Valorization of the secondary thickener wastewater from the conventional depuration plants as a substrate for microalgae biomass production

Conventional wastewater treatment plants (CWWTPs) play a key role in reducing the environmental impact of urban wastewater. But nowadays, this role goes even further with climate change, water scarcity, and the current energy crisis. After the thickening of the secondary sludge, a wastewater flow is generated, which is normally recirculated to the plant head for further treatment. This work aims to use secondary thickener wastewater (STW) as a culture medium for Chlorella vulgaris, Neochloris oleoabundans and Scenedesmus quadricauda growth. These microalgae have the capacity to remove organic and inorganic matters, particularly, in wastewater rich in nitrogen and phosphate compounds. The experiments have been carried out in stirred photobioreactors with 1 L capacity under the following common operating conditions pH 8, mechanical agitation = 200 rpm, air supply rate = 0.5 L/min, continuous artificial illumination intensity = 359 ?E m-2 s-1, and non-sterilized conditions. The net biomass generated for C. vulgaris, N. oleoabundans, and S. quadricauda was 1.89 g/L, 2.73 g/L, and 2.52 g/L, respectively. The kinetic growth study showed a maximum specific growth rate, volumetric biomass productivity and culture duration for C. vulgaris (0.0128 h-1, 0.00583 g/(L h), and 429 h), for N. oleoabundans (0.0151 h-1, 0.00456 g/(L h), and 429 h), and for S. quadricauda (0.0146 h-1 and 0.007362 g/(L h), and 509 h). The microalgae biochemical composition determined biomass rich in energetic compounds (carbohydrates and lipids contents) between 70.9 % and 86.8 % with lower proteins and lipids contents between 9.17 % and 11.5 % and 6.29 % and 13.9 %, respectively. The final water quality registered lower chemical oxygen demand (COD) less than 157 mg O2/L. The final treated water can be direct discharge or used for irrigation.