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

Publication . Oliveira, Verónica; Kirklund, G.; Horta, Carmo; Labrincha, João; Ferreira, Célia; Kirkelund, Gunvor M.

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.

2019-06Journal article

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Testing new strategies to improve the recovery of phosphorus from anaerobically digested organic fraction of municipal solid waste

Publication . Oliveira, Verónica; Ferreira, Célia; Labrincha, João; Rocha, Joana; Kirkelund, Gunvor M.

BACKGROUND: This work is focused on phosphorus (P) recovery from the anaerobically digested organic fraction of municipal solid waste (referred to as ‘digestate’) as a fertilizer. The main purpose was to propose and test modifications to the electrodialytic process that increase P extraction, improve the quality of the fertilizer by removing contaminants, and reduce hydraulic retention time to allow for smaller system footprints. Strategies tested were: (i) lowering the pH of the digestate suspension to <4.5 using the electrochemical reactions and enhance P solubilization from the waste; (ii) changing the configuration of the electrodialytic cell from three to two chambers; and (iii) stirring the sample to shorten the duration of the extraction. RESULTS: Results show that the acidification of digestate by the electrochemical reactions was effective to enhance P extraction yield. Three-chamber electrodialytic experiments enabled the removal of heavy metals from the digestate, producing P-rich solutions with low metal concentrations. This resulted in the production of high-quality fertilizer which can be used for agricultural applications. The modification of the electrodialytic cell set-up from three- to two-chamber neither resulted in an increase of the P extraction yields, nor contributed to the removal of metals from the liquid phase of digestate. Reduction of the hydraulic retention time of electrodialytic extraction of P from 16 days to 9 days was attained by the use of stirring and by electrodialytic acidification. CONCLUSIONS: An increase of P recovery was accomplished by modification of electrodialytic extraction, resulting in 90% of Pbeing successfully extracted from the digestate and transformed into struvite.

2019Journal article

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The setting up of a pilot scale pay-as-you-throw waste tariff in Aveiro, Portugal

Publication . Ferreira, Célia; Neves, Anita; Fernández Braña, Álvaro

Over the last 20 years solid waste has become an important issue in municipalities and counties and a problem for societies aspiring to economic, social and environmental sustainability. The new, recently approved Waste Framework Directive and Landfilling Directive have even higher targets related to municipal waste recycling and landfilling, and critical changes need to take place if European targets are to be realistically achieved. Pay-as-you-throw (PAYT) waste tariffs are an instrument that can help achieving recycling targets. PAYT works by linking the payment of a waste-collection tariff with the amount of waste discarded so that the payment increases when more waste is produced: “the more one discards, the more one pays”. This work shows the early stages of PAYT implementation in pilot scale in a municipality in Portugal. The changes carried out in the collection infrastructures are described, as well as other complementary measures that were simultaneously implemented, namely reinforcement of the collection of dry recyclables and promotion of home composting.

2019Journal article

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