Automating polymer dosing on municipal sludge dewatering ideally needs two measurements: inlet sludge solids (liquid, roughly 1–6 %) and cake outlet solids (semi-solid, 15–40 %).
The maturity of these two is wildly different.
The inlet: competitive and well served
Microwave, ultrasonic and optical suspended-solids methods all have mature products from many suppliers. The selection problem here is fouling resistance and maintenance load, not whether measurement is possible at all.
Ultrasonic acoustic impedance has some practical advantages on inlet sludge: a non-intrusive probe mounted flush with the bore, no moving parts, nothing to block, a flanged spool that inserts directly and does not require a permanently full pipe. But inlet sludge is heavily fouling, so an automatic flushing arrangement is mandatory (solenoid valve plus wash water or compressed air on a timer), along with a sampling port on the spool for gravimetric calibration.
The cake: only three mature products worldwide, and essentially nothing domestically
For online cake solids there are currently three mature products globally, on three different physical routes — microwave, ultrasonic transmission, and near-infrared.
Why is it so hard? Because cake is a non-uniform semi-solid whose packing density varies.
Two cakes at the same 25 % solids, one compacted and one loose, present completely different paths to any signal crossing them. Any method that measures the bulk will misread packing voids as moisture change.
The three approaches deal with this differently. The microwave route uses a ratio of two parameters to cancel packing density out. The strongest ultrasonic product uses pulse-echo backscatter — it measures only a local volume 2 to 5 cm in front of the probe, so bulk voids in the cake do not affect it. That, rather than any inherent superiority of ultrasound, is the real reason it can claim an advantage over bulk microwave density.
Advice for buyers
If you are evaluating online cake solids, the first question to put to any supplier is: how do you handle varying packing density? A proposal that cannot answer it will not hold up on the machine, however good the lab data looks.
If you only need the inlet, you have plenty of options — compare on maintenance load and anti-fouling design.
Only with both points can dosing run as a genuine feed-forward plus feedback loop. Inlet measurement alone will always be open-loop estimation.
