Arsenic is a toxic element that is present at high levels in many groundwaters, with local geology often being the source. If a drinking water supply is left untreated, the symptoms of arsenic poisoning (arsenicosis) include organ failure, nerve damage, skin scaling and an increased risk of cancer.
Being on the World Health Organisation's list of 10 chemicals of major public health concern, removing arsenic from drinking water is a top priority for governments worldwide.
RISING TO THE CHALLENGE
RISE is a student-led organisation that was launched by graduate students at Tsinghua University. Their water projects are designed to help protect the estimated 280 million people in China without access to safe drinking water.
In their latest project, RISE have been working to remove high levels of arsenic often found in rural well waters by using bio-sand filters.
Traditional bio-sand filters have shown good efficacy for removal of pathogens such as E. coli, but have struggled with chemical contaminants like arsenic. In sponsorship with Palintest, RISE have been able to modify the filters and accurately track their ability to remove arsenic from untreated ground water in real world situations.
Bio-sand filtration is an emerging technology to tackle water quality in developing regions. Cheap to build and easy to maintain, they are able to improve drinking water quality and have been shown to reduce cases of dysentery in communities where filters have been installed.
They work by slowly drawing the water through a column of sand which physically traps many of the contaminants. As the filter ages, microorganisms form a biofilm over the sand which can consume and absorb many of the biological pathogens in the water. Unlike other technologies, bio-sand filters get more effective with time, as this biofilm grows.
In order to remove naturally present arsenic in the groundwater, the team incorporated rusty nails in the filter. As iron rusts, the oxides that form can trap and adsorb arsenic from the water as it passes through the filter - the nails provided a cost effective and easily accessible form of iron oxide.
In order to be truly effective, there must be enough contact time between the water and iron oxide without stopping the oxidation or rusting process. Therefore, two designs were trialled with nails placed either above, or embedded within, the fine sand layer. Nails placed above the filter would have ample access to oxygen at the expense of a reduced contact time, whilst nails embedded in the filter would have extended contact times but may be starved of available oxygen - preventing continued rusting and diminishing performance.
THE MEASUREMENT MATTERS
The aim of the project was to establish which filter design was the most effective. The benchmark for this was to quantify which filter could reduce arsenic levels to below China's guideline level of 50 µg/L or, if possible, below the WHO's guideline value of 10 µg/L. The filters would have to be able to consistently perform when incoming arsenic levels were in excess of 200 µg/L.
The RISE students therefore needed a way of accurately measuring arsenic levels to below 10 µg/L in a portable field instrument, to both verify laboratory results and for when immediate analysis was required. By using the Arsenator DigiPAsS, the team were able to obtain rapid on-site data with digital precision, avoiding the subjectivity often associated with test strips. With Palintest's unique tri-filter arsenic trap system, harmful gasses do not escape into the atmosphere, protecting those carrying out the test.
ARSENIC REMOVAL RESULTS
The most effective filter was the bio-sand filter with nails embedded 3cm below the surface of the bulk sand (NIS Filter). The table below shows an example of the results for the bio-sand filter with nails in a diffuser basin above the sand (ABF Filter) as well as the NIS Filter, which were obtained via ICP-MS and verified on-site using the Arsenator DigiPAsS.
|Average Influent |
|ABF Filter Effluent |
|NIS Filter Effluent |
|232 ||54.7 ||21.1 |
|359 ||71.69 ||19.61 |
|345 ||90.39 ||21.1 |
|222 ||52.6 ||19.54 |
The results show that the embedded nails in sand filter was very effective at reducing arsenic levels to near the WHO guideline value of 10 µg/L, even when the source water was over 300 µg/L.
Initially there was concern that embedding the nails and preventing access to oxygen would quickly reduce the efficacy of the filter. However, dissolved oxygen in the water appears to have allowed the iron oxide products to continue forming, ensuring consistent long term performance from the filter unit. The NIS filter effluent was consistently below 50 µg/L over a six month period.
"The great thing about the Arsenator is that you can use it in the field, which means you get immediate results and can have more control over sample collection and testing. It is great for confirming lab results when you are unsure."
Kate Smith, RISE President and Doctoral Candidate at Tsinghua University's School of Environment.
The research and implementation of the embedded nails bio-sand filter has demonstrated significantly improved drinking water quality. The results obtained by the RISE team are now published in the environmental chemistry journal, Chemosphere and their work has been recognised internationally through the Veolia Student Solidarity Award.
Looking to 2017, RISE plan to return to villages that have been identified as having the highest arsenic concentrations in well water, based on Arsenator results, to learn more about the overall water quality. This will be followed by a larger trip to continue with filter installation and testing, to establish how they perform as the water quality changes between villages.
As important as the filters will be a series of workshops and surveys to improve villagers' water safety awareness and gauge their feelings with regards to local water quality and receptiveness to the bio-sand filters.
It's going to be a busy year for the RISE team and Palintest are proud to be able to provide the right analysis equipment for water quality improvement projects such as RISE.
For more information on the project, please visit www.rise-thu.org