Glyphosate degradation by immobilized bacteria: field studies with industrial wastewater effluent.

Immobilized bacteria have been shown in the laboratory to effectively remove glyphosate from wastewater effluent discharged from an activated sludge treatment system. Bacterial consortia in lab columns maintained a 99% glyphosate-degrading activity (GDA) at a hydraulic residence time of less than 20 min. In this study, a pilot plant (capacity, 45 liters/min) was used for a field demonstration. Initially, activated sludge was enriched for microbes with GDA during a 3-week biocarrier activation period. Wastewater effluent was then spiked with glyphosate and NH4Cl and recycled through the pilot plant column during start-up. Microbes with GDA were enhanced by maintaining the pH at less than 8 and adding yeast extract (less than 10 mg/liter). Once the consortia were stabilized, the column capacity for glyphosate removal was determined in a 60-day continuous-flow study. Waste containing 50 mg of glyphosate per liter was pumped at increasing flow rates until a steady state was reached. A microbial GDA of greater than 90% was achieved at a 10-min hydraulic residence time (144 hydraulic turnovers per day). Additional studies showed that microbes with GDA were recoverable within (i) 5 days of an acid shock and (ii) 3 days after a 21-day dormancy (low-flow, low-maintenance) mode. These results suggest that full-scale use of immobilized bacteria can be a cost-effective and dependable technique for the biotreatment of industrial wastewater.     

Effects of operating parameters on performances of nitrification and denitrification processes

Nitrification and denitrification of synthetic wastewater was studied by using two reactors in series. An activated sludge unit was used for nitrification followed by a downflow biofilter (packed column) for denitrification. A glucose solution was fed to the denitrification column to supply carbon source. Effects of important process variables such as sludge age, hydraulic residence time and feed ammonium concentration on system's performance were investigated. Effluent ammonium-nitrogen (NH4-N) concentration decreased with increasing sludge age and hydraulic residence time and remained constant for sludge age and hydraulic residence times greater than 12 d and 15 h, respectively. Feed ammonium-nitrogen concentration above 200 mg/l resulted in significant levels of NH4-N in the effluent at Š_c = 15 d and Š_H = 12 h in nitrification. Performance of denitrification stage was not satisfactory for feed NO₃-N concentrations above 150 mg N/l resulting in significant effluent NO₃-N levels at hydraulic residence time of Š_H = 6 h.     

Glyphosate degradation by immobilized bacteria: laboratory studies showing feasibility for glyphosate removal from waste water.

To evaluate immobilized bacteria technology for the removal of low levels of glyphosate (N-phosphonomethylglycine) from aqueous industrial effluents, microorganisms with glyphosate-degrading activity obtained from a fill and draw enrichment reactor inoculated with activated sludge were first exposed to glyphosate production wastes containing 500-2000 mg glyphosate/L. The microorganisms were then immobilized by adsorption onto a diatomaceous earth biocarrier contained in upflow Plexiglas columns. The columns were aerated, maintained at pH 7.0-8.0, incubated at 25 degrees C, supplemented with NH4NO3 (50 mg/L), and exposed to glyphosate process wastes pumped upflow through the biocarrier. Glyphosate degradation to aminomethylphosphonic acid was initially > 96% for 21 days of operation at flows yielding hydraulic residence times (HRTs) as short as 42 min. Higher flow rate studies showed > 98% removal of 50 mg glyphosate/L from the waste stream could be achieved at a HRT of 23 min. Glyphosate removal of > 99% at a 37-min HRT was achieved under similar conditions with a column inoculated with a pure culture of Pseudomonas sp. strain LBr, a bacterium known to have high glyphosate-degrading activity. After acid shocking (pH 2.8 for 18 h) of a column of immobilized bacteria, glyphosate-degrading activity was regained within 4 days without reinoculation. Although microbial growth and glyphosate degradation were not maintained under low organic nutrient conditions in the laboratory, the low levels of degradable carbon (45-94 mg/L) in the industrial effluent were sufficient to support prolonged glyphosate-degrading activity. The results demonstrated that immobilized bacteria technology is effective in removing low levels of glyphosate in high-volume liquid waste streams.     

Factors influencing antibiotic resistance burden in municipal wastewater treatment plants

Municipal wastewater treatment plants are recognized reservoirs of antibiotic-resistant bacteria. Three municipal wastewater treatment plants differing on the dimensions and bio-treatment processes were compared for the loads of amoxicillin-, tetracycline-, and ciprofloxacin-resistant heterotrophic bacteria, enterobacteria, and enterococci in the raw inflow and in the treated effluents. The sewage received by each plant, in average, corresponded to 85,000 inhabitant equivalents (IE), including pretreated industrial effluents (≤30%) in plant activated sludge, 105,000 IE, including pretreated hospital effluents (≤15%) in plant trickling filter, and 2,000 IE, exclusively of domestic sewage, in plant submerged aerated filter. The presence of pretreated industrial effluents or of pretreated hospital sewage in the raw inflow did not imply significantly higher densities (per milliliter or per IE) of antibiotic-resistant bacteria in the raw wastewater. Longer hydraulic residence periods (24 h) corresponded to higher bacterial removal rates than shorter periods (12 and 9 h), although such efficiency did not imply significant average decreases in the antibiotic resistance prevalence of the treated effluent. The bacterial loads in the treated effluent could be ranked according to the treatment efficiency, suggesting that the characteristics of the raw inflow may have less relevance on the quality of the treated wastewater than other aspects, such as the inflow volume, the type of biological treatment, or the hydraulic residence time.     

Characterization and biological treatment of pickling industry wastewater

Pickling industry wastewaters present unique difficulties in biological treatment because of high salt content (3–6% salt). Conventional activated sludge cultures disintegrate or loose microbial activity as a result of plasmolysis at salt concentrations above 1%. In order to overcome adverse effects of salt in pickling wastewater, salt tolerant bacteria (Halobacter halobium) was added to activated sludge culture and used in biological treatment of the wastewater in an activated sludge unit. After characterization and nutrient balancing of the wastewater, an activated sludge unit was used in laboratory to investigate the effects of major process variables such as sludge age and hydraulic residence time on performance of the system. Single stage and two stage activated processes were used for the treatment of the pickling wastewater. More than 95% of COD removal was obtained with a single stage process at a sludge age of θ_c=10?d and hydraulic residence time of θ_H=30?h. Similar results were obtained with the two stage process, when sludge ages and hydraulic residence times for each stage were θ_c1=θ_c2=10?d, and θ_H1=θ_H2=15?h, respectively. Kinetic coefficients were determined and the design equations were developed by using the experimental data.     

Microflora from leaf debris is suitable for treatment of starch industry wastewater

Biological treatment of industrial waste is a widely practiced technique that generates comparatively less environmentally hazardous waste than other chemical treatment processes. Wet milling of maize generates huge amount of wastewater (5 m³/ton) of low pH with organic matter and nutrients. Anaerobic methanogenic and aerobic bacteria are mostly highly sensitive to low pH. The treatment of wastewater causes huge cost of chemical neutralization or hydraulic recirculation for maintaining neutral pH. In the present study, different microbial consortia isolated from cow dung, active sludge from an anaerobic reactor for treatment of industrial wastewater, and leaf debris from benthic soil were screened for tolerance against low pH and for potential of chemical oxygen demand (COD) removal in order to find out an alternative microbial population for industrial water treatment at low pH. The most effective consortia found from leaf debris were further investigated for optimal operation. The microscopic analysis of leaf debris sludge showed abundance of Gram‐negative methanococci, which was found tolerant to low pH in plate culture method. On further investigation for COD removal from starch industry effluent, they were found to be most effective at pH 5 with highest COD removal rate of 70% and lowest biomass generation of 81%. Hence, it was concluded that the low pH‐tolerant methanogen bacteria, enriched from leaf debris sludge, is highly beneficial for anaerobic treatment of wastewater from several industries including corn starch industry by reducing cost of operation for neutralization to neutral pH and through reducing excess waste sludge production by the treatment system.     

Microbial community origin and fate through a rural wastewater treatment plant

Conventional wastewater treatment relies on a complex microbiota; however, much of this community is still to be characterized. To better understand the origin, dynamics and fate of bacteria within a wastewater treatment plant: untreated primary wastewater, activated sludge and post-treatment effluent were characterized. From 3163 exact sequence variants (ESVs), 860 were annotated to species-level. In primary wastewater, 28% of ESVs were putative bacterial species previously associated with humans, 14% with animals and 5% as common to the environment. Differential abundance analysis revealed significant relative reductions in ESVs from potentially human-associated species from primary wastewater to activated sludge, and significant increases in ESVs from species associated with nutrient cycling. Between primary wastewater and effluent, 51% of ESVs from human-associated species did not significantly differ, and species such as Bacteroides massiliensis and Bacteroides dorei increased. These findings illustrate that activated sludge increased extracellular protease and urease-producing species, ammonia and nitrite oxidizers, denitrifiers and specific phosphorus accumulators. Although many human-associated species declined, some persisted in effluent, including strains of potential health or environmental concern. Species-level microbial assessment may be useful for understanding variation in wastewater treatment efficiency as well as for monitoring the release of microbes into surface water and the wider ecosystem.     

Characterization of tannery wastewater and biomass in a membrane bioreactor using respirometric analysis

Respirometric techniques and an activated sludge model (ASM) were applied for the characterization of tannery wastewater and biomass in a pilot plant membrane bioreactor (MBR) operating at high sludge age. The traditional respirometric tests and the IWA-ASM1 were modified to take into account the specific operating conditions, the solid-liquid separation technology and the wastewater complexity. As a result the wastewater biodegradable COD was fractionated into four components: readily biodegradable, rapidly hydrolysable, slowly hydrolysable and inorganic (due to the presence of reduced sulphur compounds). The kinetic and stoichiometric parameters of the biomass (heterotrophic and nitrifying) were estimated through the integration of model simulations and respirometric tests results. In particular the ammonium and nitrite-oxidizing biomasses were separately characterized: the growth kinetics of ammonium and nitrite-oxidizing bacteria resulted noticeably lower than the traditional reference values (mu(max,AOB)=0.25d(-1)e mu(max,NOB)=0.23d(-1) at 20 degrees C, respectively). The ASM was finally used to confirm that the results of the wastewater and biomass characterization allow to properly simulate the mixed liquor suspended solids in the MBR pilot plant and the COD concentration in the effluent.     

Characterization of microbial traits associated with glyphosate biodegradation in industrial activated sludge

Summary The microorganisms from two industrial (I1, I2) activated sludges that treat glyphosate (N-phosphonomethyl glycine) wastes and one domestic (D1) sludge were enumerated by microscopic examination and by the use of eight selective media. I1 and I2 had higher total counts but fewer pseudomonads and no yeasts. The enumerations correlated directly with traditional biological performance measurements. A total of 393 microbial strains were isolated from the sludges to correlate the occurrence and relationship of glyphosate-degrading activity (GDA) to 155 biochemical and morphological characteristics. Each activated sludge contained unique bacterial populations with the microbes treating industrial wastes, capable of utilizing a wide range of carbohydrates. Numerical taxonomy (arithmetic average linkage) using simple matching and Jaccard coefficients confirmed that there were five (D1), three (I1), and 12 (I2) clusters. GDA was found in only a small portion of the industrial clusters and did not correlate with any other characteristic tested, even though the GDA strains had a large phenotypic diversity. This suggests that GDA is not a universal trait and its expression requires enrichment through specific selective pressures.     

The application of constant recycle solids concentration in activated sludge process.

The applicability of the model derived by Ramanathan and Gaudy (Biotechnol. Bioeng., 11, 207, (1969)) for completely mixed activated sludge treatment holding the recycle solids concentration as a system constant was investigated using an actual industrial organic wastewater. Short-term experiments were conducted at various dilution rates (1/8, 1/6, 1/4, 1/2, 1/1.5 hr-1) for two recycle solids concentration values (5000 and 7000 mg/liter). The influent substrate concentration was maintained at 1000 mg/liter COD and the hydraulic recycle ratio- alpha, was kept at 0.3. It was found that for bottling plant (Pepsi Cola) wastewaters, a steady state with respect to reactor biological solids and effluent COD, at different dilution rates, could be attained, lending experimental evidence to the assumption that a steady state could be reached in developing the model and also affecting the applicability of the model in industrial organic wastewater. The reactor biological solids and effluent COD calculated from the model closely agreed with the observed values at dilution rates lower than 0.5 hr-1. Operation at dilution rates higher than 0.5 hr-1 will washout the biological solids from the reactor and the recycle substrate concentration will be apparent if the concentration of XR were not increased.     

Characterization and biological treatment of ceramic industry wastewater

Ceramic industry wastewaters not only contain high suspended and total solids but also significant amounts of dissolved organics resulting in high BOD or COD loads. Suspended solids can be removed from the wastewater by chemical precipitation. However, dissolved BOD/COD compounds can only be removed by biological or chemical oxidation. Effluent wastewater from chemical sedimentation stage of EGE CERAMIC industry was characterized and subjected to biological treatment in a laboratory scale activated sludge unit. Experiments were conducted at different hydraulic and solids retention times. The best results were obtained with Š_c=20 h of hydraulic and Š_c=20 days of solids retention times (sludge age) resulting in effluent COD concentration of 40 mg/l from a feed wastewater of 720 mg/l COD content. The suspended solids content of the activated sludge effluent was approximately 52 mg/l.     

Design,Construction, and Starting-up of an Anaerobic Reactor for the Stabilisation,Handling, and Disposal of Excess Biological Sludge Generated in a Wastewater Treatment Plant

Design, construction, and starting-up of an upflow anaerobic sludge blanket reactor was carried out. This system was proposed for excess sludge stabilisation, particularly that generated at an activated sludge wastewater treatment facility installed in a sugarcane mill. The upflow anaerobic sludge blanket (UASB) reactor built, had a working volume of 22.3 m³and a hydraulic residence time of 22 days. Methane production was at a maximum of 79% volume with an average of 60% for this treatment. For starting up the anaerobic reactor, a suitable inoculum from a neighboring plant was used. As the waste characteristics in both plants were different, an acclimation procedure was followed to achieve granulation. Control and stability of anaerobic reactions were monitored with alkalinity data, using the so-called ‘alfa alkalinity’ to try to keep its value at around 0.4. Once pseudosteady-state conditions were reached (chemical oxygen demand reduction and methane-rich biogas production within ±10 percent), the organic load was steadily increased up to feeding 100% excess sludge. The UASB reactor used to stabilise the excess biomass generated a sludge with a much lower volume than that originally fed. Its design ensured adequate hydraulic flow and biogas production with a high methane content. The bacteria were attached constituting spheres and very minor maintenance operations were required.     

Biodegradation of p-nitrophenol in an aqueous waste stream by immobilized bacteria.

Microbiological analyses of activated sludge reactors after repeated exposure to 100 mg of p-nitrophenol (PNP) per liter resulted in the isolation of three Pseudomonas species able to utilize PNP as a sole source of carbon and energy. Cell suspensions of the three Pseudomonas sp., designated PNP1, PNP2, and PNP3, mineralized 70, 60, and 45% of a 70-mg/liter dose of PNP in 24, 48, and 96 h, respectively. Mass-balance analyses of PNP residues for all three cultures showed that undegraded PNP was less than 1% (less than 50 micrograms); volatile metabolites, less than 1%; cell residues, 8.4 to 14.9%; and water-soluble metabolites, 1.2 to 6.7%. A mixed culture of all three PNP-degrading Pseudomonas sp. was immobilized by adsorption onto diatomaceous earth biocarrier in a 1.75-liter Plexiglas column. The column was aerated and exposed to a synthetic waste stream containing 629 to 2,513 mg of PNP per liter at flow rates of 2 to 15 ml/min. Chemical loading studies showed that the threshold concentration for acute toxicity of PNP to the immobilized bacteria was 2,100 to 2,500 mg/liter. Further studies at PNP concentrations of 1,200 to 1,800 mg/liter showed that greater than 99 and 91 to 99% removal of PNP was achieved by immobilized bacteria at flow rates of 10 and 12 ml/min, respectively. These values represent hydraulic retention times of 48 to 58 min and PNP removal rates of 0.99 to 1.1 mg/h per g of biocarrier at 25 degrees C under optimal conditions. This study shows the successful use of immobilized bacteria technology to remove high concentrations of PNP from aqueous waste streams.     

Biodegradation of p-nitrophenol in an aqueous waste stream by immobilized bacteria

Microbiological analyses of activated sludge reactors after repeated exposure to 100 mg of p-nitrophenol (PNP) per liter resulted in the isolation of three Pseudomonas species able to utilize PNP as a sole source of carbon and energy. Cell suspensions of the three Pseudomonas sp., designated PNP1, PNP2, and PNP3, mineralized 70, 60, and 45% of a 70-mg/liter dose of PNP in 24, 48, and 96 h, respectively. Mass-balance analyses of PNP residues for all three cultures showed that undegraded PNP was less than 1% (less than 50 micrograms); volatile metabolites, less than 1%; cell residues, 8.4 to 14.9%; and water-soluble metabolites, 1.2 to 6.7%. A mixed culture of all three PNP-degrading Pseudomonas sp. was immobilized by adsorption onto diatomaceous earth biocarrier in a 1.75-liter Plexiglas column. The column was aerated and exposed to a synthetic waste stream containing 629 to 2,513 mg of PNP per liter at flow rates of 2 to 15 ml/min. Chemical loading studies showed that the threshold concentration for acute toxicity of PNP to the immobilized bacteria was 2,100 to 2,500 mg/liter. Further studies at PNP concentrations of 1,200 to 1,800 mg/liter showed that greater than 99 and 91 to 99% removal of PNP was achieved by immobilized bacteria at flow rates of 10 and 12 ml/min, respectively. These values represent hydraulic retention times of 48 to 58 min and PNP removal rates of 0.99 to 1.1 mg/h per g of biocarrier at 25 degrees C under optimal conditions. This study shows the successful use of immobilized bacteria technology to remove high concentrations of PNP from aqueous waste streams.     

Configuration of biological wastewater treatment line and influent composition as the main factors driving bacterial community structure of activated sludge

The structure of microbial consortia in wastewater treatment facilities is a resultant of environmental conditions created by the operational parameters of the purification process. In the research, activated sludge from nine Polish wastewater treatment plants (WWTPs) was investigated at a molecular level to determine the impact of the complexity of biological treatment line and the influent composition on the species structure and the diversity of bacterial consortia. The community fingerprints and technological data were subjected to the canonical correspondence and correlation analyses. The number of separated biological processes realized in the treatment line and the presence of industrial wastewater in the influent were the key factors determining the species structure of total and ammonia-oxidizing bacteria in biomass. The N₂O-reducers community composition depended significantly on the design of the facility; the highest species richness of denitrifiers was noted in the WWTPs with separated denitrification tanks. The contribution of industrial streams to the inflow affected the diversity of total and denitrifying bacterial consortia and diminished the diversity of ammonia oxidizers. The obtained data are valuable for engineers since they revealed the main factors, including the design of wastewater treatment plant, influencing the microbial groups critical for the stability of purification processes.     

Anaerobic treatment of wastewater from a food-manufacturing plant with a low concentration of organic matter and regeneration of usable pure water

Wastewater from a food-manufacturing plant with a low concentration of organic matter below 100 mg/l TOC was first treated at 37°C in an anaerobic fluidized-bed reactor (AFBR) or in an upflow anaerobic sludge blanket (UASB). The TOC removal efficiency in both reactors decreased from 85% to 65% as the influent TOC concentration decreased from 100 to 35 mg/l at a hydraulic retention time (HRT) of 6 h. Treatment at an HRT of 4 h resulted in an effluent TOC concentration of 11 to 15 mg/l. The concentration of suspended solids in the effluent could be reduced to 20 mg/l, which corresponded to 7% of that of the influent. The effluent from both reactors was then treated anaerobically in a fixed-bed reactor system. The TOC concentration and optical density (OD) of the effluent from the aerobic treatment were reduced to 5 mg/l and 0.005, respectively, at an HRT of 2 h. When anaerobically or aerobically treated effluent was passed over an activated carbon column, the effluent TOC concentration was reduced to 2 to 3 mg/l. The conductivity of 1.3 mS/cm in raw wastewater, which was not removed through the above treatments, was reduced to 0.001 mS/cm on an ion-exchange resin column. An effluent quality corresponding to that of ultra-pure water for industrial use was finally attained by the treatment in this multi-step system.     

In situ characterization of nitrifiers in an activated sludge plant: detection of Nitrobacter Spp

Aims: The purpose of this work was to investigate microbial ecology of nitrifiers at the genus level in a typical full-scale activated sludge plant. Methods and Results: Grab samples of mixed liquor were collected from a plug-flow reactor receiving domestic wastewater. Fluorescent in situ hybridization technique (FISH) was used to characterize both ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) in combination with Confocal Scanning Laser Microscope (CSLM). Fluorescently labelled, 16S rRNA-targeted oligonucleotide probes were used in this study. Both Nitrosomonas and Nitrosospira genera as AOB and Nitrobacter and Nitrospira genera as NOB were sought with genus specific probes Nsm156, Nsv443 and NIT3 and NSR1156, respectively. Conclusions: It was shown that Nitrosospira genus was dominant in the activated sludge system studied, although Nitrosomonas is usually assumed to be the dominant genus. At the same time, Nitrobacter genus was detected in activated sludge samples. Significance and Impact of the Study: Previous studies based on laboratory scale pilot plants employing synthetic wastewater suggested that only Nitrospira are found in wastewater treatment plants. We have shown that Nitrobacter genus might also be present. We think that these kinds of studies may not give a valid indication of the microbial diversity of the real full-scale plants fed with domestic wastewater.     

Effect of sludge age on performance of an activated sludge unit treating 2,4 dichlorophenol-containing synthetic wastewater

Wastewaters containing chlorophenol compounds are difficult to treat by biological means because of toxic effects of chlorophenols on microorganisms. Synthetic wastewater containing 2,4 dichlorophenol (DCP) was biologically treated in an activated sludge unit at different sludge ages varying between 5 and 30 days while the feed COD, DCP contents and hydraulic residence time (HRT) were constant. Effects of sludge age on COD, DCP and toxicity removals were investigated. Increases in sludge age caused significant increases in biomass concentration in the aeration tank, which resulted in increases in percent COD, DCP and toxicity removals. COD removal increased from 58 to 90%, while DCP and toxicity removals increased from 15 to 100% and from 38 to 100%, respectively, when the sludge age was raised from 5 to 30 days. Resazurin method based on dehydrogenase activity was used for assessment of the feed and effluent wastewater toxicity. Sludge volume index (SVI) decreased with increasing sludge age indicating improved settling characteristics of the sludge at high sludge ages. Operation at a sludge age of 25 days resulted in more than 90% COD and nearly 100% DCP and toxicity removal with an SVI value of 108 ml g⁻¹ under the experimental conditions tested.     

Limitation of sludge biotic index application for control of a wastewater treatment plant working with shock organic and ammonium loadings

This study aimed to determine the relationship between activated sludge microfauna, the sludge biotic index (SBI) and the effluent quality of a full-scale municipal wastewater treatment plant (WWTP) working with shock organic and ammonium loadings caused by periodic wastewater delivery from septic tanks. Irrespective of high/low effluent quality in terms of COD, BOD₅, ammonium and suspended solids, high SBI values (8–10), which correspond to the first quality class of sludge, were observed. High SBI values were connected with abundant taxonomic composition and the domination of crawling ciliates with shelled amoebae and attached ciliates. High SBI values, even at a low effluent quality, limit the usefulness of the index for monitoring the status of an activated sludge system and the effluent quality in municipal WWTP-treated wastewater from septic tanks. It was shown that a more sensitive indicator of effluent quality was a change in the abundance of attached ciliates with a narrow peristome (Vorticella infusionum and Opercularia coarctata), small flagellates and crawling ciliates (Acineria uncinata) feeding on flagellates.     

Performance evaluation of an anoxic/oxic activated sludge system: effects of mean cell residence time and anoxic hydraulic retention time

A laboratory investigation has been undertaken to asses the effects of two operating parameters, mean cell residence time (MCRT) and anoxic hydraulic retention time (HRT), on the performance of an anoxic/oxic activated sludge system. The performance of the system was evaluated in terms of its COD, nitrogen, and biomass characteristics. An activated sludge system is capable of producing a better effluent, in terms of COD and nitrogen characteristics, when it is operated in an anoxic/oxic fashion. A longer MCRT and an adequate anoxic HRT are desirable in the operation of an anoxic/oxic activated sludge system. For the wastewater used in this investigation, the anoxic/oxic unit was capable of producing an effluent with the following characteristics when it was operated at MCRT = 20 days, total system HRT = 10 h, and anoxic HRT = 3-5 h: COD = 15 mg/L; VSS = 10 mg/L; TKN = 1.30 mg/L; NH(3) - N = 0.60 mg/L; and NO(2) + NO(3) - N = 5.0 mg/L. A uniform distribution of biomass is achievable in an anoxic/oxic activated sludge system because of the intensive recirculation/convection maintained. The provision of an anoxic zone in the aeration tank promotes a rapid adsorption of feed COD into the biomass without an immediate utilization for cell synthesis. This, in turn, results in a high microbial activity and a lower observed biomass yield in the system. A tertiary treatment efficiency is achievable in an anoxic/oxic activated sludge system with only secondary treatment operations and costs. A conventional activated sludge system can be easily upgraded by converting to the anoxic/oxic operation with minor process modifications.