Recent efforts have been made to reduce releases of air toxics and smog precursors from wastewater treatment plants. Hydrgen sulfide is commonly the primary odor and is an important target for removal. Its oxidation, however, generates sulfuric acid and sometimes elemental sulfur, which can create substantial operational problems for biofilters. Declining pH may inhibit the organisms that degrade compounds other than hydrogen sulfide and may hasten aging of organic Biofilter media. A two‐stage biofilter has been designed and installed at the Ojai Valley Sanitary District wastewater treatment plant. The first stage is an enclosed system with a medium of small, inert, porous stones. It is called an acid gas biofilter. The second stage is a section of a traditional open biofilter filled with wood chips. The acid gas biofilter effectively removed H2S and volatile organic compounds while causing much lower headloss than traditional biofilters. However, considerable flow heterogeneity in both the acid gas biofilter and the wood chip biofilter was observed. The two‐stage system presumably will have a longer bed life because the first stage bed is inert and because the second stage is protected from acidification by removal of H2S in the first stage.
The radiance of most objects seen at a distance through the atmosphere is dominated by scattered light of a blue hue that should make the landscape appear predominately blue. However, common experience shows that people can see colors at a distance. A possible explanation of this paradox is that the visual system splits the light into a haze layer and the background landscape. A straightforward mathematical description of this splitting explains the results of a color matching study in the Great Smoky Mountains National Park. In this study, hues of objects seen through haze were found to be constant with changes in optical depth while colorfulness decreased exponentially.
Drinking water source contamination poses a great threat to human health in developing countries. Point-of-use (POU) water treatment techniques, which improve drinking water quality at the household level, offer an affordable and convenient way to obtain safe drinking water and thus can reduce the outbreaks of waterborne diseases. Ceramic water filters (CWFs), fabricated from locally sourced materials and manufactured by local labor, are one of the most socially acceptable POU water treatment technologies because of their effectiveness, low-cost and ease of use. This review concisely summarizes the critical factors that influence the performance of CWFs, including (1) CWF manufacturing process (raw material selection, firing process, silver impregnation), and (2) source water quality. Then, an in-depth discussion is presented with emphasis on key research efforts to address two major challenges of conventional CWFs, including (1) simultaneous increase of filter flow rate and bacterial removal efficiency, and (2) removal of various concerning pollutants, such as viruses and metal(loid)s. To promote the application of CWFs, future research directions can focus on: (1) investigation of pore size distribution and pore structure to achieve higher flow rates and effective pathogen removal by elucidating pathogen transport in porous ceramic and adjusting manufacture parameters; and (2) exploration of new surface modification approaches with enhanced interaction between a variety of contaminants and ceramic surfaces.
A pilot‐scale biofilter was operated at a publicly owned treatment works to remove hydrogen sulfide and a mixture of volatile organic compounds (VOCs) from waste gas streams. Lava rock was used for the medium, which averaged pH 4 during the study. The study itself was conducted in three phases. The bed volume was 9.8, 6.5, and 3.3 m3 in phases 1, 2, and 3, respectively, while the flowrate was approximately 17 m3/min throughout the study. The removal efficiency of hydrogen sulfide was consistently greater than 90% for all three phases and, despite low‐pH conditions, weighted average VOC removal was greater than 70%. The lava rock provided a hospitable environment for microorganisms while causing low head loss (25 Pa at a load rate of 3.8 m/min). However, lava rock is susceptible to dissolution at low‐pH conditions (the degree of dissolution depends on the source of the material).
In an ideal biofilter, airflowsparallel to the biofilter axis and at the same velocity through allparts of the bed. In contrast, non-unifom flow causes differentparcels of air to experience detention times above or below the average, reducing treatment efficiency. Designers and operators have long been aware that medium nonun iformity, bed compaction, fissuring, and separationjkom the vessel walls can causeflow channeling and damage biofilterperformance. H o w , as biofilters have been designed for lower headloss and higher flowrates, flow heterogeneity has also arkn, aspressure variation in the headspdce affectsflow in the medium.In an effort to investigateflow heterogeneity, a two-dimensional steady-state computationalfluid dynamics (CFD) model was developed to simulateflow through a typical biofilter. The model assumes incompressible, two-dimensional, Navier-Stokes flow in the spaces above and below the bed and DarcyJlow in the porous medium. The modeling effort demonstrated that the optimal desgn depends on thepermeability of the medium, the airflowrate, and biofilter configuration. Several simple options for inlet and outlet location were compared. In very lowpermeability media, theflow was uniform regardless of the design. With highlypermeable media, the optimal design choice depended on the Reynolds number. However, the design with side inlet and same side outlet had the lowestflow heterogeneity of thefive designs over a wide range of Reynolds numbers. The model also demonstrated that removal efJicciency decreases as flow heterogeneity increases.
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