indicated that oxygen transfer can be improved by up to 50% at the highest applied rotational speed (50 rpm) and that the additional energy demand required for the rotation can be compensated by the enhanced oxygen transfer. However, at the highest rotational speed (50 rpm), the fine bubbles bypassed the MBR module, and, consequently, could not contribute to any cleaning effect. The α-factors at different rotational speeds showed similar results. This indicates that the depletion was caused neither by surfactants nor by viscosity phenomena but rather by the floc/solid holdup of the sludge.

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.

Considering a membrane bioreactor for the treatment of vegetable oil refinery wastewaters at industrially relevant organic loading rates

The present study aims to shed more light on the use of membrane bioreactors (MBRs) for the treatment of vegetable oil refinery wastewaters (VORWs). A MBR was operated for 157 days in which it was fed with real VORW of varying composition at a range of organic loading rates (0.20 ± 0.05-3.79 ± 0.29 kg COD m^-3 day^-1). The hitherto unconsidered fate of VORW constituents through the biological process was followed using gas chromatography/mass spectrometry analysis. This analysis revealed that only 19% of the identified feed constituents remained in the MBR effluent whereas ten new compounds were formed. Linear correlation analysis attributed the effluent residual COD to soluble microbial products (SMP) and non-readily biodegradable recalcitrant oily compounds. Trend of change of MLSS, mixed liquor viscosity and SMP with increasing OLR suggested that when MBR is operated under industrial conditions for the VORW treatment, the mixed liquor fouling propensity potentially increases with increasing OLR in the range studied.

Treatment of aqueous wastes by means of Thermophilic Aerobic Membrane Reactor (TAMR) and nanofiltration (NF): process auditing of a full-scale plant

This work focuses on the Thermophilic Aerobic Membrane Reactor (TAMR) process. The research was carried out on a full-scale facility where, all along a 12-year period, daily monitoring and process audit tests were conducted for the process analysis and optimization. The plant treated -light and high-strength aqueous wastes and two different configurations were adopted: (1) thermophilic biological reactor + ultrafiltration (TAMR) and (2) TAMR + nanofiltration (TAMR + NF). In the latter case, the average chemical oxygen demand removal yield was equal to 89% and an effective denitrification (nitrogen oxides removal equal to 96%) was achieved by reducing the dissolved oxygen concentration in the bioreactor. Low specific sludge production was observed. Poor sludge settling properties were measured by a lab-scale settling test; respirometric tests (nitrogen uptake rate and ammonia uptake rate) showed the presence of denitrification and the inhibition of nitrification. Hydrodynamic tests revealed the presence of a significant dead space, thus showing room for improving the overall process performance. Finally, the rheological properties of the sludge were measured as a function of the biomass concentration, pH, temperature, and aeration scheme.

Insight into the response of anammox granule rheological intensity and size evolution to decreasing temperature and influent substrate concentration

Anammox granules are advantageous because of their relatively higher nitrogen removal rate (NRR) and biomass retention capacity in ammonia-containing wastewater treatment. However, little attention has been paid to granule rheological intensity and size evolution, especially under low temperature and substrate concentration conditions. In this study, the size evolution and variations in rheological properties associated with biochemical characteristics of anammox granules were investigated at decreasing temperatures (35 → 13 °C) and influent substrate concentrations (300 → 50 mg NH₄⁺-N L^-1). Both the specific anammox activities (SAA) and yield stress (τ_c) (or storage modulus (G')), which reflected granules' intensity, decreased with decreasing temperature and influent substrate concentration. An exponential correlation was found between SAA and τ_c (or G'). Granule size strongly decreased at low temperature (13 °C) and influent substrate concentration (50 mg NH₄⁺-N L^-1), despite slight variations in τ_c (or G'). A threshold τ_c (or G') that is closely related to the hydrodynamic shear force in the reactor may exist for the anammox granules. Once the τ_c (or G') of the anammox granules was lower than this threshold value (τ_c^∗ = 10.13-15.63 kPa), granules that could not endure hydrodynamic shear forces would disintegrate and their size would decrease substantially. Candidatus Kuenenia was the dominant genus in the expanded granular sludge bed reactor, reaching a minimum abundance of 14.6% at 16 °C because of the low-temperature shock, but increasing in abundance to 57.0% at 13 °C, indicating it has a competitive advantage at low temperatures. This contributed to achieving a high reactor nitrogen loading rate (>1.0 kg N m^-3 d^-1) even at 13 °C or with 50 mg NH₄⁺-N L^-1 influent. Overall, the results of this study will facilitate management of anammox bioreactors that run at various temperatures and influent substrate concentrations by clarifying the correlation between rheological intensity and anammox granule sludge activity and identifying the τ_c (or G') threshold.

How the nature of the compounds present in solid and liquid compartments of activated sludge impact its rheological characteristics

Although the role of the solids concentration on the rheological characteristics of sludge is greatly documented in the literature, few studies focused on the impact of the nature of these solids. How the nature of solutes can modify the solid-liquid interactions and thus the rheological properties of the sludge are also slightly explored. Thus, the objective of this study is to investigate the rheological characteristics of activated sludge in relation with the nature of the compounds present in the solid and liquid phases. Rheological measurements were carried out on raw sludge and on sludge modified by mechanical actions and/or addition of solids or solutes. The rheological properties of raw and modified sludges were measured according to flow and dynamic measurements. Results demonstrated that if suspended solid concentration affected sludge rheological parameters, the nature of the solids was quite of importance. The key role of nature and molecular weight of solutes was also highlighted. The results contribute to a better knowledge of the relationship between sludge composition and its rheological properties, which is useful for the optimization of sludge mixing, pumping or aeration and also for the improvement of sludge dewatering, notably by a relevant choice of adjuvant.

Rheological characterisation of concentrated domestic slurry

The much over-looked element in new sanitation, the transport systems which bridge the source and treatment facilities, is the focus of this study. The knowledge of rheological properties of concentrated domestic slurry is essential for the design of the waste collection and transport systems. To investigate these properties, samples were collected from a pilot sanitation system in the Netherlands. Two types of slurries were examined: black water (consisting of human faecal waste, urine, and flushed water from vacuum toilets) and black water with ground kitchen waste. Rheograms of these slurries were obtained using a narrow gap rotating rheometer and modelled using a Herschel-Bulkley model. The effect of concentration on the slurry are described through the changes in the parameters of the Herschel-Bulkley model. A detailed method is proposed on estimating the parameters for the rheological models. For the black water, yield stress and consistency index follow an increasing power law with the concentration and the behaviour index follows a decreasing power law. The influence of temperature on the viscosity of the slurry is described using an Arrhenius type relation. The viscosity of black water decreases with temperature. As for the black water mixed with ground kitchen waste, it is found that the viscosity increases with concentration and decreases with temperature. The viscosity of black-water with ground kitchen waste is found to be higher than that of black water, which can be attributed to the presence of larger particles in the slurry.

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.

Influence of gas injection on viscous and viscoelastic properties of Xanthan gum

Xanthan gum is widely used as a model fluid for sludge to mimic the rheological behaviour under various conditions including impact of gas injection in sludge. However, there is no study to show the influence of gas injection on rheological properties of xanthan gum specifically at the concentrations at which it is used as a model fluid for sludge with solids concentration above 2%. In this paper, the rheological properties of aqueous xanthan gum solutions at different concentrations were measured over a range of gas injection flow rates. The effect of gas injection on both the flow and viscoelastic behaviour of Xanthan gum (using two different methods - a creep test and a time sweep test) was evaluated. The viscosity curve of different solid concentrations of digested sludge and waste activated sludge were compared with different solid concentrations of Xanthan gum and the results showed that Xanthan gum can mimic the flow behaviour of sludge in flow regime. The results in linear viscoelastic regime showed that increasing gas flow rate increases storage modulus (G'), indicating an increase in the intermolecular associations within the material structure leading to an increase in material strength and solid behaviour. Similarly, in creep test an increase in the gas flow rate decreased strain%, signifying that the material has become more resistant to flow. Both observed behaviour is opposite to what occurs in sludge under similar conditions. The results of both the creep test and the time sweep test indicated that choosing Xanthan gum aqueous solution as a transparent model fluid for sludge in viscoelastic regime under similar conditions involving gas injection in a concentration range studied is not feasible. However Xanthan gum can be used as a model material for sludge in flow regime; because it shows a similar behaviour to sludge.

Effects of potassium permanganate conditioning on dewatering and rheological behavior of pulping activated sludge: mechanism and feasibility

The proposed mechanism of KMnO₄ conditioning on pulping sludge dewatering.

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.

Advanced Wastewater Treatment Engineering-Investigating Membrane Fouling in both Rotational and Static Membrane Bioreactor Systems Using Empirical Modelling

Advanced wastewater treatment using membranes are popular environmental system processes since they allow reuse and recycling. However, fouling is a key limiting factor and so proprietary systems such as Avanti’s RPU-185 Flexidisks membrane bioreactor (MBR) use novel rotating membranes to assist in ameliorating it. In earlier research, this rotating process was studied by creating a simulation model based on first principles and traditional fouling mechanisms. In order to directly compare the potential benefits of this rotational system, this follow-up study was carried out using Avanti’s newly developed static (non-rotating) Flexidisks MBR system. The results from operating the static pilot unit were simulated and modelled using the rotational fouling model developed earlier however with rotational switching functions turned off and rotational parameters set to a static mode. The study concluded that a rotating MBR system could increase flux throughput when compared against a similar static system. It is thought that although the slowly rotating spindle induces a weak crossflow shear, it is still able to even out cake build up across the membrane surface, thus reducing the likelihood of localised critical flux being exceeded at the micro level and lessening the potential of rapid trans-membrane pressure increases at the macro level.

Characterization of Bacterial Cellulose by Gluconacetobacter hansenii CGMCC 3917

In this study, comprehensive characterization and drying methods on properties of bacterial cellulose were analyzed. Bacterial cellulose was prepared by Gluconacetobacter hansenii CGMCC 3917, which was mutated by high hydrostatic pressure (HHP) treatment. Bacterial cellulose is mainly comprised of cellulose Iα with high crystallinity and purity. High-water holding and absorption capacity were examined by reticulated structure. Thermogravimetric analysis showed high thermal stability. High tensile strength and Young's modulus indicated its mechanical properties. The rheological analysis showed that bacterial cellulose had good consistency and viscosity. These results indicated that bacterial cellulose is a potential food additive and also could be used for a food packaging material. The high textural stability during freeze-thaw cycles makes bacterial cellulose an effective additive for frozen food products. In addition, the properties of bacterial cellulose can be affected by drying methods. Our results suggest that the bacterial cellulose produced from HHP-mutant strain has an effective characterization, which can be used for a wide range of applications in food industry.

Operation of integrated sequencing batch membrane bioreactor treating dye-containing wastewater at different SRTs: study of overall performance and fouling behavior

The main aim of the present work was to study the overall performance and the fouling behavior of the integrated sequencing batch membrane bioreactor (SBMBR) treating synthetic dye-containing wastewater at different sludge retention times (SRTs) of 10, 40, and 160 days. For this reason, the removal efficiencies of color and chemical oxygen demand (COD) and the membrane fouling rates as well as the main sludge characteristics including extracellular polymeric substances (EPS), soluble microbial products (SMP), particle size distribution (PSD), and apparent viscosity were evaluated during the operation time of three SBMBRs. According to the obtained results, the decolorization efficiency was found to increase with increasing the SRT while the COD removal efficiency was similarly high (94-97%) in all SBMBRs regardless of SRT change. The rate of rise of transmembrane pressure (TMP) was much lower in the SBMBR operating at SRT of 160 days compared with the two other SBMBRs. Cake development on the membrane surface was recognized as the predominant fouling mechanism in three SBMBRs. Among the stated sludge properties, specific EPS and uniformity coefficient of the particle size distribution were suggested to be the controlling factors of the membrane fouling rate in three SBMBRs. During the operation at prolonged SRT (160 days in this study), better decolorization efficiency and lower biofouling rate were obtained in SBMBR system.

Rheological behaviors of anaerobic granular sludge in a spiral symmetry stream anaerobic bioreactor

The rheological behaviors of the anaerobic granular sludge (AGS) in a spiral symmetry stream anaerobic bioreactor (SSSAB) were investigated. Shear-thinning behavior, thixotropic behavior, concentration-viscosity behavior and temperature-viscosity behavior were evaluated based on the constitutive equation of the AGS. The results indicated that the Herschel-Bulkley model was able to adequately describe the constitutive relation of AGS in the SSSAB. The AGS also showed shear-thinning behavior as well as thixotropic behavior. The critical shear rate and network strength of the AGS were 61.8 s(-1) and 497.0 W m(-3), respectively. The relationship between the apparent viscosity and the sludge concentration was illustrated and explained by the Woodcock formula. The relationship between apparent viscosity of the AGS and temperature could be modeled using the Arrhenius equation. The AGS was significantly thermo-sensitive and its mean energy of activation was 14.640 kJ mol(-1). Notably, it was necessary to consider such behaviors in the hydrodynamic modeling of SSSAB in which shear condition, sludge concentration and temperature were in non-uniform distribution.

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.

Evaluation of different configurations of hybrid membrane bioreactors for treatment of domestic wastewater

Four membrane bioreactors (MBRs) with the same dimensions were studied for 180 days: three hybrid growth membrane bioreactors with biofilm attached in different packing media and a conventional MBR (C-MBR). The four MBRs had an identical membrane module of hollow fiber with a nominal porous diameter of 0.4 μm. The MBRs were: (1) a C-MBR; (2) a moving bed membrane bioreactor (MB-MBR), which was packed with 2 L of carrier Kaldnes-K1, presenting an exposed surface area of 678.90 m²/m³; (3) a non-submerged organic fixed bed (OFB-MBR) packed with 6.5 L of organic packing media composed of a mixture of cylindrical pieces of wood, providing an exposed surface area of 178.05 m²/m³; and (4) an inorganic fixed bed non-submerged membrane bioreactor (IFB-MBR) packed with 6 L of spherical volcanic pumice stone with an exposed surface area of 526.80 m²/m³. The four MBRs were fed at low organic loading (0.51 ± 0.19 kgCOD/m³ d). The results were recorded according to the behavior of the total resistance, transmembrane pressure (TMP), permeability, and removal percentages of the nutrients during the experimental time. The results showed that the MB-MBR presented the better performance on membrane filtration, while the higher nutrient removals were detected in the OFB-MBR and IFB-MBR.

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.

Thixotropic behaviour of thickened sewage sludge

The aim of the work is a description of the rheological behaviour of thickened sewage sludge. The sample of thickened sludge was collected from the wastewater treatment plant, where pressure flotation unit is used for a process of thickening. The value of dry matter of collected sample was 3.52%. Subsequently the sample was diluted and the rheological properties of individual samples were obtained. Several types of rheological tests were used for the determination of the sample. At first the hysteresis loop test was performed. The next test was focused on the time-dependency, i.e. measurement of dependence of dynamic viscosity on the time at constant shear rate. Further dependence dynamic viscosity on the temperature was performed. Then the activation energy was obtained from measured values. Finally, the hysteresis areas were counted and measured values were evaluated with use of Herschel-Bulkley mathematical model.

Investigation of sludge viscosity and its effects on the performance of a vacuum rotation membrane bioreactor

Sludge characteristics of a full-scale vacuum rotation membrane (VRM) bioreactor having plate-type membranes with 0.038 microm nominal pore size and 540 m2 surface area were investigated. The VRM plant is composed of an aeration tank and a filtration chamber. The sludge floc size distribution, as determined microscopically, was mainly between 0 and 100 microm in the filter chamber with very little difference in size distribution between summer (20-25 degrees C) and winter (10-15 degrees C) seasons. Small floc size encountered in the filtration chamber was evident due to excessive shear administered by the rotation action and vigorous agitation by the scouring air. The mixed liquor suspended solids (MLSS) was variable between 5.23 and 14.10 g/L in the VRM tank and sludge behaved plastic. Viscosity ranged between 2.39 and 30 cP in the VRM tank in relation with MLSS concentration and temperature. Above 20 cP inter-plate spaces rapidly clogged leading to major clean-up.

The study of organic removal efficiency and halophilic bacterial mixed liquor characteristics in a membrane bioreactor treating hypersaline produced water at varying organic loading rates

In this study the organic pollutant removal performance and the mixed liquor characteristics of a membrane bioreactor (MBR), employing a halophilic bacterial consortium, for the treatment of hypersaline synthetic produced water - at varying organic loading rates (OLR) from 0.3 to 2.6 kg CODm(-3)d(-1) - were considered. The oil and grease (O&G) and COD removal efficiency were 95-99% and 83-93%, respectively with only transient O&G (mainly polycyclic aromatic hydrocarbons) and soluble microbial products accumulation being observed. With increasing OLR, in the range 0.9-2.6 kg COD m(-3)d(-1), as a result of change in both extracellular polymeric substances (EPS) and zeta potential, bioflocculating ability improved but the compressibility of the flocs decreased resulting in the occurrence of EPS bulking at the highest OLR studied. The latter resulted in a change in the rheology of the mixed liquor from Newtonian to non-Newtonian and the occurrence of significant membrane fouling.

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.

Photocatalytic degradation of waste activated sludge using a circulating bed photocatalytic reactor for improving biohydrogen production

In this study, a sloping trough circulating bed photocatalytic reactor (STCBPR) was developed to degrade waste activated sludge (WAS). Effects of the four factors (shape of trough, circulating speed, TiO(2) dosage and dilution multiple of WAS) on the photocatalytic degradation of WAS were examined. Under the optimum conditions, 45% of the chemical oxygen demand (COD) removal and 47% of the volatile solids (VS) removal were achieved in 7.5-fold dilution after reaction for 8h. Moreover, the WAS was pretreated by STCBPR and then fermented to produce hydrogen. The result showed that the cumulative hydrogen production from photocatalysis pretreated WAS was 3.6 times higher than that from UV-light pretreated WAS, and 17 times higher than that from raw WAS. Therefore, it could be confirmed that the STCBPR has higher photocatalytic degradation efficiency for WAS, and photocatalytic pretreatment could enhance the biohydrogen production.

Effect of carriers on sludge characteristics and mitigation of membrane fouling in attached-growth membrane bioreactor

This study aims to investigate the effect of carriers on membrane fouling. Three sets of submerged membrane bioreactors (MBRs), including one suspended-growth MBR, called SMBR, and two attached-growth MBRs (one called AMBR, and another with baffles called AMBRb) were operated at various media volume fractions. The results indicated that the membrane-fouling rates of AMBR and AMBRb were lower than that of SMBR. Moreover, the operation times of AMBR and AMBRb were 4.2 and 3.5 times longer, respectively, than that of SMBR at an optimum media volume fraction of 30%. The biochemical effects of carriers on sludge suspension significantly affected membrane fouling, unlike the physical effects of carriers. Further studies on sludge characteristics indicated that particle size, soluble microbial product (SMP), sludge volume index (SVI) and filament index (FI) did not remarkably affect membrane fouling, whereas the protein in bound EPS primarily contributed to membrane fouling.

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).

A novel approach to evaluate the permeability of cake layer during cross-flow filtration in the flocculants added membrane bioreactors

In order to obtain a better understanding of the cake layer formation mechanism in the flocculants added MBRs, a model was developed on the basis of particle packing model considering cake collapse effect and a frictional force balance equation to predict the porosity and permeability of the cake layers. The important characteristic parameters of the flocs (e.g., floc size, fractal dimensions) and operating parameters of MBRs (e.g., transmembrane pressure, cross-flow velocity) are considered in this model. With this new model, the calculated results of porosities and specific cake resistances under different MBR operational conditions agree fairly well with the experimental data.

The effect of hydraulic retention time on the performance and fouling characteristics of membrane sequencing batch reactors used for the treatment of synthetic petroleum refinery wastewater

The use of membrane sequencing batch reactors, operated at HRT of 8, 16 and 24h, was considered for the treatment of a synthetic petroleum wastewater. Increase in HRT resulted in statistically significant decrease in MLSS. Removal efficiencies higher than 97% were found for the three model hydrocarbon pollutants at all HRTs, with air stripping making a small contribution to overall removal. Particle size distribution (PSD) and microscopic analysis showed reduction in the protozoan populations in the activated sludge with decreasing HRT. PSD analysis also showed a higher proportion of larger and smaller sized particles at the lowest HRT. The rate of membrane fouling was found to increase with decreasing HRT; SMP, especially carbohydrate SMP, and mixed liquor apparent viscosity also showed a pronounced increase with decreasing HRT, whereas the concentration of EPS and its components decreased. FTIR analysis identified organic compounds as the main component of membrane pore fouling.