Arsenic in groundwater poses significant health risks, especially in regions dependent on groundwater for drinking water. This study compares the performance of two brackish water reverse osmosis membranes for arsenic (III) rejection. The study explores membrane performance under conditions not extensively addressed before, such as using natural anaerobic groundwater with arsenic concentrations of 70–80 µg/L and examining the effect of sulfide complexation on arsenic rejection. The impact of sodium sulfide and antiscalant on arsenic rejection was investigated, with Na₂S tested as a complexing agent. The aim was to determine whether antiscalant addition affects sulfide stability or arsenic rejection, and whether it could lead to decreased membrane efficiency in the short term. Results show that membrane type contributes to a 5–8 % difference in arsenic rejection. Arsenic rejection mirrored boron's trend but was consistently higher. However, the addition of sodium sulfide, with or without the antiscalant, did not enhance arsenic rejection, indicating that membrane type remains the primary factor. These findings highlight the need for tailored membrane selection to optimize arsenic rejection under anaerobic conditions. Chemical treatment strategies should align with specific contaminant profiles and feedwater conditions, with further pilot-scale validation for real-world applications.

Biological activated carbon (BAC) filtration is an important treatment step in the production of drinking water especially if drinking water is produced from surface water. The performance and processes within a BAC filter have been of interest for researchers since the 1980's, mainly because of its ability to remove natural organic matter known as disinfection precursors. A malfunction of one of the pre-treatment steps might affect the feed water quality into the BAC filters. The main objective of this study was to determine the immediate response of the BAC filters to a rapid change in feed water quality. It was shown that with the studied setup it was possible to compare the effect of different pre-treatment steps and subsequent different water qualities on the performance of the BAC filters on the long term adaptation. However, especially the immediate response was not studied in detail before. All filters were able to mitigate a sudden change in feed water quality, either through improved adsorption or increased activity of the biomass on the filter. As a result of this resilience against sudden changes, it is therefore concluded that there is no direct need for very stringent on-line monitoring and continuous adjustments of the feed water quality of the BAC filters. The addition of phosphate resulted in the lowest dissolved organic carbon (DOC) concentration in the effluent of the BAC filters. In this study the influence of intact cells in the feed water on the performance of the BAC filters was shown to be limited.

Our aim was to systematically investigate the influence of anions (HPO₄ ²⁻), cations (Ca²⁺, Mg²⁺) and neutral H₄SiO₄ on Fe flocculation and As(III) removal in the complex natural water matrix. For this purpose, three different anaerobic groundwaters were selected and manipulated by dosing of Ca²⁺, Mg²⁺, HPO₄ ²⁻, or by their removal by cation – and anion exchange. The change in Fe floc volume and of dissolved Fe and As were followed in aerated jar experiments. Fe floc growth was improved by addition of Ca²⁺ or Mg²⁺, and hindered by their removal. This hindered floc growth was more severe for groundwaters with higher P:Fe ratios, where Fe flocs carry a larger net negative surface charge, and rely stronger on Ca²⁺ or Mg²⁺ for charge neutralisation. When expressing the charge balance of the different groundwaters as the molar ratio (Ca²⁺ + Mg²⁺)/P, a linear relationship was found with the cumulative Fe floc volume, with a plateau at molar ratios >500. At environmentally relevant concentrations, H₄SiO₄ was found more likely to compete with As(III) for adsorption capacity than HPO₄ ²⁻. As(III) removal was strongly related to Fe removal - independent of Ca²⁺ or Mg²⁺ presence - indicating that As(III) is primarily adsorbed at an early stage in the flocculation process.