Start-up and process control of demonstration plant for peroxides wastewater treatment and reuse within EU-Horizon2020 AquaSPICE project

The AquaSPICE project, funded by the European Union's Horizon 2020 research and innovation initiative, aims to promote resource efficiency, provide compact solutions for industrial applications, and actualize circular water use in European Process Industries. The expected results and impacts deriving from this project include a significant reduction of the current use of freshwater resources at the five case studies, one of which is the water treatment and re- use within peroxide production units at SOLVAY, ARETUSA (CS#2, Tuscany, Italy). The AquaSPICE project supports Solvay's environmental policy by employing the WAPEREUSE pilot system, whose aim is to reuse Solvay’s wastewater for cooling towers processes. This is achieved using conventional systems instead of energy-intensive and environmentally impactful processes. Additionally, by reusing this industrial wastewater for cooling towers purposes, AquaSPICE helps reduce Solvay’s pressure on biodiversity. The aim of this thesis work is to test the WAPEREUSE pilot-scale plant installed at Falconara (AN). The pilot units tested during the internship period include the wastewater neutralization unit, GFH filtration, and MBR. The objective is to understand if the MBR can discharge the effluent wastewater into one of the 3 possible reuse options, one of which includes direct internal reuse within the Solvay plant. This is determined based on the pilot effluent concentrations of COD and nitrates (Solvay's wastewater has an average COD concentration of 1000 mgO2/L and 850 mg NO3/L). Four different Solvay stocks, each consisting of 1 m3 of wastewater, were tested during the internship. The inlet pH was always neutralized from 2 to 7. The GFH unit did not provide substantial advantages in terms of COD removal. For the biological reactor, it was confirmed that the developed technology is effective in removing nitrates. Discharge options 1 (NO3-N<30 mg/L), 2 and 3 (NO3-N<20 mg/L) for nitrates were achieved in stocks 3 and 4 by setting different cycles of denitrification-oxidation. Kinetics of nitrates removal were estimated to be around 10 mgN/(L*h). For COD, removal efficiency depends on the influent (Solvay wastewater contains a large portion of recalcitrant COD) and the technology needs enhancement to meet discharge options 2 (COD<125 mg/L) and 3 (COD<100 mg/L). In all the stocks, option 1 (COD<500 mg/L) was always achieved. Automation of the pilot system is managed by a PLC. All pilot plant's operation and treatment performance were monitored by sensors recording values from probes installed in the tanks and laboratory analysis. As surrogate on demand measurement of COD, UV absorbance at 254nm proved to be a discrete indicator. Indeed, the correlation between COD values and UV254nm value both measured with the probe and in lab, present an R2 higher than 0.7. The plots of general trend of nitrate concentrations in time indicate that the periodic calibrations are crucial for maintaining data accuracy. On average, it is suggested to calibrate and clean the probes weekly. Energy and reagents consumptions were also monitored, in particular, it was observed that during one of the optimal cycles, to treat 1 m3 of Solvay wastewater, 2.2L of NaOH and 2.92L of external carbon source were used. The energy consumption was estimated on average in the stocks analyzed as 100 kWh/m3.