It's all water under the bridge
When we titled the last Update 'Water, water everywhere ...' we hadn't meant this as a prediction that the district would experience two major storms!
However, it is exactly these sort of events that the on-site water management system is designed to handle.
In this issue we look at more water issues; we investigate the recently commissioned Reverse Osmosis Plant, report on progress at the tailings storage facility, and explain why an understanding of rock chemistry is so important in effective water management.
Reverse What?
The Reverse Osmosis Plant
Remember your Fifth Form General Science? 'Osmosis is the natural movement of a solvent from an area of low solute concentration, through a semi permeable membrane, to an area of high solute concentration'.
Put simply it's a natural process that drives a purer liquid (like water) through a membrane to the side with less pure liquid.
What's a semi permeable membrane? You might be wearing one. Gore-tex fabrics have pores big enough to let water vapour out, but too small to let liquid water in. That's why you can have a Gore-tex parka that 'breathes' and gets rid of sweat, but still keeps rain out.
It is osmosis that allows plants to absorb moisture through their roots. A dramatic example of osmosis occurs when you sprinkle salt on slugs and snails in the garden and they shrivel up.
Reverse Osmosis (RO) is the opposite of osmosis. It reverses the natural direction of movement by applying high pressure on the side of the membrane with the less pure water, forcing the water through the membrane but retaining those molecules that are larger than a water molecule. Still confused? Think of it as a filtering process similar to that used in a swimming pool, where a pump forces pool water through a filter to remove leaves and other debris. With reverse osmosis, the membrane is so fine that it filters at a molecular level. This is called nano-filtration.
Reverse osmosis is used in the desalination process to produce drinking water from seawater. It is also used in the wine and dairy industries. A kidney dialysis machine uses reverse osmosis to purify blood.
Why do we need an RO Plant now? The unexpected requirement to construct the Southern Stability Cutback in the Martha pit brought with it a literal downstream effect: more water.
That means more water to be treated prior to discharge.
The site has very specific treated water discharge quantity and quality criteria. The RO Plant filters or 'polishes' water from the existing treatment plant producing discharge water of higher quality. That means we can discharge more water without compromising river water quality.
The Reverse Osmosis Plant: filtration at a molecular level
The School of Rock
You need to know your rock. When mined, rock is classified as NAF or PAF. This isn't a subjective slang judgement as in 'That rock is so naf. The words are acronyms. NAF means Non-Acid Forming' and PAF stands for 'Potentially Acid Forming'. It is important to know which is which, as each type of rock is handled differently.
PAF material is buried in the tailings dam wall. As rehabilitation takes place a NAF clay and rock capping layer is formed that will encapsulate (or wrap up) the PAF rock so that oxygen in the air is prevented from interacting with it. This stops the formation of acid runoff.
NAF material can be used for a variety of construction and fill requirements on site without the restrictions placed on PAF.
Water running off NAF rock can contain silt, so needs to pass through settling ponds before discharging from site. But because PAF rock runoff can be acidic and can contain dissolved metals, it needs to be collected and pumped to the water treatment plant where potential contaminants are removed prior to discharge.
At present, we have larger areas of exposed PAF rock stockpiles than normal, and coupled with the current wet weather we have more water on-site requiring treatment than we would like. The new reverse osmosis plant will provide some relief in managing these additional volumes of water, but our focus is on rehabilitating these areas so that no further treatment is required.
