Dissolved Organic Carbon (DOC) is the concentration of organic carbon in the filtrate produced by passing a water sample through a 0.45 µm pore size membrane filter. The filtration step removes particulate organic carbon (POC) — including algae, bacteria, suspended solids, and colloidal material larger than 0.45 µm — so that DOC represents only the dissolved and sub-micron colloidal organic fraction. Total Organic Carbon (TOC) is the sum of DOC and POC. DOC is expressed in milligrams of carbon per litre (mg C/L).
For most drinking water, clean surface water, and groundwater samples, DOC is approximately equal to TOC because particulate organic matter is present at negligible concentrations. In these contexts, the terms are often used interchangeably in practice, though formally they represent distinct fractions. The distinction becomes significant in samples with substantial suspended solids or biological material — such as eutrophic surface water, stormwater runoff, or treated wastewater effluent — where the particulate fraction may contribute meaningfully to total organic carbon.
DOC is analytically significant because it is the fraction of organic carbon most directly relevant to drinking water treatment chemistry. Natural organic matter (NOM) in its dissolved form is the primary precursor to disinfection byproducts (DBPs): when chlorine or other disinfectants react with DOC, they form trihalomethanes (THMs), haloacetic acids (HAAs), and other regulated compounds. Monitoring DOC at intake and through each treatment stage — coagulation, flocculation, sedimentation, filtration, and disinfection — tracks precursor removal efficiency and predicts DBP formation potential.
DOC measurement follows the same analytical workflow as TOC, with the addition of a filtration step. The sample is passed through a 0.45 µm membrane filter (typically polyethersulfone (PES), cellulose acetate, or PVDF), and the filtrate is then acidified, sparged to remove all dissolved inorganic carbon (dissolved CO₂, bicarbonates, and carbonates), and oxidised for CO₂ measurement by NDIR detection. Care must be taken to minimise contamination from the filter itself — new filters should be pre-rinsed with ultrapure water before use, and filter blanks should be measured and subtracted from results where low-level DOC work requires it.
ISO 8245 provides guidelines for the determination of both TOC and DOC in water, covering sample preparation, oxidation methods, and reporting. USEPA Method 415.3 also includes DOC as a measurement option, defining DOC operationally by the 0.45 µm filtration step. In Australian environmental monitoring and drinking water quality programs, DOC is routinely measured under NATA accreditation to methods based on ISO 8245 or USEPA 415.3.
Key Points
- Organic carbon fraction passing through a 0.45 µm membrane filter; excludes particulate organic matter
- TOC = DOC + POC (particulate organic carbon); for clean water, DOC ≈ TOC
- Primary organic carbon parameter for drinking water quality and disinfection byproduct control
- Measured by the same TOC instruments as TOC, with a filtration pre-treatment step
- Standard methods: ISO 8245, USEPA 415.3
Relevant Standards
- ISO 8245 (water quality — guidelines for the determination of TOC and DOC)
- USEPA 415.3 (TOC and DOC in source water and drinking water; 0.45 µm filtration defines DOC)
- Standard Methods 5310C (persulfate-UV or heated-persulfate oxidation; applicable to DOC after filtration)
- ASTM D7573 (total and organic carbon in water — applicable to DOC fraction)
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Frequently Asked Questions
What is the difference between DOC and TOC?
TOC (Total Organic Carbon) includes all organic carbon in the sample — dissolved, colloidal, and particulate. DOC (Dissolved Organic Carbon) is the fraction that passes through a 0.45 µm filter, which removes algae, bacteria, and suspended particles. For clear drinking water and most groundwater samples, DOC and TOC are essentially equivalent. For samples with significant suspended solids or biological material (stormwater, river water during flood events, wastewater effluent), DOC will be lower than TOC because particulate carbon has been removed.
Why is DOC monitored in drinking water?
DOC is the primary precursor to disinfection byproducts (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs), which form when chlorine reacts with natural organic matter. Reducing DOC before disinfection — through enhanced coagulation, ozonation, or biological filtration — lowers DBP formation. DOC is also used as a surrogate for overall organic contamination, as an early warning indicator for catchment events, and to track treatment efficiency through each stage of the process.
How is DOC measured?
The sample is filtered through a 0.45 µm membrane filter to remove particulate matter. The filtrate is then analysed using a TOC analyser — the same instrument and method used for total organic carbon, but applied to the filtered sample. The most common approach is NPOC measurement: the filtered sample is acidified and sparged to remove all dissolved inorganic carbon (dissolved CO₂, bicarbonates, and carbonates), then oxidised (by heated persulfate, UV-persulfate, or high-temperature combustion) and measured by NDIR detection.
What filter pore size defines dissolved organic carbon?
By convention, DOC is the organic carbon fraction passing through a 0.45 µm nominal pore size membrane filter. This pore size was originally chosen to separate dissolved from suspended matter in natural water samples. It is worth noting that some colloidal organic particles smaller than 0.45 µm will pass through the filter and be included in the DOC measurement — truly dissolved molecules at the molecular scale and colloids up to 0.45 µm are both captured in the DOC fraction.