R. P. Van Hille, G. A. Boshoff, P. D. Rose and J. R. Duncan
Alkaline precipitation of heavy metals from acidic water streams is a popular and long standing treatment process. While this process is efficient it requires the continuous addition of an alkaline material, such as lime. In thelong term or when treating large volumes of effluent this process becomes expensive, with costs in the mining sector routinely exceeding millions of rands annually. The process described below utilises alkalinity generated by the alga Spirulina sp., in a continuous system to precipitate heavy metals. The design of the system separates the algal component from the metal containing stream to overcomemetal toxicity. The primary treatment process 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 from 1.8 to over 7 in the post-treatment stream. Secondary treatment and polishing steps depend on the nature of the effluenttreated. In the case of the high sulphate effluent the treated stream was passed into an anaerobic digester at a rate of 4 l/day. The combination of the primary and secondary treatments effected a removal of over 95% of all metals tested for as well as 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 culturecould be provided by using tannery effluent or a combination of saline water and sewage. This would have the additional benefit of treating either a tannery or sewage effluent as part of an integrated process.
South Africa has been and still is one of the world’s leading producers of precious metals and minerals. The pollution of surface waters by acid mine drainage (AMD) andacidic effluents from refineries is a serious problem in the country. Pollution from these sources adversely affects the aesthetic appearance of surface waters and destroys living organisms that inhabit them, making the water systems more vulnerable to organic pollution. In addition the majority of mines are located in the most highly industrialised and hence heavily populated areas, where the demandfor fresh water for domestic and industrial use is highest . The principles governing the generation of AMD are relatively well understood. Upon exposure to oxygen and water and in the presence of oxidising bacteria, pyrite and other sulphide minerals are oxidised to produce dissolved metals, sulphate and acidity . The conventional processes for the treatment of AMD or acidic, metal ladeneffluents from industry involve neutralisation by the addition of alkaline chemicals such as limestone, lime, sodium hydroxide, sodium carbonate or magnesia. The pH is increased as a result and leads to the precipitation of metals. These active systems generally require the construction of a plant with a variety of reactor systems, such as agitated reactors, precipitators, clarifiers and thickeners.Aside from the initial construction costs the continuous cost of reagents make these systems unattractive from a financial perspective [2 and 3]. A medium sized mining operation may spend well over a million rand annually on the cost of lime alone. In recent years there has been a trend toward the implementation of passive treatment schemes. These take advantage of naturally occurring geochemicaland biological processes to improve water quality with minimal operation and maintenance requirements . Constructed wetlands and anoxic limestone drains (ALDs) are two of the most popular passive treatment methods [3 and 4].
The majority of active and passive treatment systems depend on precipitation as the primary method for metal removal from acidic effluents. The precipitation is...