Dissolved organic carbon concentration changes in surface drinking water sources, treatment challenges and potential solutions: a review

High dissolved organic carbon (DOC) concentrations in drinking water sources pose substantial challenges for drinking water treatment, necessitating advancements in treatment technologies. DOC reacts with disinfectants accelerating disinfectant decay, forming disinfection by-products and the hydrophilic portion supports bacterial regrowth. Out of 1640 reported surface waters in Europe and North America, 43 % showed a significant increase (p < 0.05), 49 % showed no trend (p > 0.05), and only 8 % showed a decreasing trend (p < 0.05). DOC increase is associated by climatic (warming, extreme weather events) and non-climatic factors (anthropogenic activities, atmospheric CO₂ increase, atmospheric acid deposition recovery), however, the most reported one is climatic factors. With climate change as a global phenomenon, unreported catchments are likely to undergo substantial increases, necessitating site specific studies using appropriately monitored long-term data and improved statistical methods. Majority of terrestrially transported DOC is hydrophobic. During prolonged storage, some in-lake processes (photodegradation, biodegradation) could turn some inflowing hydrophobic DOC into hydrophilic, while algal growth could further generate new hydrophilic compounds. Traditional treatment processes are at their limits to remove high DOC, hydrophilic compounds and emerging pollutants such as pharmaceutical and personal care products and antibiotic-resistant genes. Advancing water treatment processes alone may not sufficiently address these challenges cost effectively. Alternative approaches such as catchment intervention to reduce the mobilisation of soil organic carbon, temporary storage to buffer fluctuating DOC concentrations, or frontline treatments such as riverbank filtration, suspended biological activated carbon or aerated biologically activated carbon are suggested as potential solutions but should be investigated for feasibility as they are site-specific.