Nitrogen pollution from agriculture is a major challenge facing our society today. Biological nitrogen fixation is key to combat the damage that is caused by synthetic nitrogen. Azolla spp. are ideal candidates for fast nitrogen fixation. This study aimed to investigate the optimal growth conditions for Azolla pinnata R. Brown. The growth conditions that were investigated included the growth medium type and strength, light intensity, the presence/absence of nitrogen in the medium, pH control, and humidity. Higher light intensities increased plant growth by 32%, on average. The highest humidity (90%) yielded higher growth rate values than lower humidity values (60% and 75%). The presence of nitrogen in the medium had no significant effect on the growth rate of the plants. pH control was critical under the fast growth conditions of high light intensity and high humidity, and it reduced algal growth (from visual observation). The optimal growth rate that was achieved was 0.321 day−1, with a doubling time of 2.16 days. This was achieved by using a 15% strength of the Hoagland solution, high light intensity (20,000 lx), nitrogen present in the medium, and pH control at 90% humidity. These optimised conditions could offer an improvement to the existing phytoremediation systems of Azolla pinnata and aid in the fight against synthetic nitrogen pollution.
The behaviour of a locally isolated diazotrophic consortium was investigated with the prospect of agricultural applications. A repeatable culture was obtained in a non-sterile bioreactor. Metagenomic analysis indicated Chryseobacterium ssp. and Flavobacterium ssp. were the dominant species, making up approximately 50% of the microbial community. The oxygen supply was varied and mass-transfer limited growth was attained under all experimental conditions. Negligible amounts of aqueous metabolites were formed, indicating a high selectivity towards biomass production. High oxygen availability resulted in decreased growth efficiencies i.e., the specific energy requirements for biomass synthesis. This was attributed to reduced electron transport chain efficiencies and nitrogenase protection mechanisms. Mass and energy balances indicated that sessile biomass with a high C:N served as a carbon sink. The most efficient growth was measured at an aeration feed composition of 21% oxygen and 79% nitrogen. The study presents one of the only known investigations of operational conditions on diazotrophic growth in a non-sterile bioreactor. In addition, it provides a strong foundation for the development of a Biological Nitrogen Fixation process with scaling potential.
Aquatic nitrogen pollution is one of the most urgent environmental issues requiring prevention and mitigation. Large quantities of high-ammonium wastewaters are generated by several industrial sectors, such as fertilizer and anaerobic-digestion plants. Nitrification of these wastewaters is commonly carried out, either to remove nitrogen or produce liquid fertilizers. Standard control methodologies for the efficient nitrification of high-ammonium wastewaters to produce liquid fertilizers have not yet been established and are still within their early stages of development. In this paper, novel pH-based control algorithms are presented that maintain operation at the microbial maximum reaction rate (υmax) in batch and continuous reactors. Complete conversion of ammonium to nitrate was achieved in a batch setup, and a conversion of 93% (±1%) was achieved in a continuously-stirred-tank-reactor. The unparalleled performance and affordability of the control schemes proposed offer a steppingstone to the future of sustainable fertilizer production.
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