Arsenic removal for contaminated groundwater
Tom Hall, principal technical specialist at WRc Processes, has investigated the options for removing arsenic from contaminated groundwater supplies.
A lower standard for arsenic is to be added to the EC’s drinking water directive, with the current standard of 50µg/l to be replaced by a level of 10µg/l. Arsenic is an unusual problem in the UK, with a only a few affected water sources. In some cases the contamination has been linked to coal or tin mining activities. Small boreholes where arsenic exceeds 10µg/l may be shut down, but treatment will be necessary at large sources in order to meet the new standard by 2003.
Chemical coagulation treatment is capable of removing arsenic to achieve the new standard, particularly when using iron based coagulants, and this approach is likely to be implemented at surface water treatment works. However, this type of treatment is generally unsuitable for groundwater sources, particularly for smaller sites, because of chemical and waste handling difficulties. There are two main alternatives, reverse osmosis (RO) or adsorption-based processes.
Perceived disadvantages of RO include its relatively high cost, production of large volumes of waste for disposal, and its potential to change the corrosivity of the water as a result of softening and de-alkalisation of the treated flow.
Adsorption processes are seen to be more suitable, using either activated alumina or iron-based media, some of which have been developed primarily for arsenic removal. Pilot plant trials have now been carried out in order to compare adsorption media. Software (GACman) has also been developed by WRc for modelling arsenic removal on a large scale.
In the pilot plant trials, chlorinated water was used as the feed, typically containing less than 0.1mg/l free chlorine. The arsenic concentration in the feed water was usually within the range 15 to 25µg/l, averaging around 20µg/l. With the use of chlorinated water it was believed the arsenic would largely be in the arsenate form, which is much more readily removed by adsorption (and coagulation) than arsenite.
The plant consisted of eight clear plastic columns, each 0.1m in diameter and about 1m deep. A 0.6m depth of media and a downflow of 100 l/h would provide an empty bed contact time (EBCT) of 3min. By adjusting flowrate and by operating beds in series, EBCTs of three, six and 12min were investigated. The facility to dose hydrochloric acid to the feed to some of the columns was available, so that the effects of pH could be investigated.
Performance of each media was assessed by taking daily samples of the feed and treated waters primarily for arsenic analysis, from which the capacity of the media for arsenic could be calculated.
Media investigated included:
- activated alumina of two grain sizes (14/28 and 28/48),
- a granular ferric hydroxide media (Wasserchemie GEH),
- catalytic manganese media,
- strong base anion exchange resins (including Purolite A400).
The ion exchange resin and the catalytic media showed a very low capacity for arsenic, treating less than 1,000 bed volumes (BV) to 10µg/l in the treated water. Furthermore, arsenic was displaced from the resin to much higher concentrations than in the feed water (over 50 µg/l), making it particularly unsuitable for this application. Performance of the activated alumina and GEH media are reviewed below.
Performance at each EBCT is shown in Table. 1, indicating an increasing arsenic capacity with increasing EBCT. This means that, for a given site, higher costs for larger plant to achieve longer EBCT would be offset to some extent by a lower operating cost for replacement (or regeneration) of exhausted media. Performance of activated alumina (14/28 grade) was evaluated at four pH levels, achieved by dosing acid to the feed to three columns to reduce the pH relative to that of the feed water. The results, shown in Fig. 1, demonstrate that reducing the pH gave a dramatic increase in capacity. The bed life at the unadjusted pH was around 20 days compared with over 200 days at pH 6.
The cost of greatly reduced media requirements at lower pH would be offset by the cost of pH adjustment, not only for the acidification but also for alkali to return to the original pH before distribution. Using sulphuric acid, the cost to reduce the pH of the water at this site to 6.5 was estimated to be 0.5p/m3, plus 1.5p/m3 to return the pH to 7.5 using sodium hydroxide (NaOH). Despite the dramatic increase in capacity, these costs and, more significantly, the additional chemical storage and handling requirements, particularly for small sites, did not make pH adjustment a favourable option.
When the media becomes exhausted, two options are available; either disposal of the spent media and replacement with new, or chemical regener-ation using NaOH. The latter was investigated over several runs using pilot plant columns.
Regeneration involved passing eight bed volumes of 0.5M NaOH (2% w/w) upwards through the bed at a rate of 4 BV/hr. After the first regeneration, there was roughly 20% reduction in arsenic removal capacity compared with the virgin material, although there was no further marked reduction in capacity over the subsequent six regenerations.
The initial treated water quality when a regenerated bed was brought into operation was poor, with high pH and high aluminium, which would remain above 200µg/l for up to 250 BV treated. This could be reduced to 50 BV by rinsing the bed with acid after regeneration. Cost estimates suggested that the regeneration option could be less expensive than media replacement, taking into account the chemical costs compared with media replacement and the waste disposal costs (despite the much greater volume of waste with the regeneration option). However, problems arising from chemical storage and handling, particularly for smaller and more remote sites, may make bed regeneration an unfavourable option for implementation at individual sites.
GEH media results
The very much higher capacity for the Wasserchemie GEH ferric hydroxide-based media compared with the activated alumina media is indicated in Fig. 2. Tests indicated that, as with the activated alumina, capacity could be increased considerably by pH reduction of the feed water. For example, at pH 6.5, the treated water arsenic concentration was 1µg/l after 150,000 BV, compared with 6µg/l at pH 7.5. The full potential of pH adjustment was never identified, but it is likely that well over 200,000 BV could be treated at pH 6.5 before the arsenic concentration in the treated water reached 10µg/l. However, the same arguments against pH adjustment apply for GEH as for alumina, and because the bed life is very high for GEH without pH adjustment, the potential benefits would be less marked.
Waste & disposal
For a plant treating 20Ml/d using a 3min EBCT, the total volume of media would be 42m3. Table. 2 indicates the waste volumes for disposal from this design of plant using activated alumina (with media replacement or regeneration after exhaustion) or GEH.
The regeneration option figures assume an eight BV of total chemical use (alkali plus acid). The figures also neglect the likely need to replace media after a several regenerations (which would reduce the volume of waste) and does not allow for reuse of regenerant.
An additional waste for disposal results from the initial poor quality water when virgin or regenerated media are brought into operation. Pilot trials suggested this could be as much as 50 BV for regenerated activated alumina (assuming an acid rinse after regeneration) and 10-20 BV for virgin activated alumina or GEH.
Operating costs for activated alumina with media replacement (based on the results of the pilot plant trials), are as follows;
- No pH adjust: 5.5p/m3
- pH adjust to 6.5: 3.1p/m3
- Regen. no pH adjust: 3.2p/m3
The pH adjustment costs include NaOH to return the pH to the original value. With GEH, assuming the bed life would be 15 times longer than activated alumina without pH adjustment, the operating cost would be <1p/m3 even if the media was two to three times more expensive than alumina.
|Table 1 Effect of EBCT on Arsenic Capacity
of Activated Alumina
|Empty bed contact time (EBCT)||Bed volumes (BV) to achieve 10µ/l in
|Removal capacity (gAS/kg activated alumina)|
|Table 2 Waste Production for a 20ml/d Plant
With 3 Min EBCT
|Treatment media||Bed volumes treated||Annual disposal|
|28/48 alumina (disposal)||12,000||613m3 media|
|28/48 alumina (regen.)||10,000||5840m3 regenerant|
|GEH/FeOH (disposal)||>150,000||49m3 media|
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