Question 1

Raw wastewater can potentially contain which of the following?&#10;A)Pathogens&#10;B)Microconstituents&#10;C)Both A. and B.&#10;D)None of the above

Accepted Answer

Raw wastewater can potentially contain pathogens and microconstituents. Pathogens (disease causing organisms) commonly found in wastewater include bacteria, viruses, and protozoa. Microconstituents are substances (naturally occurring and manmade) which include inorganic and organic elements that are detected in water that may have a potential impact on human health and the environment. The detection of microconstituents in an influent wastewater stream presents continuing challenges to municipal engineers responsible for designing treatment systems to remove the potentially harmful elements.

Question 2

In a typical municipal wastewater treatment plant, at which point in the treatment process is the influent flow meter located?&#10;A)Prior to pretreatment screening&#10;B)Between screens and grit removal&#10;C)Between grit removal and lift pumps&#10;D)Between lift pumps and primary treatment units

Accepted Answer

In a typical municipal wastewater treatment plant, the influent flow meter is located between the grit removal and the lift pumps. The flow meter then protected from the large debris removed by the screens (roots, rags, grease, and plastics) and the inorganic solids (sand, gravel, and eggshells) that settle out during the grit removal process. Flow meters placed on gravity lines are generally less expensive and easier to service than those placed on pressure lines. Therefore, the influent flow monitor is located before the pumps that lift the influent wastewater high enough to permit gravity flow through the primary and secondary treatment units.

Question 3

In a typical municipal wastewater treatment plant, what is the normal retention time range for primary sludge in a gravity thickener, depending on temperature?&#10;A)Less than 1 day&#10;B)1 to 2 days&#10;C)2 to 3 days&#10;D)More than 3 days

Accepted Answer

In a typical municipal treatment plant, the gravity thickener retention time normally ranges between 1 to 2 days for primary sludge. The exact retention time depends on the temperature of the sludge. The most common configuration for a gravity sludge thickener is a circular tank with a side water of 10 to 14 feet (3 to 4 m) and a designed detention period of 24 hours.

Question 4

If septicity occurs in a gravity thickener, which of the following actions would not likely improve the situation?&#10;A)Aerate the thickener feed&#10;B)Add chlorine or hydrogen peroxide&#10;C)Decrease the underflow pumping rate&#10;D)Decant the thickener feed

Accepted Answer

If septicity occurs in a gravity thickener, decreasing the sludge pumping rate would not likely improve the situation. Septicity is characterized by floating solids, foul odors and reduced underflow currents, and occurs when solids have been stored too long in a gravity thickener. A decreased underflow pumping rate may increase the solids deposition in the tank.

Question 5

How many gallons of wastewater (approximately) can be held in a rectangular wet well with the following dimensions: 10-foot (ft) liquid depth, 12 ft wide, and 15 ft long?&#10;A)240 gallons&#10;B)1,800 gallons&#10;C)2,400 gallons&#10;D)13,464 gallons

Accepted Answer

A rectangular wet well that is 12 ft wide, 15 ft long, and has a liquid depth of 10 ft can hold approximately 13,464 gallons. Solution: Volume of wet well = Length x Width x Depth Given, 1 cubic foot ( ft³) = 7.48 gallons Volume = 15 ft x 12 ft x 10 ft = 1,800 ft³ In gallons, 1,800 ft³ x 7.48 gallons/ ft³ = 13,464 gallons

Question 6

Typically, what are assumed to be appropriate sludge storage facilities for mechanical wastewater treatment plants?&#10;A)Drying beds&#10;B)Lagoons&#10;C)Tanks&#10;D)All of the above

Accepted Answer

Typically, drying beds, lagoons, and tanks are considered to be appropriate facilities in which to store sludge at mechanical wastewater treatment plants. These facilities may be used to store liquid or dried sludge. The design considerations for sludge storage facilities include odor controls for storage tanks, and aeration units and covers for lagoons. A minimum of 120 to 180 days of storage capacity is commonly required for the design life of the plant. Specific design requirements for sludge storage facilities are set by state regulating agencies.

Question 7

Why would wastewater treatment plant effluent need to be dechlorinated?&#10;A)Reduce the toxicity due to chlorine residuals&#10;B)Reduce odors&#10;C)Increase nutrients released to the receiving stream&#10;D)None of the above

Accepted Answer

Wastewater treatment plant effluent may need to be dechlorinated to reduce the toxicity due to chlorine residuals. High levels of chlorine residuals are toxic to certain kinds of aquatic life. Chlorine residuals may react with organic materials found in certain receiving waters and form carcinogenic compounds that may not be removed by conventional water treatment plants. Many times, regulatory agencies require municipalities to add dechlorination facilities to existing treatment plants and set specific design criteria. The type of dechlorination system selected is based on the following criteria: type of chemical storage required, amount of chemical needed, ease of operation, compatibility with existing equipment, and safety.

Question 8

What is the change in elevation between the upstream invert and the downstream invert of a 400-foot (ft) section of 8-inch diameter sewer pipe set at a 4.5% slope?&#10;A)<2 ft&#10;B)4.5 ft&#10;C)18 ft&#10;D)Not enough information provided

Accepted Answer

The change in elevation of a 400 ft section of 8-inch diameter sewer pipe set at a 4.5 % slope is 18 ft.
Solution: Elevation change = Length of pipe x slope
Given, 4.5 % slope = 4.5 ft / 100 ft
Elevation Change = 400 ft x 4.5 ft / 100 ft
= 18 ft

Question 9

What is considered to be the scouring velocity of gravity sanitary sewers, in feet per second (ft/sec)?&#10;A)1 ft/sec&#10;B)2 ft/sec&#10;C)4 ft/sec&#10;D)4.5 ft/sec

Accepted Answer

The scouring velocity of gravity sanitary sewers is considered to be 2 ft/sec. Typically, state regulatory agencies require that sewers be designed and constructed so that the mean velocity, when flowing full, is not less than 2.0 feet per second, based on a Manning's formula using an "n" value of 0.013. However, slopes that provide a velocity greater than the minimum 2 ft/sec are recommended so that a scouring velocity can be achieved within the pipe at low flow periods and/or to address the increasing use of low flow plumbing fixtures that contribute less flow to a collection system that may have been considered in the design per capita daily flow assumptions.

Question 10

What is the primary purpose of the use of lift stations in a municipal wastewater collection system?&#10;A)Minimize easement requirements for an alignment&#10;B)Increase the capacity of a sewer line&#10;C)Move wastewater from a lower to higher elevation&#10;D)Increase velocity of the wastewater traveling through a sewer line

Accepted Answer

The primary purpose of lift stations in a wastewater collection system is to move wastewater from lower to higher elevations. This is done so that wastewater transported by lower-lying sewers can fed into gravity sewers that are at higher elevations. Key elements of lift stations include the following:
• wastewater receiving well (wet-well)
• pumps and piping, and associated valves
• motors
• power supply system
• an equipment control and alarm system
• odor control system and ventilation system
Specific design requirements for primary settling tanks are set by state regulating agencies.

Deck 20: Waste Water