Solids retention time (SRT): The theoretical length of time, usually in days, that
solids are retained in an aeration basin, clarifier, or other structure. SRT is used to
calculate wasting rates.
Soluble BOD (SBOD): Soluble BOD is a sample that has been filtered through a
0.45 μm filter. Soluble BOD includes volatile fatty acids (VFAs) and organic
material that will readily ferment to create more VFAs in an anaerobic selector.
Soluble: Dissolved in a solution and more readily available for biochemical or
chemical reactions. For instance, soluble BOD or soluble COD are readily
available as food to microorganisms in a biochemical reaction. Soluble BOD
includes volatile fatty acids (VFAs) and organic material that will readily be
available to microorganisms in a biological process.
Step aeration activated sludge process: It is an improved plug flow activated
process system in which (a) return activated sludge (RAS) enters the head end of
the aeration tank with a portion of the wastewater influent, (b) the wastewater
influent piping is so arranged that an increment of sewage is discharged into the
aeration tank (3–5 hours DT) at subsequent steps. In doing this, the waste load is
more uniformly spread over the length of the aeration tank, resulting in better
utilization of the oxygen supplied, and the biological process system is much less
susceptible to shock load, and (c) the biological sludge is maintained highly
active throughout the aeration tank so that more organics are removed in a shorter
contact time.
Struvite: It is magnesium ammonium phosphate (MgNH 4 PO 4 (H 2 O) 6 ). Struvite
forms hard, very insoluble, white, yellowish-white, or brownish-white crystals.
Subnatant: The water or liquid remaining below a floated sludge (float) after
flotation.
Substrate: The food or chemical substance on which organisms depend for growth.
The organic matter in wastewater (as measured by the BOD 5 test) is a substrate
for the microorganisms in a biological process. For instance, in biological phosphorus removal systems, volatile fatty acids are a readily available substrate used
by phosphorus-accumulating organisms (PAOs).
Supernatant: The water or liquid remaining above a sediment or precipitate after
sedimentation.
Supracell: It is a circular dissolved air flotation (DAF) clarifier designed by the
Lenox Institute of Water Technology (LIWT) and manufactured by Krofta
Engineering Corporation (KEC).
Suspended-growth biological processes: In a suspended-growth biological process system, biomass or microorganisms are suspended in the biological reactors
for aerobic and/or anoxic biochemical reactions. They include all kinds of
activated sludge processes as well as oxidation ditch process, such as
(a) conventional activated sludge process using diffused aeration;
(b) conventional activated sludge process using mechanical aeration; (c) highrate activated sludge process using diffused aeration; (d) pure oxygen activated
sludge process using covered reactor; (e) pure oxygen activated sludge process
using uncovered reactor; (f) extended aeration activated sludge process with
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159
solids are retained in an aeration basin, clarifier, or other structure. SRT is used to
calculate wasting rates.
Soluble BOD (SBOD): Soluble BOD is a sample that has been filtered through a
0.45 μm filter. Soluble BOD includes volatile fatty acids (VFAs) and organic
material that will readily ferment to create more VFAs in an anaerobic selector.
Soluble: Dissolved in a solution and more readily available for biochemical or
chemical reactions. For instance, soluble BOD or soluble COD are readily
available as food to microorganisms in a biochemical reaction. Soluble BOD
includes volatile fatty acids (VFAs) and organic material that will readily be
available to microorganisms in a biological process.
Step aeration activated sludge process: It is an improved plug flow activated
process system in which (a) return activated sludge (RAS) enters the head end of
the aeration tank with a portion of the wastewater influent, (b) the wastewater
influent piping is so arranged that an increment of sewage is discharged into the
aeration tank (3–5 hours DT) at subsequent steps. In doing this, the waste load is
more uniformly spread over the length of the aeration tank, resulting in better
utilization of the oxygen supplied, and the biological process system is much less
susceptible to shock load, and (c) the biological sludge is maintained highly
active throughout the aeration tank so that more organics are removed in a shorter
contact time.
Struvite: It is magnesium ammonium phosphate (MgNH 4 PO 4 (H 2 O) 6 ). Struvite
forms hard, very insoluble, white, yellowish-white, or brownish-white crystals.
Subnatant: The water or liquid remaining below a floated sludge (float) after
flotation.
Substrate: The food or chemical substance on which organisms depend for growth.
The organic matter in wastewater (as measured by the BOD 5 test) is a substrate
for the microorganisms in a biological process. For instance, in biological phosphorus removal systems, volatile fatty acids are a readily available substrate used
by phosphorus-accumulating organisms (PAOs).
Supernatant: The water or liquid remaining above a sediment or precipitate after
sedimentation.
Supracell: It is a circular dissolved air flotation (DAF) clarifier designed by the
Lenox Institute of Water Technology (LIWT) and manufactured by Krofta
Engineering Corporation (KEC).
Suspended-growth biological processes: In a suspended-growth biological process system, biomass or microorganisms are suspended in the biological reactors
for aerobic and/or anoxic biochemical reactions. They include all kinds of
activated sludge processes as well as oxidation ditch process, such as
(a) conventional activated sludge process using diffused aeration;
(b) conventional activated sludge process using mechanical aeration; (c) highrate activated sludge process using diffused aeration; (d) pure oxygen activated
sludge process using covered reactor; (e) pure oxygen activated sludge process
using uncovered reactor; (f) extended aeration activated sludge process with
3 Biological Processes
159