Effect of nitrogen limitation on performance of toluene degrading biofilters

The literature reports conflicting observations regarding the need for nutrient addition to biofilters treating contaminated gases. Such conflicts are often based on quasi-steady-state performance data collected on biofilters operated under continuous loading conditions. In the studies described herein, the impact of nitrogen limitations on two toluene-fed biofilters was assessed over a 97-day period. The biofilters were packed with polyurethane foam medium and contained different initial levels of nitrate-nitrogen. Toluene and CO2 concentration profiles were monitored during both normal steady loading conditions and short-term, unsteady-state transient loading conditions (e.g., shock loads). Packing medium samples were periodically removed and analyzed to quantify changes in nitrate-nitrogen content over time. Data are presented which show that over long-time periods (several months), nutrient-induced kinetic limitations diminished biofilter performance during transient, unsteady-state conditions even when performance during normal steady loading was not adversely affected. Elemental analysis of biomass removed from the biofilters support nitrate-nitrogen and CO2 concentration profile data and clearly illustrate how kinetically limited biofilters fail during shock loads even when there is an overall stoichiometric excess of nutrients.     

H2S and VOCs abatement robustness in biofilters and air diffusion bioreactors: A comparative study

The robustness of a conventional biofilter and an air diffusion bioreactor (ADB) was comparatively evaluated in laboratory-scale plants treating a mixture of H₂S, butanone, toluene and alpha-pinene at gas residence times of 50 s. Under steady state conditions, H₂S, butanone and toluene were almost completely degraded, while alpha-pinene removal did not exhibit removal efficiencies (REs) higher than 11.0 ± 2.3%. Fluctuations in temperature from 8 °C to 30 °C did not impact significantly process performance in any of the biotechnologies tested. However, while the ADB unit was able to cope with three and six fold step increases in pollutant loadings, volatile organic compounds (VOCs) REs noticeably decreased in the biofilter when subjected to a six fold step change (i.e. 90% reduction for butanone and 30% for toluene). A process shutdown of five days resulted in the temporary loss of butanone and toluene RE in the ADB system. A lack of irrigation during five days caused a slight decrease in the biofilter REs, while a failure in the pH control system drastically affected the ADB performance. Finally, process robustness was quantified. The calculated overall risks showed that both biotechnologies were reliable for H₂S and VOCs treatment in wastewater treatment plants, ADB diffusion exhibiting a higher robustness towards fluctuations commonly found under routine operation. This robustness was further confirmed by the high stability of the DGGE profiles.     

Aerobic VS. anaerobic biological treatment of organically contaminated groundwater

This paper describes laboratory scale results of aerobic and anaerobic biological treatment studies conducted to evaluate the feasibility of treating ground water contaminated with an organic solvent consisting of an equal weight‐mixture of methylethylketone and cyclohexanone. For this purpose, three alternatives were considered, namely a single‐stage anaerobic baffled reactor, an activated sludge system, and an aerated lagoon. The study focused on determining and comparing the treatment efficiency of each of the three treatment processes under similar operating conditions. Aerobic processes proved to be more effective in treating the organically contaminated groundwater.     

Biological treatment of H(2)S using pellet activated carbon as a carrier of microorganisms in a biofilter

Biological treatment is an emerging technology for treating off-gases from wastewater treatment plants. The most commonly reported odourous compound in off-gases is hydrogen sulfide (H(2)S), which has a very low odor threshold. This study aims to evaluate the feasibility of using a biological activated carbon as a novel packing material, to achieve a performance-enhanced biofiltration processes in treating H(2)S through an optimum balance and combination of the adsorption capacity with the biodegradation of H(2)S by the bacteria immobilized on the material. The biofilm was mostly developed through culturing the bacteria in the presence of carbon pellets in mineral media. Scanning electron microscopy (SEM) was used to identify the biofilm development on carbon surface. Two identical laboratory scale biofilters, one was operated with biological activated carbon (BAC) and another with virgin carbon without bacteria immobilization. Various concentrations of H(2)S (up to 125 ppmv) were used to determine the optimum column performance. A rapid startup (a few days) was observed for H(2)S removal in the biofilter. At a volumetric loading of 1600 m(3)m(-3)h(-1) (at 87 ppmv H(2)S inlet concentration), elimination capacity of the BAC (181 gH(2)Sm(-3)h(-1)) at removal efficiency (RE) of 94% was achieved. If the inlet concentration was kept at below 30 ppmv, high H(2)S removal (over 99%) was achieved at a gas retention time (GRT) as low as 2s, a value, which is shorter than most previously reported for biofilter operations. The bacteria population in the acidic biofilter demonstrated capacity for removal of H(2)S in a broad pH range (pH 1-7). There are experimental evidences showing that the spent BAC could be re-used as packing material in a biofilter based on BAC. Overall, the results indicated that an unprecedented performance could be achieved by using BAC as the supporting media for H(2)S biofiltration.     

Distributions of respiratory contaminants from a patient with different postures and exhaling modes in a single-bed inpatient room

This study investigated contaminant transport and evaluated the ventilation performance in a single-bed inpatient room. The study performed comparative experimental analysis on the distributions of respiratory contaminants breathed out and coughed out by a patient in a full-scale chamber, which simulated a single-bed inpatient room. The contaminant exhaled by the patient was simulated by an SF₆ tracer gas and 3-μm particles at steady-state conditions. The differences in the contaminant distribution between the coughing and breathing cases were insignificant for the mixing ventilation case, while for the displacement ventilation, the contaminant concentrations in the upper part of the room were higher for the coughing case. The contaminant concentrations in the inpatient room for the case with the patient sitting on the bed were lower than those for the patient supine on the bed for the displacement ventilation under the same supply airflow rate. The SF₆ tracer gas and 3-μm particles released at a notable initial velocity for simulating a cough could give similar contaminant distributions in the inpatient room. Therefore, the experimental data can be used to validate a CFD model, and the validated CFD model can be used to investigate transient coughing and breathing processes.     

Effect of gas empty bed contact time on performances of various types of rotating drum biofilters for removal of VOCs

The effects of gas empty bed contact time (EBCT), biofilter configuration, and types of volatile organic compounds (VOCs) were evaluated to assess the performance of rotating drum biofilters (RDBs), especially at low EBCT values. Three types of pilot-scale RDBs, a single-layer RDB, a multi-layer RDB, and a hybrid RDB, were examined at various gas EBCTs but at a constant VOC loading rate. Diethyl ether, toluene, and hexane were used separately as model VOC. When EBCT increased from 5.0 to 60s at a constant VOC loading rate of 2.0kgCOD/(m(3)day), ether removal efficiency increased from 73.1% to 97.6%, from 81.6% to 99.9%, and from 84.0% to 99.9% for the single-layer RDB, the multi-layer RDB, and the hybrid RDB, respectively, and toluene removal efficiency increased from 76.4% to 99.9% and from 84.8% to 99.9% for the multi-layer RDB and the hybrid RDB, respectively. When hexane was used as the model VOC at a constant loading rate of 0.25kgCOD/(m(3)day), hexane removal efficiency increased from 31.1% to 57.0% and from 29.5% to 50.0% for the multi-layer RDB and hybrid RDB, respectively. The single-layer, multi-layer, and hybrid RDBs exhibited, respectively, the lowest, middle, and highest removal efficiencies, when operated under similar operational loading conditions. Hexane exhibited the lowest removal efficiency, while diethyl ether displayed the highest removal efficiency. The data collected at the various EBCT values correlated reasonably well with a saturation model. The sensitivity of removal efficiency to EBCT varied significantly with EBCT values, VOC properties, and biofilter configurations. Process selection and design for RDB processes should consider these factors.     

中深层套管式地埋管换热器换热性能模拟研究

建立套管式地埋管换热器数值模型,采用FLUENT对制热工况下套管式地埋管换热器的换热性能进行模拟,研究运停比、岩土热导率、地温梯度等因素对单位钻孔深度换热量的影响.岩土热导率为2.5 W/(m·K),地温梯度为0.03℃/m条件下:不同运停比的单位钻孔深度换热量均随时间推移而下降.相同供暖期,单位钻孔深度换热量由大到小...     

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.     

Response of the anaerobic fluidized bed reactor to transient changes in process parameters

Anaerobic fluidized bed reactors treating a high strength dairy type waste water were subjected to transient decreases in temperature and increases in influent flowrate or COD for 4 and 8 h periods. The quality of the effluent deteriorated during a shock load but returned to normal steady state values 6–22 h after original conditions had been restored, the stability of the reactor during increases in influent COD were found to be influenced by the bicarbonate buffering capacity of the feed.     

Long-term ammonia removal in a coconut fiber-packed biofilter: Analysis of N fractionation and reactor performance under steady-state and transient conditions

A comprehensive study of long-term ammonia removal in a biofilter packed with coconut fiber is presented under both steady-state and transient conditions. Low and high ammonia loads were applied to the reactor by varying the inlet ammonia concentration from 90 to 260 ppm_v and gas contact times ranging from 20 to 36 s. Gas samples and leachate measurements were periodically analyzed and used for characterizing biofilter performance in terms of removal efficiency (RE) and elimination capacity (EC). Also, N fractions in the leachate were quantified to both identify the experimental rates of nitritation and nitratation and to determine the N leachate distribution. Results showed stratification in the biofilter activity and, thus, most of the NH₃ removal was performed in the lower part of the reactor. An average EC of 0.5 kg N-NH₃ m⁻³ d⁻¹ was obtained for the whole reactor with a maximum local average EC of 1.7 kg N-NH₃ m⁻³ d⁻¹. Leachate analyses showed that a ratio of 1:1 of ammonium and nitrate ions in the leachate was obtained throughout steady-state operation at low ammonia loads with similar values for nitritation and nitratation rates. Low nitratation rates during high ammonia load periods occurred because large amounts of ammonium and nitrite accumulated in the packed bed, thus causing inhibition episodes on nitrite-oxidizing bacteria due to free ammonia accumulation. Mass balances showed that 50% of the ammonia fed to the reactor was oxidized to either nitrite or nitrate and the rest was recovered as ammonium indicating that sorption processes play a fundamental role in the treatment of ammonia by biofiltration.     

Large-eddy simulation of human-induced contaminant transport in room compartments

A large-eddy simulation is used to investigate contaminant transport owing to complex human and door motions and vent-system activity in room compartments where a contaminated and clean room are connected by a vestibule. Human and door motions are simulated with an immersed boundary procedure. We demonstrate the details of contaminant transport owing to human- and door-motion-induced wake development during a short-duration event involving the movement of a person (or persons) from a contaminated room, through a vestibule, into a clean room. Parametric studies that capture the effects of human walking pattern, door operation, over-pressure level, and vestibule size are systematically conducted. A faster walking speed results in less mass transport from the contaminated room into the clean room. The net effect of increasing the volume of the vestibule is to reduce the contaminant transport. The results show that swinging-door motion is the dominant transport mechanism and that human-induced wake motion enhances compartment-to-compartment transport. PRACTICAL IMPLICATIONS: The effect of human activity on contaminant transport may be important in design and operation of clean or isolation rooms in chemical or pharmaceutical industries and intensive care units for airborne infectious disease control in a hospital. The present simulations demonstrate details of contaminant transport in such indoor environments during human motion events and show that simulation-based sensitivity analysis can be utilized for the diagnosis of contaminant infiltration and for better environmental protection.     

Fluvial-controlled metal and As mobilisation, dispersal and storage in the Río Guadiamar, SW Spain and its implications for long-term contaminant fluxes to the Doñana wetlands

Flood-related contaminant (As, Cd, Cu, Pb, Zn) remobilisation, dispersal and storage in the Río Guadiamar was investigated following the 1998 Aznalcóllar tailings dam failure, along with records of floodplain contaminant loading in the decades preceding the tailings release. A series of post-spill floods resulted in the transfer of vast quantities of sediment-borne heavy metals and As towards the lower reaches of the Guadiamar and the borders of the Do?ana National Park, but over-bank flood deposits collected between May 1999 and March 2002 show a systematic fall in contaminant concentrations following successive flood events. Geochemical improvements can largely be attributed to sediment mixing of contaminated and 'clean' material derived from calcareous catchment soils. Longer-term contaminant patterns in floodplain sediment cores show higher heavy metal and As loading rates operating before the opening of the Aznalcóllar pit in 1979 and in some instances pre-dating 1954. The remobilization and dispersal of historically contaminated alluvium in the upper Guadiamar means that the post-clean-up contaminant signature in flood-transported sediments largely reflects chronic, long-term metal mining in the Guadiamar catchment, rather than the acute effects of the Aznalcóllar spill. Generally results present a cautiously optimistic prognosis for the sensitive wetlands of Do?ana, but high dissolved (aqueous) heavy metal (especially Cu and Zn) concentrations in the upper Guadiamar emphasise the need for addressing contaminant 'hotspots' in the region and for maintaining flow requirements for aquatic ecosystems. This study illustrates the importance of establishing antecedent geomorphological-geochemical conditions in a spill-impacted river system, both for assessing the impacts of a single catastrophic pollution event and for developing appropriate strategies for remediation.     

城市污水处理厂含硫恶臭气体的产生与治理

胜利油田沙营污水处理厂采用曝气除油沉砂/水解沉淀/曝气生物滤池工艺处理市政污水,出水水质达到《城镇污水处理厂污染物排放标准》(GB 18918—2002)的一级B标准。根据当地的环境条件及已有的工程实例经验,采用洗涤—生物滤床过滤联合除臭工艺,对格栅间、水解沉淀池、DN生物滤池、脱水机房及反冲洗废水池等产生有毒有害气体的构筑物进行加盖封闭,再通过管道将臭气输送到独立的除臭工艺段,处理后高空排放。在除臭构筑物周围5 m内环侧和厂界取样检测,排放气体达到《恶臭污染物排放标准》(GB 14554—93)的二级标准。     

An evaluation of solid phase microextraction for analysis of odorant emissions from stored biosolids cake

Odors are a common occurrence at wastewater treatment plants, biosolids processing facilities and biosolids recycling locations. Accurate, objective measurement techniques are needed to monitor emissions, to develop new waste handling procedures and to reduce the production of the volatile gases. The objective of this study was to evaluate the use of solid phase microextraction for measuring common odorants that are found in biosolids facilities. The odorants were collected and concentrated by solid phase microextraction (SPME) and then quantified by gas chromatography with detection by mass spectrometry. A 75-microm Carboxen-Polydimethylsiloxane coating was used for the analysis of trimethylamine, dimethyl sulfide, dimethyl disulfide and methyl mercaptan. Gaseous standards were generated for individual compounds and for dry and wet mixture from permeation apparatus. The differences in sensitivity between fibers, the competition between analytes and water vapor for the active sites on the fiber and the lack of production of artifacts make SPME suited for qualitative analysis and enables quick screening for the identification of compounds with adverse organoleptic characteristics.     

The effect of nitrate on VOC removal in trickle-bed biofilters

In response to the growing concern over volatile organic compounds (VOCs), biofiltration is becoming an established economical air pollution control technology for removing VOCs from waste air streams. Current research efforts are concentrating on improving control over key parameters that affect the performance of gas phase biofilters. This study utilized diethyl ether as a substrate, nitrate as the sole nutrient nitrogen source within two co-currently operated trickle-bed biofilters, for over 200 days. The two pelletized medium biofilters were operated at a low empty bed contact time of 25 s, inlet gas flow rates of 8.64 m³/day, nutrient liquid flow rates of 1 liter/day, and COD loading rates of 1.8 and 3.6 kg/m³ per day, respectively. Operational parameters including contaminant concentration in the gas phase, nutrient nitrate concentration in the aqueous phase, and the frequency of biomass removal were considered. Special attention was given to the effect and the role of nitrate on VOC removal. Throughout the experiment, nitrate persisted in the liquid effluent and the ether removal efficiencies improved with increasing influent nitrate concentration, which suggest that the nitrate diffusion into the biofilms is rate determining. By increasing the concentration of oxygen in the feed to this biofilter from 21% (ambient air) to 50 and 100%, while maintaining an influent ether concentration of 133 ppmv and a feed nitrate concentration of 67 mg-N/liter, the performance of the biofilter was not significantly affected. These results suggest that nitrogen was rate limiting as a growth nutrient rather than as an electron acceptor for the respiration of ether. The results also indicated that removal of excess biomass is necessary to maintain long-term performance. However, the required frequency of biomass removal depends on operating parameters such as loading.     

Outdoor Air Contaminants and Indoor Air Quality under Transient Conditions

The indoor concentrations of contaminants originating from outdoor sources have been measured and calculated under transient conditions. The results show that contaminants that are supplied to an office building via the ventilation system can reach considerably high concentration levels. The indoor/outdoor concentration ratio and time lag are dependent on the air change rate. In buildings with low air change rates the indoor concentration variations are smoothed out compared to buildings with high air change rates. The results from the theoretical model are compared to the results from both laboratory and field measurements and the model is verified for well mixed conditions in a 20 m³ test chamber. The model can be used to simulate different control strategies for reduction of indoor contaminant concentrations related to outdoor sources. One such control strategy is based on reduction of the outdoor air change rate during periods with peak outdoor contaminant concentrations.     

Response of dissolved oxygen to changes in influent organic loading to activated sludge systems

The major objective of this investigation was to examine the response of the activated sludge process to transient organic loadings and to evaluate the applicability of dissolved oxygen (DO) concentration and oxygen uptake rate as process control variables.A modified laboratory-scale activated sludge unit was used to continuously measure the ambient dissolved oxygen level. The change of the dissolved oxygen concentration responded to variations in the influent composition of the wastewater. First, the system was maintained at steady-state and system variables were measured for soluble TOC, MLVSS and oxygen uptake rate. The organic shock loading was induced by increasing or decreasing the baseline feeding of TOC concentration. As soon as a transient loading occurred the DO levels in the system were continuously monitored with the DO analyzer.A simplified material balance equation for dissolved oxygen in the system was developed and the dynamic behavior of oxygen uptake rate was examined. It was demonstrated that under transient conditions the changes in the dissolved oxygen concentration reflected the variations in the exogenous respiration rate of the biomass in the system due to fluctuations in the influent waste characteristics. Based upon the rapid response to the shock loading and the correlation of the change in the DO level to the magnitude of the shock loading, the two variables (DO concentration and oxygen uptake rate) generated from this biological monitor showed potential as a control variable for the activated sludge process.     

CO2-dispersion studies in an operation theatre under transient conditions

Maintaining air quality and thermal comfort inside an operation theatre equipped with horizontal jet flow type air-conditioning units, has been a challenge to engineers. The objective of this study is to analyze the airflow pattern in such operation theatres and the influence of location of the air-conditioners. The outcome of this study is expected to reduce the post-operation problems faced due to excess concentration of contaminants. Experimental studies were conducted in 10 different hospital operation theatres. Parameters such as air temperature and carbondioxide concentration were measured at discrete points chosen in the theatre. A 3D time-dependent numerical model was developed to simulate the airflow in terms of parameters such as velocity, temperature and CO₂ distributions in an operation theatre under transient conditions. The Eulerian approach using the volume fraction of the mixture of air and CO₂ was used to solve the numerical model. Finite volume approach was attempted in this work with PISO (pressure implicit with split operators) algorithm for the pressure correction equations. The simulated results were compared with the experimental results for validity. The locations of the air-conditioners were changed in the numerical model to analyze the airflow patterns and the contaminant distribution.     

生物滤池/生态砾石床处理含氮微污染地表水

采用生物滤池/生态砾石床组合工艺进行了微污染地表水(含低碳、高NO3--N浓度)的脱氮研究,通过投加乙酸钠为碳源考察了C/N值、温度、水力负荷对反应器脱氮效能的影响。结果表明,C/N值对反应器的脱氮效能影响较大,在C/N值为10时可以取得较高的反硝化效率(>90%)。在低温下(2～10℃)反应器的反硝化效能受到严重抑制;在13～17℃条件下,反硝化效率恢复到60%左右;当水温>20℃时,在水力负荷为8 m3/(m2.h)的条件下(此时生物滤池和生态砾石床的水力停留时间分别为15、30 min),对NO3--N的去除率能够达到90%以上。生态砾石床能够将生物滤池出水中残余的碳源去除,保证了出水的水质安全。     

Analytical solutions for contaminant diffusion in four-layer sediment-cap system for subaqueous in-situ capping

This paper presents analytical solutions for predicting one-dimensional contaminant diffusion in a four-layer sediment-cap system, which is typically encountered in subaqueous in-situ capping of contaminated sediments. The sediment-cap system is comprised of, from top to bottom, a layer of capping material (e.g., clean sand), a layer of reactive core mat (RCM), a layer of contaminated sediment and a layer of uncontaminated sediment. Two different bottom boundary conditions are considered, i.e., zero-concentration-gradient bottom boundary condition and zero-concentration bottom boundary condition, for which the method of separation of variables is used to obtain the analytical solutions. The extensively verified CST3 (Consolidation and Solute Transport 3) model is used to verify the proposed analytical solutions. Using the verified analytical solutions, parametric studies are conducted to investigate the effect of several important parameters on contaminant transport in the four-layer sediment-cap system. The results indicate that the cap thickness, the contaminated sediment thickness, the uncontaminated sediment thickness, the effect of RCM, and the RCM distribution coefficient have significant impact on contaminant diffusion in the four-layer sediment-cap system. The analytical solutions presented herein can be used to assist the design of subaqueous in-situ capping of contaminated sediments and to verify other numerical models.