Suspended Biomass Concentration Research Articles

Understanding the role of polyurethane sponges on rapid formation of aerobic granular sludge and enhanced nitrogen removal

Long formation time and limited simultaneous nitrification–denitrification (SND) are still the problems for aerobic granular sludge (AGS) application. This study presents a novel strategy to simultaneously promote sludge granulation and enhance nitrogen removal. This new approach consists of adding polyurethane sponges (PUS) and applying intensive aeration periodically. To demonstrate the effectiveness of this strategy and understand the role of PUS, two sequencing batch reactors (R1: without PUS; R2: PUS addition) were used. The results show that the granulation time in R2 (14 days) was much shorter and the nitrogen removal (80.7 %) was much higher when compared to the R1 (75 days and 64.8 %). Biofilm was rapidly formed on PUS because of the porous network structure, resulting in the low suspended biomass concentration. Thus, a close to discrete non-flocculent settling type was created, which promoted the selection of well-settling aggregates rapidly and efficiently. Meanwhile, biofilm was detached from PUS by intensive aeration, providing the nucleus (fragments of biofilm) in-suit for microbial adhesion. Under the combined effects, the aerobic granulation process was significantly accelerated. The total biomass amount was maintained during initial granulation because of the rapid biofilm formation, resulting in the stable removal of NH4+-N at a rapid start-up period. The rapid formation of biofilm and fast granulation created more anoxic zones during aeration and thereby enhanced SND. The biofilm formation also increased the relative abundances of aerobic denitrifiers (Dokdonella and Denitratisoma), autotrophic denitrifiers (Sulfuritalea), and hydrolytic/fermentativebacteria (Ferruginibacter, Opitutus, and Lactobacillus), which enhanced the SND performance and endogenous denitrification.

Fate of Emerging Contaminants in an Integrated Fixed-Film Activated Sludge Plant

Conventional sewage treatment techniques have shown low removal efficiency for emerging contaminants (ECs). Hence, more efficient treatment systems need to be investigated. The current study suggests that an Integrated Fixed-Film Activated Sludge Process (IFAS) could be one such alternative. The degradation of eight ECs in municipal sewage has been studied during the transition of a pilot IFAS plant to a steady state and compared with the removal of the eight ECs in a conventional sequencing batch reactor (SBR)–based sewage treatment plant at a steady state. In the six weeks before attaining a steady state, the outlet total biochemical oxygen demand, total chemical oxygen demand, and total suspended solids (TSS) decreased from 42 to 10 mg/L, 96 to 26 mg/L, and 38 to 10 mg/L, respectively. The suspended biomass concentration, hydraulic retention, and return sludge rate of the plant were 1,600 mg/L, 6.9 h, and 175% of inflow, respectively. During the transition to a steady state in the IFAS plant, the removal efficiency of Caffeine, Gemfibrozil, and Testosterone was found to increase by 41% (15%–56%), 26% (63%–89%), and 65% (19%–84%) respectively, due to enhanced biological degradation. The SBR was found to be more efficient in the removal of ECs, which could be due to the higher concentration of suspended biomass.

High-rate biohydrogen production from xylose using a dynamic membrane bioreactor

This study aimed a high-rate dark fermentative H2 production from xylose using a dynamic membrane module bioreactor (DMBR) with a 444-μm pore polyester mesh. 20 g xylose/L was fed continuously to the DMBR at different hydraulic retention times (HRTs) from 12 to 3 h at 37 °C. The maximum average H2 yield (HY) and H2 production rate (HPR) at 3 h HRT were found to be 1.40 ± 0.07 mol H2/mol xyloseconsumed and 30.26 ± 1.19 L H2/L-d, respectively. The short HRT resulted in the maximum suspended biomass concentration (8.92 ± 0.40 g VSS/L) along with significant attached biomass retention (7.88 ± 0.22 g VSS/L). H2 was produced by both butyric and acetic acid pathways. Low HY was concurrent with lactic acid production. The bacterial population shifted from non-H2 producers, such as Lactobacillus and Sporolactobacillus spp., to Clostridium sp., when HY increased. Thus, xylose from lignocellulose is a feasible substrate for dark fermentative H2 production using DMBR.

The importance of integrated fixed film activated sludge reactor and intermittent aeration in nitritation-anammox systems: Understanding reactor optimization for lagoon supernatant treatment

A single stage nitritation-anammox process was studied in a continuous flow reactor fed with raw lagoon municipal-sludge supernatant containing 835 ± 45 mg N L−1 of NH4+-N in the feed. Reactor operation was divided into five periods for a total of 438 days. Application of intermittent aeration with a 105 min aeration phase followed by a 15 min non-aeration phase reduced the aerobic nitritation rate by 13% but enhanced the anammox activity by 1.9 times. Under the dual effect of IFAS and intermittent aeration, the nitrogen removal rate (NRR) was improved from 0.12 ± 0.012 kg N (m3 d)−1 to 0.28 ± 0.03 kg N (m3 d)−1, representing an increased removal of NH4+ from 56 ± 4.3% to 92.2 ± 2.1%. The evolution of bacterial activity and communities showed that anammox activity significantly improved with the addition of the non-aeration phase, and the AOB population was largely improved with an increase of suspended biomass concentration in the reactor. Microbial community composition through 16S rRNA gene sequencing analysis showed that Nitrosomonas was the predominant AOB in the flocs phase, while Candidatus Brocadia was the dominant anammox bacteria in the biofilm phase.

Possible mechanism of efficient mainstream partial nitritation/anammox (PN/A) in hybrid bioreactors (IFAS)

ABSTRACT An explanation of possible mechanism of efficient PN/A in hybrid bioreactors was presented. The bottleneck process is nitritation. Surplus nitrite production by ammonium oxidizing bacteria (AOB) is required for assuring the activity of anammox bacteria and eliminating nitrite oxidizing bacteria (NOB). It will be possible if nitrogen removal rate by AOB (r N_AOB) is higher than NOB (r N_NOB). It was shown that in biofilm AnAOB bacteria should out-compete NOB, whereas nitrogen transformation rates by AOB are usually lower than NOB. However, the growth of r-AOB in activated sludge allows out-selecting NOB. Impact of ammonium-, nitrite–nitrogen and suspended biomass concentration in hybrid PN/A systems on nitrogen removal rates in the temperature ranges from 10°C to 25°C was presented and discussed. Because bulk liquid ammonium nitrogen concentration can be higher in SBR bioreactors (after certain period of time after aeration starts) or in the initial zones of plug-flow systems than in fully mixed systems, conditions for running efficient PN/A are more favourable in intermittently aerated ‘IFAS-SBR’ or ‘IFAS-plug flow’ bioreactors.

Accurate evaluation for the biofilm-activated sludge reactor using graphical techniques

A complete graphical solution is obtained for the completely mixed biofilm-activated sludge reactor (hybrid reactor). The solution consists of a series of curves deduced from the principal equations of the hybrid system after converting them in dimensionless form. The curves estimate the basic parameters of the hybrid system such as suspended biomass concentration, sludge residence time, wasted mass of sludge, and food to biomass ratio. All of these parameters can be expressed as functions of hydraulic retention time, influent substrate concentration, substrate concentration in the bulk, stagnant liquid layer thickness, and the minimum substrate concentration which can maintain the biofilm growth in addition to the basic kinetics of the activated sludge process in which all these variables are expressed in a dimensionless form. Compared to other solutions of such system these curves are simple, easy to use, and provide an accurate tool for analyzing such system based on fundamental principles. Further, these curves may be used as a quick tool to get the effect of variables change on the other parameters and the whole system.

Influence of various operating conditions on wastewater treatment in an AS-biofilm reactor and post-treatment using TiO2-based solar/UV photocatalysis

ABSTRACTIn the present study, the effect of carbon to nitrogen (C/N) ratio, suspended biomass concentration (X), hydraulic retention time (HRT) and dissolved oxygen (DO) on chemical oxygen demand (COD) and nutrient removal from wastewater was investigated in a lab-scale activated sludge (AS)-biofilm reactor. Furthermore, in order to improve the quality of the treated wastewater, photocatalysis by TiO2 was investigated as a post-treatment technology, using solar and UV irradiations. The AS-biofilm reactor provided a substantial removal efficiency in terms of COD, ammonia nitrogen , total nitrogen (TN) and total phosphorous when the system was maintained at C/N ratio 6.66, X in the range 2–2.5 g/L, HRT 10 h, DO in the range of 3.5–4.5 mg/L and organic loading rate (OLR) of 0.96 kg COD/m3 d during Run 1. Similarly, when the reactor was maintained at C/N ratio 10, X in the range of 3–3.5 g/L, HRT 8 h, DO in the range of 3.5–4.5 mg/L and OLR of 1.8 kg COD/m3 d during Run 2. The microstructure of suspended and attached biomass comprised a dense bacterial structure of cocci and bacillus microorganisms. The UV photocatalysis was found to be better than solar photocatalysis during the comparative analysis. The maximum removal efficiencies of COD, most probable number and phosphorous at optimum conditions in the case of UV and solar irradiations were 72%, 95%, 52% and 71%, 99%, 50%, respectively.

Biohydrogen production at pH below 3.0: Is it possible?

Biological hydrogen production was investigated in continuous acidogenic reactors fed with sucrose at 30 °C without pH control. In the first experimental phase, three reactors were compared: a structured fixed-bed (FB), a granular UASB (UG) and a flocculent UASB (UF-1). They were run at 3.3 h HRT and 33 gCOD L−1d−1 OLR. Hydrogen production occurred throughout the experimental period with an average effluent pH of only 2.8. The FB, UG and UF-1 reactors presented volumetric hydrogen production rates (VHPR) of 95 ± 69, 45 ± 37 and 54 ± 32 mLH2 L−1h−1, respectively; and H2 yields (HY) of 1.5 ± 0.8, 0.8 ± 0.6 and 1.2 ± 0.7 molH2 mol−1 sucroseconsumed, respectively. The UF-1 reactor showed intermediate VHPR and HY, but no declining trend, contrary to what was observed in the FB reactor. Thus, aiming at continuous and long-term H2 production, a flocculent UASB was applied in the second experimental phase. In this phase, the HRT of the acidogenic reactor, which was named UF-2, was raised to 4.6 h, resulting in an OLR of 25 gCOD L−1d−1. The VHPR and the HY increased considerably to 175 ± 44 mLH2 L−1h−1 and 3.4 ± 0.7 molH2 mol−1 sucroseconsumed, respectively. These improvements were accompanied by greater sucrose removal, higher suspended biomass concentration, less production of lactate and more of acetate, and high ethanol concentration. Contradicting the current published literature data that reports strong inhibition of H2 production by dark fermentation at pH less than 4.0, the UF-2 reactor presented stable, long-term H2 production with satisfactory yields at pH 2.7 on average. 16 S rDNA sequencing revealed that two sequences assigned as Ethanoligenens and Clostridium accounted for over 70% of the microbiota in all the reactors. The non-necessity of adding alkalizing agents and the successful H2 production under very acid conditions, demonstrated in this study, open a new field of investigation in biological hydrogen production by dark fermentation towards a more sustainable and feasible technology.

A novel application of anaerobic bio-entrapped membrane reactor for the treatment of chemical synthesis-based pharmaceutical wastewater

Pharmaceutical wastewaters are mainly generated by chemical-synthetic industries and usually contain high chemical oxygen demand (COD) and toxicity. A novel lab-scale anaerobic bio-entrapped membrane reactor (AnBEMR) packed with bio-ball carriers was constructed and compared with an anaerobic membrane bioreactor (AnMBR). The aims of this study are to investigate the treatment performances and evaluate the stability of the membrane filtration of the AnBEMR and AnMBR under the conditions of a high organic loading rate (OLR), ranging from 8.0 to 36.7kgCOD/m3d, and high total dissolved solids (TDS), ranging from 18,411 to 25,925mg/L, in the treatment of chemical synthetic-based pharmaceutical wastewater. Four different hydraulic retention times (HRTs) of 10.6, 14.1, 21.3, and 42.6h were studied. Total COD (TCOD) removal efficiency of the AnBEMR ranged from 36.7% to 50.8% at an OLR of 8.7±0.7kgCOD/m3d, while the highest OLR (34.0±2.7kgCOD/m3d) only led to TCOD removal efficiency of 10.1–20.7%. The AnBEMR achieved approximately 5–10% higher TCOD removal efficiency than the AnMBR. The influences of HRT and TDS (salinity) were also studied. The results show an increase of trans-membrane pressure (TMP) with decreasing HRT in both MBRs, where extracellular polymeric substances (EPS), soluble microbial products (SMP) and suspended biomass concentration were significantly higher at the shortest HRT of 10.6h, resulting in faster membrane fouling. Membrane fouling was improved in the AnBEMR (i.e., longer membrane filtration operating periods) due to the production of lower concentrations of EPS and SMP (protein and carbohydrate contents). Proteins, rather than carbohydrates, were the major component of EPS and SMP in both reactors. The study concludes that the application of the AnBEMR could achieve better treatment performance and reduced membrane fouling by producing lesser EPS, SMP, and suspended biomass concentration.

Effect of hydrodynamic characteristics on the performance of biofilm for degrading phenol in inverse fluidized bed biofilm reactor

The influence of some hydrodynamic effects on the performance of biofilm in inverse fluidized bed biofilm reactor (IFBBR) was studied with low-density polystyrene support particles of various sizes (2.9, 3.5, and 3.8 mm) using Pseudomonas fluorescens for the degradation of phenol. The biofilm reactor was operated under different superficial air velocities for a fixed settled bed height of particles to study the effect of hydrodynamics on biofilm thickness, biofilm dry density, bioparticle density, and attached and suspended biomass concentrations for efficient biodegradation of phenol. There is evidence that the chemical oxygen demand reduction and phenol degradation efficiency were found to be high at the optimized superficial air velocity with controlled biofilm thickness and for a stable and dense biofilm dry density. The results of the study revealed that with increase in superficial air velocity, the biofilm thickness and bioparticle density decreases while the biofilm dry density and suspended biomass concentration increases. However, above a critical superficial velocity (optimal superficial velocity) the detachment force does not control the outgrowth of the biofilm anymore and the thickness increases rapidly with decreasing suspended biomass concentration. The optimal superficial velocity for better biodegradation of phenol was found to 0.240, 0.220, and 0.230 m/s for the particle sizes of 2.9, 3.5, and 3.8 mm, respectively. The particle size of 3.5mm has been found to be the optimal particle size for efficient biodegradation of phenol in IFBBR with better hydrodynamic effects and biofilm morphology.

Investigation of decolorization kinetics and biodegradation of azo dye Acid Red 18 using sequential process of anaerobic sequencing batch reactor/moving bed sequencing batch biofilm reactor

The purpose of this study was to investigate the application of anaerobic sequencing batch reactor (An-SBR)/moving bed sequencing batch biofilm reactor (MB-SBBR) for treatment of wastewater containing Acid Red 18 (AR18). Based on statistical analysis, COD removal rate and anaerobic biomass concentration were increased with increasing initial AR18 concentration, while the specific COD removal rate was not significantly different among An-SBRs. The results revealed that the decolorization followed first-order kinetics in An-SBR1 receiving AR18 with concentration of 100 mg l−1. However, in two other reactors with the initial dye concentration of 500 and 1000 mg l−1, the dye was removed following two successive parts with two different reaction orders. The results showed that the dye decolorization rate increased with increasing its concentration in the feed solution. In MB-SBBRs, the ratio of attached-growth biofilm to suspended biomass concentration was increased gradually during the operation period. This ratio was measured 0.76, 0.70 and 0.51 in MB-SBBR1, 2 and 3 after three months of the reactors operation. Microscopic observations showed a large number of filamentous bacteria in the structure of attached-growth biofilm and suspended biomass in MB-SBBRs which speculated to be the reason for the different Sludge Volume Index (SVI) observed among these reactors.

Application of a Hybrid Process with Biofilm and Suspended Biomass for Treating Petrochemical Wastewater

To improve the efficiency of petrochemical wastewater treatment, a hybrid system with both attached and suspended biomass was proposed to replace the existing biofilm process. According to the performances of the hybrid system, the optimal suspended biomass concentration in the tank was determined around 1000 mg/L. When the average influent COD and NH4+-N concentration were 395.7 mg/L and 13.5 mg/L, the according removal efficiencies of the hybrid system were up to 84% and 69%, compared to 74% and 20% when operated with the biofilm system. The GC-MS analysis showed that both the amounts and types of refractory organics in the effluent of hybrid system were greatly reduced when compared with those of the previous biofilm process. The foaming problem was also resolved along with the enhanced pollutants removal efficiency. The hybrid process was an economic and feasible alternative for petrochemical wastewater treatment, which ensured carbonaceous oxidation, nitrification and decomposition of refractory substances.

Evaluation of potential integration of entrapped mixed microbial cell and membrane bioreactor processes for biological wastewater treatment/reuse

The challenge of biological wastewater treatment process is the design and operation of effective retention of mixed microbial cells within the reactor. Entrapped mixed microbial cell (EMMC) technology is designed to entrap the mixed microbial cells in polymeric carriers; membrane bioreactor (MBR) process utilizes membrane sheets/fibers to effectively retain the biomass in the reactor. These two biotechnologies are considered potential alternatives for conventional biological treatment/reuse because of their capability of retaining high concentration of biomass in the reactor, or in other words increasing the solid retention time (SRT). The simultaneous removal of organics and nitrogen were investigated using a modified EMMC system design. The modified EMMC system demonstrated higher organic and nitrogen removal performance due to high SRT. Compared to single-stage MBR process operated at similar conditions, the modified EMMC system was able to achieve slightly lower organic removal, comparable nitrification, and higher total nitrogen removal. One limitation in applying an EMMC only treatment process regime for potential reuse of treated wastewater is that such an operation requires the removal of pathogens and large particles if disinfection and solid/liquid separation were not followed. The major challenge of MBR process to overcome is membrane fouling, and the high energy consumption associated with fouling control. The intrinsic features of EMMC process including high SRT, low, and stabilized effluent suspended biomass concentration may significantly reduce the chance and extent of membrane fouling; while the membrane filtration can further polish the effluent quality from EMMC process. Therefore, integrating MBR and EMMC is strongly recommended because it may be a “break-through” for solving the membrane fouling problem and in improving effluent quality for potential reuse.

Mathematical Model for the Biofilm–Activated Sludge Reactor

A steady state mathematical model is developed for describing the completely mixed biofilm–activated sludge reactor (hybrid reactor). The model is derived by simultaneously considering Monod kinetics expressions and Fickian’s diffusion theory for substrate in biofilm. In addition, it includes the basic concepts, which describe both culture (suspended and attached) and the competition between them for limiting substrate. By using this model the suspended biomass concentration can be obtained for this system. Subsequently, the other remaining parameters of the system can be computed. Therefore it helps to design and operate the hybrid reactor under different conditions for any given set of kinetic parameters. The utility of the model has been explained for a given set of data and verified by comparing with another solution. It is found that for the same set of data, the model is accurate in the results. The model has been presented in more than one form, each form having an explicit solution of the system. ...

Sludge Production and Settleability in Biofilm-Activated Sludge Process

The sludge production and settleability have been estimated experimentally in a completely mixed biofilm-activated sludge reactor (hybrid reactor). A steady-state hybrid reactor was run at different stages of suspended biomass concentration (X) under constant values of influent substrate concentration ( So ) and hydraulic retention time (HRT). The values of X were gradually decreased in these stages until the system completely washed out of the suspended biomass and converted to pure biofilm reactor. As a result, the role of biofilm in the treatment gradually increased with an increase in the effluent substrate concentration (S) . The experiment was supported by a mathematical expression for describing the sludge yield in the system under the previous conditions. The experimental and theoretical studies in the present work reveal that there is a critical phase of the hybrid system at which the system produces a high rate of excess sludge. That critical phase is found at a specific ratio between the suspen...

A simplified model for the steady-state biofilm-activated sludge reactor

A simplified mathematical model is proposed to describe the steady-state completely mixed biofilm-activated sludge reactor (hybrid reactor). The model is derived based on Monod kinetic expressions and the Fickian diffusion law in biofilm. In addition, it considers all the essential concepts that describe the two types of growth (suspended and attached) and the competition between them for limiting substrate. Also the present study has been extended to investigate simple and accurate mathematical expressions for describing the substrate diffusion in biofilm ( J). The expression for substrate flux has an explicit solution, which may be useful in the proposed model and many other applications. The application of the model for the hybrid system has been explained for a given set of data and verified by comparison with another solution. Also the model was applied to experimental results for a trace level of suspended biomass concentration ( X). It was found that the biofilm flux ( J) is the key factor in the model prediction, hence the accuracy of the model output is influenced by the accuracy of J. Compared with other solutions for such systems the model is simple, easy to use, and provides an accurate tool for describing such systems based on fundamental principles.

Characteristics of phosphorus removal from wastewater by biofilm sequencing batch reactor (SBR)

Sequencing batch reactor (SBR) of biofilm formed on fibrous carriers was effective for phosphorus removal from wastewater when conditions included a 30% packing ratio for the carriers and 9 h hydraulic retention time (3 h for the anaerobic reaction phase and 6 h for the aerobic reaction phase). After 6 months of operation at chemical oxygen demand (COD) loading of 1.00 kg COD/m 3 per day, the biomass concentration in the biofilm reached a high level of 8855 mg/l in terms of MLSS and 5532 mg/l in terms of MLVSS, while as the suspended biomass concentration in the liquid phase of the reactor, it was almost 60 mg/l in terms of MLSS. In every cycle, phosphorus removal was achieved by removing the biomass detached from the biofilm (excess sludge) after 20 min sedimentation. When the influent COD loading rate increased from 0.27 to 1.32 kg COD/m 3 per day, the phosphorus removal efficiency surpassed 90%, and this process showed a strong tolerance for the increase of COD loading. The dominant bacterial species were found to be Pseudomonas, Aeromonas and Bacillus. The excess sludge detached from the biofilm had a high phosphorus content of 5.67% (w/w) in dry biomass and good settling performance with an SVI of 99.5–103.8 ml/g.

A model for oxygen consumption in aerobic heterotrophic biodegradation in dual-phase drainage systems

A mathematical model with two kinetics variations was developed for the description of oxygen consumption in aerobic heterotrophic processes in longitudinal aqueous dual-phase systems where biomass is suspended in the liquid and attached on the solid material surface. This pertains to sewers, drains, rivers and certain wastewater treatment reactors. The model parameters were related to the liquid and solid phases, such as suspended biomass concentration X, its maximum growth rate μ max, water volume V, biofilm oxygen up-take rate BUR, and the biofilm covered area WA. The Monod equation-based zero and first order kinetics were applicable depending on organics concentration. Simulation demonstrated the influence of relevant parameters on the oxygen profile. It illustrated that the suspended biomass can play a critically important role, which generally should not be neglected; the relative importance of the two biomass types is system dependent, and can be quantitatively assessed on the basis of the parameters mentioned above. The potential application of the model is discussed.

Growth of an aerobic and an anoxic toluene-degrading biofilm - a comparative study

The growth dynamics of a toluene-degrading biofilm was investigated under aerobic and under nitrate-reducing (anoxic) conditions. The study was performed with a continuously fed biofilm reactor (biodrum system). Under denitrifying conditions, the maximum toluene degradation in the reactor was achieved during the exponential growth phase of the biofilm. Under aerobic conditions, however, the maximum toluene degradation was reached during the lag phase of biofilm growth. Meanwhile, a simultaneous increase of the suspended biomass concentration in the bulk occurred concomitant with the toluene removal. The comparison of the structure of the aerobic and anoxic biofilms indicated that the anoxic biofilm was smooth and regular whereas the aerobic biofilm showed a lot of irregularity and a filamentous structure on top of the biofilm. Both the aerobic and the anoxic growth were modelled using the computer programme BIOSIM from EAWAG. The models considered active and inactive biomass. The active biomass was growing on toluene as sole carbon source, whereas the inactive biomass was composed of polymers and dead cells unable to degrade toluene. Model simulations showed that the biofilm activity was mainly concentrated in the top-layer of the biofilm under both aerobic and anoxic conditions.

Growth of an aerobic and an anoxic toluene-degrading biofilm - a comparative study

The growth dynamics of a toluene-degrading biofilm was investigated under aerobic and under nitrate-reducing (anoxic) conditions. The study was perfonned with a continuously fed biofilm reactor (biodrum system). Under denitrifying conditions, the maximum toluene degradation in the reactor was achieved during the exponential growth phase of the biofilm. Under aerobic conditions, however, the maximum toluene degradation was reached during the lag phase of biofilm growth. Meanwhile, a simultaneous increase of the suspended biomass concentration in the bulk occurred concomitant with the toluene removal. The comparison of the structure of the aerobic and anoxic biofilms indicated that the anoxic biofilm was smooth and regular whereas the aerobic biofilm showed a lot of irregularity and a filamentous structure on top of the biofilm. Both the aerobic and the anoxic growth were modelled using the computer programme BIOSIM from EAWAG. The models considered active and inactive biomass. The active biomass was growing on toluene as sole carbon source, whereas the inactive biomass was composed of polymers and dead cells unable to degrade toluene. Model simulations showed that the biofilm activity was mainly concentrated in the top-layer of the biofilm under both aerobic and anoxic conditions.