Characterization of sludge structure and activity in submerged membrane bioreactor.

Sludge characteristics of a submerged membrane bioreactor (MBR) and an activated sludge process (AS) were compared, during a first phase at the same operating conditions (low MLSS and conventional SRT) and in a second phase with a high sludge retention time (SRT) in the membrane bioreactor. During the first phase, a bimodal flocs size distribution was observed in the MBR with simultaneously a macro-flocs population (240 microm) bigger than the flocs of activated sludge due to the absence of recirculating pump, and also more microflocs (1 to 15 microm) and free suspended cells retained by the membrane. It is shown that the membrane leads to an accumulation of proteins and polysaccharides in the sludge supernatant which is probably responsible for the high fouling propensity of the sludge during the starting period of MBR. These compounds are partially degraded after 50 to 60 days of operation. In the first phase respirometric experiments didn't demonstrate a significant difference in the maximal removal rates of either MBR or AS biomass (with excess substrate), except in the dynamic period during which the membrane retention gave an advantage by increasing the biomass activity. On the other hand, the respirometry shows that the half saturation constant for nitrification was significantly higher in the MBR process, suggesting higher substrate transfer limitation. During the last phase, it is shown that an increase of SRT from 9 to 106 days leads to a diminution of average macro-flocs size in the MBR from about 240 to 70 microm. With the SRT increase, modification in the organic compounds is also observed (proteins, polysaccharides and COD) in the sludge supernatant. Increasing the SRT from 9 to 40 days seems to slightly reduce the level of organic compounds (probable biodegradation), but the concentrations increased when SRT changes from 40 days to 106 days (probable accumulation of non biodegradable compounds).     

Correlation between membrane fouling and soluble/colloidal organic substances in membrane bioreactors for municipal wastewater treatment.

Two similar membrane bioreactors of 2 m3 each were operated in parallel over two years under the same operational conditions, fed with the same municipal wastewater. The only process and operational difference between both pilot plants was the position of the denitrification zone (pre-denitrification in pilot 1 and post-denitrification in pilot 2). Despite parallel operation, the two MBRs exhibited different fouling rates and decreases in permeability. These differences could not be accounted for by MLSS concentrations, loading rates, or filtration flux. In a one-year investigation, soluble and colloidal organic material in the activated sludge of both MBR was regularly analysed by spectrophotometric and Size Exclusion Chromatography (SEC) methods. The larger organic molecules present in the sludge water phase (i.e. polysaccharides, proteins and organic colloids) originating from microbial activity (extracellular polymeric substances) were found to impact on the fouling and to explain the difference in membrane performance between the two MBR units. In both pilot plants, a linear relationship could be clearly demonstrated between the fouling rate of the membrane and the concentration of polysaccharides in the sludge water phase during a 5 month operational period at an SRT of 8 days.     

A pilot study on accelerated sludge degradation by a high-concentration membrane bioreactor coupled with sludge pretreatment.

A new sludge treatment process combining a high MLSS membrane bioreactor with sludge pretreatment techniques was studied in pilot-scale experiments. The membrane bioreactor (MBR) was adopted for high efficiency aerobic digestion. The combination of alkaline-ozone treatment of the mixed liquor in the MBR reactor accelerated the biodegradation process by enhancing biodegradability of the sludge. The hydraulic retention time (HRT) of the reactor was set as 3.1 days and the DO level was 1 mg/L on average. After 5 months of operation, the accumulative total solids reduction was more than 70%. Removal efficiency of volatile solids and non-volatile solids were 76% and 54%, respectively. It was found that a considerable portion of the non-volatile solids was dissolved into ions and then flushed out with the effluent. Also, about 41% and 28% of T-N and T-P in the raw sludge were removed although no biological nutrient removal process was adopted. The experiment was run smoothly without significant membrane fouling, even at the relatively high levels of MLSS concentration (11,000-25,000 mg/L). It is concluded that the newly proposed process can significantly increase the sludge reduction efficiency with much shorter retention times.     

The importance of measuring the sludge filterability at an MBR - introduction of a new method

Sludge properties have a strong impact on the operational aspects of membrane bioreactors (MBRs). Poor sludge properties cause stronger membrane fouling and reduce the filtration performance of MBRs. Up to now there is no general method used to measure the fouling or filtration relevant sludge properties in MBRs. The aim of this work was to develop a simple but reliable method to supply operators a tool to monitor the important sludge properties for their application and to compare this method with existing techniques. Through extensive research a new method called the sludge filtration index (SFI) has been developed to indicate the appropriate sludge parameters for MBR systems in a cheap and easy manner. The SFI can be measured with simple laboratory equipment and offers operators a powerful tool to monitor the conditions of their sludge, independent of the membrane conditions.     

Modeling of extracellular polymeric substances and soluble microbial products production in a submerged membrane bioreactor at various SRTs.

Extracelluar polymeric substances (EPSs) and soluble microbial products (SMPs) produced by microbial cultures involved in membrane biofouling have been widely investigated. A mathematical model of EPS and SMP formation and degradation was established based on the activated sludge model no. 1 (ASM1) and was applied to the membrane bioreactor sludge with different sludge retention times (SRTs). The unified theory that the distinct products from the EPS and SMP overlapped each other was integrated into the ASM1. Two components, five processes and eight parameters were newly added to set up the model. To increase the accuracy of model simulation, microbial kinetic parameters were determined by respirometry method and applied to the model instead of microbial kinetic constant offered in ASM1. From the respirometry result, both of heterotroph and autotroph showed different yield value, growth rate and decay rate from activated sludge. There was no significant effect of SRT on SMP production and the experimental results showed good agreement with the predicted values by the model simulation. With the developed unified EPS and SMP model, EPS and SMP production could be simulated so well that it can be applied for the membrane biofouling control.     

Optimal sludge retention time for a bench scale MBR treating municipal sewage.

Membrane bioreactors allow for higher sludge concentrations and improved degradation efficiencies with respect to conventional activated sludge. However, in the current practice these systems are often operated under sub-optimal conditions, since so far no precise indications have yet been issued on the optimal operating conditions of MBR for municipal wastewater treatment. This paper reports some results of four years of operation of a bench scale membrane bioreactor where steady state conditions were investigated under different sludge retention times. The whole experimental campaign was oriented towards the investigation of optimal process conditions in terms of COD removal and nitrification, biomass activity and growth, and sludge characteristics. The membrane bioreactor treated real municipal sewage, and four different sludge ages were tested (20, 40, 60, and 80 days) and compared with previous data on complete sludge retention. The results showed that the the biology of the system, as assessed by the oxygen uptake rate, is less affected than the sludge physical parameters. In particular, although the growth yield was observed to dramatically drop for SRT higher than 80 days, the biological activity was maintained under all the tested conditions. These considerations suggest that high SRT are convenient in terms of limited excess sludge production without losses of the treatment capacity. Physical characteristics such as the viscosity and the filterability appear to be negatively affected by prolonged sludge retention times, but their values remain within the ranges normally reported for conventional activated sludge.     

Quantitative analysis of biological effect on membrane fouling in submerged membrane bioreactor.

The objective of this study is to investigate solids concentration and extracellular polymeric substance (EPS) effects on the membrane fouling in the submerged membrane bioreactor. The relationship between the solids retention time (SRT) and the amount of EPS is observed in three lab-scale MBRs. Additionally, the EPS effect on membrane fouling is quantified by calculating the specific cake resistance (alpha) using an unstirred batch cell test. By observing the sludge over a long period under various SRT scenarios, a wide range of EPS and membrane fouling data is obtained. These observations provide sufficient evidence of the functional relationship between SRT, EPS and alpha. As SRT decreases, the amount of EPS bound in sludge floc becomes higher in the high MLSS condition (> 5,000 mg/L). The amount of EPS in the sludge floc has positive influence on alpha. A sigmoid trend between EPS and alpha is observed and the functional relationship obtained by dimensional analysis is consistent with the experimental results.     

Hybrid membrane bioreactor for water recycling and phosphorus recovery.

This paper deals with the performance of hybrid membrane bioreactor (MBR) combining the precoagulation/sedimentation and membrane bioreactor. The hybrid MBR not only produces the treated water with excellent permeate quality but also shows much lower membrane fouling than the conventional MBR. It may come from its extremely low F/M ratio to maintain the low viscosity even in the high MLSS concentration range of about 20,000 mg/L. Some results of microbial community analysis in MBRs was conducted to demonstrate the other reason for its lower membrane fouling. Hybrid MBR has a high potential to be used for the recycling use of the municipal wastewater. Coagulated sludge produced in the hybrid MBR is a promising phosphorus resource. This paper also contains a recent progress of phosphorus recovery technology, which uses a new phosphoric acids absorbent, i.e. the hexagonal mesostructured zirconium sulfate (ZS). The ZS has the extremely high adsorption capacity of phosphoric acids through anion exchange. The adsorbed phosphoric acids are released from the ZS in a high pH range of about 13.     

Evaluation of the importance of various operating and sludge property parameters to the fouling of membrane bioreactors

A single-fibre microfiltration system was employed to investigate the importance of various operating and sludge property parameters to the membrane fouling during sludge filtration. The sludge was obtained from a submerged membrane bioreactor (SMBR). A series of comparative and correlative filtration and fouling tests were conducted on the influence of the operating variables, sludge properties and the liquid-phase organic substances on the membrane fouling development. The test results were analysed statistically with Pearson's correlation coefficients and the stepwise multivariable linear regression. According to the statistical evaluation, the membrane fouling rate has a positive correlation with the biopolymer cluster (BPC) concentration, sludge concentration (mixed liquor suspended solids, MLSS), filtration flux and viscosity, a negative correlation with the cross-flow velocity, and a weak correlation with the extracellular polymeric substances and soluble microbial products. BPC appear to be the most important factor to membrane fouling development during the sludge filtration, followed by the filtration flux and MLSS concentration. The cross-flow rate also is important to the fouling control. It is argued that, during membrane filtration of SMBR sludge, BPC interact with sludge flocs at the membrane surface to facilitate the deposition of the sludge cake layer, leading to serious membrane fouling.     

Influence of solid retention time on the rheology of MBR sludge.

The rheological characterization is of crucial importance in sludge management both for biomass dewatering and stabilization purposes and for the definition of design parameters for sludge handling operations. The sludge retention time (SRT) has a significant influence on biomass properties in biological wastewater treatment systems and in particular in membrane bioreactors (MBR). The aim of this work is to compare the rheological behaviour of the biomass in a membrane bioreactor operated under different SRT. A bench scale MBR was operated for four years under the same conditions except for the SRT, that ranged from 20 days to complete sludge retention. The rheological properties were measured over time and the apparent viscosity was correlated with the concentration of solid material under equilibrium conditions. The three models most commonly adopted for rheological simulations were evaluated and compared in terms of their parameters. Steady state average values of these parameters were related to the equilibrium biomass concentration (MLSS). The models were tested to select the one better fitting the experimental data in terms of Mean Root Square Error (MRSE). The relationship between the apparent viscosity and the shear rate, as a function of solid concentration, was determined and proposed.     

Effects of solid retention time on the performance of submerged anoxic/oxic membrane bioreactor.

In this study, four similar bench-scale submerged Anoxic/Oxic Membrane Bioreactors (MBR) were used simultaneously to investigate the effects of solids retention time (SRT) on organic and nitrogen removal in MBR for treating domestic wastewater. COD removal efficiencies in all reactors were consistently above 94% under steady state conditions. Complete conversion of NH(4+)-N to NO(3-)-N was readily achieved over a feed NH(4+)-N concentration range of 30 to 50 mg/L. It was also observed that SRT did not significantly affect the nitrification in the MBR systems investigated. The average denitrification efficiencies for the 3, 5, 10 and 20 days SRT operations were 43.9, 32.6, 47.5 and 66.5%, respectively. In general, the average effluent nitrogen concentrations, which were mainly nitrate, were about 22.2, 27.6, 21.7 and 13.9 mg/L for the 3, 5, 10 and 20 days SRT systems, respectively. The rate of membrane fouling at 3 days SRT operation was more rapid than that observed at 5 days SRT. No fouling was noted in the 10 days and 20 days SRT systems during the entire period of study.     

Effect of sludge retention time on characteristics of dynamic membrane in sequencing bioreactors

The characteristics of self-forming dynamic membrane (DM) in sequencing bioreactors under different sludge retention times (SRT) (SRT = 5, 10, 20, 40, 60 days) were studied using a scanning electron microscope, particle sizing distribution and others. The results indicated that the SRT has an evident effect on the characteristics of DM. The content of extracellular polymeric substances and protein decreased with the increase of SRT. The change of polysaccharide was small regardless of SRT. The filtration resistance of the DM was divided into two stages: an initial slowly-rising stage and a fast-rising later stage which were both irrespective of SRT. With the increase of SRT, the filtration resistance increase extent reduced and the running cycle became longer. Compared to the lower SRT, the particle size distribution of mixed liquor and DM has a decreasing trend at higher SRT. The average particle size of DM was larger than that of the mixed liquor irrespective of SRT. The amounts and types of microorganisms on the surface of DM were more abundant as SRT increased. Low SRT produced the DM surface with some Cocci while the high SRT gave the DM dominated with Cocci, Filamentous and Bacillus.     

Effects of flux enhancing polymer on the characteristics of sludge in membrane bioreactor process.

A newly developed membrane performance enhancer (MPE) was used to prevent membrane fouling in a membrane bioreactor (MBR) process. It transpired that 1,000 mg/l of MPE reduced polysaccharide levels from 41 mg/I to 21 mg/I on average under the experimental condition. Repeated experiments also confirmed that 50-1,000 mg/l of MPE could reduce membrane fouling significantly and increase the intervals between membrane cleanings. Depending on MPE dosages and experimental conditions, trans-membrane pressure (TMP) increase was suppressed for 20-30 days, while baseline TMP surged within a few days. In addition, MPE allowed MBR operation even at 50,000 mg/l of total solid and reduced permeate COD. However, no evidence of toxicity for sludge was found from respiratory works.     

Integration of performance,molecular biology and modeling to describe the activated sludge process

To study process performance and population dynamics in activated sludge, a pilot-scale Membrane Bioreactor (MBR) was installed in a municipal wastewater treatment plant (Aubergenville, France). Since no solids losses occur in the MBR effluent, the sludge residence time (SRT) can be: i) easily controlled by means of the sludge withdrawal, and ii) dissociated from the hydraulic residence time (HRT). A complete characterization of this activated sludge system was performed at three sludge ages (5, 10 and 20 days). Raw and treated wastewater quality, as well as sludge concentration, was analyzed, nucleic probe analysts was performed to determine the heterotrophic and nitrifier populations, and the results were compared to the output from a multispecies model that integrates substrate removal kinetics and soluble microbial products (SMP) production/consumption. This paper presents an integrated analysis of the activated sludge process based on chemical, molecular biology, and mathematical tools. The model was able to describe the MBR system with a high degree of accuracy, in terms of COD removal and nitrification, as well as sludge production and population dynamics through the ratio of active nitrifters/bacteria. Both steady-state and transient conditions could be described accurately by the model, except for technical problems or sudden variations in the wastewater composition.     

磁性悬浮颗粒减缓MBR中膜污染的研究

向MBR中投加磁性悬浮颗粒,通过改善污泥混合液特性来控制膜污染,试验结果表明:最佳吸附剂为四氧化三铁/硅藻土复合颗粒,吸附剂的最佳投加量为0.1 g/L;投加复合颗粒的MBR出水的COD、氨氮和硝态氮均优于参比MBR,说明复合颗粒能改善膜出水水质;投加颗粒的MBR膜表面的EPS各组分均低于参比MBR,说明四氧化三铁/硅藻土复合颗粒能有效的吸附污泥悬浮液中的EPS,加强MBR的通量稳定性,从而延缓膜污染,减少膜清洗次数,降低运行成本。     

Performance of membrane-coupled organic acid fermentor for the resources recovery form municipal sewage sludge.

This study focused on the treatment performance of membrane-coupled organic acid fermentor (MOF) with intermittent reciprocal air/ozone backwashing for the keeping of high permeation flux as well as for the effective recovery of dissolved organics from municipal sewage sludge. Intermittent reciprocal air/ozone backwashing was effective for membrane fouling reduction. When MOF was operated under the conditions of pH 5.5, hydraulic retention time (HRT) of 2 days and 20 days of solids retention time (SRT), most favourable fermentation efficiency was attained. Great inhibition for acid producing by intermittent reciprocal air/ozone backwashing was not observed during long-term operation. MOF with intermittent reciprocal air/ozone backwashing is believed to be an effective system for the recovery of organic matter from municipal sewage sludge and membrane fouling reduction.     

Effect of 200 days' sludge retention time on performance of a pilot scale submerged membrane bioreactor for high strength industrial wastewater treatment.

A high strength industrial wastewater was treated using a pilot scale submerged membrane bioreactor (MBR) at a sludge retention time (SRT) of 200 d. The MBR was operated at a high sludge concentration of 20 g/L and a low F/M ratio of 0.11 during 300 d of operation. It was found that the MBR could achieve COD and TOC overall removal efficiencies at more than 99 and 98% TN removal. The turbidity of the permeate was consistently in the range of 0.123 to 0.136 NTU and colour254 absorbance readings varied from 0.0912 to 0.0962 a.u. cm(-1). The sludge concentration was inversely proportional to the hydraulic retention time (HRT), yielded excellent organic removal and extremely low sludge production (0.0016 kgVSS/day).     

Experimental and CFD simulation studies of wall shear stress for different impeller configurations and MBR activated sludge

Membrane bioreactors (MBRs) have been used successfully in biological wastewater treatment for effective solids-liquid separation. However, a common problem encountered with MBR systems is fouling of the membrane resulting in frequent membrane cleaning and replacement which makes the system less appealing for full-scale applications. It has been widely demonstrated that the filtration performances in MBRs can be improved by understanding the shear stress over the membrane surface. Modern tools such as computational fluid dynamics (CFD) can be used to diagnose and understand the shear stress in an MBR. Nevertheless, proper experimental validation is required to validate CFD simulation. In this work experimental measurements of shear stress induced by impellers at a membrane surface were made with an electrochemical approach and the results were used to validate CFD simulations. As good results were obtained with the CFD model (<9% error), it was extrapolated to include the non-Newtonian behaviour of activated sludge.     

Membrane bioreactor (MBR) sludge inoculation in a hybrid process scheme concept to assist overloaded conventional activated sludge (CAS) process operations

This study analyzes the effect of inoculating membrane bioreactor (MBR) sludge in a parallel-operated overloaded conventional activated sludge (CAS) system. Modelling studies that showed the beneficial effect of this inoculation were confirmed though full scale tests. Total nitrogen (TN) removal in the CAS increased and higher nitrate formation rates were achieved. During MBR sludge inoculation, the TN removal in the CAS was proven to be dependent on MBR sludge loading. Special attention was given to the effect of inoculation on sludge quality. The MBR flocs, grown without selection pressure, were clearly distinct from the more compact flocs in the CAS system and also contained more filamentous bacteria. After inoculation the MBR flocs did not evolve into good-settling compact flocs, resulting in a decreasing sludge quality. During high flow conditions the effluent CAS contained more suspended solids. Sludge volume index, however, did not increase. Laboratory tests were held to determine the threshold volume of MBR sludge to be seeded into the CAS reactor. Above 16-30%, supernatant turbidity and scum formation increased markedly.     

Fouling cake layer in a submerged anaerobic membrane bioreactor treating saline wastewaters: curse or a blessing?

The treatment of inhibitory (saline) wastewaters is known to produce considerable amounts of soluble microbial products (SMPs), and this has been implicated in membrane fouling; the fate of these SMPs was of considerable interest in this work. This study also investigated the contribution of SMPs to membrane fouling of the; (a) cake layer/biofilm layer, (b) the compounds below the biofilm/cake layer and strongly attached to the surface of the membrane, (c) the compounds in the inner pores of the membrane, and (d) the membrane. It was found that the cake/biofilm layer was the main reason for fouling of the membrane. Interestingly, the bacteria attached to the cake/biofilm layer showed higher biodegradation rates compared with the bacteria in suspension. Moreover, the bacteria attached to the cake layer showed higher amounts of attached extracellular polysaccharides (EPS) compared with the bacteria in suspension, possibly due to accumulation of the released EPS from suspended biomass in the cake/biofilm layer. Molecular weight (MW) analysis of the effluent and reactor bulk showed that the cake layer can retain a large fraction of the SMPs in the reactor and prevent them from being released into the effluent. Hence, while cake layers lead to lower fluxes in submerged anaerobic membrane bioreactors (SAMBRS), and hence higher costs, they can improve the quality of the reactor effluent.