Showing papers by "Andreas Lorke published in 2019"

Methane and nitrous oxide emission from different treatment units of municipal wastewater treatment plants in Southwest Germany.

Abstract: We measured the atmospheric emission rates of methane (CH4) and nitrous oxide (N2O) in two wastewater treatment plants in Southwest Germany, which apply different treatment technologies. Dissolved gas concentrations and fluxes were measured during all processing steps as well as in the discharge receiving streams. N2O isotopocule analysis revealed that NH2OH oxidation during nitrification contributed 86-96% of the N2O production in the nitrification tank, whereas microbial denitrification was the main production pathway in the denitrification tank in a conventional activated sludge (CAS) system. During wastewater treatment using a modified Ludzack-Ettinger system (MLE) with energy recovery, N2O was predominantly produced by the NO2- reduction by nitrifier-denitrification process. For both systems, N2O emissions were low, with emission factors of 0.008% and 0.001% for the MLE and the CAS system, respectively. In the effluent-receiving streams, bacterial denitrification and nitrification contributed nearly equally to N2O production. The CH4 emission from the MLE system was estimated as 118.1 g-C d-1, which corresponds to an emission factor of 0.004%, and was three times lower than the emission from the CAS system with 0.01%.

Laboratory and field investigations on freeze and gravity core sampling and assessment of coring disturbances with implications on gas bubble characterization

Abstract: The quantification of greenhouse gas emissions from aquatic ecosystems requires knowledge about the spatial and temporal dynamics of free gas in sediments. Freezing the sediment in situ offers a promising method for obtaining gas‐bearing sediment samples, unaffected by changes in hydrostatic pressure and sample temperature during core withdrawal and subsequent analysis. This article presents a novel freeze coring technique to preserve the in situ stratigraphy and gas bubble characteristics. Nondestructive X‐ray computed tomography (CT) scans were used to identify and characterize coring disturbances of gravity and freeze cores associated with gassy sediment, as well as the effect of the freezing process on the gas bubble characteristics. Real‐time X‐ray CT scans were conducted to visualize the progression of the freezing process. Additional experiments were conducted to determine the freezing rate to assess the probability of sediment particle/bubble migration, and gas bubble nucleation at the phase transition of pore water to ice. The performance of the freeze coring technique was evaluated under field conditions in Olsberg and Urft Reservoir (Germany). The results demonstrate the capability of the freeze coring technique for the preservation of gas‐bearing sediments and the analysis of gas bubble distribution pattern in both reservoirs. Nevertheless, the obtained cores showed that nearly all gravity and freeze cores show some degree of coring disturbances.

Effect of Cascading Reservoirs on the Flow Variation and Thermal Regime in the Lower Reaches of the Jinsha River

Abstract: We analyzed the alteration of discharge and water temperature caused by two newly established reservoirs in the lower reaches of the Jinsha River. In comparison to longer-term observations from the pre-impoundment period, the seasonal flow variability was significantly affected by the larger, upstream-located Xiluodu reservoir, with higher discharge in spring and reduced discharge in summer. The smaller, downstream located Xiangjiaba reservoir did not contribute significantly to the total hydrological alteration caused by the reservoir cascade. Thermal stratification occurred in spring and summer in the Xiluodu reservoir, but was not observed in the Xiangjiaba reservoir. The vertical structure and seasonal dynamics of thermal stratification were mainly governed by the water temperature of the inflow and the depth of the water outlet. Despite the different thermal structure, both reservoirs reduced the amplitude of annual temperature variations and delayed the seasonal temperature cycle in the downstream river water. In contrast to discharge variation, thermal effects were cumulative along the cascading reservoirs. Homogenization and delay effects can be expected to increase further with the completion of ongoing reservoir construction upstream of the two studied reservoirs. Based on our findings, we discuss the larger-scale impacts of cascading hydropower developments and emphasize the need for taking water temperature and its variation into account when developing optimized operation or mitigation strategies for these systems.

Hydrodynamic control of gas-exchange velocity in small streams

Abstract: Gas exchange is a critical component of any biogeochemical mass balance model of dissolved gases in aquatic systems, yet the magnitude and drivers of spatial and temporal variations of air-water exchange rates in shallow streams are poorly understood. We investigated the relationships between gas exchange velocity of carbon dioxide and methane and flow hydraulics at different sections along a third order stream in Southwest Germany. To cover a wide range of different flow conditions, the sections were selected based on visual categorization of the dominant surface flow type. We found that in smooth and rippled flows, gas exchange velocities followed a universal dependence on turbulent dissipation rates predicted by the small-eddy and surface renewal models. For these surface flow types, the scaling applied to both, bulk-scale dissipation rates estimated from flow geometry and dissipation rates estimated from turbulence measurements. Turbulence was strongly anisotropic under rough flow conditions and gas exchange velocities were lower than predicted from measured dissipation rates. Nevertheless, near-surface turbulence and gas exchange velocities differed among surface flow type categories, indicating that quantitative assessment and mapping of surface flow type may facilitate improved parameterizations of gas exchange velocities at larger spatial scales. We further describe a novel instrument facilitating an objective assessment of surface flow by measuring the acceleration of a small floating sphere drifting freely on the water surface. In combination, our findings may open a new road for understanding, measuring and predicting spatial and temporal variability of gas exchange in streams.

Carbon dioxide and methane fluxes over a boreal river measured with eddy covariance

Abstract: Institute for Atmospheric and Earth System Research INAR/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland. Finnish Meteorological Institute, Helsinki, Finland. Institute for Environmental Sciences, University of Koblenz-Landau, Landau, Germany. Department of Physical Geography and Ecosystem Science, Lund University, Lund, Sweden. Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California. Institute for Atmospheric and Earth System Research INAR/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, Helsinki, Finland. Faculty of Biological and Environmental Sciences, University of Helsinki, Lahti, Finland. 8Department of Earth Sciences, Uppsala University, Uppsala, Sweden.