Utilization of centrate for the production of the marine microalgae Nannochloropsis gaditana

Abstract

In this paper, the production of the microalga Nannochloropsis gaditana using centrate from the anaerobic digestion of treated urban wastewater is studied. For this, semicontinuous cultures were performed indoors at laboratory scale, under controlled conditions, supplying seawater with different centrate percentages from a real wastewater treatment plant as the culture medium. It was demonstrated that N. gaditana can be produced using solely centrate as the nutrient source but only at percentages below 50%. Above this level, inhibition is caused by an excess of ammonia, thus reducing productivity. In the 30-50% centrate range, biomass productivity was 0.4 g·l −1 ·day −1 , equal to that measured when using Algal culture medium. Moreover, the biochemical composition of the biomass was also equal to that measured when using Algal culture medium, with the protein content in the 30-40% d.wt. range; whereas the lipid content ranged from 20 to 25% d.wt. Under these conditions, phosphorus depuration from the culture medium was in the 80-90% range while nitrogen depuration was only between 20 and 40%, indicating an excess of nitrogen in the centrate with respect to phosphorus. In spite of this phosphorus limitation, in the optimal centrate range (30-50% in the culture medium), the cells performed under optimal conditions, removing up to 35 mg N ·l −1 ·day −1 and 5.7 mg P ·l −1 ·day −1 , with quantum yield values measuring 1.0-1.3 g·E −1 . By supplying additional phosphorus, it was possible to enhance productivity and increase nitrate and phosphorus depuration to over 80%. The use of centrate is confirmed as a useful method for reducing microalgae production costs while also increasing process sustainability, especially when using biomass for bioenergy applications.

Feasibility study of biogas upgrading coupled with nutrient removal from anaerobic effluents using microalgae-based processes

“…The lipid contents obtained (2.9 -11.2%) were in the low reported range in axenic microalgae cultures (5-77%, Chisti 2007), but were similar to those values reported in microalgae cultivated in wastewaters (2-23%, Serejo et al, 2015;Sepúlveda et al, 2015 (Klok et al 2013;Kandilian et al, 2014). In this context, the biomass here harvested could be used for other purposes than for biodiesel production.…”

Wastewater treatment using microalgae: how realistic a contribution might it be to significant urban wastewater treatment?

“…Although these values are relevant, the most important criteria are (i) the net removal capacity and (ii) the final outlet concentration of nitrogen and phosphorus -this last factor determining whether it is possible to safely release the water into the for N. gaditana (Sepúlveda et al, 2015). A maximum removal of 8.5 mgN l -1 day -1 was reported using Chlorella (Marcilhac et al, 2014).…”

“…With regard to phosphorus, this is essential for algal growth as it is involved in many cellular processes even though it makes up less than 1% of the biomass. The N/P ratio in wastewater is critical because excess nitrogen cannot be removed if the phosphorus content is insufficient to allow it; in fact, it is sometimes necessary to supply additional phosphorus to reduce the nitrogen level below the release limits (Ledda et al, 2015;Sepúlveda et al, 2015).…”

Utilization of centrate from wastewater treatment for the outdoor production of Nannochloropsis gaditana biomass at pilot-scale

“…Furthermore as an added value of this process, the biomass produced is energy rich and can be further processed to make biofuels or other valuable products such as biofertilizers, biopolymers, bioplastics, lubricants, paints, dyes and colorants [13]. Also, from an energetic point of view, wastewater remediation using microalgae consumes much less than using conventional systems (0.52 MJ m −3 versus 3.6 MJ m − 3 respectively), thus presenting economic and sustainability advantages [14]; notwithstanding these, the utilization of wastewater limits the production of microalgal biofuels to freshwater strains, although the utilization of seawater is the most sustainable and suitable way to produce them [4].…”