Mixing inside an anaerobic digester is often continuous and is not actively controlled. The selected mixing regime can however affect both gas production and the energy efficiency of the biogas plant. This study aims to evaluate these effects and compare three different mixing regimes, 150 RPM and 25 RPM continuous mixing and minimally intermittent mixing for both digestion of fresh substrate and post-digestion of the organic fraction of municipal solid waste. The results show that a lower mixing intensity leads to a higher biogas production rate and higher total biogas production in both cases. 25 RPM continuous mixing and minimally intermittent mixing resulted in similar biogas production after process stabilization, while 150 RPM continuous mixing resulted in lower production throughout the experiment. The lower gas production at 150 RPM could not be explained by the inhibition of volatile fatty acids. Cumulative biogas production until day 31 was 295 ± 2.9, 317 ± 1.9 and 304 ± 2.8 N ml/g VS added during digestion of fresh feed and 113 ± 1.3, 134 ± 1.1 and 130 ± 2.3 N ml/g VS added during post digestion for the 150 RPM, 25 RPM and minimally mixed intensities respectively. As well as increasing gas production, optimal mixing can improve the energy efficiency of the anaerobic digestion process. 相似文献
A laminated sediment core collected in the southern part of the Baltic Proper was analysed for DDT compounds, polychlorinated biphenyls (PCB) including the coplanar congeners, polybrominated diphenyl ethers (PBDE) and polychlorinated naphthalenes (PCN). Two sewage sludge samples were also analysed for the same compounds.
The results of the sediment analyses indicate the presence of PCB, DDT and PBDE compounds in sediment layers dating from the 1950s and later. Neither coplanar PCB nor PCN were detected at any level of the sediment core. The PCB concentrations indicate a slight increase in levels during the last decades, while there were no changes in the levels of DDT compounds. In contrast, TeBDE levels increase 4- to 8-fold and one PeBDE congeners levels increase 10- to 20-fold. The results of sewage sludge analyses showed the concentration of the individual PBDE to be at about the same level as for the individual PCB congeners. The congener pattern in sludge samples indicates low-chlorinated products to be the main PCN source. 相似文献
In a world increasingly modified by human activities, the conservation of biodiversity is essential as insurance to maintain resilient ecosystems and ensure a sustainable flow of ecosystem goods and services to society. However, existing reserves and national parks are unlikely to incorporate the long-term and large-scale dynamics of ecosystems. Hence, conservation strategies have to actively incorporate the large areas of land that are managed for human use. For ecosystems to reorganize after large-scale natural and human-induced disturbances, spatial resilience in the form of ecological memory is a prerequisite. The ecological memory is composed of the species, interactions and structures that make ecosystem reorganization possible, and its components may be found within disturbed patches as well in the surrounding landscape. Present static reserves should be complemented with dynamic reserves, such as ecological fallows and dynamic successional reserves, that are part of ecosystem management mimicking natural disturbance regimes at the landscape level. 相似文献
Ecosystems at high latitudes are highly dynamic, influenced by a multitude of large-scale disturbances. Due to global change processes these systems may be expected to be particularly vulnerable, affecting the sustained production of renewable wood resources and abundance of plants and animals on which local cultures depend. In this paper, we assess the implications of new understandings of high northern latitude ecosystems and what must be done to manage systems for resilience. We suggest that the focus of land management should shift from recovery from local disturbance to sustaining ecosystem functions in the face of change and disruption. The role of biodiversity as insurance for allowing a system to reorganize and develop during the disturbance and reorganization phases needs to be addressed in management and policy. We emphasize that the current concepts of ecological reserves and protected areas need to be reconsidered to developp dynamic tools for sustainable management of ecosystems in face of change. Characteristics of what may be considered as customary reserves at high latitudes are often consistent with a more dynamic view of reserves. We suggest new directions for addressing biodiversity management in dynamic landscapes at high latitudes, and provide empirical examples of insights from unconventional perspectives that may help improve the potential for sustainable management of biodiversity and the generation of ecosystem services. 相似文献
This study explores the linkages between trade policy, corruption, and environmental policy. We begin by presenting a theoretical model that produces several testable predictions, including: (i) the effect of trade liberalization on the stringency of environmental policy depends on the level of corruption; and (ii) corruption reduces environmental policy stringency. Using panel data from a mix of developed and developing countries from 1982 to 1992, we find evidence that supports these conjectures. We view these results as representing an attempt at understanding the myriad of complex relationships that exist in an open economy. 相似文献
Honeybee colonies reproduce by colony fission and swarming. The primary swarm leaves the nest with the mated mother queen.
Further “after-swarms” can leave the nest. These are composed of virgin queens and sister workers. Since all workers in the
primary swarm have the same relationship to the mother queen, kin recognition cannot have any effect on the worker distribution
in the swarm. Because of polyandry of the mother queen, the after-swarm is composed of super- and halfsister workers of the
virgin queen. In this case kin recognition might affect swarm composition if workers increase their inclusive fitness by preferentially
investing in a supersister queen. The distribution of workers in the mother colony, the primary and the after-swarm was analyzed
using single-locus DNA fingerprinting in two colonies of the honeybee (Apis mellifera). The colonies were composed of 21 and 24 worker subfamilies because of multiple mating of the queen. The subfamily distribution
in the mother colonies before swarming was significantly different from the subfamily frequencies in the primary swarm. This
indicates different propensities for swarming in the various subfamilies. The subfamily distribution was also significantly
different between the mother colony and the after-swarm. There was however no significant difference between the subfamily
composition of the primary and the after-swarm. The average effects of kin recognition on the distribution of the subfamilies
in the two after-swarms were less than 2%. We conclude that colony-level selection sets the evolutionary framework for swarming
behaviour.
Received: 22 May 1996 / Accepted after revision: 2 November 1996 相似文献