Importance of Evaluating Phosphate Levels in Tubewells in High Arsenic Areas of Asia Tom Mahin1,2, Tommy Ngai2, Susan Murcott1, Mohon Mondal3 1 - Massachusetts Institute of Technology (MIT) 2 - Centre for Affordable Water & Sanitation Technology (CAWST) 3 - LEDARS – BangladeshPresented at 33rd WEDC International Conference Accra, Ghana
Arsenic in Asia From: World Bank “Study: Arsenic Contamination ofGroundwater in South and East Asian Countries” 2005
Impact of Phosphates onArsenic Removal by Adsorption SystemsNumerous studies have shown thatphosphates (PO4) have a significant impacton arsenic removals by some of the mostcommon and cost effective arsenic treatment(iron-based) systems.Because PO4 has a similar chemical structureto arsenate, they compete with arsenic foradsorption sites on iron oxides.PO4 are relatively common in tubewells inhigh arsenic areas of Asia.
Origin of Phosphates in Ground WaterPhosphates are believed to result from thedecomposition of organic matter. Also similarto the release of arsenic, phosphates can bereleased from sediments in aquifers by: (1) dissolution of iron oxides under anoxic conditions (absence of dissolved oxygen), (2) desorption of phosphates from iron oxides iron (such as high pH conditions).Fertilizers & latrines can also contributephosphates to shallow groundwaters.
Phosphates Often are Elevated When Arsenic is at High Levels (red line is 50 ug/L) Pakistan India (West Bengal)Above from Nickson et al., Applied Geochemistry Above from McArthur et al., Applied Geochemistry China - Inner Mongolia Bangladesh From KTH Sweden by Fei Shi (2004) From KTH – Sweden by Jonsson & Lundell
Bangladesh PhosphateResults (DPHE/BGS)• 0.3 mg/L P (median)for 3,530 samples• But when As > 10 ppbP averaged 1.3 mg/L(median - 1 mg/L)
West Bengal(north of Barasat in southern West Bengal) PO4 ranged from (1–6.5 mg/L). “ From: McArthur et al., Natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water: the example of West Bengal and its worldwide implications” Applied Geochemistry 19 (2004) 1255–1293
China - Inner Mongolia (Huhhot Basin)> 50 ppb As Wells Have Elevated Phosphates 50 ug/L - As Wells > 100 m ( ) have higher arsenic, phosphate & pH levels From : “Mobilisation of arsenic and other trace elements in fluviolacustrine aquifers of the Huhhot Basin, Inner Mongolia” Smedley et al, Appied Geochemistry (18) 2003
Example of How Phosphate, Iron & Arsenic Levels Vary by Individual Wells (Bangladesh - Chandpur)For 2 different wells and iron levels are very similarbut phosphates differ by a factor of 10 From: Targeting safe aquifers in regions with arsenic-rich groundwater in Bangladesh Case study in Matlab Upazila - Jonsson and Lundell (2004) Swedish University of
Iron to Phosphate Ratios The ratio of iron to phosphates (Fe/PO4-P)is important because higher iron levelsincrease and higher phosphates reducearsenic % removals. So higher Fe/PO4-Pratios are associated with > arsenic %removals. Phosphate results are reported as eitherPO4-P or P, both are the same. Howeverresults reported as PO4 are about 3 timeshigher than as P. .
Red River Delta Vietnam Parameter Average Median (n=74) Arsenic* 115 ug/L 98 ug/L (range 9-382 ug/L) PO4-P* 0.75 mg/L 0.56 mg/L Fe* 13. 7 mg/L 13 mg/L Fe/PO4-P ratio 18 (avg./avg.)Conclusion: PO4-P levels high but Fe also high, sandfilters can remove arsenic for some wells in Vietnam * Data from: Berg et al. “Arsenic Removal from Groundwater by Household Sand Filters – Comparative Field Study, Model Calculations, and Health Benefits”– E, S & T
High Arsenic Area of Cambodia (Kandal Province) PO4-P (mg/L)* As (ug/L)* Fe (mg/L)* Average - 0.66 Average – 233 Average – 2.8 Range:<0.2–3.14 Range:1 -1340 Range<0.05-16 Calculated Average Fe/PO4-P Ratio 4Conclusion – Generally highPO4-P levels & moderate Felevels, supplemental ironneeds to be used not just filters* - Data from Swiss Institute of Aquatic Science andTechnology, Eawag, as published in Environ. Sci. Technol.
The Impact of Phosphates on % Removal of Arsenic by Sand Filters Pink circle = very high phosphates (P > 2.5 mg/L). * Note as iron levels increase, the % As removals increase even with very high phosphates Adapted from graph by Berg et al., “Household Sand filters for Arsenic Removal – Technical Report” – EAWAG 2004
Fe/PO4-P Ratios and % As Removal Arsenic As - % As Iron PO4-P Fe/PO4-P (As) well filtered removed (well) mg/L ratio ug/L water mg/L Vietnam 223 21 91% 11 0.05 220 137 49 64% 11 2.8 4 117 45 62% 10 2.6 4 70 9 87% 7 0.2 34 55 44 20% 6 3.7 2 Bangladesh 8 + iron 3 without nails 171 25 85% with nails from nails 2.3 17 with nailsRaw Data from: Berg et al. “Arsenic Removal from Groundwater by Household Sand Filters – Comparative Field Study, Model Calculations, and Health Benefits”– E S & T, Ratios by Mahin
ConclusionsEven where PO4 levels are generallylower, individual wells may have high PO4.Fe/PO4 ratios should be considered whenevaluating treatment performance results &when preparing sampling plans.MIT/CAWST/LEDARS pilot tested KanchanArsenic Filter (KAF) in areas of Bangladeshwith high arsenic/phosphate levels. The KAFuses nails that rust adding iron & resulting inhigh Fe/PO4 & higher As removals.