Management of Trace Elements in Ash Dams

This project was undertaken to make an assessment of the practicality of "alternative" strategies to manage trace elements, in particular, arsenic, boron, molybdenum, selenium and vanadium in ash dams. These elements form oxyanions in flyash leachates and are soluble at the pH ranges occurring in the dams. Their release can have a detrimental effect on the environment.

Although some conventional water treatment processes should be able to remove the targeted trace elements from water, there are significant capital and operational costs involved in treating the large volumes of water discharged from ash handling systems. For example, it is possible to treat ash dam water with ferric salts to induce the formation of ferric oxyhydroxide at an optimum pH. This would remove arsenate, molybdate, selenite and vanadate from the ash dam water. Borate and selenate will not be removed.

It has been reported in a study by Tarong Energy that the cost of treating ash dam water in a reverse osmosis plant was estimated to be $1023/ML. There is currently some 10 Mt of ash produced yearly in Australian bituminous coal-fired power stations. Although not all of this ash is stored in ash dams (some is utilised and much is stored as "dry" landfill), the magnitude of the expense required to treat the volumes of ash dam water or leachate that may be discharged is evident.

Preliminary work had indicated that the following two processes :

• direct addition of ferric chloride to ash
• use of zero-valent iron to treat ash dam water

have the potential to minimise the release of the trace elements, arsenic, molybdenum, selenium and vanadium.

This project continued the study commenced by CSIRO and the CRC for Coal in Sustainable Development. Further experimental work including field work was completed into the use of iron as a reductant. As well, an assessment of the use of biological treatment in bioreactors and wetlands has been included. Thermodynamic calculations using MINTEQ have also been completed both on the process of reduction with iron and by anaerobic bacteria (used in bioreactors and naturally present in wetlands).

Results
The results of the experimental work and assessment of the data indicate that:

• the "inpond" process in which ferric chloride is added directly to flyash to prevent the release of the targeted trace elements would be limited to ashes with a circum neutral pH (the pH range is critical and at greater than approximately pH 8, there is a release or poor adsorption of the elements). Further study would be required to assess the long term leach behaviour of the treated ash
• a strategy based on the use of iron metal (zero-valent iron) as a reducing agent has obvious potential. The oxidation (corrosion) of the iron acts to chemically reduce some species including selenate and also results in the formation of ferrous and ferric oxides/hydroxides on the metal's surface. Arsenate, molybdate, selenite, selenate and vanadate can be removed from ash dam water. The process requires that the water contact a large surface area of iron/iron oxide to be effective. Further work is required to design efficient operational plants that could utilise materials such as scrap iron, steel wool waste or commercially available iron powder. A disadvantage in the use of the iron metal is the release of ferrous ions and possibly trace amounts of other elements.
• constructed wetlands could be used to remove some of the targeted trace elements. The data is limited but the biological activity will remove the arsenic and selenium (certainly the selenite). Further work is required to assess the behaviour of molybdenum and vanadium
• none of the above processes were able to remove boron. It appears that even costly commercial processes such reverse osmosis have limitations in removing this trace element.

Conclusions
The two most promising strategies to manage the trace elements, arsenic, molybdenum, selenium and vanadium in ash dam water are the use of iron metal (zero-valent iron) as a reducing agent and constructed wetlands. The results indicate that there may be alternatives to use of conventional water treatment processes to treat ash dam waters. There is an obvious need for further pilot scale studies and careful assessment prior to the installation of any large scale processes.

Further work is required to identify processes that can effectively treat ash dam water to remove boron. Initially a full literature survey including the patent literature is required.