Ventilation of Enclosed Primary Clarifiers 1

[dsimms]

Our sewage treatment plant has primary clarifiers enclosed in a building. We plan to upgrade the ventilation and install an odour control system. According to the NFPA 820 standard we should be ventilating the building at 12 air changes per hour. This would mean a huge volume of air going to odour treatment.

Has anyone had experience with a similar situation? Any thoughts on how to reduce the air volume and/or demonstrate safety without meeting the NFPA 820 standard?

 

[aussiemike]

I have worked on a couple of projects where ventilation of primary clarifiers was required.

There are a couple of low cover designs that I am aware of. One consists of an HDPE sheet with suitable supports to cover the clarifier and scraper mechanisms. The other consists of pie shaped fibreglass sections supported by radial guide bars across the clarifer. Both of these are reasonably expensive, but should pay themselves off quickly compared to the size of scrubber required for a building.

In both my projects, the clients required buildings to be constructed over the clarifiers in preference to low level covers. The clients were prepared to trade off the high air extraction rates you describe for ease of maintenance.

The main problems with the low level covers perceived by our clients were:
1. Difficulty of access to the clarifers for observation and regular maintenance; and
2. The need to replace the rotating perimeter drive bridges with fixed bridge centre drives. To my knowledge, the low level covers are only suitable for centre drive clarifiers as the covers interfere with the scraper on a perimeter drive. If your clarifiers are large, I suspect they will be perimeter drive and may require significant modification to allow the covers to be installed.

The 12 changes per hour sounds about right for personnel entry, however the scrubber required for low level covers will be very much smaller. The scrubbed volume would be much less than a building, and you could reduce the extraction rate to 4 changes per hour as nobody will go in on a regular basis.

Another option is to cover the effluent weirs only as I suspect this is where most of the odour will be evolved.

I suggest you do a bit of a search on the internet for suppliers of the clarifier covers in your area.

There is a company here in Australia called Geomembrane Technology that makes the plastic covers and I think some of the mining equipment suppliers (eg.Outokumpu) make the fibreglass covers. One plant I know of that has a number of the fibreglass covers is the Ulu Pandan WWTP in Singapore but I don't know where they got their covers.

Good luck.

Aussie Mike

 

[dfman]

Hi there

Just read your questions re clarifier covers. This question is always a difficult one to answer. Different people will give you different answers depending on their experience.

Aussie Mike's comments pretty much sum up the situation in Australia. A "cover and treat" approach where the clarifier & other odorous point sources are covered & ducted to suitable process treatment such as BioTrickling Filters or BioFilters is generally far more cost-effective & provides a safer approach.

Your design on extraction of the covered space is probably 5 to 10 changes per hour. Overall the cost will be much less than treating building air directly.

I personally don't like the idea of extracting air directly from the building as potentially toxic & very wmelly air is pulled right past people accessing the area. Also from an odour control technology point of view you are going to end using centrifugal fans that cost a bomb to run (100kW per 50,000 m3/h is not unusual!)

With the odorous point sources covered & treated you can then vent the building at 15 to 20 changes per hour using axial fans directly to atmosphere, & probably not exceed boundary odour limits with dispersion.

However if you need to treat this air in-duct or stack odour treatment technologies such as Advanced Oxidation can be used to polish these discharges as needed.

If you would like more information please contact Clean TeQ in Australia

Cheers
ctairman
info@cleanteq.com