Detention time is the length of time that sewage remains in the gravity or rising mains or in pump station wet wells before it reaches the wastewater treatment plant. The longer it is detained in the system, the greater the likelihood that conditions arise that are conducive to the development of odors due to hydrogen sulfide production. Apart from producing malodorous gasess, the sewage also becomes more difficult to treat once it reaches the sewage treatment plant. Consquently, the goal is to keep the time in which raw sewage remains in the system to a minimum.
Conditions that promote the generation of hydrogen sulfide are reduced dissolved oxygen which in turn increases the activity of sulfur reducing bacteria, disulfovibrio.
Detention time can be reduced at the design stage by ensuring that appropriately sized rising mains are utilized in the system to effectively handle the expected flows. However, if detention time increases as a result of unexpected demand, odour generation can be temporarily mitigated using one or more of the following methods:
- Lime dosing of the pump station wet well to deal with excessive inflows
- Reduced wet well inflows resulting in odours due to increased detention time can be managed by adding water to the wet well via a hose until inflows increase
- Increased detention time in rising mains and consequent odour generation may be countered by oxygen or air injection to keep the sewage in an aerobic state. Alternatively, the sewage may be kept in an anoxic state by introducing nitrates into the rising main.
- The development of H2S gas may be prevented by addition of iron sulfates which will bind to any sulfides dissolved in the sewage.
- Slime development on pipe walls , which provide an environment conducive to hydrogen sulfide producing bacteria, can be controlled by injection of enzymes.