Spatial Variability of Anaerobic Processes and Wastewater pH in Force Mains.

The present study focuses on anaerobic organic matter transformation processes in force mains for the purpose of improving existing sewer process models. Wastewater samples were obtained at 100 m intervals from a 1 km long pilot scale force main and measured for several wastewater parameters. Transformation rates for selected parameters were calculated and their spatial variability analyzed. In terms of electron transfer, fermentation was the most significant process, resulting in a net volatile fatty acid formation of 0.83 mmol/L. Sulfate reduction resulted in a production of 0.73 mmol/L of inorganic sulfide. Methanogenesis was negligable in all experiments despite an anaerobic residence time of more than 30 hours. As a result of the anaerobic processes, the wastewater pH decreased by approximately one pH unit, resulting in a corresponding increase in the fraction of molecular hydrogen sulfide. A significant spatial variablilty was observed for the average transformation rates of all parameters.

Invertebrates in stormwater wet detention ponds - Sediment accumulation and bioaccumulation of heavy metals have no effect on biodiversity and community structure.

The invertebrate diversity in nine stormwater wet detention ponds (SWDP) was compared with the diversity in eleven small shallow lakes in the western part of Denmark. The SWDPs and lakes were chosen to reflect as large a gradient of pollutant loads and urbanization as possible. The invertebrates as well as the bottom sediments of the ponds and shallow lakes were analyzed for copper, iron, zinc, cadmium, chromium, lead, aluminum, nickel, arsenic and the potentially limiting nutrient, phosphorus. The Principal Component Analysis showed that invertebrates in SWDPs and lakes differed with respect to bioaccumulation of these elements, as did the sediments, albeit to a lesser degree. However, the Detrended Correspondence Analysis and the TWINSPAN showed that the invertebrate populations of the ponds and lakes could not be distinguished, with the possible exception of highway ponds presenting a distinct sub-group of wet detention ponds. The SWDPs and shallow lakes studied seemed to constitute aquatic ecosystems of similar taxon richness and composition as did the 11 small and shallow lakes. This indicates that SWDPs, originally constructed for treatment and flood protection purposes, become aquatic environments which play a local role for biodiversity similar to that of natural small and shallow lakes.

Modeling Sulfides, pH and Hydrogen Sulfide Gas in the Sewers of San Francisco.

An extensive measuring campaign targeted on sewer odor problems was undertaken in San Francisco. It was assessed whether a conceptual sewer process model could reproduce the measured concentrations of total sulfide in the wastewater and H2S gas in the sewer atmosphere, and to which degree such simulations have potential for further improving odor and sulfide management. The campaign covered measurement of wastewater sulfide by grab sampling and diurnal sampling, and H2S gas in the sewer atmosphere was logged. The tested model was based on the Wastewater Aerobic/Anaerobic Transformations in Sewers (WATS) sewer process concept, which never had been calibrated to such an extensive dataset. The study showed that the model was capable of reproducing the general levels of wastewater sulfide, wastewater pH, and sewer H2S gas. It could also reproduce the general variability of these parameters, albeit with some uncertainty. It was concluded that the model could be applied for the purpose in mind.

Modeling anaerobic organic matter transformations in the wastewater phase of sewer networks.

A conceptual model to simulate transformations of organic matter and sulfur compounds in the wastewater phase of sewer networks under anaerobic conditions was developed. For calibration and validation of the model, a series of laboratory experiments on generation of readily biodegradable organic matter and sulfide under anaerobic conditions in samples of wastewater was conducted. Compared with previous studies, the proposed model includes sulfate reduction in the bulk water as well as a revised description of fermentation processes. Substrate affinity for fermentative bacteria was found to be significantly higher than what has been proposed in the activated sludge model, resulting in higher fermentation rates at low substrate concentrations. The findings of the study are important to understand formation of volatile organic compounds and odorous substances in sewers.

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.

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.

Improved urban stormwater treatment and pollutant removal pathways in amended wet detention ponds.

Dissolved and colloidal bound pollutants are generally poorly removed from stormwater in wet detention ponds. These fractions are, however, the most bio-available, and therefore three wet detention ponds were amended with planted sand filters, sorption filters and addition of precipitation chemicals to enhance the removal of dissolved pollutants and pollutants associated with fine particles and colloids. The three systems treated runoff from industrial, residential and combined (residential and highway) catchments and had permanent volumes of 1,990, 6,900 and 2,680 m(3), respectively. The treatment performance of the ponds for elimination of total suspended solids (TSS), total nitrogen (Tot-N), total phosphorous (Tot-P), PO(4)-P, Pb, Zn, Cd, Ni, Cr, Cu, Hg were within the range typically reported for wet detention ponds, but the concentrations of most of the pollutants were efficiently reduced by the planted sand filters at the outlets. The sorption filters contributed to further decrease the concentration of PO(4)-P from 0.04 ± 0.05 to 0.01 ± 0.01 mg L(-1) and were also efficient in removing heavy metals. Dosing of iron sulphate to enrich the bottom sediment with iron and dosing of aluminium salts to the inlet water resulted in less growth of phytoplankton, but treatment performance was not significantly affected. Heavy metals (Pb, Zn, Cd, Ni, Cr and Cu) accumulated in the sediment of the ponds. The concentrations of Zn, Ni, Cu and Pb in the roots of the wetland plants were generally correlated to the concentrations in the sediments. Among 13 plant species investigated, Rumex hydrolapathum accumulated the highest concentrations of heavy metals in the roots (Concentration Factor (CF) of 4.5 and 5.9 for Zn and Ni, respectively) and Iris pseudacorus the lowest (CF < 1). The translocation of heavy metals from roots to the aboveground tissues of plants was low. Therefore the potential transfer of heavy metals from the metal-enriched sediment to the surrounding ecosystem via plant uptake and translocation is negligible.

Anaerobic transformations of organic matter in collection systems.

Anaerobic transformations of wastewater organic matter in the bulk water phase of collection system networks were investigated in laboratory-scale experiments. The wastewater was collected from three locations, which provided samples with different characteristics, ranging from young to mature. Hydrolysis, fermentation, and sulfate reduction were identified as the most important anaerobic processes. Significant quantities of readily biodegradable substrate were produced by hydrolysis of complex organic substrates. The readily biodegradable substrate was further fermented into volatile fatty acids (VFA). The rate of fermentation was found to be limited by the hydrolysis process. The readily biodegradable substrate generated was almost entirely composed of VFA, primarily acetic and propionic acids. A production of sulfide was observed in all experiments, demonstrating that part of the readily biodegradable substrate was consumed by sulfate respiration. The sulfide production was most pronounced in mature wastewater that had previously undergone extended anaerobic transport.

Growth kinetics of hydrogen sulfide oxidizing bacteria in corroded concrete from sewers.

Hydrogen sulfide oxidation by microbes present on concrete surfaces of sewer pipes is a key process in sewer corrosion. The growth of aerobic sulfur oxidizing bacteria from corroded concrete surfaces was studied in a batch reactor. Samples of corrosion products, containing sulfur oxidizing bacteria, were suspended in aqueous solution at pH similar to that of corroded concrete. Hydrogen sulfide was supplied to the reactor to provide the source of reduced sulfur. The removal of hydrogen sulfide and oxygen was monitored. The utilization rates of both hydrogen sulfide and oxygen suggested exponential bacterial growth with median growth rates of 1.25 d(-1) and 1.33 d(-1) as determined from the utilization rates of hydrogen sulfide and oxygen, respectively. Elemental sulfur was found to be the immediate product of the hydrogen sulfide oxidation. When exponential growth had been achieved, the addition of hydrogen sulfide was terminated leading to elemental sulfur oxidation. The ratio of consumed sulfur to consumed oxygen suggested that sulfuric acid was the ultimate oxidation product. To the knowledge of the authors, this is the first study to determine the growth rate of bacteria involved in concrete corrosion with hydrogen sulfide as source of reduced sulfur.

Modeling of hydrogen sulfide oxidation in concrete corrosion products from sewer pipes.

Abiotic and biotic oxidation of hydrogen sulfide related to concrete corrosion was studied in corrosion products originating from a sewer manhole. The concrete corrosion products were suspended in an acidic solution, mimicking the conditions in the pore water of corroded concrete. The removal of hydrogen sulfide and dissolved oxygen was measured in parallel in the suspension, upon which the suspension was sterilized and the measurement repeated. The results revealed the biotic oxidation to be fast compared with the abiotic oxidation. The stoichiometry of the hydrogen sulfide oxidation was evaluated using the ratio between oxygen and hydrogen sulfide uptake. The ratio for the biotic oxidation pointed in the direction of elemental sulfur being formed as an intermediate in the oxidation of hydrogen sulfide to sulfuric acid. The experimental results were applied to suggest a hypothesis and a mathematical model describing the hydrogen sulfide oxidation pathway in a matrix of corroded concrete.

Biodegradability of organic matter associated with sewer sediments during first flush.

The high pollution load in wastewater at the beginning of a rain event is commonly known to originate from the erosion of sewer sediments due to the increased flow rate under storm weather conditions. It is essential to characterize the biodegradability of organic matter during a storm event in order to quantify the effect it can have further downstream to the receiving water via discharges from Combined Sewer Overflow (CSO). The approach is to characterize the pollutograph during first flush. The pollutograph shows the variation in COD and TSS during a first flush event. These parameters measure the quantity of organic matter present. However these parameters do not indicate detailed information on the biodegradability of the organic matter. Such detailed knowledge can be obtained by dividing the total COD into fractions with different microbial properties. To do so oxygen uptake rate (OUR) measurements on batches of wastewater have shown itself to be a versatile technique. Together with a conceptual understanding of the microbial transformation taking place, OUR measurements lead to the desired fractionation of the COD. OUR results indicated that the highest biodegradability is associated with the initial part of a storm event. The information on physical and biological processes in the sewer can be used to better manage sediment in sewers which can otherwise result in depletion of dissolved oxygen in receiving waters via discharges from CSOs.

Influence of pipe material and surfaces on sulfide related odor and corrosion in sewers.

Hydrogen sulfide oxidation on sewer pipe surfaces was investigated in a pilot scale experimental setup. The experiments were aimed at replicating conditions in a gravity sewer located immediately downstream of a force main where sulfide related concrete corrosion and odor is often observed. During the experiments, hydrogen sulfide gas was injected intermittently into the headspace of partially filled concrete and plastic (PVC and HDPE) sewer pipes in concentrations of approximately 1,000 ppm(v). Between each injection, the hydrogen sulfide concentration was monitored while it decreased because of adsorption and subsequent oxidation on the pipe surfaces. The experiments showed that the rate of hydrogen sulfide oxidation was approximately two orders of magnitude faster on the concrete pipe surfaces than on the plastic pipe surfaces. Removal of the layer of reaction (corrosion) products from the concrete pipes was found to reduce the rate of hydrogen sulfide oxidation significantly. However, the rate of sulfide oxidation was restored to its background level within 10-20 days. A similar treatment had no observable effect on hydrogen sulfide removal in the plastic pipe reactors. The experimental results were used to model hydrogen sulfide oxidation under field conditions. This showed that the gas-phase hydrogen sulfide concentration in concrete sewers would typically amount to a few percent of the equilibrium concentration calculated from Henry's law. In the plastic pipe sewers, significantly higher concentrations were predicted because of the slower adsorption and oxidation kinetics on such surfaces.

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.

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.

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.

Corrosion of concrete sewers--the kinetics of hydrogen sulfide oxidation.

Hydrogen sulfide absorption and oxidation by corroding concrete surfaces was quantified in a test rig consisting of 6 concrete pipes operated under sewer conditions. The test rig was placed in an underground sewer monitoring station with access to fresh wastewater. Hydrogen sulfide gas was injected into the pipe every 2nd hour to peak concentrations around 1000 ppm. After some months of operation, the hydrogen sulfide became rapidly oxidized by the corroding concrete surfaces. At hydrogen sulfide concentrations of 1000 ppm, oxidation rates as high as 1 mg S m(-2) s(-1) were observed. The oxidation process followed simple nth order kinetics with a process order of 0.45-0.75. Extrapolating the results to gravity sewer systems showed that hydrogen sulfide oxidation by corroding concrete is a fast process compared to the release of hydrogen sulfide from the bulk water, resulting in low gas concentrations compared with equilibrium. Balancing hydrogen sulfide release with hydrogen sulfide oxidation at steady state conditions demonstrated that significant corrosion rates--several millimeters of concrete per year--can potentially occur at hydrogen sulfide gas phase concentrations well below 5-10 ppm. The results obtained in the study advances the knowledge on prediction of sewer concrete corrosion and the extent of odor problems.

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.

Kinetics and stoichiometry of sulfide oxidation by sewer biofilms.

Oxidation of sulfide under aerobic conditions by biofilms grown on municipal wastewater in 6 identical pipe reactors was investigated. The biofilms were grown at pH 7.6 and temperatures of 20 and 25 degrees C under aerobic-anaerobic transient conditions with pulse dosing of sulfide in the bulk water. The pulse dosing of sulfide served to simulate conditions in a gravity sewer located downstream of a pressure main. During growth of the biofilms, sulfide was pulse dosed in concentrations of 0, 0.5, 2.0 and 5.0 g Sm(-3) with a frequency of 1h(-1). Based on a series of batch experiments, kinetics and stoichiometry of sulfide oxidation by the sewer biofilms was investigated and a rate equation and a stoichiometric constant proposed. Sulfide oxidation kinetics was significantly faster for biofilms grown at sulfide loadings of 0.5, 2.0 and 5.0 g Sm(-3)h(-1) than for biofilms grown in the absence of sulfide. However, the kinetics of sulfide oxidation was relatively constant for biofilms grown at sulfide loadings above 0.5 g Sm(-3)h(-1). Mass balance calculations of dissolved oxygen and sulfur compounds suggested the oxidation product to be elemental sulfur. Further oxidation of elemental sulfur could not be documented.