Influence of season, inoculum density and ammonia concentration on biomass abundance and ammonia utilization of a microalgal consortium for tertiary remediation of tannery wastewater and valorization of biomass

Abstract

Phyco-remediation has been advocated as a tertiary step for simultaneous remediation of tannery wastewater and valorization of microalgal biomass. This study was based on the hypothesis that climatic factors could be important functional determinants for system performance in open phyco-remediation systems subject to seasonal variations. In this study, ammonia utilization and biomass abundance obtained by a microalgal consortium (Tetradesmus obliquus CCAP 276/1A, Tetradesmus obliquus CPUT-L1, Tetradesmus dimorphus CPUT-L2, Neochloris sp. CPUT-W1) in treated tannery wastewater were measured under simulated winter, autumn/spring and summer conditions (temperature: 8.0 °C, 17.5 °C, 27.0 °C, respectively; photoperiod: 10:14 h, 12:12 h, 14:10 h light:dark, respectively) with varied initial ammonia (7.4 mg/L to 45.6 mg/L) and inoculum size (6 % vol./vol to 34 % vol./vol.) in 500 mL flasks (350 mL working volume) in shaking incubators at 160 rpm. Season had highly significant effects (p < 0.0001) on both responses in an increasing continuum from winter to summer (biomass: 0.39 ± 0.06 g/L to 0.72 ± 0.07 g/L; ammonia utilization 1.7 ± 0.6 mg/day to 4.1 ± 1/6 mg/day). The inoculum size did not have significant effects on either response (p > 0.5), while the initial ammonia concentration had a highly significant effect (p < 0.0001) on the ammonia utilization rate, but not on biomass abundance. Despite the halo-tolerant nature of the microalgal strains, sodium removal efficiency was <1 %, indicating that it is unlikely that phyco-remediation is feasible for removal of sodium in highly saline wastewaters. Overall, this study demonstrated the importance of studying microalgal growth and remediation under climatic conditions likely to be expected in the field with a view to pilot scale implementation.