Effect of N:P Ratio on Microalgae/Nitrifying Bacteria Community in Agro-Digestate Treatment

Abstract

The role of P content on the treatment and valorization of the liquid fraction of digestate, namely centrate, through microalgae-based technologies was evaluated in this study. The performance of four column photobioreactors, which were fed on diluted centrate with corrected (10 mg N/ mg P) and not modified (129 mg N/ mg P) N:P ratio, were monitored and compared. The results demonstrated that P shortage in the centrate affected neither the total nitrogen and COD removal rate nor the volumetric biomass productivity, suggesting that expensive addition of P salts is not necessary to maximize the efficiency of the process. On the contrary, the addition of P to the centrate promoted the ammonia oxidation process as higher nitrite production was observed in the photobioreactors with adjusted N:P ratio than in the ones fed with the non-adjusted N:P ratio. These findings were confirmed by fluorescence in-situ hybridization and quantitative PCR assays, which revealed a higher number of ammonia-oxidizing bacteria in the microalgal suspensions cultivated on centrate with P addition. In conclusion, the N:P ratio in the centrate seems to have a role in controlling the nitrification process rather than in the overall nutrient removal rate and biomass productivity of the microalgae-based system.

Comprehensive assessment of the microalgae-nitrifying bacteria competition in microalgae-based wastewater treatment systems: Relevant factors, evaluation methods and control strategies

“…Different medium characteristics and environmental and operational conditions can greatly affect microalgae or nitrifying bacteria, and shift their equilibrium [31,52]. The most important factors related to both microalgae and nitrifying bacteria activity are depicted in Figure 2 and discussed below.…”

“…This behaviour was reported by González-Camejo et al [32], who observed an equilibrium between microalgae and bacteria when the ammonium loading rate was under 30-35 mg N•L -1 •d -1 , but a significant proliferation of nitrifying bacteria when it went over 40 mg N•L -1 •d -1 . Other authors have also reported nitrification control by adjusting the N:P ratio of the influent [31]. If phosphorus is scarce in the medium, nitrifying bacteria will not be able to grow.…”

“…With wastewater from anaerobic processes such as digestates, centrates and effluents from anaerobic membrane bioreactor (AnMBR) systems or aerobic secondary effluents, heterotrophic bacteria activity is usually neglected as their organic matter content is mostly recalcitrant to biodegradation [11]. As ammonium (NH4) is usually present in these wastewater streams [27,28], the competition between microalgae and autotrophic nitrifying bacteria (specifically ammonium oxidising bacteria (AOB)) for ammonium uptake plays a significant role [29][30][31]. There are several ways of dealing with microalgae-nitrifying bacteria cultivation systems to treat high ammonium-loaded wastewater streams, depending on the final goal of the process: i) Maximising microalgae activity to improve nitrogen and phosphorus removal, thus reducing the nutrient concentration of the effluent common in wastewaters with middle or low ammonium concentrations, i.e.…”

Comparison of the Performance and Microbial Community Structure of Two Outdoor Pilot-Scale Photobioreactors Treating Digestate

“…High ammonium oxidation is quite common in microalgae-based technologies treating municipal and agro-zootechnical digestate [34,54,55], especially if phosphorus is readily available [65]. The elevated P concentration in the centrate favored the activity of the AOB [66], which are the main responsible for ammonium oxidation. This hypothesis is supported also by the positive correlation between the ammonium oxidation rate and the concentration of PO 4…”