ElsevierAcevedo Juárez, B.; Camiña, C.; Corona, JE.; Borrás Falomir, L.; Barat Baviera, R. (2015). The metabolic versatility of PAOs as an opportunity to obtain a highly P-enriched stream for further P-recovery. Chemical Engineering Journal. 270:459-467. doi:10.1016/j.cej.2015

Nutrient recovery technologies are rapidly expanding due to the need for the appropriate recycling of key elements from waste resources in order to move towards a truly sustainable modern society based on the Circular Economy.Nutrient recycling is a promising strategy for reducing the depletion of non-renewable resources and the environmental impact linked to their extraction and manufacture. However, nutrient recovery technologies are not yet fully mature, as further research is needed to optimize process efficiency and enhance their commercial applicability. This paper reviews state-of-the-art of nutrient recovery, focusing on frontier technological advances and economic and environmental innovation perspectives. The potentials and limitations of different technologies are discussed, covering systems based on membranes, photosynthesis, crystallization and other physical and biological nutrient recovery systems (e.g. incineration, composting, stripping and absorption and enhanced biological phosphorus recovery).

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This research work proposes an innovative water resource recovery facility (WRRF) for the recovery of energy, nutrients and reclaimed water from sewage, which represents a promising approach towards enhanced circular economy scenarios. To this aim, anaerobic technology, microalgae cultivation, and membrane technology were combined in a dedicated platform. The proposed platform produces a high-quality solid- and coliform-free effluent that can be directly discharged to receiving water bodies identified as sensitive areas. Specifically, the content of organic matter, nitrogen and phosphorus in the effluent was 45 mg COD·L−1, 14.9 mg N·L−1 and 0.5 mg P·L−1, respectively. Harvested solar energy and carbon dioxide biofixation in the form of microalgae biomass allowed remarkable methane yields (399 STP L CH4·kg−1 CODinf) to be achieved, equivalent to theoretical electricity productions of around 0.52 kWh per m3 of wastewater entering the WRRF. Furthermore, 26.6% of total nitrogen influent load was recovered as ammonium sulphate, while nitrogen and phosphorus were recovered in the biosolids produced (650 ± 77 mg N·L−1 and 121.0 ± 7.2 mg P·L−1).

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Two outdoor photobioreactors were operated to evaluate the effect of variable ambient temperature on an indigenous microalgae-nitrifying bacteria culture dominated by Chlorella. Four experiments were carried out in different seasons, maintaining the temperature-controlled PBR at around 25 ºC (by either heating or cooling), while the temperature in the non-temperature-controlled PBR was allowed to vary with the ambient conditions. Temperatures in the range of 15-30 ºC had no significant effect on the microalgae cultivation performance. However, when the temperature rose to 30-35 ºC microalgae viability was significantly reduced. Sudden temperature rises triggered AOB growth in the indigenous microalgae culture, which worsened microalgae performance, especially when AOB activity made the system ammonium-limited. Microalgae activity could be recovered after a short temperature peak over 30 ºC once the temperature dropped, but stopped when the temperature was maintained around 28-30 ºC for several days.

A continuously aerated SHARON (single reactor high activity ammonia removal over nitrite) system has been operated to achieve partial nitritation. Two sets of batch experiments were carried out to study the effect of ammonia concentration and salinity on the activity of ammonia-oxidizing bacteria (AOB). Activity of AOB raised as free ammonia concentration was increased reaching its maximum value at 4.5 mg NH3-N l(-1). The half saturation constant for free ammonia was determined (K(NH3)=0.32 mg NH3-N l(-1)). Activity decreased at TAN (total ammonium-nitrogen) concentration over 2,000 mg NH4-N l(-1). No free ammonia inhibition was detected. The effect of salinity was studied by adding different concentrations of different salts to the biomass. No significant differences were observed between the experiments carried out with a salt containing or not containing NH4. These results support that AOB are inhibited by salinity, not by free ammonia. A mathematical expression to represent this inhibition is proposed. To compare substrate affinity and salinity inhibitory effect on different AOB populations, similar experiments were carried out with biomass from a biological nutrient removal pilot plant. The AOB activity reached its maximum value at 0.008 mg NH3-N l(-1) and decreased at TAN concentration over 400 mg NH4-N l(-1). These differences can be explained by the different AOB predominating species: Nitrosomonas europaea and N. eutropha in the SHARON biomass and Nitrosomonas oligotropha in the pilot plant.

Outdoor microalgae cultivation systems treating anaerobic membrane bioreactor (AnMBR) effluents usually present ammonium oxidising bacteria (AOB) competition with microalgae for ammonium uptake, which can cause nitrite accumulation. In literature, nitrite effects over microalgae have shown controversial results. The present study evaluates the nitrite inhibition role in a microalgae-nitrifying bacteria culture. For this purpose, pilot-and lab-scale assays were carried out. During the continuous outdoor operation of the membrane photobioreactor (MPBR) plant, biomass retention time (BRT) of 2 d favoured AOB activity, which caused nitrite accumulation. This nitrite was confirmed to inhibit microalgae performance. Specifically, continuous 5-d lab-scale assays showed a reduction in the nitrogen recovery efficiency by 32, 42 and 80% when nitrite concentration in the culture accounted for 5, 10 and 20 mg N•L-1 , respectively. On the contrary, short 30-min exposure to nitrite showed no significant differences in 2 the photosynthetic activity of microalgae under nitrite concentrations of 0, 5, 10 and 20 mg N•L-1. On the other hand, when the MPBR plant was operated at 2.5-d BRT, the nitrite concentration was reduced to negligible values due to increasing activity of microalgae and nitrite oxidising bacteria (NOB). This allowed obtaining maximum MPBR performance; i.e. nitrogen recovery rate (NRR) and biomass productivity of 19.7 ± 3.3 mg N•L-1 •d-1 and 139 ± 35 mg VSS•L-1 •d-1 , respectively; while nitrification rate (NOxR) reached the lowest value (13.5 ± 3.4 mg N•L-1 •d-1). Long BRT of 4.5 d favoured NOB growth, avoiding nitrite inhibition. However, it implied a decrease in microalgae growth and the accumulation of nitrate in the MPBR effluent. Hence, it seems that optimum BRT has to be within the range 2-4.5 d in order to favour microalgae growth with respect to AOB and NOB.