Enhanced Reverse Osmosis Concentrate Disposal and Nutrients Conversion to Microalgae Bioenergy with a Gradient-Fed Strategy

Abstract

Reverse osmosis concentrate (ROC) disposal with microalgae has attracted increasing interest because of its intrinsic advantages, like simultaneous wastewater remediation and bioenergy production. But these processes are currently suffering from inefficient nutrients removal and poor microalgae growth due to ROC toxicity. A gradient-fed strategy is proposed to enhance ROC disposal and nutrients conversion to microalgal Chlorella vulgaris lipid by adjusting the ROC feeding. Time-evolving profiles of nutrients and chemical oxygen demand (COD) during experiments as well as their influences on microalgae growth and metabolism have been investigated. Results indicate that the removal of NH4+ and COD using the gradient-fed process was improved by 104.3% and 214.2% compared to those obtained in batch mode, generating a high-quality ROC effluent with N and P nutrients almost totally removed. Microalgae biomass and lipid content were simultaneously enhanced with the gradient-fed process. Energy production analysis showed that the highest energy yield was achieved with use of the gradient-fed process at 30%–70%–100% as 39.0 kJ/L, which was higher than the volumetric heat value of natural gas (about 36.0 kJ/L) and significantly higher than that obtained in batch mode (21.6 kJ/L). This work provides a practical strategy for large-scale microalgae cultivation with ROC.