Air injection into force mains has been used to control sulfide generation. However, the design criteria have not been clearly established. In this study, the minimum concentration of dissolved oxygen (DO) required to prevent sulfide generation, and the oxygen balance in the force mains were investigated using an experimental facility. Air injection completely eliminated sulfide presence at the pipe outlet when DO at the pipe end was 0.2 mg/l or higher. Reaeration from gaseous phase to wastewater was affected by sewage flow velocity and oxygen concentration in the gaseous phase. Oxygen consumption rate in bulk water (Rr) ranged widely from 3 to 18 mg/l.h. Oxygen consumption rate in biofilm (Re) was measured using a rotating reactor. Re seemed to increase in proportion to the square root of DO.

Wastewater quality changes and re‐aeration during transport in an air‐injected pressure sewer (force main) were studied. Samples were taken from an air‐injected pressure sewer located in the northern part of Japan. The study focused on the transformations of wastewater organic matter as characterized by measurements of oxygen uptake rate. Wastewater was maintained under aerobic conditions by injecting air, which controlled sulfide formation and removed readily biodegradable substrate (SS) by aerobic microbial processes. To preserve SS, it was necessary to decrease dissolved oxygen concentration by reducing air‐injection flow rate. An aerobic–anaerobic wastewater process model for the air‐injected pressure sewer was established, and model parameters were determined based on experimental findings. It was possible to simulate transformations of wastewater organic matter, dissolved oxygen, and sulfide. The model can be applied to determine an appropriate air‐injection flow rate to control sulfide formation and reduce SS removal.

Experimental facilities comprising 1.8 km of pipeline, 100 mm in diameter and pumping equipment, were installed in a wastewater treatment plant and operated continuously for more than one year to clarify the main factors governing hydrogen sulfide generation in pressure mains. The effects of temperature, organic matter, and sulfate on sulfide generation rate were investigated based on observed values. The sulfide generation rate depended significantly on wastewater temperature. It was confirmed not empirically but experimentally that the effect of temperature (T) was expressed by (1.065)T-20. In respect of organic matter, it is considered that there is a little effect of organic matter concentration on sulfide generation rate when the fluctuation of soluble organic matter concentration is slight. However, based on observed values, it was found that sulfide generation rate clearly depended on sulfate concentration when the biofilm was rather thick like these experiments. Also, partial penetration of sulfate into biofilm was confirmed using a biofilm model. Furthermore, biofilm model as a sound method for predicting sulfide generation rate was discussed.

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