Toward enhancing the reliability of particulate organic carbon analysis in aquatic systems: A new perspective on particle settling and pretreatment improvement

Reliable total organic carbon (TOC) analysis, including particulate organic carbon (POC), is crucial for evaluating water-treatment performance and tracing the carbon cycle in aquatic systems. In this study, we investigated the impact of particle settling on the recovery and precision of POC in TOC analysis of water samples, focusing on environmental particle samples and wastewater influents. Environmental particle samples were obtained from compost, algae, and sewage sludge. Additionally, improvement strategies for reliable TOC analysis were evaluated using physicochemical pretreatment methods, including resuspension via gas injection, ultrasonication (UL), a combined ultrasonic and alkaline (CUAL) method, and dispersant injection. We confirmed that particle settling was strongly correlated with decreased precision and TOC recovery, with large particle sizes and low surface charges being significant contributing factors. The CUAL method significantly reduced TOC recovery loss due to particle settling (3.6 %–21.9 %), compared to the conventionally used UL (33.9 %–60.8 %), owing to an increased recovery of POC during CUAL pretreatment. Dispersant application increased the particle zeta potential, reducing particle settling rates by 19.6 % compared to that achieved without a dispersant. Additionally, the turbidity index (i.e., NTU₅/NTU₀) was useful for proactively evaluating particle settling, showing a strong negative correlation with the particle settling rate (r = −0.82, p < 0.01). Its validity was confirmed through sample pretreatment tests conducted on wastewater samples (n = 16). Our findings significantly improve POC recovery and TOC measurement reliability, especially for samples with high suspended solids (SS), reinforcing their critical impact on the accuracy of water-quality assessments.