The potential impact of replacing nitrate with ammonium hydroxide in microalgae production on the biomass productivity and CO2 utilization efficiency

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Ammonium hydroxide has many advantages as a CO 2 absorbent compared with other amine solutions in Carbon Capture, Utilization and Storage. In this work, the use of ammonium hydroxide was suggested to replace the nitrate as an alternative nitrogen source and to enhance the CO 2 utilization and biomass production during microalgae cultivation. Abiotic absorption experiments showed that the CO 2 absorption capacity of culture medium increased with the increase of ammonium hydroxide concentration, and it kept consistent with different kind of CO 2 absorbents. Consumption curves for sodium nitrate and ammonium hydroxide were built from batch cultivation experimental data and it was found that the microalgal cells had faster growth rates when these nutrients concentrations were kept at low levels (2 mmol/L and 4 mmol/L, respectively). In the fed-batch cultures under both indoor and outdoor operations, the maximum biomass concentration and carbon utilization efficiency using ammonium hydroxide as a nitrogen source was much higher than that of using sodium nitrate as a nitrogen source. By using pH-regulated CO 2 supplementation, the pH of the microalgae culture medium was maintained within the neutral or slightly alkaline range. Thus, the volatilization of ammonium hydroxide was negligible. These results indicated that replacing nitrate with ammonium hydroxide could be a promising approach to increase CO 2 utilization efficiency and to reduce production costs in microalgae cultivation. • Replacing sodium nitrate with ammonium hydroxide as the nitrogen source was first proposed in microalgal cultivation. • Biomass concentration and carbon utilization efficiency using ammonium hydroxide as nitrogen sources were achieved by 2.35 g/L, and 114.93%. • A 32.8 % increase in the biomass productivity using ammonium hydroxide compared to that of sodium nitrate under outdoor conditions. • Ammonium hydroxide could be a cheaper source of nitrogen that can enhance CO 2 utilization by microalgae.