The use of bioreactors and a vertical flow reactor in the treatment of circum-neutral mine affected water.

OceanaGold’s Globe Progress Mine, located in the West Coast of New Zealand, is a hard rock gold mine which ceased operation in 2015 and is now in the closure phase. During operations an active water treatment plant was used to remove contaminants before release offsite. As the mine transitioned into closure, a site-wide water balance model based on sampling data identified the need for treatment of various water sources for arsenic and iron. Field trials were established to test two methods of passive treatment, sulphate reducing bioreactors and a meso-scale vertical flow reactor to determine the most appropriate passive treatment option for post-closure. To establish the most efficient substrate mixes, four bioreactors with different combinations of organic media, with the addition of either mussel shells or biosolids were constructed. The bioreactors were fed water from the sites combined Underdrains (median chemistry: 28 g/m3 iron, 1.69g/m3 arsenic and 430 g/m3 sulphate). Results showed that in all cases biosolids treatments out preformed mussel shell treatments, but ratios of other materials had no effect. At 50 hours HRT biosolid treatments showed removal of iron at 60%, arsenic at 75%, and sulphate at 20%. Results also indicated that in most cases there was a positive relationship with HRT, but this was strongest for sulphate removal. An insulated tank was also trialled to reduce the effect of cold ambient air temperatures on the bacteria’s activity. This showed that internal temperature was dictated more by influent temperature but using an insulted tank prevented diurnal temperature fluctuations in the outside edge of substrates, and therefore maintaining a more constant temperature throughout the reactor. The vertical flow reactor (VFR), which utilises oxidation of iron rich water to precipitate onto a non- reactive gravel bed, and then allows for adsorption of other metals such as arsenic onto precipitated iron hydroxides. Water from the waste rock underdrain (median chemistry: 7.6g/m3 of iron, 0.17g/m3 of arsenic, and 575g/m3 of sulphate) was fed through the VFR. Results from this showed median iron and arsenic removal was 99.10% and 94.83%, respectively. High removal was achievable at even very low residence times, with 84% iron and 75% arsenic removal at 10.5 hrs residence time. These results showed a VFR could remove adequate iron and arsenic with a small footprint and was deemed appropriate as a full-scale system for the study site