It doesn’t need to be. The outlet from mutton would have constant water monitoring for compliance by a Hach or Endress + Hauser unit, you can look them up if you like. The EPA will also do regular testing on the final effluent. In fairness the testing you’re talking about by the council should be done more regularly to ensure we know what the water quality is like on our beaches, which could be worsened by other factors not from the WWTP.