NFPA 820 Wet Well Ventilation - Dry Volume or Air Space Above Liquid?

[m2e]

There is a fairly large wet well that I will need to ventilate per NFPA 820, 12 ACH to classify it as class 1 div 2. We also need to provide odour control to the exhaust air, so that resulted in a huge odour control unit.

NFPA 820 stated 12 ACH is required, but can we get away with only calculating the air space above the liquid, when the wet well has liquid? I would assume that if the wet well is dry, there is little to none H2S production. I would also assume the H2S production is proportional to the amount of liquid (sewage) in the wet well, but the air space is inversely proportional to the amount of liquid in the wet well. So what is the best way to determine the ACH required?

 

[bimr]

VFD pumps are commonly specified in order to use a smaller wet well. With that pumping arrangement, the volume of water is normally constant. It seems reasonable to use the volume above the water level as you are proposing.

 

[swazimatt]

H2S generation is difficult to predict as it depends on what the wastewater is made up of. A pumpstation at capacity is more likely to have low odour/H2S issues than a new pumpstation designed for a development that will be completely occupied in 10 years

a wetwell will never be dry as the pumps will shut off at a certain depth, so there will be wastewater sitting in the base festering and producing H2S

Also unless your odour unit is active ie. with an extractor fan, the flow to the odour unit will be maximum as the wetwell goes from empty to full.