An operating free chlorine residual of ______ is usually required in practice.

The requirement for an operating free chlorine residual in water treatment typically falls within the range of 1.0 to 3.0 mg/l (or ppm). This range is widely accepted as it effectively achieves disinfection and maintains a residual that helps prevent the regrowth of pathogens in the distribution system.

Maintaining a free chlorine residual within this range ensures that enough chlorine is present to eliminate harmful microorganisms while also being safe for consumers. Chlorine concentrations below this level may be insufficient for effective disinfection, potentially allowing pathogens to survive, whereas levels significantly above can lead to taste and odor issues or create harmful byproducts.

In contrast, other options presented do not align with the common practice standards for free chlorine residual in disinfection processes.