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 during approximately 4 hours of anaerobic transport in the force main was significant. The following gravity sewer was aerobic and a fast reduction of the sulfide concentration was observed due to sulfide oxidation and emission of hydrogen sulfide gas to the sewer atmosphere. During 14 minutes of transport in the gravity sewer, the sulfide concentration decreased on average 30%. Despite the continuous emission of hydrogen sulfide, the concentration in the atmosphere of the gravity sewer was generally below 3% of the concentration in equilibrium with the sulfide concentration in the wastewater. This observation demonstrated that removal of hydrogen sulfide gas by adsorption onto the sewer walls exposed to the sewer atmosphere was fast compared with emission from the wastewater. An application of a conceptual sewer process model for simulating the formation and fate of sulfide in wastewater collection systems was demonstrated. The model was able to reproduce the average conditions in the investigated intercepting sewer satisfactory, but could not account for the observed variability. Overall, the model predicted that approximately 90% of the decrease of the sulfide concentration in the gravity sewer was due to 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 per year in the gravity sewer section.
Sulfide related problems are a major concern when designing and operating wastewater collection systems. The major problems related to buildup of sulfide in sewers include corrosion of sewer structures, odor nuisance and health impacts on sewer workers (U.S. EPA., 1974). The corrosion of sewer structures result from adsorption of hydrogen sulfide gas onto the wetted surfaces exposed to the sewer atmosphere and subsequent oxidation of the hydrogen sulfide to sulfuric acid. The sulfuric acid reacts readily with the alkaline components of the concrete structures such as sewer pipes and manholes. The odor nuisance result from emission of foul gases into the urban atmosphere including hydrogen sulfide gas as well as other low molecular and volatile organic compounds (VOC’s) produced under anaerobic conditions. The health impacts relate to the buildup of toxic/lethal levels of hydrogen sulfide gas in the sewer atmosphere.
The formation and fate of sulfide in wastewater collection systems is governed by a large number of complex and interrelated processes (Pomeroy and Parkhurst, 1977). Sulfide is produced by anaerobic heterotrophic degradation of organic matter with sulfate as terminal electron acceptor. This process mainly takes place in the biofilm and sediments covering the submerged sewer walls. From here, the sulfide diffuses towards areas of lower sulfide concentration; i.e., the outer layers of the biofilm/sediments and the wastewater stream. In case dissolved oxygen (DO) is present, the sulfide is readily oxidized by chemical or biological processes. However, if anaerobic conditions prevail, the sulfide will enter the wastewater stream. Sulfide present in the flowing wastewater may then either be oxidized, react with metals to produce insoluble metal sulfides, or be emitted as hydrogen sulfide gas to the overlaying sewer atmosphere.
Models are much needed that can be applied for design and management of wastewater collection systems. The models must allow prediction of sulfide and the consequent problems, and must be applicable as decision support tools for sulfide control practices. A number of empirical models have been developed for these purposes. However, these empirical models have serious limitations in terms of applicability. A more widely applicable approach for sulfide prediction in both gravity sewers and force mains relies on a conceptual description of the sulfur cycle in terms of a sewer process model for sulfide build-up. An example of such a conceptual sewer process model is the WATS model (Wastewater Aerobic-anaerobic Transformations in Sewers) developed at Aalborg University, Denmark (Hvitved-Jacobsen, 2002).
The WATS model has successfully been calibrated and validated for simulation of the in-sewer carbon cycle against field measurements (e.g., Almeida et al., 2000; Vollertsen et al., 2005). Similarly, model simulations of sulfide buildup in force mains have also proven feasible (e.g., Nielsen et al., 1998). However, a thorough validation of the sulfur cycle transformations in gravity sewers has not yet been performed. The objective of this study was accordingly:
- To establish data on the formation of sulfide in a force main and the fate of sulfide in a downstream-located aerobic gravity sewer.
- To validate the applicability of the WATS model for simulating the formation and fate of sulfide in sewers based on the results of the field study.