Methanogenesis requires low oxidation reduction potential (ORP) -200 to -300 which is difficult to maintain in a biogas reactor working on kitchen waste treatment. By adding any ORP reducing agent one can reduce the ORP..
Hydrogen sulfide can be an effective reducing agent as long as your pH is not too acidic and your metanogens can tolerate an anaerobic environment. Another option would be the introduction of reactive metal particles, such as iron. Aluminum or magnesium particles might also be suitable as long as the pH of your system is not neutral. However, once again the issue would be the impact that the metallic reaction products might have upon the bacteria.
Typically, there is no need to add reductants to an anaerobic digestor, since in the absence of oxygen - assuming the system is gas-tight - the corresponding microbial consortium will take over sooner or later. To this end it may be recommendable to inoculate the reactor, e.g. with a small amount of municipal sewage sludge. It also may be reocommendable to follow the evolution of organic acids, since excess acids may hamper methanogenisis.
Sodium sulphide (Na2S) is sometimes used to lower the ORP, but be careful about safety issues (H2S emission at pH below 7).
Sulfides will typically inhibit your methanogens. So it might be better just to give time to the system- the oxygen will be scavenged by a few aerobs in your reactors and you will have low redox potentials.
You totally don't need to worry about removing the excess oxygen in a reactor, as Harald mentioned. Since anaerobic digestion goes through three stages - hydrolysis, acetogenesis/acidogenesis and methanogenesis - hydrolytic phase is ruled by facultative anaerobes, which consume all the oxygen during their metabolic activity. Therefore, hydrolytic bacteria will take care of methanogenic bacteria on a long term basis.
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