Managing water on site
The rainfall in Waihi is relatively high, averaging 2,147 mm/year, and unlike many mines around the world which are operated in dry climates, the Waihi site operates with a net surplus of water. Because of this net water surplus, water management is an integral part of operations, and the water treatment plant and the reverse osmosis plant exist to treat surplus water prior to discharge to the Ohinemuri River.
The water quality limits are based on protecting both the aquatic biology of the river and downstream users, and the limits are based on the USEPA guidelines for the protection of aquatic life (instream) where such appropriate limits exist, and other guidelines where appropriate. Flow measurement is an important aspect. Compliance limits for the treated water quality parameters are set based on a dilution factor.
The consents allow up to 26,000 cubic metres per day to be discharged from the water treatment plant, or up to 40% of the river flow, depending on which is the lesser. For this reason, flow meters exist to record the treated water discharge volumes. The river flows are monitored using a river level recorder. Regular river gaugings are carried out to ensure that the river level provides an accurate indication of the river flow.
The water treatment plant
Prior to building the water treatment plant, substantial laboratory trials were undertaken to develop the optimum treatment process design. This work looked at maximising the oxidation of cyanide in the form of free cyanide and weakly bound complexes of copper, nickel and zinc (WAD cyanide) which are recognised as the forms having most potential to affect aquatic biology.
The water treatment plant has performed well, with a very high level of compliance with the consent conditions. In 1999 and again in 2011 the plant was upgraded and in 2008 a reverse osmosis plant was built and commissioned to provide state-of-the-art water treatment to Waihi’s operations.
Excess water on site can be derived from a number of sources; some contain cyanide—added during the gold and silver extraction process—some do not. To manage this, the water treatment plant incorporates two distinct treatment stages:
- a cyanide destruction process, using hydrogen peroxide—a strong oxidising agent—to destroy the cyanide, and copper sulphate as a catalyst to speed up the reaction
- a metal and trace ion removal process, using ferric chloride, lime and flocculants.
The reverse osmosis plant
The addition of an $11m reverse osmosis plant has meant that the site can discharge more water as the new process treats water to a very high level of purity, far in excess of that required by regulators. This process forces water containing impurities through a membrane. The membrane filters at the molecular level, holding back anything bigger than a water molecule. This ‘nano-filtration technology’ is also used in the wine and dairy industries. A kidney dialysis machine uses the same technology to purify blood.