Production of Spirulina sp. in sea water supplemented with anaerobic effluents in outdoor raceways under temperate climatic conditions

Abstract

The use of untreated sea water supplemented with anaerobic effluents from digested pig waste and sodium bicarbonate was evaluated as a low-cost medium for semi-continuous cultivation of a mixed culture of two Spirulina strains in outdoor raceways under temperate climatic conditions (pond temperature in the range 21-26°C and light intensity in the range 225- 957μE m-2 s-1). The mixed culture had a predominant population (86.6 ± 3.9%) of an atypical Spirulina strain consisting of straight filaments, which appeared spontaneously after the strain with helicoidal trichomes had been subcultured. Morphological studies for the identification of the type and size of trichomes of the two strains (HF and SF) were carried out. The proportions of the two strains were observed to be stable during the monitoring period (30 days). Three different sets of semicontinuous cultures were carried out. Sets 1 and 2 were operated under regime 1 (a single addition of an aerobic effluents at time zero and no pH control) during the same season (June and July) of different years. Set 3 was operated under regime 2 (semi-continuous addition of anaerobic effluents and pH control) during the autumn. A minimum productivity of 3.6 g m-2 day-1 was obtained at one of the lowest temperatures (22.1 °C) and light intensities (245 μE m-2 s-1) and a maximum productivity of 10.9 g m-2 day-1 was observed at the highest temperature (25 °C) and highest average light intensity (618 μE m-2 s-1) registered for sets 1 and 2. The protein content in the Spirulina biomass harvested from these two sets varied from 17% to 65.6%. In set 3, a maximum productivity of 9.0 g m-2 day-1 was re corded at an average temperature of 24.4 °C and at an average light intensity of 668 μE m-2 s-1. The protein content in this set under regime 2 varied within a narrower range than in set 1 and set 2 (from 34.8% to 49.1%), apparently because of a continuous availability of ammonia nitrogen at a level of 30-50 mg 1-1. However, in terms of the removal of ammonia nitrogen and chemical oxygen demand, regime 1 was more efficient than regime 2.