hi,
nobody has answered this thread so I will give it a go......

- What is the sludge exactly? i.e. waste that creates methane?

- What is the exact purpose of the exhaust? life safety?

- If the tank is "sealed" with only extract point(s), little if anything would be accomplished because the extract fan would be attempting to create a vacuum.

A little more detail info. might help.

cheers,

Thanks for your reply i will add the additional info, the sludge is sewage water, it containsamonst others hydrogen sulphide which has a strong smell. that the nearby population complain about. the purpose of the extraqction is to remove the hydrogen sulphide(h2s) and chemically treat this. the air into the tank is from leakage at the tank lid it is notr prefectly sealed and other small opening. i feel that all we are doing is to locally remove the air from around the extract point. i feel that there should be move extract point say every 15-20 m this would remove more H2s from all over the tank instead of a localised area. any ideas

a few comments:

- methane is probably present fan and control gear should be rated for the atomsphere - the exact requirements depends on the local code

- the area over which any exhaust connection exterts any influence (suction) is very small indeed. The best way t imagine this is consider the difference between blow out and a candle, and trying to suck it out.

- one exhaust point properly sized will create a negative pressure (ever so slightly below atomospheric). The exhaust air will draw fresh air in through the existing openings over the sludge surface and discharge into odour scrubber.

- consider all possibilities, high sewerage level, external flooding, etc.

some food for thought.

cheers,

-

I believe that the only way that you could survive with only one extraction point would be to assure that your air intake points were in the opposite side of your extraction, in order to avoid bypasses of the fresh air going directly to extraction.
In such a big tank I would support your idea of several extractions, nevertheless you should try to identify and control the air inlet points.
As noted above, be careful with the extraction fan motor rating ( if in the air flow) since the air mixture can be flammable.
Hope that helps.
PR