Oxidation and Precipitation of Sulfide in Sewer Networks

Whenever wastewater is transported in sewer networks, it is likely that anaerobic conditions in the wastewater arise as a result of physical, microbial and chemical reactions. Anaerobic conditions in wastewater of sewer networks are often associated with a number of problems like malodors, health risks and corrosion of concrete and metals. Most of the problems relate to the buildup of hydrogen sulfide in the atmosphere of sewer networks. In this respect, the processes of the sulfur cycle are of fundamental importance in ultimately determining the extent of such problems. This study focused on oxidation and precipitation of sulfide, which are considered important processes in the sulfur cycle in wastewater and biofilms of sewer networks. Based on experimental studies, it was the objective to establish kinetics and stoichiometry of oxidation and precipitation of sulfide and to integrate the processes in an already existing sewer process model, thereby improving its capabilities for prediction of sulfide buildup in wastewater and atmosphere of sewer networks. Accordingly, efforts were made to develop experimental procedures for estimation of model parameters. Sulfide oxidation in both wastewater and biofilms of sewer networks was studied in detail with emphasis on determination of process kinetics and stoichiometry. In the water phase, sulfide oxidation may be both chemical and biological and the investigations showed that both processes were of significant importance in the sulfur cycle. In addition, it was found that aerobic sewer biofilms contributed significantly to the oxidation of sulfide present in the water phase. The oxygen uptake associated with sulfide oxidation was found to be significant and potentially even more important than the oxygen uptake resulting from heterotrophic carbon transformations. The experiments indicated that biological sulfide oxidation in the water phase and sulfide oxidation by sewer biofilms produce elemental sulfur under the conditions investigated. The stoichiometry of chemical sulfide oxidation was apparently more complex producing both thiosulfate and sulfate. The effect of temperature on oxidation kinetics was described by the widely used Arrhenius equation. Rates of chemical and biological sulfide oxidation in the wastewater were found to double with temperature increases of 10 and 7C, respectively. The biofilm experiments indicated a smaller dependency on temperature in that the biofilm sulfide oxidation rate was found to double with a temperature increase of approximately 23C. The pH dependency of chemical sulfide oxidation in wastewater represented the dissociation of sulfide, with the hydrosulfide ion being more rapidly oxidized than molecular hydrogen sulfide. Thus, the rate of chemical sulfide oxidation is significantly faster in slight alkaline wastewater than in acidic wastewater. Biological sulfide oxidation was fastest at in situ wastewater pH and remained relatively stable with moderate fluctuations in pH. Interactions between metals and sulfide were studied in both wastewater and biofilms. Particular emphasis was on the importance of iron in the sulfur cycle. Iron is typically among the dominant metals in wastewater. The experiments showed that, ferric iron (Fe(III)) that was added to anaerobic wastewater was rapidly reduced to ferrous iron (Fe(II)) and precipitated subsequently with dissolved sulfide as ferrous sulfide (FeS). The ferrous sulfide precipitation was relatively fast, but not immediate. Despite the very low solubility of ferrous sulfide, initially present iron did not react completely with sulfide. This observation was probably explained by the presence of ligands in the wastewater, which reacted with the iron. The biofilm experiments showed that sulfide accumulated along with several metals in anaerobic biofilms as the result of metal sulfide precipitation. Particularly, zinc and cupper were important for the accumulation of metal sulfides in the biofilms. This was the case even when the iron concentration in the wastewater was increased approximately ten times compared to the in situ concentration. In aerobic biofilms, iron precipitation was apparently controlled by phosphate. Based on the experimental studies, a model for prediction of sulfide buildup in wastewater and atmosphere of sewer networks was developed as an extension to an already existing sewer process model, the WATS model (Wastewater Aerobic/anaerobic Transformations in Sewers). The WATS model has been developed by Hvitved-Jacobsen and co-workers at Aalborg University for more than a decade. In the basic version, the WATS model simulates changes in dissolved oxygen (DO) and organic fractions of different biodegradability under both aerobic and anaerobic conditions. Evaluation of the model concept has demonstrated that it can be successfully calibrated and validated against field data. In the extension to the WATS model, sulfur transformations were described by six processes: 1. Sulfide production taking place in the biofilm and sediments 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 from the wastewater into the sewer atmosphere and 6. adsorption and oxidation of hydrogen sulfide on the moist sewer walls exposed to the sewer atmosphere, potentially resulting in concrete corrosion. The extended WATS model represents a major improvement over previously developed models for prediction of sulfide buildup in sewer networks. Compared to such models, the major processes governing sulfide buildup in sewer networks are integrated at a more detailed level in the extended WATS model. This allows effects of pH, temperature and hydraulic conditions on the individual processes to be accounted for. For several of the processes, model parameters were found to be highly site specific. A sound and reliable use of complex models like the extended WATS model should therefore include a careful estimation of model parameters. This can be done by adapting literature values or preferably by conducting specifically designed experiments. Experimental procedures for parameter estimation have been developed for all the major processes of the sulfur cycle in the water phase and biofilm, which make it possible to adapt the model to local conditions.