Wastewater Calculations - Mathematics and Statistics for the Environment - page 685

24.7 ACTIVATED BIOSOLIDS
The activated biosolids process is an artificial process that mimics the natural self-purification
process that takes place in streams. In essence, we can state that the activated biosolids treatment
process is a “stream in a container.” In wastewater treatment, activated biosolids processes are
used for both secondary treatment and complete aerobic treatment without primary sedimentation.
Activated biosolids refers to biological treatment systems that use a suspended growth of organisms
to remove BOD and suspended solids.
The basic components of an activated biosolids sewage treatment system include an aeration
tank and a secondary basin, settling basin, or clarifier. Primary effluent is mixed with settled solids
recycled from the secondary clarifier and is then introduced into the aeration tank. Compressed air
is injected continuously into the mixture through porous diffusers located at the bottom of the tank,
usually along one side. Wastewater is fed continuously into an aerated tank, where the microor-
ganisms metabolize and biologically flocculate the organics. Microorganisms (activated biosolids)
are settled from the aerated mixed liquor under quiescent conditions in the final clarifier and are
returned to the aeration tank. Left uncontrolled, the number of organisms would eventually become
too great; therefore, some must periodically be removed (wasted). A portion of the concentrated sol-
ids from the bottom of the settling tank must be removed from the process (waste activated sludge,
or WAS). Clear supernatant from the final settling tank is the plant effluent.

24.7.1 a Ctivated b iosolids p roCess C ontrol C alCulations
As with other wastewater treatment unit processes, process control calculations are important tools
used by the operator to control and optimize process operations. In this section, we review many of
the most frequently used activated biosolids process calculations.

24.7.1.1 Moving Averages
When performing process control calculations, the use of a 7-day moving average is recommended.
The moving average is a mathematical method for leveling the impact of any one test result. The
moving average is determined by adding the test results collected during the past 7 days and divid-
ing by the number of tests:

Test 1Test2 + + ++ Test 3 … Test 7

Moving average = (24.30)

No.oftests performed in7days

■ EXAMPLE 24.39
Problem: Calculate the 7-day moving average for days 7, 8, and 9.

Day MLSS Day MLSS

1 3340 6 2780

2 2480 7 2476

3 2398 8 2756

4 2480 9 2655

5 2558 10 2396

Solution:

1. Moving average, day 7 = (3340 + 2480 + 2398 + 2480 + 2558 + 2780 + 2476)/7 = 2645

2. Moving average, day 8 = (2480 + 2398 + 2480 + 2558 + 2780 + 2476 + 2756)/7 = 2561

3. Moving average, day 9 = (2398 + 2480 + 2558 + 2780 + 2476 + 2756 + 2655)/7 = 2586