Mass transfer mechanisms in a heterotrophic biofilm

The diffusion coefficients of three different chemical species in naturally grown, heterotrophic biofilms have been measured and a simple model of mass transfer in a biofilm matrix is developed. The mechanical structure of the biofilm matrix reduces the molecular diffusion to about 50–80% of the value in pure water. Depending on the roughness of the biofilm surface and the flow conditions eddy diffusion increased the mass transfer into the biofilm near the surface.The influence of the diffusion potential and the donnan potential on ions have been evaluated by comparing the diffusion coefficients of a positively and a negatively charged ion and a neutral molecule in experiments with different background electrolyte concentrations. Mass transfer effects by electrostatic forces are negligible at the ionic strength of waste water and tap water.     

Demonstration of mass transfer and pH effects in a nitrifying biofilm

A bench-scale nitrifying trickling filter (surface AREA = 0.5 m²) was developed to permit evaluation of diffusion of oxygen within a biofilm, the pH dependence of ammonium oxidation and external mass transfer. In addition, a biofilm model was developed and verified for homogeneous nitrifying biofilms of varied thickness and for thin nitrifying biofilms covered by heterotrophic biofilms. The model uses literature values for the pH dependence of Monod coefficients for Nitrosomonas and Nitrobacter.

The diffusion coefficient of oxygen in the biofilm was found to be 40–80% of the value in pure water. Due to mass transfer resistance, the biomass ·sees” a lower pH than is measured in the water film passing over it. The surface uptake rate of ammonia is used as an indicator of pH gradients within the biofilm system. With the help of oxygen limitation experiments, the location of nitrifying biomass within mixed biofilms (heterotrophic, autotrophic) can be determined.

The biofilm model predicts ammonium uptake rate of a trickling filter as a function of the bicarbonate concentration in the water film.     

Modelling the consumption of dissolved contaminants by biofilm periphyton in open-channel flow

An analytical model of the decrease in dissolved contaminant concentration in an open channel flow through the consumption by a botton biofilm is presented. This idealized model considers the flux of the contaminant by diffusion through the concentration boundary layer into the biofilm, with diffusion and reaction within the biofilm. Solutions for zero-order as well as first-order kinetics are derived which show that the rate of change of the concentration in the main flow is directly related to the kinetics in the biofilm. The thickness of the concentration boundary layer is shown to have a significant influence on the rate of decrease of concentration in the main flow.     

Achieving biofilm control in a membrane biofilm reactor removing total nitrogen

Membrane biofilm reactors (MBfR) utilize membrane fibers for bubble-less transfer of gas by diffusion and provide a surface for biofilm development. Nitrification and subsequent autotrophic denitrification were carried out in MBfR with pure oxygen and hydrogen supply, respectively, in order to remove nitrogen without the use of heterotrophic bacteria. Excessive biomass accumulation is typically the major cause of system failure of MBfR. No biomass accumulation was detected in the nitrification reactor as low-level discharge of solids from the system balanced out biomass generation. The average specific nitrification rate during 250 days of operation was 1.88 g N/m² d. The subsequent denitrification reactor, however, experienced decline of performance due to excessive biofilm growth, which prompted the implementation of periodic nitrogen sparging for biofilm control. The average specific denitrification rate increased from 1.50 g N/m² d to 1.92 g N/m² d with nitrogen sparging, over 190 days thus demonstrating the feasibility of stable long-term operation. Effluent suspended solids increased immediately following sparging: from an average of 2.5 mg/L to 12.7 mg/L. This periodic solids loss was found unavoidable, considering the theoretical biomass generation rates at the loadings used. A solids mass balance between the accumulating and scoured biomass was established based on the analysis of the effluent volatile solids data. Biofilm thickness was maintained at an average of 270 μm by the gas sparging biofilm control. It was concluded that biomass accumulation and scouring can be balanced in autotrophic denitrification and that long-term stable operation can be maintained.     

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.     

Bio-reduction of soluble chromate using a hydrogen-based membrane biofilm reactor

Hexavalent chromium (Cr(VI)) is a mutagen and carcinogen that is a significant concern in water and wastewater. A simple and non-hazardous means to remove Cr(VI) is bioreduction to Cr(III), which should precipitate as Cr(OH)3(s). Since Cr(VI)-reducing bacteria can use hydrogen (H2) as an electron donor, we tested the potential of the H2-based membrane biofilm reactor (MBfR) for chromate reduction and removal from water and wastewater. When Cr(VI) was added to a denitrifying MBfR, Cr(VI) reduction was immediate and increased over 11 days. Short-term experiments investigated the effects of Cr(VI) loading, H2 pressure, and nitrate loading on Cr(VI) reduction. Increasing the H2 pressure improved Cr(VI) reduction. Cr(VI) reduction also was sensitive to pH, with an optimum near 7.0, a sharp drop off below 7.0, and a gradual decline to 8.2. Cr(III) precipitated after a small upward adjustment of the pH. These experiments confirm that a denitrifying, H2-based MBfR can be used to reduce Cr(VI) to Cr(III) and remove Cr from water. The research shows that critical operational parameters include the H2 concentration, nitrate concentration, and pH.     

Simultaneous removal of organic matter and nitrogen compounds by an aerobic/anoxic membrane biofilm reactor

The hydrogen-based membrane biofilm reactor (MBfR) has been well studied and applied for denitrification of nitrate-containing water and wastewater. Adding an oxygen-based MBfR allows total-nitrogen removal when the input nitrogen is ammonium. However, most wastewaters also contain a significant concentration or organic material, measured as chemical oxygen demand (COD). This study describes experiments to investigate the removal of organic and nitrogenous compounds in the combined Aerobic/Anoxic MBfR, in which an Aerobic MBfR (Aer MBfR) precedes an Anoxic MBfR (An MBfR). The experiments demonstrate that the Aer/An MBfR combination accomplished COD oxidation and nitrogen removal for a total oxygen demand flux (i.e., from COD and NH(4) oxidations) in the range of 1.2-7.2gO(2)/m(2)-d for 4.5psi (0.3atm) oxygen pressure to the Aer MBfR, but was overloaded and did not accomplish nitrification for the total oxygen demand load higher than 14gO(2)/m(2)-d. Total-nitrogen removal was controlled by nitrification in the Aer MBfR, because the An MBfR denitrified all NO(3)(-) provided to it by the Aer MBfR. The overload of total oxygen demand did not affect COD oxidation in the Aer MBfR, but caused a small increase of COD in the An MBfR due to net release of soluble microbial products (SMP).     

饮用水生物强化过滤工艺生物膜特性研究

以水厂沉淀池出水为原水,对生物强化过滤工艺的生物膜形成过程进行了研究,探讨了不同滤料介质组成的生物滤柱的生物膜特性,分析了膜形成过程中污染物的去除效果和滤柱生物量的变化情况,并对膜形成过程的影响因素进行了讨论.结果表明,在活性炭-石英砂滤料上生物膜形成效果要优于无烟煤-石英砂双层滤料和石英砂单层滤料;反冲洗水含氯对生物膜形成有负面影响,对无烟煤-石英砂滤柱的影响尤为显著;可以CODMn和NO-2的去除率作为生物膜成熟的评价指标.     

Approximate solution for a fluidized-bed biofilm model

An approximate algebraic solution for a fluidized-bed biofilm model, based on Monod kinetics, was developed using a one-point orthogonal collocation method. The solution was compared with an accurate numerical solution and was found to be accurate for the case where biofilm is “shallow”.     

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.     

Biofilm growth and characteristics in an alternating pumped sequencing batch biofilm reactor (APSBBR)

A novel biofilm reactor-alternating pumped sequencing batch biofilm reactor (APSBBR)-was developed to treat synthetic dairy wastewater at a volumetric chemical oxygen demand (COD) loading rate of 487 g COD m(-3) d(-1) and an areal loading rate of 5.4 g COD m(-2) d(-1). This biofilm reactor comprised two tanks, Tanks 1 and 2, with two identical plastic biofilm modules in each tank. The maximum volume of bulk fluid in the two-tank reactor was the volume of one tank. The APSBBR was operated as a sequencing batch biofilm reactor with five operational phases-fill (25 min), anoxic (9 h), aerobic (9 h), settle (6 h) and draw (5 min). The fill, anoxic, settle and draw phases occurred in Tank 1. In the aerobic phase, the wastewater was circulated between the two tanks with centrifugal pumps and aeration was mainly achieved through oxygen absorption by micro-organisms in the biofilms when they were exposed to the air. In this paper, the biofilm growth and characteristics in the APSBBR were studied in a 98-day laboratory-scale experiment. During the course of the study, it was found that the biofilm thickness (delta) in Tank 1 ranged from 1.2 to 7.2 mm and that in Tank 2 from 0.5 to 2.2 mm; the biofilm growth against time (t) can be simulated as delta=0.07t0.99 (R2 = 0.97, P = 0.002) in Tank 1 and delta = 0.08t0.66 (R2 = 0.81, P = 0.04) in Tank 2. The biomass yield coefficient, Y, was 0.18 g volatile solids (VS) g(-1) COD removal. The biofilm density in both tanks, X, decreased as the biofilm thickness increased and can be correlated to the biofilm thickness, delta .     

Mass transfer in a membrane aerated biofilm

We present experimental results of mass transfer of a non reactive tracer gas (neon) measured in aerobic heterotrophic biofilm developed from activated sludge. Biofilms are grown in various hydrodynamic conditions and the effective diffusivity is used to quantify the mass transfer through the biofilm. Beyond some cross-flow conditions, the effective diffusivity through the biofilm seems larger than in the bulk. This can be explained by a dispersion generated by convection inside the biofilm, as supported by an analytical flow model and in accordance to the numerical simulation proposed by Aspa et al. (2011).     

Respiration inhibition kinetic analysis

A protocol was developed for measuring the inhibitory effects of synthetic organic compounds on the biodegradation of naturally occurring (biogenic) organic matter. A tiered approach based on respirometry was employed. In the first tier, compounds were screened to determine if their effects were sufficiently inhibitory to warrant further testing. This was done at an inhibitor concentration of 1000 mg 1⁻¹, or the solubility limit if lower, and at both high and low biogenic substrate concentrations. If a compound caused more than 50% inhibition in a screening test, the stability of its effect was determined next. Finally, compounds causing stable responses were subjected to the respiration inhibition kinetic analysis in which the Monod kinetic parameters describing the biodegradation of butyric acid were measured in the presence of several inhibitor concentrations by using a technique based on that of Cech et al. (Wat. Sci. Technol. 17 (2/3), 259–272, 1985), thereby allowing the effects of inhibitor concentration on biodegradation kinetics to be quantified. Examples of application of the protocol are given.     

The effect of tracer diffusion in biofilm on residence time distributions

Estimating fixed-film biological reactor characteristics is often complicated by interaction of the tracer that is used in residence time distribution (RTD) studies with the biofilm in a way that causes the RTD curve to have a long tail. This tail can result from hold-up of tracer within the biofilm. A reactor model that ignores the diffusion rate at which tracer enters and leaves the biofilm may be seriously in error. Data from RTD studies in a fluidized bed reactor are presented and a model that accounts for diffusion of tracer in the biofilm is used to explain the experimental data. A material balance on inorganic nitrogen is also given to verify the organic dye tracer results. The conditions under which diffusion of tracer will cause large distortions are defined.     

Degradation of non-diffusible organic matter in biofilm reactors

A simple laboratory test has been developed in order to demonstrate qualitatively, that the removal of non-diffusible organics in a biofilm reactor requires hydrolysis by extracellular enzymes in the bulk water of the reactor. The results demonstrate the effect of changing volume of bulk water on the over all removal.     

Evaluation of kinetic parameters of a sulfur-limestone autotrophic denitrification biofilm process

In this study, four kinetic parameters of autotrophic denitrifiers in fixed-bed sulfur–limestone autotrophic denitrification (SLAD) columns were evaluated. The curve-matching method was used by conducting 22 non-steady-state tests for estimation of half-velocity constant, K_s and maximum specific substrate utilization rate, k. To estimate the bacteria yield coefficient, Y and the decay coefficient, k_d, two short term batch tests (before and after the starvation of the autotrophic denitrifiers) were conducted using a fixed-bed SLAD column where the biofilm was fully penetrated by nitrate-N. It was found that K_s=0.398 mg NO₃⁻–N/l, k=0.15 d⁻¹, k_d=0.09–0.12 d⁻¹, and Y=0.85–1.11 g VSS/g NO₃⁻–N. Our results are consistent with those obtained from SLAD biofilm processes, but different from those obtained from suspended-growth systems with thiosulfate or sulfur powders as the S source. The method developed in this study might be useful for estimation of four Monod-type kinetic parameters in other biofilm processes. However, cautions must be given when the estimated parameters are used because the measurements of the biomass and the biofilm thickness could be further improved, and the assumption of sulfur being a non-limiting substrate needs to be proved.     

Kinetics of nitrate and perchlorate removal and biofilm stratification in an ion exchange membrane bioreactor

The biological degradation of nitrate and perchlorate was investigated in an ion exchange membrane bioreactor (IEMB) using a mixed anoxic microbial culture and ethanol as the carbon source. In this process, a membrane-supported biofilm reduces nitrate and perchlorate delivered through an anion exchange membrane from a polluted water stream, containing 60 mg/L of NO₃⁻ and 100 μg/L of ClO₄⁻. Under ammonia limiting conditions, the perchlorate reduction rate decreased by 10%, whereas the nitrate reduction rate was unaffected. Though nitrate and perchlorate accumulated in the bioreactor, their concentrations in the treated water (2.8 ± 0.5 mg/L of NO₃⁻ and 7.0 ± 0.8 μg/L of ClO₄⁻, respectively) were always below the drinking water regulatory levels, due to Donnan dialysis control of the ionic transport in the system.Kinetic parameters determined for the mixed microbial culture in suspension showed that the nitrate reduction rate was 35 times higher than the maximum perchlorate reduction rate. It was found that perchlorate reduction was inhibited by nitrate, since after nitrate depletion perchlorate reduction rate increased by 77%. The biofilm developed in the IEMB was cryosectioned and the microbial population was analyzed by fluorescence in situ hybridization (FISH). The results obtained seem to indicate that the kinetic advantage of nitrate reduction favored accumulation of denitrifiers near the membrane, whereas per(chlorate) reducing bacteria were mainly positioned at the biofilm outer surface, contacting the biomedium. As a consequence of the biofilm stratification, the reduction of perchlorate and nitrate occur sequentially in space allowing for the removal of both ions in the IEMB.     

Development of high-rate anaerobic ammonium-oxidizing (anammox) biofilm reactors

To promptly establish anaerobic ammonium oxidation (anammox) reactors, appropriate seeding sludge with high abundance and activity of anammox bacteria was selected by quantifying 16S rRNA gene copy numbers of anammox bacteria by real-time quantitative PCR (RTQ-PCR) and batch culture experiments. The selected sludge was then inoculated into up-flow fixed-bed biofilm column reactors with nonwoven fabric sheets as biomass carrier and the reactor performances were monitored over 1 year. The anammox reaction was observed within 50 days and a total nitrogen removal rate of 26.0 kg-Nm(-3)day(-1) was obtained after 247 days. To our knowledge, such a high rate has never been reported before. Hydraulic retention time (HRT) and influent NH(4)(+) to NO(2)(-) molar ratio could be important determinant factors for efficient nitrogen removal in this study. The higher nitrogen removal rate was obtained at the shorter HRT and higher influent NH(4)(+)/NO(2)(-) molar ratio. After anammox reactors were fully developed, the community structure, spatial organization and in situ activity of the anammox biofilms were analyzed by the combined use of a full-cycle of 16S rRNA approach and microelectrodes. In situ hybridization results revealed that the probe Amx820-hybridized anaerobic anammox bacteria were distributed throughout the biofilm (accounting for more than 70% of total bacteria). They were associated with Nitrosomonas-like aerobic ammonia-oxidizing bacteria (AAOB) in the surface biofilm. The anammox bacteria present in this study were distantly related to the Candidatus Brocadia anammoxidans with the sequence similarity of 95%. Microelectrode measurements showed that a high in situ anammox activity (i.e., simultaneous consumption of NH(4)(+) and NO(2)(-)) of 4.45 g-N of (NH(4)(+)+NO(2)(-))m(-2)day(-1) was detected in the upper 800 microm of the biofilm, which was consistent with the spatial distribution of anammox bacteria.     

Theoretical investigation of particle deposition in biofilm systems

Experience has shown that the performance of biofilm reactors is strongly influenced by the size distribution of the constituents being biologically oxidized. Processes of diffusion, interception, and sedimentation that affect the transport of solid particles are presented and applied to biofilm systems. Biofilm systems with media of small diameter or long hydraulic residence time (fixed-bed, soil treatment, and fluidized-bed) can be effective for removing submicron particles through diffusion and interception. In systems with horizontal biofilm surface area, large particles (> 10–50 μm) can be removed by sedimentation. The slow mass transfer of particulate BOD can cause biodegradation rates to be much slower than for soluble species. An understanding of particle behavior will facilitate development of better models of biofilm systems. Areas that merit further research are indicated.