Activated Sludge Process - Wastewater Operators - page 294

mean cell residence time (mCrT) —The average length of time particles
of mixed liquor suspended solids remains in the activated sludge
process; may also be referred to as sludge retention time (SrT) :

SolidsinActivated Sludge Proc ess (lb)

Solids RemovedfromProcess (lb/day)

MCRT (days) =

mixed liquor —The contribution of return activated sludge and waste-
water (either influent or primary effluent) that flows into the aera-
tion tank.

mixed liquor suspended solids (mlSS)— The suspended solids con-
centration of the mixed liquor. Many references use this concentra-
tion to represent the amount of organisms in the liquor.

mixed liquor volatile suspended solids (mlvSS) —The organic matter
in the mixed liquor suspended solids; can also be used to represent
the amount of organisms in the process.

nematodes— Microscopic worms that may appear in biological waste
treatment systems.

nutrients— Substances required to support plant organisms. Major
nutrients are carbon, hydrogen, oxygen, sulfur, nitrogen, and
phosphorus.

Protozoa— Single-cell animals that are easily observed under the
microscope at a magnification of 100×. Bacteria and algae are prime
sources of food for advanced forms of protozoa.

return activated sludge (raS)— The solids returned from the settling
tank to the head of the aeration tank.

rising sludge— Occurs in the secondary clarifiers or activated sludge
plant when the sludge settles to the bottom of the clarifier, is com-
pacted, and then rises to the surface in a relatively short time.

rotifers— Multicellular animals with flexible bodies and cilia near
their mouths used to attract food. Bacteria and algae are their
major source of food.

Secondary treatment— A wastewater treatment process used to
convert dissolved or suspended materials into a form that can be
removed.

Settleability— A process control test used to evaluate the settling
characteristics of the activated sludge. Readings taken at 30 to 60
minutes are used to calculate the settled sludge volume (SSV) and
the sludge volume index (SVI).

Settled sludge volume (SSv)— The volume (mL/L or percent) occu-
pied by an activated sludge sample after 30 or 60 min of settling.
Normally written as SSV with a subscript to indicate the time of the
reading used for calculation (e.g., SSV 30 or SSV 60 ).
Shock load— The arrival at a plant of a waste toxic to organisms, in
sufficient quantity or strength to cause operating problems, such
as odor or sloughing off of the growth of slime on the trickling filter
media. Organic overloads also can cause a shock load.