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.     

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.     

Simultaneous nitrification/denitrification in a biofilm airlift suspension (BAS) reactor with biodegradable carrier material

Simultaneous nitrification and denitrification in one reactor has been realized with different methods in the past. The usage of biodegradable biocompounds as biofilm carriers is new. The biocompounds were designed out of two polymers having different degradability. Together with suspended autotrophic biomass the biocompound particles were fluidized in an airlift reactor. Process water from sludge dewatering with a mean ammonium nitrogen concentration of 1150 mg L⁻¹ was treated in a two stage system which achieved a nitrogen removal of 75%. Batch experiments clearly indicate that nitrification can be localized in the suspended biomass and denitrification in the pore structure of the slowly degraded biocompounds. Images taken with CLSM prove the concept of the pore structure within the biocompounds, which provide both a heterotrophic biofilm and carbon source.     

Characterization of biofilm structure and its effect on membrane permeability in MBR for dye wastewater treatment

Two membrane bioreactors were operated at aerobic (DO=6.0mg/L) and anoxic (DO<0.3mg/L) conditions for the treatment of synthetic dye wastewater to determine the effect of dissolved oxygen on membrane filterability. The rate of membrane fouling for the anoxic MBR was five times faster than that for the aerobic MBR. Differences in the nature of the biofilm that was formed on the membrane surface as the result of different DO level was the main factor in the different fouling rates. The biofilm structure was characterized using digital image analysis techniques. Biofilm images were obtained using confocal laser scanning microscopy (CLSM) at various operation points. Structural parameters were then computed from these images using an image analysis software (ISA-2). The structural parameters indicated that the anoxic biofilm was thinner than the aerobic biofilm but the anoxic biofilm was spread out on the membrane surface more uniformly and densely, resulting in the higher membrane fouling. Based on the extracellular polymeric substances (EPS) visualization and quantification, it was also found that EPS, key membrane foulants were spread out more uniformly in the anoxic biofilm in spite of lower amount of EPS compared to that in the aerobic biofilm.     

Nitrogen dynamics and removal in a horizontal flow biofilm reactor for wastewater treatment

A horizontal flow biofilm reactor (HFBR) designed for the treatment of synthetic wastewater (SWW) was studied to examine the spatial distribution and dynamics of nitrogen transformation processes. Detailed analyses of bulk water and biomass samples, giving substrate and proportions of ammonia oxidising bacteria (AOB) and nitrite oxidising bacteria (NOB) gradients in the HFBR, were carried out using chemical analyses, sensor rate measurements and molecular techniques. Based on these results, proposals for the design of HFBR systems are presented.The HFBR comprised a stack of 60 polystyrene sheets with 10-mm deep frustums. SWW was intermittently dosed at two points, Sheets 1 and 38, in a 2 to 1 volume ratio respectively. Removals of 85.7% COD, 97.4% 5-day biochemical oxygen demand (BOD₅) and 61.7% TN were recorded during the study.In the nitrification zones of the HFBR, which were separated by a step-feed zone, little variation in nitrification activity was found, despite decreasing in situ ammonia concentrations. The results further indicate significant simultaneous nitrification and denitrification (SND) activity in the nitrifying zones of the HFBR. Sensor measurements showed a linear increase in potential nitrification rates at temperatures between 7 and 16 °C, and similar rates of nitrification were measured at concentrations between 1 and 20 mg NH₄-N/l. These results can be used to optimise HFBR reactor design. The HFBR technology could provide an alternative, low maintenance, economically efficient system for carbon and nitrogen removal for low flow wastewater discharges.     

Applying a novel autohydrogenotrophic hollow-fiber membrane biofilm reactor for denitrification of drinking water

We conducted a series of pseudo-steady-state experiments on a novel hollow-fiber membrane biofilm reactor used for denitrification of oligotrophic waters, such as drinking water. We applied a range of nitrate loadings and hydrogen pressures to establish under what conditions the system could attain three goodness-of-performance criteria: partial nitrate removal, minimization of hydrogen wasting, and low nitrite accumulation. The hollow-fiber membrane biofilm reactor could meet drinking-water standards for nitrate and nitrite while minimizing the amount of hydrogen wasted in the effluent when it was operated under hydrogen-limited conditions. For example, the system could achieve partial nitrate removals between 39% and 92%, effluent nitrate between 0.4 and 9.1 mg N/l, effluent nitrite less than 1 mg N/l, and effluent hydrogen below 0.1 mg H2/l. High fluxes of nitrate and hydrogen made it possible to have a short liquid retention time (42 min), compared with 1-13 h in other studies with hydrogen used as the electron donor for denitrification. The fluxes and concentrations for hydrogen, nitrate, and nitrite obtained in this study can be used as practical guidelines for system design.     

Role of shear stress on composition, diversity and dynamics of biofilm bacterial communities

This article evaluates the effect of shear stress on the composition of biofilm bacterial communities. For the first time, a Conical Couette-Taylor Reactor (CCTR) was used to develop biofilms at varying shear stresses (from 0.055 to 0.27 Pa) and provided a useful model for studying the effect of hydrodynamics on biofilms. The composition, diversity and dynamics of biofilm bacterial communities were analysed using the PCR-SSCP fingerprint method. Results clearly demonstrate a link between shear stress and composition of the microbial communities. High shear stresses decrease biofilm diversity and the analysis of biofilm community dynamics suggests that shear stress would slow down biofilm maturation and tend to maintain a young biofilm.     

Impact of colloidal and soluble organic material on membrane performance in membrane bioreactors for municipal wastewater treatment

Two parallel membrane bioreactors (2 m3 each) were operated over a period of 2 years. Both pilots were optimised for nitrification, denitrification, and enhanced biological phosphorous elimination, treating identical municipal wastewater under comparable operating conditions. The only constructional difference between the pilots was the position of the denitrification zone (pre-denitrification in pilot 1 and post-denitrification in pilot 2). Despite identical modules and conditions, the two MBRs showed different permeabilities and fouling rates. The differences were not related to the denitrification scheme. In order to find an explanation for the different membrane performances, a one-year investigation was initiated and the membrane performance as well as the operating regime and characteristics of the activated sludge were closely studied. MLSS concentrations, solid retention time, loading rates, and filtration flux were found not to be responsible for the different performance of the submerged modules. These parameters were kept identical in the two pilot plants. Instead, the non-settable fraction of the sludges (soluble and colloidal material, i.e. polysaccharides, proteins and organic colloids) was found to impact fouling and to cause the difference in membrane performance between the two MBR. This fraction was analysed by spectrophotometric and size exclusion chromatography (SEC) methods. In a second step, the origin of these substances was investigated. The results point to microbiologically produced substances such as extracellular polymeric substances (EPS) or soluble microbial products (SMP).     

Multidimensional modeling of biofilm development and fluid dynamics in a hydrogen-based,membrane biofilm reactor (MBfR)

A two-dimensional, particle-based biofilm model coupled with mass transport and computational fluid dynamics was developed to simulate autotrophic denitrification in a spiral-wound membrane biofilm reactor (MBfR), where hydrogen is supplied via hollow-fiber membrane fabric. The spiral-wound configuration consists of alternating layers of plastic spacer net and membrane fabric that create rows of flow channels, with the top and bottom walls comprised of membranes. The transversal filaments of the spacer partially obstruct the channel flow, producing complex mixing and shear patterns that require multidimensional representation. This study investigated the effect of hydrogen and nitrate concentrations, as well as spacer configuration, on biofilm development and denitrification fluxes. The model results indicate that the cavity spacer filaments, which rest on the bottom membranes, cause uneven biofilm growth. Most biofilm resided on the bottom membranes, only in the wake of the filaments where low shear zones formed. In this way, filament configuration may help achieve a desired biofilm thickness. For the conditions tested in this study, the highest nitrate fluxes were attained by minimizing the filament diameter and maximizing the filament spacing. This lowered the shear stress at the top membranes, allowing for more biofilm growth. For the scenarios studied, biomass limitation at the top membranes hindered performance more significantly than diffusion limitation in the thick biofilms at the bottom membranes. The results also highlighted the importance of two-dimensional modeling to capture uneven biofilm growth on a substratum with geometrical complexity.     

Impact of ozonation on the genotoxic activity of tertiary treated municipal wastewater

Ozonation is an emerging technology for the removal of micropollutants from treated wastewater. Aim of the present study was to investigate the impact of ozone treatment on genotoxic and acute toxic effects of tertiary treated municipal wastewater. It is known that DNA-damaging chemicals cause adverse effects in the environment and that exposure to humans leads to cancer and other diseases. Toxicity was tested in organisms from three trophic levels namely in bacteria (Salmonella/microsome assays) which enable the detection of gene mutations, in a plant bioassay (micronucleus assay with root tip cells of Allium cepa) which reflects clastogenic and aneugenic effects and in single cell gel electrophoresis (SCGE) tests with mammalian cells which detect DNA migration caused by single-, double strand breaks and alkali labile sites. In the bacterial tests negative results were obtained with untreated samples but after concentration with C₁₈ cartridges a positive result was found in strains TA1537 and TA98 which are sensitive to frameshift mutagens while no mutations were induced in other tester strains (TA100, TA102 and YG1024). Ozone treatment led to a decrease of the mutagenic activity of the samples. In the SCGE experiments, DNA migration was detected with the unconcentrated effluent of the treatment plant and ozonation led to a substantial decrease of this effect. In the plant bioassays, negative results were obtained with the effluent and ozone treatment did not cause an alteration of the micronucleus frequencies. Also acute toxic effects were monitored in the different indicator organisms under all experimental conditions. The bacteriocidal/bacteriostatic effects which were seen with the concentrated samples were reduced by ozonation. In the experiments with the eukaryotic (plant and animal) cells no acute toxicity was seen with the effluents and ozonation had no impact on their viability. In conclusion findings of this study indicate that ozonation of tertiary effluents of a municipal treatment plant reduces the adverse effects caused by release of mutagens in aquatic ecosystems and does not decrease the viability of bacteria and eukaryotic cells. However, future research is required to find out if, and to which extent these findings can be generalized and which mechanisms account for the detoxification of the wastewater.     

Effect of DO concentration on biofilm structure and membrane filterability in submerged membrane bioreactor

The structures of biofilms deposited on the membrane surface under different dissolved oxygen (DO) conditions were characterized to identify its relation to membrane filterability in membrane bioreactors (MBR). The rate of membrane fouling for the low DO (LDO) reactor was 7.5 times faster than that for the high DO (HDO) reactor. Even though the biofilm deposited on the membrane surface in the HDO was thicker than in the LDO at the operating terminated (TMP reached 30 Kpa), biofilm resistance in both reactors were similar. Exactly, specific cake resistance of the HDO was lower than that of the LDO. Difference in biofilm characteristics as a result of different DO level was main factor affecting biofouling for both MBRs. The number of small particles ranging from 2-5mum in the biofilm as well as in the bulk solution for the LDO was greater than those for the HDO. The small particles in the bulk solution of the LDO more preferentially deposited on the membrane surface than those of HDO did. Hence, the biofilm porosity in the LDO (0.65) was smaller than that in HDO (0.85). The reduced porosity of LDO biofilm resulted in lower filterability than the HDO. The porosity data obtained from analysis of images of biofilm using confocal scanning laser microscopy (CLSM) was verified in terms of specific cake resistance (alpha) by comparing the experimentally measured values with the semi-theoretically computed values.     

Experimental results and mathematical modelling of an autotrophic and heterotrophic biofilm in a sand filter treating landfill leachate and municipal wastewater

A better understanding of wastewater treatment with soil filters is important to optimise plant operation and reduce the risk of clogging. The article presents results of a treatment concept which uses a combination of SBR and vertical-flow sand filter technology. The SBR was mainly used for denitrification and sedimentation of substances in particulate form. Efficient nitrification was achieved by the planted sand filter. Degradation rates of 10gNH(4)-N/(m(2)xd) were measured for periods with peak loadings. The two-dimensional dynamic model reproduces the biofilm growth and decay of heterotrophic and autotrophic biomass. It is capable of describing the clogging of the sand filter by combining a biochemical and a geometric model. After calibration, the model was used for the calculation of maximum nitrogen degradation performances. Maximum degradation rates of 12gNH(4)-N/(m(2)xd) can be achieved if the COD/TKN ratio is reduced before to a level lower than that of municipal wastewater. The COD was further degraded in the filter than we expected comparing it with activated sludge plants. Within the soil filter a biofilm thickness of up to 110mum is simulated depending on the embankment of gravel and grains of sand. Sensitivity analysis of model parameters showed the high impact of the maximum autotrophic growth rate, the autotrophic yield, the diffusion coefficient for oxygen and the number of contact points of the single grains of sand.     

Impacts of hydrodynamic shear force on nucleation of flocculent sludge in anaerobic reactor

The Sludge granulation in an anaerobic reactor consists of two steps: nucleation and maturation of nuclei. Nucleation as the starting point is of particular importance. In this paper, the nucleation of flocculent sludge as seed under weak, strong and violent hydrodynamic shear conditions is studied with an original quantitative method, and then the satisfactory linear correlations between the average sludge diameters and the operation time during the nucleation are demonstrated. Nucleation under strong shear conditions with a shear rate of about 8.28 s⁻¹, corresponding to superficial liquid and gas velocities of 2.66 and 0.24 m h⁻¹, develops fastest compared to weak shear conditions with a shear rate of about 0.04 s⁻¹ and violent shear conditions with a shear rate of about 12.42 s⁻¹ with the average augmentation rate of average sludge diameter of 0.57, 0.40 and 0.41 μm day⁻¹ respectively. One of the major mechanisms of the shear force on nucleation is that a high shear force accelerates the extracellular protein secretion of sludge. Although high extracellular protein content benefits nucleation, it is also shown that the extracellular proteins over-produced above around 80.5 mg gVSS⁻¹ leads nuclei to weaken and inhibit nucleation. So the violent shear force would result in disruption and wash-out of nuclei. However, the high extracellular polymers could intensify the shear force by raising the viscosity in the reactor, thus, in practice, it is important to monitor the shear conditions and extracellular protein content of sludge simultaneously in high rate reactors for stable operation.     

Effect of shear force on the initial stiffness of top and seat angle connections with double web angles

This study focused on the moment-rotation behavior of bolted top and seat angles with double web angle connections, especially the initial stiffness of this type of connection under the combination of shear force and moment. Several 3D parametric finite element models are presented in this regard, with the geometrical and mechanical properties of connections are as parameters. In the models, all the connection components such as beam, column, angles and bolts are modeled using eight node brick elements. The effects of all component interactions, such as slippage of bolts and frictional forces, are modeled using a surface contact algorithm, and to evaluate the connection behavior more precisely, bolt pre-tensioning is applied on the bolts shanks as the first load case. The results of numerical modeling are compared with test results of experiments that have been done by researchers, and show good agreement with them. To evaluate the effect of shear force on the behavior of such connections, several models were analyzed under different magnitudes of shear force, and the results of the analyses showed that the shear force has a reducing effect on the initial stiffness of bolted angle connections. Therefore, an equation is proposed to determine the reduction factor of a connection’s initial stiffness in terms of the connection’s initial stiffness and yield moment in the case of no shear force, and any expected shear force that might be applied.     

Effects of increase modes of shear force on granule disruption in upflow anaerobic reactors

Sludge washout is listed among the top practical problems of the high rate upflow anaerobic reactors. This study investigated quantitatively two sludge washout processes operated under different hydrodynamic shear increase modes with the intervals of 1 and 10 days respectively. The results reveal that the sludge washout accompanying with large-scale granule disruption could lead to performance failure with heavy sludge loss ratio of about 46.1% at sludge loss rate about 0.35 gVSS L⁻¹ d⁻¹ during the process with shear increase interval of 1 day, while the highest sludge loss rate was only 0.12 gVSS L⁻¹ d⁻¹ during the process with 10-day interval. The intensified shear conditions could weaken the granules through inhibiting the extracellular polymers production and bioactivity. As consequences, an outbreak of large-scale granule disruption would raise and then significantly accelerate the sludge washout. Since long interval could provide the granules the opportunity to recover from these negative effects to some extent, the shear increase strategy of long interval over 10 days is favorably recommended to operate full-scale reactors during the start-up and shock load periods. The pioneer use of the micro particle image velocimetry in this study offers the possibility to discover the real hydrodynamic conditions around granules at microscale for the first time and reveals that the shear force exerts directly on the granular surface as a mechanical disruption force and big granules undergo high disruption force. The granule disruption is a result of the competition between the granule and the ambient hydrodynamic shear conditions rather than a process with shear force as a sole dominant factor. These could facilitate the understanding of the complicated interactions between the hydrodynamics and reactor performance and favor then a better control of the full-scale reactors.     

中高层框剪结构侧向力分布模式的研究

通过选取5条Ⅱ类场地地震波,对所选结构进行非线性时程分析得到基底剪力和顶层位移关系曲线,按照一种侧向力分布模式对所选结构进行Pushover分析得到基底剪力和顶层位移关系曲线及层间位移角,然后和时程分析进行比较,得到一种较理想的侧向力分布模式。     

The effect of organic loading on process performance and membrane fouling in a submerged membrane bioreactor treating municipal wastewater

The results of experiments on municipal wastewater primary effluent are presented for a pilot-scale submerged membrane bioreactor (SMBR). The SMBR pilot plant employed an ultrafiltration membrane with a nominal pore size of 0.035 microm and was operated at a constant membrane flux of 30 L/m(2)h. The mixed liquor suspended solids (MLSS) concentration was maintained at 8+/-2 g/L and steady-state fouling rates were determined for 10, 5, 4, 3, and 2-d MCRTs, corresponding to food-to-microorganism (F/M) ratios of 0.34, 0.55, 0.73, 0.84, and 1.41 gCOD/gVSS d, respectively. Membrane fouling rates increased as the F/M was increased. Steady-state membrane fouling rates were correlated with total soluble microbial products (SMP) concentrations. The membrane fouling rates did not correlate well with soluble COD measured on a 0.45 microm membrane filtrate of mixed liquor or with soluble COD rejection (effluent COD/soluble COD).     

Effects of pH and precipitation on autohydrogenotrophic denitrification using the hollow-fiber membrane-biofilm reactor

Experiments carried out in a hollow-fiber, membrane-biofilm reactor (HFMBR) showed that the optimum pH for autotrophic denitrification was in the range 7.7-8.6, with the maximum efficiency at 8.4. Increasing the pH above 8.6 caused a significant decrease in nitrate removal rate and a dramatic increase in nitrite accumulation. The pH rose by 1.2 units when a large buffer was not added, suggesting that some field applications may require pH control. Precipitation of Ca(2+) occurred in every experiment. Precipitation was the largest sink for carbonate, and it also offset alkalinity production by denitrification. Although the alkalinity increased in most cases, systems with a high carbonate buffer and high pH accentuated precipitation, and the net change in alkalinity was negative. The long-term success of field applications of the HFMBR may depend upon the interactions among calcium concentration, total carbonate concentration, pH, and alkalinity changes.     

Behavior of bolted angle connections subjected to combined shear force and moment

In this paper, the effect of web angle dimensions on moment-rotation behavior of bolted top and seat angle connections, with double web angles is studied. Several 3D parametric finite element (FE) models are presented in this study whose geometrical and mechanical properties are used as parameters. In these models, all of the connection components, such as beam, column, angles and bolts are modeled using solid elements. The effect of interactions between components, such as slippage of bolts and frictional forces, are modeled using a surface contact algorithm. To evaluate the behavior of connection more precisely, bolt pretensioning force is applied on bolt shanks as the first load case. The results of this numerical modeling are compared with the results of experimental works done by other researchers and good agreement was observed. To study the influence of shear force on behavior of these connections, several models were analyzed using different values of shear force. The effect of important parameters, especially the effect of web angle dimension, is studied then. An equation is proposed to determine the reduction factor for initial rotational stiffness of connection using connection initial rotational stiffness, yield moment, the expected shear force and web angle dimension. The proposed equation is compared with other existing formulations and it was observed that the proposed model is a better estimator of connection behavior.