If its going to a soakaway chemicals mess up the ecosystem and enable it to turn to blackwater if its standing. Personally recommend getting a set of drain rods from ebay. They come with a screw fitting and usually a puller flap(Ok I don't know the technical word for it but that's what it does!).
They also double as chimney rods. Pretty cheap too.
Out of interest is this the same place your bathroom/washing machine greywaste goes and does the launderer use any of the marsalla washing powders?
I have some info that might help avoid this in the future but its 36mb so I can't attach it in email. Any suggestions?
Bruixot