## 2 Materials and Methods
### Semi-Continuous Experiment  
The substrate and inoculum used were the same as those used in the biodegradability tests. The substrate was kept in a cold room at a controlled temperature between 3 \({}^{\circ}\)C and 7 \({}^{\circ}\)C to avoid freezing and degradation. Fresh sludge was periodically brought in from the WWTP to avoid using highly evolved sludge. The inoculum-substrate mixture is detailed in Table S1 and was prepared for an inoculum-substrate ratio of 1.4 on a VS basis. To reduce the acidity and facilitate the start-up of the digesters, sodium bicarbonate (NaHCO\({}_{3}\)) was added at a rate of 2 g/L and the system was allowed to stabilise for 4 days. Experiments were conducted in three cylindrical reactors, each with a volume of 3 L (Figure 1), being arranged as follows:  
* Digester R1: Eight hundred porous PLA/Carbon Black (NB) spherical pellets (6 mm in diameter) were used as fillers. It was not possible to use the PLA/Graphene pellets because the material that they were printed from has been discontinued. Prior to their use, a pre-treatment with acetone followed the protocol developed in 2022 by [PERSON] et al. [31]. The digester was agitated at a constant speed of 60 rpm with stirring paddles.
* Digester R2: An external 0.5 L bioelectrochemical cell connected by rubber tubing to the main digester was added so that the digestate was continuously recirculated through it by means of a peristaltic pump (Dosiper, model Timlab-1R, Villablino, Leon, Spain). The cell contained two 8 \(\times\) 4 cm carbon felt electrodes that served as (bio)--anode and (bio)--cathode respectively (SGL Group, Wiesvaden, Germany). The potential difference between the electrodes was set at 1 V using a potentiostat (Biologic VSP potentiostat, Seyssinet-Pariset, France). No paddle stirrer was used andagitation was provided by the recirculation of the digestate, which was introduced and extracted from the digester at opposite sides.
* Digester R3: This digester was used as a control, without any modification. Agitation was carried out in the same way as in R1.  
The temperature was kept constant throughout the experiment using a bath with a temperature ranging between 34 \({}^{\circ}\)C and 36 \({}^{\circ}\)C. The reactors were fed in semicontinuous mode, and the volume of sludge fed to the reactors was calculated to maintain identical hydraulic retention time (HRT) in all reactors. To do that, the same volume is removed from each of the digesters before feeding. Both withdrawal and feeding were carried out through the bottom port of the digesters, discarding the first 100 mL of extraction each time the process was carried out (Figure S2). Experiments were carried out at three different HRTs of 20, 15 and 10 days. The duration of each trial is approximately three and a half times the corresponding HRT. It should be noted that there were problems with the operation of digester R2 due to complications with the pumping and recirculation system. This led to digestate leakage and especially to the accumulation of solids at the bottom of the reactor at certain times during the experiments. These problems occurred mainly at the beginning and end of the experiment.