Only part of the carpet touched sewage. Can it simply be cleaned instead of replaced?

Do not judge sewage contamination by the visible patch or whether an odor disappears. EPA recommends a professional experienced in contaminated-water work. Ask them to assess the carpet, backing, padding and underlying materials and explain what can actually be cleaned and dried. Porous contaminated materials may require removal. A spray or routine carpet cleaning alone does not demonstrate that the whole affected assembly has been safely restored.