Explosion in a waste water tank

This incident occurred at a waste water tank of an unspecified chemical plant. An explosive atmosphere formed inside the wastewater tank due to the accidental production of hydrogen. This occurred either during wastewater treatment or due to the reaction of excess sodium borohydride from the remaining wastewater in the tank with additional wastewater added to the tank later.
The hydrogen-air explosive atmosphere was forced through an overflow pipe and through a water seal and released into the surrounding area, where it was ignited by manual work. Subsequently, a flashback occurred through the overflow pipe into the wastewater tank, resulting in an explosion which destroyed the tank.

The INITIATING CAUSE was an accidental formation of hydrogen by a chemical reaction.
The ROOT CAUSE was the lack of effective monitoring and control of the process values, specifically the lack of pH adjustment during filling of the wastewater tank. This hint at shortcoming of the operative procedures, of risk assessment and/or of proper training of operators.

water seal,
oveflow pipe, sodium borohydride tank

The tank and othe plant componets were destroyed by the explosion and subsequent fire.

As preventive measure, the wastewater treatment process was modified.
As control measures, (i) a HAZOP analysis of wastewater treatment was implemented and wastewater flows were monitored.