Innovative aspects of environmental chemistry and technology regarding air,water, and soil pollution

Environmental Science and Pollution Research -     

Effect of sewer headspace air-flow on hydrogen sulfide removal by corroding concrete surfaces

Hydrogen sulfide adsorption and oxidation by corroding concrete surfaces at different air-flows were quantified using a pilot-scale sewer reactor. The setup was installed in an underground sewer research station with direct access to wastewater. Hydrogen sulfide gas was injected into the headspace of the sewer reactor once per hour in peak concentrations of approximately 500 ppmv. The investigated range of sewer air-flows was representative for natural ventilated sewer systems, and covered both laminar and turbulent conditions. The experiments demonstrated a significant effect of sewer air-flow on the kinetics of hydrogen sulfide removal from the sewer headspace. From the lowest to the highest air-flow investigated, the rate of adsorption and oxidation increased more than threefold. At all air-flows, the reaction kinetics followed a simple n-th order rate equation with a reaction order of 0.8. The effect of air-flow on hydrogen sulfide adsorption and oxidation kinetics was quantified by a simple empirical equation.     

Aerobic and anaerobic transformations of sulfide in a sewer system--field study and model simulations.

The formation and fate of sulfide in a force main and a downstream-located gravity sewer were investigated in an extensive field study. Sulfide formation in the force main was significant. However, during 14 minutes of transport in the gravity sewer, the sulfide concentration decreased 30%, on average. An application of a conceptual sewer process model for simulating the formation and fate of sulfide was demonstrated. Overall, the model predicted that approximately 90% of the decrease of the sulfide concentration in the gravity sewer was the result of sulfide oxidation and that only a small fraction entered the sewer atmosphere, causing odor and corrosion. Even so, the model predicted concrete corrosion rates of up to 1.2 mm/y in the gravity sewer section.     

Modeling the formation and fate of odorous substances in collection systems.

A conceptual model that simulates the formation and fate of odorous substances in branched collection systems is presented. The model predicts the activity of the relevant biomass phenotypes under aerobic, anoxic, and anaerobic conditions in force mains and gravity sewers. The formation and fate of individual, malodorous substances in the bulk water, biofilms, and sediments are modeled. The release of odorous compounds from the bulk water to the sewer gas phase, their fate in the gas phase, and their subsequent release into the urban atmosphere is simulated. Examples of model application include the prediction of hydrogen sulfide and malodorous fermentation products from force mains and gravity sewers.     

Gas phase transport in gravity sewers--A methodology for determination of horizontal gas transport and ventilation.

A method was developed for determination of horizontal gas transport and ventilation in gravity sewers. This was achieved by changing the composition of the sewer atmosphere by pulse injection of oxygen gas and subsequently measuring the oxygen concentration in a downstream manhole. Conventional tracer techniques may require sampling and may also affect the environment. The method developed is simple, based on direct monitoring and without environmental or toxic effects. The method was developed based on measurements in an intercepting gravity sewer. The horizontal gas transport processes were quantified by measuring the velocity and dispersion of the gas in the sewer atmosphere. Based on 54 measurements, the gas velocity was found to vary between 0.05 and 0.22 m/s. The coefficients of dispersion were calculated to be in the range 0.05 to 1.1 m2/s. Climatic conditions did not significantly influence the gas phase transport.     

Sorption media for stormwater treatment--a laboratory evaluation of five low-cost media for their ability to remove metals and phosphorus from artificial stormwater

Five sorption materials were studied with a focus on polishing pretreated stormwater: crushed limestone, shell-sand, zeolite, and two granulates of olivine. These materials are commercially available at comparatively low cost and have been subjected to a minimum of modification from their natural states. The sorbents were tested for phosphorus, arsenic, cadmium, chromium, copper, nickel, lead, and zinc at concentration and conditions relevant for typical stormwater. The materials were tested for sorption capacity and kinetics. Desorption was tested under neutral and alkaline conditions and in the presence of chloride. For most sorbent/sorbate combinations, significant sorption occurred within the first minutes of contact between sorbent and sorbate. Treatment to the low microgram per liter range could be achieved by contact times of less than 1 hour. The study indicated that sorption filters can be designed for long life expectancy at comparatively low cost by applying the materials tested.     

Stochastic modeling of chemical oxygen demand transformations in gravity sewers.

Wastewater quality characteristics in terms of biomass, its substrates, and the corresponding kinetic and stoichiometric parameters were determined based on 109 wastewater samples originating from five different campaigns in four different sewer networks. Quality parameters were determined by model calibration of measured wastewater oxygen uptake rates applying a model that describes the aerobic breakdown of wastewater organic matter. Thereafter, the distributions of the parameters were analyzed. Two of the five datasets were obtained at the upstream end of a five-km-long, intercepting gravity sewer. For each of these upstream wastewater samples, downstream samples were collected with a delay corresponding to the residence time. The upstream distributions of the wastewater composition were used as boundary conditions for a Monte Carlo simulation. The calculated downstream distributions were compared to the measured downstream distributions and good agreement was observed.     

Kinetics and stoichiometry of aerobic sulfide oxidation in wastewater from sewers-effects of pH and temperature.

Kinetics and stoichiometry of aerobic chemical and biological sulfide oxidation in wastewater from sewer networks were studied. In this respect, the effects of temperature and pH were investigated in the ranges 10 to 20 degrees C and 5 to 9, respectively. The temperature dependency of sulfide oxidation kinetics was described using an Arrhenius relationship. The effect of pH on the rate of chemical sulfide oxidation is related to the dissociation of hydrogen sulfide (H2S) to hydrogen sulfide ion (HS(-)), with HS(-) being more readily oxidized than H2S. Biological sulfide oxidation exhibited the highest rates at ambient wastewater pH, and the reaction was inhibited at both low and high pH values. Chemical sulfide oxidation was found to produce thiosulfate and sulfate, while elemental sulfur was the main product of biological sulfide oxidation. Based on the investigations, general rate equations and stoichiometric constants were determined, enabling the processes to be incorporated to conceptual sewer process models.     

Effects of pH and iron concentrations on sulfide precipitation in wastewater collection systems.

Sulfide precipitation by addition of iron salts is a widely used strategy for sulfide control in wastewater collection systems. Several parameters, such as pH, oxidation-reduction conditions, and reactant concentrations, are known to affect the feasibility of the method. However, their combined effects are difficult to predict for complex media, such as wastewater. This study investigates the effect of pH and reactant concentrations on the efficiency of iron sulfide precipitation in anaerobic municipal wastewater. Laboratory experiments showed that, when the pH was below 7, typically less than 40% of the added ferrous iron reacted by sulfide precipitation, although sulfide was in excess. However, when the pH was above 8, almost complete precipitation of all the added ferrous iron was observed. Varying the ferric-iron-to-ferrous-iron ratio demonstrated that improved efficiency could be achieved when using a 1:1 mixture of ferric chloride and ferrous sulfate.