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.     

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.     

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.     

Metal Bioavailability and Trivalent Chromium Removal in ABR

Trace metal bioavailability and chromium(III) removal were investigated in an anaerobic baffled reactor (ABR) treating a synthetic waste [COD = 4,000 mg∕L, COD:SO42? ratio 8:1 and 40:1, Cr(III) = 50 mg∕L]. Sulfide precipitation reduced nutrient bioavailability as follows (most deficient first): Fe > Co > Ni. However, the metals had the following affinity for complexation: Ni > Co >> Fe; complexation potential was greatest at the front and rear of the reactor. At the front, it was hypothesized that high concentrations of soluble microbial products provided an excess of potential ligands, which may have sequestered the metals. However, in the last compartment, a higher pH would have altered the competition between protons and metal cations for complexing sites in favor of the metal cations. The complexing potential in the ABR increased with sulfide levels, and this coincided with higher residual COD production, probably to provide a diffusion barrier to the harsh environmental conditions. Although the addition of chromium (50 mg∕L) had no observable effect on reactor performance∕stability, its removal was controlled by the following factors (most influential first): solubility > complexation > absorption > physical adsorption.     

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.     

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.     

Biological treatment of dye wastewaters using an anaerobic-oxic system

Three dye solutions, namely, C.I. Acid Yellow 17, C.I. Basic Blue 3, and C.I. Basic Red 2, were treated in an upflow anaerobic sludge blanket (UASB) reactor followed by a semi-continuous aerobic activated sludge tank. When hydraulic retention time was about 12 hours, no significant color removal was observed in the aerobic stage. In the anaerobic stage, Acid Yellow 17, Basic Blue 3, and Basic Red 2 were removed by 20%, 72%, and 78%, respectively. To treat wastewater from a dye manufacturing factor with COD concentration of 1200 mg/l and Color of 500 degrees (dilution factor), an UASB reactor (4.5 liters) and an activated sludge tank (5 liters, adjustable), COD and color were removed by more than 83% and 90% at a COD loading rate of 5.3 kg COD/m3-day in the anaerobic stage, and at the hydraulic retention time of 6-10 hours for the anaerobic stage and 6.5 for the aerobic stage. The anaerobic stage of the A/O system removes both color and COD. In addition, it also improves biodegradability of dyes for further aeroic treatment.     

Evaluation of Influent Medium Composition and Concentration in a Batch Reactor for Effective Startup of UASB Reactors

This study demonstrated the use of batch bioreactor experiments as an evaluation tool to determine appropriate influent medium composition and concentration for effective startup of an upflow anaerobic sludge blanket (UASB) reactor. Using seed sludge from a domestic treatment plant, the weight ratio of glucose and volatile fatty acids (VFA) present in the influent synthetic medium was varied and the batch performance was assessed in terms of pH variation during the course of the experiment, chemical oxygen demand (COD) reduction, specific substrate uptake rates, methanogenic activity, and methane yield. The performance was also evaluated by increasing the concentration of influent COD. Medium composition with glucose to VFA weight ratio at or below 2:8 and COD concentration at 11?g/L was determined to be optimum for effective startup of the UASB reactors. Utilization of the optimized influent medium provided a pH variation from 6.5 to 7.8 over the length of the UASB reactor and resulted in granule formation, high methanogenic activity, and methane yield. The evaluation method provided a practical approach to determine the applicability of seed sludge from a particular source and the desired influent characteristics for reduced startup duration in UASB processes.     

Treatment of Landfill Leachate by a Combined Anaerobic Fluidized Bed and Zeolite Column System

The use of a combined anaerobic fluidized bed and zeolite fixed bed system in sanitary landfill leachate treatment was investigated. Anaerobic treatability studies were successfully performed in the anaerobic fluidized bed reactor. The chemical oxygen demand (COD) removal was attained up to 90% with increasing organic loading rates as high as 18?g?COD/L?day after 80?days of operation. Good biogas production yield (Ygas) of 0.53?L biogas per gram removed COD with methane (CH4) content of 75% was obtained. The attached biomass concentration increased along the column height from bottom to top, and its mean value was found 6,065?mg/L after 100?days of operation. The anaerobically treated landfill leachate was further treated by a zeolite fixed bed reactor. While excellent ammonia removal (>90%) was obtained with the untreated zeolite, the regenerated zeolites showed higher performance. Consequently, this combined anaerobic and adsorption system is an effective tool to remove high COD and high ammonia in landfill leachate.     

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%.     

Effect of Aeration on the Quality of Effluent from UASB Reactor Treating Sewage

The treatment of effluent of pilot- and full-scale upflow anaerobic sludge blanket (UASB) reactors operating at steady state was studied in an aeration-settling system. The fine pore submerged diffusers were used to aerate the effluent of UASB reactors under different operating conditions. Forty to 55% of the biochemical oxygen demand (BOD) and the chemical oxygen demand (COD) removal efficiencies were achieved by the direct aeration of the UASB effluent in the laboratory. The maximum removal efficiencies were achieved at 30?min hydraulic retention time (HRT) and a dissolved oxygen (DO) of 5–6??mg/L or high KLa (vigorous aeration). Batch experiments on nitrogen purging and the aeration of sulfides, volatile organic compounds (VOCs), and nonpurgeable organic carbons (NPOCs) were performed to ascertain the mechanism of BOD/COD removal. During aeration, BOD and COD were reduced by the stripping of H2S and VOCs and by the chemical oxidation of total sulfides and organic carbon. The stripping and chemical oxidation depended on the HRT and DO. The performance of a full-scale surface aeration system was compared to the performance of a pilot-scale diffused aeration system. Final sedimentation was effective only in removing the solids from the effluent of the aeration system. The results were confirmed by organic mass balance.     

Effects of Stressed Loading on Startup and Granulation in Upflow Anaerobic Sludge Blanket Reactors

The successful operation of an upflow anaerobic sludge blanket (UASB) process depends on the formation of settleable and active granular sludge. As the anaerobic bacteria are slow-growing microorganisms, a common problem encountered in UASB operation is the long startup period and the development of biogranules. In the present study, an unconventional approach to accelerate startup and granulation processes in UASB reactors has been developed by stressing the organic loading rate (OLR) without having to reach steady-state conditions. Three UASB reactors treating a synthetic feed with chemical oxygen demand (COD) of 2,500 mg/L, at a mesophilic temprature of 35°C were studied. One reactor (R1) served as a control, while the other two (R2 and R3) were operated at different stress levels upon reaching COD removal efficiency of 75 and 85%, respectively. Experimental results indicated that under stressed loading conditions, the startup, and granule development were accelerated by 45 and 33%, respectively, along with the formation of granules of superior characteristics without deteriorating loading capacity. The operating time to reach designated OLRs was also shortened by at least 30 days in the stressed reactors. The results presented indicate that the unconventional startup approach could offer a practical solution for the inherent long start-up in UASB systems with concomitant saving in time and cost.     

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.     

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.     

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.     

Molecular Mechanism of Granulation. II: Proton Translocating Activity

A laboratory-scale upflow anaerobic sludge blanket (UASB) reactor was used in this study to produce granular sludge at mesophilic temperatures (35 ± 1°C). After more than 150 days of operation, a COD removal efficiency of 95% was achieved with an organic loading rate of 8.73 gCOD∕L∕day. At the same time, the sludge granulation process was observed. The mature granules were examined for their stability in terms of the presence of calcium ion, surfactant, pH (buffer and H2SO4∕NaOH solution), metabolic inhibitor (iodoacetic acid and sodium fluoride), and proton translocator (carbonyl cyanide m-chlorophenyl-hydrazone). The results showed that bacterial surface dehydration, biological metabolic activity, and proton translocating activity were directly related to the strength of UASB granules. This indicated that the proton translocating activity on bacterial surfaces was the crucial factor in sludge granulation and, as a consequence, supported the proton translocation-dehydration theory. Experimental results from other studies were also used to support this new theory.     

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.     

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.     

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.