WASTEWATER

By Rick Lallish

What laboratory tests must be done on grab samples?

A. BOD

B. pH

C. TSS

D. Total nitrogen

Answer: B. Proper sampling is necessary for accurate laboratory analysis. Tests such as pH and dissolved oxygen must be done on grab samples as the samples degrade drastically over a brief period. Some analyses such as BOD and TSS can be done on composite samples, which are drawn over periods of time and do not change as much. If you are unsure of what tests require grab or composite samples, simply review your NPDES permit. More information can be found in Standard Methods for the Examination of Water and Wastewater (Current Edition).

DRINKING WATER

By Drew Hoelscher

Which disinfectant is least likely to develop trihalomethanes (THMs)?

A. Chlorine gas

B. Sodium hypochlorite

C. Calcium hypochlorite

D. Chlorine dioxide

Answer: D. Regarding disinfection, hypochlorite and chlorine gas are similar. The most notable difference between the two is the side chemical reactions of the end products. Chlorine gas tends to lower the pH and hypochlorite tends to raise the pH of the treated water. Chlorine dioxide does not react with organics like chlorine gas and/or hypochlorite, so there is less concern with developing THMs. Chlorine dioxide is also an effective disinfectant at elevated pH levels.

About the author(s)

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.