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 – Bangladesh

Presented at 33rd WEDC International Conference Accra, Ghana

2008

Arsenic in Asia

From: World Bank “Study: Arsenic Contamination of

Groundwater in South and East Asian Countries” 2005

Impact of Phosphates on

Arsenic Removal by Adsorption Systems

Numerous studies have shown that phosphates (PO4) have a significant impact on arsenic removals by some of the most common and cost effective arsenic treatment (iron-based) systems.

Because PO4 has a similar chemical structure to arsenate, they compete with arsenic for adsorption sites on iron oxides.

PO4 are relatively common in tubewells in high arsenic areas of Asia.

Origin of Phosphates in Ground Water

Phosphates are believed to result from the decomposition of organic matter. Also similar to the release of arsenic, phosphates can be released 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 contribute phosphates to shallow groundwaters.

Phosphates Often are Elevated When

Arsenic is at High Levels (red line is 50 ug/L)

Pakistan India (West Bengal)

China - Inner Mongolia

Above from McArthur et al., Applied Geochemistry

Bangladesh

From KTH – Sweden by Jonsson & Lundell

Above from Nickson et al., Applied Geochemistry

From KTH Sweden by Fei Shi (2004)

Bangladesh Phosphate

Results (DPHE/BGS)

• 0.3 mg/L P (median)

for 3,530 samples

• But when As > 10 ppb

P averaged 1.3 mg/L

(median - 1 mg/L)

West Bengal

(north of Barasat in southern West Bengal)

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

PO4 ranged from

(1–6.5 mg/L).

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)

From: Targeting safe aquifers in regions with arsenic-rich groundwater in Bangladesh

Case study in Matlab Upazila - Jonsson and Lundell (2004) Swedish University of

Agricultural Sciences

For 2 different wells and iron levels are very similar

but phosphates differ by a factor of 10

Iron to Phosphate Ratios

The ratio of iron to phosphates (Fe/PO4-P) is important because higher iron levels increase and higher phosphates reduce arsenic % removals. So higher Fe/PO4-P ratios are associated with > arsenic % removals.

Phosphate results are reported as either PO4-P or P, both are the same. However results reported as PO4 are about 3 times higher than as P.

.

Red River Delta Vietnam

Parameter Average

(n=74) Median

Arsenic*

(range 9-382 ug/L)

115 ug/L 98 ug/L

PO4-P* 0.75 mg/L 0.56 mg/L

Fe* 13. 7 mg/L 13 mg/L

Fe/PO4-P ratio

(avg./avg.)

18

* Data from: Berg et al. “Arsenic Removal from Groundwater by Household Sand Filters –

Comparative Field Study, Model Calculations, and Health Benefits”– E, S & T

Conclusion: PO4-P levels high but Fe also high, sand

filters can remove arsenic for some wells in Vietnam

High Arsenic Area of Cambodia (Kandal Province)

PO4-P (mg/L)* As (ug/L)* Fe (mg/L)*

Average - 0.66

Range:<0.2–3.14

Average – 233

Range:1 -1340

Average – 2.8

Range<0.05-16

Calculated Average Fe/PO4-P Ratio 4

* - Data from Swiss Institute of Aquatic Science and

Technology, Eawag, as published in Environ. Sci. Technol.

2007, 41

Conclusion – Generally high

PO4-P levels & moderate Fe

levels, supplemental iron

needs to be used not just filters

The Impact of Phosphates on % Removal

of Arsenic by Sand Filters

.

Pink circle = very high phosphates (P > 2.5 mg/L)

Adapted from graph by Berg et al., “Household Sand filters for

Arsenic Removal – Technical Report” – EAWAG 2004

* Note as iron levels increase, the

% As removals increase even with

very high phosphates

Fe/PO4-P Ratios and % As Removal

Raw 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

Arsenic

(As) well ug/L

As - filtered

water

% As

removed

Iron (well) mg/L

PO4-P

mg/L

Fe/PO4-P

ratio

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

Vietnam

171 25 85% with nails 8 + iron

from nails 2.3 3 without nails

17 with nails

Conclusions

Even where PO4 levels are generally

lower, individual wells may have high PO4.

Fe/PO4 ratios should be considered when

evaluating treatment performance results &

when preparing sampling plans.

MIT/CAWST/LEDARS pilot tested Kanchan

Arsenic Filter (KAF) in areas of Bangladesh

with high arsenic/phosphate levels. The KAF

uses nails that rust adding iron & resulting in

high Fe/PO4 & higher As removals.