*2.1. Materials*

Preparation of wastewater treatment plant sludge solution (WTPS): The WTPS was obtained from the main wastewater treatment plant of Ljubljana. The purification procedure was carried out according to a slightly modified method described by Fang and Jia in 1996 [29]. The purification was based on dissolution of WTPS in a 2% EDTA solution in water and centrifugation. Fifty grams of the absolute dry mass of WTPS was firstly cut down into pieces with a size of ~1 cm × 1 cm. The pieces were then dissolved in a 300 mL EDTA solution and stirred in a 500 mL beaker (500 rpm, 24 h). Centrifugation (16,800× *g*, 30 min) and separation of the usable supernatant were the final steps of the process. A series of solutions with different viscosities (20,000, 5000, and 2000 mPa) were prepared by water evaporation from the initial solution of WTPS by heating in a water bath (85 ◦C, ~240 min) while stirring at 250 rpm in order to optimize the process of electrospinning, since it was impossible to obtain continuous nanofibers from the initial solution.
