As I understand this question the sigma referred to here is the standard deviation of the bell shaped probability curve, such that at a 1 sigma probability (that is about 95 times out of 100) the required water quality has been achieved. At the six sigma level of water quality, the successful pass rate is expected to be something like 99.99966 % and you are ‘claiming’ that out of tolerance water is very unlikely (probability 100 -99.99966%) to interfere with production.
It is a case of improve the process that need DMAIC (Define-Measure-Analyse-Improve-Control) it is the best problem solving method than others.
Important steps are:-
voice of the customer (Internal >> Employee) referred to International standard of safety (( ISO 45001 focuses on identifying and controlling risks rather than hazards, as is currently required in OHSAS 18001 )).
MSA & sample: To ensure that device , person have no wrong errors during measuring the water samples regarding biological & chemical components.