Is your concern how to reduce the hardness and sulfate, or are you perhaps concerned with treatment of metals and acidity in waters that also have high hardness and sulfate?

Some of the methods (e.g., ultra-filtration of one sort and another) will be effective at reducing [Ca], [Mg] as well as [SO4]. Others probably less so. CSIRO's hydrotalcite-based treatment approach, which offers some prospects for removing Mg.

You might try running some numerical experiments to evaluate conceptual pathways, and then devise some conceptual models for experimentation that would address the efficiency (or even effectiveness) of the nominated approaches.

The only complete method of treating (and managing) water for hardness and sulphate is the very low cost bio-electro oxidation process of Southern Engineering Services in Australia. This process:

•Removes the metals normally found in this type of water as oxides that can be recycled.

•Removes the sulphates biologically, resulting in elemental sulphur and carbonates that can be recycled.

•The Ca is removed to zero calcium carbonate precipitation potential and usually a significant amount of the Mg.

•The only major remaining compound in the water is sodium carbonate which is a natural water softener.

This process is favored by the miners and regulators as it is low cost, does not import chemicals (other than the organic carbon electron donor which is fully utilized), produces an environmentally safe water, and also allows the recovery (and removal from site) of all the contaminants from the water.

In the case of acid mine drainage water generated by waste rock heaps, this patented process also permanently removes the source of the acidity and sulphates without expensive cover systems.

The response is certainly incomplete. There are several physical treatment processes that will remove Ca, Mg and also SO4. Ultra-filtration, such as RO, and evaporation are obvious examples.

Sulfate reduction can, of course, reduce the sulfate concentration, and to the extent that it is based on a carbon-based source for bacteria, it will produce alkalinity that can, in appropriate circumstances reduce Ca by precipitation of CaCO3. There are many approaches to this reaction sequence, and you will want to understand the advantages and disadvantages of the alternatives - most of which you already know from your dissertation research.

Just to clarify, there are many systems that separate the water from the sulphate and causes of hardness.

However, these treatment methods by themselves are no longer adequate as the mining industry and regulators usually call for a complete management solution that involves the complete treatment of the total water contaminants, the removal of the treatment products from the site, and the abatement of acidity and sulphate production.

I don't believe there are any single water management systems out there that achieve all this. Unfortunately, filtration systems and evaporation only separate the water from the contaminants. It is important that the contaminants in the water are transformed into a recyclable product for total removal from the environment; otherwise the miner cannot get rid of it from his site.

I am unaware of the disadvantages/advantages you speak of regarding bacteria reduction of sulphate. Perhaps I can be of assistance in clarifying the issue.