The 1998 European Drinking Water Directive includes an arsenic standard of 10 ?g/l, reduced from 50 ?g/l in the 1980 Directive. This has meant that arsenic-removal plants are having to be considered for some British water sources. Some arsenic removal may occur in treatment plants using ion exchange, activated carbon adsorption or coagulation and filtration for iron-removal plants, but it has not been necessary in the past to design plants for arsenic removal.

The processes that can be used for arsenic removal are either adsorption onto a suitable media or reverse osmosis. Unless there are other water-quality issues that require reverse osmosis treatment, adsorption will normally be preferred. The process involves passing water through a bed of adsorbent media. Possible adsorbents include:

• granular ferric hydroxide;

• activated alumina; and

• bone char.

Ion-exchange resins and activated carbon may also be used.

Activated alumina and granular ferric hydroxide are considered to be the most likely adsorbents for a dedicated arsenic-removal plant. The design issues that need to be considered are the same as for other adsorptive processes: contact time; hydraulic loading rate, absorptive capacity; and backwashing/regeneration. The quality of the feed water needs to be high, with low turbidity and solids so adsorption is appropriate only for high-quality groundwater or treated surface waters. EBCT is normally a minimum of around 5 min. The permissible hydraulic loading rate depends on the particular media used, maxima of 10 20 m/h are appropriate. However in practice the acceptable hydraulic loading rate may be...