Sustainable water treatment at Century Mine : draft.

Abstract

Ecological effluent treatment consists of promoting and supporting growth of indigenous biota which bio-mineralize contaminants, leading to containment and stabilization of contaminants within the waste management area. With two field investigations (2006 and 2007) of the Zinifex Mine site, indigenous microbial/algal material was found to contain an average of 11.6% Ca, 12.0% S and 2.5% Mg, the main salts of the effluents. Utilizing literature growth rates, it is estimated, that about 900 g of dry weight biomass grows per square meter per dry season. The growth rates translate into an extraction of about 1.0 t of Ca, 1.1 t of S and 0.22 t of Mg per ha surface area from the effluent. Selection of suitable growth substrates along with growth rates specific to the site are needed to define design parameters for the pilot scale tests, both in Dam # 3 and in Evaporation Pond. The bio-mineralization capacity of the biota is lower in Evaporation Pond than in Dam #3. In Evaporation pond the accumulation of Ca is 0.51 t per ha per dry season and lower for S (0.14 t) and Mg (0.046 t). Improvement of growth and uptake of elements can be expected when growth limitations of the biota are known and ameliorated. Nutrient ratios of healthy algae biomass are well documented and are used to indicate growth limitations. For the algal mats in Evaporation Pond the ratio of carbon to nitrogen suggest that nitrogen is more limiting than carbon. On the other hand, nitrogen and phosphorus ratios for the mats and the sediment, suggest that nitrogen is not as limiting as phosphorus. Growth promotion trough the addition of phosphorus will increase growth. Geochemical modeling indicates that precipitation of calcium phosphate is likely to take place. This could delay a rapid growth response, as the precipitate is relegated to the sediment where it will be converted back to the water by microbes. Measurements of pH, Eh, electrical conductivity, oxygen at different depths were obtained in Evaporation Pond, which suggest the development of a chemical stratification, supported by reductions in sulphate concentrations in the water at 6 m depth. The extent, origin and effect of the chemical stratification need to be verified through an extensive sampling campaign. Test with phosphate additions in limno-corrals are suggested. The monitoring data base (2004 -2006 V-notch and 2000 to 2006 Evaporation Pond) of the chemical composition of effluents associated with the mine waste management area was used to derive a framework for annual contaminant load to be treated. Material balances and water usage were estimated per annum. Several questions remain open and require resolution. Preliminary assessments of the seasonal contaminant loads suggest that promotion of the existing bio-mineralization process will facilitate salt management pre- and post closure, implemented through a series of well defined milestones.