Macroscale and microscale analyses of nitrification and denitrification in biofilms attached on membrane aerated biofilm reactors

A membrane aerated biofilm reactor (MABR), in which O(2) was supplied from the bottom of the biofilm and NH(4)(+) and organic carbon were supplied from the biofilm surface, was operated at different organic carbon loading rates and intra-membrane air pressures to investigate the occurrence of simultaneous chemical oxygen demand (COD) removal, nitrification and denitrification. The spatial distribution of nitrification and denitrification zones in the biofilms was measured with microelectrodes for O(2), NH(4)(+), NO(2)(-), NO(3)(-) and pH. When the MABR was operated at approximately 1.0 g-COD/m(2)/day of COD loading rate, simultaneous COD removal, nitrification and denitrification could be achieved. The COD loading rates and the intra-membrane air pressures applied in this study had no effect on the start-up and the maximum rates of NH(4)(+) oxidation in the MABRs. Microelectrode measurements showed that O(2) was supplied from the bottom of the MABR biofilm and penetrated the whole biofilm. Because the biofilm thickness increased during the operations, an anoxic layer developed in the upper parts of the mature biofilms while an oxic layer was restricted to the deeper parts of the biofilms. The development of the anoxic zones in the biofilms coincided with increase in the denitrification rates. Nitrification occurred in the zones from membrane surface to a point of ca. 60microm. Denitrification mainly occurred just above the nitrification zones. The COD loading rates and the intra-membrane air pressures applied in this study had no effect on location of the nitrification and denitrification zones.     

Mass balance and isotope effects during nitrogen transport through septic tank systems with packed-bed (sand) filters

Septic tank systems are an important source of NO3(-) to many aquifers, yet characterization of N mass balance and isotope systematics following septic tank effluent discharge into unsaturated sediments has received limited attention. In this study, samples of septic tank effluent before and after transport through single-pass packed-bed filters (sand filters) were evaluated to elucidate mass balance and isotope effects associated with septic tank effluent discharge to unsaturated sediments. Chemical and isotopic data from five newly installed pairs and ten established pairs of septic tanks and packed-bed filters serving single homes in Oregon indicate that aqueous solute concentrations are affected by variations in recharge (precipitation, evapotranspiration), NH4+ sorption (primarily in immature systems), nitrification, and gaseous N loss via NH3 volatilization and(or) N2 or N2O release during nitrification/denitrification. Substantial NH4+ sorption capacity was also observed in laboratory columns with synthetic effluent. Septic tank effluent delta15N-NH4+ values were almost constant and averaged +4.9 per thousand+/-0.4 per thousand (1 sigma). In contrast, delta15N values of NO3(-) leaving mature packed-bed filters were variable (+0.8 to +14.4 per thousand) and averaged +7.2 per thousand+/-2.6 per thousand. Net N loss in the two networks of packed-bed filters was indicated by average 10-30% decreases in Cl(-)-normalized N concentrations and 2-3 per thousand increases in delta15N, consistent with fractionation accompanying gaseous N losses and corroborating established links between septic tank effluent and NO3(-) in a local, shallow aquifer. Values of delta18O-NO3(-) leaving mature packed-bed filters ranged from -10.2 to -2.3 per thousand (mean -6.4 per thousand+/-1.8 per thousand), and were intermediate between a 2/3 H2O-O+1/3 O2-O conceptualization and a 100% H2O-O conceptualization of delta18O-NO3(-) generation during nitrification.     

Nitrogen-alkalinity interactions in the highly polluted Scheldt basin (Belgium)

We present results of one year observations in highly heterotrophic and oxygen-depleted rivers of the polluted Scheldt basin. Monthly measurements revealed a high variability for dissolved inorganic carbon and nitrogen, with the following strong parallelism: highest alkalinity and NH4+ were associated with lowest NO3- and oxygen and vice-versa. In river water incubations, nitrification lowered the alkalinity whereas denitrification raised it; in an anoxic, NO3(-)-free incubation an increase of alkalinity was observed, partially due to ammonification. A stoichiometric analysis, taking into account the amount of protons produced or consumed by each process involving nitrogen, revealed that monthly variations of NO3- and NH4+ with ammonification, nitrification and denitrification could explain the 28 and 62% alkalinity variations at all stations, except one. The remaining part of the alkalinity variations was attributed to other anaerobic processes (Mn-, Fe- and SO4-reductions). This trend seems to be the result of the whole catchment metabolism (riverine waters and sediments, sewage networks and agricultural soils). The observed HCO3- concentrations in the Scheldt basin were 2-10 times higher than the representative concentrations reported in pristine basins and used in chemical weathering models. This suggests the existence of an anthropogenic source, originating from organic matter decomposition. We conclude that in highly polluted basins, nitrogen transformations strongly influence the acid-base properties of water.     

Simultaneous carbon and nitrogen removal from high strength domestic wastewater in an aerobic RBC biofilm

High strength domestic wastewater discharges after no/partial treatment through sewage treatment plants or septic tank seepage field systems have resulted in a large build-up of groundwater nitrates in Rajasthan, India. The groundwater table is very deep and nitrate concentrations of 500-750 mg/l (113-169 as NO3(-)-N) are commonly found. A novel biofilm in a 3-stage lab-scale rotating biological contactor (RBC) was developed by the incorporation of a sulphur oxidising bacterium Thiosphaera pantotropha which exhibited high simultaneous removal of carbon and nitrogen in fully aerobic conditions. T. pantotropha has been shown to be capable of simultaneous heterotrophic nitrification and aerobic denitrification thereby helping the steps of carbon oxidation, nitrification and denitrification to be carried out concurrently. The first stage having T. pantotropha dominated biofilm showed high carbon and NH4(+)-N removal rates of 8.7-25.9 g COD/m2 d and 0.81-1.85 g N/m2 d for the corresponding loadings of 10.0-32.0 g COD/m2 d and 1.0-3.35 g N/m2 d. The ratio of carbon removed to nitrogen removed was close to 12.0. The nitrification rate increased from 0.81 to 1.8 g N/m2 d with the increasing nitrogen loading rates despite a high simultaneous organic loading rate. However, it fell to 1.53 g N/m2 d at a high load of 3.35 g N/m2 d and 32 g COD/m2 d showing a possible inhibition of the process. A simultaneous 44-63% removal of nitrogen was also achieved without any significant NO2(-)-N or NO3(-)-N build-up. The second and third stages, almost devoid of any organic carbon, acted only as autotrophic nitrification units, converting the NH4(+)-N from stage 1 to nitrite and nitrate. Such a system would not need a separate carbon oxidation step to increase nitrification rates and no external carbon source for denitrification. The alkalinity compensation during denitrification for that destroyed in nitrification may also result in a high economy.     

矿化垃圾填料对污水中氮磷去除能力的动力学研究

矿化垃圾填料具备良好的粒径级配,表面为不规则的多面体,Fe、Al和Ca成分含量高,具备成为优良磷库的条件。培养实验结果可采用Langmuir吸附等温线模拟,计算所得矿化垃圾磷的饱和吸附量为2 914 mg.kg-1。矿化垃圾吸附磷的饱和吸附量和吸附速率均为粘土的3倍多,磷的解析率仅约为30%。硝化培养实验前24 h内,矿化垃圾中氨氮的浓度从129 mg N.kg-1下降到83.0 mgN.kg-1;硝酸盐氮含量相应地从137 mg N.kg-1上升到170 mg N.kg-1。而同期内粘土中氨氮的浓度下降和硝酸盐氮含量的上升幅度分别为矿化垃圾的1/2和1/6。反硝化培养过程中,矿化垃圾中硝酸盐氮零级动力学降解速率常数K值为粘土7.5倍。     

Simultaneous organic and nitrogen removal from municipal landfill leachate using an anaerobic-aerobic system

An anaerobic-aerobic system including simultaneous methanogenesis and denitrification was introduced to treat organic and nitrogen compounds in immature leachate from a landfill site. Denitrification and methanogenesis were successfully carried out in the anaerobic reactor while the organic removal and nitrification of NH4+,-N were carried out in the aerobic reactor when rich organic substrate was supplied with appropriate hydraulic retention time. The maximum organic removal rate was 15.2 kg COD/m3 d in the anaerobic reactor while the maximum NH4+-N removal rate and maximum nitrification rate were 0.84kg NH4+-N/m3/d and 0.50kg NO3--N/m3/d, respectively, in the aerobic reactor. The pH range for proper nitrification was 6-8.8 in the aerobic reactor. The organic compounds inhibited nitrification so that the organic removal in the anaerobic reactor could enhance the nitrification rate in the following aerobic reactor. The gas production rate was 0.33 m3/kg COD and the biogas compositions of CH4, CO2, and N2 were kept relatively constant, 66-75, 22-32, and 2-3%, respectively.     

Potential nitrate removal in a coastal freshwater sediment (Haringvliet Lake, The Netherlands) and response to salinization

Nitrogen transformations and their response to salinization were studied in bottom sediment of a coastal freshwater lake (Haringvliet Lake, The Netherlands). The lake was formed as the result of a river impoundment along the south-western coast of the Netherlands, and is currently targeted for restoration of estuarine conditions. Nitrate porewater profiles indicate complete removal of NO(3)(-) within the upper few millimeters of sediment. Rapid NO(3)(-) consumption is consistent with the high potential rates of nitrate reduction (up to 200 nmol N cm(-3) h(-1)) measured with flow-through reactors (FTRs) on intact sediment slices. Acetylene-block FTR experiments indicate that complete denitrification accounts for approximately half of the nitrate reducing activity. The remaining NO(3)(-) reduction is due to incomplete denitrification and alternative reaction pathways, most likely dissimilatory nitrate reduction to NH(4)(+) (DNRA). Results of FTR experiments further indicate that increasing bottom water salinity may lead to a transient release of NH(4)(+) and dissolved organic carbon from the sediment, and enhance the rates of nitrate reduction and nitrite production. Increased salinity may thus, at least temporarily, increase the efflux of NH(4)(+) from the sediment to the surface water. This work shows that salinity affects the relative importance of denitrification compared to alternative nitrate reduction pathways, limiting the ability of denitrification to remove bioavailable nitrogen from aquatic ecosystems.     

Evaluating the sources and fate of anthropogenic dissolved inorganic nitrogen (DIN) in two contrasting North Sea estuaries

Nitrogen isotope ratios (delta(15)N) were used to help elucidate the sources and fate of ammonium (NH(4)(+)) and nitrate (NO(3)(-)) in two northeastern English estuaries. The dominant feature of NH(4)(+) in the heavily urbanised Tyne estuary was a plume arising from a single point source; a large sewage works. Although NH(4)(+) concentrations (ranging from 30-150 microM) near the sewage outfall varied considerably between surveys, the sewage-derived delta(15)N-NH(4)(+) signature was remarkably constant (+10.6+/-0.5 per thousand) and could be tracked across the estuary. As indirectly supported by (15)N-depleted delta(15)N-NO(3)(-) values observed close to the mouth of the Tyne, this sewage-derived NH(4)(+) was thought to initiate lower estuarine and coastal zone nitrification. In the more rural Tweed, NH(4)(+) concentrations were low (<7 microM) compared to those in the Tyne and delta(15)N-NH(4)(+) values were consistent with mixing between riverine and marine sources. The dominant form of dissolved inorganic nitrogen (DIN) in the Tweed was agricultural soil-derived NO(3)(-). A decrease in riverine NO(3)(-) flux during the summer coinciding with an increase in delta(15)N-NO(3)(-) values was mainly attributed to enhanced watershed nutrient processing. In the Tyne, where agricultural inputs are less important compared to the Tweed, light delta(15)N-NO(3)(-) (ca. 0 per thousand) detected in the estuary during one winter survey pointed to a larger contribution from precipitation-derived NO(3)(-) during high river discharge. Regardless of the dominant sources, in both estuaries most of the variability in DIN concentrations and delta(15)N values was explained by simple end-member mixing models, implying very little estuarine processing.     

Piggery wastewater treatment using Alcaligenes faecalis strain No. 4 with heterotrophic nitrification and aerobic denitrification

Alcaligenes faecalis strain No. 4, which has heterotrophic nitrification and aerobic denitrification abilities, was used to treat actual piggery wastewater containing high-strength ammonium under aerobic conditions. In a continuous experiment using a solids-free wastewater (SFW) mixed with feces, almost all of the 2000 NH4+ -N mg/L and 12,000 COD mg/L in the wastewater was removed and the ammonium removal rate was approximately 30 mg-N/L/h, which was 5-10 times higher than the rates achieved by other bacteria with the same abilities. The denitrification ratio was more than 65% of removed NH4+ -N, indicating that strain No. 4 exhibited its heterotrophic nitrification and aerobic denitrification abilities in the piggery wastewater.     

好氧反硝化在短程硝化反硝化工艺中的作用研究

采用SBR反应器处理垃圾渗滤液，研究了短程硝化反硝化过程中好氧反硝化的作用。结果表明，SBR反应器的亚硝化效果良好，氨氮几乎完全被氧化为NO2^- -N；该系统的活性污泥中同时存在能还原NO3^- -N和NO2^- -N的好氧反硝化菌，还原NO3^- -N的好氧反硝化菌和氨氧化菌的数量及其总活性高于NO2^- -N氧化菌，这是SBR反应器能够长期维持亚硝化状态的重要原因；有机物浓度越高则好氧反硝化速率越快，此时氨氮均被氧化为NO2^- -N，当有机物浓度达到某临界值时，好氧反硝化速率几乎保持不变；溶解氧浓度越低则好氧反硝化速率越快，释放出的OH^-会导致pH值升高。好氧反硝化对于维持和促进SBR反应器的短程硝化反硝化具有重要的作用。     

Evaluation of the impact of bioaugmentation and biostimulation by in situ hybridization and microelectrode

Three rotating disk biofilm reactors were operated to evaluate whether bioaugmentation and biostimulation can be used to improve the start-up of microbial nitrification. The first reactor was bioaugmented during start-up period with an enrichment culture of nitrifying bacteria, the second reactor received a synthetic medium containing NH(4)(+) and NO(2)(-) to facilitate concomitant proliferation of ammonia- and nitrite-oxidizing bacteria, and the third reactor was used as a control. To evaluate the effectiveness of bioaugmentation and biostimulation approaches, time-dependent developments of nitrifying bacterial community and in situ nitrifying activity in biofilms were monitored by fluorescence in situ hybridization (FISH) technique and microelectrode measurements of NH(4)(+), NO(2)(-), NO(3)(-), and O(2). In situ hybridization results revealed that addition of the enrichment culture of nitrifying bacteria significantly facilitated development of dense nitrifying bacterial populations in the biofilm shortly after, which led to a rapid start-up and enhancement of in situ nitrification activity. The inoculated bacteria could proliferate and/or survive in the biofilm. In addition, the addition of nitrifying bacteria increased the abundance of nitrifying bacteria in the surface of the biofilm, resulting in the higher nitrification rate. On the other hand, the addition of 2.1mM NO(2)(-) did not stimulate the growth of nitrite-oxidizing bacteria and did inhibit the proliferation of ammonia-oxidizing bacteria instead. Thus, the start-up of NO(2)(-) oxidation was unchanged, and the start-up of NH(4)(+) oxidation was delayed. In all the three biofilm reactors, data sets of time series analyses on population dynamics of nitrifying bacteria determined by FISH, in situ nitrifying activities determined by microelectrode measurements, and the reactor performances revealed an approximate agreement between the appearance of nitrifying bacteria and the initiation of nitrification activity, suggesting that the combination of these techniques was a very powerful monitoring tool to evaluate the effectiveness of bioaugmentation and biostimulation strategies.     

Biological nitrogen removal from municipal landfill leachate: low-cost nitrification in biofilters and laboratory scale in-situ denitrification

The slow leaching of nitrogen from solid waste in landfills, resulting in high concentrations of ammonia in the landfill leachate, may last for several decades. The removal of nitrogen from leachate is desirable as nitrogen can trigger eutrophication in lakes and rivers. In the present study, a low-cost nitrification-denitrification process was developed to reduce nitrogen load especially in leachates from small landfills. Nitrification was studied in laboratory and on-site pilot aerobic biofilters with waste materials as filter media (crushed brick in upflow filters and bulking agent of compost in a downflow filter) while denitrification was studied in a laboratory anoxic/anaerobic column filled with landfill waste. In the laboratory nitrification filters, start-up of nitrification took less than 3 weeks and over 90% nitrification of leachate (NH4-N between 60 and 170mg N l(-1), COD between 230 and 1,300 mg l(-1)) was obtained with loading rates between 100 and 130 mgNH4-N l(-1) d at 25 degrees C. In an on-site pilot study a level of nitrification of leachate (NH4-N between 160 and 270 mg N l(-1), COD between 1,300 and 1,600 mg l(-1)) above 90% was achieved in a crushed brick biofilter with a loading rate of 50mg NH4-N l(-1) d even at temperatures as low as 5-10 degrees C. Ammonium concentrations in all biofilter effluents were usually below the detection limit. In the denitrification column. denitrification started within 2 weeks and total oxidised nitrogen in nitrified leachate (TON between 50 and 150mg N l(-1)) usually declined below the detection limit at 25 degrees C, whereas some ammonium, probably originating from the landfill waste used in the column, was detected in the effluent. No adverse effect was observed on the methanation of waste in the denitrification column with a loading rate of 3.8 g TON-N/t-TS(waste) d. In conclusion, nitrification in a low-cost biofilter followed by denitrification in a landfill body appears applicable for the removal of nitrogen in landfill leachate in colder climates.     

Sources of nitrate and ammonium contamination in groundwater under developing Asian megacities

The status of nitrate (NO(3)(-)), nitrite (NO(2)(-)) and ammonium (NH(4)(+)) contamination in the water systems, and the mechanisms controlling their sources, pathways, and distributions were investigated for the Southeast Asian cities of Metro Manila, Bangkok, and Jakarta. GIS-based monitoring and dual isotope approach (nitrate delta(15)N and delta(18)O) suggested that human waste via severe sewer leakage was the major source of nutrient contaminants in Metro Manila and Jakarta urban areas. Furthermore, the characteristics of the nutrient contamination differed depending on the agricultural land use pattern in the suburban areas: high nitrate contamination was observed in Jakarta (dry fields), and relatively lower nutrients consisting mainly of ammonium were detected in Bangkok (paddy fields). The exponential increase in NO(3)(-)-delta(15)N along with the NO(3)(-) reduction and clear delta(18)O/delta(15)N slopes of NO(3)(-) ( approximately 0.5) indicated the occurrence of denitrification. An anoxic subsurface system associated with the natural geological setting (e.g., the old tidal plain at Bangkok) and artificial pavement coverage served to buffer NO(3)(-) contamination via active denitrification and reduced nitrification. Our results showed that NO(3)(-) and NH(4)(+) contamination of the aquifers in Metro Manila, Bangkok, and Jakarta was not excessive, suggesting low risk of drinking groundwater to human health, at present. However, the increased nitrogen load and increased per capita gross domestic product (GDP) in these developing cities may increase this contamination in the very near future. Continuous monitoring and management of the groundwater system is needed to minimize groundwater pollution in these areas, and this information should be shared among adjacent countries with similar geographic and cultural settings.     

Application of the 15N technique to determine the contributions of nitrification and denitrification to the flux of nitrous oxide from aerated pig slurry.

In order to elucidate and quantify nitrogen transformations occurring during aerobic treatment of pig slurry, two laboratory experiments were carried out with contrasting levels of aeration, high level (experiment 1) and low level (experiment 2) of aeration. During these experiments, after reaching steady-state conditions, a single pulse of NO3(-)-15N tracer was added directly to the reactor. When nitrification conditions were prevailing (experiment 1), no transformation of added NO3(-)-15N occurred (98.7% 15N-recovery as nitrates in the liquid slurry). Moreover, the unlabelled nitrous oxide (N2O) measured (7.4% of the total nitrogen content of the raw slurry) strongly demonstrated that the source of N2O emitted was nitrification. During simultaneous nitrification and denitrification conditions (experiment 2), the added NO3(-)-15N was found in gaseous nitrogen forms (N2O-15N: 27%; N2-15N: 18%) and in the liquid (TN-15N: 54%). From this result, it was evident that N2O was mainly emitted by denitrification process. Using the decrease of the isotopic excess of the NO3(-)-15N we calculated that 92% of NO2(-)-N was directly denitrified (as gaseous forms) without any previous oxidation to nitrate.     

低温条件下微生物对沉积物氮释放的影响研究

通过室内静态模拟实验,研究了沉积物中氮的释放特性。将反应器密封后自然达到厌氧状态,温度与水库底部相近（7～8℃）,进行未灭菌和灭菌两个系列对比试验,连续监测两个系列装置中NO3^-N、NO^2-N、NH4^＋-N、TN、DO和pH的变化,并对试验前后沉积物中的总氮含量进行测定。结果表明,未灭菌装置沉积物中总氮减少量明显多于灭菌装置,而上覆水中可监测总氮少于灭菌装置,说明在厌氧条件下氮元素会在微生物作用下从沉积物中释放出来,并通过反硝化和厌氧氨氧化等作用以气态形式释放出水体。试验前后对底泥表面的硝化、亚硝化、反硝化和氨化细菌的数量通过计数进行了比较,发现厌氧后反硝化细菌和氨化细菌明显增多,说明厌氧过程中对氮释放起主要作用的是反硝化细菌和氨化细菌。     

Simultaneous assessment of nitrification and denitrification on freshwater epilithic biofilms by acetylene block method

Acetylene (C2H2) inhibits key enzymes involved in nitrification (Ammonium monooxygenase) and denitrification (N2O reductases). Thus an injection of C2H2 at mid time of a batch type incubation make it possible to assess denitrification by measurement of the N2O accumulation as well as nitrification, calculated from the variations of the ammonium flux. As estimated by the "acetylene block technique", denitrification is known to be only a measure of the denitrification rate supported by nitrate diffusing from the water column (Dw). This paper presents a first application on river epilithic biofilms which proved that the simultaneous measurement of Dw and nitrification allows the estimation of the order of magnitude of total denitrification (Dt) when nitrification is detected in the tested sample. This approach appears to be an easy tool for determination of nitrification and denitrification in natural samples and as thus presents an alternative to isotopic 15N methods.     

Effects of drying on nitrification activity in zeoponic medium used for long-term space missions

One component of a proposed life support system is the use of zeoponic substrates, which slowly release NH4+ into "soil" solution, for the production of plants. Nitrifying bacteria that convert NH4+ to NO3- are among the important microbial components of these systems. Survival of nitrifying bacteria in dry zeoponic substrates is needed, because the substrate would likely be stored in an air-dry state between croppings. Substrate was enriched for nitrifying bacteria and allowed to air-dry in a laminar flow hood. Stored substrate was analyzed for nitrifier survivability by measuring nitrifier activity at the beginning, 3 days, 1, 2, and 3 weeks. After rewetting, activity was approximately 9 micrograms N g-1 h-1 regardless of storage time. Nitrification rates did not decrease during storage. It seems unlikely that drying between plantings would result in practical reductions in nitrification, and reinoculation with nitrifying bacteria would not be necessary.     

序批法工艺设计几个问题的探讨

本文从工艺设计出发,讨论了SBR法用于城市污水处理工程的工艺计算中如:系统泥龄,产泥量,NH4+ -N的硝化,NO3- -N的反硝化,容积负荷及运行工序等几个方面的问题     

MBR中DO对同步硝化反硝化的影响

膜生物反应器（MBR）中,在DO为1mg/L左右,MLSS为8000-9000mg/L,温度为24℃,进水pH值为7.2,COD、NH3-N分别为523-700mg/L和17.24-24mg/L的相对稳定条件下,对COD、NH3-N、TN的去除率分别为96%、95%、92%。详细分析了在控制DO的条件下,MBR发生同步硝化、反硝化的原因,并提出了在单级好氧反应器中控制DO可发生短程硝化一反硝化生物脱氮的机制。     

Kinetic and metabolic study of benzene, toluene and m-xylene in nitrifying batch cultures

The effect of benzene, toluene, and m-xylene (BTX) compounds on the nitrifying activity of a sludge produced in steady-state nitrification was evaluated in batch cultures. Benzene and m-xylene at 10 mg C/L decreased ammonium consumption efficiency by 57% and 26%, respectively, whereas toluene did not affect the ammonium oxidation process. The consumed NH4+-N was totally oxidized to NO3- -N. There was no significant effect at 5 mg C/L of each aromatic compound. BTX (5-20mg C/L) induced a significant decrease in the values for specific rates of NH4+ -N consumption (76-99%) and NO3- -N production (45-98%). At 10 mg C/L of BTX compounds, the inhibition order on nitrate production was: benzene > m-xylene > toluene while at 20 mg C/L, the sequence changed to m-xylene > toluene > benzene for both nitrification inhibition and BTX compounds persistence. At 5 mg C/L of BTX compounds, there was no toxic effect on the sludge whereas from 10 to 50 mgC/L, bacteria did not totally recover their nitrifying activity. At a concentration of 5 mg C/L, toluene was first oxidized to benzyl alcohol, which was later oxidized to butyrate while m-xylene was oxidized to acetate and butyrate.