Arsenic removal from drinking water by a household sand filter in Vietnam — Effect of filter usage practices on arsenic removal efficiency and microbiological water quality

One Liner: 
Household sand filter in Northern Vietnam consistently reduces arsenic from groundwater by 95% irrespectively of usage practices and sand usage duration. The findings are probably transferable to other sand filters, such as the biosand filter, if similar groundwater compositions (e.g. iron:arsenic ratio >50, phosphate <2.5 mg/L), geological settings, well and sand filter properties exist.
TitleArsenic removal from drinking water by a household sand filter in Vietnam — Effect of filter usage practices on arsenic removal efficiency and microbiological water quality
Publication TypeJournal Article
Year of Publication2015
AuthorsNitzsche, KSonja, Lan, VMai, Trang, PThi Kim, Viet, PHung, Berg, M, Voegelin, A, Planer-Friedrich, B, Zahoransky, J, üller, S-K, Byrne, JMartin, öder, C, Behrens, S, Kappler, A
JournalScience of The Total Environment
Pagination526 - 536
Date PublishedJan-01-2015
Keywordsarsenate, arsenite, fecal indicator bacteria, groundwater, iron minerals, sorption
AbstractHousehold sand filters are applied to treat arsenic- and iron-containing anoxic groundwater that is used as drinking water in rural areas of North Vietnam. These filters immobilize poisonous arsenic (As) via co-oxidation with Fe(II) and sorption to or co-precipitationwith the formed Fe(III) (oxyhydr)oxides. However, information is lacking regarding the effect of the frequency and duration of filter use aswell as of filter sand replacement on the residual As concentrations in the filtered water and on the presence of potentially pathogenic bacteria in the filtered and stored water.We therefore scrutinized a household sand filter with respect to As removal efficiency and the presence of fecal indicator bacteria in treatedwater as a function of filter operation before and after sand replacement. Quantification of As in the filtered water showed that periods of intense daily use followed by periods of non-use and even sand replacement did not significantly (p b 0.05) affect As removal efficiency. The As
concentration was reduced during filtration from115.1±3.4 μg L−1 in the groundwater to 5.3±0.7 μg L−1 in the filteredwater (95% removal). The first flush of water fromthe filter contained As concentrations belowthe drinking water limit and suggests that this water can be used without risk for human health. Colony forming units (CFUs) of coliform bacteria increased during filtration and storage from 5 ± 4 per 100 mL in the groundwater to 5.1 ± 1.5 × 103 and 15 ± 1.4 × 103 per 100 mL in the filtered water and in the water from the storage tank, respectively. After filter sand replacement, CFUs of Escherichia coli of b100 per 100 mL were quantified. None of the samples contained CFUs of Enterococcus spp. No critical enrichment of fecal indicator bacteria belonging to E. coli or Enterococcus spp. was observed in the treated drinking water by qPCR targeting the 23S rRNA gene. The results demonstrate the efficient and reliable performance of household sand filters regarding As removal, but indicate a potential risk for human health arising from the enrichment of coliform bacteria during filtration and from E. coli cells that are introduced by sand replacement.
Short TitleScience of The Total Environment
CAWST Analysis: 
This study provides further evidence that iron-sand treatment system is effective in arsenic removal.  For ideal arsenic removal, iron:arsenic ratio should be >50; competing species such as phosphate should be <2.5 mg/L; and sand depth should be > 30 cm.

In this study site, the ground water contains high levels of iron (about 16 mg/L), arsenic (about 115 ug/L), and manganese (about 1200 ug/L).  When the ground water is pumped above ground and is poured into a household sand filter, all of these 3 species, namely dissolved iron in the form of Fe(II), dissolved arsenic in the form of As(III), and dissolved manganese in the form of Mn(II) are oxidized by atmospheric oxygen.

Fe(II) is oxidized into insoluable Fe(III).  As(III) is oxidized into arsenate, or As(V), and is removed either by sorption to or co-precipitation with Fe(III) phases. Manganese  is also oxidized by aeration initially, into Mn(IV). However, due to the excess of As(III) and Fe(II), especially in the top 20 cm of sand, Mn(IV) is subsequently used as an oxidant for As(III) and Fe(II), turning them into As(V) and Fe(III), and subsequently immobilizing arsenic and iron. Consequently, Mn(IV) is turned into dissolved Mn(II) form in the top 20 cm of sand.

At sand depth of about 20 to 30 cm, the study found manganese occur in the form of Mn(IV), indicating that no Fe(II) and As(III) are present anymore. Mn(II) has oxidized into Mn(IV) due to dissolved oxygen at this depth, catalyzed by microbial activities at this depth.

So, in simple terms, iron and arsenic are mainly trapped in the top 20 cm of the sand, with highest concentrations in the top 2 cm. Very low level of arsenic and iron is found beyond 20 cm depth.

Furthermore, the study found arsenic concentration in the filtered water to be consistently within the WHO guideline of 10 ug/L.  Effluent arsenic is not affected by (1) increasing the frequency raw water is poured into the filter per day, (2) stopping filter use for 3 days in a row, and (3) replacing the entire sand layer.

This study also mentioned that coliform bacteria increases significantly after filtration.  However, it should be noted that the household sand filter studied is not a biosand filter.  The sand filter studied is not designed for microbial removal. 

There are 3 main differences between the sand filter studied and a biosand filter. First the outlet of the sand filter studied is at the bottom. Thus, water drains completely, sand is not saturated with water, and there is no standing water level as in a biosand filter.  Second, the sand filter studied has a sand depth of only 32 cm, rather than 55cm as in a biosand filter version 10.  Finally, the sand filter studied has sand grain size much larger than the filtration sand in a biosand filter.  About 1/3 of the sand has grain size > 1mm, and 2/3 of the sand < 1mm.  The biosand filter requires all filtration sand to have grain size <0.7 mm.

Due to these differences, the sand filter studied does not remove microbiological contaminants.

Nevertheless, the main contribution of this study is its clarification on how arsenic can be effective removed by a sand filter in the presence of iron.  Therefore, sand filters can be an excellent option in arsenic removal when designed properly.