Andre, the metabisulfite ...

Andre, the metabisulfite anion is unstable under anaerobic conditions and will be reduced to hydrogen sulfide. This will happen rather quickly - probably less than an hour.   There will be a corresponding reduction in methane formation, in proportion to the metabisulfite,  as the organic load is diverted to sulfur reduction,    Then hydrogen sulfide (H2S) will appear in the biogas and offgas streams.  This will also happen quickly.   The H2S will partition into the gas and liquid effluent at about 50/50 ratio depending on the pH of the anaerobic reactor.   To mitigate the impact of H2S you can add ferrous chloride or ferric chloride which will precipitate the H2S as ferrous sulfide (FeS.)  These iron salts are strong acids which may require additional alkalinity.   

The aerobic process which receives the anaerobic effluent will experience a heavy non-carbonaceous oxygen demand as the sulfide is oxidized to sulfate.  And this  oxidation will release acidity so there may be a pH depression. 

All of these effects are concentration-dependent.  There should not be any permanent toxicity to either anaerobic or the aerobic process.  

Kind regards,