Iron (Fe²⁺), manganese (Mn²⁺), and ammonium (NH₄⁺) are the three most common contaminants in anaerobic groundwater and are typically removed in rapid sand filters in a series of simultaneous, uncontrolled, and interconnected redox reactions. In this study, we demonstrated separation of these oxidation processes, including reversing the order of NH₄⁺and Mn²⁺oxidation, allowing Mn²⁺to oxidize before NH₄⁺. To achieve this uncommon sequence, the filter was operated with low O₂ concentrations (∼0.02 mmol/L, ∼0.5 mg/L) and a high pH (∼8). Under these conditions, Mn²⁺ oxidation is consuming all available O₂, suppressing the occurrence of NH₄⁺oxidation. In the filter with low O₂ (0.08 mmol/L, ∼3 mg/L) and low pH (∼6.8), the opposite was observed, as Mn²⁺ oxidation was delayed under these conditions, resulting in complete O₂ consumption by NH₄⁺-oxidizing bacteria. Reactive transport modelling and parameter estimation revealed that Mn²⁺ oxidation is one order of magnitude faster in absence of NH₄⁺ oxidation (1.4 × 10⁻² vs 2.5 × 10⁻³ mmol/L), whereas NH₄⁺ oxidation seemed to be accelerated by simultaneous Mn²⁺ oxidation (6.8 × 10⁻³ vs 2.9 × 10⁻² s⁻¹). This interconnection between Mn²⁺ and NH₄⁺ oxidation was further emphasized by the observation of Mn²⁺ release in the presence of NO₂⁻. In conclusion, this study has shown that a shift from conventional aerated groundwater treatment to sequential oxidation in separate filters offers (i) a more controllable system, (ii) the potential to optimize the rates of each oxidation process separately, which would ultimately result in higher flows and less backwashing.

Groundwater is one of the major sources for drinking water supply worldwide. Conventional iron removal via aeration-filtration produces about 72,802 t of iron sludge annually in the Netherlands alone. Iron sludge comprises low-density flocs of little to no commercial value. The current study explored a novel concept for iron removal, namely anoxic iron sulfides formation in a fixed bed continuous flow reactor. Iron sulfides usually form dense structures and offer a wider range of re-use applications. A packed bed up-flow column reactor filled with pyrite granules was fed iron and sulfide containing solutions. Produced solids were analyzed applying X-ray diffraction analysis, Raman spectroscopy, digital microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. Rapid iron sulfides formation was observed after < 10 min. The formed minerals were partially retained by the pyrite granules. The molar ratio of removed Fe(II) to removed S(-II) equaled up to 0.76 ± 0.16 mol Fe(II)_rem/(mol S(-II)_rem). Our results show that iron sulfides formation can present an interesting alternative to iron removal via aeration-filtration due to its compact particle sizes and fast formation rates.

To meet the increasing drinking water demand, membrane technologies are used to treat previously unavailable water sources. A byproduct of membrane technologies is the concentrate stream, containing valuable resources in higher concentrations. We studied the recovery of iron from different groundwater matrices and anaerobic reverse osmosis (RO) concentrates via precipitation of vivianite and the co-removal of other common groundwater divalent cations Mn²⁺, Mg²⁺ and Ca²⁺ during vivianite precipitation. The formed precipitates were characterized using X-Ray Diffraction and Scanning Electron Microscopy. Vivianite precipitation removed a maximum of 89 % of Fe²⁺ in raw groundwater and 52 % Fe²⁺ from RO concentrate. Substantial co-removal of Mn²⁺ (max 91 %) and limited co-removal of Mg²⁺ (max 7 %) were found, without hindering Fe removal efficiencies or altering morphological changes of the vivianite crystal. In contrast, co-removal of Ca²⁺ occurred at the expense of iron removal, forming amorphous calcium phosphate precipitates. This study shows the potential of vivianite precipitation for iron recovery across a wide range of groundwater matrices and highlights the need for further research to optimize this novel method to treat concentrate streams that are challenging to dispose of.

Removal of carcinogenic arsenic (As) from groundwater is essential for providing safe drinking water. Arsenate (As(V)) is more effectively removed in groundwater filters than arsenite (As(III)), making the oxidation of As(III) to As(V) a key step in the treatment process. This study distinguishes between surface-catalytic and biological As(III) oxidation on natural manganese oxide (MnO_x) coated filter sand, since it is unknown which pathway dominates in filters. The MnO_xcoated sand was collected from a full-scale groundwater filter and consisted of a mixture of different abiotically and biologically formed Mn oxides, such as Birnessite and Todorokite. A lab-scale filter setup was operated with As(III)-containing water. Within 3 weeks, a shift from surface-catalytic to biological As(III) oxidation was observed. Initially, surface-catalytic As(III) oxidation (k_CHEM= 0.318 min^–1) was coupled to Mn(II) release at a ratio of 0.96, approximating the stoichiometric ratio of 1. This coupling disappeared over time, indicating the biological nature of the reaction, as confirmed by microbial inhibition. An increase in relative abundance of the known As-oxidizing families Comamonadaceae, with Polaromonas as the dominant genus, and Microscillaceae were found post experiments. Except for these changes, the microbial community on the sand grains stayed relatively similar prior to and post experiments. No significant changes in the physical-chemical properties of the MnO_xcoating were found post experiments. A first-order biological As(III) oxidation rate constant k_BIOof 4.64 min^–1was found, yielding a half-life of 9 s. This represents a 14-fold acceleration compared with surface-catalytic oxidation, revealing that kinetic limitations rather than surface passivation can be attributed to the loss of surface-catalytic oxidation. Our study demonstrates that biological oxidation of As(III) can outpace the acknowledged oxidizing power of MnO_x, offering a potential new pathway for the development of effective As removal systems.

Iron-based adsorbents are commonly used to remove arsenic (As) from water for drinking water purposes. Here, we study the role of biological As(III) oxidation on iron-based adsorbents in filters and its effect on overall As uptake. A lab-scale filter with iron oxide coated sand (IOCS), a commonly used adsorbent, was operated with water containing As(III) and As(V), while water samples were taken periodically over its height. As(III) oxidation initiated after approximately 10 days and increased to a first order rate constant of 0.09 s⁻¹ after 57 days resulting in full oxidation of As(III) in <50 s. Consequently, the filter shifted from an As(III) to an As(V) adsorbing filter. Oxidation was not observed after inhibiting the microbial activity using sodium azide confirming its biogenic nature. This implies that As(III) oxidizing biomass can grow on iron-based adsorbents in water filters without requiring inoculation. As the experimental conditions were similar to full-scale As treatment plants, we believe that biological As(III) oxidation is widely overlooked in these systems. Occurrence of biological oxidation is, however, beneficial for removal, as at pH <8 the adsorption capacity for As(V) can be up to 10-fold higher than for As(III). With these new insights, arsenic treatment using iron-based adsorbents can be further optimized. We suggest a more robust new design with a biological active As(III) oxidizing top layer and an As(V) adsorbing bottom layer.

Iron (Fe²⁺), manganese (Mn²⁺), and ammonium (NH₄⁺) oxidation processes were studied in three single media and three dual media full-scale rapid sand filters (RSFs) using reactive transport modelling (RTM) in PHREEQC and parameter estimation using PEST. Here, we present the insights gained into the spatial distribution of Fe and Mn mineral coatings in RSFs and its influence on the oxidation sequence and rates. Fe²⁺ and Mn²⁺ oxidation predominantly occurred simultaneously in the RSFs, contrary to the expected sequential oxidation based on Gibbs free energy calculations. During backwashing, RSF grains become fully mixed, which initiates heterogeneous Mn²⁺ oxidation on Mn-coated grains that end up in the top layer. The resulting grains have a mixed Fe/Mn mineral coating, which is limiting heterogeneous Mn²⁺ oxidation due to the limited Mn mineral surface available. Mixed coatings did not seem to affect Fe²⁺ oxidation rates, instead oxidation rates were increasing at lower pH. We found that RSFs can be designed to spatially separate Fe²⁺ and Mn²⁺ oxidation, which results in optimal conditions for Mn²⁺ oxidation. The RSF needs to consist of two layers with varying density to inhibit mixing and complete Fe²⁺ oxidation should occur in the top layer. The developed RTM can be used to estimate the depth at which Fe²⁺ oxidation is complete, and thus the ideal intersection depth of the two layers. A novel perspective is provided on how mineral coating distribution in single and dual media filters influence removal rates and the sequence of oxidation, which contributes to the design of more efficient groundwater filters.

Long-term consumption of groundwater containing elevated levels of arsenic (As) can have severe health consequences, including cancer. To effectively remove As, conventional treatment technologies require expensive chemical oxidants to oxidise neutral arsenite (As(III)) in groundwater to negatively charged arsenate (As(V)), which is more easily removed. Rapid sand filter beds used in conventional aeration-filtration to treat anaerobic groundwater can naturally oxidise As(III) through biological processes but require an additional step to remove the generated As(V), adding complexity and cost. This study introduces a novel approach where As(V), produced through biological As(III) oxidation in a sand filter, is effectively removed within the same filter by embedding and operating an iron electrocoagulation (FeEC) system inside the filter. Operating FeEC within the biological filter achieved higher As(III) removal (81 %) compared to operating FeEC in the filter supernatant (67 %). This performance was similar to an analogous embedded-FeEC system treating As(V)-contaminated water (85 %), confirming the benefits of incorporating FeEC in a biological bed for comparable As(III) and As(V) removal. However, operating FeEC in the sand matrix consumed more energy (14 Wh/m³) compared to FeEC operated in a water matrix (7 Wh/m³). The efficiency of As removal increased and energy requirements decreased in such embedded-FeEC systems by deep-bed infiltration of Fe(III)-precipitates, which can be controlled by adjusting flow rate and pH. This study is one of the first to demonstrate the feasibility of embedding FeEC systems in sand filters for groundwater arsenic removal. Such systems capitalise on biological As(III) oxidation in aeration-filtration, effectively eliminating As(V) within the same setup without the need for chemicals or major modifications.

The mechanisms and by-product formation of electrochemical oxidation (EO) for As(III) oxidation in drinking water treatment using groundwater was investigated. Experiments were carried out using a flowthrough system, with an RuO ₂/IrO ₂ MMO Ti anode electrode, fed with synthetic and natural groundwater containing As(III) concentrations in a range of around 75 and 2 µg/L, respectively. Oxidation was dependent on charge dosage (CD) [C/L] and current density [A/m ²], with the latter showing plateau behaviour for increasing intensity. As(III) concentrations of <0.3 µg/L were obtained, indicating oxidation of 99.9 % of influent As(III). Achieving this required a higher charge dosage for the natural groundwater (>40 C/L) compared to the oxidation in the synthetic water matrix (20 C/L), indicating reaction with natural organic matter or other compounds. As(III) oxidation in groundwater required an energy consumption of 0.09 and 0.21 kWh/m ³, for current densities of 20 and 60 A/m ², respectively. At EO settings relevant for As(III) oxidation, in the 30–100 C/L CD range, the formation of anodic by-products, as trihalomethanes (THMs) (0.11–0.75 µg/L) and bromate (<0.2 µg/L) was investigated. Interestingly, concentrations of the formed by-products did not exceed strictest regulatory standards of 1 µg/L, applicable to Dutch tap water. This study showed the promising perspective of EO as electrochemical advanced oxidation process (eAOP) in drinking water treatment as alternative for the conventional use of strong oxidizing chemicals.

Rapid sand filters (RSF) are an established and widely applied technology for the removal of dissolved iron (Fe²⁺) and ammonium (NH₄⁺) among other contaminants in groundwater treatment. Most often, biological NH₄⁺oxidation is spatially delayed and starts only upon complete Fe²⁺ depletion. However, the mechanism(s) responsible for the inhibition of NH₄⁺oxidation by Fe²⁺ or its oxidation (by)products remains elusive, hindering further process control and optimization. We used batch assays, lab-scale columns, and full-scale filter characterizations to resolve the individual impact of the main Fe²⁺ oxidizing mechanisms and the resulting products on biological NH₄⁺ oxidation. modeling of the obtained datasets allowed to quantitatively assess the hydraulic implications of Fe²⁺ oxidation. Dissolved Fe²⁺ and the reactive oxygen species formed as byproducts during Fe²⁺ oxidation had no direct effect on ammonia oxidation. The Fe³⁺ oxides on the sand grain coating, commonly assumed to be the main cause for inhibited ammonia oxidation, seemed instead to enhance it. modeling allowed to exclude mass transfer limitations induced by accumulation of iron flocs and consequent filter clogging as the cause for delayed ammonia oxidation. We unequivocally identify the inhibition of NH₄⁺oxidizing organisms by the Fe³⁺ flocs generated during Fe²⁺ oxidation as the main cause for the commonly observed spatial delay in ammonia oxidation. The addition of Fe³⁺ flocs inhibited NH₄⁺oxidation both in batch and column tests, and the removal of Fe³⁺ flocs by backwashing completely re-established the NH₄⁺removal capacity, suggesting that the inhibition is reversible. In conclusion, our findings not only identify the iron form that causes the inhibition, albeit the biological mechanism remains to be identified, but also highlight the ecological importance of iron cycling in nitrifying environments.

Slow Sand Filtration is popular in drinking water treatment for the removal of a wide range of contaminants (e.g., particles, organic matter, and microorganisms). The Schmutzdecke in slow sand filters (SSFs) is known to be essential for pathogen removal, however, this layer is also responsible for increased head loss. Since the role of deeper layers in bacteria and virus removal is poorly understood, this research investigated the removal of E.coli WR1 and PhiX 174 at different depths of a full-scale SSF. Filter material from top (0–5 cm), middle (5–20 cm) and deep (20–35 cm) layers of an established filter was used in an innovative experimental set-up to differentiate physical-chemical and biological removal processes. In the analysis, we distinguished between removal by biological activity, biofilm and just sand. In addition, we modelled processes by a one-side kinetic model. The different layers contributed substantially to overall log removal of E.coli WR1 (1.4–1.7 log₁₀) and PhiX 174 (0.4–0.6 log₁₀). For E.coli WR1, biological activity caused major removal, followed by removal within biofilm and sand, whereas, removal of PhiX 174 mainly occurred within sand, followed by biofilm and biological activity. Narrow pore radii in the top layer obtained by micro-computed tomography scanner suggested enhanced retention of bacteria due to constrained transport. The retention rates of E.coli WR1 and PhiX 174 in top layer were four and five times higher than deeper layers, respectively (k_ret 1.09 min⁻¹ vs 0.26 min⁻¹ for E.coli WR1 and k_ret 0.32 min⁻¹ vs of 0.06 min⁻¹ for PhiX 174). While this higher rate was restricted to the Schmutzdecke alone (top 5 cm), the deeper layers extend to around 1 m in full-scale filters. Therefore, the contribution of deeper layers of established SSFs to the overall log removal of bacteria and viruses is much more substantial than the Schmutzdecke.