After invasive inspection of the discharge line on my system we uncovered the field lines and I was told that it is an older type system that used 8 inch lines with a "sock" liner on the outside of the pipe. The field was then covered with sand not gravel and then covered with plastic and a couple of feet of topsoil. 

The contractor wants to remove the four runs of eight inch pipe that are forty feet long each and replace with four inch pipe and a gravel bed. Is this the only alternative or can a product like Septic Scrub help with this problem?