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    • Basically the treatment of AMD is based

    2018-10-26

    Basically, the treatment of AMD is based on either separation such as precipitation, adsorption and filtration or any other separation or reaction process used for water and wastewater treatment. The weakness of these conventional processes includes low removal capacity, lack of selectivity and intolerance to organic substances. Furthermore, the generation of large quantities of waste is an issue that requires attention due to high capital and operating costs (Eccles, 1999). After numerous studies, it is possible to confirm that the most commonly used methods are high rates algal ponds (HRAP) (Oswald, 1988; Mehrabadi et al., 2015). Furthermore, a patented method known as Algal Turf Scrubber (ATS) was developed to remediate AMD and polluted groundwater. It uses suspended tryptophan hydroxylase of common green algae species such as Chlorella, Scenedesmus, Cladophora or cyanobacteria such as Spirulina, Oscillatoria, Anabaena or consortia of species (Craggs et al., 1996; Adey et al., 1996). This structured approach was patented as an Algal Turf Scrubber® (ATS™) in subsequent U.S. Patents with the following details: 4,333,263 – 1982; 4,966,096 – 1990; 5,097,795 – 1992: and 5,851,398 – 1997. This patented method has proved the effectiveness of this technology regarding the efficient removal of heavy metals to acceptable levels in AMD or any other industrial wastewater, and also an effective removal of organic compounds such as chlorinated and aromatic organic compounds was recorded (Craggs et al., 1996; Adey et al., 1996). In a comparative study between a waste stabilization ponds system (WSP) and HRAP for the treatment of urban polluted water with lower ion concentrations of Zn, Cu and Pb Toumi et al. (2000) reported that the HRAP had a higher removal rate per unit volume per day with values up to 10 times more efficient in the case of Cu. High pH values were recorded and they enhanced metal precipitation; the high pH values are the consequence of effective algal photosynthesis. A hybrid process that led to a patent was reported by Rose et al. (1998), the patent emerged from a combination of HRAP and SRB. In this process, the precipitation of heavy metals is achieved by direct input of AMD into HRAP with high pH values and the biomass having adsorbing properties. In addition, the biomass of the HRAP is recovered and used as a carbon source for SRB (Perales-Vela et al., 2006). For effective removal of Mn and stability of a neutralizing pH, non-agitated algal ponds have been suggested to remediate challenges that constructed wetland technologies are facing (Phillips et al., 1995). In this study it was shown that consortia of algae and cyanobacteria could effectively reduce prohibitive Mn concentrations to an environmentally safe level. In this case Mn was removed by means of biomass adsorption, high pH precipitation and immobilization (Perales-Vela et al., 2006). All these previous studies point to a promising future for algae bioremediation based treatment for AMD and wastewaters. Algae can produce oxygen at a lower costs and it is also very effective in the removal of pollutants through phycoremediation (Wang et al., 2016). The existence of various species creates opportunities that can be explored for treatment of AMD. Spirulina sp. has been very effective regarding the removal of heavy metals from AMD. Also, there is a variety of acidophilic organisms growing easily in AMD but there is little literature about their use for AMD treatment (Das et al., 2009; Sanchez-Andrea, 2014). The ability of aquatic plants to remove heavy metals efficiently from contaminated acid mine drainage water can be traced back to 1973 (Dinardo et al., 1991). Several algae strains have been successful in removal of heavy metals from AMD or wastewaters, but most of the investigations were completed on batch process in which microalgal species are grown. The function of algae as essential components of a wetland with regard to remediation of AMD cannot be overlooked as heavy metal contamination sinks through bioaccumulation. This function plays a major role in the design of passive bioreactor systems for the removal of sulphates in AMD (Zagury et al., 2007). A passive bioreactor system operates on the same principle as a large activated carbon filter with AMD coming into the surface area, percolating through a specially constructed barrier layer and exiting the system from below. Algae and other aquatic plants are major components for the initial stage of the bioreactor. Both Van Hille et al. (1999) and Balaji et al. (2014) described the use of Spirulina sp. in the remediation of AMD effluent; they discovered that rapid uptake of various heavy metals through direct contact with the AMD effluent stream reached saturation in 30 min. After this time the algae began to exhibit toxicity effects. However, these species of algae continuously generate alkaline chemical materials that act to neutralize the acidity of the AMD through the production of inorganic bicarbonate salts.