Arthrospira (Spirulina) in tannery wastewaters. Part 2: Evaluation of tannery wastewater as production media for the mass culture of Arthrospira biomass
Mass blooms of Arthrospira (Spirulina) have been reported in waste stabilisation ponds treating tannery wastewaters and have been linked to a reduction in odour emissions in these systems. However, these blooms are unstable and unreliable, forming and disappearing in an apparently unpredictable manner, and they have remained poorly understood. Controlled production of Arthrospira biomass in this medium could not only be used to enable a more predictable control of odour in these systems (as detailed in Part 1 of this report), but could also provide a biomass product with external value. Techno-economic studies of microalgal biomass production have identified the cost of growth media formulation as a critical driver in the profitability of the algal biotechnology enterprise. Apart from the potential feed value of Arthrospira biomass as a product, the renewed interest in, and possibly marginal economics of, biofuels production from the microalgae has refocused attention on the possible advantages of wastewater use as low-cost production media. Part 2 of this study reports the investigation of factors regulating Arthrospira growth in the tannery wastewater medium and thus requiring active control in order to optimise biomass production. It was shown that Arthrospira growth in this high-protein, low-carbonate medium is under ammonia control, rather than nutrient limitation, as may previously have been thought. It was also shown that an effective mass culture strategy in this medium would require a maximum effluent loading rate that operates as a function of the optimised ammonia removal rate. Growth optima were demonstrated for ammonia and bicarbonate levels of 20 mg∙ℓ-1 and 12–17 g∙ℓ-1, respectively, and inhibition of growth was demonstrated at ammonia levels above 60 mg∙ℓ-1. Both autotrophic and mixotrophic growth of Arthrospira was observed and organic uptake may contribute to a stimulation of biomass production compared to growth in defined inorganic media. Heavy metal accumulation may present a toxicity hazard where biomass is targeted for use in animal feed rations. A heavy metals removal step was investigated involving the passage of the tannery effluent through an anaerobic sulphide-generating compartment in a primary pond, prior to its use in Arthrospira production. An acceptable Arthrospira feed-grade biomass was produced in this way. These results indicate potential cost-benefit advantages in the use of tannery effluent-based growth media for Arthrospira biomass production, and waste nutrient recovery may mitigate negative energy yield problems where the biomass is further processed in biofuels manufacture.
Keywords: Spirulina; Arthrospira; microalgal biomass; biofuels; tannery; wastewater; waste stabilisation ponds