Fine Bubble Diffusers Research Articles

Stability Analysis of an Activated Sludge Bioreactor at a Petrochemical Plant at Different Temperatures

The Mexican petrochemical industry, Morelos S.A. de C.V. is one of the biggest and most important petroleum industries in Mexico and in Latin America. It has an activated sludge system to treat its wastewater flow, which is about 7000m3/d. The wastewater contains volatile organic carbon substances classified as toxic. The old surface aeration system was changed for fine bubble diffusers; however, one major drawback of the new aeration system is that the temperature in the bioreactor has increased due to the compression of the air, which at the compressor exit reaches up to 85oC. This effect produces a temperature in the aerated bioreactor of 32oC during the fall, whereas in the spring and summer, the bioreactor temperature reaches higher values than 40oC. The high temperatures reduce the microorganisms’ activity and cause higher volatilization rates of volatile compounds, among other effects, which affect the performance of the biological treatment. In this work we performed a stability analysis at different equilibrium points in different scenarios of temperatures, recycle flowrate and wastage flowrate. The eigenvalues of the state matrix of dynamical model of the petrochemical wastewater treatment were evaluated. The model describes the effect of the temperature on the soluble and volatile pollutants removal rate, heterotrophic organism growth rate, oxygen uptake rate and biomass concentration in the settler. The model was obtained from tests that were done in laboratory reactors with 14 L of capacity, which were operated at different temperatures (from 30oC to 45oC), with the same conditions as the actual system. Simulations were done based on the model obtained manipulating the operation variables Qw and Qr. The stability analysis was also done to the reactor without recycle (Qr = 0 m3/d). In all the cases the stability analyses was performed. The stripping of VOC´s is an important process of COD removal at high operation temperatures.

Investigation of an Ozone Membrane Contactor System

Ozone mass transfer rates were determined for nine expanded porous Teflon membranes that had different pore size, thickness, and pore volume, a nonporous Teflon membrane, and a PVDF membrane. The mass transfer coefficient was 7.6 ± 0.5 × 10−5 m/s at Re of 2000 for all membranes tested even though pore sizes ranged from 0.07 to 6 μm and thickness from 0.076 to 0.25 mm. Mass transfer increased with liquid side Reynolds number. Therefore, it is likely that ozone mass transfer is liquid phase controlling and not membrane limited. For a hypothetical case of 4000 m3/d and 2 mg/L ozone transferred, plate and frame membrane and hollow fiber contactors are approximately one and two orders of magnitude smaller, respectively, than fine-bubble diffusers.

Fifteen Years of Offgas Transfer Efficiency Measurements on Fine‐Pore Aerators: Key Role of Sludge Age and Normalized Air Flux

Fine-pore diffusers, often called fine-bubble diffusers, have nearly replaced coarse bubble diffusers in municipal wastewater treatment over the past 20 years. The rapid increases in energy costs, which began in the 1970s, created financial incentives to upgrade to this more expensive and maintenance-intensive method of aeration. Fine-pore diffusers have the added benefit of reducing volatile organic compound stripping and reduced aeration heat loss. This paper summarizes 15 years of oxygen transfer efficiency measurements using the offgas technique. Efficiencies are shown for different types of diffusers at various tank geometries (depth, diffuser size, and number), airflow rates, and mean cell retention times (MCRT or sludge age). By normalizing the airflow rates per unit of depth and diffusing area, efficiencies measured in different plants can be compared. The results show that aeration efficiencies are logarithmically related to the ratio between MCRT and the normalized air flux, with transfer rates suppressed by low MCRT or high normalized air flux systems. There is no evidence for different alpha factors among the different types of fine-bubble diffuser types.

A new low-cost biofilm carrier for the treatment of municipal wastewater in a moving bed reactor

The Moving Bed Biofilm Reactor has proven to be an efficient system in wastewater treatment and has become a viable solution for small treatment plants. The main objective of this research was to analyse the performance of a moving bed reactor using low-cost local material when fed with municipal wastewater. A pilot reactor with a total volume of 900 litres was built and it was fed continuously with municipal wastewater. The operation of the system was adjusted to six different organic loading rates. The biofilm carrier was polyethylene tubing with internal diameter of 1.1 cm, cut into pieces of 1.2 cm. The tested material offered a specific surface area of 590 m2/m3. Air was provided with a fine-bubble diffuser. The main results show that the reactor performance was stable and predictable. The COD removal confidently behaves according to a general hyperbolic kinetic equation. The maximal total COD removal attained was 81%. Nitrification was observed only for organic loads with values under 5.7 gCOD/m2 x d. Good adherence of the microorganisms was observed for the applied organic loading rates. After several months of operation, the material showed no signs of abrasion or deformation. The sludge production behaved linearly with the organic load reaching 979 gTSS/d with the highest organic load of 35.7 gCOD/m2 x d. The amount of microorganisms attached to the carrier increased with the organic load tending to an asymptotical maximal value of 17.3 g/m2 (as dry solids). Mean cellular retention times from 2.0 to 23.1 days were determined.

Clogging and cleaning of fine-pore membrane diffusers

Causes for the increase of the pressure loss of fine bubble diffusers made of elastomers were investigated. Methods are described for removing clogging material from membrane diffusers and/or attenuating or even avoiding the formation of clogging deposits. On the basis of some practical examples the procedure is discussed and obtained reductions of pressure losses are specified.

Biological coating of EPDM-membranes of fine bubble diffusers

Biological coatings on EPDM-membranes are a problem on many large wastewater treatment plants, as the oxygen supply of the micro-organisms is no longer guaranteed. Investigations prove that the pressure loss and the Shore A-hardness of the EPDM-membranes increase while on the other hand their softener content decreases accordingly. The detected coatings on the membrane surfaces and in the slits or holes of the membranes show extra-cellular organic substances (EPS), which, compared with fibrillar/filamented EPS usually found on surfaces in wastewater treatment plants, are viscous to a much greater extent. As, besides primary organic parts (carbon), the coatings on the membranes as well as in the slits or holes also consist of inorganic constituents (magnesium, silicon, and others), the authors assume that, the separating agent (and also inactive filler) talcum (magnesium silicate), used when producing the membranes, supports at least a first beginning of the coating. Superfine dust constituents and fibres, input via the compressed air, will build up inside the coating and consequently lead to a gradual clogging of the holes or slits. Besides chemical cleaning measures, the exchange of the EPDM-membranes against membranes of silicone would also be a possible measure to solve this problem. The market will decide, if, in the future, a cleaning or an exchange of the EPDM-membranes against membranes of silicone will be applied, but it has to be considered that the loss of softener is irreversible.

Oxidation of Aged Raw Landfill Leachate with O3 Only and O3/H2O2: Treatment Efficiency and Molecular Size Distribution Analysis

ABSTRACT An aged raw landfill leachate was taken from the equalization storage tank at Clover Bar Landfill Leachate Treatment Plant, Edmonton, Alberta, Canada. The average quality parameters of this leachate were: COD=1,090mg/L, BOD5=39mg/L, color=1,130 TCU, NH3-N=455mgN/L, alkalinity=4,030mg/L as CaCO3and pH=8.30. The major fraction of this leachate was large refractory organic compounds. Ozone (O3) only and O3 combined with hydrogen peroxide (O3/H2O2) were applied to treat this leachate, aiming at enhancing COD and color reduction and increasing its biodegradability (i.e., the ratio of BOD5/COD). All of the O3 only and O3/H2O2 oxidation experiments were performed in a gas washing bottle equipped with a fine bubble diffuser. The used ozone dose ranged from 1.2 to 12.5g O3/L leachate for O3 only treatment, and 1.8 to 13.8g O3/L leachate for O3/H2O2 treatment. H2O2 dose for O3/H2O2 treatment was 0.63g H2O2/L leachate. COD, BOD5, color, NH3-N, nitrite+nitrate, and alkalinity were measured before and after treatment. Meanwhile the molecular size distribution of the leachate, before and after treatment, was analyzed by using a high-performance liquid chromatograph (HPLC) with gel filtration column and UV detector at 254nm. The addition of H2O2 had an insignificant effect (at 5% significance level) on enhancing COD and color reduction. After oxidation, the maximum BOD5 increase was about 110% for O3/H2O2 treatment and about 141% for O3 only treatment at a used ozone dose of 3.6g O3/L leachate and 2.6g O3/L leachate for O3/H2O2 and O3 only, respectively. As the used ozone dose increased, NH3-N and alkalinity decreased considerably, and nitrite+nitrate increased accordingly. Treatment efficiency models, which describe the changes in COD, BOD5/COD, NH3-N, nitrite+nitrate, and alkalinity as functions of the used ozone dose, were developed. Statistically (at 5% significance level), the treatment efficiency models for both treatments are not different. According to the results of molecular size distribution analysis, no correlation was observed between the BOD5 increase and oxidation by-products’ formation.

Aerosol-bound emissions of polycyclic aromatic hydrocarbons and sterols from aeration tanks of a municipal waste water treatment plant.

Aeration tanks of wastewater treatment plants (WWTP) are a potential source of atmospheric aerosol particles. Several groups of organic compounds (sterols, polycyclic aromatic hydrocarbons, estrogens) were analyzed in aerosol particles sampled at a municipal WWTP, and the particle size distribution was measured directly with optical particle counters. Aerosol emissions from an activated treatment tank equipped with fine bubble diffusers were low; however, at the preaeration tank equipped with coarse bubble diffusers, sterol concentrations up to 14 ng m(-3) were measured. Directly next to the tank, sterols were associated mainly to particles with aerodynamic diameter >1.35 microm. The results suggest that coprostanol could be a useful tracer for monitoring the emission of aerosol particles from WWTPs. Moreover, wastewater treatment could contribute substantially to the atmospheric concentrations of cholesterol and 24-ethylcholesterol. Aeration tanks with fine bubble diffusers are no major source of atmospheric aerosol particles, whereas coarse bubbling devices seem to emit considerable amounts of aerosol particles.

308 超微細気泡散気装置による地球温暖化抑制効果

The total power consumption of sewage treatment plants in Japan consists of 0.7% of that of Japan as a whole. Therefore, from a greenhouse effect prevention point of view, it is important to reduce the energy consumption of blowers in sewage treatment plants to reduce CO_2 emission. The Ultra Fine Bubble Diffuser is made of a special membrane punched with many small holes, which enables the generation of ultra-fine bubbles. This enables the Diffuser to have a very high oxygen transfer efficiency, leading to the reduction of required air for the activated sludge process and the reduction of power consumption of the blower. In this paper, the structure and features of the Ultra Fine Bubble Diffuser, along with a case study of an actual application, are introduced.

Comparing Different Designs and Scales of Bubble Columns for Their Effectiveness in Treating Kraft Pulp Mill Effluents

The experimental results obtained in three different types of ozone contactors were analyzed to study the effects of the ozone contactor design, configuration, operating conditions, and scale-up on the: (1) ozonation process induced reduction efficiencies of color, AOX, COD, and TOC from biologically treated Kraft pulp mill effluents; (2) the increase in biodegradability of this type of wastewater; and (3) the dynamics of the ozone gas absorption process. The three types of ozone contactors included: (1) an extra-coarse-bubble diffuser ozone contactor; (2) an impinging-jet ozone contactor; and (3) a fine-bubble diffuser ozone contactor. Similar treatment levels were achieved in those ozone contactors although the impinging-jet bubble column was more effective in treating Kraft pulp mill effluents due to its smaller reactor volume and lower off-gas ozone concentrations. Consequently, the operating costs of an ozonation process and ozone off-gas destruction facilities will be greatly reduced when using the impinging-jet bubble column design for treating Kraft pulp mill effluents.

Transfer number in fine bubble diffused aeration systems

On the basis of full-scale data from 58 clean water tests performed in 26 activated sludge tanks equipped with fine bubble diffusers and of a theoretical approach, it can be stated that fine bubble aeration systems with total floor coverage arrangement provide higher kLa values and the lowest spiral liquid circulation. An efficiency criterion for oxygen transfer (NT) was defined on the basis of the dimensional analysis. The transfer number NT allows us to take account of the impact of vertical liquid circulation movements on oxygen transfer. The values of NT calculated from the results of full scale nonsteady-state clean water tests vary from 5.3 x 10(-5) to 9.1 x 10(-5) and are directly dependent upon the arrangement of air diffusers. It has been shown that the highest transfer numbers corresponded to the total floor coverage arrangement and the average calculated NT values is 7.7 x 10(-5), independently of the diffuser density and of the gas velocity, over the ranges studied. The lowest transfer numbers are obtained when the diffusers are located in separate grids, and the transfer number is reduced with increasing air flow rate.

Effect of air flow rate on oxygen transfer in an oxidation ditch equipped with fine bubble diffusers and slow speed mixers

The effect of air flow rate on oxygen transfer efficiency was examined in clean water and under process conditions in an oxidation ditch equipped with fine bubble membrane diffusers and large blade slow speed mixers. Under process conditions, an increase in the air flow rate resulted in a decrease of the oxygen transfer efficiency similar to that observed in clean water. Consequently, the value of the alpha factor was in the order of 0.58 independently of the air flow rate (between 45 and 110 m 3 h −1 per m 2 of membrane). The combined influence of the air flow rate and of the horizontal liquid velocity on the oxygen transfer efficiency was studied. Results have evidenced that regrouping the diffuser grids and applying a horizontal flow could lead to a significant limitation in the impact of the air flow rate on the oxygen transfer efficiency. Finally, the use of the off gas method to determine the oxygen transfer efficiency of an aeration system at a given air flow rate led to the definition of a relationship, that enabled the evaluation of the system oxygenation capacities at air flow rates different from the one at which the measurements were initially performed.

Biological odour control by diffusion into activated sludge basins

Diffusion of odourous air into activated sludge basins is an effective and economical approach for odour control at wastewater treatment plants. Over 30 facilities in North America practice this technique. Most of these applications are for dilute or moderate strength air streams with relatively low hydrogen sulfide (H2S) levels. Unfortunately, the lack of data documenting the effectiveness of the process and concerns for blower corrosion have limited its acceptance as a method for controlling odour emissions. This paper presents a compilation and review of wastewater treatment facilities that are currently using activated sludge diffusion. Performance data are tabulated, and information on operation and maintenance of the blower/diffuser systems are summarized. New data are presented that compare the effectiveness of coarse vs. fine bubble diffusers for treatment of a high strength, high-H2S air stream from sludge holding tanks at Concord, NH. Designand performance information is presented on a dedicated odour control system installed at Phoenixville, Pennsylvania in 1996 that collects high-H2S air streams from an influent chamber and primary clarifiers and diffuses it into a mechanically-aerated activated sludge basin. A cost-effectiveness analysis is presented for a blower/diffuser system installed exclusively for odour control. Methods used to overcome initial operational problems are discussed.

Testing of fine-bubble aeration diffusers as CO2 saturation elements for outdoor microalgal cultures

Testing of fine-bubble aeration diffusers as CO2 saturation elements for outdoor microalgal cultures

Alpha correction factors for static aerators and fine bubble diffusers used in municipal facultative aerated lagoons

The alpha correction factor (KLa process water/KLa clean water; where KLa is the volumetric oxygen mass transfer coefficient) was evaluated for 4 kinds of static aerators and 2 kinds of fine bubble diffusers used in municipal facultative aerated lagoons. For this purpose, a 40 m3 steel tank was filled (3.1 m side water depth) with clean or process water. The process water consisted of “mixed liquor” from a municipal facultative aerated lagoon divided into a cascade of four identical basins that were 3.5 m deep. Results showed that in the last three basins, the alpha correction factors were relatively high, being between 0.85 to 0.95 for any type of aeration device. In the first lagoon, however, the alpha correction factors were between 0.70 and 0.90 for static aerators and about 0.70 for fine bubble diffusers. Furthermore, at the inlet of the first basin, the alpha factor was as low as 0.59 for a static aerator and 0.26 for a fine bubble diffuser, due to the composition of the “mixed liquor”. It was shown that the alpha correction factor that should be used for the design of aeration systems for facultative aerated lagoons should be lower in the first basin of a series of basins and could be higher than 0.85 for the downstream basins.

Alpha correction factors for static aerators and fine bubble diffusers used in municipal facultative aerated lagoons

The alpha correction factor (KLa process water/KLa clean water; where KLa is the volumetric oxygen mass transfer coefficient) was evaluated for 4 kinds of static aerators and 2 kinds of fine bubble diffusers used in municipal facultative aerated lagoons. For this purpose, a 40 m3 steel tank was filled (3.1 m side water depth) with clean or process water. The process water consisted of “mixed liquor” from a municipal facultative aerated lagoon divided into a cascade of four identical basins that were 3.5 m deep. Results showed that in the last three basins, the alpha correction factors were relatively high, being between 0.85 to 0.95 for any type of aeration device. In the first lagoon, however, the alpha correction factors were between 0.70 and 0.90 for static aerators and about 0.70 for fine bubble diffusere. Furthermore, at the inlet of the first basin, the alpha factor was as low as 0.59 for a static aerator and 0.26 for a fine bubble diffuser, due to the composition of the “mixed liquor”. It was shown that the alpha correction factor that should be used for the design of aeration systems for facultative aerated lagoons should be lower in the first basin of a series of basins and could be higher than 0.85 for the downstream basins.

Aeration investigation of biological aerated filters using off-gas analysis

The bulk of the operating costs for biological aerated filters are associated with aeration, accordingly, any efficiency improvements which may be made to this aspect of the process could result in significant savings operationally. This study used off-gas analysis to determine the effects on oxygen transfer efficiency of variations to the air flow and substrate loading rates to two pilot scale granular media biological aerated filters (BAFs) treating primary settled sewage. The columns provided a good representation of full scale units, providing up to 99% ammonia reduction. Incremental increases in the mass transfer coefficient (KLa) were observed, as reported elsewhere and attributed to direct interfacial transfer between the bubbles and the biofilm. The OTE varied with flow rate but was far more sensitive at the lower end of the experimental range, rising sharply for air flow rates below 5 to 10 m/h. Increasing the organic loading rate also improved the OTE. The use of fine bubble diffusers did not improve the OTE, however, probably as a result of the packing media used.

Aeration investigation of biological aerated filters using off-gas analysis

The bulk of the operating costs for biological aerated filters are associated with aeration, accordingly, any efficiency improvements which may be made to this aspect of the process could result in significant savings operationally. This study used off-gas analysis to determine the effects on oxygen transfer efficiency of variations to the air flow and substrate loading rates to two pilot scale granular media biological aerated filters (BAFs) treating primary settled sewage. The columns provided a good representation of full scale units, providing up to 99% ammonia reduction. Incremental increases in the mass transfer coefficient (KLa) were observed, as reported elsewhere and attributed to direct interfacial transfer between the bubbles and the biofilm. The OTE varied with flow rate but was far more sensitive at the lower end of the experimental range, rising sharply for air flow rates below 5 to 10 m/h. Increasing the organic loading rate also improved the OTE. The use of fine bubble diffusers did not improve the OTE, however, probably as a result of the packing media used.

Aerated Anoxic Oxidation-Denitrification Process

The conventional practice for an anoxic denitrification basin has been to minimize oxygen input on the basis that it is detrimental to the process. For existing secondary treatment systems, allotting 25–35% of the aeration volume for an unaerated anoxic zone will significantly reduce plant capacity. Further, a group has held that bulking sludge control is best achieved by eliminating all forms of oxygen, including nitrates, from the initial contact or biological selector zones. The 6.75 m 3 /s Phoenix 91st Avenue wastewater-treatment plant (WWTP) was designed with nitrate recycle to the selector zones for bulking sludge control and the anoxic zones were provided with fine-bubble diffusers to increase oxidation capacity while simultaneously removing the recycled nitrates. This paper provides the process, design rationale, plant-layout features, plant modifications to existing structures, and the operating results for the facility. The successful performance of the 1.3 m 3 /s prototype plant resulted in the initiation of the design for conversion of the other four plants, totaling 5.4 m 3 /s capacity, to the aerated anoxic design.

Effects of operational variables on the oxygen transfer performance of ceramic diffusers

Controlled changes in the operation of two side‐by‐side activated sludge pilot plants investigated the effect of operational variables on the performance of ceramic (fine bubble) diffusers. The experimental plan manipulated the following operational variables: aeration system volumetric organic loading rate, sludge age, recycle rate, and the addition of an upstream aerobic or anoxic biological selector. Each pilot plant consisted of a full depth aeration column, aerated using a single ceramic dome diffuser, and a dedicated clarifier for sludge acclimation. The aeration influent channel at a large municipal wastewater treatment plant provided the influent wastewater to the pilot plants. An off‐gas method determined aeration system oxygen transfer efficiencies. Fine bubble oxygen transfer efficiencies fluctuated throughout the study. A nitrifying aeration system, operated at an 8‐day sludge age, exhibited significantly higher αF values compared with a nonnitrifying system operated at a 2‐day sludge age. Differences in organic loading, over the range investigated (0.50 to 1.2 kg biochemical oxygen demand [BOD]5/m3·d), did not show an effect on fine bubble αF performance. At a short sludge age (2 days) and a high volumetric organic loading rate, a pilot system operated at a higher recycle rate (100% versus 33%) exhibited improved aF values. The addition of either an aerobic or an anoxic upstream biological selector (13‐minute hydraulic retention time) improved ceramic diffuser oxygen transfer performance at high volumetric organic loading rates. For the site‐specific wastewater, ceramic diffuser oxygen transfer performance improved as the extent of nitrification increased. Ceramic αF values also increased as the soluble organic content of the mixed liquor decreased, although it was not possible to predict an αF value based solely on the mixed liquor soluble chemical oxygen demand level or the extent of nitrification. Because of the higher αF values in nitrified versus nonnitrified mixed liquors, a much smaller than expected increase in process airflows was required to meet the increased oxygen demand of nitrification, lowering process energy consumption, and aeration system emissions.