Hach-Lange WTOS P-RTC - Manual (Page 31)

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Parameterization and operation
4.5 Explanations
4.5.1 Ortho-phosphate and total phosphate
The goal of phosphate control is to reduce the total phosphate in
the effluent stream at a waste water treatment plant. However,
the precipitation only affects the ortho-phosphate content. The
PO
4
-P target value specifies the ortho-phosphate value to be
maintained in the precipitation reactor. Therefore, this value must
be lower than the value to be maintained in the effluent.
Note: If CLOSED-LOOP is set in the controller, the setpoint is effective
immediately. If OPEN-LOOP is set in the controller, the desired PO
4
-P
effluent concentration of the reactor is required.
4.5.2 Recycling the chemical sludge
Precipitant savings can be achieved, particularly in activated
sludge plants, if existing chemical sludge is reused for
phosphorus elimination. In the case of precipitant overdosage,
the stoichiometrically excessive metal content (for metal
phosphate formation) is reused in the formation of metal
phosphate upon renewed contact with dissolved phosphate.
Renewed adsorption of phosphate, by precipitates already
formed, is possible. The existing precipitant·sludge portion thus
represents a reserve for the precipitation of P-influent peaks or
for a short-term failure of the dosing devices; however, other
authors consider this reserve to be minimal.
The precipitant savings automatically take effect for closed-loop
controls. To be able to use the reserve for open-loop controls
as well, the correction (
CORR FACTOR) ) can be used. A negative
input reduces the precipitant dosing:
Example:
A correction value of –50 % is set.
This reduces the precipitant dosing by half.
In the event of negative influences on the phosphate
precipitation, a positive correction value can be entered:
Example:
A correction value of +100 % is set.
This doubles the precipitant dosing.
4.5.3 Biological phosphate elimination
The BIO-P parameter is used to take biological phosphate
elimination into account. If no phosphate elimination is effective at
the phosphate measuring point, the
BIO-P factor can be used to
specify the percentage of influent phosphates that is biologically
incorporated into the sludge on an empirical basis. In this case, it
is important to differentiate between the measurement of
ortho-phosphate and total phosphate. For total phosphate, the
"unavoidable" biological phosphate elimination must always be
taken into account. This can be set at 1 % of the BOD
5
. An
increased biological phosphate elimination can be added in all
cases and must be estimated on the basis of empirical values.