Real-time, low-maintenance DOC monitoring using the Proteus

Proteus Instruments Launches New Proteus Range of Sensors

Proteus Instruments have launched the new Proteus range of sensors that enable the accurate real-time measurement of parameters that were previously difficult to measure at high frequency, including dissolved organic carbon (DOC), Biochemical Oxygen Demand (BOD), refined oils, and chlorophyll a, alongside traditional water quality parameters.

Background

DOC is operationally defined as the amount of organic carbon compounds that will pass through a 0.45 µm filter. In natural river systems, DOC quantity is of interest to river basin managers as shifts in concentration can alter nutrient supply, pH, light absorbance, and photochemistry. In addition, high DOC concentration has implications for drinking water treatment by affecting coagulant demand, filter backwashes and runtime, disinfectant dose, and the formation of disinfectant by-products (Trihalomethanes – THMs). Trihalomethanes may have long-term effects on health and should be considered when chlorinating drinking water high in DOC. However, to date, real-time DOC measurement has required expensive monitoring cabinets with high reagent costs and maintenance requirements. As a result, many industrial and environmental monitoring regimes have consisted of sporadic grab samples with subsequent laboratory analysis.

The Real-time Measurement Solution

Fluorescence spectroscopy is a selective and sensitive optical technique enabling in-situ, real-time measurement of dissolved organic matter. Molecules absorb light of a specific wavelength and orbiting electrons are excited to a higher energy state. The electrons then emit light of a specific wavelength to return to the base state. The use of fluorescence spectroscopy as a technique for rapid assessment of organic matter quantity, particularly DOC, has been widely used by academic researchers. A subset of the coloured dissolved organic matter (CDOM) pool is fluorescent (FDOM) and has a distinct fluorescence signal (Fig. 1). This peak is associated with humic and fluvic acids, by-products of microbial degradation of vascular plant material. The intensity of the FDOM fluorescence signal is strongly related to DOC concentration (Fig. 2).

The CDOM/FDOM peak is generally associated with excitation at ~365nm and emission at ~470nm, as shown in the red box on the excitation emission matrix (Fig 1). However, until recently, analysis of CDOM required sample collection and transport to a laboratory for analysis on expensive, large, power-hungry spectrofluorometers.

Over the last 3 years, RS Hydro has developed and rigorously tested the Proteus DOC, a fluorescence-based real-time DOC monitoring platform. By combining a miniaturized LED-based CDOM sensor, thermistor, and turbidity sensor, the Proteus DOC provides a highly accurate and reliable real-time indication of DOC.

The Proteus DOC is unique as it has a strong scientific underpinning with a research article in a leading international scientific journal (e.g., Khamis et al., 2017). In addition, the sensor includes robust correction algorithms to account for signal interference associated with temperature and turbidity variability, providing unrivaled accuracy and repeatability. The Proteus DOC also comes with a standard factory DOC calibration derived from installations across a diverse range of applications, which can then be tailored for specific monitoring sites for optimal accuracy. RS Hydro provides consultation on all aspects of installation and calibration, striving to provide optimal solutions for all clients. Furthermore, the Proteus DOC includes an integrated wiper for cleaning all optical windows, reducing the need for user intervention and ensuring a stable baseline for long-term deployments.

Applications

The Proteus DOC has been deployed for extended periods in urban river systems with wastewater drainage problems (e.g., CSOs and cross-connections). In these systems, the Proteus DOC accurately measures DOC during both base flow and storm flow conditions (see Figs 4 and 5). The sensor must be corrected for temperature quenching (reduction in signal) and turbidity to obtain strong relationships with laboratory DOC. Once established, excellent continuous records of DOC can be obtained (Fig. 5). The sensor is sensitive enough to detect daily oscillations of DOC associated with photodegradation, production, and uptake cycles.

In addition to urban applications, the Proteus DOC has been used on an inlet to a water treatment works, achieving excellent correlations between CDOM and TOC (Total Organic Carbon). Several research organizations have also purchased the sensor package for studying reactive organic matter dynamics. The Birmingham Institute of Forest Research (BIFOR) purchased two units for a long-term monitoring project, expressing satisfaction with the sensor's stability and accuracy. Dr. Phillip Blaen, BIFOR Research Fellow, states, “The low maintenance requirements due to the integrated wiper and infrequent calibration requirements make the Proteus ideal for long-term monitoring of organic matter.”

Added Benefits of the Proteus DOC

• Real-time data – easy integration with telemetry and SCADA for alarms/alerts.
• Easily deployable – suitable for a wide range of environments (e.g., inlets, effluents, reservoirs, boreholes).
• Versatile – modular design allows concurrent recording of additional parameters (e.g., Temperature, turbidity, pH, conductivity, optical DO, chlorophyll a, ORP, ammonia).
• Portable – Bluetooth option available.
• Sensitive – detects very low concentrations for use in clean water systems.
• Low maintenance – self-cleaning with automatic wiper, requiring simple calibration only every 6–12 months.

Numerous Applications

• Monitoring effluents for compliance
• Environmental monitoring
• Monitoring inflow to water treatment works
• Research tool for academia

References

1. Lee E-J, Yoo G-Y, Jeong Y, Kim K-U, Park J-H, Oh N-H. 2015. Comparison of UV–VIS and FDOM sensors for in situ monitoring of stream DOC concentrations. Biogeosciences 12: 3109–3118. DOI: 10.5194/bg-12-3109-2015.

2. Khamis K, Bradley C, Stevens R, Hannah DM. 2017. Continuous field estimation of dissolved organic carbon concentration and biochemical oxygen demand using dual-wavelength fluorescence, turbidity, and temperature. Hydrological Processes 31: 540–555. DOI: 10.1002/hyp.11040.