Arsenic Removal from Ground Water by Sand Filtration during Biological Iron Oxidation

Influence of iron, phosphate, and silicate on arsenic removal from groundwater using a low-cost ceramic filter

The ceramic filter amended with iron (Fe) has proven to be a potential low-cost method for arsenic (As) removal from groundwater. The presence of Fe, phosphate (P), and silicate (Si) significantly affects the As removal efficiency of the ceramic filter, which has not been passably investigated. The present research aimed to examine the effect of Fe, P, and (or) Si presence as single or in combination on As (III) removal from synthetics groundwater by a low-cost iron amended ceramic filter (IACF). Laboratory-scale filtration experiments at different compositions of Fe, P, Si, and As (III) were conducted by the IACF fabricated with a ceramic candle and iron netting box. Fe (II) in synthetic groundwater positively impacted As (III) removal. At a concentration of 2 mg/L of Fe (II), the As levels in the effluent decreased to less than the maximum contamination level (MCL) of 50 μg/L. Groundwater P concentration needed less than 3 mg/L or Si concentrations required less than 35 mg/L to effectively reduce As (III) to below the MCL at 5 mg/L of groundwater Fe (II). The cumulative effect of P and Si on As removal was found to be more significant than distinct contributions. The presence of 2 mg/L P and 35 mg/L or higher Si in the groundwater cumulatively reduced the As removal performance from 92% to 63%, and the MCL was not met. The negative impact of P and Si on As (III) removal followed the order of (P + Si) > P > Si. P competed with As for adsorption sites during the process, while Si inhibited the Fe release and floc formation, significantly reducing As removal performance. The study findings can potentially contribute to optimizing IACF as a low-cost method for As removal from groundwater.

Evaluating configuration of dual unit ceramic filter for arsenic removal from highly contaminated groundwater

Iron (Fe) amended dual unit ceramic filters (DUCF) can be a viable treatment option for arsenic (As) removal from highly contaminated groundwater. The present field study investigated the effect of filter configurations, the separate-unit dual filter (SUDF) and connect-unit dual filter (CUDF), on As removal from groundwater having As concentration of 475 μg/L. SUDF was configured by placing 1st and 2nd filter units side-by-side, whereas the 1st filter unit was placed on the top of the 2nd filter unit in CUDF configuration. Comparing the two filter configurations, SUDF achieving As concentration in the effluent below 50 μg/L (standard value) was found more effective due to sufficient Fe²⁺ in the 2nd filter. Average As concentrations in the final product (effluent of 2nd filter) were 43 μg/L from SUDF and 111 μg/L from CUDF. The short hydraulic residence time (3.3 min) in the 2nd filter of CUDF, along with limited contact between water and the iron net, lead to inadequate soluble Fe²⁺ resulting in poor As removal. Both filter configurations effectively removed Fe, P, and Mn with more than 90% reduction of these parameters by the 1st filter. Analysis of insoluble hydrous ferric oxides flocs through XAFS L3-edge spectra confirmed the oxidation of As(III) to As(V) in both the SUDF and CUDF systems resulting in enhanced As removal efficiency. The study results found SUDF as an appropriate configuration for filter design to treat highly contaminated groundwater in rural areas of developing countries.

Effect of pre-aeration on the removal of arsenic and iron from natural groundwater in household based ceramic filters

Maintenance of existing household arsenic (As) removal technologies are comparatively difficult due to the use of the sand beds as a filter. Moreover, pre-aeration of groundwater is avoided during filter operation that may affect the removal efficiency. This study investigated the effect of pre-aeration on the efficacy of simple iron nested ceramic filter (CF) for the removal of As and Fe from the natural groundwater. Five CFs at 5 households in the Bagerhat district of Bangladesh were tested for 31 days with pre-aerated groundwater (AGW system) and non-aerated groundwater (NAGW system). Pe-aeration of groundwater significantly improved (p > 0.5) the removal efficiency of As and Fe in the CFs. The filters effectively removed As in the groundwater from 203 - 231 μg/L to 29-40 μg/L in the AGW system whereas the effluent As were >50 μg/L in the NAGW system. Iron (Fe) was also removed effectively and the overall As and Fe removal efficiency were more than 82% and 99%, respectively in the AGW system. Removal of Mn and PO₄-P were significantly enhanced achieving more than 56% and 99% removal, respectively in the AGW system. X-ray absorption fine structure (XAFS) analysis indicated that the oxidation of Fe²⁺ and As(III) and subsequent adoption/precipitation are the main processes controlling the removals of As and Fe in the CFs. Two stages oxidation of Fe²⁺ and As(III) in the AGW system facilitated to increase As and Fe removal efficiency. The findings of this study suggest that the iron net nested ceramic filters with pre-aeration step is an effective method and can be employed at the household level in As contaminated region.

Arsenic removal from iron-containing groundwater by delayed aeration in dual-media sand filters

Generally, abstracted groundwater is aerated, leading to iron (Fe²⁺) oxidation to Fe³⁺ and precipitation as Fe³⁺-(hydr)oxide (HFO) flocs. This practice of passive groundwater treatment, however, is not considered a barrier for arsenic (As), as removal efficiencies vary widely (15-95%), depending on Fe/As ratio. This study hypothesizes that full utilization of the adsorption capacity of groundwater native-Fe²⁺ based HFO flocs is hampered by rapid Fe²⁺ oxidation-precipitation during aeration before or after storage. Therefore, delaying Fe²⁺ oxidation by the introduction of an anoxic storage step before aeration-filtration was investigated for As(III) oxidation and removal in Rajshahi (Bangladesh) with natural groundwater containing 329(±0.05) µgAs/L. The results indicated that As(III) oxidation in the oxic storage was higher with complete and rapid Fe²⁺ oxidation (2±0.01 mg/L) than in the anoxic storage system, where Fe²⁺ oxidation was partial (1.03±0.32 mg/L), but the oxidized As(V)/Fe removal ratio was comparatively higher for the anoxic storage system. The low pH (6.9) and dissolved oxygen (DO) concentration (0.24 mg/L) in the anoxic storage limited the rapid oxidation of Fe²⁺ and facilitated more As(V) removal. The groundwater native-Fe²⁺ (2.33±0.03 mg/L) removed 61% of As in the oxic system (storage-aeration-filtration), whereas 92% As removal was achieved in the anoxic system.

Arsenic removal by iron-oxidizing bacteria in a fixed-bed coconut husk column: Experimental study and numerical modeling

Groundwater in several parts of the world, particularly in developing countries, has been contaminated with Arsenic (As). In search of low-cost As removal methods, the biological oxidation of As(III) and Fe(II) followed by co-precipitation requires detailed investigation for the practical implementation of this technology. The present study investigated the biological oxidation of As(III) and Fe(II) through a combination of laboratory experiments and reactive transport modeling. Batch experiments were conducted to evaluate the As(III) oxidation by Fe-oxidizing bacteria, mainly Leptothrix spp. A fixed-bed down-flow biological column containing inexpensive and readily available coconut husk support media was used to evaluate the combined removal of As(III) and Fe(II) from synthetic groundwater. Oxidation and co-precipitation processes effectively reduced the concentration of As(III) from 500 μg/L to < 10 μg/L with a hydraulic retention time of 120 min. A one-dimensional reactive transport model was developed based on the microbially mediated biochemical reactions of As(III) and Fe(II). The model successfully reproduced the observed As(III) and Fe(II) removal trends in the column experiments. The modeling results showed that the top 20 cm aerobic layer of the column played a primary role in the microbial oxidation of Fe(II) and As(III). The model calibration identified the hydraulic residence time as the most significant process parameter for the removal of Fe and As in the column. The developed model can effectively predict As concentrations in the effluent and provide design guidelines for the biological treatment of As. The model would also be useful for understanding the biogeochemical behavior of Fe and As under aerobic conditions.