Convert process black pdf pages to pure black - Adobe Acrobat

Hi Guys

We have a multi colored document we are printing (some pages have colour some are b&w) our digital press can detect b&w pages and charge us that price but because the document has process black printing on the pages that are B&W its detecting them as colour. Is there a way to tell pdf or convert the black in the pdf to a pure black (100% k / grayscale etc.)

Thanks In Advance
Samuel