It's a real engineering target with a specific meaning — not a slogan.
Zero liquid discharge (ZLD, sometimes called zero effluent discharge) is an established industrial wastewater engineering target: a facility treats and recycles essentially all of its process wastewater internally, so no liquid effluent is released to municipal systems or the environment untreated.
It's a genuinely different kind of requirement than the consumer-facing claims elsewhere on this site, and it's worth being explicit about that: ZLD is a facility engineering specification MPFREE audits as part of the processing/factory supply-chain scope, not itself a claim about the product's microplastic content. A facility can meet a ZLD requirement and still fail Pillar 1 testing on its product, and vice versa — they're independent checks, audited separately, for exactly the reason that conflating "this facility manages its wastewater responsibly" with "this product tests below the detection threshold" would be the kind of unscoped claim the MPFREE claims policy exists to prevent.