Key Takeaways & Executive Findings
- •• • Replacing 70% of activated carbon achieves >85% of the removal efficiency of full replacement for most DOM fractions (except LMWC) and >95% removal for most pesticide-related contaminants, while saving ~30% of new carbon cost. • • UV254 removal reaches 70% at 70% replacement ratio, indicating effective removal of aromatic organic matter, a key DBP precursor. • • Removal of THMs and HAAs is insensitive to replacement ratio, whereas HALs removal improves significantly with increasing ratio, highlighting the importance of adsorbate polarity and molecular structure. • • Triazole pesticides exhibit only ~60% removal even at 70% replacement due to high water solubility and low octanol-water partition coefficient, necessitating alternative treatment or higher replacement ratios.
Abstract
Activated carbon (AC) filters in drinking water treatment plants (DWTPs) experience significant adsorption performance decline over extended operation, yet complete media replacement is a major cost. To evaluate cost-effective strategies, pilot-scale column experiments with five AC replacement ratios (0%, 30%, 50%, 70%, and 100%) were conducted to assess removal of disinfection by-product (DBP) precursors and pesticide-related emerging contaminants. For dissolved organic matter (DOM), all fractions except low molecular weight compounds (LMWC) achieved >85% of the removal obtained with full replacement when 70% new AC was used, with UV254 removal reaching 70%. LMWC, due to small molecular size and low adsorption energy, required higher replacement ratios or full replacement for substantial removal. For DBPs, removal of trihalomethanes (THMs) and haloacetic acids (HAAs) was insensitive to replacement ratio, while haloacetaldehydes (HALs) removal improved markedly with increasing ratio, indicating structural selectivity. For pesticide-related contaminants, all except triazoles achieved >95% removal at 70% replacement; triazoles, due to high water solubility, high polarity, and low octanol-water partition coefficient, achieved only ~60% removal. Overall, replacing 70% of AC restored treatment performance to >80% of that with full replacement, ensuring effluent quality while saving ~30% of new carbon cost. Molecular structure, polarity, and pore size matching are key determinants of removal efficiency; optimizing replacement ratio balances water quality and economic benefits.
1. Introduction
Long-term operation of activated carbon (AC) filters in drinking water treatment plants (DWTPs) leads to gradual saturation of adsorption sites and biofilm development, resulting in diminished capacity for removing dissolved organic matter (DOM), disinfection by-product (DBP) precursors, and trace organic contaminants. Complete replacement of AC media is a capital-intensive operation, often accounting for a significant fraction of annual operational budgets. Utilities face the dilemma of balancing effluent quality against replacement frequency, yet limited quantitative guidance exists on optimal replacement ratios that achieve near-full performance at reduced cost.
This study addresses this bottleneck by systematically evaluating five replacement ratios (0%, 30%, 50%, 70%, and 100%) in pilot-scale columns, measuring removal efficiencies for DOM fractions, DBP precursors (THMs, HAAs, HALs), and pesticide-related emerging contaminants. The experimental design directly quantifies the trade-off between replacement ratio and treatment performance, providing actionable data for cost-effective AC management. By identifying that 70% replacement restores >80% of full replacement performance for most contaminants, the study offers a practical strategy to reduce operational costs while safeguarding public health.
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QIU Fuguo, DU Yanlong, JI Wenxiang, JIA Tingfang, WANG Zhenyu, JIANG Ling, LIANG Hong, JIANG Caifang, DONG Huiyu (2026). Effect of Activated Carbon Replacement Ratio in Drinking Water Treatment Plant Activated Carbon Filters on the Removal of Disinfection By-Product Precursors and Pesticide-Related Emerging Contaminants. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025092802
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Frequently Asked Questions
What is the minimum replacement ratio that ensures acceptable removal of DBP precursors without compromising regulatory compliance?
Based on the study, a 70% replacement ratio achieves >85% of the removal efficiency of full replacement for most DOM fractions and >95% for most pesticide-related contaminants, with UV254 removal at 70%. This suggests that 70% replacement is sufficient to maintain DBP precursor removal at levels close to full replacement, likely meeting regulatory limits, while saving ~30% of new carbon cost.
How does the molecular weight and polarity of contaminants influence their removal at different replacement ratios?
Low molecular weight compounds (LMWC) and polar compounds such as triazole pesticides are less effectively removed at lower replacement ratios due to their small size and high water solubility, which reduce adsorption affinity. For LMWC, removal only becomes substantial at higher replacement ratios or full replacement. In contrast, larger, less polar compounds are effectively removed even at 70% replacement, indicating that pore size distribution and surface chemistry of fresh AC are critical for adsorbing smaller, more polar molecules.
What are the cost implications of adopting a 70% replacement strategy compared to full replacement?
Adopting a 70% replacement ratio reduces new carbon procurement by 30%, directly lowering material costs. Given that AC replacement is a major operational expense, this strategy offers significant cost savings while maintaining treatment performance at >80% of full replacement for most contaminants. The study suggests that the 70% ratio is a cost-effective compromise, but site-specific water quality and regulatory requirements should be considered.
Are there any contaminants that are not adequately removed at 70% replacement, and what alternatives exist?
Triazole pesticides are only ~60% removed at 70% replacement due to their high polarity and low octanol-water partition coefficient. For such compounds, higher replacement ratios (e.g., 100%) or additional treatment processes such as ozonation, advanced oxidation, or specialized adsorbents may be necessary to achieve desired removal. The study highlights the need for contaminant-specific assessment when determining replacement strategies.
How does the performance of partially replaced AC filters change over time, and what is the expected service life?
The study provides immediate post-replacement performance data, but long-term performance will depend on the rate of fouling and adsorption capacity exhaustion. Typically, AC filters require periodic replacement every 3-5 years, but partial replacement may extend the interval between full replacements. However, the study does not provide long-term data; thus, pilot testing and monitoring are recommended to determine optimal replacement frequency for specific plant conditions.
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