Water quality monitoring system enabled by optical MEMS technology

by John Williamson
Contributing Editor, MEMS Investor Journal

Real-time detection of degradation in the quality of public water supplies currently relies on bulky monitoring stations which typically contain a range of traditional, non-MEMS sensors and also require use of chemicals and continual maintenance.  This makes current systems too expensive to provide comprehensive monitoring coverage for an entire water distribution network, according to Optiqua Technologies, a company based in Singapore and The Netherlands.

To address this problem, Optiqua has developed a compact integrated early warning system for water quality.  The system features sensor nodes that are of low cost and high durability and therefore provide feasible economics to create a high density monitoring network.

Schematic showing the structure and main working principle of the sensor;   (1) wave guides (4 µm width) edged in chip surface are a path for laser light; (2) top cladding is removed to expose sensing window to sample.  Refractive index (RI) changes cause phase shift in laser light; (3) light beam is split in two paths; inference between sensing beam and reference beam reveals RI-induced phase difference; (4) thermo-optic phase modulator enables accurate measurement of the phase difference, which is directly proportional to RI change.

Optiqua’s lab-on-a-chip refractive index sensor detects small concentrations of contaminants in public water supplies.

The system incorporates lab-on-a-chip sensors and data transmission module and can send quality monitoring alerts to drinking water utilities in real time at a fraction of the cost of traditional monitoring systems.  Optiqua is currently continuing its system development work with their partners –- PUB, Singapore's national water agency, and Vitens, the Netherlands’ largest water supplier.

Real-time water quality data is acquired directly from the distribution network using online sensors.  Low cost and high durability allow for an economically viable network covering the entire distribution grid.  With optimized sensor placement and intelligent data analysis software, detection of contaminations close to the source is guaranteed, minimizing incident response times and recovery costs.

Optiqua’s technology is based on the Mach-Zehnder interferometer principle, according to Pieter Brascamp, the company’s product development manager in Singapore.  He explains, “[Our] modules are positioned at many places along the water distribution system to draw continual samples for analysis.  A silicon chip, about 4 by 60 millimeters in size, is at the core of these modules.  Laser light is directed through waveguides in the chip.  A sensing window in one of the waveguides is exposed to the water sample. Changes in refractive index cause a phase difference between the two light paths, which is directly proportional to the refractive index change.”

The sensor can be attached to any drainage point using a bypass system.  An integrated flow chamber guides water to sensor surface.  Flexible electronic and optical fibers allow versatile installation, while small external electronics unit allows sensor to be placed in remote positions.

An “event” is signaled when sensors detect a deviation from background signals, which are created by regularly occurring natural variations in the waters’ composition.  These patterns, for example diurnal and seasonal variations, are stored as historical baseline information.

“Every chemical substance has a specific refractive index,” Brascamp continues. “Our sensor can respond to all relevant chemical contaminants, and has the sensitivity to measure index changes caused by contaminants at the parts per million level.”  He adds, “Rather than identifying a specific contaminant, [our sensors] instead signal a condition that falls outside the natural baseline data.  Water company personnel can then respond swiftly, exactly to the affected area, to conduct a thorough analysis. ”

Communications between the monitoring stations and headquarters depend on the water company’s requirements and available infrastructure.  “Right now we are focusing on water quality monitoring in urban areas, where a combination between wired and wireless communications, such as general packet radio service,  is the most likely option,” Brascamp says.  “Alert displays also depend on customer requirements and can be integrated into existing supervisory control and data acquisition, or SCADA, systems.  For this, we are collaborating with our technology partner Schneider Electric, world leader in SCADA systems.”

Although Optiqua’s sensors units are already in place for testing in Singapore and the Netherlands, work at the company continues to optimize performance, packaging and service life.  “Sensor lifetime, especially in a high-density urban water system, is important,” Brascamp says.  “Since our sensors use no chemicals or other disposables, they are able to function continually for a long time before replacement.”

Copyright 2010 MEMS Investor Journal

Comments

[PRAKASH NARAIN TANDON]

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