4. IN-SITU REMOVAL OF ARSENIC
Arsenic contamination of groundwater poses serious health threats and mitigation of this contamination is of high importance in affected regions. The World Health Organisation has set 0.01 mg/L as limit value for arsenic in drinking water, which has also been taken up by Vietnamese drinking water standards (TCVN 5502:2003).
Mitigation strategies generally focus on use of alternative safe water sources, such as surface water, rain water or different aquifer layers unaffected by arsenic contamination, or on the removal of arsenic from contaminated sources. Removal of arsenic from groundwater can be done on a medium to large scale level in community or municipal treatment plants or on household level. A wide range of treatment processes are established or being developed; these can be grouped under either oxidation and sedimentation, coagulation and filtration, sorptive filtration or membrane filtration.
Several arsenic removal processes make use of the effect of arsenic binding to the positively-charged surface of iron hydroxide and oxides, e.g. in coagulation with ferric sulfate or ferric chloride or by arsenic adsorption to granular iron hydroxide/oxide. The same material is formed in the aquifers when in-situ iron removal of iron causes iron hydroxide and oxide to develop around particles of the aquifer matrix. This can in principle also be used for in-situ adsorption of arsenic.
In many cases, high iron and manganese concentrations are also present when groundwater contains high arsenic content. Reduction of arsenic concentrations can therefore be a welcoming side-effect of in-situ iron and manganese removal. In-situ treatment may also be specifically conceived for arsenic removal, making use of all the advantages of the in-situ treatment technology, such as lower cost or avoiding producing waste sludge. Avoiding the production of waste streams is strong advantage in arsenic removal, due to the potentially hazardous nature of arsenic rich waste.
In-situ arsenic removal has been tested in Europe, USA, India and Bangladesh. Studies in Europe with co-removal of As during in-situ iron removal found a reduction of As to below the WHO limit (Rott et al. 2002). The company FERMANOX reports that one of its plants installed in Bavaria, Germany, reduces Arsenic concentrations in groundwater from 0.027 mg/L to 0.002 mg/L (FERMANOX website). A study in West-Bengal examined six small scale in-situ arsenic removal plants (Sen Gupta et al., 2009). The authors found that the plants could produce water satisfying the WHO limit, however, at a rather low efficiency of 2 m3 water extracted per m3 of water infiltrated. A study examining different aspects of in-situ arsenic removal in household water treatment in Bangladesh concluded that the technology is less suitable in handpump applications and vulnerable to diverse geochemical conditions. (D. van Halem, 2011). Although several large scale applications of in-situ removal of arsenic seem to be under way, including in Vietnam, there is no documented experience available in literature to date. People at FERMANOX are convinced that in-situ arsenic removal has a very high potential to succeed, but think that good pilot applications with both small and large-scale applications are still largely missing (personal communication T. Winkelnkemper).
The review of documented experience with in-situ arsenic removal indicates that that in principle, arsenic concentrations can be successfully reduced to acceptable levels by in-situ removal. Available studies are cautious about the actual potential for household level applications. On the other hand, medium to large scale applications appear most promising. Research, real-scale piloting and documentation of experience is definitely needed to develop the technology and to showcase its feasibility.
5. CONCLUSION: POTENTIAL OF IN-SITU TREATMENT OF IRON, MANGANESE AND ARSENIC IN VIETNAM
5.1 Groundwater in Vietnam
The large majority of the population and economic activities in Vietnam are concentrated in the two large deltas in the north and south of the country and in the narrow coastal strip. Groundwater is the most important resource for drinking water supply there and is also often used for industrial applications. Iron and manganese is frequently present in groundwater in this region, the removal of iron and manganese is a routine treatment requirement, both for household level supply and in municipal or large-scale industrial applications.
Arsenic contamination of groundwater has been identified more than a decade ago to be a serious issue in both of the two large river deltas. About 10 million persons are estimated to be affected in the Red River Delta and up to 1 million in the Mekong delta (Michael Berg et al., 2007). While the rural population using individual tube wells for water supply is under the highest threat, also the populations of large cities like Hanoi risk to consume water contaminated with arsenic. Finding sound mitigation strategies and treatment technologies for securing the water supply of rural and urban populations is an important and still largely unsolved challenge.
5.2 Potential for in-situ removal of iron and manganese in Vietnam
The potential for in-situ removal of iron and manganese in Vietnam is obvious and huge. The low requirements in operation make the technology especially suitable for small rural water providers with low capacities for running complex facilities. Of course, its cost advantages will also be beneficial to large municipal utilities or industries.
Despite the obvious potential, in-situ treatment is not yet established in Vietnam. As with all technologies that are new in a country, up-take does not happen by itself, but requires a concerted effort of piloting, evaluating, documentation, dissemination of experience, lobbying towards authorities, revising norms and standards, educating engineers, raising awareness of end users and much more. This will evidently require considerable effort and patience by pioneers among scientists, water works and industrial enterprises investing into the introduction of in-situ removal technology to Vietnam, but the chances to succeed are good.
5.3 Potential for in-situ removal of arsenic in Vietnam
The previous chapter has highlighted that in-situ removal of iron and manganese has a strong potential for application in Vietnam. It can be expected that in-situ arsenic removal, which naturally goes along with in-situ removal of iron and manganese, will receive equal attention among Vietnam’s water engineers and scientists.
The technology of removing arsenic with in-situ treatment in aquifers is still in its infancy and more research is needed for both understanding basics of the process as well as developing the technology into a routine application. However, the treatment technology has strong potential advantages like cost-efficiency and zero waste streams and it definitely should be considered as one of the priorities in research and development for arsenic mitigation in Vietnam and worldwide.
European researchers and companies have decades of experience with in-situ iron and manganese removal. They are highly interested in cooperating with Vietnamese partners for bringing in-situ arsenic removal to maturity and contributing to making groundwater in Vietnam’s deltas a safer source for drinking water supply.
REFERENCES
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