**1. Introduction**

The extensive consumption of natural resources in recent years is reflected in the increasing consumption of packaging [1], which consequently causes the increase of the amount of plastic waste. According to the World Economic Forum, there will be more plastic than fish in the oceans by 2050 if human habits persist [2]. From a different perspective, the current global health crisis has highlighted the importance of using local and easily accessible raw materials that do not have to travel long distances to reach the consumer. These facts indicate that all future packaging materials should be renewable, environmentally friendly and, if possible, made from alternative raw materials (e.g., industrial waste) [3,4]. In general, waste is described as a product or by-product substance derived from industrial or agricultural processes or other activities with end-use purposes [5]. Waste can be a direct result of processing technology or the product of secondary treatment of waste streams, for example, wastewater, which produces several types of wastewater treatment plant sludge.

Depending on the treatment stage, primary, secondary, tertiary and chemical sludges are produced. Primary sludge is formed during primary treatment (screening, grate removal, flotation, precipitation and sedimentation) when heavy solids, grease and oils are

**Citation:** Lavriˇc, G.; Mileti´c, A.; Pili´c, B.; Medvešˇcek, D.; Nastran, S.; Vrabiˇc-Brodnjak, U. Development of Electrospun Films from Wastewater Treatment Plant Sludge. *Coatings* **2021**, *11*, 733. https://doi.org/ 10.3390/coatings11060733

Academic Editor: Sandra Dirè

Received: 21 May 2021 Accepted: 17 June 2021 Published: 18 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

separated from raw wastewater [6,7]. Usually, the primary sludge contains 2–9% solids. The remaining 90% (sometimes 99.5%) is water. Secondary sludge (waste activated sludge) is formed during biological treatment when the biodegradable organic content of wastewater is degraded by microorganisms. The total concentration of solids ranges from 0.8% to 3.3%, depending on the type of biological treatment process, with the remainder being water [7]. The organic fraction of waste activated sludge contains 50–55% carbon, 25–30% oxygen, 10–15% nitrogen, 6–10% hydrogen, 1–3% phosphorus and 0.5–1.5% sulfur [8]. Tertiary sludge is produced in advanced wastewater treatment stages when nutrients (nitrogen and phosphorus) need to be removed [6]. Usually, nutrient removal is carried out simultaneously with organic matter removal. Chemical sludge is produced by chemical processes carried out at the municipal wastewater treatment plant, such as chemically assisted primary sedimentation. In this process, an appropriate coagulant is added to the primary clarifier to reduce the organic load for further biological treatment. The qualitative and quantitative properties of the sludge depend on the reagen<sup>t</sup> used and the dosage. Typical reagents are hydrated lime, ferric chloride, aluminum sulfate and chitosan. Chemical sludge may contain non-negligible quantities of metals, due in part to the inorganic coagulants used. Chemical sludge can also be produced by coagulation–flocculation of the supernatant thickener and by backwashing after sedimentation/chemical–physical treatment [9].

Due to legislation restricting the use of landfills and land as sludge disposal methods, many researchers have attempted to reuse and recycle sludge in the most sustainable possible way [9–25]. Taking into account that the organic components of sludge present a rich source vein in terms of energy and nutrients waiting to be tapped, various studies show that, in the context of the circular economy, an important advantage of energy and fuels derived from waste is that they can replace other energy resources and limit the associated CO2 emissions. From a scientific point of view, the challenge of sewage sludge is one of the most studied in the last 30 years. The concepts of material and energy recovery, which are milestones of today's circular economy, have already been addressed by several working groups [19–22]. Most of the studies on the material utilization of wastewater sludge are based on the extraction of polyhydroxyalkanoates [9].

Over the years, it has been shown that wastes generated mainly from processes in the agricultural, food, textile and paper industries have a high end-use potential [10]. A good example of high-end potential from the agri-food sector is waste from coffee production, as described by Figueroa et al. and Malara et al. [11,12]. As a waste solution, an anthocyaninbased milk freshness indicator or sensor that could be used as an indicator of actual milk quality was shown by Weston et al. [13]. Researchers have also presented available technologies and materials for exploited cooking oil recycling, which has a significant impact on household waste solutions [14–16]. In recent years, substantial progress has been made in waste processing in the textile and paper industries [17–22].

In the past, studies on the utilization of wastewater treatment plant sludge (WTPS) for various purposes have been published [9,23–25]. Most of them are based on the extraction of polyhydroxyalkanoates (PHAs) from WTPS. PHAs are a class of bio-based and biodegradable polymers produced by bacterial fermentation of complex organic substrates [25]. They belong to the class of polyesters and can be thermoplasts or elastomers, and depending on the structure, they have similar properties to the conventional plastics, which makes PHAs suitable candidates for their substitution [25]. Despite the extensive literature review, no report of the direct use of WTPS for material preparation by electrospinning has been published so far. The materials produced by electrospinning technology are intended for a variety of applications in many fields such as medicine, pharmaceuticals, biotechnology, sustainable engineering materials and even packaging [26–28]. The advantage of the above technology lies in the production of nanofibers and their small diameter, large specific surface area and high porosity. Meanwhile, the rapid development of nanotechnology has enabled new applications for electrospun materials. In general, electrospinning technology is a new strategy for environmentally friendly nanomaterials

with special properties and a promising solution for wastewater sludge as well. Accordingly, at least a partial presence of various biopolymers in a local WTPS was expected. With the main objective of producing a new, local, circular, renewable and environmentally friendly packaging material, the sludge was chemically characterized at the beginning of the research. The main advantage of our process is that the electrospinning solution was prepared using water and ethylenediaminetetraacetic acid (EDTA). However, the aim of the research was to clean and treat the wastewater treatment plant sludge in such a way that it is suitable for the electrospinning process, thus creating a new material.

#### **2. Materials and Methods**
