Question 1

What is the purpose of an air gap?&#10;A)Reduces corrosion within the distribution system&#10;B)Eliminates the need for lift station check valves&#10;C)Prevents contamination of a potable water supply&#10;D)Both A and C

Accepted Answer

Air gaps are used to prevent the contamination of a potable water supply. Air gaps are physical separations between supply pipes and receiving vessels, and are required by state drinking water cross-connection control regulations for instances where backsiphonage or backpressure conditions may exist. An example of an air gap in household plumbing is the space between a faucet and a sink. Typically, air gaps are sized vertically to be at least two pipe diameters (minimum 1-inch). There is no available data that suggests the use of air gaps may reduce corrosion within a distribution system or eliminate the need for lift station checkvalves.

Question 2

Concerning atmospheric vacuum breakers (ABV), which of the following statements is incorrect?&#10;A)The AVB is always located downstream from all shut-off valves&#10;B)The AVB air inlet valve is open when water flows in the normal direction&#10;C)AVB valves do not operate properly when under continuous pressure for more than 12 hours in a 24-hour period&#10;D)None of the above

Accepted Answer

The statement "The AVB air inlet valve is open when water flows in the normal direction" is incorrect. The air inlet valve is closed when water flows in the normal direction. This device is used to protect against backsiphonage and contamination, and relies on air inlet valve to open when backpressure is present. AVB valves must be placed in a location where shut-off valves or downstream obstructions will not keep the device under pressure.

Question 3

What is the purpose of a thrust block when designing water distribution systems?&#10;A)Prevents pipe movement when a pressure system is activated and pressurized&#10;B)Prevents backsiphonage and contamination&#10;C)Used to isolate pressure pipe sections&#10;D)None of the above

Accepted Answer

The purpose of thrust blocks is to prevent pipe movement when a pressure system is activated and pressurized. Typically, thrust blocking is required at all points where the direction of a pressure pipe changes. Thrust blocking is normally required where fittings, valves, and hydrants are installed. Concrete is commonly used for blocking, and the size of the block is based on pipe size, working pressure, fitting type, degree of directional change, and local soil conditions.

Question 4

Which criteria are used to determine the width of the top of an excavated trench when installing waterlines?&#10;A)Depth and diameter of pipe&#10;B)Local soil type and groundwater elevation&#10;C)Depth of frost line&#10;D)All of the above

Accepted Answer

The width at the top of an excavated varies, depending on the depth and diameter of the installed pipe, local soil type and groundwater elevation, and depth of the frost line. The Occupational Safety and Health Administration (OSHA) sets the safety regulations for trenching and shoring which ultimately defines how trenches are to be sloped, supported, or drained so that workers can safely install waterlines. Generally, local and state design requirements dictate excavated trench widths based on pipe diameters. Waterlines are commonly required to be installed at least 6 inches below the frost line (when applicable).

Question 5

Which of the following is not recommended when testing polyvinyl chloride (PVC) pressure pipe?&#10;A)Locate testing connection at the lowest point of the line&#10;B)Properly backfill pipe trench&#10;C)Use air pressure for testing&#10;D)Use hydrostatic pressure for testing

Accepted Answer

The use of air pressure is not recommended for testing PVC pressure pipes. The manufacturers of PVC have indicated that air pressure testing of PVC pipes is extremely dangerous because of its explosion potential. When PVC piping explodes, plastic shrapnel is sent in all directions and can cause serious injuries or death. Consequently, hydrostatic pressure testing is the method employed by municipal water agencies for testing PVC pressure pipe.

Question 6

Flocculation occurs during which of the following water treatment processes?&#10;A)Coagulation&#10;B)Sedimentation&#10;C)Filtration&#10;D)Disinfection

Accepted Answer

Flocculation occurs during the coagulation process. During the coagulation process, alum or other chemicals are added to the water to form "floc" particles that attract dirt. As the combined particles continue to attract and grow (flocculate), their combined weight will cause them to fall out of suspension and settle during the sedimentation process.

Question 7

What is the approximate capacity (in gallons) of a 12-foot diameter circular lime softening tank that has a 10 foot water depth?&#10;A)898 gallons&#10;B)8,460 gallons&#10;C)10,770 gallons&#10;D)Not enough information provided

Accepted Answer

A circular lime softening tank that is 12-foot in diameter and a 10-foot water depth can hold approximately 8,460 gallons. Solution: Capacity of circular tank (on end) = area x height of tank Where Circular tank area = diameter2 x 0.7854 = 144 ft² x 0.7854 = 133.10 ft² Given 7.48 gallons = 1 ft³ Capacity = 133.10 ft² x 10 ft x 7.48 gallons/ ft³ = 8,460 gallons (approximate)

Question 8

What is the required chlorinator setting (in pounds per day (lbs/day Cl₂)) to treat a flow of 1.78 million gallons per day (MGD) with a chlorine dose of 4 milligrams/liter (mg/L)? A)7.12 lbs/day Cl₂ B)14.8 lbs/day Cl₂ C)59.4 lbs/day Cl₂ D)Not enough information provided

Accepted Answer

The required chlorinator setting to treat 1.78 MGD with a chlorine dose of 4 mg/L is 59.4 lbs/day Cl₂. Solution: Chlorinator setting = chlorine dosage x flow Given Weight of water = 8.34 lbs/gallon Conversion from mg/L to lbs/day: (mg/L Cl₂) (flow in MGD) (8.34 lbs/gal) = lbs/day Cl₂ Chlorinator setting = (4 mg/L Cl₂) (1.78 MGD) (8.34 lbs/gal) = 59.4 lbs/day Cl₂

Question 9

How much water (in approximate million gallons per day (MGD)) can be treated in a detention basin with the dimensions 15 feet by 10 feet by 8 feet deep and a required detention time of 60 minutes?&#10;A)0.22 MGD&#10;B)12.93 MGD&#10;C)149.6 MGD&#10;D)None of the above

Accepted Answer

A 15-foot by 10 foot by 8-foot deep detention basin with a required detention time of 60 minutes can treat approximately 0.22 MGD.
Solution: Basin Volume = length x width x depth
Given 7.48 gallons = 1 cubic foot
Basin Volume = (15 ft) x (10 ft) x (8 ft) x (7.48 gallons/cubic feet)
= 8,976 gallons
Flow Rate = 8,976 gallons/60 minutes
= 149.6 gallons/minute
149.6 gallons/minute x 1,440 minutes/day = 215,424 gallons
(approx. 0.22 MGD)

Question 10

What is the average filtration rate (in GPM/sq. ft.) for a plant with three 10-foot square filters that treats 1 million gallons in 24 hours of continuous operation?&#10;A)0.33 GPM/sq. ft.&#10;B)2.32 GPM/sq. ft.&#10;C)6.95 GPM/sq. ft.&#10;D)Not enough information provided

Accepted Answer

The average filtration rate for a plant with three 10-foot square filters that treats 1 million gallons per day is 2.32 GPM/sq. ft.
Solution: Filtration rate = total gallons treated per unit of time / total filter surface area
= (1,000,000 gallons/day x (1 day/1,440 minutes)) / (3 x 10 ft x 10 ft)
= (695 gallons/minute (GPM)) / (300 square feet)
= 2.32 GPM/sq. ft.

Deck 21: Water Treatment