Wastewater pumpstation ventilation/odour control

[swazimatt]

The local wastewater pumpstation code has the following requirements for ventilation and odour control for WWPS:
The following summarises the air flow rates for different purposes.

Fresh air inlet (continuous): Six air changes per hour of the volume above first pump start level
Sewer air exhaust : At least sufficient to produce an air velocity of 0.2 m/sec through the equivalent of 75% of the cross sectional area of the incoming sewer.
Storage tank : Two changes of air per hour

Total exhaust (continuous):
The greater of:
 Sewer and storage tank exhaust plus fresh air inlet
 12 changes per hour of the wet well

I was wondering if anyone had information about where this information may have originated? I understand that part of it relates to the flow of the wastewater in the pipe creating an associated air flow, but the resulting numbers appear quite large. I would imagine that the real air change at the WWPS would be equivalent to the peak wet weather flow in the sewer as this is how quickly the wet well will fill.
using the above option i end up with a flow rate of 107 l/s and the PWWF is only 27 l/s
I understand that using PWWF i am assuming that it is a sealed system. By including an exhaust to a carbon filter i am allowing an air flow so the water flow may come into play

I also want to know if this can be ventilated passively or needs to be forced, is there a way to calculate the passive flow

 

[bimr]

Don't know where those specific numbers came from but in the states, the recommended numbers come from NFPA 820.

NFPA 820

 

[FacEngrPE]

NFPA Standard for Fire Protection in Wastewater Treatment and Collection Facilities can be viewed at no charge on NFPA's site.
Your work-to specification is a bit more detailed than NFPA 820, but the general idea is that if there is a risk of sewage fermentation, then measures must be applied to eliminate risk of igniting sewer gas.
Depending on the amount of fermentation you may also need to eliminate a risk of worker H[sub]2[/sub]S exposure. OSHA Hydrogen sulfide

 

[swazimatt]

Thanks the NFPA refers to 12 air changes per hour as well so similar lines. The spec i posted is for New Zealand.

 

[sjohns4]

I highly doubt you'll be able to passively vent the wet well. Any time I've seen an installation where we had to provide that kind of air flow there was some forced induction. Besides, it's the really low dry weather flow when everything is stagnant that's really causing the gas problem and resulting need for ventilation.
I used a blower /carbon filter tank skid from Evoqua for several projects where ventilation and odor control were required. Prety simple setup. I'm sure there are multiple manufacturer's out there making similar equipment.