Biofiltration of gas-phase hexane and toluene mixture under intermittent loading conditions

This paper reports the performance of a compost biofilter subjected to periodic intermittent loads of gas-phase hexane and toluene. The biofilter was operated for 10 h per day, at different empty bed residence times (4, 2 and 1.3 min), and at different inlet concentrations of hexane and toluene, varying between 2 and 3.8 g m^?3, respectively. Steady-state removal efficiency profiles, reaching more than 90% for both the pollutants, was observed after 44 days of operation. Periodic operation of the compost biofilter was characterized by an adsorption step, followed by biological conversion of the pollutants by the microorganisms inherent to the compost. After resuming daily biofilter operation, the required times for biochemical reaction to dominate the initial adsorption step was observed to be 2.5 and 1 h, respectively, for toluene and hexane. The maximum elimination capacity due to the biological step was found to be 61.6 g m^?3 h^?1. The results from this study showed the effectiveness of the biofilter to handle mixtures of gas-phase pollutants, subjected to regular intermittent operations, thus proving their worthiness for industrial use.     

Comparative response of two organic biofilters treating ethylbenzene and toluene after prolonged exposure

The response of two biofilters filled with an organic waste material for treating ethylbenzene and toluene was studied for 415 and 472 operating days, respectively. The peak elimination capacities (EC_MAX) recorded were 170 g m^?3 h^?1 for ethylbenzene (EC_MAX) and 138 g m^?3 h^?1 for toluene. Regarding the degradation profile through the biofilters, an increase in the inlet concentration displaced the degradation profile into the downstream section. A sudden decrease in the performance of both biofilters occurred when the moisture content (MC) of the packing material exceeded 37% for ethylbenzene and 30% for toluene. Thus, a recommended MC value was established in the 15–30% range. Given the bioreactor was operated at a low MC level, fungi prevailed over bacteria. Nevertheless, synergism was detected between both microorganism types for the mineralization of the aromatic hydrocarbons.     

Removal of H2S by co-immobilized bacteria and fungi biocatalysts in a bio-trickling filter

Biological control of odor gases has gained more attention in recent years. In this study, removal performance of a vertical bio-trickling filter inoculated with bacteria and fungi was studied. Bacteria and fungi were isolated from activated sludge in a sewage treatment plant. By adopting “three step immobilization method”, the bio-trickling filter could degrade pollutant immediately once hydrogen sulfide (H₂S) passed. The optimal empty bed resident time was 20 s. The optimal elimination capacity was about 60 g H₂S m^?3 h^?1 with removal efficiency of 95%. And the maximum elimination capacity was 170 g H₂S m^?3 h^?1. Pressure drop was ranged between 5 and 15 mm H₂O per bed over the whole operation. Removal efficiency was not affected obviously after terminating nutrient supply. The bio-trickling filter could recover back after shut down H₂S gaseous and liquid supplies simultaneously. Microbial community structure in the bio-trickling filter was not changed significantly.Combining bacteria and fungi would be a better choice for inoculation into a bio-trickling filter because of the quickly degradation of H₂S and rapid recovery under shut-down experiment. This is the first study attempting to combine bacteria and fungi for removal of H₂S in a bio-trickling filter.     

Partial nitritation of landfill leachate with varying influent composition under intermittent aeration conditions

The start-up and operation of a partial nitritation sequencing batch reactor for the treatment of landfill leachate were carried out on intermittent aeration mode. Partial nitrite accumulation was established in 15 days after the mode was changed from continuous aeration to intermittent aeration. Despite the varying influent composition, partial nitritation could be maintained by adjusting the hydraulic retention time (HRT) and the air flow rate. An increase in the air flow rate together with a decrease in air off duration can improve the partial nitritation capacity and eventually result in the development of granular sludge with fine diameters. A nitrogen loading rate of 0.71 ± 0.14 kg/m³/d and a COD removal rate of 2.21 ± 0.13 kg/m³/d were achieved under the conditions of an air flow rate of 19.36 ± 1.71 m³ air/m³/h and an air on/off duration of 1.5 min/0.7 min. When the ratio of total air flux (TAF) to the influent loading rate (ILR) was controlled at the range of 163–256 m³ air/kg COD, a stable effluent NO₃^?–N/NO_x^?–N (NO₂^?–N plus NO₃^?–N) ratio below 13% was achieved. Interestingly, the effluent pH was found to be a good indicator of the effluent NO₂^?–N/NH₄⁺–N ratio, which is an essential parameter for a subsequent anaerobic ammonium oxidation (Anammox) reactor.     

Pretreatment of municipal landfill leachate by a combined process

Biodegradability enhancement of landfill leachate using air stripping followed by coagulation/ultrafiltration (UF) processes was introduced. The air stripping process obtained a removal efficiency of 88.6% for ammonia nitrogen (NH₄–N) at air-to-liquid ratio of 3500 (pH 11) for stripping 18 h. The single coagulation process increased BOD/COD ratio by 0.089 with the FeCl₃ dosage of 570 mg l^?1 at pH 7.0, and the single UF process increased the BOD/COD ratio to 0.311 from 0.049. However, the combined process of coagulation/UF increased the BOD/COD ratio from 0.049 to 0.43, and the final biological oxygen demand (BOD), chemical oxygen demand (COD), NH₄–N and colour of leachate were 1223.6 mg l^?1, 2845.5 mg l^?1, 145.1 mg l^?1 and 2056.8, respectively, when 3 kDa molecular weight cut-off (MWCO) membrane was used at the operating pressure 0.7 MPa. In ultrafiltration process, the average solution flux (J_V), concentration multiple (M_C) and retention rate (R) for COD was 107.3 l m^?2 h^?1, 6.3% and 84.2%, respectively.     

Removal mechanism of trace oxytetracycline by aerobic sludge

To investigate the mechanism of removal of selected pharmaceuticals in activated sludge systems, laboratory-scale batch experiments were conducted to assess the adsorption and degradation behavior of trace oxytetracycline (OTC). The adsorption equilibrium of OTC was observed in 30 min and the adsorption process could be well described by a pseudo-second-order model with a rate of 0.362 L μg^?1 min^?1. The OTC adsorption rate decreased with increasing temperature and could be fitted by the Freundlich isotherm. The linear partition coefficients (K_d) were 1.19, 0.999, and 0.841 L g^?1 at temperatures of 15, 20, and 25 °C, respectively. Thermodynamic analysis revealed that the adsorption of OTC onto the inactivated sludge was spontaneous (ΔG = ?16.7 to ?17.0 kJ mol^?1), enthalpy-driven (ΔH = ?24.9 kJ mol^?1), entropy-retarded (ΔS = ?27.4 J (mol K)^?1), and predominantly a physical adsorption.     

Kinetics of nitrate reductive denitrification by nanoscale zero-valent iron

Nanoscale zero-valent iron (Fe⁰) was synthesized for nitrate denitrification. The reduction efficiency of nitrate decreased quickly with increasing initial pH value, increased considerably with the increasing dosage of nanoscale Fe⁰, and did not vary much with initial nitrate concentrations changing from 20 to 50 mg l^?1 when the excessive amount of nanoscale Fe⁰ was utilized. With reductive denitrification of nitrate by nanoscale Fe⁰, the removal rate of nitrate reached 96.4% in 30 min with nanoscale Fe⁰ dosage of 1.0 g l^?1 and pH_in 6.7, and more than 85% of the nitrate was transformed into ammonia. Kinetics analysis in batch studies demonstrates that the denitrification of nitrate by nanoscale Fe⁰ involves reaction on the metal surface, which fits well the pseudo-first order reaction with respect to nitrate concentration. The observed reaction rate constant of reductive denitrification of nitrate was determined to be 0.086 min^?1 with a nanoscale Fe⁰ dosage of 1.0 g l^?1 and pH_in 6.7. Fast and highly effective denitrification can be achieved by nanoscale Fe⁰ compared with commercial Fe⁰ powder, this is due to the extremely high surface area and high reactivity for nanoscale Fe⁰, which can enhance the denitrification efficiencies remarkably.     

Rotating biological contactors for wastewater treatment – A review

Rotating biological contactors (RBCs) for wastewater treatment began in the 1970s. Removal of organic matter has been targeted within organic loading rates of up to 120 g m⁻² d⁻¹ with an optimum at around 15 g m⁻² d⁻¹ for combined BOD and ammonia removal. Full nitrification is achievable under appropriate process conditions with oxidation rates of up to 6 g m⁻² d⁻¹ reported for municipal wastewater. The RBC process has been adapted for denitrification with reported removal rates of up to 14 g m⁻² d⁻¹ with nitrogen rich wastewaters. Different media types can be used to improve organic/nitrogen loading rates through selecting for different bacterial groups. The RBC has been applied with only limited success for enhanced biological phosphorus removal and attained up to 70% total phosphorus removal. Compared to other biofilm processes, RBCs had 35% lower energy costs than trickling filters but higher demand than wetland systems. However, the land footprint for the same treatment is lower than these alternatives. The RBC process has been used for removal of priority pollutants such as pharmaceuticals and personal care products. The RBC system has been shown to eliminate 99% of faecal coliforms and the majority of other wastewater pathogens. Novel RBC reactors include systems for energy generation such as algae, methane production and microbial fuel cells for direct current generation. Issues such as scale up remain challenging for the future application of RBC technology and topics such as phosphorus removal and denitrification still require further research. High volumetric removal rate, solids retention, low footprint, hydraulic residence times are characteristics of RBCs. The RBC is therefore an ideal candidate for hybrid processes for upgrading works maximising efficiency of existing infrastructure and minimising energy consumption for nutrient removal. This review will provide a link between disciplines and discuss recent developments in RBC research and comparison of recent process designs are provided (Section 2). The microbial features of the RBC biofilm are highlighted (Section 3) and topics such as biological nitrogen removal and priority pollutant remediation are discussed (Sections 4 Biological nutrient removal in RBCs, 5 Priority pollutant remediation in RBCs). Developments in kinetics and modelling are highlighted (Section 6) and future research themes are mentioned.     

Effects of applied voltage on hydrogen production rate of a single reactor BML with Clostridium sp.

This study aimed to explore the influences of single-chamber systems with different applied voltage on bio-hydrogen (H₂) production. The reactor used was the bio-electrochemically assisted microbial reactor (BEAMR) membrane-less (BEAMR-membrane-less, BML). The microbial dark fermentative H₂ production method was adopted. After the hot screening process and the DNA sequencing, the domesticated dominant microflora was Clostridium sp. This study discussed the influences of the cases with (continuous and intermittent) and without applied voltage separately. The results showed that, the H₂ production rate of the case with intermittent applied voltage (117 mL/h g VSS) of 0.24 V was increased of 1.7 folds higher than the without applied voltage (69 mL/h g VSS) and 1.3 folds higher than the case with continuous applied voltage (88.2 mL/h g VSS) of 0.24 V. The produced H₂ concentration with intermittent applied voltage was 18.9% (18.6–19.1%) higher than the without applied voltage, while there was no significant difference with continuous applied voltage.     

Removal of VOCs emitted from p-xylene liquid storage tanks by a full-scale compost biofilter

A full-scale compost biofilter system (CBS) was built to treat vapor losses emitted from three analogous vertical fixed roof p-xylene (p-X) liquid storage tanks of 7500 m³ total working volume. The CBS showed a stable and efficient treatment of standing loss (L_S) and working loss (L_W) with an average removal efficiency of 95.4% through 18-month standing storage and filling operation of the tanks. The weight of activated carbon employed in the concentration buffer influences the CBS performance for L_W treatment under high organic loads. The daily average pressure drops across biofilter bed displayed very low with a maximum of 7.6 mm-H₂O. This reflects that the CBS can be operated without stress and compression of biofilter bed and thus reduce maintenance frequency. No additional inputs of chemicals and fuels along with low electricity and water usage helped in its prolonged operation and increased its cost effectiveness. These advantages make the CSB become a promising control technology of vapor losses emitted from fixed roof organic liquid storage tanks.     

Electrostatic ignition of sensitive flammable mixtures: Is charge generation due to bubble bursting in aqueous solutions a credible hazard?

Experiments have been conducted to gain insight into the credibility of sparging aqueous solutions as an electrostatic ignition hazard for sensitive hydrogen/air or fuel/oxygen mixtures (Minimum Ignition Energies of ∼0.017 mJ and ∼0.002 mJ, respectively, compared to ∼0.25 mJ for hydrocarbon/air mixtures). Tests performed in a 0.5 m³ ullage produced electric field strengths between 125 and 560 V m⁻¹ for air flows of 5–60 l min⁻¹, respectively, comprised of 2–4 mm diameter bubbles. Field strength can be related to the space charge and fitting to an exponential accumulation curve enabled the charge generation rate from the air flows to be estimated. This was observed to be directly proportional to the air flow and its magnitude was consistent with literature data for bubble bursts. The charge accumulation observed at laboratory scale would not be a cause for concern. On the basis of a simple model, the charge accumulation in a 27 m³ ullage was predicted for a range of air flows. It is apparent from such calculations that ignition of hydrocarbon/air mixtures would not be expected. However, it would seem possible that field strengths might be sufficient to cause a risk of incendive spark or corona discharges in moderately sized vessels with sensitive flammable mixtures.     

Artificial neural network modelling of As(III) removal from water by novel hybrid material

The present study reported a method for removal of As(III) from water solution by a novel hybrid material (Ce-HAHCl). The hybrid material was synthesized by sol–gel method and was characterized by XRD, FTIR, SEM–EDS and TGA–DTA. Batch adsorption experiments were conducted as a function of different variables like adsorbent dose, pH, contact time, agitation speed, initial concentration and temperature. The experimental studies revealed that maximum removal percentage is 98.85 at optimum condition: pH = 5.0, agitation speed = 180 rpm, temperature = 60 °C and contact time = 80 min using 9 g L⁻¹ of adsorbent dose for initial As(III) concentration of 10 mg L⁻¹. Using adsorbent dose of 10 g L⁻¹, the maximum removal percentage remains same with initial As(III) concentration of 25 mg L⁻¹ (or 50 mg L⁻¹). The maximum adsorption capacity of the material is found to be 182.6 mg g⁻¹. Subsequently, the experimental results are used for developing a valid model based on back propagation (BP) learning algorithm with artificial neural networking (BP-ANN) for prediction of removal efficiency. The adequacy of the model (BP-ANN) is checked by value of the absolute relative percentage error (0.293) and correlation coefficient (R² = 0.975). Comparison of experimental and predictive model results show that the model can predict the adsorption efficiency with acceptable accuracy.     

Effects of high ammonia loads on nitrogen mass balance and treatment performance of a biotrickling filter

A biotrickling filter packed with coal slag as packing medium was continuously used for more than 9 months under high ammonia loading rates of up to 140 g/m³/h. Nitrogen mass balance and microbial community analysis were conducted to evaluate the inhibitory effects of high ammonia concentration and metabolic by-products on the rates of nitrification. Ammonia removal efficiency reached above 99% at an empty bed retention time of as low as 8 s when inlet concentrations were below 350 ppm. The maximum and critical elimination capacities of the biotrickling filter were 118 g/m³/h and 108.1 g/m³/h, respectively. Kinetics analysis results showed that less than 2.5 s was required for the biotrickling filter with pH control to treat ammonia at concentrations of up to 500 ppm in compliance with the Taiwan EPA standard (outlet NH₃ < 1 ppm). Results of mass balance and microbial community analysis indicated that complete removal was mainly contributed by the activities of autotrophic ammonia oxidizing bacteria and not by physical absorption or adsorption at low loading rates. However, at high inlet loadings, ammonium became the dominant by-product due to inhibitory effects of high ammonia concentration on the bacterial community.     

Performance of a pilot scale up-flow septic tank for on-site decentralized treatment of residential wastewater

The aim of this research was to study the on-site anaerobic treatment of a medium-strength residential wastewater in a pilot-scale up-flow septic tank (UST). The effects of three different hydraulic retention times (HRTs) of 24, 12 and 6 h on the UST performance were investigated. The UST removed 85, 77, and 86% of biochemical oxygen demand (BOD₅), chemical oxygen demand (COD) and total suspended solids (TSS), respectively, at steady state operation and with a 24 h HRT. Decreasing the HRT to 12 and then 6 h resulted in deteriorated effluent quality and significantly reduced reactor performance. The sludge showed a high specific methanogenic activity (SMA) of 15.2 mL CH₄ g^?1 VSS d^?1 with raw wastewater substrate. The solids accumulated in the tank by the end of the experiment had a VSS/TSS of 0.57, demonstrating significant stabilization. Overall, the UST is concluded to be a technically and economically promising alternative to conventional septic tanks for the on-site decentralized treatment of residential wastewater, particularly in the rural communities of developing countries.     

Toluene removal biofilter modeling: Optimization and case study

Based on the model proposed by De Visscher and Van Cleemput for methane oxidation in landfill cover soils, a simulation model for biofiltration of toluene-contaminated air has been developed for biofilters with substrate inhibition. A convenient way to optimize biofilter performance was developed assuming Haldane kinetics. It was calculated that for a typical oilsands operation emitting 200 ton of toluene annually, 90% of the toluene can be removed by a 740 m³ biofilter, if the waste gas sent to the biofilter has a toluene concentration of 2.25 g m⁻³. The optimal initial concentration increases with increasing target efficiency.     

Application of Modified Carbon Anodes in Microbial Fuel Cells

The effect of different carbon anodes was examined in a new design of single chambered microbial fuel cell (SCMFC). The new cell design used a low-cost hydrophilic membrane to replace costly proton exchange membranes and carbon felt and a range of carbon and modified carbon anodes were investigated. The fuel for the SCMFC was brewery wastewater which was diluted with domestic wastewater and the presented microflora acts as a source of electro-active bacteria. The membrane acts as a separator between the anode chamber and an air cathode and allows the transfer of ions based on the wastewater's natural conductivity. The air cathode was carbon black (Ketjen Black EC 300J) which was deposited (1 mg cm⁻² concentration) directly onto the surface of the separator (one side of the membrane). Steady state polarization demonstrated maximum power densities of up to 30 mW m⁻² and a steady state power density of 20 mW cm⁻² at a current density of 110 mA m⁻² was achieved. The best performing anodes were made from carbon modified with quinone/quinoid groups. With unmodified graphite felt (the control anode material) as anode, the maximal power density obtained was 9.5 mW m⁻².     

Pb(II) adsorption from aqueous solutions by raw and treated biomass of maize stover – A comparative study

The potential to remove Pb(II) ion from wastewater treatment systems using raw and treated maize stover through adsorption was investigated in batch experiments. To achieve this, batch mode experiments were conducted choosing specific parameters such as pH (2–8), dosage concentration (2–30 g L⁻¹), contact time (5–180 min), temperature (20–45 °C) and metal ion concentrations (10–50 mg L⁻¹). Adsorption was pH-dependent showing a maximum at pH value 5. The equilibrium sorption capacities of raw and treated maize stover were 19.65 and 27.10 mg g⁻¹, respectively. The adsorption data fitted well to the Langmuir isotherm model. Kinetic studies revealed that the adsorption process followed pseudo-second-order model. The calculated thermodynamic parameters showed that the adsorption of Pb(II) was spontaneous and exothermic in nature. Consequently, this study demonstrated that both raw and treated maize stover could be used as adsorbents for the treatment of Pb(II) from industrial wastewaters.     

Evaluating the efficacy of alumina/carbon nanotube hybrid adsorbents in removing Azo Reactive Red 198 and Blue 19 dyes from aqueous solutions

Concerning the high volume of wastewater containing dye in Iran and its adverse effects, it is necessary to develop scientific solutions for treating these wastewaters. The aim of this study was to evaluate the efficiency of the alumina-coated multi-walled carbon nanotubes in removing the Reactive Red 198 (RR 198) and Blue 19 (RB 19) dyes. Synthetic samples including dye with different concentrations were prepared. These samples were put in contact with different contents of alumina/multi-walled carbon nanotubes, in different pH values, in different contact times, different temperatures and the presence of sodium sulfate or sodium carbonate. The optimum pH, dye concentration and temperature for removal of the two dyes was 3, 50 mg l⁻¹ and 25 °C, respectively. The optimum adsorbent dose for removal the RR 198 dye was 0.5 g l⁻¹ and for Blue 19 was 0.4 g l⁻¹. The optimum contact time for RR 198 was 150 min and RB 19 was 180 min. In this condition, maximum removal efficiency for RR 198 and RB 19 was 91.54% and 93.51%, respectively. The adsorption study was analyzed kinetically, and the results revealed that the adsorption fitted a pseudo-second order kinetic model. According to these results alumina/multi-walled carbon nanotubes can effectively remove RR 198 and RB 19 from aqueous solutions.     

Sensitivity analysis and optimization for gasoline vapor condensation recovery

Volatile organic compounds (VOCs) are easily evaporated and discharged from everywhere into the atmosphere, especially in various operations of gasoline. The emission of VOCs is always a significant environmental problem, and the control of VOCs pollution has been a hot topic in the field of air purification. In this paper, the condensation separation method for gasoline vapor recovery was investigated and four gasoline vapors of S1–S4 were selected for the sensitivity analysis and optimization of the condensation process, using the Model Analysis Tools from Aspen Plus. Generally, to control VOCs pollution efficiently, both the vapor recovery efficiency and the outlet vapor concentration of the condensation recovery system should be simultaneously considered. Then an optimized three-stage condensation process was proposed, whose condensation temperatures were optimized and designed at 1 °C, −40 °C and −110 °C, respectively. Further, based on the comprehensive consideration of both meeting the more strict VOCs emission standard and ensuring the condensation recovery system work stably and economically, it was recommended that the maximum total vapor recovery efficiencies for S1–S4 should be 99.73%, 99.79%, 99.82% and 99.19%, and the minimum outlet vapor concentrations be 2.87 g/m³, 2.75 g/m³, 3.04 g/m³ and 16.98 g/m³, respectively. Accordingly, the condensation temperature of the copious cooling stage should be set at −130 °C. Moreover, the total cooling duties for the single-stage and three-stage condensation processes were investigated and compared when the condensation temperature of the recovery system ranged from 20 °C to −110 °C. The total cooling duties of the three-stage condensation process for S1–S4 would be saved by 12.23%, 15.68%, 13.96% and 15.65%, respectively. Finally, a three-stage condensation system was developed for the industrial gasoline vapor recovery, which has performed well since its installation.     

Decolorization of synthetic Methyl Orange wastewater by electrocoagulation with periodic reversal of electrodes and optimization by RSM

Treatment of Methyl Orange (MO), an azo dye, synthetic wastewater by electrocoagulation with periodic reversal of the electrodes (PREC) was examined. Response Surface Methodology (RSM) was used to optimize the influence of experimental conditions for color removal (CR), energy consumption (ENC), electrode consumption (ELC) and sludge production (SP) per kg MO removed (kg(MO_r)) with optimal conditions being found to be pH 7.4, solution conductivity (к) 9.4 mS cm⁻¹, cell voltage (U) 4.4 V, current density (j) 185 mA cm⁻², electrocoagulation time (T) 14 min, cycle of periodic reversal of electrodes (t) 15 s, inter-electrode distance (d) 3.5 cm and initial MO concentration of 125 mg L⁻¹. Under these conditions, 97 ± 2% color was removed and ENC, ELC and SP were 44 ± 3 kWh kg(MO_r)⁻¹, 4.1 ± 0.2 kg(Al) kg(MO_r)⁻¹ and 17.2 ± 0.9 kg(sludge) kg(MO_r)⁻¹, respectively. With the enhanced electrochemical efficiency resulting from the periodic electrode reversal, the coefficients of increased resistance and decreased current density between the two electrodes in the PREC setup were 2.48 × 10⁻⁴ Ω cm⁻² min⁻¹ and 0.29 mA cm⁻² min⁻¹, respectively, as compared to 7.72 × 10⁻⁴ Ω cm⁻² min⁻¹ and 0.79 mA cm⁻² min⁻¹ as measured for the traditional electrocoagulation process. The rate constant of decolorization was also enhanced by 20.4% from 0.152 min⁻¹ in the traditional electrocoagulation process to 0.183 min⁻¹ in the PREC process. These performance characteristics indicate that the PREC approach may be more promising in terms of practical application, as a cost-effective treatment, than conventional electrocoagulation for textile dye removals.