WASTEWATER

By: Rick Lallish

It is generally accepted that the sludge blanket depth in secondary clarifiers should not exceed 2 feet (0.6 meters). By allowing a deeper sludge blanket, what undesirable outcome may occur?

A. Nitrification may occur in the secondary clarifier.

B. Deeper blankets inhibit settleability of the sludge.

C. Premature digestion will occur in the secondary clarifier.

D. Solids may be washed out and into the effluent flow.

Answer: D. Monitoring of the sludge blanket is a key process control parameter. The rule of thumb is never to allow the sludge blanket to reach more than 25 percent of the clarifier sidewall. By allowing a deeper sludge blanket, operators may encounter troubles with blanket washout due to excess flows (heavy rain event) or denitrification (sludge too long in the clarifier). Either condition could affect effluent solids readings and violate NPDES permit limits.

DRINKING WATER

By: Drew Hoelscher

Feeding aluminum sulfate as a coagulant will cause the raw water alkalinity to:

A. Decrease

B. Increase

C. Stay the same

D. Dissolve

Answer: A. Primary coagulation is commonly achieved by dosing the water with metallic salts such as aluminum sulfate, ferric sulfate or ferric chloride. The chemical and physical reaction that takes place between the cationic metallic salts, alkalinity, and suspended solids happens very quickly, resulting in microsized sticky particles called floc. Floc particles gain mass and become heavy enough to settle out during sedimentation. If the source water is too low in alkalinity to produce aluminum hydroxide precipitation, an operator may choose to add lime to enhance the floc-forming ability of the aluminum sulfate. However, it is important to note that aluminum sulfate has specific pH ranges for optimal performance, and lime will raise the water’s pH. 

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.