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David M. Struve, SFWMD, Meifang Zhou, SFWMD, Tom Baber, Litkenhaus AssociatesABSTRACT
A remote P analyzer has been designed and optimized using UV/thermal induced
persulfate digestion of total phosphorus in water and ascorbic acid-
phosphomolybdenum blue method for TP and TRP determination. The analytical
parameters were remotely monitored and the data were remotely processed. This
remote P analyzer successfully provided the near real-time TP and TRP data of the
water at the outlet of a STA of the SFWMD. The P concentration results from this
analyzer were comparable with the lab analysis and the method detection limit was
also similar (4 g/L) to the lab method. The diurnal fluctuation of TP and TRP
concentrations in the water at the STA was observed. Not only may this analyzer be
very useful for optimizing the operation of the STA, but also has the potential
application in surface water P TMDL calculation and monitoring.
INTRODUCTION
The traditional method of measuring P concentration in water involves field grab
sampling with subsequent lab analysis. As more STA’s are being constructed for
protecting the ecosystem and the TMDL concept is being adopted for various water
bodies, it is increasingly important to be able to obtain near real-time P
concentration data for optimization of the STA operation and P budget studies.
MATERIALS AND METHODS
RESULTS AND DISCUSSION
The remote TP analyzer is a batch system that processes only one sample at a time. The
water sample was digested with persulfate /H2SO4 without neutralization. The final optimum
acid concentration ([H]) and [H]/[Mo] were 0.4 N and 75 respectively. The reaction time
(RT) was optimized by determining the FCD time at different P concentration. The FCD time
increased as the phosphorus concentration decreased (Table 1). Ten minutes RT was selected
to insure the FCD of the low P concentration water samples.
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[P],
g
/L
Grab TPO4 TPO4TRP, unbackground color corrected Grab OPO4 8 per. Mov. Avg. (TPO4) 8 per. Mov. Avg. (TRP, unbackground color corrected)2 per. Mov. Avg. (Grab TPO4 ) 2 per. Mov. Avg. (Grab OPO4 )
Figure 5 Figure 6Figure 3
Figure 4Figure 7
Analyzer Monitoring and Data Process: The test scheduler, analytical parameters (such as the
digestor and cabinet temperatures, the responses of the standards and samples, and the level
of reagents and waste in the bottles) and data were remotely monitored or processed.
Instrument Location and Site: The instrument
was located at the outlet of the STA1W just
upstream of the outflow pump station G310.
Reagents: The standard curves were prepared
from the reference P standard solution. The
60ppb QC sample was prepared from phytic
acid and a reference standard. The color
reagents were prepared according to our lab
study and the Standard Method.
Full color development (FCD) time: FCD time
was determined using the Time Drive program
at 880 nm in the lab.
The P concentration dynamic in the outflow water during small and large pulse
flows at the STA were quite different, and the P dynamic at medium pulse flow
was somewhat similar to the large pulse flow (Figure 3, 4, 5). When the water flow
rate was nearly constant, a diurnal fluctuation of total phosphorus with the
maximum near midday and the minimum near midnight in the water was observed
(Figure 6). The different P concentration dynamics with different water flow
patterns may be a combination effect of residence time in the STA and the diurnal
fluctuation of the TP in water (Figure 3, 4, 5). When the water flow was stopped,
the TP concentration in the canal water gradually decreased and exhibited almost
no daily fluctuation (Figure 7). When the water flow rate was increased quickly
from 0 to near 2100 ft3/s, the TP concentration spiked and then decreased to
typical levels.
Table 1. The full color development time (FCDT) and the methodreaction time (MRT).
10 ppb P 32 ppb P 64 ppb P sampleFCDT (min) 4.90.2a 3.60.3a 3.20.1a -MRT (min) - - - 10
a: n = 2
The TP recovery of a 60 ppb QC sample was 95% (Table 2). The TP, TRP and MDL of the
field method were very similar to those of the lab method (Table 2). The responses of the
standards of the 642 sets of standard curves were very stable. The linear correlation
coefficient of the standard curves was higher than the lab QA/QC requirement of 0.995.
The TP and TRP results from the field and lab methods are shown in Figure 1 and the hourly
mean water flow rates of the STA outlet are plotted in Figure 2. The overall TP and TRP
results from field and lab methods were very similar. However, the field method showed the
all the detail of the P concentration dynamics in the STA effluent water.
In order to examine the effect of flow rate on TP concentration in outflow water of the STA,
five sections of the TP results were selected according to the flow patterns, such as small
pulse, medium pulse, large pulse, constant, and no water flow out of the STA (Figure 1, 2,
3, 4, 5, 6, 7).
Table 2. The method performance (MDL, STD curve correlationcoefficient, and SD)
Method TPa TRP MDL ppb
r2
Field 571.3b 361.1b 3-4 0.99960.0016c
Lab 57 37 2-4 -a: 60 ppb organic and inorganic P QC sample; b: MDL of the fieldmethod was estimated from results of the QC sample; c: n = 642
CONCLUSION
The near real time TP data from the inlet
and outlet of the STA will enhance
optimization of STA operations. The high
temporal resolution data can be used to
refine/develop/calibrate the models for
STA design/operation, and will also be
very useful for TP loading and TMDL
calculations from storm water runoff.
Additionally, the field TP analyzer can provide a tool for research on the dynamic
biogeochemistry of the P in the ecosystem.
Figure 1. TP and TRP results from field and lab method Figure 2. Water flow rate at the outlet of STA1W
Figure 3. TP results at small pulse water flow Figure 4. TP results at medium pulse water flow
Figure 5. TP results at large pulse water flow Figure 6. TP results at near constant water flow
Figure 7. TP results at near zero water flow
Digestion and analysis: The sample was digested
sequentially for 37 minutes via thermal digestion at
90 C and 30 minutes of UV digestion with
persulfate/H2SO4. After 10 minutes of color
development in the reaction chamber, the colorintensity of the sample was measured using a minispectrophotometer.
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