Abstract
In the present study, the microalga Scenedesmus obliquus CCAP 276/38 has been applied in the treatment of a real wastewater (RWW) derived from an anaerobic digestion process of corn silage and livestock wastewater. The liquid phase of the digestate showed a low viscosity value and a high content of ammonia up to 3 g/L. In a preliminary phase, the experimental tests were carried out in Erlenmeyer flasks in order to identify the optimal RWW concentration and, in particular, its influence on biomass growth and productivity. The tests were carried out at 20°C at an artificial dark/light cycles of 12 hours. Three different concentrations were tested: 1, 2 and 3% of RWW in water, in the presence of sodium bicarbonate (NaHCO3, 50 mM) as inorganic carbon source. The obtained results showed that S. obliquus was able to grow in all the tested RWW concentrations even if a higher growth rate and biomass production were observed in the cultures containing 1% RWW. In order to test the influence of N/P ratio on microalgal growth, two different salts, KNO3 (0,2 g/L) and K2HPO4 (0,02 g/L), were added to the medium containing 1% RWW (N/P = 84.4), to correct the N/P value. The biomass growth rate increased in the medium with the lower N/P value (N/P = 27.9). The microalgal production process was scaled-up in a stirred tank photobioreactor (working volume 5 L), in the same temperature and illumination conditions using a medium with the composition optimized in flask tests. The culture was carried out for 124 hours, fed-batch addition of RWW (1%) was done during the fermentation in order to replace the carbon source. The results (growth rate, biomass dry weight and productivity) were compared with those obtained in presence of a synthetic medium with sodium bicarbonate 50 mM as carbon source. The work clearly demonstrated the capability of S. obliquus CCAP 276/38 to grow in alkaline wastewater and the possibility to employ this species in the treatment of effluents containing high ammonia concentration.
