Arsenic removal
In addition to iron and manganese, arsenic is another trace element that occurs particularly in reduced groundwater (deep groundwater). Arsenic contamination of drinking water is widespread worldwide. The problem is that low concentrations of arsenic are not detectable by taste. At the same time, arsenic accumulates in the body and can lead to gradual poisoning with a range of symptoms, including dark skin discoloration and skin cancer on the extremities.
Due to its toxic and carcinogenic effects, drinking water with elevated arsenic levels must be treated. The limit value of < 0.010 mg/L applies to water supply systems that were put into operation before 12 January 2028 and remains valid until the end of 11 January 2036.
For systems commissioned on or after 12 January 2028, as well as for all systems from 12 January 2036 onwards, a stricter limit value of < 0.004 mg/L applies.
The causes of elevated arsenic levels in water are usually natural arsenic deposits. Arsenic can be dissolved from rock by infiltrating water, where it mainly occurs in two chemical forms: trivalent arsenite (As³⁺) and pentavalent arsenate (As⁵⁺). Arsenic concentrations in raw water can reach up to 0.1 mg/L or even higher.
Water with an arsenic concentration above 0.010 mg/L or more than 0.004 mg/l from 2028 must be treated. However, arsenic can be removed from water relatively easily and reliably.
Processes for arsenic removal
Depending on the arsenic concentration and the water matrix, various methods are available for removing arsenic. Arsenic has a high affinity for iron. If the iron concentration is also elevated, arsenic can be completely or at least partially removed through oxidation and subsequent filtration.
Removal through oxidation
By using ozone or other strong oxidising agents, arsenic removal can be combined with iron and manganese removal in a single filtration step. In this process, arsenite is oxidised to arsenate, which is retained in the filter. Due to its high affinity for iron, arsenate binds to iron hydroxide and is removed from the filter bed together with the iron and manganese sludge during filter backwashing.
If the water does not contain iron, it may be necessary to dose iron-based flocculants.
Removal through adsorption
At low arsenic concentrations, filtration using adsorptive filter media such as granular ferric hydroxide (GFH/GEH) is possible. Arsenic binds to the GEH material. This requires that iron and manganese be completely removed from the water prior to adsorption. Adsorption filters can reduce arsenic concentrations to below the detection limit.
Adsorption filters cannot be backwashed. Once the medium is exhausted, it must be completely replaced. The typical service life is two to three years. Advantages include a simple system design, low maintenance requirements and ease of operation.
Removal through flocculation
Another method (particularly suitable for large-scale plants) is flocculation/precipitation by dosing iron salts upstream of a filtration stage. Similar to adsorption, arsenic binds to the resulting iron sludge and is subsequently removed by filtration.
A disadvantage of this method is the comparatively high volume of sludge produced, which generally requires complex sludge treatment including dewatering and pressing.
Removal by Nanofiltration (NF) or Reverse Osmosis (RO)
Both forms of arsenic can be reliably removed from water using reverse osmosis. This process is particularly suitable when fully desalinated water (e.g. for process water) is required. However, the high energy demand, increased wastewater volume and up to 30% higher water production must be taken into account. In addition, the use of chemical additives is required.
Consulting and Planning
Are you interested? We would be pleased to support you in the design of your water treatment system and in selecting the optimal process. Please contact us.