Broken wastewater treatment

My wastewater treatment plant appears to have stopped working, so all the settlement's waste water is getting flushed out to the ocean.
Here's the save:
https://drive.google.com/file/d/1AwQP5niHI2uzHTnvncd15pz5m8GfI9_s/view?usp=sharing
It's stuck on 'missing input'. It has plenty of both wastewater and sand. If I turn 'truck import' to off, it will only take 80 wastewater from the pipe and then stop; if I turn 'truck import' on, it will take 320 wastewater and process a few more cycles, then stop.
No idea what's going on here - this plant has worked fine for its whole lifetime. I tried demolishing and rebuilding it, but the exact same behaviour reoccurred.

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