Water Quality, Disinfection, and Cross-Connection Control
Core concepts
Understand disinfection fundamentals: chlorine residual (free vs. total), CT (concentration × contact time), and the difference between primary and secondary disinfection. Know the disinfectant residual must be maintained throughout distribution. Grasp cross-connection control: the definition of a cross-connection, actual vs. potential, and the hierarchy of backflow prevention - air gap (best), reduced-pressure-zone (RPZ) assembly, double check valve, and vacuum breakers - matched to hazard level (high vs. low). Understand backpressure vs. backsiphonage. Know coliform as the regulatory indicator and the meaning of total coliform positives and E. coli. Understand nitrification in chloraminated systems and DBP (disinfection byproduct) concerns. Know main flushing, storage turnover, and biofilm control.
Traps to watch
The most common trap: choosing a device that is inadequate for a high-hazard connection (e.g., a double check where an RPZ or air gap is required). Another: confusing backpressure (downstream pressure exceeds supply) with backsiphonage (negative pressure in the supply). Also watch free vs. total chlorine residual questions and the CT concept.
Worked example
An irrigation system connected to a potable main with chemical injection is a high-hazard cross-connection. The correct protection is an air gap or an RPZ - not a double check valve. If asked which is preferred, the air gap provides the highest protection.
Rule of thumb
Match protection to hazard: air gap > RPZ > double check > vacuum breaker. High hazard demands air gap or RPZ.