Using the Kinetics of Biological Flocculation and the Limiting Flux Theory for the Preliminary Design of Activated Sludge Systems. II: Experimental Verification

Current activated sludge models consider that the removal of biodegradable organics by suspended growth includes: rapid enmeshment of the organic particles in the microbial floc, hydrolysis of the complex organic molecules into readily biodegradable organic substances, and oxidation of dissolved organic substances. All of the models assume that hydrolysis is the rate-limiting step, but none considers the role that the kinetics of biological flocculation and the sludge-settling characteristics may play in defining the activated sludge operating parameters. Several researchers have studied the kinetics of biological flocculation, and have analyzed its role on the removal of particulate chemical oxygen demand in suspended growth reactors. It has been demonstrated that a large proportion of the organic matter present in sewage can be removed by biological flocculation using short hydraulic retention times and subsequent settling. The first paper demonstrates that the one-dimensional limiting flux theory may be useful for coupling the sludge-settling properties with the aeration tank behavior, and the second paper presents experimental evidence that the proposed model is a reasonable first approximation that can be used for activated sludge system design and operation.     

Dynamic Growth Rates of Microbial Populations in Activated Sludge Systems

Results of mathematical modeling and whole cell 16S ribosomal RNA-targeted fluorescence in situ hybridizations challenge the widely held perception that microbial populations in “steady-state” activated sludge systems share a common net growth rate that is proportional to the inverse of the mean cell residence time. Our results are significant because they encourage bioprocess engineers to appreciate the differences in growth physiology among individual microbial populations in complex mixed microbial communities such as suspended growth activated sludge bioreactor systems.     

Robust Optimal Design of Activated Sludge Bioreactors

This paper proposes an algorithm for a robust optimal design of the biological reactor and secondary settling facilities in suspended growth nitrogen and phosphorus removal systems. Robust optimization includes uncertainty in the decision-making procedure and seeks a solution that remains “close” to optimal for all potential operation scenarios. It thus differs fundamentally from the deterministic and stochastic approaches, where uncertainty is ignored or a solution based on either the most likely scenario or the average performance over all potential scenarios is produced. The robust optimization of a suspended growth system is a multiobjective optimization problem concerned with minimization of the global costs and variability of the system’s performance around the optimal. The proposed robust optimization approach uses the ASM3 model, making use of its performance prediction capabilities to produce a powerful tool for designing activated sludge systems. The algorithm was applied to the design of the biological reactor and secondary settling facilities for the Vila Real municipal wastewater treatment plant (Portugal).     

Evidence and Long-Term Feasibility of Enhanced Biological Phosphorus Removal in Oxidation-Ditch Type of Aerated-Anoxic Activated Sludge Systems

This study investigated the potential of four full-scale oxidation ditches to accomplish enhanced biological phosphorus removal (EBPR). Despite the fact that none of the tested oxidation ditches were designed to perform EBPR, mixed liquors from all four ditches showed good specific phosphorus release and uptake rates, a typical characteristic of a typical EBPR biomass. The specific phosphorus release rates ranged from 0.042- to 0.254-mg P/mg VSS-d and the specific phosphorus uptake rates ranged from 0.023- to 0.125-mg P/mg VSS-d for the tested full-scale plants. The EBPR potential of one of the full-scale plants (Central Davis Sewer District) was further studied by changing the aeration patterns in the ditch. The mixed liquor in this full-scale plant exhibited good phosphorus release and uptake trends and dissolved phosphorus, as low as 1.26 mg/L, could be accomplished in the final effluent of this plant as a result of this optimization. The long-term feasibility of the EBPR in this full-scale was tested by running a bench-scale EBPR reactor, in which the anaerobic phase was replaced with aerated-anaerobic phase to simulate the mixed liquor environment that exists in Central Davis mixed liquor and, in general, in all oxidation-ditch-type activated sludge configurations. The bench-scale reactor showed consistent EBPR activity in the absence of an anaerobic environment and more than 85% phosphorus removal could be maintained in the reactor for more than 400 days. The intrafloc microanaerobic/anoxic zones, which may be present in the mixed liquor, did not seem to add to the EBPR efficiency under aerated-anaerobic conditions. Cloning and sequencing using Rhodocyclus specific forward primer RHC439 showed the abundance of organisms most closely falling in Rhodocyclaceae family but not related to Candidatus Accumulibacter phosphatis. Simultaneous 4′-6–Diamidino-2–phenylindole (DAPI) staining and fluorescent in situ hybridization (FISH) using RHC439 probe clearly demonstrated the participation of polyphosphate accumulating organism (PAOs) targeted by RHC439 (i.e., in Rhodocyclaceae family). Microautoradiography assisted FISH using RHC439 further confirmed the participation of PAOs in Rhodocyclaceae family.     

Modeling a Combined Anaerobic/Anoxic Oxide and Rotating Biological Contactors Process under Dissolved Oxygen Variation by Using an Activated Sludge-Biofilm Hybrid Model

A hybrid model which incorporated a biofilm model into the general dynamic model was developed to predict the effluent quality of a combined activated sludge and biofilm process—Taiwan National Central University Process 1. The system was performed under three different dissolved oxygen (DO) conditions in the oxic tank, including 2.0, 1.0, and 0.5 mg/L. When the DO increased from 0.5 to 2.0 mg/L, the soluble biodegradable substrate (SS) and soluble phosphate (PO4) in the effluent were not significantly influenced. Their removal efficiencies were above 92 and 94%. Ammonia–nitrogen (NH3) removal efficiency increased from 36 to 83% and nitrate–nitrogen (NO3) increased from 1.7 to 2.9 mg/L. In biofilm, when the DO was 2.0 mg/L, the active autotrophic biomass (ZA) fraction was 15.7% (surface) to 12.9% (substratum). But when the DO was 0.5 mg/L, the ZA fraction became lower and the fraction was 6.2% (surface) to 3.5% (substratum). The fraction of active nonpoly-P heterotrophic biomass (ZH) in the biofilm did not vary significantly, the values were about 28–35%. ZI decreased as the DO increased. SS in the biofilm did not vary significantly and was maintained at about 2.0 mg/L. When DO increased, NO3 also increased, NH3 decreased from 13.1 to 1.8 mg/L in biofilm.     

Biological Kinetic Data Evaluation of an Activated Sludge System Coupled with an Ultrafiltration Membrane

A membrane bioreactor (MBR) system treating wastewater containing high molecular weight compounds was operated at solids retention times (SRTs) ranging from 30 to 2 days. Chemical oxygen demand removal efficiencies exceeded 99% and effective nitrification was obtained at SRTs between 30 and 5 days. A significant shift in the biological population structure was observed at the 2 days SRT as the content of gram-negative microorganisms increased and nitrifying bacteria were washed out. At this low SRT, limitations in the biological reaction kinetics resulted in incomplete degradation of the feed protein increasing the presence of soluble organic matter in the effluent. Furthermore, the diluted mixed liquor prevented the formation of a filtration cake on the membrane surface, further deteriorating effluent quality. Biological kinetic data parameters were analyzed using three different representations for biomass: volatile suspended solids, lipid phosphates, and total enzymatic activity. All three indicators exhibited similar trends resulting in very comparable estimates for endogenous decay coefficients, thus demonstrating the reliability of volatile suspended solids as a measure for biological activity in activated sludge. Lower than typical endogenous decay rates in the MBR suggested favorable environmental conditions for respiration and a lower potential for self oxidation and predation. The true yield coefficient was in the range of conventional activated sludge systems, refuting previous suggestions of lower yields in MBRs.     

Kinetics of Phosphorus Release and Uptake in a Membrane-Assisted Biological Phosphorus Removal Process

A long-term comparative study on the kinetics of enhanced biological phosphorus removal (EBPR) was carried out in pilot scale membrane-assisted and conventional biological phosphorus removal processes, by monitoring system performance, phosphorus mass balances, and maximum specific rates in off-line batch tests. The two systems exhibited similar performance in the removal of soluble phosphorus (P) from the influent wastewater, in the specific P release observed in the anaerobic zone, and in the maximum specific P release and volatile fatty acid (VFA) uptake rates. However, when the VFA in the influent was limiting, the conventional EBPR (CEBPR) process performed significantly better than the membrane (MEBPR) counterpart, and this behavior was also reflected in the kinetics of P release. Denitrifying dephosphatation was observed to be significant in both processes during periods of satisfactory P removal. When the aerobic recycle ratio was reduced to a minimum level, the anoxic P uptake activity in the CEBPR sludge was lower than that of the MEBPR sludge. Finally, the biomass decay rates of the two sludge types were estimated to be comparable, with significant reduction of the decay under unaerated conditions.     

Effect of Ozone on Filamentous Bulking in a Laboratory Scale Activated Sludge Reactor Using Respirometry and INT-Dehydrogenase Activity

Filamentous bulking can be controlled by the addition of oxidant chemical agents such as ozone. To evaluate the ozone effect on activated sludge from a laboratory scale reactor, different techniques were applied: Settleability test, respirometry (oxygen uptake rate), and the INT-dehydrogenase activity test carried out both by spectrophotometry (DHAa) and image analysis (DHAi). In activated sludge, the respirometric technique and the spectrophotometric DHAa quantified ozone action on the total respiratory activity of flocs; in contrast, the image DHAi test was applied to evaluate the specific action of ozone on filamentous microorganisms. The conditions for application of the INT-dehydrogenase activity test were standardized using pure cultures of a filamentous microorganism (Sphaerotilus natans) and a floc-forming bacterium (Acinetobacter anitratus). For activated sludge with filamentous bulking, ozone doses and treatment times necessary to improve settleability were established. Ozone dose levels and contact times influence the viability of bacteria in flocs and filaments and the finding of appropriate parameters to preserve floc viability and, at the same time, inhibiting filaments is essential to filament control by ozonation.     

Utilization of an Activated Sludge for the Improvement of an Existing Thermophilic Wastewater Treatment System

A pilot-scale activated sludge system was started to determine its effectiveness in treating the thermophilic biological effluent from an existing organic chemical industrial wastewater treatment system. Preliminary results demonstrated that an additional 95% biological oxygen demand and 65% dissolved organic carbon removal was achieved. In addition, significant biodegradation of the volatile organic compounds and organic nitrogen was observed.     

Mathematical Model for the Biofilm–Activated Sludge Reactor

A steady state mathematical model is developed for describing the completely mixed biofilm–activated sludge reactor (hybrid reactor). The model is derived by simultaneously considering Monod kinetics expressions and Fickian’s diffusion theory for substrate in biofilm. In addition, it includes the basic concepts, which describe both culture (suspended and attached) and the competition between them for limiting substrate. By using this model the suspended biomass concentration can be obtained for this system. Subsequently, the other remaining parameters of the system can be computed. Therefore it helps to design and operate the hybrid reactor under different conditions for any given set of kinetic parameters. The utility of the model has been explained for a given set of data and verified by comparing with another solution. It is found that for the same set of data, the model is accurate in the results. The model has been presented in more than one form, each form having an explicit solution of the system. Compared with other solutions of such a system, the model provides a good tool for describing such a system based on fundamental principles.     

Thermal Treatment of Waste Activated Sludge Using Liquid Boiling

In this work we investigated the use of a short time, low superheat boiling process to treat biological sludge. The treated sludge would exhibit a deteriorated filterability, and a marked increase in soluble organics content. A large portion of extracellular polymers was released from the solid phase by boiling. The microbial density levels of the total coliform bacteria and the heterotrophic bacteria were reduced after treatment. Dilution in sludge concentration could allow more organic compounds to be hydrolyzed and a greater fraction of microbes to be disinfected when compared with the undiluted samples.     

Comparison of Bacterial Bioluminescence with Activated Sludge Oxygen Uptake Rates during Zinc Toxic Shock Loads in a Wastewater Treatment System

The use of a bioengineered bioluminescent bacterium (Shk1) for monitoring zinc toxicity was evaluated with samples from a municipal activated sludge wastewater treatment plant and in a bench-scale activated sludge system. Bioluminescent measurements were compared with oxygen uptake rates of activated sludge samples. In batch experiments with activated sludge, the Zn EC50 for Shk1 bioluminescence was 16 mg/L, while the Zn EC50 for activated sludge OURs was approximately 58 mg/L. In the bench-scale system, the influent Zn concentrations tested were 50 and 200 mg/L in toxic shock loads of about 4 h duration. Soluble Zn transport through the influent, aeration basin, and clarifier was able to be monitored by the decrease in Shk1 bioluminescence. However, bioluminescence in samples from the aeration basin decreased faster than activated sludge specific oxygen uptake rates. Differences in responses of Shk1 and the activated sludge community may be due to differences in the assay conditions, the growth forms, physiology of the organisms, or previous cultivation conditions.     

In Situ Microscale Analyses of Activated Sludge Flocs in the Enhanced Biological Phosphate Removal Process by the Use of Microelectrodes and Fluorescent In Situ Hybridization

Phosphate concentration microprofiles were measured within activated sludge flocs in the enhanced biological phosphate removal (EBPR) process, and a fluorescent in situ hybridization and clone library analysis were conducted to indentify polyphosphate accumulating organisms (PAOs). The center of the flocs had the highest phosphate concentrations, and the stratification of the flocs found by microprofiling indicated that the PAOs were probably distributed evenly throughout the flocs. Under the assumption that the phosphate, which was generated because of phosphate release by microbial activity, was not consumed by microbes and was only transferred from the flocs to the bulk by diffusion during anaerobic conditions, the effective diffusion coefficient (Df) for phosphate release within the flocs was calculated to be 3.33×10?7?cm2/s at the end of the anaerobic phase of the EBPR process. These results provide a better understanding of the phosphate removal mechanism, and this understanding of the internal function of flocs can lead to improvement in the modeling, design, and operation of the biological phosphorus removal process.     

Activated Sludge Process Modification for Sludge Yield Reduction Using Pulp and Paper Wastewater

Implications of conventional activated sludge (CAS) process modification to a low sludge production (LSP) process have been studied for treating pulp and paper wastewaters. The activated sludge process is modified to a two-stage design to establish a microbial food chain that would result in reduced sludge production. The return activated sludge in the LSP process bypasses the first (dispersed growth) stage to be received only by the second (predatory) stage. The resulting once-through operation of the dispersed growth (DG) stage makes it potentially susceptible to bacterial washout under hydraulic shock conditions. A sensitivity analysis of the DG stage operation was performed by varying its hydraulic residence time. The experimental data revealed that the optimal DG stage hydraulic residence is between 3 and 5?h, with bacterial washout likely to be initiated within 2?h. Based on laboratory results, it appears that a well-designed LSP system is likely to be able to handle day-to-day variations in hydraulic and organic loading rates. The LSP process produced 36% less sludge than the CAS process while consuming approximately 25% more oxygen. The treatment performance of the two systems was comparable except that the LSP sludge had much better settling and dewatering properties.     

Replacing Outliers and Missing Values from Activated Sludge Data Using Kohonen Self-Organizing Map

Modeling the activated sludge wastewater treatment plant plays an important role in improving its performance. However, there are many limitations of the available data for model identification, calibration, and verification, such as the presence of missing values and outliers. Because available data are generally short, these gaps and outliers in data cannot be discarded but must be replaced by more reasonable estimates. The aim of this study is to use the Kohonen self-organizing map (KSOM), unsupervised neural networks, to predict the missing values and replace outliers in time series data for an activated sludge wastewater treatment plant in Edinburgh, U.K. The method is simple, computationally efficient and highly accurate. The results demonstrated that the KSOM is an excellent tool for replacing outliers and missing values from a high-dimensional data set. A comparison of the KSOM with multiple regression analysis and back-propagation artificial neural networks showed that the KSOM is superior in performance to either of the two latter approaches.     

Computer Program Development for the Design of Integrated Fixed Film Activated Sludge Wastewater Treatment Processes

The Integrated Fixed Film Activated Sludge (IFAS) wastewater treatment systems are activated sludge biological nutrient removal processes that have been enhanced by the addition of biofilm support media into the aerobic zone of the system to obtain year round nitrification in activated sludge systems that otherwise could not support it. The objective of this study was to develop a computer package called “IFAS” that allows steady-state simulation of IFAS wastewater treatment processes based on the International Association Water Quality general model for activated sludge and empirical equations for chemical oxygen demand (COD) uptake and nitrification on integrated fixed film developed at Virginia Tech. The current version of the IFAS program supports only sponge-type media; however, the model could be modified for other media if the appropriate equations and required parameters values are known. Data obtained from IFAS sponge media pilot scale plants treating a weak municipal wastewater supplemented by sodium acetate, urea, sodium bicarbonate, and potassium phosphates and operated at different aerobic mean cells residence times were used to evaluate the model with parameter values for nitrification and COD uptake rates developed in batch studies. The model-generated ammonia and soluble COD profiles were insignificantly different statistically from the experimental data. The IFAS model satisfactorily predicts carbonaceous removal and nitrification, and has the potential to be a useful tool for scientists and engineers seeking to design and optimize either IFAS or conventional biological nutrient removal activated sludge systems.     

Contrasting the Benefits of Primary Clarification versus Prefermentation in Activated Sludge Biological Nutrient Removal Systems

The potential benefits prefermentation can provide to biological nutrient removal are measured and compared to the costs of excess oxygen consumption and sludge production incurred by an activated sludge system that utilizes prefermentation, instead of primary clarification. Prefermentation was found to produce superior performance in regards to enhanced biological phosphorus removal. A lower soluble orthophosphorus effluent value [3.2?mg/L for the prefermented activated sludge (PAS) train versus 4.6?mg/L for the control train with primary clarification (PCAS)] and a higher percent phosphorus (% P) content of the biomass (9.0% for the PAS train versus 7.8% for the PCAS train) were both found to be statistically significant (P values of 4.26×10?5 and 0.0082, respectively). In addition statistically significant improvements in denitrification rates and reduced observed yields were observed due to prefermentation. However statistically significant increases in solids inventory and in particular oxygen uptake rates offset these improvements. Waste activated sludge production was slightly higher in the PAS train but was not found to be statistically significant.     

Effect of Cd(II) on Different Bacterial Species Present in a Single Sludge Activated Sludge Process for Carbon and Nutrient Removal

Heavy metal cadmium(II) was added stepwise into an A2O pilot plant to investigate the toxic effects of Cd(II) on the removal efficiencies, kinetic parameters (yield coefficients and maximum specific growth rates) and reaction rates of carbon, nitrogen and phosphate for the acclimatized heterotrophic and autotrophic bacteria. Results showed that 2?mg/L Cd(II) initially affected the biological reaction of phosphate removal. At Cd(II) 5?mg/L, the efficiencies of total nitrogen removal and nitrification were substantially dropped. At the same time, the yield coefficient and maximum specific growth rate of heterotrophs were significantly decreased from 0.8?g?COD/g?COD and 6.44?day?1 to 0.54?g?COD/g?COD and 4.67?day?1, respectively. And, the denitrification rate was inhibited by about 61%. The inhibition percentages of anaerobic release, anoxic and aerobic uptake rates of phosphate were about 76, 64, and 90%, respectively. When Cd(II) concentration was continually increased up to 35?mg/L, removal efficiency of chemical oxygen demand (COD) was significantly dropped. However, there was no obvious inhibition on the biological reactions of anaerobic ammonification.     

User-Friendly Mathematical Model for the Design of Sulfate Reducing H2/CO2 Fed Bioreactors

This paper presents three steady-state mathematical models for the design of H2/CO2 fed gas-lift reactors aimed at biological sulfate reduction to remove sulfate from wastewater. Models 1A and 1B are based on heterotrophic sulfate reducing bacteria (HSRB), while Model 2 is based on autotrophic sulfate reducing bacteria (ASRB) as the dominant group of sulfate reducers in the gas-lift reactor. Once the influent wastewater characteristics are known and the desired sulfate removal efficiency is fixed, all models give explicit mathematical relationships to determine the bioreactor volume and the effluent concentrations of substrates and products. The derived explicit relationships make application of the models very easy, fast, and no iterative procedures are required. Model simulations show that the size of the H2/CO2 fed gas-lift reactors aimed at biological sulfate removal from wastewater highly depends on the number and type of trophic groups growing in the bioreactor. In particular, if the biological sulfate reduction is performed in a bioreactor where ASRB prevail, the required bioreactor volume is much smaller than that needed with HSRB. This is because ASRB can out-compete methanogenic archaea (MA) for H2 (assuming sulfate concentrations are not limiting), whereas HSRB do not necessarily out-compete MA due to their dependence on homoacetogenic bacteria (HB) for organic carbon. The reactor sizes to reach the same sulfate removal efficiency by HSRB and ASRB are only comparable when methanogenesis is inhibited. Moreover, model results indicate that acetate supply to the reactor influent does not affect the HSRB biomass required in the reactor, but favors the dominance of MA on HB as a consequence of a lower HB requirement for acetate supply.     

Spheroidizing Kinetics and Optimization of Heat Hreatment Parameters in CK60 Steel Using Taguchi Robust Design

 When processing parts made from medium carbon steel, toughness and flexibility are of importance. Therefore, to achieve these properties, the cementite in the steels is spheroidized through heat treatment. Different parameters such as the time and temperature of spheroidizing and the initial microstructure of the steel affect the amount of spheroidized cementite. In the present work, the percent of contribution of two parameters, i.e. initial microstructure and spheroidizing time, on the percent of spheroidization in CK60 steel was investigated using Taguchi robust design. Based on the number of parameters (two) and the number of their levels (four), a L16 orthogonal array, which consists of 16 experiments, was chosen. The initial microstructures consisted of Martensite (M), Coarse Pearlite (C. P.), Fine Pearlite (F. P.), and Bainite (B) and the chosen spheroidization times were 4, 8, 12, 16 hours. Spheroidizing was done at the constant temperature 700 oC. After spheroidizing was completed, the samples were prepared in order to view their microstructure under an optical microscope and to determine the spheroidized percent using MIP? (Metallographic Image Processing) software. It was found that the spheroidizing time had the most influence (58.5%) on spheroidized percent and the initial microstructure only had a 31.1% contribution. Finally, the instantaneous growth rate of the carbide was also deduced.