Most water problems that persist do so for one reason: biofilm and resistant pathogens survive the chemicals thrown at them.
Biofilm shelters pathogens and is roughly a thousand times more resistant to conventional disinfectants than free-floating cells, so chlorine-based routines can dose for years without clearing the source. The result is recurring Legionella and pathogen risk, failed tests and rising chemical cost.
In wastewater, the same conventional approach is energy-hungry and chemical-heavy, with aeration alone taking 45 to 75% of a plant's energy, while effluent consents keep tightening.
The cost is paid twice: in the chemicals and energy spent, and in the risk and compliance exposure that remains.