COD Removal Kinetics Research Articles

UV light and [formula omitted] catalysed COD removal of AO 8 using NaOCl as oxidant

The present study aims to establish NaOCl as a potential oxidant in the COD removal of Acid Orange 8 using UVC light (λ = 254 nm) and Fe2+ as catalysts. The different systems used in this study are NaOCl, Fe2+/NaOCl, UV/NaOCl, and Fe2+/NaOCl/UV. All these process were found to be operative in acidic, neutral and basic medium. The initial decolorisation and COD removal efficiency (CODeff) for different systems follow the order: Fe2+/NaOCl/UV > UV/NaOCl >Fe2+/NaOCl > NaOCl. Nevertheless, NaOCl can alone be used in the treatment process considering its CODeff to the extent of 95% in 90 min. The change in pH of the solutions after treatment is an important observation – for non-UV systems it remained around 11.0 and 7.0 in other systems. Thus, UV systems are environmental benign. The effect of various anions on CODeff was tested in Fe2+ systems. Presence of F− ions were found to accelerate CODeff in both the systems. However, the effect is more pronounced in Fe2+/ NaOCl/UV, where complete CODeff was observed in the presence of 9.0 gl−1 of F−. The COD removal kinetics for all systems was studied using zero–order, first–order, second–order, and BMG kinetic models. BMG model was found to be more suitable among all and is in good agreement with CODeff of all systems. It is, therefore, established that NaOCl can serve as a powerful oxidant in the advanced oxidation process.

Two-phase (acidogenic-methanogenic) anaerobic fixed bed biofilm reactor enhances the biological domestic sewage treatment: Perspectives for recovering bioenergy and value-added by-products.

The organic matter bioconversion into methane during anaerobic digestion (AD) comprises different steps, the acidogenic and methanogenic phases being clearly distinct in terms of metabolic activities. In this work, new configurations of anaerobic fixed bed biofilm reactors (AFBBR) were operated under conventional methanogenic conditions (single phase - SP-AFBBR, M1R), and in a sequential two-phase system, acidogenic reactor followed by methanogenic reactor (TP-AFBBR, AcR+M2R), in order to verify the impact of the AD phase separation on the overall system performance in operational, kinetics and microbiological aspects. The results indicated that feeding the methanogenic reactor with the acidogenic effluent stream provided a shorter operating start-up period (11 and 32 days for SP and TP-AFBBR, respectively), a greater alkalinity generation (0.14 and 0.41g-CaCO3·g-CODremoved-1 for M1R and M2R, respectively), and the optimization of biomethane production (methane yield of 95 and 154N-mLCH4·g-CODremoved-1 for M1R and M2R, respectively). The COD removal kinetics was also favored in the TP-AFBBR (k1-COD=1.4 and 2.9 h-1 for M1R and M2R, respectively), since the soluble fermentation products were readily bioavailable to the biomass in the reactor. Hydrogenotrophic methanogenesis was the predominant pathway in the M2R, while the Methanosaeta-driven acetoclastic pathway predominated in the M1R. The greater diversity of Bacteria and Archaea in M2R denotes a better balance between the species that degrade volatile organic acids from AcR (i.e. Syntrophorhabdus, Syntrophus and Syntrophobacter) and the hydrogenotrophic methanogens (Methanoregula, Methanolinea and Methanospirillum) that consume the biodegradation products. The estimated bioenergy generation potential (range of 0.39-0.64kWh·m-3-sewage considering the COD removed) for full-scale TP-sewage treatment plants evidences the feasibility of energetic recovery in the domestic sewage anaerobic treatment.

Anaerobic domestic wastewater treatment in a sequencing granular UASB bioreactor: Feasibility study of the temperature effect on the process performance

A laboratory-scale sequencing granular up-flow anaerobic sludge blanket (UASB) reactor was applied for the treatment of synthetic domestic wastewater at 15, 25 and 35 °C. The experiments were performed at different hydraulic retention times (gradually decreasing from a maximum of 22 h to a minimum of 9 h). Results at 25 and 35 °C showed similar COD removal efficiency and specific biogas production in the range of 84–94% and 0.14–0.27 m3/kgCODremoved, respectively. At 15 °C lower COD removal kinetics were observed, and a minimum hydraulic retention time of 14 h was necessary to obtain an effluent in accordance with current Italian legislation. At all temperatures, high quality effluent was achieved in terms of total suspended solids (TSS) concentration, while nitrogen and phosphorus exceeded the Italian law limits, as they are not removed during the anaerobic digestion process. Moreover, dissolved methane in the liquid phase was analysed: methane lost with the effluents was on average ≈ 37% at 25 °C and ≈ 24% at 35 °C of the produced amount, thus revealing higher losses at lower temperatures for the increase of gas solubility. These results show the need of research on technologies aimed at removing or recovering such energetic greenhouse gas. However, effective biogas production and potentialities for nutrients’ recovery demonstrated in this study confirm the feasibility of the anaerobic process as sustainable treatment of low-strength wastewaters.

Effect of controlled and uncontrolled pH on tannery wastewater treatment by the electrocoagulation process

ABSTRACT In this study, the effects of the pH of tannery wastewater obtained from a local leather processing factory on the removal of COD and turbidity, and cost of theelectrocoagulation process was investigated. Studies were carried out under two different operatingconditions. The highest COD removal efficiency was obtained as 83.5% when the initial pH was set to 3, while the energy consumption at this pH was found to be 3.88 kwh m−3. In studies where pH was kept constant, the highest removal efficiency obtained was 83.33% at pH = 5. In this case, the energy consumption was determined as 2.44 kWh m−3. The total cost of the system was calculated as 1.0899 $ m−3 under controlled pH conditions and 0.8761 $ m−3with uncontrolled pH. Studies show that the COD removal kinetics are more suited to 2nd degree kinetics. The highest removal rate for uncontrolled pH was k2 = 0.0791 L g−1 min−1 at pH = 3. The highest rate for controlled pH was k2 = 0.0397 L g−1 min−1 at pH = 5.

Removal of chemical oxygen demand from industrial estate sewage over hybridized anatase-graphene oxide-carbon nanotubes nanocomposite under solar irradiation

The aim of present work was to investigate the performance of hybridized materials, containing anatase, graphene oxide (GO), and carbon nanotubes (CNTs) in the photocatalytic treatment of sewages discharged from the industrial estate. The obtained results demonstrated the ability of hybridized nanocomposite for the solar treatment of sewages collected from different stages of industrial recovery process, between upstream of anaerobic system, and downstream of sand filter. The appropriate proportions of GO, and CNTs were determined, as 3.33 %. The effects of variables like initial COD, irradiation time, sewage collection position, and pH were evaluated on the treatment performance. If the initial COD is controlled at the level of 780 mg.l−1, the COD removal efficiency reaches to ∼90 % which is the prohibitive condition in the industrial scale. The maximal photoactivity was achieved within 20 min by the control of sewage pH at the level of 8. The COD removal kinetics were related to the load of anatase nanoparticles, < 20 nm, specific surface area, 60 m2. g−1, and band gap energy, 2.1 eV, of prepared nanocomposite which is a promising material for the wastewater cleaning in the real condition.

Electron flow assisted COD removal in wastewater under continuous flow conditions using microbial electrochemical system

Microbial electrochemical system (MES) was an emerging technology for synchronous power generation and pollution removal. During current generation, the COD removal performance of air-cathode MES fed with domestic wastewater was greatly enhanced, compared with the condition absent of current. Based on the simulation of first-order kinetics, the COD removal rate was investigated with the increasing of current densities from 0 to 1.61 ± 0.13 A m−2 by external resistances regulation. In this process, the overall COD removal kinetics constants were increased by 110% from 0.075 ± 0.005 to 0.157 ± 0.010 h−1. The background COD reduction unrelated to current generation was greatly stimulated by the initial rising of current density (from 0 to 0.273 ± 0.003 A m−2) and then limited by the substrate competition with exoelectrogens. The effluent with low COD concentration was found to restrict the power output of downstream anodes with a COD limit level of 200 mg L−1 and SCOD of 120 mg L−1. In MES, only a small fraction (< 3.8%) of energy could be harvest as electricity. Nevertheless, very little energy (< 7.2%) from reduced COD was used for biomass synthesis, while most of the reduced COD was mineralized by microbial metabolism. The MES was observed a low sludge yield of 0.028 ± 0.006 g-sludge g−1-COD with a sludge retention time of 30 days. Therefore, the low sludge yield and electron-flow-assisted COD removal should be recognized as the two main advantages of MES rather than power generation in wastewater treatment.

Comparative study on treatment of methylene blue dye wastewater by different internal electrolysis systems and COD removal kinetics, thermodynamics and mechanism

In this study, Fe/Cu, Fe/Al/Cu, Fe/Cu/C and Fe/Al/Cu/C internal electrolysis systems (IESs) were constructed and used to treat methylene blue dye (MB) wastewater. The effects of filler mass ratio, filler dosage, solution pH, reaction time and reaction temperature on COD removal were discussed, while the kinetics, thermodynamics and mechanism of COD removal were also investigated. The results showed that when the COD removal rates were basically the same, the reaction times of Fe/Al/Cu, Fe/Cu/C and Fe/Al/Cu/C IESs were shorter, and the filler dosages were lower. For the four systems, the appropriate pH was around 5, while the suitable reaction temperature was in the range of 20–25 °C. The COD removals of these four IESs were generally greater than 90%. The COD removal processes of the four systems could be better described by the improved pseudo-second-kinetic model, and the liquid film diffusion was the rate-controlling step. Moreover, the COD removal was a spontaneous and endothermic process. MB was degraded into inorganic substances in four steps. In addition, the FTIR characterization of the fillers before and after reaction suggests the four IESs have good stability.

BIOREACTOR LANDFILLS: COMPARISON AND KINETICS OF THE DIFFERENT SYSTEMS

The need for more sustainable landfilling has increased interest in bioreactor landfills as a suitable tool for optimising degradation processes. Bioreactors can be categorised as follows: anaerobic, aerobic, semi aerobic and hybrid. The choice of a specific bioreactor can be strongly influenced by the desired treatment objectives (i.e., energy recovery, increased rate of waste stabilisation, washing) as well as by the specific site conditions (e.g., waste characteristics, climate and social/economic situation, regulations). However, the increased rate of waste stabilisation should be the primary driving principle in the bioreactor landfill design (Cossu, 2010). Full-scale bioreactor landfills are still uncommon and one of the reasons is the perception that the effectiveness of this technology is not well demonstrated. This paper aims to contribute to filling this knowledge gap by analysing and comparing the lab scale applications of different types of bioreactors available in the literature and providing a survey of the different methods by considering their respective advantages and disadvantages. Qualitative analysis of the main types of bioreactor landfills is provided according to a few selected characteristics (i.e. energy recovery, biochemical kinetics, technological complexity, costs). Considering landfill sustainability, the discussion is primarily focused on the quantification of the stabilisation capability of the different bioreactors which is calculated in terms of COD and ammonia removal kinetics. The results demonstrate that the optimisation of COD removal kinetics is the highest in aerated bioreactors, while ammonia removal kinetics is maximum in hybrid bioreactors (i.e., 6 and 10 times higher, respectively, compared to the anaerobic bioreactors).

Treatment of dinitrodiazophenol production wastewater by Fe/C and Fe/Cu internal electrolysis and the COD removal kinetics

In this study, Fe/C and Fe/Cu internal electrolysis (IE) was used to treat dinitrodiazophenol (DDNP) wastewater. COD removal was selected as evaluation index to discuss the difference of the two systems. The results of orthogonal experiment showed that under the same operational conditions, the COD removals of Fe/C IE were all greater than Fe/Cu IE, and the pH of wastewater had the greatest significant impact on COD removal. According to orthogonal and single factor affecting experiments, the optimum operational conditions of Fe/C and Fe/Cu IE were as follow: pH = 1, reaction time (RT) = 2 d, medium molar ratio (R) = 1.37 and reaction temperature (TR) = 25 °C (Fe/C IE); pH = 1, RT = 4 d, R = 1.14 and TR = 30 °C (Fe/Cu IE), and the COD removals of Fe/C and Fe/Cu IE under the optimum operational conditions were 86.65% and 82.24%, respectively. The COD removal kinetics by Fe/C and Fe/Cu IE was also investigated by classical reaction kinetics, traditional pseudo-first-order kinetic and innovative pseudo-kinetics models. The results showed that COD removal processes were well described by the novel pseudo-second-order kinetic model, and the liquid film diffusion was the potential rate-controlling step.

Simultaneous application of copper and PbO2 anodes for electrochemical treatment of olive oil mill wastewater

Olive oil mill wastewater (OOMW) is a complex effluent that must be treated before discharge into the environment. In this study, a combination of various mechanisms was conducted in an electrochemical cell. Copper and PbO2 anodes were used, while hydrogen peroxide was electro-generated at an activated carbon fiber cathode. Fenton-like reaction and electrooxidation occurred during electrolysis for organic compounds degradation in OOMW. The effects of pH, dilution of OOMW, and electrical current on COD and phenol removals were investigated. The results showed that 78.5% phenol and 60% COD removals were obtained at optimum conditions of 300 mA electrical current, 150 min electrolysis, and pH 3–3.5 (regulated). The regulation of pH during electrolysis significantly developed the treatment efficiency of electrochemical process. Phenol and COD removal kinetics were likely to follow second-order model and first-order model, respectively. Simultaneous application of two anodes had synergic effect on removal efficiency of electrochemical process. Biodegradability (BOD5/COD) was dramatically improved from 0.18 to 0.468 after electrochemical oxidation. It was concluded that the two mechanisms of production of active oxygen and chlorine had synergic effect on electrochemical degradation.

Fenton treatment of landfill leachate under different COD loading factors

The application of Fenton treatment technology for treatment of landfill leachate greatly depends on the optimum Fenton operating conditions for a specific leachate. Determining optimum Fenton conditions requires multiple experiments using variable reaction parameters (pH, temperature, and H2O2 and Fe2+ doses) and previous researches show a wide range of optimal operating conditions. In this study, the applicability of the dimensionless loading factor (LCOD), which is defined as the initial COD (COD0) of leachate with respect to available O2 for oxidation, was examined to derive optimum Fenton oxidant dose using reduced set of experiments. The Fenton experiments were conducted using leachates with three different COD0 concentrations, 1092, 546, and 273mgL−1, LCOD in the range of 0.25–1.0, and H2O2/Fe2+ 1.8 (w/w). The experimental data were analyzed to determine the correlation between the LCOD factor and selected feasibility parameters, amongst which were: (i) the COD removal kinetics, (ii) the total COD removal, (iii) the usability of H2O2 with respect to COD removal, (iv) leachate biodegradability, and (v) treatment cost incurred by chemical usage. The experimental COD removal with respect to the amount of O2 supplied by H2O2 was compared with respect to the optimum COD removal efficiency by the equation: ηFPoptimum=0.733LCOD-0.182 as developed by Singh and Tang (2013) and a LCOD of 0.75 was determined to be the optimum LCOD for leachate treatment.

Microbial Fuel Cells for Direct Electrical Energy Recovery from Urban Wastewaters

Application of microbial fuel cells (MFCs) to wastewater treatment for direct recovery of electric energy appears to provide a potentially attractive alternative to traditional treatment processes, in an optic of costs reduction, and tapping of sustainable energy sources that characterizes current trends in technology. This work focuses on a laboratory-scale, air-cathode, and single-chamber MFC, with internal volume of 6.9 L, operating in batch mode. The MFC was fed with different types of substrates. This study evaluates the MFC behaviour, in terms of organic matter removal efficiency, which reached 86% (on average) with a hydraulic retention time of 150 hours. The MFC produced an average power density of 13.2 mW/m3, with a Coulombic efficiency ranging from 0.8 to 1.9%. The amount of data collected allowed an accurate analysis of the repeatability of MFC electrochemical behaviour, with regards to both COD removal kinetics and electric energy production.

Kinetic Studies for an Aerobic Packed Bed Biofilm Reactor for Treatment of Organic Wastewater with and without Phenol

A laboratory scale aerobic fixed film bioreactor packed with glass beads for biofilm growth was used to evaluate the removal efficiencies of COD and phenol for a carbohydrate—phenol mixture in wastewater. It was done by an indigenous mixed culture inoculums developed after collecting sludge from a return line of an activated sludge plant. The test result on continuous flow in the above biofilm reactor indicated an optimum hydraulic loading range of 4-6.4 m3day-1m-2 for attainment of reasonable amount of COD removal in case of carbohydrate substrate only. The COD removal efficiency, however, gradually depleted from 100% to 54% by gradual increase in organic loading (OLR) from 0.72-4.32 kgday-1m-3, beyond which removal was not significant. For the identical loading conditions, in presence of phenol in the substrate along with carbohydrate, the COD removal was observed varying from 100-40% in the above organic loading range. The COD removal kinetics in presence of phenol also shows a decreasing trend compared to data obtained without the presence of phenol in wastewater that reveals biological inhibition. The experimental data were fitted in a simple plug flow model for evaluating the zero order, first order and Monod form of rate equations to evaluate the kinetics. It was found that Monod type rate equations combining a zero and first order rate expression is the best fit for the above hydraulic and organic loading that gives a best fit half velocity constant value of 35 mgL-1 (R2 = 0.9612).

Treatment of potato chips manufacturing wastewater by electrocoagulation

Treatment of wastewater from potato chips manufacturing by electrocoagulation (EC) was investigated. Experiments were conducted to determine the optimum operating conditions such as electrode type, pH, current density and retention time. Aluminium and iron electrodes were used, and aluminium electrodes were found to be more suitable since it had a higher removal rate of COD, turbidity and suspended solids than the iron electrode. The removal efficiencies of COD and turbidity were high, being 60% and 98%, respectively, with retention time < 40 min. 0.05–1.75 kg (per kg COD removed) of dried sludge was removed. COD removal kinetics during EC process was described by a macro-kinetics model. Results from the kinetic studies showed that the kinetic data fit the second-order kinetic model well. The operating costs investigated in the present study were the energy cost of EC and the material cost due to the consumption of aluminium electrode. Operating costs were varied in the range of 0.48 to 5.42 $/m 3 and 0.62 to 6.32 $/m 3 wastewater treated at 20–300 A/m 2 and 5–40 min, respectively. The energy consumption was 4 kWh/m 3 for wastewater treated less than 8 min under typical operating conditions.

High-carbohydrate wastewater treatment by IAL-CHS with immobilized Candida tropicalis

A novel immobilized-cell bioreactor was developed and utilized for treatment of high-carbohydrate wastewater and production of single-cell protein (SCP) simultaneously. The system is the internal airlift loop reactor with ceramic honeycomb support for biofilm (IAL-CHS), and the microorganisms tested was Candida tropicalis, which has a high protein yield. The optimum temperature for C. tropicalis was 35–40 °C, and bioreactor experiments were performed at 35 °C. The internal airlift loop reactor with ceramic honeycomb supports allowed a large and dense accumulation of biomass, but also generated significant SCP. Mass-transport resistance did not limit the overall COD-removal kinetics and oxygen mass transfer may have been accelerated to the biofilm. Fermentation of carbohydrates to organic acids lowered the pH below the optimum range of 4.5–6.5, and pH below 4.5 retarded or stopped the oxidation reactions for the organic acids. Since organic-acid oxidation is necessary for attaining COD removal from the wastewater, pH control may be essential for effective treatment in some cases with high-carbohydrate wastewaters. The addition of 0.35 g/L urea could increase the COD removal rate. Continuous wastewater was carried out and the organic loading reached 15 kg COD/(m 3 day).

High-carbohydrate wastewater treatment by IAL-CHS with immobilized Candida tropicalis

A novel immobilized-cell bioreactor was developed and utilized for treatment of high-carbohydrate wastewater and production of single-cell protein (SCP) simultaneously. The system is the internal airlift loop reactor with ceramic honeycomb support for biofilm (IAL-CHS), and the microorganisms tested was Candida tropicalis, which has a high protein yield. The optimum temperature for C. tropicalis was 35–40 °C, and bioreactor experiments were performed at 35 °C. The internal airlift loop reactor with ceramic honeycomb supports allowed a large and dense accumulation of biomass, but also generated significant SCP. Mass-transport resistance did not limit the overall COD-removal kinetics and oxygen mass transfer may have been accelerated to the biofilm. Fermentation of carbohydrates to organic acids lowered the pH below the optimum range of 4.5–6.5, and pH below 4.5 retarded or stopped the oxidation reactions for the organic acids. Since organic-acid oxidation is necessary for attaining COD removal from the wastewater, pH control may be essential for effective treatment in some cases with high-carbohydrate wastewaters. The addition of 0.35 g/L urea could increase the COD removal rate. Continuous wastewater was carried out and the organic loading reached 15 kg COD/(m3 day).

Dairy wastewater treatment in a moving bed biofilm reactor

Dairy raw wastewater is characterised by high concentrations and fluctuations of organic matter and nutrient loads related to the discontinuity in the cheese production cycle and machinery washing. The applicability of a Moving Bed Biofilm Reactor (MBBR) filled with FLOCOR-RMP plastic media to the treatment of dairy wastewater was evaluated in a pilot-plant. COD fractionation of influent wastewater, MBBR performance on COD and nutrient removal were investigated. A removal efficiency of total COD over 80% was obtained with an applied load up to 52.7 gCOD m-2 d-1 (corresponding to 5 kgCOD m-3d-1). The COD removal kinetics for the MBBR system was assessed. The order of the kinetics resulted very close to half-order in the case of a biofilm partially penetrated by the substrate. The nitrogen removal efficiency varied widely between 13.3 and 96.2% due to the bacterial synthesis requirement. The application of a MBBR system to dairy wastewater treatment may be appropriate when upgrading overloaded activated sludge plants or in order to minimise reactor volumes in a pre-treatment.

Indirect Electrochemical Treatment of Saline Dyestuff Wastewater

Dyestuff wastewater is always found to contain highly-concentrated sodium chloride. In many cases, biological processes are not available to dispose saline dyestuff wastewater. In this paper, both direct and indirect electrochemical oxidation of saline dyestuff wastewater were carried out and the accessibility of indirect electrochemical oxidation by electrochemically generated hypochlorite was investigated. It was found that the indirect electrochemical process removed COD by 87% and color by 100% in 50 minutes while the direct electrochemical process removed COD by 47% and color by 50% in 5 hours. The rate-limiting step for hypochlorite evolution at the anode was found to be diffusion-controlled. For the two processes, COD removal kinetics could be simplified as the pseudo first-order kinetics. The rate constants of COD removal were 0.038 min−1 and 0.0018 min−1 for the indirect and direct oxidation respectively. Current efficiency of the indirect process was 1 while that of the direct process was 0.16; cost efficiency of the two processes was 8000 mg-COD (kWh)−1 and 1000 mg-COD (kWh)−1 respectively. It can be concluded that the indirect electrochemical process is a promising method for the disposal of saline dyestuff wastewater.

Upgrading Wastewater Treatment by Water Hyacinth in Developing Countries

With increasing stress on existing wastewater treatment systems, it is necessary either to upgrade the treatment unit(s) or install an entirely new treatment plant. Obviously, the upgrading is preferred over the alternative of having a new system. Keeping this in view, in the present project, an attempt has been made to explore the possibility of upgrading existing facultative ponds using water hyacinth. Bench-scale batch studies were designed to compare the performance of hyacinth treatment system with facultative ponds. Investigations were carried out with synthetic wastewater having COD in the range of 32.5-1090 mg/l. The efficiency of COD removal in water hyacinth ponds was 15-20 percent more than the facultative ponds. Based on the results, an empirical model has been proposed for COD removal kinetics. In the second phase of the project a hyacinth pond was continuously operated. BOD, COD, TS, TN, TP, pH, and DO were regularly monitored. However, the DO of the effluent from hyacinth treatment system was considerably reduced. Effluent should be aerated before it is discharged. The results indicate that the existing facultative ponds can be stalked with water hyacinth to improve their performance as well as hyacinth treatment systems can be installed to support the conventional treatment.

Mechanisms and Kinetics of COD Removal and Simultaneous Oxygen Consumption in a Rotating Biological Contactor Biofilm

A study on aerobic biofilm COD removal and simultaneous respiration is made using a Wazu respirationmeter unit. In a closed recirculating system completely submerged RBC biofilm samples have been examined in short-term batch experiments, using the corresponding waste water or dissolved acetate. The removal of dissolved organic substrate and the respiration were found to answer the ½-order/0-order kinetic model, whereas the removal of suspended solids COD (CODs) showed 1-order kinetics. Under conditions usually met in practice the 0-order rates for dissolved COD(CODd) removal and respiration were 30–40 gm−2d−1 and 10–16 gm−2d−1, respectively; in absence of organic substrate a certain background respiration was observed (3.5–9.0 gm−2d−1). As a result of the 1-order kinetics over a wide concentration range, extremely high CODs removal rates were observed even exceeding 300 gm−2d−1. The difference between the maximum respiration and the CODs removal rates is attributed to physical biofilm adsorption processes. Biofilm COD removal kinetics for waste water containing both CODs and CODd are discussed, illustrating that the kinetic order of the combined process and the reaction constant strongly depends on the CODs/CODd ratio. Conclusion is that a RBC biofilm loaded with a CODs/CODd mixture oxidizes some dissolved organic matter and acts as a catalyst in the coagulation and flocculation of fine suspended solids.