Cultivation of Nannochloropsis oculata in saline oil & gas wastewater supplemented with anaerobic digestion effluent as nutrient source

This work investigates the feasibility of growing marine microalgae Nannochloropsis oculata in a mix of seawater and saline produced water obtained during oil & gas extraction, supplemented with liquid digestate from anaerobic digestion process as source of nutrients. In particular, three-stage cultures were conducted by varying the produced water loading in the culture media (from 0 up to 50% v/v), supplemented with 5% v/v of digestate and seawater. Growth parameters as well as nitrogen (ammonium) and organic carbon (expressed as chemical oxygen demand) removal efficiencies were monitored. Results revealed that N. oculata is perfectly able to grow in a seawater containing produced water from 10 up to 30% v/v. A slower growth was observed for 40 and 50% v/v of produced water, because of the salinity higher than 60 g·L−1. Maximal growth rates obtained were 0.35, 0.27 and 0.16 day−1, with a maximal optical density of 6.3, 5.2 and 3.2 for 10, 20 and 30% v/v of produced water, respectively. Nannochloropsis oculata showed better removal efficiencies for ammonium nitrogen (around 100%) than for organic carbon (approximately 40% after one step of acclimation), regardless of the produced water loading, most chemical oxygen demand being volatilized and/or degraded by bacteria during the first two days of a culture. Regardless of the loading, > 90% of iron brought by produced water and digestate was pre- cipitated and/or assimilated/adsorbed by N. oculata.