Dissolved/Total Organic Carbon Sensors

Dissolved/Total Organic Carbon Sensors

Background
Dissolved Organic Carbon (DOC) is operationally defined as the amount of organic carbon based compounds that can pass through a 0.45 µm filter. Measurement is usually conducted in the laboratory using expensive benchtop analysers that oxidize organic carbon in the water sample to form carbon dioxide. There are two different methods for the oxidation of organic carbon to CO2: (1)combustion in an oxidizing gas; and (2) UV or heat driven chemical oxidation with a persulfate solution. A conductivity detector or an infrared detector then detects the released CO. Unfortunately these techniques require expensive and power hungry laboratory equipment with high reagent costs.


Why is it important
In natural river systems, the quantity of DOC is of interest to river basin managers as shifts in concentration can alter nutrient levels,pH, light absorbance and photochemistry of the river system. In addition, high DOC concentration poses many problems for drinking water treatment. Inparticular, it can influence coagulant demand, filter backwash runtime,disinfectant dose and the formation rate of disinfectant by-products(Trihalomethanes – THMs). THMs have long-term negative implications for health and formation potential is a critical consideration when chlorinating drinking water high in DOC. Furthermore, high DOC can lead to water discolouration  However, to date real-time DOC measurement hasrequired expensive monitoring cabinets with high maintenance requirements and reagents costs. Hence, many industrial and environmental monitoring regimes have been comprised of sporadic grab samples with analysis subsequently undertaken in a laboratory. 


Challenges associated with DOC monitoring
Despite the test being entrenched in legislation there are numerous problems and challenges associated with use of the test:

  • There is a lag until results are available (transportation to lab + analytical test time 1h),hence damage can occur before the data is available;
  • The test require expensive laboratory equipment;
  • The test involves dangerous chemicals that need careful disposal and are potentially harmful to operators;

It is clear that a move from traditional laboratory testing to in-situ (real-time) monitoring would help to alleviate some of the problems outlined above. It would immediately address points I - III and would help to improve spatial temporal resolution of monitoring that would be directly beneficial to basin managers, water companies and legislators alike. 


Proteus the real-time solution for DOC monitoring
The Proteus is a new product launched by Proteus instruments providing users with a robust,repeatable, low maintenance sensor platform for measuring DOC in real-time. The The Proteus is underpinned by comprehensive research exploring the use of in-situ fluorescence as a technique for real-time DOC measurement. The Proteus for DOC monitoring (See Fig. 1) is a multi-parameter instrument that incorporates a CDOM fluorometer, turbidity sensor and thermistor to provide real time measurement of the bulk dissolved organic load, negating the need for couriers and laboratory analysis. Using a robust correction algorithm the CDOM signal is corrected, in real time, for temperature interference. The result is a repeatable measurement that can provide instantaneous DOC measurement with a simple site specific calibration for turbidity and CDOM relationship with DOC (Fig. 3). 


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 dissolved organic matter pool can be mapped in optical spaced based on its fluorescent properties (see Fig. 2). A subset of the coloured dissolved organic matter (CDOM) pool is fluorescent and has a distinct fluorescence peak (Fig.2). This peak is primarily associated with humic and fluvic acids, the by-products of microbial degradation of vascular plant material. The intensity of the CDOM fluorescence signal, often reported in quinine sulphate (QSE)units, is strongly correlated to DOC concentration and TOC. Numerous published studies from a wide range of geographical locations have correlated CDOM fluorescence with DOC and our site-specific calibrations can provide users with accurate and highly repeatable measurements (see Fig. 3).


Applications

  • Catchment monitoring (upland / peatland)
  • Assessing organic load through water treatment works
  • Process control - Filter management and coagulation control
  • Monitoring disinfection by-product formation potential
  • Monitoring raw water intake