Liquid-Gas Mass Transfer of Volatile Substances in an Energy Dissipating Structure.

Mass transfer of a range of volatile substances was studied under highly turbulent conditions. The applied setup mimicked drop structures, where the release of volatile organic carbons likely occurs at a high rate. The experiments covered several substances in a range of resistances from residing entirely in the liquid film to being fully in the gas film. The two-film theory yielded a good prediction of the whole measured range. This allowed the experimental validation of a method where two reference substances are applied, to determine the transfer of any other substance - independently of where its resistance to mass transfer resides. One finding was that the range of dimensionless Henry's constants, where both films contributed by more than 5%, was 0.0027 to 1.05, which is over five times higher than the accepted rule of thumb (0.0005-0.18). Another finding was that the ratio between the liquid and the gas film mass transfer coefficients of the reference substances was similar for the two drop configurations studied. If this holds true over a wider range of configurations, such a ratio constitutes a valuable shortcut to the current practice of ignoring gas film resistance in the estimation of mass transfer rates.

Assessment of sulfide production in a full scale wastewater sludge rising main.

Production and build-up of sulfide in wastewater systems, especially downstream of rising mains, may lead to severe odour nuisance, toxic environments and high risk of corrosion. Due to increased population migration towards cities and lower area availability for treatment facilities, rising mains for the conveyance of wastewater sludge are becoming more frequent, and research on sulfide build-up in such cases is needed. In this paper the findings of the work carried out in a full scale wastewater sludge rising main, operated during different seasons and under distinct conditions are presented (comprising both the start-up and normal operation stages of the facility). Results showed a sulfide build-up rate of 3.24 g S-2 m-2 d-1 in the summer and of 2.30 g S-2 m-2 d-1 during the winter. The ratio of sulfate reduction to sulfide production (SO4-2/S-2) was of roughly 3 to 1, as expected. Furthermore, obtained results allowed adjusting a second order polynomial empirical equation for the forecasting of sulfide build-up within the sludge rising main. The obtained equation for sludge significantly differs from existing equations obtained for wastewater. Moreover, this work also allowed obtaining new insight into the positive influence of biofilm and hydraulic retention time in the biological sulfide generation, as well as into its variation along the length of the rising main.

Release of hydrogen sulfide under intermittent flow conditions - the potential of simulation models.

For engineering purposes it is especially useful to be able to predict and control sewer corrosion rates and odor impacts as well as to design effective measures aiming to reduce effects related to hydrogen sulfide formation and release. Doing so, it is important to use modeling tools that are capable of assessing variations of dissolved oxygen, dissolved sulfide and hydrogen sulfide gas concentrations for a wide range of environmental scenarios. Two such models were assessed: AEROSEPT, an empirical formulation, and WATS, a conceptual and more complex approach. The models were applied to evaluate the effects of transitions between pressure mains and gravity sewers in the air-liquid mass transfer of hydrogen sulfide at the Ericeira sewer system in Portugal. This network is known to have odor and corrosion problems, especially during summer. Despite the unavoidable uncertainties due to the unsteady flow rate and the quantification of air velocity and turbulence, the simulation results obtained with both models have been shown to adequately predict the overall behavior of the system.

Liquid-gas mass transfer at drop structures.

Over the last decades, considerable progress has been made in the understanding of the sulfur cycle in sewer systems. In spite of a wealth of experimental and field studies that have addressed the release of hydrogen sulfide from free surface flows in gravity sewers and the corresponding air-water mass transfer, little is known about hydrogen sulfide emission under highly turbulent conditions (e.g., drop structures, hydraulic jumps). In this study, experimental work was carried out to analyze the influence of characteristics of drops on reaeration. Physical models were built, mimicking typical sewer drop structures and allowing different types of drops, drop heights, tailwater depths and flow rates. In total, 125 tests were performed. Based on their results, empirical expressions translating the relationship between the mass transfer of oxygen and physical parameters of drop structures were established. Then, by applying the two-film theory with two-reference substances, the relation to hydrogen sulfide release was defined. The experiments confirmed that the choice of the type of drop structure is critical to determine the uptake/emission rates. By quantifying the air-water mass transfer rates between free-fall and backdrop types of drop, the latter resulted in considerably lower oxygen uptake rates.

Release of hydrogen sulfide in a sewer system under intermittent flow conditions: the Eiceira case study, in Portugal.

The presence and fate of hydrogen sulfide in wastewater systems were studied in two stretches of an intercepting sewer system located in a coastal village, in Portugal. A range of hydraulic parameters were obtained and liquid and gas phase measurements were carried out, both continuously and through intensive sampling campaigns. Upstream, where flow rates were relatively low, dissolved sulfide concentrations around 12 mg S L-1 and hydrogen sulfide gas concentrations above 250 ppm were observed, along with limited corrosion damage. It is believed this is due to the low relative humidity detected along the atmosphere of the studied sewer system. Downstream, gas concentrations were always below 40 ppm. Despite that, high signs of corrosion were detected, particularly in manholes with drop structures. It is thought that condensation of spray produced by the fall is the main cause of the phenomenon. Another relevant observation was the rapid decline in dissolved sulfide contents along gravity trunk sewers following the discharge of rising mains, with loss rates as high as 40 mg S L-1 h-1. Air-flow velocities corresponded to 15-50% of wastewater flows, an observation which agrees with other authors' publications addressing relatively small pipes and moderate water flows.

Erratum: Water Science and Technology 75 (10), 2257-2267: Liquid-gas mass transfer at drop structures, Natércia Matias, Asbjørn Haaning Nielsen, Jes Vollertsen, Filipa Ferreira and José Saldanha Matos, doi: 10.2166/wst.2017.103.

Over the last decades, considerable progress has been made in the understanding of the sulfur cycle in sewer systems. In spite of a wealth of experimental and field studies that have addressed the release of hydrogen sulfide from free surface flows in gravity sewers and the corresponding air-water mass transfer, little is known about hydrogen sulfide emission under highly turbulent conditions (e.g., drop structures, hydraulic jumps). In this study, experimental work was carried out to analyze the influence of characteristics of drops on reaeration. Physical models were built, mimicking typical sewer drop structures and allowing different types of drops, drop heights, tailwater depths and flow rates. In total, 125 tests were performed. Based on their results, empirical expressions translating the relationship between the mass transfer of oxygen and physical parameters of drop structures were established. Then, by applying the two-film theory with two-reference substances, the relation to hydrogen sulfide release was defined. The experiments confirmed that the choice of the type of drop structure is critical to determine the uptake/emission rates. By quantifying the air-water mass transfer rates between free-fall and backdrop types of drop, the latter resulted in considerably lower oxygen uptake rates.

Dynamic Modeling of Hydrogen Sulfide within Enclosed Environments in Biosolids Recovery Facilities.

Hydrogen sulfide emissions from wastewater affect human health and equipment durability, thus presenting a complex issue for utilities. Several VOC emission models have been used before to predict H2S in collection systems and water resources recovery operations, even if with restrictions. By contrast, fewer studies focus on biosolids emissions and modelling. This paper presents a dynamic modelling approach to predict H2S concentration in a tank headspace of a wastewater biosolids recovery facility. Data from one of the largest Portuguese water resources recovery facilities was collected under different facility operating modes. The developed model adequately predicted H2S concentration, with R2 values of 0.89 and 0.78, for different periods of the year, thus showing how modelling may reliably contribute to utility operation decisions.