Assessment of a deep, LED-enhanced high-rate algal pond for the treatment of digestate

• Published in: Algal research (Amsterdam) 2021-11, Vol.59, p.102444, Article 102444
• Main Authors: Torres-Franco, A.F., Figueredo, C.C., Barros, L., Gücker, B., Boëchat, I.G., Muñoz, R., Mota, C.R.
• Format: Article
• Language: English
• Subjects: Digestate; LED lighting; Microalgae; Nutrient removal and recovery; Wastewater treatment
• ISSN: 2211-9264; 2211-9264
• DOI: 10.1016/j.algal.2021.102444

High Rate Algal Ponds (HRAP) are a sustainable alternative for wastewater treatment. However, HRAPs must be shallow (h = 0.2–0.3 m) for adequate sunlight penetration and require relatively long hydraulic retention times (HRT, >5 d) for effective pollutant removal, which results in high area footprints. In this context, a deep HRAP (h = 1.2 m) was equipped with red light-emitting diodes (LED, 660 nm) to provide photosynthetic active radiation in the aphotic zone. A conventional HRAP (HRAP1) and the LED-enhanced HRAP (HRAP2) were operated with identical working volumes (1.25 m3) and HRT (10 d), treating digestate dilutions of 5% (Stages S-0, S-I), 25% (S-II, S-III) and 50% (v/v) (S-IV to S-VI). The HRAP2 used only 25% of the area needed by the HRAP1 but LEDs used 1.4 (S-0 to S-II), 2.3 (S-III, S-IV) and 3.4 (S-V) times the energy consumed in HRAP1. HRAP2 presented similar performance to HRAP1 during the treatment of 5% and 25%-digestate, achieving removals above 75% for TSS, COD, TAN, and TKN. However, HRAP1 showed better treatment performance at 50%-digestate, likely due to more intense light radiation induced by its shallow depth, resulting in higher microalgal activity. Biomass productivity decreased with increasing digestate concentration, showing averages in HRAP1 and HRAP2 of 9 and 3 gVSS·m−2d−1, respectively, during feeding with 5%-digestate. With 25%, average productivities ranged from 2 to 5 and 2 to 6 gVSS·m−2d−1, and from 0 to 6 and 0 to 4 gVSS·m−2d−1 with 50%-digestate, respectively. Both systems showed reasonable nitrogen and phosphorus recoveries in the biomass (TN: 8–17% and 6–34%; TP: 10–45% and 5–48% for HRAP1 and HRAP2, respectively). Nitrogen volatilization, likely as NH3, N2O, and N2, was caused by stripping and nitrification-denitrification in both systems. Our data suggest that using LED-lighting to complement sunlight in deeper HRAPs is a promising alternative for enhancing microalgae-based reactors for digestate treatment. [Display omitted] •Smaller area footprint is necessary for wider application of high rate algal ponds.•A new LED-enhanced high rate algal pond was assessed for digestate treatment.•LED-enhanced pond required only 25% of the area needed for the conventional high rate pond.•Both ponds showed similar performance for 5% and 25% digestate dilution rates.•Treatment efficiencies above 75% were achieved for TSS, COD and ammonia.