A rusty looking deposit in the pipework from the mixing chamber to the balance flow and through A02.
This is the second one we have found and after stripping down the first one it appeared to originate in the mixing chamber so wondered if it was something to do with the S02 perhaps?
We used to have horrendous problems with this in the COBE C3 mixer chamber in some accounts,it's almost certainly iron oxide which is so small it passes through membranes and comes out of solution when the pH changes in the machine and can be removed fairly easily using oxalic acid or if its still available a product called Super Iron Out.
If it is iron oxide the 3 possible sources we found after investigating various CS3 customer complaints about 10 years ago were:
1/RO input raw water tank which on inspection was antiquated and grossly contaminated (iron bacteria),problem resolved after in depth clean from inside tank.
2/Bicarb cartridge (not Gambro) contamination which was visible as a grey line when fluid filled,there were 2 raw material suppliers identified Solvay and Brunner Mond but I can't recall which was the offending product but it was a short term problem due to lack of high quality medical grade bicarb raw material from normal supplier.
3/Sodium hypochlorite 5% - contamination having been produced in a steel tank.
4/Also from my Sheffield NHS days either groundwater sources (most common) or reservoirs fed by streams running through iron deposits can have high levels,they try to reduce this by manual dosing with an aluminum oxide as a flocculant which was problematical as it caused brain damage before it was discovered and removed.
We have a similar discolouration of Gambro dialysate hoses. However, it is not deposits, it is actually the tubing changing colour. We questioned whether it was using Clinell cleaning wipes but when we stopped, the discolouration continued. Remains unresolved!