Exam Study Guide: Advantages of Plastic Media; and Treating Colder Water

Welcome back to TPO magazine's Exam Study Guide Series, which offers a pair of water/wastewater study questions with in-depth explanations of the answers. Last time, we covered a set of wastewater and drinking water treatment questions on the topics of Primary Clarifiers; and Microsand Processes. This time, you can test your knowledge about plastic media in a trickling filter, and treating colder water.

Wastewater Treatment Sample Question:

What is an advantage of using plastic media in place of traditional rock-type media in a trickling filter?

A) More surface area for filter flies to breed

B) Increased surface area for increased biofilm growth

C) Increased opportunities for ponding

D) The media bed is less aerobic

Answer: Many of today’s active trickling filter treatment plants have removed and replaced the traditional rock media with plastic or synthetic media. Plastic media offers much more room for biological growth per cubic foot than the same amount of rock media. This increased volume of biological slime allows more microorganisms to grow which in turn allows more organic and hydraulic loading per cubic foot. So, the answer is B, increased surface area for increased biofilm growth.

Plastic media also helps the biological slimes that break away from the media (called sloughing) to flow downward to the underdrain and out with the flow to the secondary clarifier, whereas with rock media this sloughing can get clogged within the rock media bed. Clogged media can result in anaerobic conditions near the clogged area which may lead to odors and poor performance.

The shape of the plastic media also contributes to more available openings for air to circulate throughout the media. This increased air circulation can help maintain aerobic conditions and potentially reduce odors from the filter itself. The open void space of a filter using synthetic media can be as much as 95% of the trickling filter’s total volume.

I hope that answers A and C are obviously incorrect; we try hard to prevent filter fly infestations at trickling filter plants and ponding of the media prevents the flow of the primary effluent down through the bed, creating stagnant conditions and reducing the overall treatment efficiency. Switching to plastic media may help reduce the issues rather than increase them.

Water Treatment Sample Question:

Which statement about treating colder water is accurate? 

A) The colder the water, the longer it takes particles to settle out

B) Colder water requires less time for solids to float to the surface

C) The colder the water, less speed is required of the flash mixing equipment

D) As water temperatures drop, less time is needed to accomplish settling

Answer: Think about the change in density of water as it freezes in your kitchen freezer or ice-maker. The answer is A, the colder the water, the longer it takes particles to settle out. As the temperature decreases, the activity of the water molecules slow down. The water becomes dense enough to change from a liquid state to being solid. On the other hand, if we boil the same water on the stove-top, we can cause water to begin evaporating into steam, which is water converting from liquid to a vapor state.

In a settling basin, the settling velocity of particles is affected by the same changes in water density due to temperature variations. Water molecules move slower as the water becomes denser, causing chemical reactions to become slower. The increased water density will cause the floc particles in the settling basin to settle at a slower rate, meaning we will need more detention time to achieve the target effluent turbidity and suspended solids goals. Most often, operators find that more energy is required to thoroughly mix the coagulants into the raw water, and increased flocculator speed is sometimes needed to assure efficient particle collisions.

About the author: Ron Trygar is the senior training specialist for water and wastewater programs at the University of Florida's TREEO Center. Previously, he was the wastewater process control specialist at Hillsborough County Public Utilities in Tampa, Florida. He has worked in the wastewater industry for more than 30 years in a variety of locations and positions. Trygar became a Certified Environmental Trainer (CET) in 1998 and has since provided training for associations and regulatory agencies such as Florida Department of Environmental Protection (FDEP); Florida Water and Pollution Control Operators Association Short Schools; USABlueBook; Florida Water Environment Association sponsored training events; and local school environmental programs. Working alongside the FDEP Northeast District, Trygar helped begin the Florida Rural Water Association and FDEP joint operator certification review classes that are still given around the state today. He holds a Florida Class A wastewater treatment operator’s license and a Florida Class B drinking water operator’s license.


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