Dissolved Organic Carbon (DOC) is a term used to refer to organic carbon that is sized between 0.22 µm and 0.7 µm.
DOC in aquatic environments can come from autochthonous (produced within the environment) and allochthonous (produced external to the aquatic environment) sources. Allochthonous origins include humus produced during plant and leaf litter decomposition as well as soil OM decomposition. It can be washed into the water source via overland flow or through leaching.
Within aquatic systems, DOC can be generated from aquatic plants and algae through respiration. DOC in water can also originate from humus produced as a result of plant and leaf litter decomposition.
DOC encompasses a range of substances from the simplistic to more complex, including humic and fulvic acids.
Total Organic Carbon (TOC) refers to the sum of the organic carbon in the water, considering both particulate and dissolved forms. By ‘organic carbon’ we are specifically referring to the carbon contained within organic compounds.
DOC
In environmental waters, DOC is a major component of the carbon cycle and will serve as the primary source of food for most aquatic life. DOC is a backbone of the functioning of both marine and freshwater systems, with firm connectivity between the two as DOC can enter freshwaters from the terrestrial environment and then be transported into the marine. Aquatic environments are also one of the major carbon storage areas and important in global carbon sequestration e.g. carbon sequestration through exchange with sediments.
In a clean water treatment context, DOC can interact differently depending on the disinfection process utilised. There can be issues with the formation of harmful by-products during disinfection with Chlorine as Chlorine can react with fulvic and humic acids in the water causing the formation of Trihalomethanes and HAA5, which are linked with health concerns including reproductive issues, birth defects and some cancers.
In treated water, the presence of higher levels of DOC cause a significant risk of bacterial regrowth in treatment and distribution systems (e.g. biofilm formation) as well as often causing discoloration. In wastewater treatment specifically, DOC that is not biodegradable will often be released into the environment as part of the treated effluent.
TOC
TOC has important implications across both the water treatment industry and water used for specific industry processes such as cooling, pharmaceutical uses and sensitive manufacturing processes. TOC is more of a sensitive measure than BOD/COD for organic carbon in water. For water uses which rely on high purity water, TOC is an essential parameter to ensure the water meets the standards required for the use. As with DOC, TOC is susceptible to having the base molecules required to form undesirable disinfection by-products.
Typical DOC & TOC measurement can take several forms from analysers in laboratory settings that rely on manual sampling or online analysers, usually using a flow-injection technique; online analysers tend to be more suited to applications where TOC controls are extremely stringent.
Typical analysers will oxidise the organic compounds present in a sample to CO² and measure the amount of CO² that is subsequently produced. Regardless of the technique employed for the oxidation process, the compounds will be oxidised in a sequential pattern with different fractions defined for each stage. TOC analysers have two main areas in which they tend to differ, often depending on the properties of the water intended for analysis:
A. How they oxidise the organic matter. Common methods include photo-oxidation, photo-chemical oxidation, catalytic oxidation, thermo-chemical oxidation and high temperature combustion.
B. How they detect the carbon. Common methods include non-dispersive infrared (NDIR) or a conductometric technique (either selective/membrane or non-selective/direct).
Carbon in water can be separated into multiple fractions based on physical and physicochemical properties. Analysers will use the different fractions of carbon within a sample, through the addition and subtraction of the different fractions to calculate either TOC or DOC as required.
Carbon fractions:
The combination of methods chosen and whether lab based or online can affect the accuracy and reliability of the TOC readings produced, with each method having its own ‘pros and cons’. An undeniable issue with either a lab or online pathway is the need for consumables and regular maintenance with items such as UV lamps, filters, pump heads all requiring replacement and disposal; UV lamps can be of particular problem as many contain mercury which requires specific disposal procedures to prevent contamination.
Fluorescence on particular known wavelengths can be used as a proxy for organic matter activity in water, which is supported by a wealth of scientific literature. Chromogenic (coloured) Dissolved Organic Matter (commonly known as CDOM) has distinct peaks on a fluorescence Excitation-Emission Matrix. These peaks are primarily associated with humic and fulvic acids, key components of DOC & TOC. The peaks encompass a wide range of compounds that fluoresce at these specific excitation-emission wavelengths. Numerous scientific studies have then correlated this peak with both TOC & DOC.
The Proteus technology harnesses this relationship and uses our patented algorithm to use the optical CDOM fluorescence signal to provide both TOC and DOC readings in real-time without the need for reagents or lengthy laboratory processes. Proteus combines CDOM with temperature and turbidity and an initial paired sampling campaign to correlate the DOC and TOC with the CDOM signal to produce high accuracy, site-specific data.
Proteus requires minimal maintenance with very few consumables, key to keeping running costs low, with no specialist disposal requirements. Optical LED sensors have a far higher operating life than traditional UV lamps and have minimal drift, meaning calibration frequency is relatively low. A Proteus multiprobe can be set to take readings at whatever frequency is desired, allowing far more data to be collected more easily than with other, more laborious methods.
Proteus is designed to enable clients to have more control over their systems by having high accuracy data available in real-time to make decisions over their processes as needed. Real-time feedback is key in process efficiency and Proteus multiprobes can go one step further as a range of other physicochemical and fluorescence-based parameters (i.e. BOD, COD, coliforms) can be collected simultaneously.
