What can be saved and what must go
Often, yes. Porous items soaked by sewage — like carpet, mattresses, upholstered furniture, and many fabrics — usually cannot be safely cleaned and should be discarded. Some hard, non-porous items can be disinfected and kept.
Because sewage carries harmful germs, items that absorb it are generally considered unsafe to keep, no matter how much you clean them. Hard surfaces and some sealed items may be safely disinfected, but soaked soft goods are usually a loss.
Before you throw anything away, photograph it with dates and keep a written list, since those losses may be recoverable if the landlord was responsible for the backup. Do not risk your health trying to salvage contaminated items. If a sewage backup destroyed your belongings because of a problem your landlord ignored, a free review can help you understand your options.
This is general information about California tenant rights, not legal advice. Every situation is different.