Certification Corner

Certification Corner lets Opflow readers sharpen their skills with a quick quiz of terminology, regulations, practices, and formulas pertinent to water/wastewater collection, treatment and distribution system operations. Check out the questions below and in each issue of Opflow to test your knowledge. Follow us on Instagram for weekly #TestYourKnowledgeTuesday trivia.

For additional questions, purchase the Water Operator Certification Study Guide and the Wastewater Operator Certification Study Guide from the AWWA Store.

Question 1

When opening a chlorine cylinder valve,
a. pull or tug at the wrench
b. use a wrench that's at least 12 in. long for added leverage
c. use common bleach to check for any chlorine leaks
d. open only one turn to permit maximum withdrawal rate


d. open only one turn to permit maximum withdrawal rate.

Ton containers are transported, hoisted, stored, and used in the horizontal position. When they are to be used, they must be positioned so the two valves are oriented with one directly above the other. In this position, the top valve delivers chlorine gas, and the bottom delivers liquid chlorine. A single ton container can deliver chlorine gas at rates up to 400 LB/D (360 KG/D) against a backpressure of 35 PSIG (240 KPA) at room temperature without frosting. Liquid chlorine can be delivered at rates up to 9,600 LB/D (4,400 KG/D) if any evaporator is being used. If a plant's requirements exceed 400 LB/D (180 KG/D), the plant can still operate on gas withdrawal by connecting two or more containers to feed at the same time. 

The exact maximum withdrawal rate of gas for vacuum systems can be determined from the following formula: (Chlorine room temp. °F - threshold temp. °F) Withdrawal factor = maximum withdrawal rate. 

The withdrawal factor depends on the size and shape of the cylinder or container. A cylinder or container has been installed and the flexible connector attached, the valve can be opened and the lines checked for leakage according to standard procedures. If no leaks are found, open the cylinder valve. One complete turn will permit the maximum withdrawal rate.

Source: AWWA, WSO: Water Treatment, Fourth Edition, pages 199, 202 (catalog No. 1956).

Question 2

The floating cover on an anaerobic digester should be inspected
a. daily.
b. weekly.
c. bimonthly.
d. monthly.


a. daily.
Never allow the floating cover of an anaerobic digester to be raised above the maximum level, because sludge could flow over the digester walls or the rollers could leave the roller guides. Do not permit the level of the digesting sludge to drop so low that the cover rests on the corbels, except when the digester is cleaned. If the gas seal breaks and air enters the digester, an explosive atmosphere can exist in the digester. 

Inspect the cover frequently (daily) to prevent the cover from becoming crooked, tipped, stuck, or jammed. Carefully control digester sludge feed and withdrawal rates. An additional inspection of the cover every time the level of the digester contents is changed is a good idea.

Source: Office of Water Programs, California State University, Sacramento. Operation of Wastewater Treatment Plants, A Field Study Training Program, volume 2, seventh edition, page 181 (catalog no. 20719).

Question 3

What's the height limit to which siphoned water can be lifted at sea level?
a. 22.4 ft
b. 32 ft
c. 33.9 ft
d. 34 ft


c. 33.9 ft
Backsiphonage is a condition in which pressure in the distribution system is less than the atmospheric pressure; that is, something is “sucked” into the system because the main is under a vacuum. Because backsiphonage is caused by atmospheric pressure, the height to which siphoned water can be lifted is limited to 33.9 ft (10.3 m) at sea level, the point at which downward pressure caused by the water column’s weight equals the upward force of atmospheric pressure.

Source: AWWA, Principles and Practices of Water Supply Operations, Water Transmission and Distribution, fourth edition, pages 271, 278 (catalog No. 1957).