Granular activated carbon (GAC) has been widely introduced to advanced drinking water purification plants to remove organic matter and ammonium. Backwashing, which is the routine practice for GAC maintenance, is an important operational factor influencing the performance of GAC and its microbial biomass. In this study, the effects of backwashing on the ammonium removal potential of GAC were evaluated. In addition, abundances of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) on GAC were analyzed. GAC samples before and after backwashing were collected from a full-scale drinking water purification plant. Samplings were conducted before and after implementation of prechlorination of raw water. The results showed that the ammonium removal potential of the GAC increased by 12% after backwashing before prechlorination (p < 0.01). After implementing the prechlorination, the ammonium removal potential of the GAC decreased by 12% even after backwashing (p < 0.01). The AOA was predominant on the GAC in the two samplings. Regardless of prechlorination, the amounts of the AOA and the AOB remained at the same level before and after backwashing. Analysis of the backwashing water indicated that the amounts of the AOA and AOB washed out from the GAC were negligible (0.08%–0.26%) compared with their original amounts on the GAC. These results revealed the marginal role of backwashing on the biomass of ammonia oxidizers on GAC. However, the results also revealed that backwashing could have a negative impact on the ammonium removal potential of GAC during prechlorination.

Nitrification is a major ecological process occurring in biological activated carbon (BAC) filters used for advanced drinking water treatment. Ammonia oxidation is a rate-limiting step in nitrification that depends on the activity of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) associated with BAC. However, the growth competition between AOA and AOB remains unclear. In this study, time-course DNA-stable isotope probing (DNA-SIP) was applied to compare the growth activities of AOA and AOB for ammonium and urea in a BAC filter, in which AOA were more abundant than AOB. When a low level of ammonium (0.14 mg N L⁻¹) was continuously supplied to BAC, AOA demonstrated higher autotrophic growth activity. A similar trend was noticed in a microcosm supplied with a low concentration of urea (0.14 mg N L⁻¹) and ¹³C-labeled bicarbonate (H¹³CO₃ ⁻). However, the microcosm supplied with only ¹³C-urea as the sole nitrogen and carbon source indicated faster assimilation of carbon derived from urea by AOB. It is likely that AOB could directly incorporate and hydrolyze urea for nitrification. On the other hand, AOA demonstrated growth activity only when bicarbonate was added to urea, suggesting that they did not directly incorporate urea for nitrification. Thus, the different substrate affinities of AOA and AOB to ammonium and urea seem to be the reason for the niche separation on BAC. Understanding the different responses of ammonia oxidizers to nitrogen substrates may contribute to the improvement of BAC design and operation in actual advanced drinking water treatment.

The enrichment of cadmium (Cd) in farmland soil poses serious risks to agricultural safety and remains challenging to remediate. This study evaluated CaAl-layered double hydroxide (CaAl-LDH) as a highly efficient and stable passivator for Cd-contaminated soil. Laboratory adsorption tests demonstrated that Cd²⁺ adsorption on CaAl-LDH followed pseudo-second-order kinetics and the Langmuir model, indicating monolayer chemisorption, with a maximum capacity of 469.48 mg·g⁻¹ at pH 6. The adsorption mechanisms include surface complexation, interlayer anion exchange, dissolution–precipitation, and isomorphic substitution. A three-year field trial in Yongkang City, China showed that CaAl-LDH promoted the transformation of Cd in rhizosphere soil from the ion exchange state (F2) to the residual state (F7) and Fe–Mn oxidized state (F5), reducing the exchangeable Cd content by 26.71%. Consequently, Cd content in rice grains decreased by 68.42% in the first year and remained over 37% lower in the second year, consistently below the national food safety limit. Future research should focus on the optimization of material’s stability and application protocol. The results demonstrate that CaAl-LDH provides a cost-effective and sustainable strategy for the in situ passivation remediation of Cd-contaminated farmland, contributing to food safety and sustainable agriculture.

Biofilms in drinking water distribution systems pose significant risks by harboring chlorine-resistant bacteria. This study isolated five bacterial strains ( Sphingomonas ursincola , Sphingobium amiense , Gordonia amicalis , Microbacterium saccharophilum , Hydrogenophaga laconesensis ) from municipal pipelines to evaluate differences in biofilm formation and chlorine resistance. Biofilm formation ability varied notably, with S. ursincola showing the strongest capacity and H. laconesensis the weakest. Quorum sensing (QS) signal molecules (C ₆ -HSL, 3-OXO-C ₁₄ -HSL; 0.2240–0.2481 μg/L) and extracellular polymeric substances (EPSs: 19.940–32.407 mg/L) were critical in biofilm regulation, where QS molecules influenced EPS composition. Chlorine resistance assays revealed species-specific tolerance: at ≤1.0 mg/L, resistance ranked M. saccharophilum > S. amiense > G. amicalis > S. ursincola > H. laconesensis ; above 1.0 mg/L, S. ursincola and S. amiense exhibited robust resistance. Low chlorine levels (0.6 mg/L) damaged only 25.91%–34.80% of bacteria, insufficient to control the biofilm formation of the tested isolates. Optimal disinfection occurred at 1.0–1.5 mg/L, effectively controlling biofilm biomass. These findings highlight EPS-driven chlorine resistance mechanisms and QS-mediated biofilm regulation, providing actionable strategies for pipeline management. IMPORTANCE: This study addressed a critical gap in understanding different bacterial biofilm dynamics and chlorine resistance mechanisms in drinking water systems. By linking quorum sensing to extracellular polymeric substance production, it reveals how bacteria modulate biofilm resilience, elucidating species-specific mechanisms underlying biofilm resilience to chlorine disinfection. The identification of chlorine-resistant species ( Sphingomonas ursincola , Sphingobium amiense ) and optimal disinfection thresholds (1.0–1.5 mg/L) directly informs municipal water treatment protocols, providing a practical chlorine concentration range (1.0–1.5 mg/L) that effectively controls biofilms while avoiding excessive disinfectant use. These results are pivotal for mitigating secondary contamination risks and safeguarding public health, particularly in aging infrastructure where biofilm-related outbreaks are prevalent.