Liquid Digester from Urban Wastewater Treatment Plants for Chlorella vulgaris’ Growth and Nutrient Recirculation

Abstract

Wastewater treatment is an important issue because it directly impacts the environment and is directly related to climatic change. However, urban wastewater treatment plants just now cannot complete the treatment from the viewpoint of the circular economy. This work uses the digested liquid generated from the anaerobic digester as culture media to grow the green microalga Chlorella vulgaris. Different culture media at different effluent concentrations are prepared with tap water (5, 10, 25, 35, and 50%, v/v). The experiments were performed in stirred photobioreactors in batch mode with 1L capacity. The common operating conditions used were pH of the culture media = 8, mechanical agitation = 200 rpm, air supply rate = 0.5 L/min, and continuous artificial light at illumination intensity = 359 μE m⁻² s⁻¹. The experimental results show that the kinetic growth parameters maximum specific growth rate (0.0204 h⁻¹) and volumetric biomass productivity (0.00860 g L⁻¹ h⁻¹) values were determined in the culture at 25% (v/v) of digested liquid. Furthermore, the wastewater treated quality in terms of removal chemical oxygen demand percentage (26.6%) and total nitrogen removal percentage (94.7%) were determined in the cultures with 50 and 10% (v/v) of liquid digester from wastewater treatment plant (LD-WWTP). The highest net harvest biomass concentration (x–x_o = 2.54 ± 0.0155 g/L), total lipid content (13.1%), and %CO₂ removal (72.2%) at the end of the cultures were registered in culture operated at 25% (v/v) of LD-WWTP. These results show the possible recovery and recirculation of nutrients from LD-WWTP.