Organic Removal of Anaerobically Treated Leachate by Fenton Coagulation

The leachate from a Hong Kong landfill, containing 15,700 mg∕L of chemical oxygen demand (COD) and 2,260 mg∕L of ammonia nitrogen (NH3–N), was first treated in a UASB (upflow anaerobic sludge blanket) reactor at 37°C. The process on average removed 90.4% of COD with 6.6 days of hydraulic retention at an organic loading rate of 2.37 g of COD∕L?day. The UASB effluent was further treated by the Fenton coagulation process using H2O2 and Fe2+. Under the optimal condition of 200 mg of H2O2∕L and 300 mg of Fe2+∕L and an initial pH of 6.0, 70% of residual COD in the UASB effluent was removed, of which 56% was removed by coagulation∕precipitation and only 14% by free radical oxidation. It is obvious that H2O2 and Fe2+ had a strong synergistic effect on coagulation. The average COD in the final effluent was 447 mg∕L. Removing each gram of COD required 0.28 g of Fe2+ and 0.18 g of H2O2.     

Combined System for Biological Removal of Nitrogen and Carbon from a Fish Cannery Wastewater

A combined system composed of three sequentially arranged reactors, anaerobic-anoxic-aerobic reactors, was used to treat the wastewater generated in the tuna cookers of a fish canning factory. These wastewaters are characterized by high chemical oxygen demand (COD) and nitrogen concentrations. The anaerobic process was performed in an upflow anaerobic sludge blanket reactor operated in two steps. During Step I different influent COD concentrations were applied and organic loading rates (OLRs) up to 4 g COD/(L?d) were achieved. During Step II hydraulic retention time (HRT) was varied from 0.5 to 0.8 days while COD concentration in the influent was constant at 6 g COD/L. The OLRs treated were up to 15 g COD/(L?d). When HRTs longer than 0.8 days were used, COD removal percentages of 60% were obtained and these values decreased to 40% for a HRT of 0.5 days. The denitrification process carried out in an upflow anoxic filter was clearly influenced by the amount of carbon source supplied. When available carbon was present, the necessary COD/N ratio for complete denitrification was around 4 and denitrification percentages of 80% were obtained. The nitrification process was successful and was almost unaffected by the presence of organic carbon (0.2–0.8 g TOC/L), with ammonia removal percentages of 100%. Three recycling ratios (R/F) between the denitrification and nitrification reactors were applied at 1, 2, and 2.5. The overall balance of the combined system indicated that COD and N removal percentages of 90% and up to 60%, respectively, were achieved when the R/F ratio was between 2 and 2.5.     

Experimental Validation of the Static Granular Bed Reactor for Industrial Waste Anaerobic Treatment

The static granular bed reactor (SGBR) is a unique high-rate anaerobic reactor designed to operate in a simple downflow manner, offering high chemical oxygen demand (COD) removal efficiencies (greater than 90%) resulting from high biomass retention in the system. A study was performed to evaluate the SGBR versus a control system, the upflow anaerobic sludge blanket (UASB) reactor, and to evaluate performance idiosyncrasies of the SGBR and the control. The two reactors were operated at three different hydraulic retention times (HRTs): 8, 16, and 24 h. The reactors treated synthetic wastewater, intended to simulate food industry waste, composed of sucrose and nonfat dry milk. Overall, COD removal was higher for the SGBR than for the UASB reactor. In particular, at a HRT of 8 h, the SGBR achieved a COD removal of 90.7% and the UASB reactor reduced the COD concentration by 77.5%. The UASB reactor’s specific COD loading factor proved rate limiting with values ranging from 0.19 to 0.94?gCOD/(gVS?d) versus 0.11 to 0.34?gCOD/(gVS?d) for the SGBR. A tracer study idealized hydraulics within the two systems, and the results showed minimal dead volume and 4–6% short circuiting for both reactors.     

Biodegradation of Phenol with Wastewater as a Cosubstrate in Upflow Anaerobic Sludge Blanket

Anaerobic degradation of phenol mixed with a readily degradable synthetic wastewater (DSWW) as a cosubstrate was studied in a 12?L upflow anaerobic sludge blanket reactor at 30±2°C over a period of 632?days. DSWW was prepared by diluting sugar cane based molasses. The biomass was acclimatized to high phenol concentration by gradually decreasing the DSWW chemical oxygen demand (COD) of 4,000?mg/L. Feed made up of phenol COD and DSWW COD in the ratio of 7:3 (phenol concentration = 1,176?mg/L) was successfully treated at a hydraulic retention time (HRT) of 12?h and organic loading rate (OLR) of 8?g?COD/L?day. Phenol removal ranged from 99.9 to 84% at phenol COD varying from 10 to 70% in the feed. During the entire operation, COD removal varied from about 74 to 91.3%. The influent COD was distributed into CH4–COD ( ～ 72%), effluent COD ( ～ 17%), and sludge and unaccounted COD ( ～ 11%). The process failure occurred at 4:1 phenol COD: DSWW COD. Specific methanogenic activity of granular sludge exhibited uniform activity up to phenol COD of 70%. The performance of the reactor could not be maintained beyond 70% phenol COD even by reducing the sludge loading rate, increasing HRT, or decreasing OLR.     

Automatic Start-Up of UASB Reactors

A control system to automate the start-up of anaerobic wastewater treatment reactors is presented. The system controls the feedflow rate, using the biogas production or the biogas flow rate as the only on-line variable. Furthermore, other off-line parameters, such as methane content in the biogas and COD of influent are also necessary to calculate the COD balance in the reactor. Two different start-up strategies were applied: fed batch and continuous operation. In the fed-batch operation the frequency of feeding is automatically set depending on the efficiency in COD removal. The residual organic load fraction (the fraction of influent COD not degraded to methane), is the key parameter enabling the controller the modification of feeding frequency. In order to improve reliability of the system, a second parameter k representing the gas flow rate, was introduced. By using this strategy, starting from an organic loading rate (OLR) lower than 0.5 kg COD∕(m3?day), a load higher than 8 kg COD∕(m3?day) was achieved in only 33 days, with an efficiency, in terms of COD removal, higher than 90%. When the system was operated in continuous mode, the key parameter applied is again the residual organic loading rate fraction, expressed as the percentage of COD that remained undegraded in methane. Two extreme values—“minimum” and “maximum”—have to be defined. Once the system reaches the minimum, the controller increases the feeding pump speed automatically, and when it attains the maximum value, the feeding flow is decreased proportionally. In order to ensure stable operation, a further parameter, the waiting time between the moment at which the threshold value is reached and the modification of the feeding flow rate, was introduced. 24 h has proven to be an excellent value for this purpose. By means of this strategy, starting from an influent OLR lower than 0.5 kg COD∕(m3?day), an OLR of 9–12 kg COD∕(m3?day) was achieved in 40 days, with COD removal efficiency higher than 95%.     

Biodegradation of Mixtures of Phenolic Compounds in an Upward-Flow Anaerobic Sludge Blanket Reactor

The anaerobic biodegradability of mixtures of phenolic compounds was studied under continuous and batch systems. Continuous experiments were carried out in up-flow anaerobic sludge bed (UASB) reactors degrading a mixture of phenol and p-cresol as the main carbon and energy sources. The total chemical oxygen demand (COD) removal above 90% was achieved even at organic loading rates as high as 7 kg COD/m3/day. Batch experiments were conducted with mixtures of phenolic compounds (phenol, p-cresol, and o-cresol) to determine the specific biodegradation rates using unadapted and adapted anaerobic granular sludge. Phenol and p-cresol were mineralized by adapted sludge with rates several orders of magnitude higher than unadapted sludge. Additionally, an UASB reactor was operated with the mixture phenol, p-cresol, and o-cresol. After 54 days of operation, 80% of o-cresol (supplied at 132 mg/L) was eliminated. The phenol biodegradation was not affected by the presence of o-cresol. These results demonstrate that major phenolic components in petrochemical effluents can be biodegraded simultaneously during anaerobic treatment.     

Effect of Increasing Nitrobenzene Loading Rates on the Performance of AMBR and Sequential AMBR/CSTR Reactor System

A laboratory scale sequential anaerobic migrating blanket reactor (AMBR)/aerobic completely stirred tank reactor (CSTR) system was operated to investigate the effect of increasing nitrobenzene (NB) concentrations on the performance of AMBR/CSTR reactor system. The reactor system was operated at increasing NB loading rates from 1.93?to?38.54?g?NB?m?3?day?1 and at a constant hydraulic retention time of 10.38?days. In this study, chemical oxygen demand (COD) and NB removal efficiencies, variations of bicarbonate alkalinity (Bic.Alk.), total volatile fatty acid (TVFA), and total methane gases were monitored. COD removal efficiencies were 93–94% until a NB loading rate of 5.78?g?m?3?day?1 in the AMBR reactor. For maximum COD removal, the optimum NB loading rate and NB concentration were found to be 5.78?g?m?3?day?1 and 60?mg?L?1, respectively. COD removal efficiencies decreased from 94 to 87% and to 85% at NB loading rates of 1.93–28.90 and 38.54?g?m?3?day?1, respectively. COD was mainly removed in the first compartment. NB removal efficiencies also were approximately 100% at all NB loading rates in the effluent of the AMBR reactor. The maximum total gas and methane gas productions were found to be 2.8?L?day?1 and 1.3?mL?day?1, respectively, at a NB loading rate of 5.78?g?m?3?day?1. The TVFA concentration in the effluent of AMBR was low (17?mg?L?1) at a NB loading rate as high as 38.54?g?m?3?day?1. Overall COD removal efficiencies were found to be 99 and 96% at NB loading rates of 1.93 and 38.54?g?m?3?day?1, respectively, in a sequential AMBR/CSTR reactor system. In this study, NB was reduced to aniline under anaerobic conditions. Aniline removal efficiencies were 100% until a NB loading rate of 17.34?g?m?3?day?1 in aerobic CSTR reactor while aniline removal efficiency decreased to 90% at a NB loading rate of 38.54?g?m?3?day?1 in an aerobic reactor. In the aerobic step, aniline was mineralized to catechol. The contribution of aerobic step is not only the degradation of aniline, it may also increase the COD removals from 85 to 99% at a NB loading rate as high as 38.54?g?m?3?day?1.     

Anaerobic Treatment of Synthetic Textile Wastewater Containing a Reactive Azo Dye

In this study, anaerobic treatment of synthetic textile wastewater containing a reactive azo dye, namely, Remazol Brilliant Violet 5R, was investigated. A fluidized bed reactor (FBR) was used in the study. Before the operation period, start-up of the FBR was completed in 128 days with an immobilized microorganism level of 0.069 g volatile suspended solids per g support material (pumice). Anaerobic treatment of synthetic textile wastewater revealed that 300 mg/L dye was removed in the FBR system. Chemical oxygen demand (COD) and color reduction in the system were approximately 60 and 94%, respectively. Under anaerobic conditions, formation of two sulfonated aromatic amines (SAAs) was detected due to anaerobic reduction of the dye. The SAAs were not degraded under anaerobic conditions. In addition to the anaerobic treatment, the effectiveness of aerobic treatment was investigated in order to further reduce the COD after the anaerobic treatment.     

Combined Removal of Carbon and Nitrogen in an Integrated UASB-Jet Loop Reactor Bioreactor System

An innovative anaerobic–aerobic integrated bioreactor system consisting of an upflow anaerobic sludge blanket (UASB) and a jet loop reactor was developed to investigate the feasibility of combined removal of carbon and nitrogen for a low-strength wastewater at different hydraulic retention times (HRTs) and recycle ratios. Total chemical oxygen demand (COD) removal of the integrated system increased from 87 to 92%, at a combined system HRT of 44?h, when the recycle ratio was increased from 100 to 400%, respectively. Denitrification efficiency of the integrated system increased from 49 to 86%, at all HRTs, when the recycle ratio was increased from 100 to 400%. The integrated system, on average, achieved more than 78% of total nitrogen at all HRTs. Nitrogen content of the biogas produced from the UASB reactor increased with increase in recycle ratios while the methane content exhibited a reverse trend, irrespective of the HRTs. Sludge volume index of the UASB reactor increased from 15?to?42?mL/g total suspended solids at the end of the study. Specific methanogenic activity of the granular sludge decreased from 1.3 to 0.8 g CH4–COD/g volatile suspended solids per day at the end of the study. Nitrogen and COD mass balance of the integrated system indicated that a substantial amount of influent nitrogen and COD was lost in the effluent as dissolved form.     

Treatment of Dilute Wastewaters Using a Novel Submerged Anaerobic Membrane Bioreactor

Three 3?L laboratory scale submerged anaerobic membrane bioreactors (SAMBRs) with in situ membrane cleaning due to the bubbling of recycled biogas underneath them were studied for their ability to treat dilute wastewaters. Both Mitsubishi Rayon hollow-fiber and Kubota flat sheet membranes made of polyethylene with a pore size of 0.4?μm were used in this study, and the effect of different substrates (460?mg/L of glucose or synthetic) on chemical oxygen demand (COD) performance in the SAMBR was investigated. It was found that both membranes resulted in similar COD removals (>90% soluble COD at a hydraulic retention time of 3?h), but that the transmembrane pressure across the hollow fiber membranes was higher under similar conditions. Molecular weight analysis of the feed, reactor contents, effluent, and extracellular polymers using high pressure liquid chromatography showed that the membrane filtered out most of the high MW soluble organics, resulting in high COD removals. The experimental results from the SAMBR show the potential benefits of using this novel reactor design in a biological wastewater treatment process to minimize energy use and sludge production.     

Pilot-Scale Simulation of Landfill Bioreactor and Controlled Dumping of Fresh and Partially Stabilized Municipal Solid Waste in a Tropical Developing Country

Four pilot-scale lysimeters were used to study the benefits of landfill operation with and without leachate recirculation in tropical weather conditions. Young and old landfills were simulated by filling lysimeters with a segregated fraction of fresh municipal solid waste (MSW) and MSW mined from an open dump site, respectively, and periodically monitoring leachate quantity and quality and biogas quality. For each substrate, one lysimeter was operated as a bioreactor with leachate recirculation and another lysimeter was operated as a controlled dump, for a period of 10 months. Densities between 652 and 825??kg/m3 could be achieved with fresh and mined MSW. Despite such compaction during waste placement, bioreactor technology helps in leachate management, especially in the case of the young landfill lysimeter operated in tropical weather. The benefits of leachate recirculation in the young landfill lysimeter were evident from the significant decrease in chemical oxygen demand (COD) (86%), biochemical oxygen demand (BOD) (82%), dissolved organic carbon (DOC) (85%), and volatile solids (75%) in leachates. However, ammonia nitrogen (amm-N) and chlorides in the leachates accumulated in bioreactor landfills. Operating an old landfill lysimeter as a bioreactor seemed to have no exceptional advantage in the context of leachate management, although leachate recirculation enhanced the methane potential of both fresh and mined MSW.     

Effect of Carbon Source on PCE Dehalogenation

A laboratory study using the upflow anaerobic sludge blanket reactor for treating high-strength wastewater containing tetrachloroethene (PCE) was carried out to study the effect of carbon source, recycle, and shock loading on dehalogenation of PCE and process performance. The PCE was dehalogenated to trichloroethylene, cis-1,2-dichloroethylene, vinyl chloride, and ethylene. During the study on the effect of carbon source, the PCE and COD removal up to 97% and biogas production of 0.518–0.47 m3∕kg CODrem with methane content up to 66% were achieved under steady-state operating conditions. An increase in the influent COD from 2,000 to 4,000 mg∕L did not show any improvement in the PCE removal. Recycling of effluent at 50% showed the decrease in COD removal and increase in the effluent concentration of dichloroethylene and vinyl chlorides. Around 1–3.5% of influent PCE stripping to biogas was observed. It was observed that methanol has the stimulatory effect on the dehalogenation of PCE. A shock loading study showed that the upflow anaerobic sludge blanket reactor could assimilate 1.5–2 times the original PCE concentration (50 mg∕L) without much effect on the process performance.     

Treatment of High-Strength Pharmaceutical Wastewater and Removal of Antibiotics in Anaerobic and Aerobic Biological Treatment Processes

Anaerobic and aerobic treatment of high-strength pharmaceutical wastewater was evaluated in this study. A batch test was performed to study the biodegradability of the wastewater, and the result indicated that a combination anaerobic-aerobic treatment system was effective in removing organic matter from the high-strength pharmaceutical wastewater. Based on the batch test, a pilot-scale system composed of an anaerobic baffled reactor followed by a biofilm airlift suspension reactor was designed. At a stable operational period, effluent chemical oxygen demand (COD) from the anaerobic baffled reactor ranged from 1,432 to 2,397?mg/L at a hydraulic retention time (HRT) of 1.25 day, and 979 to 1,749?mg/L at an HRT of 2.5 day, respectively, when influent COD ranged from 9,736 to 19,862?mg/L. As a result, effluent COD of the biofilm airlift suspension reactor varied between 256 and 355?mg/L at HRTs of from 5.0 to 12.5 h. The antibiotics ampicillin and aureomycin, with influent concentrations of 3.2 and 1.0?mg/L, respectively, could be partially degraded in the anaerobic baffled reactor: ampicillin and aureomycin removal efficiencies were 16.4 and 25.9% with an HRT of 1.25 day, and 42.1 and 31.3% with HRT of 2.5 day, respectively. Although effective in COD removal, the biofilm airlift suspension reactor did not display significant antibiotic removal, and the removal efficiencies of the two antibiotics were less than 10%.     

Biotransformation of Carbon Tetrachloride and Anaerobic Granulation in a Upflow Anaerobic Sludge Blanket Reactor

Carbon tetrachloride (CT) in a synthetic wastewater was effectively degraded in a 2?l upflow anaerobic sludge blanket reactor during the granulation process by increasing the chemical oxygen demand (COD) and CT loadings. The effect of operational parameters such as influent CT concentrations, COD, CT loading, food to mass (F/M) ratio, and specific methanogenic activity (SMA) were also detected during granulation. Over 97% of CT was removed at 37°C, at a COD loading rate of 10?g/L?day. Chemical oxygen demand and CT removal efficiencies of 92 and 88% were achieved when the reactor was operating at CT and COD loading rates of 17.5?mg/L?day and 12.5?g/L?day, respectively. This corresponds to an hydraulic retention time of 0.28?day and an F/M ratio of 0.57?g?COD/g?volatile?suspended?solids?(VSS)?day. In 4?weeks, the seed sludge developed the CT degrading capability that was not very sensitive to shocks. The granular sludge cultivated had a maximum diameter of 2.5?mm and SMA of 1.64?g?COD/g?VSS?day. Glucose biodegradation by CT acclimated anaerobic granules was expressed with competitive inhibition. However the competitive inhibition was not significant since the competitive inhibition coefficient (Ki) was as high as 18.72?mg/L. Kinetic coefficients of k (maximum specific substrate utilization rate), Ks (half velocity coefficient), Y (growth yield coefficient), and b (decay coefficient) were determined as 0.6/day, 1.1?mg/L, 0.23?g?VSS/g glucose-COD, and 0.01/day, respectively, based on growth substrate glucose–COD during CT biotransformation. The CT was treated via biodegradation and this contributed to 89% of the total removal. The removal contributions from biomass adsorption, abiotic transformation, and volatilization were negligible. Adsorption and volatilization accounted for only 0.8 and 0.5% of the total removal, respectively.     

Combined Anaerobic/Aerobic Secondary Municipal Wastewater Treatment: Pilot-Plant Demonstration of the UASB/Aerobic Solids Contact System

Anaerobic pretreatment followed by aerobic posttreatment of municipal wastewater is being used more frequently. Recent investigations in this field using an anaerobic fluidized bed reactor/aerobic solids contact combination demonstrated the technical feasibility of this process. The investigation presented herein describes the use of a combined upflow anaerobic sludge bed (UASB)/aerobic solids contact system for the treatment of municipal wastewater and attempts to demonstrate the technical feasibility of using the UASB process as both a pretreatment unit and a waste activated sludge digestion system. The results indicate that the UASB reactor has a total chemical oxygen demand removal efficiency of 34%, and a total suspended solids removal efficiency of about 36%. Of the solids removed by the unit, 33% were degraded by the action of microorganisms, and 4.6% accumulated in the reactor. This low solids accumulation rate allowed operating the UASB reactor for three months without sludge wasting. The long solids retention time in this unit is comparable to the one normally used in conventional sludge digestion units, thus allowing the stabilization of the waste activated sludge returned to the UASB reactor. Particle flocculation was very poor in the UASB reactor, and therefore, it required postaeration periods of at least 100?min to proceed successfully in the aerobic unit. Polymer generation, which is necessary for efficient biological flocculation, was practically nonexistent in the anaerobic unit; therefore, it was necessary to maintain dissolved oxygen levels greater than 1.5?mg/L in the aerobic solids contact chamber for polymer generation to proceed at optimum levels. Once these conditions were attained, the quality of the settled solids contact chamber effluent always met the 30?mg BOD/L, 30?mg SS/L secondary effluent guidelines.     

2,4,6 Tri-Chlorophenol Containing Wastewater Treatment Using a Hybrid-Loop Bioreactor System

A hybrid-loop bioreactor system consisting of a packed column biofilm and an aerated tank bioreactor with an effluent recycle was used for biological treatment of 2,4,6 tri-chlorophenol (TCP) containing synthetic wastewater. The effects of sludge age (solids retention time) on chemical oxygen demand (COD), TCP, and toxicity removal performance of the system were investigated for sludge ages between 5 and 30?days, while the feed COD (2600±100?mg?L?1), TCP (370±10?mg?L?1), and the hydraulic residence time (25?h) were constant. Percent TCP, COD, and toxicity removals increased with increasing sludge age resulting in nearly complete COD, TCP, and toxicity removal at sludge ages above 20?days. Biomass concentrations in the packed column and in the aeration tank increased with increasing sludge age resulting in low reactor TCP concentrations, and therefore, high TCP, COD, and toxicity removals. More than 95% of COD, TCP, and toxicity removal took place in the packed column reactor. Volumetric rates of TCP and COD removal increased due to increasing biomass and decreasing effluent TCP and COD concentrations with increasing sludge age. The specific rate of TCP removal was maximum (120?mg?TCP?gX?1?day?1) at a sludge age of 20?days. TCP inhibition was eliminated by operation of the system at sludge age above 20?days to obtain nearly complete COD, TCP, and toxicity removal.     

UASB Treatment of Tapioca Starch Wastewater

Feasibility of the upflow anaerobic sludge blanket (UASB) process was investigated for the treatment of tapioca starch industry wastewater. After removal of suspended solids by simple gravity settling, starch wastewater was used as a feed. Start-up of a 21.5-L reactor with diluted feed of approximately 3,000 mg∕L chemical oxygen demand (COD) was accomplished in about 6 weeks using seed sludge from an anaerobic pond treating tapioca starch wastewater. By the end of the start-up period, gas productivity of 4–5 m3/m3r?day was obtained. Undiluted supernatant wastewater with a COD concentration of 12,000–24,000 mg∕L was fed during steady-state reactor operation at an organic loading rate of 10–16 kg COD/m3r?day. The upflow velocity was maintained at 0.5 m∕h with a recirculation ratio of 4:1. COD conversion efficiencies >95% and gas productivity of 5–8 m3/m3r?day were obtained. These results indicated that removal of starch solids from wastewater by simple gravity settling was sufficient to obtain satisfactory performance of the UASB process.     

Biodegradation of PCE in a Hybrid Membrane Aerated Biofilm Reactor

A continuous flow flat sheet hybrid membrane aerated biofilm reactor (MABR) was used to treat a synthetic wastewater containing perchloroethylene (PCE); 1.25–2.5?g chemical oxygen demand (COD)/L of glucose was also added to the synthetic wastewater as a source of COD representative of a real wastewater. The reactor was able to biodegrade 70?mg?L?1 of PCE in 9?h without the accumulation of any intermediate compounds, resulting in a removal rate of 247?mmol of PCE?h?1?m?3 in a reactor with a specific membrane area of 4.048?m2?m?3. MABRs have never been used before for PCE degradation, and this rate is one of the highest volumetric PCE degradation rates reported in the literature. COD removal was also good and varied from 85 to 92%. Since very few volatile fatty acids accumulated in the system, most of the residual COD was attributed to soluble microbial products as reported by previous researchers. A mass balance on chloride during this study showed that only 72–81% of it could be accounted for. It is probable that some of the chlorinated ethenes were adsorbed onto the biofilm or that aerobic intermediates of low-chlorinated compounds such as trichloroethanol, dichloroacetyl, and chloroacetaldehyde were produced in the system. Nevertheless the chloride mass balance in this work compares well with the literature. Due to their high PCE and COD removal rates, hybrid MABRs have the potential to be used for a number of refractory organics which require combined anaerobic/aerobic biological treatment for degradation.     

Comparative Study between a Hybrid System and a Biofilm System for the Treatment of Ammonia and Organic Matter in Wastewaters

The efficiency of two similar gas-lift bioreactors, a biofilm reactor and a hybrid circulating floating bed reactor (CFBR), were studied and compared. In the biofilm CFBR the biomass grew preferably adhered on a plastic granular support, whereas in the hybrid CFBR both suspended biomass and biofilms were allowed to grow in the reactor. COD/NH4+ ratio (COD=chemical oxygen demand) was manipulated between 0.0 and 8.0?g/g, maintaining the ammonia influent concentration around 50?mg N–NH4+/L, the ammonia loading rate at 0.9?kg N–NH4+/m3?day and the hydraulic retention time at 1.36?h. At low COD/NH4+ ratio (0 and 0.5?g/g) both systems behaved similarly, achieving ammonia removal percentages higher than 95%. In the biofilm CFBR a reduction of the nitrification percentage from 95 to 20% was observed when a COD/N–NH4+ ratio up to 8?g/g was applied in the influent. However, at the same operational conditions, the nitrification process in the hybrid CFBR was slightly affected. In the hybrid-CFBR reactor heterotrophs growing in suspension consumed the COD source faster than those growing in biofilms as was monitored. The growth of heterotrophic microorganism in suspension had a beneficial effect for the nitrifying population growing in the biofilm of the hybrid CFBR. Nitrifying activity of the biofilm was not limited by the presence of heterotrophs consuming dissolved oxygen, displacing the nitrifying bacteria or creating mass transfer resistance as was observed in the biofilm CFBR.