Exam Study Guide

WASTEWATER - By Rick Lallish

Chlorine contact time varies from state to state. Most states require either 15 or 30 minutes at the design average flow rate. Why is a minimum contact time so important for disinfection?

A. Chlorine gas is hazardous and has to be regulated

B. To allow excess chlorine gas to dissipate

C. To allow the chlorine time to inactivate the microorganisms

D. To allow the BOD time to bleed off before disinfection

Answer: C. Disinfection using chlorine is based on contact time. It takes time for the chlorine to react with the components of the microorganism. Several factors affect the time needed to achieve disinfection: concentration, temperature, pH, mixing and level of disinfection required. The contact time is based on the amount of flow moving through the facility and the contact basin size. The operator cannot control either of these, but can control the dosage to achieve proper disinfection. This knowledge is important for many reasons, such as meeting chlorine residual limits and fecal coliform reporting. Understanding how to control the chlorine dosage is an important process control tool for successful disinfection.


What is the U.S. EPA primary maximum contaminant level for iron?

A. 1.0 mg/L

B. 0.3 mg/L

C. 0.0 mg/L

D. Iron is not regulated by a maximum contaminant level

Answer: D. There is not a U.S. EPA maximum contaminant level for iron, which is regarded as an aesthetic issue rather than a health issue. However, the EPA does have iron on the secondary maximum contaminant level list as 0.3 mg/L. This is a recommended level and is not enforceable. Studies have shown that a water system typically experiences red water complaints at or above 0.3 mg/L for iron.

About the authors

Rick Lallish is water pollution control program director and Drew Hoelscher is program director of drinking water operations at the Environmental Resources Training Center of Southern Illinois University Edwardsville.  


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