## 1 Introduction  
Wastewater treatment plants (WWTPs) represent an indirect source of MP emissions into the environment, due to the amounts of MPs still contained in the treated water [1, 2]. Because MPs have a multifactorial origin, municipal wastewater is significantly polluted by them. In particular, MP concentrations between 0.28 and 3.14\(\cdot\)10\({}^{4}\) MPs/L have been reported in the influent of municipal WWTPs [3, 4, 5]. The treatment processes carried out in these facilities can remove more than 90% of the MPs initially contained in the wastewater; however, the majority of these MPs accumulate in the sewage sludge [1, 6]. Very different concentrations of MPs have been measured in sewage sludge, ranging from 400 to 170,000 MPs/kg [7, 8, 9]. Taking action at these facilities to improve the MP removal efficiency is key to preventing their release into the aquatic environment [10]. However, at the same time, the sludge containing the removed MPs is applied to agricultural soils, introducing the MPs into the environment. In WWTPs, sewage sludge is mainly produced during the primary and secondary settling processes. Additionally, coagulation and flocculation are an essential part of drinking water treatment and can also be used to improve the performance of some tertiary wastewater treatments [11, 12, 13, 14]. Coagulant and flocculant agents are used to destabilize present colloids and form larger \"particles\" capable of settling [15; 16; 17]. Physical-chemical processes, which occur simultaneously when these chemicals are added, may enhance MP retention in sewage sludge [15; 16; 18]. The efficiency of MP entrapment in the flocs is highly influenced by the type and surface charge of the MPs, coagulant/flocculant dosages, mixing speed, pH, characteristics of the flocs, the presence of extracellular polymeric substances (EPS), and even the presence of other pollutants [19; 20; 21]. It has been reported that the use of chemicals achieves MP removal rates between 44 and 98% from wastewater [22; 23]. For example, a coagulation/flocculation step after secondary treatment removed up to 93% of the MPs. Using polyaluminum chloride (PAC) as a coagulant in the range of 36-72 mg/L, wastewater samples from the primary clarifier, secondary clarifier, and final effluent outflow were examined; the MP removal rate ranged from 23 to 93%. According to another investigation, an aluminum-based coagulant concentration of 405 mg/L resulted in MP removal efficiencies within the range of 47.1% to 81.6% [23; 24]. At a laboratory scale, a study investigated the removal efficiency of polyethylene MPs (PE-MPs) from wastewaters, using FeCl\({}_{3}\cdot\)6H\({}_{2}\)O and AlCl\({}_{3}\) with cationic and anionic polyacrylamide (PAM) at different concentrations (coagulation with jar tests; after maintaining the rapid mixing speed at 300 r/min for 1 min, it was lowered to 100 r/min for 14 min and then allowed to settle for 30 min). In terms of PE-MP removal efficiency, Al-based salts outperformed Fe-based salts in the experiment. PE-MP retention increased to over 91% with the addition of anionic and cationic PAM and high AlCl\({}_{3}\) concentrations [23; 25]. With the use of cationic PAM, the removal efficiency of PE particles increased, especially for small-sized PE-MP particles. However, when anionic PAM was used, the removal rate of PE-MPs was significantly greater (61%) [23; 25]. The reason behind this is the development of thick Al-based flocs, which improved PE-MP adsorption and sedimentation throughout the coagulation phase. Other studies also reported better performances for the anionic PAM compared to the cationic one [26]. Additionally, in laboratory studies employing a batch approach, it was observed that the combination of MgCl\({}_{2}\) and anionic PAM produced larger and more stable flocs, resulting in the best MP retention efficiency (84.9%) [12; 27]. All the above studies were carried out with synthetic waters or after different treatments in the WWTPs, including a new coagulation-flocculation step.