Question

1. how does uv light effect microorganisms?
2. describe the primary reason that chlorine may not be used for drinking water disinfection at a particular drinking water facility.

Explanation:

1. UV light (specifically UV-C) penetrates microbial cells and induces cross-linking in their genetic material (DNA/RNA), creating thymine dimers that stop the microbe from reproducing, effectively neutralizing it without killing it outright.
2. When chlorine is added to water with high NOM levels, the chlorine reacts with organic compounds to form DBPs. These compounds are linked to adverse health effects, so facilities with high NOM in their raw water may avoid chlorine to meet drinking water safety regulations, opting for alternative disinfection methods instead.

Answer:

1. UV light damages the DNA/RNA of microorganisms, preventing them from replicating and rendering them non-infectious.
2. The primary reason is the presence of natural organic matter (NOM) in the source water that reacts with chlorine to form harmful disinfection byproducts (DBPs) like trihalomethanes (THMs), which exceed regulatory safety limits for drinking water.