Mine Waters and Effluents Part 5 - Zero Liquid Effluent Discharge
Many water treatment processes are excellent at removing contaminants from water, but where do these contaminants ultimately go?
Some common technologies, such as Reverse Osmosis and Ion Exchange, transfer the contaminants to a concentrated liquid stream called ‘brine’. This brine stream is small in volume but difficult and costly to treat due to the high contaminant load.
Other technologies take an alternative approach and precipitate the contaminants into a solid form. This is beneficial because solids are easily separated from liquids, making them cheaper to dispose than brines.
Many water treatment processes use a combination of brine-producing and precipitation technologies – as discussed in Part 3 of this series.
Brine minimisation in water treatment is a common pursuit due to the following advantages:
Higher water recovery – less water escaping with the brine allows more treated water production.
Lower brine handling costs – smaller brine volumes are cheaper to treat or dispose.
Reduced environmental footprint – smaller brine volumes require less resources for treatment and disposal.
Creek in New Caledonia contaminated by brines from local mining operations. Source: Wikimedia Commons
The ultimate goal of producing no liquid brine is called Zero Liquid Effluent Discharge (ZLED), which has historically been difficult and costly to accomplish.
Today, many mine sites have limited space and resources for handling brine; and strict licences regulate the contaminant load of liquids that can be discharged into the environment.
This has led to the development of several viable ZLED water treatment processes, which can be implemented by mining and other industries.
The following diagram shows an example of a ZLED water treatment process for mine effluent. It uses four main technologies to convert all the contaminants to solids: Lime Precipitation, Ion Exchange, Reverse Osmosis and Evaporation.
Example of a Zero Liquid Effluent Discharge (ZLED) process
Nearly all the liquid from the effluent is recovered as high-quality treated water, which can be re-used at the mine or safely discharged to the environment. The solids are usually combined with tailings or used as landfill.
It is also important to recognise that there is value in many common mine water contaminants.
The precious metals (gold, silver, platinum) are an obvious example; however other metals – such as copper, antimony, nickel and cobalt – can also become valuable by-products.
A smartly designed water treatment plant will intentionally concentrate and recover these elements during the treatment process, to reduce the amount of brine and help pay for costs of the plant.
Another example is calcium and sulphate, which are common contaminants in Acid Mine Drainage. These elements can combine to make gypsum – a common soil conditioner used by farmers and household gardeners.
Nitrates are also found in mine waters and can be converted to valuable fertilisers such as potassium nitrate or calcium nitrate.
Mine water contaminants can be converted to gypsum and fertilisers used by farmers. Source: Wikimedia Commons
By adopting the challenging goal of brine minimisation or elimination (ZLED) and converting contaminants, where possible, into valuable by-products, mine water treatment plants can provide a number of benefits: improving site efficiencies, reducing environmental footprints, lowering costs – and ultimately increasing the amount of clean water in the world.
Part 1 – Sources of mine waters and effluents
Part 2 – Typical contaminants in mine water
Part 3 – Mine water treatment technologies
Part 5 – Zero liquid discharge