WASTEWATER

By Rick Lallish

What operational parameters are able to be controlled by the operator in an activated sludge treatment process?

A. BOD load, hydraulic load and detention times

B. Influent solids, flow and solids retention time

C. Dissolved oxygen, return rate and wasting rate

D. Weather, flow and demand 

ANSWER: C. The activated sludge process has many variables to be aware of, but operators have total control over only these three. Operators may set the DO to the point necessary to maintain aerobic conditions and fulfill the needs of the microorganism population. They also can set the return rate to ensure that the sludge can settle in the clarifiers to an acceptable detention time. Operators also can decide how much sludge needs to be wasted from the system. Operators who understand these parameters can make the necessary choices to ensure proper treatment. Many things are outside operators’ control, but should be considered in making changes in the process.

More information may be found in the Office of Water Programs, CSU-Sacramento textbook Operation of Wastewater Treatment Plants, Seventh Edition, Volume 2, Chapter 11.

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DRINKING WATER

By Drew Hoelscher

What is the maximum residual disinfectant level for chlorine dioxide?

A. 4.0 mg/L as ClO2

B. 1.0 mg/L as ClO2

C. 0.8 mg/L as ClO2

D. 0.5 mg/L as ClO2

ANSWER: C. Chlorine dioxide is an effective primary disinfectant, if Cryptosporidium or the development of trihalomethanes are of concern. However, chlorine dioxide is unstable at elevated concentrations, so on-site generation by reacting sodium chlorite with chlorine is required. Feeding chlorine dioxide creates a scenario where elevated chlorite levels may hinder the quality of the finished water. To ensure that the finished water quality meets the national primary drinking water regulations, operators sample the water at the distribution entry point and have it analyzed for chlorite. Chlorite has a maximum contaminant level of 1.0 mg/L.

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.