Solids Capture in Dewatering

A dewatering machine takes one stream in and sends two out, so a mass balance on the solids pins down how much of the feed actually left in the cake. Feed at 4% solids producing a 22% cake and a filtrate carrying 0.1% solids captures 22(4 − 0.1)/[4(22 − 0.1)] = 97.9%. The elegance of this form is that it needs no flow measurement at all — three laboratory solids tests, taken at the same moment, are enough.

Good belt presses and centrifuges run 90–98% capture; below 90% the recycle stream is doing real damage. That is the number the calculation is really for: everything the machine misses goes back to the head of the plant in the filtrate or centrate, so a press losing 10% of a 10-tonne-a-day solids stream is quietly adding a tonne a day of fine, hard-to-settle solids to the influent load, showing up as rising MLSS, deteriorating settleability and a clarifier that will not behave. The usual cause is polymer — wrong product, wrong dose, wrong mixing energy, or a batch made up hours earlier and gone stale — and the fix costs less than the recycle. Sample the three streams at the same time and after the machine has run steadily for at least twenty minutes, or the balance will simply describe a transient.