In my view, sewage sludge can't be used in Organic farming. As it is clear from the name "organic", which means without any inorganic fertilizers. While Sewage sludge is a mixture of organic and inorganic nutrients with numbers of pathogens, heavy metal, nanoparticles, toxic materials etc.
It may be used after some treatments and recycling methods, as a term used "Biosolids", but that only kills pathogens, Not the heavy metal and inorganic nutrients.
In my opinion "Organic farming" refers to chemicals free (synthetic/processed chemicals) farming. So, the sewage sludge of the industries (residual, semi-solid by-product material that comes from the industrial waste) may not be used, however that get from the houses or municipalities may be used after refinement (separation of toxics).
Sewage sludge can be permitted in organic farming as a source of nutrients however it should not be industrial and chemical waste. Otherwise it can not become organic. Organic means the sludge containing organic matter.
formed from living things after their decomposition/degradation..