Smartphone-based turbidity reader

Abstract

Water quality is undergoing significant deterioration due to bacteria, pollutants and other harmful particles, damaging aquatic life and lowering the quality of drinking water. It is, therefore, important to be able to rapidly and accurately measure water quality in a cost-effective manner using e.g., a turbidimeter. Turbidimeters typically use different illumination angles to measure the scattering and transmittance of light through a sample and translate these readings into a measurement based on the standard nephelometric turbidity unit (NTU). Traditional turbidimeters have high sensitivity and specificity, but they are not field-portable and require electricity to operate in field settings. Here we present a field-portable and cost effective turbidimeter that is based on a smartphone. This mobile turbidimeter contains an opto-mechanical attachment coupled to the rear camera of the smartphone, which contains two white light-emitting-diodes to illuminate the water sample, optical fibers to transmit the light collected from the sample to the camera, an external lens for image formation, and diffusers for uniform illumination of the sample. Including the smartphone, this cost-effective device weighs only ~350 g. In our mobile turbidimeter design, we combined two illumination approaches: transmittance, in which the optical fibers were placed directly below the sample cuvette at 180° with respect to the light source, and nephelometry in which the optical fibers were placed on the sides of the sample cuvette at a 90^° angle with respect to the to the light source. Images of the end facets of these fiber optic cables were captured using the smart phone and processed using a custom written image processing algorithm to automatically quantify the turbidity of each sample. Using transmittance and nephelometric readings, our mobile turbidimeter achieved accurate measurements over a large dynamic range, from 0.3 NTU to 2000 NTU. The accurate performance of our smartphone-based turbidimeter was also confirmed with various water samples collected in Los Angeles (USA), bacteria spiked water samples, as well as diesel fuel contaminated water samples. Having a detection limit of ~0.3 NTU, this cost-effective smartphone-based turbidimeter can be a useful analytical tool for screening of water quality in resource limited settings.

Figure 1

Smartphone based turbidimeter. (a) A photograph of the device. (b) Schematic of the device. (c) Schematic of the illumination path and the locations of the optical fibers for two illumination modes (i.e., nephelometry and transmittance).

Figure 2

Turbidity measurement steps using a custom developed smartphone application. (a) Main menu of the application. It shows four buttons that allow the user to capture an image, select an image, view history or exit the application (b) A screenshot of the application after two raw format images from the photo library are selected. (c) After assigning a job name to a measurement (d) the selected images are ready to be uploaded and processed at our servers. (e) The warning box that displays the screen after the images are uploaded to the servers. (f) The application informs the user about the status of the images at the server. (g) Image processing steps performed at the server. (h) The result of a turbidity measurement. It displays the turbidity level in NTU, along with the GPS location of where the image was captured at, the upload time and the image name.

Figure 3

Images of the end facets of the fiber optic cables captured using the portable turbidimeter at different water turbidity levels: (a) 0.05 NTU, (b) 15 NTU, (c) 320 NTU, and (d) 750 NTU at an exposure time of 0.8 s. Yellow boxes show the nephelometric fibers and red box shows the transmittance fibers.

Figure 4

Measured intensities of turbid water samples using (a) nephelometry and (c) transmittance modes of our mobile turbidimeter. (b) Zoomed in version of the red box area in (a). (d) Zoomed in version of the red box area in (c).

Figure 5

(a) Turbidity measurements using the smartphone-based turbidimeter compared against the measurements of an EPA-certified benchtop turbidimeter. (b) Zoomed in version of the red box area in (a).

Figure 6

Field testing. (a) Turbidity of water samples taken from different sources. (b) Turbidity of non-potable water samples spiked with diesel fuel at different concentrations. At both images, blue bar demonstrates the turbidity measured using the smartphone turbidimeter and red bar demonstrated the turbidity measured using the benchtop turbidimeter.

Figure 7

(a) Turbidity measured using the smartphone turbidimeter as a function of E. coli concentration. (b) Zoomed in version of (a) for low concentrations of bacteria. (c) Turbidity measured using the smartphone turbidimeter as a function of ampicillin concentration. (d) Zoomed in version of (c) for 0–8 mg/L of ampicillin concentration. (e) Turbidity measured using the smartphone turbidimeter as a function of cephalexin concentration. (f) Zoomed in version of (e) for 0–20 mg/L of cephalexin.