Environmental Engineering Reference
In-Depth Information
In water bodies, as plants and animals excrete wastes or die, the organic
phosphorus they contain sinks to the bottom, where bacterial decomposi-
tion converts it back to inorganic phosphorus, both dissolved and attached to
particles. This inorganic phosphorus gets back into the water column when
the bottom is stirred up by animals, human activity, chemical interactions,
or water currents. Then it is taken up by plants and the cycle begins again.
In a stream system, the phosphorus cycle tends to move phosphorus down-
stream as the current carries decomposing plant and animal tissue and dis-
solved phosphorus. It becomes stationary only when it is taken up by plants
or is bound to particles that settle to the bottom of ponds.
In the field of water quality chemistry, phosphorus is described by sev-
eral terms. Some of these terms are chemistry based (referring to chemically
based compounds), and others are methods based (they describe what is mea-
sured by a particular method). The term
orthophosphate
is a chemistry-based
term that refers to the phosphate molecule all by itself.
Reactive phosphorus
is
a corresponding method-based term that describes what is actually being
measured when the test for orthophosphate is being performed. Because the
lab procedure is not quite perfect, mostly orthophosphate is obtained along
with a small fraction of some other forms. More complex inorganic phos-
phate compounds are referred to as
condensed phosphates
or
polyphosphates
.
The method-based term for these forms is
acid hydrolyzable
.
Monitoring Phosphorus
Monitoring phosphorus is challenging because it involves measuring very
low concentrations—down to 0.01 mg/L or even lower. Even such very low
concentrations of phosphorus can have a dramatic impact on streams. Less
sensitive methods should be used only to identify serious problem areas.
Although many tests for phosphorus exist, only four are likely to be per-
formed by most monitors:
1. The
total orthophosphate
test is largely a measure of orthophosphate.
Because the sample is not filtered, the procedure measures both
dissolved and suspended orthophosphate. The USEPA-approved
method for measuring total orthophosphate is known as the
ascorbic
acid method
. Briefly, a reagent (either liquid or powder) containing
ascorbic acid and ammonium molybdate reacts with orthophos-
phate in the sample to form a blue compound. The intensity of the
blue color is directly proportional to the amount of orthophosphate
in the water.
2. The
total phosphorus
test measures all the forms of phosphorus in the
sample (orthophosphate, condensed phosphate, and organic phos-
phate) by first “digesting” (heating and acidifying) the sample to
convert all the other forms to orthophosphate. The orthophosphate
