Evaluation of hydraulic and solid retention time on the removal of nutrients and organic matter from a municipal wastewater using filamentous microalgae

NASHIRO A.; MARCHESE S.; MARSILI S.; CAGNONE R.; LOPEZ MAYER G.; GOROSITO S.; CABAÑAS D.; GARCIA A.; IORIO A. F. DE; REARTE T.A.

Ciudad Autónoma de Buenos Aires

Congreso; 11th World Congress of Chemical Engineering; 2023

World Chemical Enginering Council. Asociación Argentina de Ingenieros Químicos

In recent years, interest has been given to the use of microalgae for wastewater treatment, since it has lower costs and allows the economic valorization of biomass. However, its application at commercial scales is still incipient due to three main limitations: high surface demand, hydraulic retention time and biomass harvesting costs. Filamentous microalgae show great potential since they can be harvested from the culture with simple and inexpensive processes by sieving, and they enable the separation of the solid (i.e. cellular) retention time (SRT) from the hydraulic retention time (HRT), thus, HRT can be reduced. The objective of this work was to evaluate the removal capacity of dissolved nutrients (N and P) and organic matter from a primary municipal wastewater using filamentous microalgae with different HRT and SRT. Two strains of filamentous microalgae were investigated: Stigeoclonium sp. and Oedogonium sp. The wastewater to be treated presented 70 mg.L-1 of chemical oxygen demand (as a measure of organic matter), 40 mg.L-1 of NH4+, 0,8 mg.L- 1 of NO3- and 2,7 mg.L-1 of soluble reactive phosphorus. It was used as a culture medium for microalgae, expecting that they would form consortiums with native bacteria from the effluent, so that a synergistic removal of organic matter (attributed to bacteria mainly) as well as nutrients would occur.The experiments were carried out in glass bubbled-columns with 220 mL of capacity in a semicontinuos regimen, with injection of CO2 by pulses of 1 second every 15 minutes, a photoperiod of 14 hours of light and 10 hours of darkness and a room temperature of 24 ± 2 ºC. The treatments resulted from a combination of different HRT (24 and 12 hours) and SRT (10; 5; 3.3; 2.5 days) for each strain. A filter with a pore size of 166 µm was used to hold back the biomass inside the reactors.Biomass productivity was higher in Stigeoclonium sp., reaching values around 0,46 g. L-1.d-1 with 3,3 d SRT and 24 h HRT. Oedogonium sp. attained a maximum value of 0,41 g. L-1.d-1 with 3,3 d SRT and 12 h HRT. Stigeoclonium sp. exhibited a removal of 95% in the case of P, 77% in NH4+, and 45% in nitrate, achieving values for discharge in waterbodies by the current legislation. Regarding the values of organic matter, Stigeoclonium sp. presented removal percentages between 25% and 75% in all the treatments with 12 h HRT, while it did not exhibit any removal for HRT of 24 h except for 2,5 d SRT whose value was around 44%.Oedogonium sp. obtained lower percentages of removal of nutrients. In the case of nitrate, it overcame the initial values in every treatment, except for 10 d SRT and 24 h, which could indicate the existence of nitrification process. There was neither removal of organic matter.In conclusion, with low values of HRT, Stigeoclonium sp. proved to be a good candidate for municipal wastewater treatment.