Potential improvement to a citric wastewater treatment plant using bio-hydrogen and a hybrid energy system

Treatment of highly concentrated organic wastewater is characterized as cost-consuming. The conventional technology uses the anaerobic-anoxic-oxic process (A²/O), which does not produce hydrogen. There is potential for energy saving using hydrogen utilization associated with wastewater treatment because hydrogen can be produced from organic wastewater using anaerobic fermentation. A 50 m³ pilot bio-reactor for hydrogen production was constructed in Shandong Province, China in 2006 but to date the hydrogen produced has not been utilized. In this work, a technical-economic model based on hydrogen utilization is presented and analyzed to estimate the potential improvement to a citric wastewater plant. The model assesses the size, capital cost, annual cost, system efficiency and electricity cost under different configurations. In a stand-alone situation, the power production from hydrogen is not sufficient for the required load, thus a photovoltaic array (PV) is employed as the power supply. The simulated results show that the combination of solar and bio-hydrogen has a much higher cost compared with the A²/O process. When the grid is connected, the system cost achieved is 0.238 US$ t⁻¹ wastewater, which is lower than 0.257 US$ t⁻¹ by the A²/O process. The results reveal that a simulated improvement by using bio-hydrogen and a FC system is effective and feasible for the citric wastewater plant, even when compared to the current cost of the A²/O process. In addition, lead acid and vanadium flow batteries were compared for energy storage service. The results show that a vanadium battery has lower cost and higher efficiency due to its long lifespan and energy efficiency. Additionally, the cost distribution of components shows that the PV dominates the cost in the stand-alone situation, while the bio-reactor is the main cost component in the parallel grid.