Design Flow

Sanitary sewers are designed to carry peak residential, commercial, institutional, and industrial flows, as well as infiltration and inflow. Gravity sewers are designed to flow full at the design peak flow. Design flows are based on various types of developments.

If it is assumed that the average velocity of the wastewater in the collection system is 2.89 feet per second. How long does it take the wastewater in hours to reach the treatment plant if the maximum distance from the treatment plant to the farthest lateral is 12 miles?

First calculate the number of feet in 12 miles. Then divide that distance by the velocity to determine the distance.

# of feet in 12 miles 12 x 5,280 ft = 63,360 ft

(63,360 ft) / (2.89 ft/sec) = 21,923.88 sec

Now convert sec into hrs (21,923.88 sec) / (3,600 sec/hr) = 6.09 hrs

Minimum Pipe Size

A minimum pipe size is dictated in gravity sewer design to reduce the possibility of clogging. The minimum pipe diameter recommended by the Ten State Standards is 8 inches. Though the Ten State Standards are adopted by ten specific states (Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, New York, Ohio, Pennsylvania, and Wisconsin) and the Province of Ontario, they often provide the basis for other state standards.

AVERAGE DESIGN FLOWS FOR DEVELOPMENT TYPES

Calculating areas What is the area of the top of a tank that is 55 ft long 70 ft wide and 60 ft deep?

The tank is a rectangle. So the area will be length X width.

Area = 55 ft X 70 ft = 3,850 ft2

What is the area of the face in square feet of a 30 inch diameter pipe?

A pipe face is a circle. So the area would be 0.785 X D X D OR 3.14 X R X R (R is 1/2 D)

Diameter = (30 inch) / (12 inches/ft) = 2.5 ft D OR (2.5 ft) / (2) = 1.25 R

Area = 0.785 x 2.5 ft x 2.5 ft = 4.9 ft2 OR 3.14 X 1.25 X 1.25 = 4.9 ft2

Velocity

The velocity of wastewater is an important parameter in a sewer design. A minimum velocity must be maintained to reduce solids deposition in the sewer, and most states specify a minimum velocity that must be maintained under low flow conditions. The typical design velocity for low flow conditions is 1 foot/second. During peak dry weather conditions the sewer lines must attain a velocity greater than 2 feet/second to ensure that the lines will be self-cleaning (i.e., they will be flushed out once or twice a day by a higher velocity). Velocities higher than 10 feet/second should be avoided because they may cause erosion and damage to sewers and manholes.

Calculating Flow in a tee connection

The flow entering the leg of a tee is 25 gal/min. If the flow through one branch is 17 gal/min, what is the flow through the other branch?

The flow through the leg is equal to the sum of the branches. Subtract the branch from the leg to solve for the missing branch flow.

Flow in 2nd branch = 25 gal/min – 17 gal/min = 8 gal/min

Calculating Average Daily Flow

What is the daily average flow for a lift station in MGD given the following data?

Date: 4/12/2011 Time: 8:00 a.m. Meter Reading: 3,567,657 gallons

Date: 4/19/2011 Time: 8:00 a.m. Meter Reading: 6,763,269 gallons

Take total flow and divide it by 7 since the time period is 7 days.

(6,763,269 – 3,567,657) / (7) = 456,516.00 gallons = 0.46 MGD