The preculture of algal flocs was created using activated sludge from an aeration tank cultured in secondary effluent in a batch reactor under static conditions. In this case, the biomass concentration was 694 mg L−1 after 30 days, and the mean diameter of algal flocs was 0.994 mm (Biliani and Manariotis 2022 ). The present study examined the formation of photogranules in six configurations (Fig. S1). In each case, 85 mL of the preculture was placed in a 1-L glass beaker, and a final volume of 1000 mL was reached with primary effluent wastewater (PE) (3 reactors) or secondary effluent (SE) (3 reactors). Every reactor had different mixing and fill-draw conditions, as shown in Table S1 . The six reactors were placed in a room next to the window at a temperature of 20 ± 5 °C for 240 days. Primary and secondary effluent was sampled from the wastewater treatment plant (WWTP) of the University of Patras campus at Rio, and their characteristics are given in Table 1 . Mixing was not used in the static reactors (PE Static and SE Static), while mixing at 30 rpm was applied in the rest of the reactors (PE Mixing 1, PE Mixing 2, SE Mixing 3 and SE Mixing 4). During the 2nd period, the hydraulic reaction time (HRT) was doubled from 8 to 16 days in the PE Static and PE Mixing 1 reactor, 4 to 8 days PE in the Mixing 2 reactor, 2 to 4 days in the SE Static and SE Mixing 3 reactor and 1.3 to 2.7 days in the SE Mixing 4 reactor. In the 3rd period, the volume withdrawn decreased to half in the SE cultures but remained the same in the PE cultures. In the 4th period, the mixing velocity increased from 30 to 50 rpm for the mixing cultures (PE Mixing 1, PE Mixing 2, SE Mixing 3 and SE Mixing 4) and the HRT of the SE cultures was decreased to half. The light intensity in the 4th period increased by adding led lamps (12 V, RGB SPECTRUM, USA) with a photoperiod of 24 h yielding an irradiance of 100 μmol photons m−2 s−1, compared to 50 photons m−2 s−1 during the 3rd period. In the 5th period, the HRT was further decreased in the SE but remained the same in the PE cultures. The operation cycle consisted of feeding (10 min), rapid mixing (2 min), reaction (0.98 to 1.98 days), mixing (2 min), settling (10 min) and decanting (10 min). Samples were systematically taken from the supernatant at the end of the settling phase. Less frequently, samples were taken from the mixed liquor before the settling phase.
Characteristics of wastewater and microalgae preculture
| Nutrient | Units | Primary effluent | Secondary effluent | Microalgae Preculture | |||||
|---|---|---|---|---|---|---|---|---|---|
| Range | Mean | SD* | Range | Mean | SD | Mean | SD | ||
| COD | mg L−1 | 200–220 | 205 | 10 | 50–60 | 54 | 5 | - | - |
| TSS | mg L−1 | 30–40 | 33 | 5 | 20–30 | 21 | 5 | 200 | 2.6 |
| Chl-a | mg m−3 | - | - | - | - | - | - | 597 | 0.002 |
| Total-P | mg L−1 | 4.5–5.5 | 5.0 | 0.5 | 3.5–5.5 | 4.2 | 0.75 | 7.8 | 0.1 |
| NO3-N | mg L−1 | 0.0–1.1 | 0.57 | 0.5 | 5.5–12 | 7.9 | 3.25 | 0.4 | 0.06 |
| NH3-N | mg L−1 | 10–30 | 22 | 10 | 2.0–10 | 5.3 | 4 | 4.7 | 0.03 |
| pH | - | 7.0–8.0 | 7.7 | 0.5 | 7.0–8.0 | 7.4 | 0.5 | 8.9 | - |
*SD: standard deviation
