Water/Wastewater Laboratory Calculations - Basic Mathematics for Water and Wastewater Operators - page 233

■ Example 16.4
Problem: At a flow rate of 0.5 gpm, how long (in minutes and seconds) will it take to
flush a 60-ft length of 3/4-in. service line?

Solution:

3/4-inch diameter = 0.06 ft.

2 3

0.785 ×× × D Length (ft) 7.48 gal /ft × 2

Flushing time =

Flow rate (gpm)

2 3 ×

0.785 × (. ) 006 × 60 ft × 7.48 gal/ft 2

=

0.5 gpm

= 5.1 min

Convert the fractional part of a minute (0.1) to seconds:

0.1 min × 60 sec/min = 6 sec

5.1 min = 5 min, 6 sec

COMPOSITE SAMPLING CALCULATION
(PROPORTIONING FACTOR)
When preparing oven-baked food, the cook sets the correct oven temperature and
then usually moves on to some other chore. The oven thermostat maintains the
correct temperature, and that is that. Unlike the cook, in water and wastewater
treatment plant operations the operator does not have the luxury of setting a plant
parameter and then walking off and forgetting about it. To optimize plant opera-
tions, various adjustments to unit processes must be made on an ongoing basis.
The operator makes unit process adjustments based on local knowledge (experi-
ence) and on lab test results. However, before lab tests can be performed, samples
must be taken. The two basic types of samples are grab samples and composite
samples . The type of sample taken depends on the specific test, the reason the
sample is being collected, and the requirements in the plant discharge permit.
A grab sample is a discrete sample collected at one time and one location. It is
primarily used for any parameter whose concentration can change quickly (e.g.,
dissolved oxygen, pH, temperature, total chlorine residual) and is representative
only of the conditions at the time of collection.
A composite sample consists of a series of individual grab samples taken at
specified time intervals and in proportion to flow. The individual grab samples are
mixed together in proportion to the flow rate at the time the sample was collected
to form the composite sample. The composite sample represents the character of
the water/wastewater over a period of time.