Assuming the wastewater we are talking about is from households, that water is typically conveyed through pipes to a wastewater treatment plant.  In many situations, the wastewater is collected from areas at elevations below the location of the treatment plant.  Those waters are collected in tanks called lift stations.  Those lift stations pump out based on the levels in the tank. If the flow into the lift stations is low, as it often is in the middle of the night, the lift stations don't pump out frequently enough. As a result the oxygen in the water is used up by aerobic (oxygen using) bacteria.   Once the water is anaerobic, with little to no oxygen, the anaerobic bacteria become more active. That is the process described previously.   I have performed many odor investigations over the years and have found that this condition is almost always the source of significant sulfur odor issues.

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Assuming the wastewater we are talking about is from households, that water is typically conveyed through pipes to a wastewater treatment plant.  In many situations, the wastewater is collected from areas at elevations below the location of the treatment plant.  Those waters are collected in tanks called lift stations.  Those lift stations pump out based on the levels in the tank. If the flow into the lift stations is low, as it often is in the middle of the night, the lift stations don't pump out frequently enough. As a result the oxygen in the water is used up by aerobic (oxygen using) bacteria.  

Once the water is anaerobic, with little to no oxygen, the anaerobic bacteria become more active. That is the process described previously.  

I have performed many odor investigations over the years and have found that this condition is almost always the source of significant sulfur odor issues.