Microalgae recycling improves biomass recovery from wastewater treatment high rate algal ponds

Abstract

Microalgal biomass harvesting by inducing spontaneous flocculation (bioflocculation) sets an attractive approach, since neither chemicals nor energy are needed. Indeed, bioflocculation may be promoted by recycling part of the harvested microalgal biomass to the photobioreactor in order to increase the predominance of rapidly settling microalgae species. The aim of the present study was to improve the recovery of microalgal biomass produced in wastewater treatment high rate algal ponds (HRAPs) by recycling part of the harvested microalgal biomass. The recirculation of 2% and 10% (dry weight) of the HRAPs microalgal biomass was tested over one year in an experimental HRAP treating real urban wastewater. Results indicated that biomass recycling had a positive effect on the harvesting efficiency, obtaining higher biomass recovery in the HRAP with recycling (R-HRAP) (92-94%) than in the control HRAP without recycling (C-HRAP) (75-89%). Microalgal biomass production was similar in both systems, ranging between 3.3 and 25.8 g TSS/md, depending on the weather conditions. Concerning the microalgae species, Chlorella sp. was dominant overall the experimental period in both HRAPs (abundance>60%). However, when the recycling rate was increased to 10%, Chlorella sp. dominance decreased from 97.6 to 88.1%; while increasing the abundance of rapidly settling species such as Stigeoclonium sp. (16.8%, only present in the HRAP with biomass recycling) and diatoms (from 0.7 to 7.3%). Concerning the secondary treatment of the HRAPs, high removals of COD (80%) and N-NH (97%) were found in both HRAPs. Moreover, by increasing the biomass recovery in the R-HRAP the effluent total suspended solids (TSS) concentration was decreased to less than 35 mg/L, meeting effluent quality requirements for discharge. This study shows that microalgal biomass recycling (10% dry weight) increases biomass recovery up to 94% by selecting the most rapidly settling microalgae species without compromising the biomass production and improving the wastewater treatment in terms of TSS removal.

Life cycle assessment of high rate algal ponds for wastewater treatment and resource recovery

“…In this case, due to the more favourable climatic conditions for microalgae growth compared to Catalonia, the HRT was the same over the year (HRT = 3 days). It has to be noted that, for the same reason, the microalgal biomass production is considerably higher in the system implemented in Andalucía with respect to the one located in Catalonia (3-26 gTSS m-2 d-1 vs. 15-30 gTSS m-2 d-1, respectively) (Gutiérrez et al, 2016;Morales-Amaral et al, 2015a).…”

“…Treated water is then discharged into a surface water body. Part of the harvested microalgal biomass (2 and 10 % on a dry weight basis in summer and winter, respectively) is recycled in order to enhance spontaneous flocculation (bioflocculation) and increase microalgae harvesting efficiency (Gutiérrez et al, 2016). The remaining harvested biomass is thickened (HRT = 24 h), thermally pretreated (75 °C, 10 h) and codigested with primary sludge (35 °C, 20 days).…”

“…In this context, the HRT of each HRAP has to be modified over the year (8, 6 and 4 days) in accordance with the weather conditions (i.e. solar radiation and temperature) in order to accomplish wastewater treatment and meet effluent quality requirements for discharge (García et al, 2000;Gutiérrez et al, 2016). For this reason, it was considered that during summer months (from May to July) only two HRAPs work in parallel (HRT = 4 days), whereas all of them are operated during winter months (from November to April) (HRT = 8 days).…”

“…In the case of HRAP systems coupled with biogas and biofertilizer production (Scenarios 1 and 2), inventory data regarding construction materials and operation were based on the detailed engineering designs performed in the frame of this study. Treated wastewater characteristics were estimated considering the removal efficiencies and experimental results obtained in the pilot systems implemented at the Universitat Politècnica de Catalunya-BarcelonaTech (UPC) (5 m2) (Gutiérrez et al, 2016) and at the Las Palmerillas Experimental Station (100 m2) (Morales-Amaral et al, 2015a) for Scenarios 1 and 2, respectively. NH4+ volatilization was estimated through nitrogen mass balance.…”

“…Since Metal Depletion Potential is mainly influenced by construction materials, the lower environmental performance of Scenario 1 is owing to the larger surface area required for its implementation compared to Scenario 2 (4 m2 p.e.-1 vs. 3 m2 p.e.-1, respectively). As mentioned above, in the system implemented in Catalonia (Scenario 1), a higher HRT is needed (especially during winter months) compared to that implemented in Andalucía (Scenario 2) in order to obtain a effluent quality suitable for discharge (García et al, 2000;Gutiérrez et al, 2016, Morales-Amaral et al 2015a;…”