Laboratory-scale digesters were used to perform side-by-side evaluations of alternative digestion systems including thermophilic-mesophilic-phased digestion (TPAD), acid/methane-phased digestion, mesophilic digestion, and the performance of the thermophilic stage of TPAD alone. Total detention times in the systems evaluated varied from 8 to 20 days. Temperatures in the mesophilic stage of TPAD were varied from 35 to 43.3 degrees C and temperatures in the thermophilic stage were varied from 55 to 58.9 degrees C. The purpose of the study was to evaluate volatile solids (VS) reduction and thickening properties of the different systems. Parameters evaluated included solids, volatile acids, gas production and quality, various chemical constituents of the sludge, and thickening properties using polymer and ferric chloride plus polymer. The TPAD systems were found capable of higher VS destruction at total detention times of 10 and 12.5 days compared to single-stage mesophilic or thermophilic digestion at 15 to 20 days. A laboratory-scale thickening test was performed to evaluate thickening properties. Based on obtaining 95% capture in the test, the polymer demand of mesophilic and acid-methane-phased digestion was the lowest. The TPAD system required approximately 180% of the dosage of the mesophilic sludge and thermophilic sludge required approximately 310% of the dosage. Temperatures of 40.5 and 43.3 degrees C in the mesophilic stage of TPAD were found to increase the polymer requirement and decrease the VS reduction of the TPAD system. Higher temperatures (above 56.1 degrees C) and lower detention times (less than 15 days) in the thermophilic digesters seemed to result in problems with degradation of volatile acids, particularly propionic acid. 相似文献
Hazard assessment is an essential element in the evaluation of the potential effects of chemical substances on the environment. To date, most work has focused on hazard assessment schemes for the aquatic environment, but in recent years, a number of proposals have been developed for other environmental compartments. Due to limited datasets, the suitability of the toxicity cut-off values in these schemes has not been fully determined and the practicalities associated with using these approaches have not been fully established. This study, which focused on the soil compartment, was performed to examine cut-off values proposed by two terrestrial hazard assessment schemes and establish the availability of data. Data on earthworms indicated that current proposals for toxicity cut-off values are appropriate. However, analysis of IUCLID (International Uniform ChemicaL Information Database) indicates that even for commonly used high production volume chemicals, insufficient data are available to enable classification. Whilst the necessary data may already be available for selected groups of substances (e.g. pesticides and veterinary medicines), a significant experimental testing programme would therefore be required before a terrestrial classification system could be applied widely. Such data may become available in the future as a result of initiatives such as REACH. 相似文献
Determining the reproductive processes of benthic invertebrates is central to our understanding of their recruitment and population
dynamics. Sexual reproduction of the gonochoric and viviparous Great Barrier Reef sponge, Luffariella variabilis (Poléjaeff 1884) was quantified from histological samples collected over two reproductive seasons (2004 and 2005). Gametogenesis
commenced for females at a water temperature of 21°C, the lowest water temperature of the year. Spermatogenesis occurred above
22.5°C with sperm asynchronously developed and released from August or September to October. Oocytes developed asynchronously
from July to September, embryos from September to December, and larvae from November to December. Female reproduction terminated
in December (after larval release) prior to the highest mean annual water temperature of 30°C in January. There was a significant
(35%) decrease in female reproductive output in 2005 compared to 2004, as measured by the reproductive index (0.68 ± 0.12
female reproductive propagules mm−2 of mesohyl in 2005 compared with 1.05 ± 0.10 mm−2 in 2004). This corresponded with delayed oogenesis and spermatogenesis, and a shortened larval development cycle corresponding
with a delayed minimum temperature (21°C) in August of 2005 compared with July 2004. Accordingly, the maximum percentage of
the mesohyl occupied by female reproductive propagules (eggs, embryos and larvae) was also reduced by 60% in 2005 (overall
mean of 13.04% in October 2004 compared with 5.35% in October 2005). However, the mean sizes of individual female propagules
remained the same from year to year. Males in contrast, showed no overall difference in either reproductive index or percentage
occupation of the mesohyl between 2004 and 2005. The lowered reproductive output (∼35%) of females of L. variabilis associated with delayed minimum water temperatures may have important implications for population reproductive success where
oogenesis and spermatogenesis and larval release are cued by minimum and maximum water temperatures, respectively. 相似文献
Abstract Microtox and Ames bioassays were employed to assess acute toxicity and mutagenicity of water soluble components of class–fractionated oils extracted from one creosote–and four petroleum–contaminated soils. Microtox results revealed that potential acute toxicity resides mainly in the polar class fractions at three sites and indicated potential synergistic and antagonistic effects between compounds in the total extracts at two sites. Ames Salmonella/microsome testing indicated that the polyaromatic fractions at two sites exhibit weak mutagenicity with enzymatic activation, while the polar fractions at two sites are weakly mutagenic without enzyme activation. Further chemical characterization of the polar and polyaromatic fractions is required to fully assess the potential of health and ecological risks at the creosote–and petroleum–contaminated sites exhibiting these toxic responses. 相似文献
Objectives: Mixed-use urban environments, such as arterial roads with adjacent commercial land uses, represent crash locations with the highest risk. These locations are often characterized by high volumes of motor vehicle traffic, on-street parking, and interactions with multiple road user groups such as pedestrians, cyclists, and public transportation. The objective of this study was to investigate previously identified crash risk factors for mixed-use urban environments and assess how parking occupancy, center medians, and cyclist volume influence performance and workload in a driving simulator study.
Methods: Thirty participants were recruited for the study. Participants completed 6 drives that presented different combinations of cyclist volume, median condition, and parking occupancy. Incorporated into the simulator drives was a secondary peripheral detection task (PDT) designed to measure mental workload. Participants provided subjective assessments of workload using the Rating Scale Mental Effort (RSME).
Results: Mean lateral lane position was found to significantly vary across the 3 independent variables of parking occupancy, cyclist volume, and median conditions. No significant changes were identified for mean speed across the conditions. Subjective and objective measures of workload identified changes due to the presence of cyclists with slower reaction times for the PDT task when cyclists were present.
Conclusion: The findings provide insight into the interaction of road design elements in mixed-use urban road environments and demonstrate that increasingly complex environments increase driver demand. This has important road design implications for mixed-use arterial roads, which are often characterized by complex interactions between multiple road user groups. 相似文献
To assist transport modeling in assessments of the radiological impact of a geological repository for radioactive wastes, the mobility of various elements was studied in arable and wetland soils in the Forsmark region, Sweden. Pore water and total element contents were determined for five types of unconsolidated deposits (regolith), spanning a wide range of soil properties with respect to pH and organic matter content. Two soil depths were sampled to capture element mobility in regolith layers affected and unaffected by soil-forming processes. The solid/liquid partition coefficients (Kd values) for most elements varied significantly among regolith types. For most elements, the observed variations in Kd values could be explained by variations in soil properties. For many elements, mobility increased with decreasing soil pH. The results provide a significant addition of data on radionuclide retention in soils, taking account of soil properties and processes. 相似文献
Archaeological and paleontological datasets are used in conservation to add time-depth to ecology. In central Texas, several
top carnivores including prehistoric Native American hunters have been extirpated or have had their historic ranges restricted,
which has resulted in pest-level white-tailed deer (Odocoileus virginianus texana) populations in some areas. Differences in body size of deer between prehistory and modernity are expected, given that a
lack of predation likely has increased intraspecific competition for forage among deer, resulting in smaller body size today.
In fact, modern deer from settings without harvest pressure are significantly smaller than those from harvested areas and
from prehistoric deer. From a natural history perspective, this research highlights potential evolutionary causes and effects
of top-predator removal on deer populations and related components of biological communities in central Texas. 相似文献