Improving the energy efficiency of an electrodialytic process to extract phosphorus from municipal solid waste digestate through different strategies

This work has been funded by Portuguese National Funds through FCT – Portuguese Foundation for Science and Technology under CERNAS (UID/AMB/00681/2013). Celia Dias-Ferreira and Verónica Oliveira have been funded through FCT “Fundação para a Ciência e para a Tecnologia” by POCH – Programa Operacional Capital Humano within ESF – European Social Fund and by national funds from MCTES (SFRH/BPD/100717/2014; SFRH/BD/115312/2016). The authors would like to thank Sabrina Hvid for carrying out ICP analyses.This work assesses the possibility of energy optimisation during the electrodialytic extraction of phosphorus from the municipal solid waste digestate. Strategies tested for energy optimisation consisted in (i) using a dualstage extraction approach; (ii) replacement of continuous stirring by pulse stirring and; (iii) utilisation of pulse electric current as an alternative to a constant current. Experiments were carried out using different stirring profiles (continuous stirring, pulsed stirring, manual stirring and no stirring) and different profiles for generating the electric field (continuous current and pulsed current). Dual-stage approach decreased the energy consumption by 30%. Energy was mainly used in the stirring operation (80%). Applying this strategy caused a faster acidification of the waste suspension and reduced the time required for the extraction. As for the second strategy, use of pulse stirring, it is possible to save energy by operating the stirrer 25% of the time, without compromising the phosphorus extraction values, which are close to 90%. Corresponding energy savings reached 70%. The third strategy tested, use of pulse current, had as side-effect a significant decrease of the phosphorus extracted, so the implementation of pulse current is not recommended. Overall, the energy efficiency of the electrodialytic extraction of phosphorus can be improved by 80% through the use of the dual-stage approach and the use of stirring for 25% of the time. There is also a potential in further reducing the energy use by further improving the stirring system.