and the ongoing disposal of the sludge generated by the plant on a ‘ continuous little and often ’ basis . Active treatment plants either use lime to raise the pH and precipitate the dissolved metals in the form of hydroxides , or use biological activity in the form of sulphate reducing bacteria to effectively reverse the pyrite oxidation process which caused the contamination and precipitate the dissolved metals as insoluble sulphides .
Challenges which remain However , some challenges remain , including the treatment of waters containing low level concentrations of zinc and cadmium , tightening of the treatment standards for arsenic and selenium and reduction of sulphate concentrations to sufficiently low concentrations .
Zinc and cadmium cannot be adequately removed using the methods commonly employed for the removal of iron and aluminium . For low levels of contamination , anaerobic wetlands can be used , but this has been restricted due to odour issues caused by the excess hydrogen sulphide gas created by sulphate-reducing bacteria venting to atmosphere . Research is focusing on both alternative methods for the treatment of low level zinc and cadmium concentrations .
Over recent years , the discharge standard for arsenic has been reduced due to concerns about short and long-term toxicity , making treatment more challenging . Dissolved arsenic is present as either Arsenite ( As3 +) or Arsenate ( As5 +). Arsenate is more easily removed from solution than Arsenite , thus most processes rely on an oxidation stage ( using a reagent such as hydrogen peroxide ) prior to its removal from solution . Removal is then typically achieved using co-precipitation with the iron , usually already present in the mine water . Alternative methods include ionic exchange , reverse osmosis or the use of adsorptive media such as granulated ferric hydroxide . The challenge is to better understand the optimum conditions for arsenic removal and develop control philosophies that achieve this at minimum cost .
The discharge standard for selenium has also been reduced , forcing some mines to modify their
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