Performance evaluation of an anoxic/oxic activated sludge system: effects of mean cell residence time and anoxic hydraulic retention time

A laboratory investigation has been undertaken to asses the effects of two operating parameters, mean cell residence time (MCRT) and anoxic hydraulic retention time (HRT), on the performance of an anoxic/oxic activated sludge system. The performance of the system was evaluated in terms of its COD, nitrogen, and biomass characteristics. An activated sludge system is capable of producing a better effluent, in terms of COD and nitrogen characteristics, when it is operated in an anoxic/oxic fashion. A longer MCRT and an adequate anoxic HRT are desirable in the operation of an anoxic/oxic activated sludge system. For the wastewater used in this investigation, the anoxic/oxic unit was capable of producing an effluent with the following characteristics when it was operated at MCRT = 20 days, total system HRT = 10 h, and anoxic HRT = 3-5 h: COD = 15 mg/L; VSS = 10 mg/L; TKN = 1.30 mg/L; NH(3) - N = 0.60 mg/L; and NO(2) + NO(3) - N = 5.0 mg/L. A uniform distribution of biomass is achievable in an anoxic/oxic activated sludge system because of the intensive recirculation/convection maintained. The provision of an anoxic zone in the aeration tank promotes a rapid adsorption of feed COD into the biomass without an immediate utilization for cell synthesis. This, in turn, results in a high microbial activity and a lower observed biomass yield in the system. A tertiary treatment efficiency is achievable in an anoxic/oxic activated sludge system with only secondary treatment operations and costs. A conventional activated sludge system can be easily upgraded by converting to the anoxic/oxic operation with minor process modifications.     

Cost optimization of activated sludge systems

Activated sludge is a widely used aerobic biological waste-water treatment process. A rational approach to least cost design of an integrated system is described which includes the following processes: activated sludge reactor, final settling tanks, gravity thickening, and aerobic sludge digestion. Both capital and operation and maintenance costs are considered. Biological reactor design is based on microbial kinetic concepts and continuous culture of microorganisms theory. Biological solids retention time (θ_c) is utilized as the primary independent design variable to which system performance is related, e.g., effluent quality, ammonia oxidation, and excess sludge production. Liquid-biomass separation is based on the batch flux technique, a rational approach to design of gravity separators (final settling tanks). Trade-offs among reactor volume, clarifier size, recycle pumping capacity, thickener capacity, digester volume, air requirements, and sludge production are discussed. The optimum design is taken as the combination of these parameters within the acceptable design domain, determined by effluent quality criteria, that results in minimum cost. While the method described is general, design of a given treatment system depends on availability, from lab or pilot studies, of system specific numerical values for biological growth coefficients and biomass setting characteristics. A design example illustrates the approach.     

Upgrading activated sludge systems and reduction in excess sludge

Most of 200 Activated Sludge Plant in Iran are overloaded and as a result, their efficiency is low. In this work, a pilot plant is manufactured and put into operation in one of the wastewater treatment plants in the west of Tehran. Instead of conventional activated sludge, a membrane bioreactor and an upflow anaerobic sludge blanket reactor used as a pretreatment unit in this pilot. For the sake of data accuracy and precision, an enriched municipal wastewater was opted as an influent to the pilot. Based on the attained result, the optimum retention time in this system was 4h, and the overall COD removal efficiency was 98%. As a whole, the application of this retrofit would increase the plant's capacity by a factor of 5 and reducing the excess sludge by a factor of 10. The sludge volume index in the anaerobic reactor was about 12 after granulation occurred.     

Wave characterization for mammalian cell culture: residence time distribution

The high dose requirements of biopharmaceutical products led to the development of mammalian cell culture technologies that increase biomanufacturing capacity. The disposable Wave bioreactor is one of the most promising technologies, providing ease of operation and no cross-contamination, and using an innovative undulation movement that ensures good mixing and oxygen transfer without cell damage. However, its recentness demands further characterization. This study evaluated the residence time distribution (RTD) in Wave, allowing the characterization of mixing and flow and the comparison with ideal models and a Stirred tank reactor (STR) used for mammalian cell culture. RTD was determined using methylene blue with pulse input methodology, at three flow rates common in mammalian cell culture (3.3×10(-5)m(3)/h, 7.9×10(-5)m(3)/h, and 1.25×10(-4)m(3)/h) and one typical of microbial culture (5×10(-3)m(3)/h). Samples were taken periodically and the absorbance read at 660nm. It was observed that Wave behavior diverted from ideal models, but was similar to STR. Therefore, the deviations are not related to the particular Wave rocking mechanism, but could be associated with the inadequacy of these reactors to operate in continuous mode or to a possible inability of the theoretical models to properly describe the behavior of reactors designed for mammalian cell culture. Thus, the development of new theoretical models could better characterize the performance of these reactors.     

Prediction of effluent pollution level in activated sludge systems

This paper deals with theoretical and practical problems of modeling activated sludge wastewater treatment processes. A substrate removal model suitable for predicting the concentration of poorly biodegradable substances in purified water, based on batch laboratory experiments, is presented.     

Liquid residence time distributions in immobilized cell bioreactors

Previous work has demonstrated that high ethanol productivities can be achieved using yeast or bacterial cells adsorbed onto the surface of ion exchange resin in vertical packed bed bioreactors. The present work quantitatively characterizes the overall degree of backmixing in such reactors at two scales of operation: 2.0 and 8.0 L. Stimulus-response experiments, using two solvents (2,3-butanediol and 2-ethoxyethanol) as tracers, were performed to measure the liquid phase residence time distribution (RTD) during continuous ethanol fermentations using the yeast Saccharomyces cerevisiae and the bacterium Zymomonas mobilis at the 2-L scale, and with S. cerevisiae at the 8-L scale. In order to separately determine the effects of liquid flow rate and gas evolution on the degree of mixing, stimulus-response experiments were also performed in the systems without microbial cells present. The evolution of CO(2) was found to dramatically increase the extent of mixing; however, the tanks-in-series model for non-ideal flow represented the systems adequately. The packed beds were equivalent to over 70 tanks-in-series during abiotic operation while during fermentations, with similar liquid flow rates, they ranged in equivalence from 35 to 15 tanks-in-series. This increased knowledge of the overall degree of mixing in packed bed, immobilized cell bioreactors will allow for more accurate kinetic modelling and efficient scale up of the process.     

The interpretation of mean transit time measurements for multiphase tissue systems

A model-independent proof of the central-volume principle for multiphase tissue systems is presented. This derivation emphasizes the transport processes which occur within the system, and the physical constraints which the system must satisfy for valid application of the principle. These constraints include: (i) the fluid flowing into the system must be equivalentlylabelled; (ii) the system under study must be part of a larger system which has no diffusive inlets or outlets. The derivation shows that the definition of the volume of distribution and the choice of appropriate partition coefficients for evaluation of this quantity, are independent of any assumption concerning tracer equilibrium between phases as a whole. A less restrictive definition of equivalent labelling also results from the proof.The relationship between the mean transit times measured by global residue detection and by “snapshot” outflow detection is derived. If diffusion of tracer across the boundaries of the monitored system is significant compared to convective transport, then these two transit times will not have identical values.The conditions under which valid measurements of regional, i.e. local, physiological variables can be performed by residue detection are also discussed, and it is shown that regional residue-detection measurements may be used to assess variations in anatomical structure throughout a larger region. However, the use of the height/area method for calculating regional perfusion can lead to error when tracer enters the detector field by diffusion, as opposed to convection, or when a significant amount of diffusion occurs before tracer enters the observed region.     

Diversity of isolates of Acinetobacter from activated sludge systems based on their whole cell protein patterns

Whole cell protein extracts from strains of the currently recognized genomic species of Acinetobacter, together with those from a range of isolates of several genomic species identified using the Biolog system and obtained from a biological nutrient-removal activated sludge plant were analysed by SDS-PAGE. The dendrograms obtained after numerical analysis for the known genomic species generally supported the taxonomic relationships suggested from earlier DNA–DNA hybridisation data. In some cases the activated sludge isolates identified to genomic species level clustered closely with the corresponding genomic species reference strains, although isolates 5 and 8/9 were scattered throughout the dendrogram. Considerable variations were seen in the protein patterns of the 27 different environmental isolates of genomic species 7 that were analysed. Three unidentified Acinetobacter isolates examined formed their own subcluster. Received 06 September 1996/ Accepted in revised form 10 December 1996     

Metabolic model for glycogen-accumulating organisms in anaerobic/aerobic activated sludge systems

Glycogen-accumulating organisms (GAO) have the potential to directly compete with polyphosphate-accumulating organisms (PAO) in EBPR systems as both are able to take up VFA anaerobically and grow on the intracellular storage products aerobically. Under anaerobic conditions GAO hydrolyse glycogen to gain energy and reducing equivalents to take up VFA and to synthesise polyhydroxyalkanoate (PHA). In the subsequent aerobic stage, PHA is being oxidised to gain energy for glycogen replenishment (from PHA) and for cell growth. This article describes a complete anaerobic and aerobic model for GAO based on the understanding of their metabolic pathways. The anaerobic model has been developed and reported previously, while the aerobic metabolic model was developed in this study. It is based on the assumption that acetyl-CoA and propionyl-CoA go through the catabolic and anabolic processes independently. Experimental validation shows that the integrated model can predict the anaerobic and aerobic results very well. It was found in this study that at pH 7 the maximum acetate uptake rate of GAO was slower than that reported for PAO in the anaerobic stage. On the other hand, the net biomass production per C-mol acetate added is about 9% higher for GAO than for PAO. This would indicate that PAO and GAO each have certain competitive advantages during different parts of the anaerobic/aerobic process cycle.     

The microbiology of biological phosphorus removal in activated sludge systems

Activated sludge systems are designed and operated globally to remove phosphorus microbiologically, a process called enhanced biological phosphorus removal (EBPR). Yet little is still known about the ecology of EBPR processes, the microbes involved, their functions there and the possible reasons why they often perform unreliably. The application of rRNA-based methods to analyze EBPR community structure has changed dramatically our understanding of the microbial populations responsible for EBPR, but many substantial gaps in our knowledge of the population dynamics of EBPR and its underlying mechanisms remain. This review critically examines what we once thought we knew about the microbial ecology of EBPR, what we think we now know, and what still needs to be elucidated before these processes can be operated and controlled more reliably than is currently possible. It looks at the history of EBPR, the currently available biochemical models, the structure of the microbial communities found in EBPR systems, possible identities of the bacteria responsible, and the evidence why these systems might operate suboptimally. The review stresses the need to extend what have been predominantly laboratory-based studies to full-scale operating plants. It aims to encourage microbiologists and process engineers to collaborate more closely and to bring an interdisciplinary approach to bear on this complex ecosystem.     

The operating line for activated sludge

The mass balance for O2 on an activated sludge process has been re-arranged to identify the O2 limited boundary for any given treatment installation and for any combination of feed flow rate and input bCOD. The boundary is described graphically as the operating line.     

Analytical study of three-compartment pharmacokinetic models: concentration,area under curves,mean residence time

The analytical expression for the drug concentration in each of the three compartments of a pharmacokinetics model, following any type of administration, is obtained from the transfer or elimination coefficients alone, without numerical approximation. Important applications of this result are the fitting of experimental data and the calculation of the areas under concentration curves or of the mean residence time of the drug in a given compartment or in the system as a whole.     

荧光定量PCR监测五氯酚对好氧颗粒污泥和活性污泥中氨氧化细菌数量的影响

通过特异引物扩增环境中氨氧化细菌16S rDNAV2保守区域,将该片段克隆到T-easy载体上,PCR产物经测序和定量PCR扩增体系鉴定,证实PCR扩增产物为氨氧化细菌16S rDNA保守序列,以含该序列的重组质粒作为定量PCR监测氨氧化细菌数量的DNA标准品。用荧光定量PCR技术比较了五氯酚(PCP)对好氧颗粒污泥和活性污泥中氨氧化细菌数量的影响。结果表明,不加PCP的反应器中,好氧颗粒污泥和活性污泥中氨氧化细菌的数量分别为4.28×107±5.44×106cells/(g干污泥)和2.51×109±8.61×108cells/(g干污泥)。随着PCP浓度的增加(0~50mg/L),PCP对氨氧化细菌数量的影响不大(P>0.05),而且,污泥中氨氧化细菌的数量与氨氮的去除率无直接的正相关关系(P>0.05),PCP主要是抑制氨氧化细菌的代谢活性导致污泥氨氮去除效率降低。     

Steady-state model for activated sludge with constant recycle sludge concentration

In a previous report it was concluded that steady-state operation of completely mixed reactors for growth of heterogeneous microbial populations, i.e., activated sludge processes, was extremely difficult to attain if maintenance of a constant sludge recycle ratio, c, was required, and equations were devised in which the concentration of cells in the recycle, x_R, rather than the recycle ratio, was constant. In this report the equations are developed and computational analysis shows the effect on substrate and cell concentrations in the reactor of operational variables such as inflowing feed concentration, hydraulic recycle ratio, recycle sludge concentration, dilution rate, and the biological “constants” μ_m, k_s, and Y. The stabilizing effect of operating with constant x_R on the dilute-out pattern is shown.     

Aerobic selectors in slaughterhouse activated sludge systems: a preliminary investigation

Filamentous bulking at a slaughterhouse activated sludge treatment plant significantly reduced mixed liquor settling properties, which caused many operational problems and worsening in effluent quality. The main cause of this condition was attributed to significant levels of influent readily biodegradable COD, which was present primarily in the form of organic acids. An aerobic selector was chosen to eradicate the usual bulking incidents of slaughterhouse wastewater treatment plants. Other plant enhancements included increased aeration batch reactor volume, and provision of step feed capability. Comparison of data before and after aerobic selector installation showed a significant improvement in mixed liquor settleability, which eradicated the need for chemicals that had been used to control filaments and to control effluent solids loss. The additional volume of the aeration and chemicals eliminations from the activated sludge system also served to eliminate aquatic toxicity in the treated effluent.     

A general kinetic model for biological nutrient removal activated sludge systems: model evaluation

Hu et al. (2007) presented a general kinetic model for biological nutrient removal (BNR) activated sludge (AS) systems in general, but for external nitrification (EN) BNRAS (ENBNRAS) systems in particular. In this article, this model is evaluated against a large number of experimental data sets. In this evaluation, the model is first used to simulate a wide variety of conventional internal nitrification (IN) BNRAS systems to evaluate its predictions and also evaluate the model parameters suggested by Hu et al. (2007), and to calibrate those constants for which values are not available in the literature. Simulation results indicate that the model, with appropriately calibrated parameters, is capable of predicting COD removal, nitrification and denitrification and two types of biological excess phosphorus removal (BEPR), namely aerobic and anoxic/aerobic P uptake BEPR. The model is then used to simulate the ENBNRAS systems to evaluate its capacity of simulating the behaviour of this system. Simulation results show that the model is capable of simulating the behaviour of the ENBNRAS systems, including COD, nitrification, denitrification and BEPR, particularly anoxic P uptake BEPR, with the values of kinetic and stoichiometric parameters obtained in modelling conventional BNRAS systems, except for micro(NIT), K(MP), eta(PAO) and eta(H) which required calibration.     

A general kinetic model for biological nutrient removal activated sludge systems: model development

In this article, a kinetic model is developed and presented for biological nutrient removal (BNR) activated sludge (BNRAS) systems in general, but for external nitrification (EN) BNRAS (ENBNRAS) systems in particular. The model is based on the UCTPHO model, but includes some significant modifications, such as anoxic P uptake and associated denitrification by phosphorus accumulating organisms (PAOs). Some key features of the model are described and discussed before the model is presented. Model evaluation will be addressed in another article (Hu et al., 2007).     

Active heterotrophic biomass and sludge retention time (SRT) as determining factors for biodegradation kinetics of pharmaceuticals in activated sludge

The present study investigates the biodegradation of pharmaceutically active compounds (PhACs) by active biomass in activated sludge. Active heterotrophs (X_bh) which are known to govern COD removal are suggested as a determining factor for biological PhAC removal as well. Biodegradation kinetics of five polar PhACs were determined in activated sludge of two wastewater treatment plants which differed in size, layout and sludge retention time (SRT).Results showed that active fractions of the total suspended solids (TSS) differed significantly between the two sludges, indicating that TSS does not reveal information about heterotrophic activity. Furthermore, PhAC removal was significantly faster in the presence of high numbers of heterotrophs and a low SRT. Pseudo first-order kinetics were modified to include X_bh and used to describe decreasing PhAC elimination with increasing SRT.