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This study is related to distribution temperature sensing (DTS) in sewers for tracing illicit or unintended inflows to foul sewers. A DTS measurement is performed with a fiber optic cable that is installed at the invert of a sewer pipe in combination with a standalone laser/computer instrument. This set-up generates in-sewer temperature measurements with high resolutions in time (every minute) and space (every metre) along the cable over long periods of time (weeks on end). The prolonged monitoring period in combination with the high level of detail in the dataset allows the study of anomalies (i.e., unexpected temperatures and/or temperature variations at certain locations), even if these only occur very infrequently. The objective of this paper is to develop an automated tool to analyze the large data masses and identify anomalies caused by illicit or unintended inflows. In this study, an algorithm for detecting the temperature changes that are caused by both wastewater discharge and inflow of stormwater are developed. A comparison of the results of the automated procedure to the results of a manual assessment of the datasets (Elmehaven, Denmark) shows that the automated procedure performs very well.

Traditionally, sulfide abatement has been done by adding e.g. ferrous or ferric iron salts to the start of sewer force mains. Iron dosage must hence correspond to an estimate of how much sulfide will form while the wastewater stays in the main, which is not straightforward. Adding iron salts at the end of the main has the advantage that the exact amount of sulfide to precipitate, in principle, can be known. A drawback is that the reaction time is short compared to start-of-pipe treatment. Sulfide precipitation rates and the concentration of sulfide left after the process had run to completion were measured using an online hydrogen sulfide probe to resolve the fast precipitation reaction. Experiments were conducted in anaerobic wastewaters spiked with sulfide, and carried out under different pH conditions and with various iron-to-sulfide ratios. Sulfide precipitation rates were demonstrated to be faster with Fe(III) than with Fe(II). Experiments furthermore showed that for Fe(III), pH was the controlling parameter, whereas for Fe(II) both pH and the iron-to-sulfide ratio were important. Proposed are model equations to predict variables for the rate equations, which can be adopted by practitioners dealing with sulfide abatement.

Addition of ferrous and ferric iron salts to wastewater is a commonly used practice for sulfide abatement in sewer force mains. When iron is added to wastewater where sulfate respiration takes place, it produces ferrous sulfide precipitates with the formed sulfide. The effect of iron addition has traditionally been focused on solely from the perspective of reaction stoichiometry. Possible influences on the microbial communities in biofilms growing in force mains have largely been neglected. In this study the activity and microbiome was examined in three pilot scale force mains conveying real wastewater, two subjected to iron treatment and one operated as an untreated control. Activity was measured on suspended biofilm samples extracted from the experimental setup. The microbiome of the biofilm was analyzed by V3 + V4 16S rDNA sequencing. Correlation analysis of chemical composition of the biofilms and activity measurements for operational taxonomic units of relevance to sulfide and methane production were performed. In conclusion, it was found that both ferrous and ferric treatment reduced sulfate reduction and methane production, and that both iron salts induced significant changes to force main biofilm microbiomes.

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.

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.

Laboratory experiments were conducted to investigate the fate of the antimicrobial agent triclosan (TCS) in a conventional septic tank. The main mechanism of TCS removal from wastewater was identified to be rapid TCS sorption to suspended particles followed by settling of these particles to the bottom of the septic tank. Sorption to particles was completed within minutes while the settling took several days. Therefore, in a septic tank the removal of TCS from wastewater is mainly determined by the removal of suspended particles by sedimentation. Over 5 days of hydraulic residence time the initial dissolved TCS concentration of 100 μg L(-1) was reduced by 87 ± 8%. During the first 24 hours, 66-86% of all removed TCS was retained, whereas during the remainder of the experiment a slight but steady decrease in TCS concentration was observed. This was most likely caused by TCS diffusion and its subsequent sorption onto the septic sludge.

Laboratory experiments were conducted to investigate the kinetics of aerobic chemical and biological oxidation of selected odorous volatile sulfur compounds (VSCs) by wastewater and biofilm from sewers. The VSCs included methyl mercaptan (MeSH), ethyl mercaptan (EtSH), dimethyl sulfide (DMS) and total inorganic sulfide, which have all been reported as the main constituents of foul sewer gas. Samples of wastewater and biofilm for the experiments were obtained from two locations that differed significantly with respect to the occurrence of VSCs. One location represented an odor hot-spot downstream of a force main and the other was a gravity sewer transporting young aerobic wastewater. The kinetics of VSC oxidation for both wastewater and suspended biofilm samples followed a first-order rate equation. The average values of the reaction rate constants demonstrated the following order of reactivity: total inorganic sulfide > EtSH ≥ MeSH >> DMS. Except for total inorganic sulfide oxidation in wastewater, kinetic parameters for each VSC were of similar magnitude for the two locations. In the wastewater from the odor hot-spot, sulfide inorganic oxidation rates were approximately 12 times faster than in the aerobic wastewater.

A method for on-line measurement of the organic carbon oxidation level (OXC) during aerobic heterotrophic respiration in domestic wastewater was developed and tested. The method is based on batch incubation of sewer wastewater in an intermittently aerated respirometric reactor. Between aeration cycles, measured pH, dissolved oxygen (DO) and dissolved carbon dioxide (CO2) were used to calculate electron flow accepted by DO and the resulting production of dissolved inorganic carbon (DIC). The CO2 production was measured using a novel fiber-optic sensor based on luminescence quenching. The method was tested on domestic wastewater with a relatively high pH and alkalinity. From the DO and DIC measurements, it was possible to evaluate substrate oxidation levels with a temporal resolution of less than an hour. Addition of organic substrates during the experiments confirmed the method's applicability. The substrates tested included ethanol (OXC = -2), glucose (OXC = 0) and oxalic acid (OXC = 3).

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.

Concentrations of 6 different heavy metals and total polycyclic aromatic hydrocarbons (PAHs) were determined in stormwater runoff and in the pond water of two Danish wet detention ponds. The pond water samples were analyzed for toxic effects, using the algae Selenastrum capricornutum as a test organism. Stormwater and pond water from a catchment with light industry showed high levels of heavy metals, especially zinc and copper. The pond water showed high toxic effects and copper were found to be the main toxicant. Additionally, a large part of the copper was suspected to be complex bound, reducing the potential toxicity of the metal. Another catchment (residential) produced stormwater and pond water with moderate concentration of heavy metals. The pond water occasionally showed toxic effects but no correlation between heavy metals and toxicity was identified. PAHs concentrations were for both catchments low and no correlations between PAH concentrations in the pond and toxicity were found.

The collection system of a major city at the Persian Gulf was simulated for bulk water hydrogen sulfide and the release of sewer gas to the urban atmosphere. Geometry data on 870 km of sanitary sewer and data on dry weather flow entering all nodes in the catchment was exported from a Mike Urban database and imported to the sewer process model WATS. The process model then routed sewage and sewer gas through the system and simulated relevant physical, chemical and biological processes. In its non-calibrated state, the model was used as a planning tool to identify problem areas and to identify locations to install monitoring equipment and make preliminary choices for control strategies in terms of dosing of nitrate and iron salts. The monitoring equipment consisted of flow meters, level gauges, UV-Vis spectroscopes, and H2S gas sensors. Data from the first set of installed monitoring equipment were applied to calibrate and validate the model. It was illustrated how the calibrated model can be applied to assess compliance with quantitative formulated service levels and to design control strategies in terms of dosing of iron and nitrate salts.

The startup of a wet retention pond designed for extended stormwater treatment was monitored by more than one year of continual measurement of hydraulic parameters, nutrients and quality parameters in the pond itself (pH, temperature, dissolved oxygen, turbidity). The data revealed that photosynthesis played an important role for dissolved oxygen and pH for most of the year. Another important observation was that the pond behaved more like a completely mixed reactor than like a plug flow reactor--even though the length to width ratio was as high as 4.5:1. The pond was equipped with sand filters and sorption filters whereby very good nutrient removal efficiencies were achieved.

The activity of hydrogen sulfide oxidizing bacteria within corroded concrete from a sewer manhole was investigated. The bacteria were exposed to hydrogen sulfide starvation for up till 18 months, upon which their hydrogen sulfide oxidizing activity was measured. It was tested whether the observed reduction in biological activity was caused by a biological lag phase or by decay of the bacteria. The results showed that the bacterial activity declined with approximately 40% pr. month during the first two months of hydrogen sulfide starvation. After 2-3 months of starvation, the activity stabilized. Even after 6 months of starvation, exposure to hydrogen sulfide for 6 hours a day on three successive days could restore the bacteriological activity to about 80% of the initial activity. After 12 months of starvation, the activity could, however, not be restored, and after 18 months the biological activity approached zero. The long-term survival aspect of concrete corroding bacteria has implications for predicting hydrogen sulfide corrosion in sewer systems subject to irregular hydrogen sulfide loadings, e.g. as they occur in temperate climates where hydrogen sulfide often is a summer-problem only.

Sulfide buildup in sewer networks is associated with several problems, including health impacts, corrosion of sewer structures and odor nuisance. In recent years, significant advances in the knowledge of the major processes governing sulfide buildup in sewer networks have been made. This paper summarizes this newly obtained knowledge and emphasizes important implications of the findings. Model simulations of the in-sewer processes important for the sulfur cycle showed that sulfide oxidation in the wetted biofilm is typically the most important sink for dissolved sulfide in gravity sewers. However, sulfide emission and thereby potential hydrogen sulfide buildup in the sewer atmosphere is of particular importance in sewers constructed with large diameter pipes, in sewers constructed with steep slopes and in sewers conveying low pH wastewater. Precipitation of metal sulfides is only important when the sulfide concentration in the wastewater is low; i.e. less than 1 g Sm(-3).

Transformations of organic matter, nitrogen and sulfur in sewers can be simulated taking into account the relevant transformation and transport processes. One objective of such simulation is the assessment and management of hydrogen sulfide formation and corrosion. Sulfide is formed in the biofilms and sediments of the water phase, but corrosion occurs on the moist surfaces of the sewer gas phase. Consequently, both phases and the transport of volatile substances between these phases must be included. Furthermore, wastewater composition and transformations in sewers are complex and subject to high, natural variability. This paper presents the latest developments of the WATS model concept, allowing integrated aerobic, anoxic and anaerobic simulation of the water phase and of gas phase processes. The resulting model is complex and with high parameter variability. An example applying stochastic modeling shows how this complexity and variability can be taken into account.

A model concept for prediction of sulfide buildup in sewer networks is presented. The model concept is an extension to--and a further development of--the WATS model (Wastewater Aerobic-anaerobic Transformations in Sewers), which has been developed by Hvitved-Jacobsen and co-workers at Aalborg University. In addition to the sulfur cycle, the WATS model simulates changes in dissolved oxygen and carbon fractions of different biodegradability. The sulfur cycle was introduced via six processes: 1. sulfide production taking place in the biofilm covering the permanently wetted sewer walls; 2. biological sulfide oxidation in the permanently wetted biofilm; 3. chemical and biological sulfide oxidation in the water phase; 4. sulfide precipitation with metals present in the wastewater; 5. emission of hydrogen sulfide to the sewer atmosphere and 6. adsorption and oxidation of hydrogen sulfide on the moist sewer walls where concrete corrosion may take place.

In this study, the kinetics and stoichiometry of chemical sulfide oxidation of wastewater from sewer networks were investigated. Based on experiments, it was shown that the stoichiometry could be considered identical for wastewater from two sampling sites. However, the kinetics differed significantly among the wastewaters from the two sites. Effects of pH and temperature were investigated in the pH and temperature ranges 5-9 and 5-25 degrees C, respectively. The rate of chemical sulfide oxidation could be related to the dissociation of H2S to HS-, with HS- being oxidized at a higher rate than H2S. The temperature dependency of the chemical sulfide oxidation rate was described using an Arrhenius relationship. The oxidation rate was found to double with a temperature increase of 12 degrees C. The stoichiometry of the chemical oxidation was not significantly affected by varying pH and temperature. Based on the experiments, a general rate equation, including a stoichiometric coefficient describing chemical sulfide oxidation in wastewater was proposed, enabling the process to be incorporated into sewer process models that can predict odor and corrosion problems.