2.4.1. PAUF Pilot Plant

For pilot-scale PAUF experiments, a tangential flow ultrafiltration plant according to Figure 1 was used (Andreas Junghans GmbH, Frankenberg/Sachsen, Germany). Three ceramic multichannel tubular membranes (CA-TiO2/Al2O3; Inopor, Veilsdorf, Germany) with a mean pore width of 10 nm were mounted in the membrane module.

**Figure 1.** Scheme and operating data of tangential flow ultrafiltration pilot scale plant used for Cu(II) enrichment and separation (Andreas Junghans GmbH, Frankenberg/Sachsen, Germany). PP, pressure pump; CF, cross-flow pump.

The pilot plant was operated with HB-PEI (Lupasol HF, MW ~25 kDa, 56 wt.% in H2O). For startup, the polymer solution was added into the feed tank and fed to the filtration circuit. Polymer solutions were pretreated with water in order to flush out the membrane-permeant size fraction until polymer retention > 99.8% was reached. In operation, the polymer was trapped in the filtration circuit (volume 11 L) and the feed reservoir was polymer-free. Polymer solutions were reused in various experiments after regeneration at pH 1.

The parts of the pilot plant that were in contact with polymer containing media were made of PVC. A gear pump was used to feed solutions from the feed reservoir into the filtration circuit and to release the buildup of filtration pressure. Cross-flow was generated by a centrifugal pump and regulated by automatic flow measurement. The pH value in the filtration circuit was continuously measured and controlled by automatic dosing of 6*M* hydrochloric acid and 10*M* sodium hydroxide solution directly into the filtration circuit. For tempering of the pilot plant, a heat exchanger was optionally mounted in the filtration circuit but removed during the experiments in order to avoid additional flow resistance. Heat generated by the cross-flow pump was sufficient to keep the temperature in the filtration circuit at 40 ◦C. The pilot plant provided a human–machine interface based on Siemens SIMATIC S7.

### 2.4.2. Fly Ash Extracts Used for Pilot-Scale PAUF Experiments

For investigations with the pilot plant, fly ash extract from the acid washing at the Ingolstadt MSWI plant was used. Suspended matter was allowed to settle and the fly ash extract was then filtered through a bag filter (50 μm). The extract taken from the MSWI plant contained 0.13 g L−<sup>1</sup> of Cu(II). In order to additionally investigate the PAUF treatment of fly ash extract with a higher Cu(II) concentration, a second extract with 0.70 g L−<sup>1</sup> of Cu(II) was achieved by adding Cu(NO3)2·3H2O to the extract taken from the MSWI plant. The evaluation was focused on the elements with the highest concentration in the fly ash extract (Pb, Zn, Ca, and Cl−, listed in Table 1).

**Table 1.** Concentration of elements Zn, Pb, Cu, Ca, and Cl− in municipal solid waste incineration (MSWI) fly ash extract used for retention experiments. \* Achieved by addition of CuNO3.

