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A rotating biological contactor (RBC) wastewater treatment plant experiences a utility power failure and standby power is unavailable. Local emergency management officials determine the power will be restored in 36 to 48 hours. What should the operator do to protect the growth on the media and the RBC units themselves?

A. Immediately wash the attached growth off the RBC discs with a power washer.

B. Manually rotate the RBC drum a quarter-turn every four hours to prevent the slime growth from only growing on the bottom portions.

C. Manually rotate the RBC drum completely until power is restored to maintain permit compliance.

D. Drain the RBC tanks and flush with fresh water.
Answer: B. If the power will not be restored before four hours, the RBC shaft should be rotated manually to ensure even slime growth on the discs. Rotating the drum a quarter-turn every four hours is recommended to even the pattern of the attached growth. This should be done in the safest manner possible. If the RBC drum is not rotated and the slime growth has only continued to grow on the portion that remains submerged, the weight and balance of the drum will be very uneven.

If the weight of the growth is not heavy enough to prevent the rotation completely, the unit will struggle to reach the apex point of the rotation and then move very quickly toward the bottom, causing excessive strain and wear on motors and gearboxes (if applicable) and on the RBC drum shafts and bearing mounts. Keep the biomass wet by hosing the slime growth occasionally, but do not wash off or pressure wash the media. You’ll need the active biomass to treat all the wastewater that will come rushing in once power is restored. One other note: Ensure that standby generators at the treatment plant are working in good order and capable of keeping critical unit processes functioning.


In the lime softening process, when calcium hydroxide is added to water to be softened and the pH is increasing, which chemical reaction occurs?

A. Alkalinity converts from the bicarbonate form to the carbonate form, and then calcium can be precipitated as calcium carbonate.

B. Hardness is converted to alkalinity, becoming permanent hardness.

C. Hardness converts to bicarbonate hardness, and noncarbonate hardness precipitates.

D. Alkalinity converts from the bicarbonate form to carbon dioxide, and magnesium precipitates as magnesium carbonate.

Answer: A. As the pH of the raw water increases with the addition of lime (calcium hydroxide), the alkalinity begins to convert from the bicarbonate to the carbonate form. At around pH 10.2, the calcium in the raw water that caused the initial hardness bonds with the excess carbonate to become a calcium carbonate precipitate (CaCO3). The heavier-than-water precipitate settles to the bottom of the lime softening unit for removal by sludge scrapers or other removal mechanisms.
Alkalinity and hardness are two distinctly different parameters and tests, but they have some things in common. Calcium and magnesium, which make up hardness, also contribute to elevated alkalinity in the water. Alkalinity is sometimes described as the ability of water to resist a change in pH, or as a buffer against acids that cause a decrease in pH. Calcium and magnesium, found in the Earth’s crust as calcium carbonate and magnesium bicarbonate, readily dissolve in water, giving groundwater natural alkalinity and hardness.

About the author

Ron Trygar, a certified environmental trainer, is the senior training specialist for water and wastewater programs at the University of Florida’s TREEO Center. He has worked in the wastewater industry for more than 30 years in a variety of locations and positions. He holds a Florida Class A wastewater treatment operator license and a Florida Class B drinking water operator license.


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