Co-Cultivation between the Microalga Tetradesmus obliquus and Filamentous Fungus Cunninghamella echinulata Improves Tertiary Treatment of Cheese Whey Effluent in Semicontinuous Mode

Abstract

Cheese whey (CW), a byproduct resulting from dairy processing, requires proper treatment and disposal. The use of microalgae during tertiary treatment emerges as a promising option due to its efficiency to remove chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) from effluents obtained after secondary treatment processes. The present study was focused on treating CW effluent at two organic loads (C1 and C2—with different concentrations of COD, TN, and TP) (550–2200 mg·L−1 of COD, 14–56 mg·L−1 of TN, and 4.5–18 mg·L−1 of TP) using the microalga Tetradesmus obliquus alone (CM1 and CM2) and in a consortium with the fungus Cuninghamella echinulata (CMF1 and CMF2), evaluating the residual values of COD, TN, and TP and removal efficiency. The experiments were carried out in an open system with a volumetric replacement ratio (VRR) of 40 and 60%. The CM treatment showed residual values of COD in the range of 190–410 mg·L−1 (removal efficiency: 57–68%), TN in the range of 6–24 mg·L−1 (removal efficiency: 29–35%), and TP in the range of 0.90–3.0 mg·L−1 (removal efficiency: 65–68%), after 7 days of volumetric replacement time (VRT) in a semicontinuous mode. In contrast, the consortium (CMF) showed greater stability and efficiency in contaminant removal compared to the treatment system containing only the microalga, showing residual values of COD in the range of 61–226 mg·L−1 (removal efficiency: 75–77%), TN in the range of 1.8–9.5 mg·L−1 (removal efficiency: 70–74%), and TP in the range of 0.6–3.5 mg·L−1 (removal efficiency: 66–70%), applying a lower VRT of 3 days and reaching the legislation standard for discharge to CMF1 (VRR: 40 and 60%) and CMF2 (VRR: 40%). The cell dry weight of 290–850 mg·L−1 was obtained (microalga and microalga–fungus cultivation), which can be a valuable biomass for biotechnological applications. Finally, during microalga–fungus co-cultivation, there was greater system buffering (with less pH variation), ensuring a better system stability.