A continuous process for the biological treatment of heavy metal contaminated acid mine water

Alkaline precipitation of heavy metals from acidic water streams is a popul ar and long standing treatment process. While this process is efficient it requires the continuous addition of an alkaline material, such as lime. In the long term or when treating large volumes of effluent this process becom es expensive, with costs in the mining sector routinely exceeding millions of rands annually. The process described below utilises alkalinity generate d by the alga Spirulina sp., in a continuous system to precipitate heavy me tals. The design of the system separates the algal component from the metal containing stream to overcome metal toxicity. The primary treatment proces s consistently removed over 99% of the iron (98.9 mg/l) and between 80 and 95% of the zinc (7.16 mg/l) and lead (2.35 mg/l) over a 14-day period (20 l effluent treated). In addition the pH of the raw effluent was increased fr om 1.8 to over 7 in the post-treatment stream. Secondary treatment and poli shing steps depend on the nature of the effluent treated. In the case of th e high sulphate effluent the treated stream was passed into an anaerobic di gester at a rate of 4 l/day. The combination of the primary and secondary t reatments effected a removal of over 95% of all metals tested for as well a s a 90% reduction in the sulphate load. The running cost of such a process would be low as the salinity and nutrient requirements for the algal cultur e could be provided by using tannery effluent or a combination of saline wa ter and sewage. This would have the additional benefit of treating either a tannery or sewage effluent as part of an integrated process. (C) 1999 Else vier Science B.V. All rights reserved.