Effects of biofilm formation on membrane performance in submerged membrane bioreactors

The effects of biofilm formation on membrane performance were evaluated for a submerged membrane bioreactor (sMBR) system with six different types of micro- and ultrafiltration membranes (working volume = 19 l). After operation for 24 h the permeability of the membranes with a larger pore size (microfiltration) decreased to that of the membranes with a much smaller pore size (ultrafiltration). Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) confirmed that biofilms could reduce the influence of the membrane surface properties. The chemical oxygen demand (COD) removal efficiency was 95% for the oily wastewater treatment in the sMBR where the filtration process made an important contribution (47% based on feed COD). Significant enhancement in COD removal occurred at the initial filtration stage because of biofilm formation and the dynamic member role of the biofilm layer. Membranes with various pore sizes had approximately the same permeate quality that was attributed to the biofilm on the membrane surfaces. Nevertheless, the ultrafiltration membranes had 43% more COD removal efficiency than the other applied membranes at the beginning of filtration (before biofilm formation) because of the smaller pore sizes and better sieving.     

A biofilm model to understand the onset of sulfate reduction in denitrifying membrane biofilm reactors

This work presents a multispecies biofilm model that describes the co‐existence of nitrate‐ and sulfate‐reducing bacteria in the H₂‐based membrane biofilm reactor (MBfR). The new model adapts the framework of a biofilm model for simultaneous nitrate and perchlorate removal by considering the unique metabolic and physiological characteristics of autotrophic sulfate‐reducing bacteria that use H₂ as their electron donor. To evaluate the model, the simulated effluent H₂, UAP (substrate‐utilization‐associated products), and BAP (biomass‐associated products) concentrations are compared to experimental results, and the simulated biomass distributions are compared to real‐time quantitative polymerase chain reaction (qPCR) data in the experiments for parameter optimization. Model outputs and experimental results match for all major trends and explain when sulfate reduction does or does not occur in parallel with denitrification. The onset of sulfate reduction occurs only when the nitrate concentration at the fiber's outer surface is low enough so that the growth rate of the denitrifying bacteria is equal to that of the sulfate‐reducing bacteria. An example shows how to use the model to design an MBfR that achieves satisfactory nitrate reduction, but suppresses sulfate reduction. Biotechnol. Bioeng. 2013; 110: 763–772. © 2012 Wiley Periodicals, Inc.     

On-line biofilm strength detection in cross-flow membrane filtration systems

A fluid dynamic gauging (FDG) technique was used for on-line and in situ measurements of Pseudomonas aeruginosa PAO1 biofilm thickness and strength on flat sheet polyethersulphone membranes. The measurements are the first to be successfully conducted in a membrane cross-flow filtration system under constant permeation. In addition, FDG was used to demonstrate the removal behaviour of biofilms through local biofilm strength and removal energy estimation, which other conventional measurements such as flux and TMP cannot provide. The findings suggest that FDG can provide valuable additional information related to biofilm properties that have not been measured by other monitoring methods.     

Formation of nitrifying biofilms on small suspended particles in airlift reactors

For a stable and reliable operation of a BAS-reactor a high, active biomass concentration is required with mainly biofilm-covered carriers. The effect of reactor conditions on the formation of nitrifying biofilms in BAS-reactors was investigated in this article. A start-up strategy to obtain predominantly biofilm-covered carriers, based on the balancing of detachment and a biomass production per carrier surface area, proved tp be very successful. The amount of biomass and the fraction of covered carrier were high and development of nitrification activity was fast, leading to a volumetric conversion of 5 kg(N) . m(-3) . d(-1) at a hydraulic retention time of 1h. A 1-week, continuous inoculation with suspended purely nitrifying microorganisms resulted in a swift start-up compared with batch addition of a small number of biofilms with some nitrification activity. The development of nitrifying biofilms was very similar to the formation of heterotrophic biofilms. In contrast to heterotrophic bio-films, the diameter of nitrifying biofilms increased during start-up. The detachment rate from nitrifying biofilms decreased with lower concentrations of bare carrier, in a fashion comparable with heterotrophic biofilms, but the nitrifying biofilms were much more robust and resistant. Standard diffusion theory combined with reaction kinetics are capable of predicting the activity and conversion of biofilms on small suspended particles. (c) 1995 John Wiley & Sons Inc.     

Model-based comparative performance analysis of membrane aerated biofilm reactor configurations

The potential of the membrane aerated biofilm reactor (MABR) for high-rate bio-oxidation was investigated. A reaction-diffusion model was combined with a preliminary hollow-fiber MABR process model to investigate reaction rate-limiting regime and to perform comparative analysis on prospective designs and operational parameters. High oxidation fluxes can be attained in the MABR if the intra-membrane oxygen pressure is sufficiently high, however the volumetric oxidation rate is highly dependent on the membrane specific surface area and therefore the maximum performance, in volumetric terms, was achieved in MABRs with relatively thin fibers. The results show that unless the carbon substrate concentration is particularly high, there does not appear to be an advantage to be gained by designing MABRs on the basis of thick biofilms even if oxygen limitations can be overcome.     

Kinetics of anaerobic methane oxidation coupled to denitrification in the membrane biofilm reactor

Anaerobic oxidation of methane coupled to denitrification (AOM-D) in a membrane biofilm reactor (MBfR), a platform used for efficiently coupling gas delivery and biofilm development, has attracted attention in recent years due to the low cost and high availability of methane. However, experimental studies have shown that the nitrate-removal flux in the CH₄-based MBfR (<1.0 g N/m²-day) is about one order of magnitude smaller than that in the H₂-based MBfR (1.1–6.7 g N/m²-day). A one-dimensional multispecies biofilm model predicts that the nitrate-removal flux in the CH₄-based MBfR is limited to <1.7 g N/m²-day, consistent with the experimental studies reported in the literature. The model also determines the two major limiting factors for the nitrate-removal flux: The methane half-maximum-rate concentration (K₂) and the specific maximum methane utilization rate of the AOM-D syntrophic consortium (k_max2), with k_max2 being more important. Model simulations show that increasing k_max2 to >3 g chemical oxygen demand (COD)/g cell-day (from its current 1.8 g COD/g cell-day) and developing a new membrane with doubled methane-delivery capacity (D_m) could bring the nitrate-removal flux to ≥4.0 g N/m²-day, which is close to the nitrate-removal flux for the H₂-based MBfR. Further increase of the maximum nitrate-removal flux can be achieved when D_m and k_max2 increase together.     

Effective biofilm control in a membrane biofilm reactor using a quenching bacterium (Rhodococcus sp. BH4)

The biofilm thickness in membrane biofilm reactors (MBfRs) is an important factor affecting system performance because excessive biofilm formation on the membrane surface inhibits gas diffusion to the interior of the biofilm, resulting in a significant reduction in the performance of contaminant removal. This study provides innovative insights into the control of biofilm thickness in O₂-based MBfRs by using the quorum quenching (QQ) method. The study was carried out in MBfRs operated at different gas pressures and hydraulic retention times (HRTs) using QQ beads containing Rhodococcus sp. BH4 at different amounts. The highest performance was observed in reactors operated with 0.21 ml QQ bead/cm² membrane surface area, 12 HRTs and 1.40 atm. Over this period, the performance increase in chemical oxygen demand (COD) removal was 25%, while the biofilm thickness on the membrane surface was determined to be 250 μm. Moreover, acetate and equivalent oxygen flux results reached 6080 and 10 640 mg·m⁻²·d⁻¹ maximum values, respectively. The extracellular polymeric substances of the biofilm decreased significantly with the increase of gas pressure and QQ beads amount. Polymerase chain reaction denaturing gradient gel electrophoresis results showed that the microbial community in the MBfR system changed depending on operating conditions and bead amount. The results showed that the QQ method was an effective method to control the biofilm thickness in MBfR and provide insights for future research.     

Electron-acceptor loadings affect chloroform dechlorination in a hydrogen-based membrane biofilm reactor

Chloroform (CF) can undergo reductive dechlorination to dichloromethane, chloromethane, and methane. However, competition for hydrogen (H₂), the electron-donor substrate, may cause poor dechlorination when multiple electron acceptors are present. Common acceptors in anaerobic environments are nitrate (NO₃⁻), sulfate (SO₄²⁻), and bicarbonate (HCO₃⁻). We evaluated CF dechlorination in the presence of HCO₃⁻ at 1.56 e⁻ Eq/m²-day, then NO₃⁻ at 0.04–0.15 e⁻ Eq/m²-day, and finally NO₃⁻ (0.04 e⁻ Eq/m²-day) along with SO₄²⁻ at 0.33 e⁻ Eq/m²-day in an H₂-based membrane biofilm reactor (MBfR). When the biofilm was initiated with CF-dechlorination conditions (no NO₃⁻ or SO₄²⁻), it yielded a CF flux of 0.14 e⁻ Eq/m²-day and acetate production via homoacetogenesis up to 0.26 e⁻ eq/m²-day. Subsequent addition of NO₃⁻ at 0.05 e⁻ Eq/m²-day maintained full CF dechlorination and homoacetogenesis, but NO₃⁻ input at 0.15 e⁻ Eq/m²-day caused CF to remain in the reactor's effluent and led to negligible acetate production. The addition of SO₄²⁻ did not affect CF reduction, but SO₄²⁻ reduction significantly altered the microbial community by introducing sulfate-reducing Desulfovibrio and more sulfur-oxidizing Arcobacter. Dechloromonas appeared to carry out CF dechlorination and denitrification, whereas Acetobacterium (homoacetogen) may have been involved with hydrolytic dechlorination. Modifications to the electron acceptors fed to the MBfR caused the microbial community to undergo changes in structure that reflected changes in the removal fluxes.     

Bio-reductive dechlorination of 1,1,1-trichloroethane and chloroform using a hydrogen-based membrane biofilm reactor

A H(2)-based, denitrifying and sulfate-reducing membrane biofilm reactor (MBfR) was effective for removing 1,1,1-trichloroethane (TCA) and chloroform (CF) by reductive dechlorination. When either TCA or CF was first added to the MBfR, reductive dechlorination took place immediately and then increased over 3 weeks, suggesting enrichment for TCA- or CF-dechlorinating bacteria. Increasing the H(2) pressure increased the dechlorination rates of TCA or CF, and it also increased the rate of sulfate reduction. Increased sulfate loading allowed more sulfate reduction, and this competed with reductive dechlorination, particularly the second steps. The acceptor flux normalized by effluent concentration can be an efficient indicator to gauge the intrinsic kinetics of the MBfR biofilms for the different reduction reactions. The analysis of normalized rates showed that the kinetics for reductive-dechlorination reactions were slowed by reduced H(2) bio-availability caused by a low H(2) pressure or competition from sulfate reduction.     

Hydrogenotrophic denitrification and perchlorate reduction in ion exchange brines using membrane biofilm reactors

Halophilic (salt loving), hydrogenotrophic (H₂ oxidizing) denitrifying bacteria were investigated for treatment of nitrate <$>({\rm NO}_3^ ‐ )<$> and perchlorate <$>({\rm ClO}_4^ ‐ )<$> contaminated groundwater and ion exchange (IX) brines. Hydrogenotrophic denitrifying bacteria were enriched from a denitrifying wastewater seed under both halophilc and non‐halophilc conditions. The cultures were inoculated into bench‐scale membrane biofilm reactors (MBfRs) with an “outside in” configuration, with contaminated water supplied to the lumen of the membranes and H₂ supplied to the shell. Abiotic mass transfer tests showed that H₂ mass transfer coefficients were lower in brines than in tap water at highest Reynolds number, possibly due to increased transport of salts and decreased H₂ solubility at the membrane/liquid interface. An average <$>{\rm NO}_3^ ‐ <$> removal efficiency of 93% was observed for the MBfR operated in continuous flow mode with synthetic contaminated groundwater. Removal efficiencies of 30% for <$>{\rm NO}_3^ ‐ <$> and 42% for <$>{\rm ClO}_4^ ‐ <$> were observed for the MBfR operated with synthetic IX brine in batch operating mode with a reaction time of 53 h. Phylogenetic analysis focused on the active microbial community and revealed that halotolerant, <$>{\rm NO}_3^ ‐ <$> ‐reducing bacteria of the bacterial classes Gamma‐Proteobacteria and Sphingobacteria were the metabolically dominant members within the stabilized biofilm. This study shows that, despite decreased H₂ transfer under high salt conditions, hydrogenotrophic biological reduction may be successfully used for the treatment of <$>{\rm NO}_3^ ‐ <$> and <$>{\rm ClO}_4^ ‐ <$> in a MBfR. Biotechnol. Bioeng. 2009; 104: 483–491 © 2009 Wiley Periodicals, Inc.     

Inhibition of the metabolism of nitrifying bacteria by direct electric current

The nitrifying bacteria in activated sludge and biofilms consisting of the bacteria immobilized on polypropylene packing were subjected to an electric current via two electrodes. In activated sludge, the metabolism of nitrifying bacteria was inhibited when the applied current was over 2.5 A m^–2, whilst in biofilms, inhibition began when the current reached 5 A m^–2. At 15 A m^–2, the nitrification rate of NH₄ ⁺-N in a biofilm with a bacterial density of 1.62 g total solids, dry wt m^–2 decreased to about 80% of its initial value. Ninety-two % of the initial biomass on the packing was retained after 36 h.     

Sequencing batch membrane biofilm reactor for simultaneous nitrogen and phosphorus removal: novel application of membrane-aerated biofilm

A sequencing batch membrane biofilm reactor (SBMBfR) was developed for simultaneous carbon, nitrogen, and phosphorus removal from wastewater. This reactor was composed of two functional parts: (1) a gas-permeable membrane on which a nitrifying biofilm formed and (2) a bulk solution in which bacteria, mainly denitrifying polyphosphate-accumulating organisms (DNPAOs), were suspended. The reactor was operated sequentially under anaerobic condition and then under membrane aeration condition in one cycle. During the anaerobic period, organic carbon was consumed by DNPAOs; this was accompanied by phosphate release. During the subsequent membrane aeration period, nitrifying bacteria utilized oxygen supplied directly to them from the inside of the membrane. Consequently, the nitrite and nitrate products diffused into the bulk solution, where they were used by DNPAOs as electron acceptors for phosphate uptake. In a long-term sequencing batch operation, the mean removal efficiencies of total organic carbon (TOC), total nitrogen (T-N), and total phosphorus (T-P) under steady-state condition were 99%, 96%, and 90%, respectively. In addition, fluorescence in situ hybridization (FISH) clearly demonstrated the difference in bacterial community structure between the membrane biofilm and the suspended sludge: ammonia-oxidizing bacteria belonging to the Nitrosomonas group were dominant in the region adjacent to the membrane throughout the operation, and the occupation ratio of the well-known polyphosphate-accumulating organism (PAO) Candidatus "Accumulibacter phosphates" in the suspended sludge gradually increased to a maximum of 37%.     

Effect of bulk liquid BOD concentration on activity and microbial community structure of a nitrifying,membrane-aerated biofilm

Membrane-aerated biofilms (MABs) are an effective means to achieve nitrification and denitrification of wastewater. In this research, microsensors, fluorescence in situ hybridization (FISH), and modeling were used to assess the impact of bulk liquid biological oxygen demand (BOD) concentrations on the activity and microbial community structure of nitrifying MABs. With 1 g m⁻³ BOD in the bulk liquid, the nitrification rate was 1.3 g N m⁻² day⁻¹, slightly lower than the 1.5 g N m⁻² day⁻¹ reported for no bulk liquid BOD. With bulk liquid BOD concentrations of 3 and 10 g m⁻³, the rates decreased to 1 and 0.4 g N m⁻² day⁻¹, respectively. The percent denitrification increased from 20% to 100% when the BOD increased from 1 to 10 g m⁻³ BOD. FISH results indicated increasing abundance of heterotrophs with increasing bulk liquid BOD, consistent with the increased denitrification rates. Modeling was used to assess the effect of BOD on nitrification rates and to compare an MAB to a conventional biofilm. The model-predicted nitrification rates were consistent with the experimental results. Also, nitrification in the MAB was much less sensitive to BOD inhibition than the conventional biofilm. The MAB achieved concurrent nitrification and denitrification, whereas little denitrification occurred in the conventional biofilm.     

Managing methanogens and homoacetogens to promote reductive dechlorination of trichloroethene with direct delivery of H2 in a membrane biofilm reactor

A study with H(2)-based membrane biofilm reactors (MBfRs) was undertaken to examine the effectiveness of direct H(2) delivery in ex-situ reductive dechlorination of chlorinated ethenes. Trichloroethene (TCE) could be reductively dechlorinated to ethene with up to 95% efficiency as long as the pH-increase effects of methanogens and homoacetogens were managed and dechlorinators were selected for during start-up by creating H(2) limitation. Based on quantitative PCR, the dominant bacterial groups in the biofilm at the end of reactor operation were Dehalococcoides, Geobacter, and homoacetogens. Pyrosequencing confirmed the dominance of the dechlorinators and identified Acetobacterium as the key homoacetogen. Homoacetogens outcompeted methanogens for bicarbonate, based on the effluent concentration of acetate, by suppressing methanogens during batch start-up. This was corroborated by the methanogenesis functional gene mcrA, which was 1-2 orders of magnitude lower than the FTHFS functional gene for homoacetogens. Imaging of the MBfR fibers using scanning electron microscopy showed a distinct Dehalococcoides-like morphology in the fiber biofilm. These results support that direct addition of H(2) can allow for efficient and complete reductive dechlorination, and they shed light into how H(2)-fed biofilms, when operated to manage methanogenic and homoacetogenic activity, can be used for ex-situ bioremediation of chlorinated ethenes.     

The accumulation of fixed biomass increases the observed growth yield of a nitrifying biofilm

The observed growth yield (Y _obs) of a nitrifying biofilm metabolizing ammonia in a continuous flow reactor was constant below a fixed biomass concentration of 40–50 g COD-biomass cm^–2. Beyond this range, an increased Y _obs with the additional accumulation of fixed biomass could be due to a considerable accumulation of inactive materials within the nitrifying biofilm.     

3D finite element model of biofilm detachment using real biofilm structures from CLSM data

In this work, a three‐dimensional model of fluid–structure interactions (FSI) in biofilm systems is developed in order to simulate biofilm detachment as a result of mechanical processes. Therein, fluid flow past the biofilm surface results in a mechanical load on the structure which in turn causes internal stresses in the biofilm matrix. When the strength of the matrix is exceeded parts of the structure are detached. The model is used to investigate the influence of several parameters related to the mechanical strength of the biofilm matrix, Young's modulus, Reynolds number, and biofilm structure on biofilm detachment. Variations in biofilm strength and flow conditions significantly influence the simulation outcome. With respect to structural properties the model is widely independent from a change of Young's modulus. A further result of this work indicates that the change of biofilm structure due to growth or other processes will significantly change the stress distribution in the biofilm and thereby the detachment rate. An increase of the mechanical load by increasing fluid flow results in a flat surface of the remaining biofilm structure. It is concluded that the change of structure during biofilm development is the key determinant in terms of the detachment behavior. Biotechnol. Bioeng. 2009;103: 177–186. © 2008 Wiley Periodicals, Inc.