Reactor Hydraulics Research Articles

Dynamic Model for UASB Reactor Including Reactor Hydraulics, Reaction, and Diffusion

Dynamic Model for UASB Reactor Including Reactor Hydraulics, Reaction, and Diffusion

Modelling phytoplankton dynamics of the eutrophic Lake Võrtsjärv, Estonia

The aim of the study was to assess the effect of nutrient loading and water level regulation of water quality in Lake Vortsjarv. The lake is highly eutrophic and it has a high value for fisheries and recreation. A water quality model was developed in order to make predictions of the effects of the different possible restoration measures (reduction of nutrient loading, water level regulation) planned to be carried out. Two phytoplankton groups, diatoms and cyanobacteria, were included in the model. Their contribution to the total algal biomass is dominating. The other state variables of the model are concentration of soluble reactive phosphorus (SRP), concentration of inorganic nitrogen and concentration of dissolved oxygen. The ecological model was constructed using continuously stirred tank reactor (CSTR) hydraulics. The zero-dimensional approach could be used because the lake is shallow and has no thermal stratification. According to the model, external nutrient loading has a very small effect on the biomass of cyanobacteria, whereas the effect is clearer when diatoms are concerned. The contribution of water level to biomass is very significant for both diatoms and cyanobacteria.

Evaluation of UV disinfection systems for large‐scale secondary effluent

A study at the Southeast Water Pollution Control Plant, San Francisco, California, was undertaken to evaluate the feasibility of replacing its current secondary effluent chlorination system with a 6.57 m3/s ultraviolet (UV) disinfection system. Two vertical lamp units and one horizontal lamp unit were pilot tested in parallel. The effective UV dose for coliform removal and the effects of feedwater characteristics and reactor hydraulics were examined. The horizontal pilot unit gave a more consistent performance than the vertical pilot units. Based on the test results, a UV dose of 65 mW·s/cm2 would be required for the plant to achieve the target effluent total coliform level (240 CFU/100 mL) 95% of the time. The large fluctuation of UV disinfection results could be attributed to the wide range of feedwater quality inherent to a combined sewer system. High suspended solids, characteristic of the plant's secondary effluent during storm events, significantly increased the UV dose required to achieve the target coliform level. Existing UV inactivation models were evaluated. The hydraulic behavior of the pilot units was found to significantly affect their test results. Higher virus removal efficiency was observed with the UV systems than the full‐scale chlorination system.

Dynamic Model for UASB Reactor Including Reactor Hydraulics, Reaction, and Diffusion

A dynamic model has been developed to describe upflow anaerobic sludge blanket (UASB) reactors from several aspects including reactor hydraulics, biological reaction kinetics, and mass transfer within anaerobic granules. A flow model of a nonideal continuously stirred tank reactor (CSTR) followed by a dispersion plug flow reactor (PFR) was used to simulate the reactor hydraulics as observed from a LiCl tracer study. The dynamic model based on this flow model was then evaluated by a set of acetate impulse data and verified with a data set from a two-step acetate feed increase experiment from a bench-scale UASB reactor. The model describes UASB reactor performance well. Simulation results indicate significant effects of reactor nonideal flow, diffusional resistance, as well as degradation kinetics on overall substrate utilization rate. Sensitivity analyses on model parameters Ks, km, KL, D, R, and nonideal flow factors revealed granule size has a strong impact on the reactor performance. The effect of KL is not significant. Reactor mixing was improved by an increase in biogas production.

Analyzing Disinfection Kinetics and Reactor Design: A Conceptual Approach Versus the SWTR

The Surface Water Treatment Rule (SWTK) specifies C x T values for achieving various degrees of inactivation of pathogenic microorganisms, with C being the concentration of the disinfectant in the effluent from any reactor and T being a characteristic contact time dependent on reactor hydraulics. The disinfection credit calculated for reactors using the SWTR‐prescribed C x T approach is compared with predictions based on accepted disinfection kinetics and fundamental reactor behavior. This analysis demonstrates that the C x T approach lacks a sound conceptual basis, tends to be somewhat conservative, and is unnecessarily costly for many water treatment plants. A dimensionless parameter, k' Cθ, the product of a disinfection rate constant, the effluent disinfectant concentration, and the theoretical detention time, provides a systematic approach for evaluating disinfection conditions and plant modifications. The decay of chlorine in the reactor is considered, and an error in a previous Journal article is corrected.

Application of Biofilm Kinetics to Anaerobic Fixed Bed Reactors

Two basic phenomena, reactor hydraulics and mass transport through biofilm coupled with kinetic expressions for substrate transformations were accounted for in order to describe the soluble COD removal mechanism in anaerobic fixed bed reactors. To provide necessary verification, experimental results from the long term operation of the pilot scale anaerobic reactor treating molasses wastewater were used. Theoretical evaluations verified by these experimental studies showed that a bulk zero-order removal rate expression modified by diffusional resistance leading to bulk half-order and first-order rates together with the particular hydraulic conditions could adequately define the overall soluble COD removal mechanism in an anaerobic fixed bed reactor. The experimental results were also used to determine the kinetic constants for practical application. In view of the complexity of the phenomena involved it is found remarkable that a simple simulation model based on biofilm kinetics is a powerful tool for design and operation of anaerobic fixed bed reactors.

The effect of reactor hydraulics on the performance of activated sludge systems-I. The traditional modelling approach: by D. Orhonet al., Wat. Res.23, 1511–1518 (1989)

The effect of reactor hydraulics on the performance of activated sludge systems-I. The traditional modelling approach: by D. Orhonet al., Wat. Res.23, 1511–1518 (1989)

The effect of reactor hydraulics on the performance of activated sludge systems—II. The formation of microbial products

A new model involving the concept of soluble residual microbial products formation was used to investigate the effect of reactor hydraulics on the substrate removal efficiencies of activated sludge systems. Strong experimental evidence in the literature suggests that what is measured in most studies is not the remaining portion of the influent degradable substrate, but organic matter of microbial origin which is residual, at least for the operating conditions considered. An appropriate simulation approach was formulated to account for the formation of these products, by a simple mechanistic modification of the newly proposed task group model. This model showed no practical difference between the performances of completely mixed and plug flow activated sludge systems, because they produced almost equal amounts of these microbial products, under similar operating conditions.

The effect of reactor hydraulics on the performance of activated sludge systems—I. The traditional modelling approach

The traditional modelling approach involving only overall substrate and biomass parameters and a Monod-type of a rate expression was used to investigate the effect of reactor hydraulics on the substrate removal efficiencies of activated sludge systems. The traditional kinetics was observed to give a useful, although not a completely accurate picture of performances of different types of reactors: it showed basically that the relative performance of a given hydraulic configuration was significantly affected by the kinetic constants as well as by operating parameters. The results of model simulations indicated that most experimental studies in this area, were designed to operate at a range which should secure complete removal of the growth limiting substrate; therefore, the related experimental data so far available, do not necessarily provide conclusive evidence on the effect of reactor hydraulics.

Growth and control of filamentous microbes in activated sludge: an integrated hypothesis

Since its initial development and application, the activated sludge process has undergone a continual evolution aimed at maximizing process efficiency and controlling population selection. Organic loading rate, dissolved oxygen concentration and reactor configuration have been implicated as key process variables affecting sludge settleability, their combined effects determining which microbe or microbes are best able to grow and survive in a given system. Using available pure and mixed culture observations, a conceptual hypothesis centered around three model organisms has been developed to explain the growth and control of filamentous organisms in activated sludge. Organism selection in continuously fed systems has been hypothesized to be dominated by filamentous organisms with high sustainable growth rates at low reactor substrate concentrations/organic loading rates and filaments best able to compete for dissolved oxygen at elevated reactor substrate concentrations/organic loading rates. Intermittently fed systems, on the other hand, enrich for nonfilamentous organisms which both rapidly extract substrate from solution and maintain peak activity during extended periods of endogeneous metabolism. While both types of reactor induced feeding patterns can control filamentous organism growth, intermittently fed systems are capable of operation over a wider range of operating conditions by accentuating differences in organism physiology. Additional selection pressures such as substrate composition, non-ideal reactor hydraulics and time-variable influent waste streams were also discussed in relation to their impact on idealized systems.