Agricultural crop residues (for example, straw or stalks), cereal wastes of food production or fallen leaves with high C:N ratio can be added as co-substrates. Alternatively, carbon sources such as fat waste or glycerol in small amounts can be added. The most easy way is to add glucose or other carbohydrate but it depends on the purpose of the study. Some references:

https://openagriculturejournal.com/VOLUME/14/PAGE/219/FULLTEXT/

https://www.sciencedirect.com/science/article/pii/B9780081025284000031

Article Co-digestion of sewage sludge with glycerol to boost biogas production

Article Anaerobic digestion of crude glycerol: a review

You might include Fecal sludge to improve the C: N ratio for your co-digestion. You might find the attached article interesting for improving the biogas production rate.

Addition of food waste increase the C: N ratio significantly due to the its complex composition.