The supplier must have accreditation according to LST EN ISO/IEC 17025:2018.
Specific tests must be performed (e.g., determination of pH, suspended solids, COD, BOD7, petroleum products, active silicic acid, ammonia, phthalates, total nitrogen, total phosphorus).
Strict limits of determination are set for some tests (Suspended Solids, BOD7, Petroleum Products) and accredited methods are required.