Quantification of high molecular weight organic carbon concentrations with LC-OCD and PHMOC for biological stability investigation of drinking water produced from surface water

The presence of aquatic biopolymeric organic carbon of high (> 10 - 20 kDa) molecular weight (high-MW OC) in drinking water produced from surface water affects its biological stability which may cause regrowth in disinfectant-free distribution. This study compares two analytical methods for determining the concentration of aquatic high-MW OC, namely LC-OCD (liquid chromatography - organic carbon detection) and PHMOC (particulate and colloidal high-molecular weight OC). LC-OCD entails prefiltration of the water sample, chromatographical separation of the relevant biopolymer (BP) OC-fraction, and in-line OC detection. PHMOC is based on the total OC content of the concentrate obtained after 30 kDa crossflow ultrafiltration of the water sample. LC-OCD BP and PHMOC showed a good linear correlation (R² 0.87) for a suite of treated surface water matrices (except raw water) in the 10 - 200 µg/L concentration range, with PHMOC values being 10% - 30% higher than the corresponding LC-OCD BP value, without a clear impact of other water matrix constituents. The indicative yields and selectivities of both methods for indigenous high-MW OC obtained from the PHMOC concentrate were high (≥ 70% - 88%) but not fully complete, which may explain the observed higher PHMOC values and scatter in the PHMOC - LC-OCD BP correlation. LC-OCD BP and PHMOC displayed similar values and trends across the different seasons and treatment stages, with treated ground water and infiltrated water having the lowest (< 10 µg/L) values. Regrowth (as Aeromonas) levels in disinfectant-free distribution networks corresponded with the high-MW OC concentration in the treated drinking water. Overall, the two methods equivalently quantify the concentration of aquatic high-MW OC. Both methods are suitable for use in biological stability studies. The small sample volume renders LC-OCD more practical, whereas the PHMOC method enables further experimentation and characterization of the high-MW OC fraction.