Physical characteristics of the sludge in a complete retention membrane bioreactor

Sludge physical characteristics play an important role in the operation of membrane bioreactors (MBR) due to their influence on filtration and their effects on handling of excess sludge. These systems are designed to maintain high solid concentrations, thus limiting sludge production and the related operational costs of the process. In this study, the sludge from a bench scale MBR operated for about 1 year with complete solid retention was investigated to assess its physical and rheological properties. Concentrations of mixed liquor suspended solids (MLSS) up to 24 gTSSL(-1) affected the diluted sludge volume index (DSVI), the capillary suction time (CST), the specific resistance to filtration (SRF) and the compressibility (s). The MBR sludge displayed similar dewatering properties of conventional waste activated sludge, suggesting that the upgrade of wastewater treatment plants with the MBR technology would not affect the behaviour of the dewatering equipment. The apparent viscosity was expressed as a function of the MLSS and the experimental data were interpreted by comparing different models. Ostwald model was chosen, and two equations for viscosity were proposed. The thixotropy of MBR sludge was also evaluated by measuring the reduced hysteresis area (rHa) and relating this parameter to the characteristics of the sludge. The evaluation of energy consumption for mixing evidenced that, under the tested conditions, the increase of solid concentration from 3 to 30 gTSSL(-1) resulted in a limited increase of energy requirements (25-30%).     

Effect of sludge retention time on inclined plate function,treatment performance and sludge characteristics of inclined plate membrane bioreactors treating municipal wastewater

Two identical bench scale inclined plate membrane bioreactors (ip‐MBRs) were operated for the treatment of real municipal wastewater for 1 year. Sludge retention time (SRT) was varied over the course of operation to investigate the effects on inclined plate function, treatment performance and sludge characteristic. Removal rates of chemical oxygen demand and ammonia over 90% and of total nitrogen (TN) more than 70% were achieved when ip‐MBRs were operated under SRTs between infinite and 40 days while short SRT (20 days) negatively affected TN removal. When the sludge concentration in anoxic tank exceeded 15 g/L, the failure of the inclined plate function was observed, resulting in no difference in sludge concentrations between aerobic and anoxic tanks. To avoid severe effects on inclined plate function and treatment performance, an SRT range of 40–80 days was recommended for ip‐MBRs. Moreover, sludge floc size under prolonged SRT became smaller than that under short SRT due to increased attrition among the sludge floc particles caused by strong aeration needed for keeping a sufficient dissolved oxygen level.     

The solids retention time-a suitable design parameter to evaluate the capacity of wastewater treatment plants to remove micropollutants

Micropollutants as endocrine disrupting compounds (EDC) or pharmaceuticals are of increased interest in water pollution control. Wastewater treatment plant (WWTP) effluents are relevant point sources for residues of these compounds in the aquatic environment. The solids retention time (SRT) is one important parameter for the design of WWTPs, relating to growth rate of microorganisms and to effluent concentrations. If a specific substance is degraded in dependency on the SRT, a critical value for the sludge age can be determined. In WWTPs operating SRTs below this critical value, effluent concentrations in the range of influent concentrations or a distribution according to the adsorption equilibrium have to be expected, whereas in WWTPs operating at SRTs higher than the critical value degradation will occur. Critical SRTs were determined for different micropollutants, indicating that the design criteria based on the sludge age allows an estimation of emissions. Different treatment technologies as conventional activated sludge systems and a membrane bioreactor were considered and no significant differences in the treatment efficiency were detected when operated at comparable SRT. The results of the investigations lead to the conclusion that low effluent concentrations can be achieved in WWTPs operating SRTs higher than 10 days (referred to a temperature of 10 degrees C). This corresponds to the requirements for WWTPs situated in sensitive areas according to the urban wastewater directive of the European Community (91/271/EEC) in moderate climatic zones.     

Bioaugmentation for nitrification at cold temperatures

Bioaugmentation of nitrifying bacteria for short solids retention time (short-SRT) nitrification is an attractive alternative for wastewater treatment plants in cold climates or for those in the process of upgrading to include nitrification. One possible source of ammonia for the production of nitrifying bacteria is the liquor generated during the dewatering of anaerobically digested sludges. The objectives of this study was to determine the impact of sudden decrease in temperature on nitrification rates and to determine if nitrification could be accomplished in sequencing batch reactors (SBRs) at 10 degrees C by seeding nitrifying bacteria acclimated to 20 degrees C. In this research, biomass produced during warm nitrification of dewatering liquor was seeded into cold SBRs at various hydraulic retention times from 43.3 to 96 h. The average decreases in nitrification rates were 58%, 71% and 82% for biomass cooled to 10 degrees C when the biomass was acclimated to 20 degrees C, 25 degrees C and 30 degrees C, respectively. The seeded SRTs of the cold SBRs were raised above the minimum solids retention time (SRT(min)) required for nitrification. Full ammonia nitrogen removal was achieved in cold SBRs that were operated at an apparent SRT less than SRT(min).     

Soluble microbial products in membrane bioreactor operation: Behaviors, characteristics, and fouling potential

This paper presents an experimental study on soluble microbial products (SMP) in membrane bioreactor (MBR) operation at different sludge retention times (SRTs). A laboratory-scale MBR was operated at SRT of 10, 20, and 40 days for treatment of readily biodegradable synthetic wastewater. The accumulation, composition, characteristics, and fouling potential of SMP at each SRT were examined. It was found that accumulation of SMP in the MBR became more pronounced at short SRTs. Carbohydrates and proteins appeared to be the components of SMP prone to accumulate in the MBR compared with aromatic compounds. The proportions of SMP with large molecular weight in supernatants and in effluents were almost identical, implying that membrane sieving did not work for most SMP. In addition, the majority of SMP was found to be composed of hydrophobic components, whose proportion in total SMP gradually increased as SRT lengthened. However, fouling potentials of SMP were relatively low at long SRTs. The hydrophilic neutrals (e.g., carbohydrates) were most likely the main foulants responsible for high fouling potentials of SMP observed at short SRTs.     

Impact of solids residence time on biological nutrient removal performance of membrane bioreactor

Impact of long solids residence times (SRTs) on nutrient removal was investigated using a submerged plate-frame membrane bioreactor with anaerobic and anoxic tanks. The system was operated at 10, 25, 50 and 75 days SRTs with hydraulic retention times (HRTs) of 2 h each for the anaerobic and anoxic tanks and 8 h for the oxic tank. Recirculation of oxic tank mixed liquor into the anaerobic tank and permeate into the anoxic tank were fixed at 100% each of the influent flow. For all SRTs, percent removals of soluble chemical oxygen demand were more than 93% and nitrification was more than 98.5% but total nitrogen percent removal seemed to peak at 81% at 50 days SRT while total phosphorus (TP) percent removal showed a deterioration from approximately 80% at 50 days SRT to 60% at 75 days SRT. Before calibrating the Biowin^® model to the experimental data, a sensitivity analysis of the model was conducted which indicated that heterotrophic anoxic yield, anaerobic hydrolysis factors of heterotrophs, heterotrophic hydrolysis, oxic endogenous decay rate for heterotrophs and oxic endogenous decay rate of PAOs had the most impact on predicted effluent TP concentration. The final values of kinetic parameters obtained in the calibration seemed to imply that nitrogen and phosphorus removal increased with SRT due to an increase in anoxic and anaerobic hydrolysis factors up to 50 days SRT but beyond that removal of phosphorus deteriorated due to high oxic endogenous decay rates. This indirectly imply that the decrease in phosphorus removal at 75 days SRT may be due to an increase in lysis of microbial cells at high SRTs along with the low food/microorganisms ratio as a result of high suspended solids in the oxic tank. Several polynomial correlations relating the various calibrated kinetic parameters with SRTs were derived. The Biowin^® model and the kinetic parameters predicted by the polynomial correlations were verified and found to predict well the effluent water quality of the MBR at 35 days SRT.     

Surface properties of sludge and their role in bioflocculation and settleability

The influence of sludge retention time (SRT) on the extracellular polymeric substances (EPS) and physicochemical properties (hydrophobicity and surface charge) of sludge was studied using laboratory-scale sequencing batch reactors (SBRs) fed a synthetic wastewater containing glucose and inorganic salts. Sludge surfaces were more hydrophobic (larger contact angle) and less negatively charged at higher SRTs (16 and 20 d) than at lower SRTs (4 and 9 d). The ratio of proteins to carbohydrates within the EPS of the sludges increased as the SRT increased from 4 to 12 d corresponding to the changes in the physicochemical properties of the sludge. The protein:carbohydrate ratio remained constant at SRTs of 16 and 20 d. A transition in sludge properties appeared to occur between the upper range of low- (9 d) and lower range of high-SRTs. The total EPS content, however, was independent of the SRT. A higher sludge volume index (SVI), an indication of poorer settleability or compression, was associated with a larger amount of total EPS but no significant correlation between SVI and the surface properties of sludge was observed. A more hydrophobic and less negatively charged surface corresponded to lower levels of ESS. These results indicate that it is the surface properties, hydrophobicity, surface charge and composition of EPS, of sludge, rather than the quantity of EPS, that govern bioflocculation. In contrast, the EPS content is more important in controlling the settleability of sludge.     

Relation between EPS adherence, viscoelastic properties, and MBR operation: Biofouling study with QCM-D

Membrane fouling is one of the main constraints of the wide use of membrane bioreactor (MBR) technology. The biomass in MBR systems includes extracellular polymeric substances (EPS), metabolic products of active microbial secretion that adversely affect the membrane performance. Solids retention time (SRT) in the MBR is one of the most important parameters affecting membrane fouling in MBR systems, where fouling is minimized at optimal SRT. Among the operating parameters in MBR systems, SRT is known to strongly influence the ratio of proteins to polysaccharides in the EPS matrix. In this study, we have direct evidence for changes in EPS adherence and viscoelastic properties due to changes in the sludge removal rate that strongly correlate with the membrane fouling rate and EPS composition. EPS were extracted from a UF membrane in a hybrid growth MBR operated at sludge removal rates of 59, 35.4, 17.7, and 5.9 L day^-1 (corresponding SRT of 3, 5, 10, and 30 days, respectively). The EPS adherence and adsorption kinetics were carried out in a quartz crystal microbalance with dissipation monitoring (QCM-D) technology in several adsorption measurements to a gold sensor coated with Polyvinylidene Fluoride (PVDF). EPS adsorption to the sensor surface is characterized by a decrease of the oscillation frequency and an increase in the dissipation energy of the sensor during parallel flow of aqueous media, supplemented with EPS, above the sensor surface. The results from these experiments were further modeled using the Voigt based model, in which the thickness, shear modulus, and shear viscosity values of the adsorbed EPS layers on the PVDF crystal were calculated. The observations in the QCM-D suggested that the elevated fouling of the UF membrane is due to higher adherence of the EPS as well as reduction in viscosity and elasticity of the EPS adsorbed layer and elevation of the EPS fluidity. These results corroborate with confocal laser scanning microscopy (CLSM) image analysis showing thicker EPS in close proximity to the membrane surface operated at reactor conditions which induced more fouling at elevated sludge removal rates.     

Influence of loosely bound extracellular polymeric substances (EPS) on the flocculation, sedimentation and dewaterability of activated sludge

Laboratory experiments on the activated sludge (AS) process were carried out to investigate the influence of microbial extracellular polymeric substances (EPS), including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), on biomass flocculation, sludge settlement and dewaterability. The heat EPS extraction method was modified to include a mild step and a harsh step for extracting the LB-EPS and TB-EPS, respectively, from the sludge suspension. Six lab-scale AS reactors were used to grow AS with different carbon sources of glucose and sodium acetate, and different sludge retention times (SRTs) of 5, 10 and 20 days. The variation in the bioreactor condition produced sludge with different abundances of EPS and different flocculation and separation characteristics. The sludge that was fed on glucose had more EPS than the sludge that was fed on acetate. For any of the feeding substrates, the sludge had a nearly consistent TB-EPS value regardless of the SRT, and an LB-EPS content that decreased with the SRT. The acetate-fed sludge performed better than the glucose-fed sludge in terms of bioflocculation, sludge sedimentation and compression, and sludge dewaterability. The sludge flocculation and separation improved considerably as the SRT lengthened. The results demonstrate that the LB-EPS had a negative effect on bioflocculation and sludge-water separation. The parameters for the performance of sludge-water separation were much more closely correlated with the amount of LB-EPS than with the amount of TB-EPS. It is argued that although EPS is essential to sludge floc formation, excessive EPS in the form of LB-EPS could weaken cell attachment and the floc structure, resulting in poor bioflocculation, greater cell erosion and retarded sludge-water separation.     

Membrane bioreactor operation at short solids retention times: performance and biomass characteristics

This study investigated the performance and biomass characteristics of a membrane bioreactor (MBR) and a completely mixed activated sludge (CMAS) system operated at short solids retention times (SRT) ranging from 0.25 to 5 d and hydraulic retention times of 3 and 6 h. The lab-scale reactors were fed with synthetic wastewater to ensure consistency in feed composition. Results show the MBR was capable of achieving excellent quality effluent regardless of the extremely short SRT. The MBR removal efficiencies ranged from approximately 97.3-98.4% (TCOD) in the MBR, compared to 77.5-93.8% (TCOD) and 94.1-97.0% (SCOD) in the CMAS. Nitrification completely ceased when SRT was < 2.5 d. The MBR biomass was composed of small, weak and uniform-sized flocs with large mass of short filamentous organisms and mainly dispersed microorganisms at SRT of 5 and 0.25 d, respectively. In contrast, the CMAS sludge was composed of large flocs with filamentous organisms as a backbone at SRT > 2.5 d. The CMAS flocs were smaller and weaker at shorter SRT. The MBR sludge contained a much higher fraction of non-flocculating microorganisms. This fraction increased significantly with decreasing SRT. It was found that the concentrations of protein and carbohydrates in the exocellular polymeric substances for both the MBR and the CMAS decreased with increasing F/M ratio or decreasing SRT. The combination of increasing amounts of non-flocculating microorganisms and a reduction of EPS at shorter SRT in both reactors contributed to deteriorating sludge settling properties. A significant presence of dispersed biomass and small flocs in MBR contributed to better reactor performance probably due to less mass transfer resistance.     

Removal of selected pharmaceuticals, fragrances and endocrine disrupting compounds in a membrane bioreactor and conventional wastewater treatment plants

Eight pharmaceuticals, two polycyclic musk fragrances and nine endocrine disrupting chemicals were analysed in several waste water treatment plants (WWTPs). A membrane bioreactor in pilot scale was operated at different solid retention times (SRTs) and the results obtained are compared to conventional activated sludge plants (CASP) operated at different SRTs. The SRT is an important design parameter and its impact on achievable treatment efficiencies was evaluated. Different behaviours were observed for the different investigated compounds. Some compounds as the antiepileptic drug carbamazepine were not removed in any of the sampled treatment facilities and effluent concentrations in the range of influent concentrations were measured. Other compounds as bisphenol-A, the analgesic ibuprofen or the lipid regulator bezafibrate were nearly completely removed (removal rates >90%). The operation of WWTPs with SRTs suitable for nitrogen removal (SRT>10 days at 10 degrees C) also increases the removal potential regarding selected micropollutants. No differences in treatment efficiencies were detected between the two treatment techniques. As in conventional WWTP also the removal potential of MBRs depends on the SRT. Ultrafiltration membranes do not allow any additional detention of the investigated substances due to size exclusion. However, MBRs achieve a high SRT within a compact reactor. Nonylphenolpolyehtoxylates were removed in higher extend in very low-loaded conventional WWTPs, due to variations of redox conditions, necessary for the degradation of those compounds.     

Effect of solids retention time on structure and characteristics of sludge flocs in sequencing batch reactors

The effect of solids retention time (SRT) (4-20 d) on sludge floc structure, size distribution and morphology in laboratory-scale sequencing batch reactors receiving a glucose-based synthetic wastewater was studied using image analysis in a long-term experiment over one year. Floc size distribution (>10 microm) could be characterized by a log-normal model for no bulking situations, but a bi-modal distribution of floc size was observed for modest bulking situations. In each operating cycle of the SBRs, the variation in food /microorganisms ratio (0.03-1.0) had no significant influence on floc size distribution and morphology. The results from a long-term study over one year showed that no clear relationship existed between SRT and median floc size based on frequency. However, sludge flocs at the lower SRTs (4-9 d) were much more irregular and more variable in size with time than those at higher SRTs (16 and 20 d). The level of effluent-suspended solids at lower SRTs was higher than that at higher SRTs.     

Monitoring thiocyanate-degrading microbial community in relation to changes in process performance in mixed culture systems near washout

Changes in microbial community structure, associated with changes in process performance, were investigated with respect to the sludge retention time (SRT) in bioreactors treating thiocyanate. Among the seven reactors operated at 0.8-3.0 d SRTs, respectively, the reactor at 2.0 d SRT displayed the maximal thiocyanate removal rate of 240.2mg/L/d. However, the thiocyanate removal efficiency suddenly decreased from 96.1% to 43.1% when the SRT was reduced from 2.0 to 1.8d, corresponding to a 50.1% drop in the removal rate. Microbial communities in the reactors operated at short SRTs, near washout, were analyzed by denaturing gradient gel electrophoresis (DGGE) based on bacterial 16S rRNA genes. All band sequences recovered were assigned to two phyla, Proteobacteria and Bacteriodetes. A Thiobacillus-like microorganism was commonly detected in all the reactors and is suggested to be the main organism responsible for thiocyanate decomposition. Several DGGE band sequences were closely related to the environmental clones detected in environments rich in sulfur and/or nitrogen compounds. Statistical analysis of the DGGE profiles demonstrated that the structure of thiocyanate-degrading communities, as well as the process performance, changed with change in SRT. The microbial community profiles were not always more closely related to those at similar SRT than those at less similar SRT on the non-metric multidimensional scaling (NMDS) map. This was also supported by clustering analysis. These results were contrary to the general notion that the community structures in continuous systems will be controlled by the washout of microbial populations. Our experimental results suggest that the structure of a microbial thiocyanate-degrading community at a given SRT would not be determined only by the washout effect.     

Anaerobic stabilisation and conversion of biopolymers in primary sludge--effect of temperature and sludge retention time

The effect of sludge retention time (SRT) and process temperature on the hydrolysis, acidification and methanogenesis of primary sludge was investigated in completely stirred tank reactors (CSTRs). The CSTRs were operated to maintain SRTs of 10, 15, 20 and 30 days at process temperatures of 25 degrees C and 35 degrees C. The rates of hydrolysis and the biodegradability of primary sludge were assessed in batch reactors incubated at 15 degrees C, 25 degrees C and 35 degrees C. The results revealed that the major amount of sludge stabilisation occurred between 0 and 10 days at 35 degrees C and 10 and 15 days at 25 degrees C. Hydrolysis was found to be the rate limiting-step of the overall digestion process, for the reactors operated at 35 degrees C and 25 degrees C, except for the reactor operated at 10 days and 25 degrees C. At the latter conditions, methanogenesis was the rate-limiting step of the overall digestion process. Proteins hydrolysis was limited to a maximum value of 39% at 30 days and 35 degrees C due to proteins availability in the form of biomass. The biodegradability of primary sludge was around 60%, and showed no temperature dependency. The hydrolysis of the main biopolymers and overall particulate COD of the primary sludge digested in CSTRs were well described by first-order kinetics, in case hydrolysis was the rate-limiting step. Similarly, the hydrolysis of the overall particulate COD of the primary sludge digested in batch reactors were described by first-order kinetics and revealed strong temperature dependency, which follows Arrhenius equation.     

The rheological behaviour of anaerobic digested sludge

Producing biogas energy from the anaerobic digestion of wastewater sludge is one of the most challenging tasks facing engineers, because they are dealing with vast quantities of fundamentally scientifically poorly understood and unpredictable materials; while digesters need constant flow properties to operate efficiently. An accurate estimate of sludge rheological properties is required for the design and efficient operation of digestion, including mixing and pumping. In this paper, we have determined the rheological behaviour of digested sludge at different concentrations, and highlighted common features. At low shear stress, digested sludge behaves as a linear viscoelastic solid, but shear banding can occur and modify the apparent behaviour. At very high shear stress, the behaviour fits well to the Bingham model. Finally, we show that the rheological behaviour of digested sludge is qualitatively the same at different solids concentrations, and depends only on the yield stress and Bingham viscosity, both parameters being closely linked to the solids concentration.     

Modelling nitrification, heterotrophic growth and predation in activated sludge

A mathematical model describing the interaction between nitrifiers, heterotrophs and predators in wastewater treatment has been developed. The inclusion of a predation mechanism is a new addition to the existing activated sludge models. The developed model considered multi-substrate consumption and multi-species growth, maintenance and decay in a culture where nitrifiers, heterotrophs and predators (protozoa and metazoa) are coexisting. Two laboratory-scale sequenced batch reactors (SBRs) operated at different sludge retention time (SRT) of 30 and 100 days for a period of 4 years were used to calibrate and validate the model. Moreover, to assess the predator activity, a simple procedure was developed, based on measuring the respiration rate with and without the presence of the predators. The model successfully described the performance of two SBRs systems. The fraction of active biomass (ammonia oxidisers, nitrite oxidisers and heterotrophs) predicted by the proposed model was only 33% and 14% at SRT of 30 and 100 days, respectively. The high fraction of inert biomass predicted by the model was in accordance with the microscopic investigations of biomass viability in both reactors. The presented model was used to investigate the effect of increasing sludge age and the role of predators on the biomass composition of the tested SBR system.     

Correlation of EPS content in activated sludge at different sludge retention times with membrane fouling phenomena

In this study, activated sludge characteristics were studied with regard to membrane fouling in membrane bioreactors (MBRs) for two pilot plants and one full-scale plant treating municipal wastewater. For the full-scale MBR, concentrations of extracellular polymeric substances (EPS) bound to sludge flocs were shown to have seasonal variations from as low as 17mgg(-1) dry matter (DM) in summer up to 51mg(gDM)(-1) in winter, which correlated with an increased occurrence of filamentous bacteria in the colder season. Therefore, it was investigated at pilot-scale MBRs with different sludge retention times (SRTs) whether different EPS contents and corresponding sludge properties influence membrane fouling. Activated sludge from the pilot MBR with low SRT (23d) was found to have worse filterability, settleability and dewaterability. Photometric analysis of EPS extracts as well as LC-OCD measurements showed that it contained significantly higher concentrations of floc-bound EPS than sludge at higher SRT (40d) The formation of fouling layers on the membranes, characterised by SEM-EDX as well as photometric analysis of EPS extracts, was more distinct at lower SRT where concentrations of deposited EPS were 40-fold higher for proteins and 5-fold higher for carbohydrates compared with the membrane at higher SRT. Floc-bound EPS and metals were suggested to play a role in the fouling process at the full-scale MBR and this was confirmed by the pilot-scale study. However, despite the different sludge properties, the permeability of membranes was found to be similar.     

Evaluation of continuous mesophilic, thermophilic and temperature phased anaerobic digestion of microwaved activated sludge

The effects of microwave (MW) pretreatment, staging and digestion temperature on anaerobic digestion were investigated in a setup of ten reactors. A mesophilic reactor was used as a control. Its performance was compared to single-stage mesophilic and thermophilic reactors treating pretreated and non-pretreated sludge, temperature-phased (TPAD) thermophilic-mesophilic reactors treating pretreated and non-pretreated sludge and thermophilic-thermophilic reactors also treating pretreated and non-pretreated sludge. Four different sludge retention times (SRTs) (20, 15, 10 and 5 d) were tested for all reactors. Two-stage thermo-thermo reactors treating pretreated sludge produced more biogas than all other reactors and removed more volatile solids. Maximum volatile solids (VS) removal was 53.1% at an SRT of 15 d and maximum biogas increase relative to control was 106% at the shortest SRT tested. Both the maximum VS removal and biogas relative increase were measured for a system with thermophilic acidogenic reactor and thermophilic methanogenic reactor. All the two-stage systems treating microwaved sludge produced sludge free of pathogen indicator bacteria, at all tested conditions even at a total system SRT of only 5 d. MW pretreatment and staging reactors allowed the application of very short SRT (5 d) with no significant decrease in performance in terms of VS removal in comparison with the control reactor. MW pretreatment caused the solubilization of organic material in sludge but also allowed more extensive hydrolysis of organic material in downstream reactors. The association of MW pretreatment and thermophilic operation improves dewaterability of digested sludge.     

Modelling the production and degradation of soluble microbial products (SMP) in membrane bioreactors (MBR)

MBR biochemical conditions have an effect on membrane fouling and SMP have been attributed to be the main MBR foulant. Thus, predicting the SMP concentration is essential for understanding and controlling MBR fouling. However, existing SMP models are mostly too complex and over-parameterized, resulting in inadequate or absent parameter estimation and validation. This study extends the existing activated sludge model No. 2d (ASM2d) to ASM2dSMP with introduction of only 4 additional SMP-related parameters. Dynamic batch experimental results were used for SMP parameter estimation leading to reasonable parameter confidence intervals. Finally, the ASM2dSMP model was used to predict the impact of operational parameters on SMP concentration. It would found that solid retention time (SRT) is the key parameter controlling the SMP concentration. A lower SRT increased the utilization associated products (UAP) concentration, but decreased the biomass associated products (BAP) concentration and vice versa. A SRT resulting in minimum total SMP concentration can be predicted, and is found to be a relatively low value in the MBR. If MBRs operate under dynamic conditions and biological nutrient removal is required, a moderate SRT condition should be applied.     

Waste activated sludge fermentation: Effect of solids retention time and biomass concentration

Laboratory scale, room temperature, semi-continuous reactors were set-up to investigate the effect of solids retention time (SRT, equal to HRT hydraulic retention time) and biomass concentration on generation of volatile fatty acids (VFA) from the non-methanogenic fermentation of waste activated sludge (WAS) originating from an enhanced biological phosphorus removal process. It was found that VFA yields increased with SRT. At the longest SRT (10 d), improved biomass degradation resulted in the highest soluble to total COD ratio and the highest VFA yield from the influent COD (0.14 g VFA-COD/g TCOD). It was also observed that under the same SRT, VFA yields increased when the biomass concentration decreased. At a 10 d SRT the VFA yield increased by 46%, when the biomass concentration decreased from 13 g/L to 4.8 g/L. Relatively high nutrient release was observed during fermentation. The average phosphorus release was 17.3 mg PO₄-P/g TCOD and nitrogen release was 25.8 mg NH₄-N/g TCOD.