Exam Study Guide: Trickling Filters; and Lime Treatments

Maintaining your education is important, especially in a career that demands licensing exams. Prove you’re an expert operator by answering these questions and others from our Exam Study Guide Series.
Exam Study Guide: Trickling Filters; and Lime Treatments

Welcome back to TPO magazine's new and improved Exam Study Guide Series, which offers a pair of water/wastewater study questions each week with in-depth explanations of the answers. We covered a set of wastewater and drinking water treatment questions last week on the topics of Volatile Acid to Alkalinity Ratio, and Odor-Reducing Chemicals. This week, you can test your knowledge about trickling filters and lime treatments. Take a look at the multiple-choice sample questions and answer explanations below.

Sample Question No. 1:

Which of the following is a method of increasing the efficiency of a trickling filter?

A) Add an additional 7 vertical feet of rock media.

B) Increase the rotational speed of the distributor arms.

C) Add anaerobic digester supernatant to the filter feed flow.

D) Recirculate filter effluent back to the filter feed.

Answer: The answer is D, recirculate filter effluent back to the filter feed. Recirculating trickling filter effluent back to the filter feed increases the efficiency several ways:

  • Contact time with the organic material in the waste flow is increased, essentially allowing some of the BOD to be re-treated.
  • The additional flow increases the total volume per cubic foot of media, which encourages more uniform and increased sloughing of zoogleal growth.
  • Increased sloughing of media improves aerobic conditions within the media, helps prevent ponding (clogging).
  • Recirculation helps reduce filter fly and snail growth.

In some cases, recirculation is necessary to keep the rotary distributor moving and maintain wetted conditions on the media. Intermittent wet/dry conditions can encourage filter fly and snail growth and reduce overall BOD removal efficiency. Recirculation also helps maintain a continuous flushing of the media, which encourages ventilation within the media bed. Answer B states “increase the rotational speed of the distributor arms.” Simply rotating the arms faster does not necessarily equate to an increase of gallons of filter feed per cubic foot of media. There are recommended flushing rates for trickling filters to enhance uniform sloughing of zoogleal growth which prevents ponding and maintains aerobic conditions within the media depths. Proper flushing includes keeping the speed slow and constant while applying many gallons per minute of filter feed flow. 

Sample Question No. 2: 

Which softening process shown treats roughly 80 percent of the raw water with an excess amount of lime to remove magnesium at a pH greater than 11, then pre-filtered source water (about 20 percent) is added in the next basin to lower the pH?

A) Caustic soda softening

B) Excess lime softening

C) Split lime softening

D) Ion exchange softening

Answer: The answer is C, split lime softening. When source water contains a high level of magnesium, a process called split lime treatment can be used. Split lime treatment is used with lime treatment alone or in lime/soda ash treatment. With split lime softening, a certain percent of the total water (about 80 percent) is treated with an excess amount of lime to remove the magnesium as magnesium hydroxide at a pH of over 11. Filtered source water is then added in the next basin to neutralize the excess lime treated portion. 

A benefit of split lime treatment softening is the need for recarbonation is reduced or eliminated entirely, which reduces overall treatment costs.

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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