Use of modelling for optimization and upgrade of a tropical wastewater treatment plant in a developing country.

This paper presents results of a novel application of coupling the Activated Sludge Model No. 3 (ASM3) and the Anaerobic Digestion Model No.1 (ADM1) to assess a tropical wastewater treatment plant in a developing country (Surat, India). In general, the coupled model was very capable of predicting current plant operation. The model proved to be a useful tool in investigating various scenarios for optimising treatment performance under present conditions and examination of upgrade options to meet stricter and upcoming effluent discharge criteria regarding N removal. It appears that use of plant-wide modelling of wastewater treatment plants is a promising approach towards addressing often complex interactions within the plant itself. It can also create an enabling environment for the implementations of the novel side processes for treatment of nutrient-rich, side-streams (reject water) from sludge treatment.     

Impact of reactive settler models on simulated WWTP performance.

Including a reactive settler model in a wastewater treatment plant model allows representation of the biological reactions taking place in the sludge blanket in the settler, something that is neglected in many simulation studies. The idea of including a reactive settler model is investigated for an ASM1 case study. Simulations with a whole plant model including the non-reactive Takács settler model are used as a reference, and are compared to simulation results considering two reactive settler models. The first is a return sludge model block removing oxygen and a user-defined fraction of nitrate, combined with a non-reactive Takács settler. The second is a fully reactive ASM1 Takács settler model. Simulations with the ASM1 reactive settler model predicted a 15.3% and 7.4% improvement of the simulated N removal performance, for constant (steady-state) and dynamic influent conditions respectively. The oxygen/nitrate return sludge model block predicts a 10% improvement of N removal performance under dynamic conditions, and might be the better modelling option for ASM1 plants: it is computationally more efficient and it will not overrate the importance of decay processes in the settler.     

Analyzing wet weather flow management using state of the art tools.

Optimal secondary clarifier performance is crucial to meet treatment requirements, especially when treating peak wet weather flows (PWWFs), to prevent high effluent suspended solids (ESS) concentrations and elevated sludge blankets. A state-of-the-art computational fluid dynamic (CFD) model was successfully used as a design and diagnostic tool to optimize performance for municipal wastewater treatment plants subject to significant PWWFs. Two case studies are presented. For Case Study 1, the model was used to determine the number of secondary clarifiers that will be necessary to treat future PWWF conditions for a plant under design. For Case Study 2, the model was used to identify modifications that are currently being made to increase the clarifier capacity for handling PWWF.     

Continuity-based interfacing of models for wastewater systems described by Petersen matrices.

In this paper the Petersen and composition matrices that modellers are now familiar with are used as a basis to construct interfacing models between subsystems considered in wastewater treatment. Starting from continuity considerations and a set of transformation reactions between components used in the two models of the subsystems to be interfaced, a set of linear algebraic equations needs to be solved. The theoretical development is illustrated using a simplified integrated model of an activated sludge system coupled to an anaerobic digester. Continuity-guaranteed interfacing of subsystems will facilitate optimization studies of the within-the-fence process units of a wastewater treatment plant or of the integrated urban wastewater system.     

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.     

Performance intensification of Prague wastewater treatment plant.

Prague wastewater treatment plant was intensified during 1994--1997 by construction of new regeneration tank and four new secondary settling tanks. Nevertheless, more stringent effluent limits and operational problems gave rise to necessity for further intensification and optimisation of plant performance. This paper describes principal operational problems of the plant and shows solutions and achieved results that have lead to plant performance stabilisation. The following items are discussed: low nitrification capacity, nitrification bioaugmentation, activated sludge bulking, insufficient sludge disposal capacity, chemical precipitation of raw wastewater, simultaneous precipitation, sludge chlorination, installation of denitrification zones, sludge rising in secondary settling tanks due to denitrification, dosage of cationic polymeric organic flocculant to secondary settling tanks, thermophilic operation of digestors, surplus activated sludge pre-thickening, mathematical modelling.     

Membrane bioreactor versus conventional activated sludge system: population dynamics of nitrifiers.

Although membrane bioreactors have attracted increasing attention in recent years, little research has been undertaken on the influence of the membrane separation on the microbial community composition. This paper compares the startup behaviour and the performance of the subsequent eight months of a membrane bioreactor with a conventional activated sludge pilot plant. Both plants were operated in parallel at the same sludge age and treated the same domestic wastewater. The identification of the nitrifying community composition using fluorescent in situ hybridization revealed only minor differences between the two reactors for both ammonia-oxidizing bacteria and nitrite-oxidizing bacteria. Accordingly, both systems exhibited the same maximum nitrification rates. Confocal laser scanning microscopy showed that the aggregates formed by nitrifying bacteria were located mostly in the inner part of the flocs and were overgrown by heterotrophic bacteria. It is concluded that the membrane separation itself does affect neither the nitrifying community composition nor the nitrification performance. However, impacts on kinetic parameters are emphasized.     

Evaluating the stabilisation degree of digested sewage sludge: investigations at four municipal wastewater treatment plants.

Further reduction of volatile suspended solids (VSS) during a post-stabilisation step was applied to evaluate the stabilisation degree of digested sewage sludge. For this purpose digested sludge was collected at four municipal wastewater treatment plants (WWTPs) and further stabilised in lab-scale chemostat reactors either under anaerobic or aerobic conditions. Experimental results showed that even in adequately digested sludge a consistent amount of VSS was degraded during aerobic post-stabilisation. It seems that aerobic conditions play a significant role during degradation of residual VSS. Additionally, specific VSS production (gVSS/peCOD110.d) as well as specific oxygen uptake rate were shown to be suitable parameters to assess the degree of sludge stabilisation at WWTPs. Fourier transform infrared spectroscopy was used to reveal changes in the sludge composition. Spectra of treated and untreated sludge samples indicated that the major component of residual VSS in stabilised sludge for instance consisted of biomass, while cellulose was absent.     

Bulking control by low-dose ozonation of returned activated sludge in a full-scale wastewater treatment plant

Sludge bulking is still a problem in the operation of state-of-the-art wastewater treatment plants (WWTPs). The ozonation of returned activated sludge (RAS) is an innovative option as a non-specific measure for the control of filament growth. The applicability of sludge ozonation for bulking control of a large wastewater treatment plant was investigated. At a full-scale WWTP one lane was equipped with a sludge ozonation plant for RAS. The implemented sludge ozonation of RAS was tested against the two identical references lanes of the same WWTP. The positive effect on settleability could be clearly proven. Low-dose sludge ozonation could be a technical alternative in comparison with the established chemical measures for bulking control.     

Modelling seasonal dynamics of "Microthrix parvicella".

The filamentous bacteria "Microthrix parvicella" can cause serious bulking and scumming in wastewater treatment plants (WWTPs) all over the world. Decades of research have identified Microthrix as a specialized lipid consumer but could not clarify the processes that allow this organism to successfully compete in activated sludge systems. In this study we developed a model, based on ASM3, that describes the pronounced seasonal variations of Microthrix abundance observed in a full-scale WWTP. We hypothesize that low temperatures reduce the solubility of lipids and inhibit their uptake by non-specialized bacteria. The presented model structure and parameters successfully fit the measured data; however they do not necessarily reflect the only and true selection mechanism for Microthrix. This model is not yet to be used for prediction; it is rather a valuable research tool to coordinate the discussion and plan future research activities in order to identify the relevant selection mechanisms favoring Microthrix in activated sludge systems.     

Modelling carbon oxidation in pulp mill activated sludge systems: calibration of Activated Sludge Model No 3.

Activated Sludge Model No 3 (ASM3) was chosen to model an activated sludge system treating effluents from a mechanical pulp and paper mill. The high COD concentration and the high content of readily biodegradable substrates of the wastewater make this model appropriate for this system. ASM3 was calibrated based on batch respirometric tests using fresh wastewater and sludge from the treatment plant, and on analytical measurements of COD, TSS and VSS. The model, developed for municipal wastewater, was found suitable for fitting a variety of respirometric batch tests, performed at different temperatures and food to microorganism ratios (F/M). Therefore, a set of calibrated parameters, as well as the wastewater COD fractions, was estimated for this industrial wastewater. The majority of the calibrated parameters were in the range of those found in the literature.     

Analysing sludge balance in activated sludge systems with a novel mass transport model

In activated sludge systems the mechanically treated wastewater is biologically cleaned by biomass (activated sludge). The basic requirement of an efficient biological wastewater treatment is to have as a high biomass concentration in the biological reactor (BR) as possible. The activated sludge balance in activated sludge systems is controlled by the settling, thickening, scraper mechanism in the secondary settling tank (SST) and sludge returning. These processes aim at keeping maximum sludge mass in the BR and minimum sludge mass in the SST even in peak flow events (storm water flow). It can be, however, only reached by a high SST performance. The main physical processes and boundary conditions such as inhomogeneous turbulent flow, geometrical features of the SST, wastewater treatment plant (WWTP) load, return sludge flow, sludge volume index etc. all influence settling thickening and sludge returning. In the paper a novel mass transport model of an activated sludge system is presented which involves a 2-dimensional SST model coupled with a mixed reactor model of the biological reactor. It makes possible to investigate different sludge returning strategies and their influence on the sludge balance of the investigated activated sludge system, furthermore, the processes determining the flow and concentration patterns in the SST. The paper gives an overview on the first promising model results of a prevailing peak flow event investigation at the WWTP of Graz.     

Influence of temperature and sludge loading on activated sludge settling,especially on Microthrix parvicella

During recent years modern full scale wastewater treatment plants with biological nitrification, denitrification and phosphorus removal have had increasing problems with foam formation on the surfaces of aerobic tanks and with bulking activated sludge. The results of a survey in 1995 (Kunst and Knoop, 1996) showed that most often the filamentous bacterium Microthrix parvicella is responsible for these problems. Up to today there is only little knowledge about its selection criteria in activated sludge. Therefore several expenments were done in full scale activated sludge plants and in laboratory systems under defined conditions to investigate the influence of low (< 0.1 kg/(kg·d)) and high (≤ 0.2 kg/(kg·d)) BOD₅-sludge loading rates on the growth and morphology of M. parvicella and the settlement of activated sludge. Furthermore the influence of temperatures of 5°C, 12°C and 20°C on the growth of M. parvicella was investigated. It was shown that M. parvicella grows at low BOD₅-sludge loading rate and low temperature and is the main causative organism of bulking and foaming sludge in nutrient removal plants. On the basis of this investigation it was concluded that the growth of M. parvicella and the settling problems of the activated sludge resulting from excessive growth of this filament will always appear in modern municipal wastewater treatment plants with BOD₅-sludge loading rate ≤ 0.1 kg/(kg·d) especially under low temperature conditions.     

Development and exploitation of a multipurpose CFD tool for optimisation of microbial reaction and sludge flow.

A numerical analysis technique for optimisation of microbial reaction and sludge flow has been developed in this study. The technique is based on the 3D multiphase Navier-Stokes solver with turbulence models. In order to make numerical analyses of the total processes in wastewater treatment plants possible, four numerical models, the microbial reaction model, a sludge settling model, oxygen mass transfer model from coarse bubbles, and a model from fine bubbles, are added to the solver. All parameters included in those models are calibrated in accordance with experimental results, and good agreements between calculated results and experimental results are found. Finally, this study shows that the numerical technique can be used to optimise wastewater treatment plants with an example of the operational optimisation of an intermittent agitation in anoxic reactors by coarse bubbles. With a proper appreciation of its limit and advantages, the exploitation of the CFD efficiently leads us to the right direction even though it is not quantitatively perfect.     

The main wastewater treatment plant of Vienna: an example of cost effective wastewater treatment for large cities.

Two-stage activated sludge plants succeed in stable treatment efficiency concerning carbon removal and nitrification with far less reactor tank volume than conventional single stage systems. In case of large treatment plants this fact is of great economic relevance. Because of the very small specific volume of these two-stage treatment plants in comparison with low loaded single-stage plants, internal cycles have to be applied to ensure sufficient nitrogen removal. Due to these internal cycles two stage activated sludge plants offer many possibilities in terms of process management which results in new process optimisation procedures as compared to conventional single-stage nutrient removal treatment plants. The proposed extension concept for the Main Treatment Plant of Vienna was validated with pilot plant investigations especially with regard to nitrogen removal where it proved to comply with the legal requirements. The operation of the treatment plant can easily be adapted to changes in temperature and sludge volume index occurring in full scale practice. Sludge retention time and aerobic volume in the second stage are controlled in order to secure sufficient nitrification capacity and to optimise nitrogen removal by means of the variation of the loading conditions for the two stages. The investigations confirmed that the specific two-stage activated sludge concept applied in Vienna is an economically advantageous alternative for large wastewater treatment plants with stringent requirements for nitrification and nutrient removal.     

活性污泥中原生动物的特征和作用

污水处理厂的处理效果主要取决于水中的微生物。原生动物能用来改善水质、评价和指示污水厂的运行和处理效果。根据原生动物的生物特征及其在不同环境条件下的种属、数量、活性等表现 ,通过其组成和数量的分析 ,可以迅速为活性污泥工艺提供有益的指示信息 ,从而指导污水厂的生产运行。