monitoring of phosphate levels in wastewater using an autonomous ...

MONITORING OF PHOSPHATE LEVELS IN WASTEWATER USING AN AUTONOMOUS MICROFLUIDIC SENSOR

J. Cleary1, C. Slater1, D. Kim1, W.S. Yerazunis2 and D. Diamond1 1

National Centre for Sensor Research, Dublin City University, IRELAND, 2 Mitsubishi Electric Research Laboratories, USA.

ABSTRACT An autonomous sensor for monitoring phosphate levels in aqueous samples has been developed. The sensor is based on the molybdenum yellow method for detection of phosphate, which involves the formation of a yellow-coloured compound when a phosphate-containing sample is mixed with an acidic reagent containing ammonium molybdate and ammonium metavanadate. The reaction is carried out in a microfluidic chip and the colour change is detected using a LED (light-emitting diode) light source and a photodiode detector. The sensor has been applied to the analysis of effluent at a municipal wastewater treatment plant in Co. Kildare, Ireland. KEYWORDS: Microfluidic, Sensor, Autonomous, Phosphate INTRODUCTION Monitoring of phosphate levels in environmental waters is a priority for monitoring agencies in many countries due to its importance as a contributor to nutrient pollution (eutrophication) of rivers, lakes and coastal waters. Phosphate pollution originates from mainly anthropogenic sources including agriculture (fertilizers, runoff from animal waste), industry, and municipal wastewater discharges. A field-deployable system for long-term monitoring of phosphate levels in water has been developed, incorporating sampling, pumping, reagent and waste storage, optical detection, and wireless communications into a robust and portable device [1]. A GSM modem allows wireless, remote control of the system and retrieval of collected data. In this paper the results of a trial in which the phosphate sensor was applied to the analysis of final effluent at a municipal wastewater treatment plant in Co. Kildare, Ireland, are described. The trial was carried out over a 23-day period in February and March 2008, and the data collected by the phosphate sensor was compared to that collected by the plant’s existing online phosphorus monitor. SENSOR DESIGN The sensor is based on the yellow method for phosphate detection, a colorimetric technique involving the formation of vanadomolybdophosphoric acid when a phosphate-containing sample is mixed in a 1:1 ratio with an acidic reagent containing ammonium molybdate and ammonium metavanadate [2]. The resulting solution is yellow and absorbs strongly below 400 nm. The absorbance is proportional to the concentration of phosphate in the original sample. Figure 1 illustrates the design of the microfluidic chip where the colorimetric reaction and detection are carried out. The chip is manufactured from PMMA (poly methyl methacrylate) sheet (Goodfellow, England). 200 µm channels are milled into the sheet using a CAT-3D-M6 Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences October 12 - 16, 2008, San Diego, California, USA 978-0-9798064-1-4/µTAS2008/$20©2008CBMS

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micromill (DATRON Electronic, Germany), and the chip layers are sealed using a pressure sensitive adhesive (ARcare® 8890, Adhesives Research, Ireland). PEEK (poly ether ether ketone) tubes are inserted into the inlet and outlet ports and are held in place using epoxy adhesive. The optical measurement is carried out by aligning a 370 nm LED light source (NSHU550E, Nichia Corporation, Japan) and a photodiode detector (S1227-33BR, Hamamatsu Photonics Ltd., UK) at opposite ends of the optical cuvette which is 5 mm in length and 1 mm in diameter. The chip dimensions (excluding PEEK tubing) are 35 x 20 x 7 mm (L x W x H).

Figure 1. Schematic and photograph of the microfluidic chip used in the phosphate sensor. The inlets are used to deliver sample (1), reagent (4), calibration solutions (2 and 5) and cleaning solution (3 and 6) to the chip. Waste exits the chip via the outlet (7) and the optical detection is carried out at the optical cuvette (8). Figure 2 shows the complete sensing system in situ during the field trial. Sample was diverted from the line delivering treated effluent to the wastewater plant’s online phosphorus monitor (Aztec P100, Severn Trent Services, UK) and sample was drawn through a membrane filter (0.45 µm pore size) and into the sensor itself using one of the solenoid pumps.

Figure 2. Photograph of the phosphate sensor during the trial. Visible components of the system include: ruggedised enclosure (1), microfluidic chip holder (2), GSM modem (3), control and memory board (4), sample inlet filter (5) and sample collection for validation purposes (6).

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The system contains bottles for storing the reagent, calibration solutions and cleaner; a sample port for collecting the water sample to be analysed; an array of solenoid pumps for pumping the required liquids through the microfluidic chip, and a 12V, 7Ah lead-acid battery which powers the system. The microfluidic chip allows for the mixing of reagent and sample, and also presents the reacted sample to the LED and photodiode for an absorbance measurement. The analyzed sample is then pumped to the waste storage container. A two point calibration procedure is carried out for each sample measured. All of the fluid handling and analytical components are controlled by a microcontroller that also performs the data acquisition and stores the data in a flash memory unit. A GSM modem (Siemens MC35i, Radionics Ltd., Ireland) is used to communicate the data via SMS protocol to a laptop computer. The complete system is contained within a rugged, watertight and airtight enclosure. RESULTS AND DISCUSSION Figure 3 shows output from the prototype phosphate sensor, and from the wastewater treatment plant’s existing online monitor.

Figure 3. Output from the prototype sensor and the existing phosphorus monitor at the wastewater treatment plant. Numbered features on the graph refer to: occasional discrepancies due to bubble interference (1), periods where the plant monitor malfunctioned (2), tracking of an event where low-quality effluent was released with elevated phosphate levels detected by both the prototype sensor and the plant monitor (3), and a calibration run using 10 mgL-1 standard phosphate solution (4).


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The sensor operated for 23 days with a measurement frequency of 1 sample/45 mins and the total number of measurements performed during the trial was 675. For the sake of clarity only the first 400 measurements are shown in Figure 3. Figure 3 shows that the data obtained from the prototype phosphate sensor was generally in very good agreement with that obtained from the existing online monitoring system. A number of discrepancies (feature 1 in Figure 3) were observed during the trial due to interference by minute air bubbles within the microfluidic chip. These occurred due to imperfect connections in the sample line and interfered with the passage of light through the optical cuvette, giving rise to spurious peaks in the phosphate levels. Despite these occasional interferences, the overall reliability of the prototype sensor exceeded that of the plant’s online monitor during the period in question, as the latter malfunctioned on a number of occasions (feature 2 in Figure 3). While the phosphate concentrations in the effluent were generally between 1 and 5 mgL-1, effluent of lower quality (i.e. higher phosphate concentration) was released over a period spanning measurements 280 – 310 approx. This event (feature 3 in Figure 3) was recorded by both the prototype phosphate sensor and the plant’s existing online monitoring system. The prototype sensor automatically performs a two-point calibration procedure, using 0 and 10 mgL-1 phosphate standard solutions, for each sample measurement. This is to compensate for the effect of diurnal temperature variations, as the detector output is slightly affected by temperature. The calibration also compensates for any drift occurring, or for possible decrease in the light intensity emitted by the LED, over longer periods. A seperate calibration run was also carried out during the trial by introducing a 10 mgL-1 phosphate solution through the sample inlet. The resulting output (feature 4 in Figure 3) was in good agreement with the known value of the phosphate solution. CONCLUSIONS An autonomous phosphate sensor has been developed by combining colorimetric reagent chemistry with microfluidic technology and a low-powered LED/photodiode detection system. In this paper we have shown that the sensor can be successfully applied to the analysis of municipal wastewater treatment plant discharge over extended periods. Further optimisation of the system to improve the detection limit (currently 0.3 mgL-1) and to eliminate interference due to bubbles is ongoing. The sensor is expected to have significant applications in the area of environmental water quality monitoring due to its low power consumption and long deployable lifetime. ACKNOWLEDGEMENTS The authors would like to acknowledge the cooperation and assistance of personnel at the wastewater treatment plant where the sensor trial was carried out. REFERENCES [1] C. McGraw, S. Stitzel, J. Cleary, C. Slater, and D. Diamond, Autonomous microfluidic system for phosphate detection. Talanta 71, pp. 1180-1185 (2007). [2] M. Sequeira, M. Bowden, E. Minogue and D. Diamond, Towards autonomous environmental monitoring systems, Talanta 56, pp. 355-363 (2002).


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