Black Bits when Flushing Thetford Porta Pottie?

Dick Dastardly

When we flush the chemical toilet the flushing water contains black bits that remain in the bowl.  I'm using the thetford fluids in the tanks but I can't seem to get rid of the black bits.  Is this just part of caravanning's rich tapestry or can I resolve
the issue easily?  Many thanks in anticipation!

DD