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).

A series of eight experiments were carried out to analyse the effects of light intensity, light duration and photoperiods on a microalgae culture for treating AnMBR effluent at an outdoor photobioreactor (PBR) plant.Improved performance was achieved in terms of nutrient recovery rates, biomass productivity and effluent nutrient concentrations at a higher net photon flux. However, the higher irradiance was also responsible for lower biomass productivity:light irradiance ratios.None of the experiments with different lighting regimes and the same net photon flux showed any significant differences. The data obtained suggest that microalgae performance in this system did not depend on the time of day when light was applied or the length of the photoperiods, but on the net photon flux. No photoinhibiton was * Constant energy consumption of 0.8 KwhSolar light is the most economical option for outdoor microalgae cultivation [24,39], but variations in the weather, day:night cycles and seasonal changes affect light intensity

The aim of this study was to evaluate the effect of light intensity and temperature on nutrient removal and biomass productivity in a microalgae-bacteria culture and their effects on the microalgae-bacteria competition. Three experiments were carried out at constant temperature and various light intensities: 40, 85 and 125 µE m s. Other two experiments were carried out at variable temperatures: 23 ± 2°C and 28 ± 2°C at light intensity of 85 and 125 µE m s, respectively. The photobioreactor was fed by the effluent from an anaerobic membrane bioreactor. High nitrogen and phosphorus removal efficiencies (about 99%) were achieved under the following operating conditions: 85-125 µE m s and 22 ± 1°C. In the microalgae-bacteria culture studied, increasing light intensity favoured microalgae growth and limited the nitrification process. However, a non-graduated temperature increase (up to 32°C) under the light intensities studied caused the proliferation of nitrifying bacteria and the nitrite and nitrate accumulation. Hence, light intensity and temperature are key parameters in the control of the microalgae-bacteria competition. Biomass productivity significantly increased with light intensity, reaching 50.5 ± 9.6, 80.3 ± 6.5 and 94.3 ± 7.9 mgVSS L d for a light intensity of 40, 85 and 125 µE m s, respectively.

The operation of an outdoor membrane photobioreactor plant which treated the effluent of an anaerobic membrane bioreactor was optimised. Biomass retention times of 4.5, 6, and 9 days were tested. At a biomass retention time of 4.5 days, maximum nitrogen recovery rate:light irradiance ratios, photosynthetic efficiencies and carbon biofixations of 51.7 ± 14.3 mg Nꞏmol -1 , 4.4 ± 1.6 % and 0.50 ± 0.05 kg CO2ꞏm 3 influent, respectively, were attained. Minimum membrane fouling rates were achieved when operating at the shortest biomass retention time because of the lower solid concentration and the negligible amount of cyanobacteria and protozoa.Hydraulic retention times of 3.5, 2, and 1.5 days were tested at the optimum biomass retention times of 4.5 days under non-nutrient limited conditions, showing no significant differences in the nutrient recovery rates, photosynthetic efficiencies and membrane fouling rates. However, nitrogen recovery rate:light irradiance ratios and photosynthetic efficiency significantly decreased when hydraulic retention time was 2 further shortened to 1 day, probably due to a rise in the substrate turbidity which reduced the light availability in the culture. Optimal carbon biofixations and theoretical energy recoveries from the biomass were obtained at hydraulic retention time of 3.5 days, which accounted for 0.55 ± 0.05 kg CO2ꞏm -3 influent and 0.443 ± 0.103 kWhꞏm -3 influent, respectively.

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.