you'd have to liquify it using a cold condensor- and it would have to be very cold to liquify methane and other hydrocarbons (around -280f). probably a multi-stage condensation to remove unwanted impurities or perhaps a fractional distillation, since it would contain hydrogen sulfide and other nasty stuff you don't want.
overall not sure how viable this idea is since you'd have to process a large volume of sewer vapor to get any usable quantity of gas.
i would just feed it into a furnace then. that way the energy is taken from the sewer gas and the hazardous chemicals can be incinerated with the help of a common catyalitic converter found on many home wood stoves.
methane is like hydrogen a little bit and maybe a adsorbson tank of magnesium hydride pellets could adsorb the methane while not haveing preference for the hydrogen sulfide and from a series of fill and purge thetanks can seperate the methane.
doped zeolites like activated carbon also might work but the production of such a zeolite would have to be in-house because a system would go through it so fast.
another idea is chemically enhanced sparging equipment but what chemicals to use idk
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