Fatty acids production from hydrogen and carbon dioxide by mixed culture in the membrane biofilm reactor

Gasification of waste to syngas (H₂/CO₂) is seen as a promising route to a circular economy. Biological conversion of the gaseous compounds into a liquid fuel or chemical, preferably medium chain fatty acids (caproate and caprylate) is an attractive concept. This study for the first time demonstrated in-situ production of medium chain fatty acids from H₂ and CO₂ in a hollow-fiber membrane biofilm reactor by mixed microbial culture. The hydrogen was for 100% utilized within the biofilms attached on the outer surface of the hollow-fiber membrane. The obtained concentrations of acetate, butyrate, caproate and caprylate were 7.4, 1.8, 0.98 and 0.42 g/L, respectively. The biomass specific production rate of caproate (31.4 mmol-C/(L day g-biomass)) was similar to literature reports for suspended cell cultures while for caprylate the rate (19.1 mmol-C/(L day g-biomass)) was more than 6 times higher. Microbial community analysis showed the biofilms were dominated by Clostridium spp., such as Clostridium ljungdahlii and Clostridium kluyveri. This study demonstrates a potential technology for syngas fermentation in the hollow-fiber membrane biofilm reactors.     

Specific activity of nitrifying biofilm in water nitrification process

The effect of the accumulation of fixed biomass on the specific activity of nitrifying biofilm was studied in a continuous flow reactor. The specific activity of nitrifying biofilm was described by the specific removal rate of ammonium-N (q_obs). The observed relationship between q_obs and the film thickness was apparent to an inverse V-shaped curve. The maximum specific activity of biofilm was attained at a film thickness in the range 15–25 μm, at which a steady state was established in the liquid phase for different influent ammonium-N concentrations (S₀). Beyond such a range, the specific activity began to decline significantly with the additional accumulation of biofilm. It was demonstrated from both experimental and theoretical approaches that reduction in the specific activity of biofilm was closely related to the ratio of active biomass to the accumulation of inactive materials within the biofilm.     

Epuration hydrocarbonne en couche fluidisee triphasique etude experimentale et modelisationHydrocarbon removal in a three-phase fluidized experimental bed and modelling of the process

Biological treatment by immobilized cells on a submerged support allows important volume reduction of units owing to the high micro-organism concentration in the process. The efficiency of classical reactors, i.e. preoxygenated fixed bed, is, nevertheless, limited by oxygen supply to the reactive medium and the unit must be worked discontinuously because of the bed clogging.A 9.4 cm i.d. three-phase fluidized reactor has been tested in an attempt to solve these problems; the turbulence induced by air injection should remove the excess biomass. Step injections of synthetic sewage have been made for different values of the working parameters. Interesting efficiency results have been observed: elimination efficiency of 10 kg BOD₅ m⁻³ day⁻¹ without recycle, is independent of space-time over a wide range.Limitations of a three-phase fluidized reactor have also been found: insufficient oxygen transfer to the liquid phase due to rapid bubble coalescence is a consequence of fluidizing small particles and inefficient sloughing which allows the reactor to be segregated depending on biofilm thickness and, sometimes, clogged in the upper part of the column.A model based on previous work in fixed bed has been proposed. Interphase mass transfer and zero order reaction within the biofilm are taken into account and the liquid phase is assumed to be perfectly mixed. The two parameters of the model are a combination of the numbers of transfer units within the biofilm and through the boundary layer and of the number of reaction units in the biofilm. The limiting substrate may be oxygen, organic carbon or any other nutrient.Experimental efficiencies can be predicted by the model with 15% accuracy. Particles size distribution must be corrected with regard to segregation.Physicobiochemical assumptions can be used in the modelling of a reactor which would not be perfectly macromixed and could be basically used in a unified theory.     

Biosorption of procaine on biologically active carbon

The article has studied the dynamics of biosorption purification of water of procaine adapted by biomass of active sludge immobilized on activated carbon. The greatest constant of the rate of biodestruction of procaine (0.151 h^–1) suspended by adapted biomass is observed at the concentration of the matter 100 mg/dm³. At biosorption removal of procaine on activated carbon under conditions of stirring the constants of the rates of biodestruction increase two–five times compared with suspended biomass. The adapted biofilm noticeably extend the service life of carbon compared with the spontaneously emerging biofilm at the sake of higher destruction activity.     

Epuration hydrocarbonne en couche fluidisee triphasique etude experimentale et modelisation

Biological treatment by immobilized cells on a submerged support allows important volume reduction of units owing to the high micro-organism concentration in the process. The efficiency of classical reactors, i.e. preoxygenated fixed bed, is, nevertheless, limited by oxygen supply to the reactive medium and the unit must be worked discontinuously because of the bed clogging.A 9.4 cm i.d. three-phase fluidized reactor has been tested in an attempt to solve these problems; the turbulence induced by air injection should remove the excess biomass. Step injections of synthetic sewage have been made for different values of the working parameters. Interesting efficiency results have been observed: elimination efficiency of 10 kg BOD₅ m^?3 day^?1 without recycle, is independent of space-time over a wide range.Limitations of a three-phase fluidized reactor have also been found: insufficient oxygen transfer to the liquid phase due to rapid bubble coalescence is a consequence of fluidizing small particles and inefficient sloughing which allows the reactor to be segregated depending on biofilm thickness and, sometimes, clogged in the upper part of the column.A model based on previous work in fixed bed has been proposed. Interphase mass transfer and zero order reaction within the biofilm are taken into account and the liquid phase is assumed to be perfectly mixed. The two parameters of the model are a combination of the numbers of transfer units within the biofilm and through the boundary layer and of the number of reaction units in the biofilm. The limiting substrate may be oxygen, organic carbon or any other nutrient.Experimental efficiencies can be predicted by the model with 15% accuracy. Particles size distribution must be corrected with regard to segregation.Physicobiochemical assumptions can be used in the modelling of a reactor which would not be perfectly macromixed and could be basically used in a unified theory.     

Role of mass transfer in overall substrate removal rate in a sequential aerobic sludge blanket reactor treating a non-inhibitory substrate

A laboratory study was undertaken to explore the role of mass transfer in overall substrate removal rate and the subsequent kinetic behavior in a glucose-fed sequential aerobic sludge blanket (SASB) reactor. At the organic loading rates (OLRs) of 2-8 kg chemical oxygen demand (COD)/m³-d, the SASB reactor removed over 98% of COD from wastewater. With an increase in OLR, the average granule diameter (d_p = 1.1-1.9 mm) and the specific oxygen utilization rate increased; whereas biomass density of granules and solids retention time decreased (13-32 d). The intrinsic and apparent kinetic parameters were evaluated using break-up and intact granules, respectively. The calculated COD removal efficiencies using the kinetic model (incorporating intrinsic kinetics) and empirical model (incorporating apparent kinetics) agreed well with the experimental results, implying that both models can properly describe the overall substrate removal rate in the SASB reactor. By applying the validated kinetic model, the calculated mass transfer parameter values and the simulated substrate concentration profiles in the granule showed that the overall substrate removal rate is intra-granular diffusion controlled. By varying different d_p within a range of 0.1-3.5 mm, the simulated COD removal efficiencies disclosed that the optimal granular size could be no greater than 2.5 mm.     

Nitritation performance in membrane-aerated biofilm reactors differs from conventional biofilm systems

Nitrogen removal via nitrite has gained increasing attention in recent years due to its potential cost savings. Membrane-aerated biofilm reactors (MABRs) are one potential technology suitable to achieve nitritation. In this study we compared lab scale MABRs with conventional biofilm reactors to evaluate the influence of environmental conditions and operational parameters on nitritation performance. The oxygen mass transfer rate is postulated as a crucial parameter to control nitritation in the MABR: Clean water measurements showed significant underestimation of the total oxygen mass transfer, however, accurate determination of the oxygen mass transfer coefficient (k_m) of the system could be achieved by adjusting the liquid-phase mass transfer resistance in the constructed model. Batch experiments at different initial ammonium concentrations revealed that the conventional biofilm geometry was superior for nitritation compared to MABRs. These differences were reflected well in estimates of the oxygen affinity constants of the key microbial players, AOB and NOB (K_O,AOB < K_O,NOB (in both systems) and K_O,NOB values smaller in the MABR vs. the conventional biofilm system). It also appeared that – in addition to oxygen limitation – the absolute and relative substrate concentrations in the biofilm (esp. of oxygen) are very important for successful nitritation. Initial biomass composition, furthermore, impacted reactor performance in the MABR systems indicating the need for appropriate inoculum choice.     

Evaluation and modeling of benzalkonium chloride inhibition and biodegradation in activated sludge

The inhibitory effect and biodegradation of benzalkonium chloride (BAC), a mixture of alkyl benzyl dimethyl ammonium chlorides with different alkyl chain lengths, was investigated at a concentration range from 5 to 20 mg/L and different biomass concentrations in an activated sludge system. A solution containing glucose and mineral salts was used as the wastewater in all the assays performed. The inhibition of respiratory enzymes was identified as the mode of action of BAC as a result of oxygen uptake rate analysis performed at BAC concentrations ranging between 5 and 70 mg/L. The glucose degradation in the activated sludge at different BAC and biomass concentrations was well-described with Monod kinetics with competitive inhibition. The half-saturation inhibition constant (K_I) which is equivalent to EC₅₀ of BAC for the activated sludge tested ranged between 0.12 and 3.60 mg/L. The high K_I values were recorded at low BAC-to-biomass ratios, i.e. less than 10 mg BAC/g VSS, at which BAC was almost totally adsorbed to biomass and not bioavailable. BAC degradation started as soon as glucose was totally consumed. Although BAC was almost totally adsorbed on the biomass, it was degraded completely. Therefore, BAC degradation was modeled using two-phase biodegradation kinetics developed in this study. This model involves rapid partitioning of BAC to biomass and consecutive degradation in both aqueous and solid phases. The aqueous phase BAC degradation rate was twenty times, on average, higher than the solid phase degradation rate. The specific aqueous (k_I1) and solid (k_I2) phase BAC utilization rate constants were 1.25 and 0.31 mg BAC/g VSS h, respectively. The findings of this study would help to understand the reason of extensive distribution of quaternary ammonium compounds in wastewater treatment plant effluents and in natural water systems although QACs are biodegradable, and develop strategies to avoid their release and accumulation in the environment.     

Nitritation performance and biofilm development of co- and counter-diffusion biofilm reactors: Modeling and experimental comparison

A comparative study was conducted on the start-up performance and biofilm development in two different biofilm reactors with aim of obtaining partial nitritation. The reactors were both operated under oxygen limited conditions, but differed in geometry. While substrates (O₂, NH₃) co-diffused in one geometry, they counter-diffused in the other. Mathematical simulations of these two geometries were implemented in two 1-D multispecies biofilm models using the AQUASIM software. Sensitivity analysis results showed that the oxygen mass transfer coefficient (K_i) and maximum specific growth rate of ammonia-oxidizing (AOB) and nitrite-oxidizing bacteria (NOB) were the determinant parameters in nitrogen conversion simulations. The modeling simulations demonstrated that K_i had stronger effects on nitrogen conversion at lower (0-10 m d⁻¹) than at the higher values (>10 m d⁻¹). The experimental results showed that the counter-diffusion biofilms developed faster and attained a larger maximum biofilm thickness than the co-diffusion biofilms. Under oxygen limited condition (DO < 0.1 mg L⁻¹) and high pH (8.0-8.3), nitrite accumulation was triggered more significantly in co-diffusion than counter-diffusion biofilms by increasing the applied ammonia loading from 0.21 to 0.78 g NH₄⁺-N L⁻¹ d⁻¹. The co- and counter-diffusion biofilms displayed very different spatial structures and population distributions after 120 days of operation. AOB were dominant throughout the biofilm depth in co-diffusion biofilms, while the counter-diffusion biofilms presented a stratified structure with an abundance of AOB and NOB at the base and putative heterotrophs at the surface of the biofilm, respectively.     

Oxygenation par le peroxyde d'hydrogene des biomasses fixees utilisees en traitement biologique des eauxOxygenation by hydrogen peroxide of the fixed biomass used in biological water treatment

Because of their advantages as compared to flocculated biomass processes, there is now a revival of interest in fixed biomass processes:no mishaps due to bad flocculation, particularly with filamentous organisms (bulking)compact equipment owing to the ability to obtain greater biomass concentrations (several g l^?1), which is impossible in flocculated biomass.In this paper, we will consider mainly bio-discs and submerged fixed bed filters. In bio-disc investigations, Hoehn and Ray's (1973), Kornegay and Andrew's (1968) now classical results showed that the bacterial film only acts on the surface, over a thickness which, at best, does not exceed 150 μm. At the same time, Bungay's (1969) very accurate measurements showed that the film active thickness coincides with the depth where the oxygen concentration in the film is higher than the critical oxygen concentration. In submerged filters, Elmaleh (1976) and Grasmick's (1978) theoretical studies permit one to define a Useful Column Height (UCH) which corresponds to the active part of the reactor and which is superposed on the height where oxygen concentration is higher than the critical oxygen concentration. In classical devices, the UCH is relatively low: approx. 0.50-1 m. In both cases, the system is provided with oxygen through an exchange between the air and the effluent to be treated, at a gas-liquid interface. This procedure limits the O₂ concentration to about 9 mg O₂ l^?1, at the ambient temperature. Therefore, to increase the UCH of a submerged reactor or the active thickness of a bio-disc film by increasing the oxygen penetrating depth, the oxygen partial pressure in the gas phase should be increased by either using pure oxygen or increasing total gas phase pressure.These two methods are somewhat difficult to use and we prefer to use another method: bringing dissolved oxygen directly into the liquid phase without the exchange at the gas-liquid interface. This is feasible by using an oxygen liberating labile chemical reagent i.e. hydrogen peroxide. We consider two types of fixed biomasses: the bio-discs and the submerged filters.Bio-discs. The apparatus used is shown in Fig. 1. The utilization of H₂O₂ resulted in a very sharp increase in the substrate removal efficiency. It is observed that the substrate removal efficiency (Figs 5 and 6) and the reduced pollution flux (Figs 4 and 7) show a maximum when these are plotted as a function of the ratio: equivalent quantity of O₂ given by H₂O₂/O₂ demanded by the effluent and as a function of dissolved oxygen in the liquid phase. Moreover, these curves suggest that oxygen acts as an inhibitor and different attempts at modeling, based on standard models of inhibiting effects, lead to the exponential model giving the lowest deviation (Fig. 8).Submerged packed reactors. The apparatus used is shown in Fig. 3. This unit is fed by urban effluents and the oxygenation in the reactor is carried out by using diluted H₂O₂ (0.5-1.5 N).     

Effect of free ammonia concentration on monochloramine penetration within a nitrifying biofilm and its effect on activity, viability, and recovery

Chloramine has replaced free chorine for secondary disinfection at many water utilities because of disinfection by-product (DBP) regulations. Because chloramination provides a source of ammonia, there is a potential for nitrification when using chloramines. Nitrification in drinking water distribution systems is undesirable and may result in degradation of water quality and subsequent non-compliance with existing regulations. Thus, nitrification control is a major issue and likely to become increasingly important as chloramine use increases. In this study, monochloramine penetration and its effect on nitrifying biofilm activity, viability, and recovery was investigated and evaluated using microelectrodes and confocal laser scanning microscopy (CLSM). Monochloramine was applied to nitrifying biofilm for 24 h at two different chlorine to nitrogen (Cl₂:N) mass ratios (4:1 [4.4 mg Cl₂/L] or 1:1 Cl₂:N [5.3 mg Cl₂/L]), resulting in either a low (0.23 mg N/L) or high (4.2 mg N/L) free ammonia concentration. Subsequently, these biofilm samples were allowed to recover without monochloramine and receiving 4.2 mg N/L free ammonia. Under both monochloramine application conditions, monochloramine fully penetrated into the nitrifying biofilm within 24 h. Despite this complete monochloramine penetration, complete viability loss did not occur, and both biofilm samples subsequently recovered aerobic activity when fed only free ammonia. When monochloramine was applied with a low free ammonia concentration, dissolved oxygen (DO) fully penetrated, but with a high free ammonia concentration, complete cessation of aerobic activity (i.e., oxygen utilization) did not occur and subsequent analysis indicated that oxygen consumption still remained near the substratum. During the ammonia only recovery phase, different spatial recoveries were seen in each of the samples, based on oxygen utilization. It appears that the presence of higher free ammonia concentration allowed a larger biomass to remain active during monochloramine application, particularly the organisms deeper within the biofilm, leading to faster recovery in oxygen utilization when monochloramine was removed. These results suggest that limiting the free ammonia concentration during monochloramine application will slow the onset of nitrification episodes by maintaining the biofilm biomass at a state of lower activity.     

Evaluation on the impacts of predators on biomass components and oxygen uptake in sequencing batch reactor and continuous systems

An expanded unified model for the biomass fractions, soluble-organic fractions, and oxygen-uptake rates considering extracellular polymeric substances (EPS), intracellular storage products (X_STO), and predators for activated sludge is used to study the impacts of predators on biomass components and oxygen uptake. The new model is applied to evaluate how predation affects the oxygen-uptake rate (OUR) and the different forms of biomass: active bacteria (X_H), X_EPS, and X_STO, under dynamic feast-and-famine and continuous conditions. For the dynamic conditions of a sequencing batch reactor (SBR), eliminating predators from the model increases X_H and X_EPS fractions significantly, and this causes the substantial increases in OUR and MLVSS once the famine period begins. An analysis of how the OUR is distributed among the several respiration processes shows that the predation of X_H is the most significant oxygen utilization rate process in the system under famine conditions of an SBR. Application of the model to simulate the long-term operation of an SBR indicates that predators reach their maximum fraction in the MLVSS (∼4% of MLVSS) at a solids retention time of about 13 days, but they are washed out at a solids retention time less than ∼3 days. Simulation for a continuous system indicates that predators take more time (about 800 h) to reach steady state and reach their maximum fraction (∼5.5%) at an SRT of ∼14 days. Comparison of SBR and continuous systems reveals that the predators have greater impact in the continuous system because the permanent near-famine condition accentuates predation processes.     

Nitrate removal and biofilm characteristics in methanotrophic membrane biofilm reactors with various gas supply regimes

Aerobic methanotrophs can contribute to nitrate removal from contaminated waters, wastewaters, or landfill leachate by assimilatory reduction and by producing soluble organics that can be utilized by coexisting denitrifiers. The goal of this study was to investigate nitrate removal and biofilm characteristics in membrane biofilm reactors (MBfR) with various supply regimes of oxygen and methane gas. Three MBfR configurations were developed and they achieved significantly higher nitrate removal efficiencies in terms of methane utilization (values ranging from 0.25 to 0.36 mol N mol⁻¹ CH₄) than have previously been observed with suspended cultures. The biofilm characteristics were investigated in two MBfRs with varying modes of oxygen supply. The biofilms differed in structure, but both were dominated by Type I methanotrophs growing close to the membrane surface. Detection of the nitrite reductase genes, nirS and nirK, suggested genetic potential for denitrification was present in the mixed culture biofilms.     

Early fouling biofilm formation in a turbulent flow system: Overall kinetics

An empirical expression is presented which describes the rate of fouling biofilm development from clean surface conditions to the onset of fluid frictional resistance increase. Experiments were conducted in a CSTR with internal recycle; a system which provided control of biological activity in the bulk fluid while simulating turbulent flow conditions. Primary biofilm accumulation rate is described by a first order expression in which the rate coefficient is a function of suspended biomass concentration, Reynolds number, and suspended biomass growth rate.     

Reacteur a cellules nitrifiantes immobilisees sur garnissage aere a co- ou contre-courant. Etude experimentale et modelisationNitrification by attached-cell reactors aerated at co- or counter-current. Experimental data and modelling

Attached-cell reactors using a bed of granular material for wastewater treatment develop a high biomass concentration which allows an important reduction of the required residence time (Jeris et al., 1977; Elmaleh, 1982). In nitrification of ammonia containing wastewater, oxygen is currently the limiting substrate; in theory, 4.18 g of oxygen are required per 1 g of nitrogen (Painter, 1970). Oxygen can be added with hydrogen peroxide (Grigoropolou, 1980; Seropian, 1980; Yahi et al., 1982) which is nevertheless expensive and it seems better to transfer oxygen from a gas phase, i.e. air, to the liquid phase through a fixed bed (Charpentier, 1976).Two attached-cell reactors (Fig. 1) were operated in parallel for nitrification of ammonia containing synthetic wastewater (Table 2). Air was upflowed through a granular packing (Table 1) maintained in fixed bed while the liquid influent was injected at co- or counter-current.

1. (1) Owing to the high oxygen transfer properties of the system and to the fact that the thickness of biofilm is always less than 100 μm, the whole process was not limited by oxygen concentration of which remained larger than 7 mg l⁻¹ (Fig. 2a) (Bungay et al., 1969). Oxidised nitrogen ammonia is completely converted into nitrate (Fig. 2b). Experimental conditions are given in Table 3.

2. (2) The plot of ammonia conversion against air superficial velocity shows a maximum (Fig. 3) after which conversion decreases rapidly by overloading of the packing (Prost, 1965). Experimental conditions are given in Table 4.

3. (3) Process efficiency decreases when superficial upflow velocity is increased (Fig. 4).

4. (4) Complete abatement of inlet pollution is reached when nitrogen concentration is less than 25 mg l⁻¹ (Fig. 5) which corresponds to a volumetric loading up to 0.6 kg N (NH₄⁺) m⁻³ day⁻¹.

Moreover, the experimental data were fitted to a model based on classical assumptions (Roques, 1980; Grady, 1982; Atkinson and Fowler, 1974; Grasmick et al., 1979; Grasmick, 1982; Harremoes, 1976, 1978; Jennings et al., 1976; Williamson and MacCarty, 1976); i.e. zero order intrinsic kinetics and diffusion transport (Table 5), and recently developed (Grasmick, 1982; Rodrigues et al., 1984). This model provides, particularly, a very easy method to check its own use—in reaction regime and in diffusion regime—when time spans or inlet concentration are changed; experimental results can indeed be plotted in such a way that straight lines are obtained (Table 6). Figures 6 and 7 show the data obtained with the counter-current nitrification reactor when respectively inlet concentration and time spans are varied. The plotted straight lines show that the overall reaction is zero order and that, therefore, the biofilm is fully penetrated. A critical time span θ_c and a critical inlet concentration C_c, for which complete conversion is achieved, are then calculated, θ_c is theoretically proportional to C₁ which is verified in Fig. 8. The straight line θ_c vs C₁ can then be used in reactor design.     

A comprehensive study on the biological treatabilities of phenol and methanol—I. Analysis of bacterial growth and substrate removal kinetics by a statistical method

An unbiassed statistical method was developed to evaluate kinetic parameters in the biological oxidation of wastewaters. Through the statistical analyses of the biological oxidation kinetics, it was shown that the kinetic equations satisfactorily described the bacterial growth and substrate removal kinetics where X is biomass concentration, S is substrate concentration, t is time, a is cell yield coefficient, k_d is cell decay coefficient, K_s is Michaelis-Menten constant, and k is substrate removal rate coefficient. The coefficients K_s and a changed with temperature insignificantly while k and k_d were closely related to it. The temperature independent coefficients K_s and a were estimated to be 236 mg 1⁻¹ (standard deviation, σ = 70 mg 1⁻¹) and 1.21 (σ = 0.06) respectively for phenol, and 2330 mg 1⁻¹ (σ = 1410 mg 1⁻¹) and 1.25 (σ = 0.45) respectively for methanol based on total organic carbon (TOC) and volatile suspended solids (VSS). The oxygen utilization rate can be formulated as where Rr is the oxygen utilization rate (mg 1⁻¹ O₂ time⁻¹), as′ is a coefficient designating oxygen requirement per substrate utilized, and b′ is a coefficient designating oxygen requirement per biomass for endogeneous respiration. The coefficient a′ was 1.39 for phenol and 2.23 for methanol, and b′ was 1.42 k_d for both substances based on TOC and VSS.     

Long-term formation of microbial products in a sequencing batch reactor

Activated sludge in a sequencing batch reactor (SBR) is subjected to alternating feast-and-famine conditions, which may result in the enhanced production of microbial products: extracellular polymeric substances (EPS), soluble microbial products (SMP), and internal storage products (X_STO). In this work, the long-term formation of these three microbial products by activated sludge in an SBR is investigated using an expanded unified model with a parallel experimental study. We also use the model to compare the impacts in an SBR to those in a continuous-flow activated sludge system. The model captures all experimental trends for all components with solids retention time (SRT) for global steady state and within a cycle. At an SRT of 20 days, the active microorganisms constitute about 28% of the mixed liquor volatile suspended solids (MLVSS); the remaining biomass is comprised of residual inert biomass (X_I) of 40%, EPS of 31%, and X_STO of ∼1%. The active biomass becomes a smaller fraction with the increasing SRT, while the inert biomass becomes increasingly dominant. For soluble components, effluent chemical oxygen demand (COD) is dominated by SMP, which varies to some degree in a cycle, peaking as external substrate becomes depleted. Within the SBR cycle, external substrate (S) declines strongly in the first part of the cycle, and SMP shows a small peak at the time of S depletion. X_STO is the only biomass component that varies significantly during the cycle. It peaks at the time that the input substrate (S) is depleted. Simulation for a continuous-flow activated sludge system and comparison with an SBR reveals that the constant “famine” conditions of the continuous system lead to lower EPS and X_STO, but higher MLVSS and X_I.     

Non-steady state simulation of BOM removal in drinking water biofilters: model development

A numerical model was developed to simulate the non-steady-state behavior of biologically-active filters used for drinking water treatment. The biofilter simulation model called "BIOFILT" simulates the substrate (biodegradable organic matter or BOM) and biomass (both attached and suspended) profiles in a biofilter as a function of time. One of the innovative features of BIOFILT compared to previous biofilm models is the ability to simulate the effects of a sudden loss in attached biomass or biofilm due to filter backwash on substrate removal performance. A sensitivity analysis of the model input parameters indicated that the model simulations were most sensitive to the values of parameters that controlled substrate degradation and biofilm growth and accumulation including the substrate diffusion coefficient, the maximum rate of substrate degradation, the microbial yield coefficient, and a dimensionless shear loss coefficient. Variation of the hydraulic loading rate or other parameters that controlled the deposition of biomass via filtration did not significantly impact the simulation results.     

Threshold concentrations of biomass and iron for pressure drop increase in spiral-wound membrane elements

In a model feed channel for spiral-wound membranes the quantitative relationship of biomass and iron accumulation with pressure drop development was assessed. Biofouling was stimulated by the use of tap water enriched with acetate at a range of concentrations (1-1000 μg C l⁻¹). Autopsies were performed to quantify biomass concentrations in the fouled feed channel at a range of Normalized Pressure Drop increase values (NPD_i). Active biomass was determined with adenosinetriphosphate (ATP) and the concentration of bacterial cells with Total Direct Cell count (TDC). Carbohydrates (CH) were measured to include accumulated extracellular polymeric substances (EPS). The paired ATP and CH concentrations in the biofilm samples were significantly (p < 0.001; R² = 0.62) correlated and both parameters were also significantly correlated with NPD_i (p < 0.001). TDC was not correlated with the pressure drop in this study. The threshold concentration for an NPD_i of 100% was 3.7 ng ATP cm⁻² and for CH 8.1 μg CH cm⁻². Both parameters are recommended for diagnostic membrane autopsy studies. Iron concentrations of 100-400 mg m⁻² accumulated in the biofilm by adsorption were not correlated with the observed NPD_i, thus indicating a minor role of Fe particulates at these concentrations in fouling of spiral-wound membrane.