Comparison of startup and anaerobic wastewater treatment in UASB, hybrid and baffled reactor

An experimental study was carried out to compare the performance of selected anaerobic high rate reactors operated simultaneously at 37?°C. The three reactors, namely upflow anaerobic sludge bed reactor (UASB), hybrid of UASB reactor and anaerobic filter (anaerobic hybrid reactor – AHR) and anaerobic baffled reactor (ABR), were inoculated with the anaerobic digested sludge from municipal wastewater treatment plant and tested with synthetic wastewater. This wastewater contained sodium acetate and glucose with balanced nutrients and trace elements (COD 6000?mg?·?l^?1). Organic loading rate (B _v ) was increased gradually from an initial 0.5?kg?·?m^?3?·?d^?1 to 15?kg?·?m^?3?·?d^?1 in all the reactors. From the comparison of the reactors' performance, the lowest biomass wash-out resulted from ABR. In the UASB, significant biomass wash-out was observed at the B _v 6?kg?·?m^?3?·?d^?1, and in the AHR at the B _v 12?kg?·?m^?3?·?d^?1. The demand of sodium bicarbonate for pH maintenance in ABR was two times higher as for UASB and AHR. The efficiency of COD removal was comparable for all three reactors – 80–90%. A faster biomass granulation was observed in the ABR than in the other two reactors. This fact is explained by the kinetic selection of filamentous bacteria of the Methanotrix sp. under a high (over 1.5?g?·?l^?1) acetate concentration.     

Biofilm Growth and Bed Fluidization in a Fluidized Bed Reactor Packed with Support Materials of Low Density

Support materials of low‐density for fluidized bed reactors provide several operational advantages, including lower energy requirements and proper biofilm growth balance. The aim of this investigation was to study the extent of biofilm growth and bed fluidization in an experimental reactor, using polyester resin (ρ_pr = 1220 kg/m³) and vitrified expanded perlite (ρ_vep = 1710 kg/m³) as alternative support materials to conventional silica sand. A noteworthy amount of biofilm was observed to be attached to both support materials from the very beginning of the bioreactor operation. Nevertheless, there were significant variations in biofilm growth and activity over the course of the experimental trials. For both perlite and polyester beds, the highest biofilm mass and the highest total number of mesophilic bacteria were observed between the 7^th and the 10^th day, showing a steady state trend at the end of the experimental runs. The chemical oxygen demand (COD) removal levels were concomitant with biofilm mass and total mesophilic bacteria changes, although the polyester bed efficiency was slightly higher than that for the perlite bed. As expected, the polyester bed was fluidized at a lower re‐circulation flow compared to the perlite bed. Reactor back‐washing was not required for these support materials since biomass excess was adequately separated by means of a special internal device. The efficiencies of removal of organic matter achieved were acceptable (up to 78 %) despite the low volume of the support material (25 %) and the low hydraulic retention time (30 min).     

Capacities and limits of three different technologies for the biological treatment of leachate from solid waste landfill sites

Leachate from a municipal waste landfill site was treated using an activated sludge bioreactor, a fluidized bed biofilm reactor and a packed-bed column reactor (trickling filter). The leachate contained high organic matter (2.0–2.6 g/l of COD), high ammonium (300–700 mg/l) and sulphide (200–800 mg/l) concentrations, as well as low metal concentrations. The continuously operating reactors were employed to study the effects of TOC loading on the removal of TOC as well as on the nitrification and denitrification processes. Among the three biological treatment technologies investigated, the fluidized bed biofilm reactor was best with respect to removing ammonia and TOC. More than 90% of TOC and 99% of ammonia were removed when TOC loading was less than 0.5 kg/m³ × d. At a TOC loading of 4 kg/m³ × d, the removal of TOC and ammonia was 80% and 99%, respectively. In contrast, the treatment of leachate with the packed-bed reactor was successful in TOC removing only at TOC loading less than 0.3 kg/m³ × d (TOC elimination decreased from 86% at 0.06 kg/m³ × d to 60% at 0.3 kg/m³ × d). However, the reactor was active in nitrification even at a higher TOC loading (more than a 98% ammonia elimination at a TOC loading of 0.5 kg/m³ × d). Leachate was processed in the activated sludge reactor when TOC loading was less than 0.5 kg/m³ × d (with a removal of TOC and ammonia up to 83% and 99%, respectively). The activated sludge reactor was also effective in TOC removal at a higher TOC loading (e.g. a 74% TOC removal at a TOC loading of 1 kg/m³ × d), but for ammonia elimination, the activity continuously decreased (less than 60% ammonia removal at a TOC loading of 1 kg/m³ × d). Overloading in the activated sludge system was indicated by a high concentration of ammonia and nitrite in the effluent. In the packed bed reactor, overloading was characterized by a progressively incomplete TOC removal. No significant overloading was found in the fluidized bed reactor up to a TOC loading of 4 kg/m³ × d.     

Improvement of Process Stability of Microbiological Quinoline Degradation in a Three‐Phase Fluidized Bed Reactor

The biological degradation of quinoline by suspended and immobilized Comamonas acidovorans was studied under continuous and discontinuous operating conditions in a three‐phase fluidized bed reactor. C. acidovorans degrades quinoline into biomass and carbon dioxide. Quinoline and the intermediates of its metabolic pathway are found only by quinoline shockloads. The continuous degradation of quinoline by suspended biomass was only possible, if the dilution rate was less than the growth rate (μ_max =0.42 h^–1) and the concentration of a shockload was less than 1 kg/m³. A concentration greater than 1 kg/m³ led to an irreversible damage of the cells. Hence, two different carrier materials were used for immobilization by attachment, to increase the stability of the process. Using immobilization of biomass on carriers decouples the hydrodynamic retention time and the growth rate of the microorganisms. A comparison of the carrier material showed no differences with respect of activity and stability of the biofilm. The process stability of a three‐phase fluidized bed reactor was increased by immobilized biomass. The degradation of toxic shockloads was only possible with immobilized biomass. A dynamic model has been developed to describe the concentration profile of quinoline, 2‐hydroxyquinoline as metabolite and the suspended biomass. A comparison of the measured and calculated values showed good agreement.     

Sorbitol and Gluconate Production in a Hollow Fibre Membrane Reactor by Immobilized Zymomonas Mobilis

This study describes the results of a hollow fibre membrane reactor with immobilized treated cells of Zymomonas mobilis which produced sorbitol and gluconic acid continuously from fructose and glucose respectively. A productivity of 10–20 g sorbitol · L^-1 · h^-1 and 10–20 gluconate · L^-1 · h^-1 (based on total bioreactor volume) from a feed of 100 g · L^-1 each of glucose and fructose was possible at high dilution rates. Kinetic parameters describing the reaction rate of treated cells in batch reactors were used to analyse the performance of the hollow fibre membrane reactor employing significant convective mass transfer. No significant mass transfer limitation was apparent.     

Granulation in anaerobic sludge bed reactors treating food industry wastes

Three parallel laboratory reactors, R1, R2 and R3, received food industry wastewater: R1 unadulterated; R2 supplemented with calcium and phosphate; R3 supplemented with ferric chloride and traces of nickel and cobalt. Reactors were packed with active granular sludge from a large scale pilot reactor treating the same wastewater. Addition of calcium and phosphate was found to be detrimental to the granule formation at naturally established reactor pH = 6·9–7·4 in R2 while iron promoted granulation in R3. Conditions of upflow velocities of 1·5–6 m h⁻¹, rapid increase of loads up to 15 kg COD m⁻³ day⁻¹ and ratios of recycle to raw waste feed of 20:1–80:1 were imposed on all reactors. The granules in R1 and R2 disintegrated, from 70–100 g liter⁻¹ VSS to a flocculant sludge at 1·5–3 g liter⁻¹. In spite of such severe washout, reactors R1 and R2 were able to maintain a steady COD removal of over 90% at a load of 10kg m⁻³ day⁻¹. R3 retained a VSS concentration around 100 g liter⁻¹ and maintained COD removal at over 95%. R3 exhibited a more stable performance and was less vulnerable to the shock treatment to which all reactors were subjected.     

Development of a simultaneous partial nitrification and anaerobic ammonia oxidation process in a single reactor

Up-flow oxygen-controlled biofilm reactors equipped with a non-woven fabric support were used as a single reactor system for autotrophic nitrogen removal based on a combined partial nitrification and anaerobic ammonium oxidation (anammox) reaction. The up-flow biofilm reactors were initiated as either a partial nitrifying reactor or an anammox reactor, respectively, and simultaneous partial nitrification and anammox was established by careful control of the aeration rate. The combined partial nitrification and anammox reaction was successfully developed in both biofilm reactors without additional biomass inoculation. The reactor initiated as the anammox reactor gave a slightly higher and more stable mean nitrogen removal rate of 0.35 (± 0.19) kg-N m⁻³ d⁻¹ than the reactor initiated as the partial nitrifying reactor (0.23 (± 0.16) kg-N m⁻³ d⁻¹). FISH analysis revealed that the biofilm in the reactor started as the anammox reactor were composed of anammox bacteria located in inner anoxic layers that were surrounded by surface aerobic AOB layers, whereas AOB and anammox bacteria were mixed without a distinguishable niche in the biofilm in the reactor started as the partial nitrifying reactor. However, it was difficult to efficiently maintain the stable partial nitrification owing to inefficient aeration in the reactor, which is a key to development of the combined partial nitrification and anammox reaction in a single biofilm reactor.     

Biomass control in a fixed-bed anaerobic reactor

In order to improve the reliability of fixed-bed anaerobic reactors, the effects of a media and biomass control method were studied using bench scale equipment and mathematical simulation. Test results showed that the allowable volumetric loading rates (kg COD/m³·d) were directly proportional to the packed ratio of the media. Although a reactor completely filled with media (packed media ratio: 100%) could handle an 80% reduction in COD at a loading rate of 11 kg COD/m³·d, a packed reactor half-filled with media (packed media ratio: 50%) could not handle more than 5.5 kg COD/m³·d to achieve the same degree of reduction in COD. Simulation results were based on actual commercial plant operation and showed a practical correlation between COD removal and effective void volume ratio. To achieve a steady reduction of more than 80% in COD, the range of the void volume ratio was presumed to be 0.6 ∼ 0.85.     

Startup of anaerobic biofilm fluidized bed reactors on molasses and phenol under various conditions

Fluidized sand bed anaerobic biofilm reactors were operated in parallel to study the effects of inoculum, loading, residence time and carrier type on the startup dynamics for the degradation of molasses and phenol. Degradation rates generally depended most directly on concentrations rather than on other operating variables. Residence times did not appear to directly influence startup. Short residence times and high loadings gave the highest specific activities for both substrates. The type of inoculum was found to be most important for the molasses system, and inoculation on fresh carrier was found to be better than reinoculation. The two times higher specific biomass retention on Siran porous glass gave essentially the same degradation rates on a volume basis.List of Symbols L kg/h loading of reactor - M kg/kg biomass per carrier mass - Red. % reduction of feed concentration due to degradation - R kg/(m³ · h) reaction rate - S kg/m³ substrate concentration in reactor and effluent - S ₀ kg/m³ substrate concentration in feed - t h time     

Mathematical model for the fluidized bed biofilm reactor

A mathematical model is proposed for the fluidized bed biofilm reactor (FBBR). For individual biofilm-covered particles (bioparticles) within the reactor, an analysis of intrabiofilm mass transfer and simultaneous intrinsic zero order reaction yields an effectiveness factor expression which is a function of the modified, zero order Thiele modulus, Φ_0,m. This expression is linked to a one-dimensional reactor flow model and a fluidization model to yield an overall reactor model describing convective transport and simultaneous biochemical conversion of substrate within a FBBR. For Φ_0,m<1.15, FBBR is mass transfer limited and 0.45 order kinetics are observed. For Φ_0,m<1.15, mass transfer limitations are insignificant and intrinsic zero order kinetics are observed. A sensitivity analysis using the proposed mathematical model indicates that biofilm thickness and media size are the two most important operating parameters. These two parameters can be optimized simultaneously for a specific application. The proposed model provides a rational approach for FBBR design.     

The Rotary Trickle‐Bed Reactor – A New Reactor Concept for Biological Gas Purification

A new type of reactor employed to the biological gas purification is presented. The avoidance of clogging in the carrier packing is achieved by i) the use of a structured, rotating carrier packing, ii) a definite liquid irrigation regime during start‐up, operation and clean‐up time phases, iii) an on‐line determination and control of the fixed biofilm mass. A uniform biofilm thickness is generated by an optimized liquid irrigation of the carrier packing with spray nozzles. The detachment of the fixed biomass is accomplished by liquid shear forces generated with jet nozzles. The time‐scheduled operation regime of the reactor is founded on the on‐line quantification of the immobilized biomass, which results in a new quality of process governing of biotrickling reactors applied to gas purification. This is proved by the experimental results of pressure drop, dynamic liquid holdup as well as the volumetric degradation rates. The degradation of styrene was investigated in laboratory and field experiments showing a maximal volumetric degradation rate of 150 g m^–3 h^–1 at a pollutant load of 200 g m^–3 h^–1. The feasibility of this reactor prototype is demonstrated by employing it to the elimination of industrial waste gas.     

Comparative study of the nitrification characteristics of two different nitrifier immobilization methods

The research investigated the nitrification characteristics of two different immobilization methods: nitrifier encapsulation in polyethylene glycol (PEG) gel pellets and nitrifier biofilm attachment on elastic plastic filler. The two carriers were placed in identical reactors. They reached a maximum nitrification rate of 39 and 25 mgN/L·h 30 days after start-up. The results showed that the nitrification efficiency in the PEG reactor was higher than in the biofilm reactor under the same conditions. Variations in temperature decreased the nitrification rate by approximately 55% in the PEG reactor from 28 to 8°C, while 74.2% in the biofilm reactor. When the COD loading rate was increased to 0.8 kg/m³ day, the nitrification efficiency in the biofilm reactor dropped sharply to 23%, and that of PEG reactor remained over 80%. PEG pellets with a high nitrification rate under all conditions showed promise as an immobilization medium, and are likely to be utilized in the nitrification of high-strength ammonia and COD wastewater during long-term operation.     

Autohydrogenotrophic Denitrifying Microbial Community in a Glass Beads Biofilm Reactor

A glass bead biofilm reactor was operated using H₂ as an electron donor to remove nitrate at 150 mg NO₃–N l⁻¹ to below detection level. The microbial community in the glass beads biofilm reactor was investigated by using denaturing gradient gel electrophoresis (DGGE) and phylogenetic analysis. In DGGE analysis of the biofilm, five bands were dominant and indicated the presence of eight β-proteobacteria, one γ-proteobacteria and twelve clostridia. An unculturable Hydrogenophaga sp., which is a new genus of hydrogen-oxidizing bacterium was dominant in microbial community of the biofilm reactor.     

Anammox start-up in sequencing batch biofilm reactors using different inoculating sludge

In this study, anammox bacteria were rapidly enriched in sequencing batch biofilm reactors (SBBRs) with different inoculations. The activated sludge taken from a sequencing batch reactor was used and inoculated to SBBR1, while SBBR2 was seeded with stored anaerobic sludge from an upflow anaerobic fixed bed (2-year stored at 5–15 °C). Nitrogen removal performance, anammox activity, biofilm characteristics and variation of the microbial community were evaluated. The maximum total nitrogen loading rate (NLR) of SBBR1 gradually reached to 1.62 kg?N/(m³/day) with a removal efficiency higher than 88 % and the NLR of SBBR2 reached to 1.43 kg?N/(m³/day) with a removal efficiency of 86 %. SBBR2 was more stable compared to SBBR1. These results, combined with molecular techniques such as scanning electron microscope, fluorescence in situ hybridization, and terminal restriction fragment length polymorphism, indicated that different genera of anammox bacteria became dominant. This research also demonstrates that SBBR is a promising bioreactor for starting up and enriching anammox bacteria.     

Application of UASB-reactors to industrial waste-water treatment; performance data and results in granulation control

Several types of high organic matter pollutants containing (COD-range: 3–50 kg · m^?3) industrial waste waters were treated in laboratory scale (1.2–23 dm³) sludge blanket (UASB) and UASB-fixed bed hybrid (UBF) reactors. In most cases higher than 80% of COD-removal efficiency has been attained. The CO₂ content of the biogas developed was mainly influenced by the neutralization (base to acid) ratio related to feed pH. Cell immobilization by granule formation was considered as a change in microbial population: enrichment and aggregate formation of Methanotrix-like filamentous microorganisms. Based on physiological and physical indexes of microbial selection and with regard to the different sensitivities of microorganisms to substrate inhibition, a new start-up method was developed for rapid (40–45 days) granulation of raw digested sludge.     

Reactor comparison and scale-up for the microaerobic production of 2,3-butanediol by Enterobacter aerogenes at constant oxygen transfer rate

Stirred tank (STR), bubble column (BCR) and airlift (ALR) bioreactors of 0.05 and 1.5 m³ total volume were compared for the production of 2,3-butanediol using Enterobacter aerogenes under microaerobic conditions. Batch fermentations were carried out at constant oxygen transfer rate (OTR=35 mmol/lh). At 0.05 m³ scale, the STR reactor achieved much higher biomass and product concentrations than the BCR and ALR reactors. At 1.5 m³ scale, however, exactly the same biomass and product concentrations could be obtained in both STR and ALR reactors. The 1.5 m³ ALR reactor performed also much better than its counterpart at small scale, achieving a productivity 2.4-fold as high as that of the 0.05 m³ BCL and ALR reactors. No differences in performances were observed between BCR and ALR. As compared to STR the tower reactors have a 12 time higher energetic efficiency (referred to product formation) and thus should be the choice for large scale production of 2,3-butanediol.The criterion of constant OTR or constant k _L a is not applicable for the scale-up of this oxygen-sensitive culture due to strong influence of reactor hydrodynamics under microaerobic conditions. The effects of mixing and circulation time on growth and metabolism of E. aerogenes were quantitatively studied in scaled-down experiments with continuous culture. For a successful scale-up of this microaerobic culture it is necessary to have an homogeneous oxygen supply over the entire reactor volume. Under conditions of inhomogeneous oxygen supply an optimum liquid circulation time exists which gives a maximum production of 2,3-butanediol.List of Symbols BD 2,3-butanediol - [mmol/l] saturation value of dissolved oxygen - D [h^–1] dilution rate - D [mm] reactor diameter - D _K [mm] top section diameter - D _R [mm] stirrer diameter - D _S [mm] draft tube diameter - EtOH ethanol - E _P [kg/kWh] energy efficiency refered to product formation - H [mm] height of reactor - HAc acetate - H _L [mm] height of liquid - k _L a [h^–1] volumetric oxygen transfer coefficient - N [rpm=min^–1] stirrer speed - OTR [mmol/lh] oxygen transfer rate - OUR [mmol/lh] oxygen uptake rate - p [Pa] pressure - P [kW] power input - P/V _L [kW/m³] specific power input - [mmHg] oxygen partial pressure (mmHg) or - [mmol/l] dissolved oxygen (mmol/l) - [mmol/gh] specific oxygen uptake rate - q _P [mmol/gh] specific productivity - R [Nm/kgK] gas constant, R = 287.06 - RQ respiration quotient - t _c [s] liquid circulation time - T [°C or K] temperature - TCA tricarboxylic acid - u _G [cm/s] mean superficial gas velocity - v _G [m/s] gas velocity at nozzels of gas distributor - V_G [l/h] aeration rate at inlet - V [m³ or l] total volume - V _L [m³ or l] liquid volume - V _N [l/mol] gas mole volume under normal conditions, V _N = 24.4116 - X [g/l] biomass concentration - CO₂ mole fraction in the effluent gas - O₂ mole fraction in the effluent gas - inlet (above the gas distributor) - ratio of oxygen consumed through TCA cycle to the total oxygen uptake rate - [g/l or kg/m³] density - [%] degree homogeneity - outlet of fermenter or top of the dispersion phase Dedicated to the 65th birthday of Professor Fritz Wagner.We thank Dr. C. Posten and T. Gabel for support with the computer control system UBICON. T.-G. Byun gratefully acknowledges financial support by DAAD.     

Reactor design for the enzymatic isomerization of glucose to fructose

A comprehensive methodology is presented for the design of reactors using immobilized enzymes as catalysts. The design is based on material balances and rate equations for enzyme action and decay and considers the effect of mass transfer limitations on the expression of enzyme activity. The enzymatic isomerization of glucose into fructose with a commercial immobilized glucose isomerase was selected as a case study. Results obtained are consistent with data obtained from existing high-fructose syrup plants. The methodology may be extended to other cases, provided sound expressions for enzyme action and decay are available and a simple flow pattern within the reactor might be assumed.List of Symbols C kat/kg specific activity of the catalyst - D m²/s substrate diffusivity within the catalyst particle - Dr m reactor diameter - d d operating time of each reactor - E kat initial enzyme activity - E _i kat initial enzyme activity in each reactor - F m³/s process flowrate - F _i m³/s reactor feed flowrate at a given time - F ₀ m³/s initial feed flowrate to each reactor - H number of enzyme half-lives used in the reactors - K mole/m³ equilibrium constant - K _S mole/m³ Michaelis constant for substrate - K _P mole/m³ Michaelis constant for product - K _m mole/m³ apparent Michaelis constant f(K, K _s, K_p, s₀) - k mole/s · kat reaction rate constant - k _d d^–1 first-order thermal inactivation rate constant - L m reactor height - L _r m height of catalyst bed - N _R number of reactors - P _i kg catalyst weight in each reactor - p mole/m³ product concentration - R m particle radius - R _P ratio of minimum to maximum process flowrate - r m distance to the center of the spherical particle - s mole/m³ substrate concentration - s _0i mole/m³ substrate concentration at reactor inlet - s ₀ mole/m³ bulk substrate concentration - s mole/m³ apparent substrate concentration - T K temperature - t d time - t _i d operating time for reactor i - t _s d time elapsed between two successive charges of each reactor - V m³ reactor volumen - V _m mole/m³ s maximum apparent reaction rate - V _p mole/m³ s maximum reaction rate for product - V _R m³ actual volume of catalyst bed - V _r m³ calculated volume of catalyst bed - V _S mol/m³ s maximum reaction rate for substrate - v mol/m³ s initial reaction rate - v _i m/s linear velocity - v _m mol/m³ s apparent initial reaction rate f(K_m, s,V_m) - X substrate conversion - X _eq substrate conversion at equilibrium - =s/K dimensionless substrate concentration - ₀=s₀/K bulk dimensionless substrate concentration - _eq=s_eq/K dimensionless substrate concentration at equilibrium - local effectiveness factor - mean integrated effectiveness factor - Thiéle modulus - =r/R dimensionless radius - _s kg/m³ hydrated support density - substrate protection factor - s residence time     

Inoculum effects on community composition and nitritation performance of autotrophic nitrifying biofilm reactors with counter‐diffusion geometry

The link between nitritation success in a membrane‐aerated biofilm reactor (MABR) and the composition of the initial ammonia‐ and nitrite‐oxidizing bacterial (AOB and NOB) population was investigated. Four identically operated flat‐sheet type MABRs were initiated with two different inocula: from an autotrophic nitrifying bioreactor (Inoculum A) or from a municipal wastewater treatment plant (Inoculum B). Higher nitritation efficiencies (NO₂^‐‐N/NH₄⁺‐N) were obtained in the Inoculum B‐ (55.2–56.4%) versus the Inoculum A‐ (20.2–22.1%) initiated reactors. The biofilms had similar oxygen penetration depths (100–150 µm), but the AOB profiles [based on 16S rRNA gene targeted real‐time quantitative PCR (qPCR)] revealed different peak densities at or distant from the membrane surface in the Inoculum B‐ versus A‐initiated reactors, respectively. Quantitative fluorescence in situ hybridization (FISH) revealed that the predominant AOB in the Inoculum A‐ and B‐initiated reactors were Nitrosospira spp. (48.9–61.2%) versus halophilic and halotolerant Nitrosomonas spp. (54.8–63.7%), respectively. The latter biofilm displayed a higher specific AOB activity than the former biofilm (1.65 fmol cell^?1 h^?1 versus 0.79 fmol cell^?1 h^?1). These observations suggest that the AOB and NOB population compositions of the inoculum may determine dominant AOB in the MABR biofilm, which in turn affects the degree of attainable nitritation in an MABR.     

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.     

Sodium/potassium selectivity and pleiotropy in stl2, a highly salt-tolerant mutation of Ceratopteris richardii

The roles of Na⁺ and K⁺ (Rb⁺) uptake were further studied in a NaCl-tolerant strain of Ceratopteris richardii containing the stl2 mutation by direct comparison with the wild-type strain. In addition to Na⁺ tolerance, stl2 also confers tolerance to Mg²⁺ and sensitivity to K⁺. In addition to higher K⁺ (Rb⁺) uptake at concentrations commonly associated with low-affinity K⁺ transport, stl2 maintained higher uptake down to 0·1 mol m^–3 Rb⁺. Up to a 25-fold excess of Na⁺ had little effect in either genotype on K⁺ (Rb⁺) uptake at low concentrations, i.e. 0·2 and 0·5 mol m^–3 RbCl. Pretreatment with K⁺ (20 mol m^–3) inhibited uptake of K⁺ (Rb⁺) in the wild type, whereas concurrent inclusion of K⁺ inhibited uptake of Rb⁺ more in stl2. In the absence of K⁺, Na⁺ uptake (0·01–60 mol m^–3) was nearly identical in the wild type and stl2. K⁺ inhibited Na⁺ uptake more effectively in stl2 than the wild type, especially at 60 mol m^–3 Na⁺. Greater inhibition of K⁺ uptake in stl2 occurred with MgCl₂ or TEA (tetraethylammonium chloride) preincubation or with simultaneous inclusion of Al³⁺ (Al₂SO₄). The higher effective velocity of K⁺ uptake at a wide range of concentrations and the enhanced selectivity for K⁺ and against Na⁺ contribute to the preservation of higher cytosolic K⁺ and lower Na⁺ under salinity stress.