Aeration plays an important role in the activated sludge process; it must supply enough oxygen to maintain the metabolic reactions of microorganisms and provide sufficient mixing in the aeration tank. During aeration, oxygen is transferred to the aeration tank while volatile organic compounds (VOCs) are stripped from the tank and cause air-pollution problems. In this study, a series of batch VOC emission tests were performed in a 500-L tank equipped with coarsebubble diffusers at 1.64-3.28 m 3 /h diffused air-flow rates and 297-299 K water temperatures in the absence and presence of commercial surfactant. The unsteady-state dissolved concentrations of p-xylene were measured during the tests and compared with the results predicted by both the American Society of Civil Engineers (ASCE)-based model and the two-zone model. According to the experimental results, the VOC emission rate increased with increasing airflow rate and decreasing surfactant concentration. The results also confirmed that the twozone model could give a better prediction of the VOC emission rates while the ASCE-based model underestimated the VOC emission rates.
By bombarding samples with 1.1-MeV 4He+ ions and observing the energy spectra of the backscattered ions, it has been possible to determine the concentrations and lattice locations of arsenic and antimony impurities diffused into silicon. For the samples investigated it was found that only 60–75% of the arsenic atoms were in substitutional sites, even at impurity concentrations well below solid solubility. About 90% of the antimony atoms were found to be in substitutional sites.
In wastewater treatment plants, many operation units such as equalization and aeration involve
oxygen transfer between wastewater and air. While oxygen is transferred from air to wastewater,
volatile organic compounds (VOCs) are stripped from wastewater to air. Because of increasingly
stringent environmental regulations, wastewater treatment operators have to do VOC inventory
of their facilities. A new mass-transfer model has been developed to predict the VOC emission
rates from batch and continuous aeration tanks with mechanical surface aerators. The model
takes into consideration that the VOC mass transfer occurs in two separate mass-transfer zones
instead of lumping the overall VOC transfer in the whole aeration tank as is done in the
conventional ASCE-based model. The predictive capabilities of the two-zone and the ASCE-based models were examined by calculating the emission rates of 10 priority pollutants from
aeration tanks. The effects of the hydraulic retention time, the Henry's law constant, gas-phase
resistance, and the water and air environmental conditions on the VOC emission rates were
predicted by the two models.
The enhanced solubility and lattice location of gold impurities in a heavily phosphorus−diffused layer of silicon have been investigated using the ion−backscattering technique. At a phosphorus surface concentration of 3.3×1020 cm−3, the corresponding gold solubility was found to be about 7×1018 cm−3 at 1100 °C, with the gold impurities being predominantly in substitutional sites. A lightly doped wafer with no phosphorus diffusion had a surface gold concentration of 1.5×1018 cm−3 after 1100 °C gold diffusion, again with the gold impurities being predominantly in substitutional sites. The results are interpreted by considering the contributions of electronic interactions and ion pairing to the enhanced gold solubility in the phosphorus−doped layer, using the simple theory of Reiss and assuming the gold and phosphorus atoms of the ion pair to be in adjacent substitutional sites, as suggested by the experimental data.
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