Wastewater (WW) acts as a vector by transferring a large part of the primary and secondary MPs from the source (e.g., domestic environment, industrial environment, roads, etc.) to the destination (surface water bodies, soils) ^{[11][16][17][18]}[14,19,20,21]. Therefore, the wastewater treatment plants (WWTPs) represent the last barrier before these substances are released into the surrounding environment ^[2][3].

In past years, most of the attention was placed on the final effluent and, therefore, the studies focused primarily on the effectiveness of the treatments conventionally present in the water line on the MPs ^[19][20][21][22,23,24]. The results showed that a conventional WWTP is able to remove up to 90% of MPs (depending on the characteristics of the influencing WW and the types of processes adopted) ^[2][3].

On the contrary, more than 90% of MPs found in WW are accumulated in sewage sludge (SeS), which in turn is used for land applications: the annual amount of MPs entering the soil in this way is greater than that enters the oceans ^[22][23][25,26]. Liu et al. ^[24][28] also highlighted the high concentration of MPs in SeS (up to 2.40 × 10⁵ particles kg⁻¹) due to an accumulation phenomenon. For this reason, it is very important to isolate the MPs, to detect their chemical nature, their physico-chemical properties, the amounts of the MPs particles, and to investigate their effects in the SeS itself and soil amended with SeS.

2. Effect on Sludge Properties

The effect of the MPs’ presence on the SeS characteristics and behavior are still under investigation and vary on the basis of the MPs composition and particle size. Li et al. ^[25][84] studied the adsorption efficiency of different metals by MPs in SeS, reporting the following affinity scale: Pb > Cd > Zn > Cu > Co > Ni. In particular, considering Cd, one of the most toxic metals due to its solubility, mobility, and biological accumulation and its ability to disrupt proteins in the cells, they noted an important increase (one order of magnitude) of adsorption by MPs after the wastewater treatment process. Three different families of MPs were found in the investigated sludge, i.e., polyamide, rubbery MPs (polyethylene and polypropylene) and glassy MPs (polyvinyl chloride and polystyrene), having decreasing adsorption efficiency towards Cd. The highest efficiency was attributed to the higher specific surface area (average value of about 5 m² g⁻¹) and to the wrinkled and aggregated structures present on the MPs surfaces in the SeS with respect to the virgin particles. Moreover, micro-Fourier transform infrared (FTIR)TIR measurements revealed higher intensity for the bands due to vibration of the O-containing groups (namely, C-O and O-H) on the sludge based MPs, probably due to the oxidative degradation of the MPs ^[26][85] and the subsequent attachment of organic matter on them ^[27][86]. This finding is in agreement with Turner et al. ^[28][87] and seems to point out that the attachment of organic matters on the MPs during the weathering strongly affects the metals adsorption. The 2D IR spectra correlation maps showed N-H bonds of the MPs as another preferential site for the metals adsorption, in agreement with the polymers efficiency order experimentally determined. The Cd adsorption was found influenced also by pH, with the highest efficiency depending from the chemical nature of the MPs but detectable in the range 6.0–7.7, hence close to the common pH values for SeS. In another study, Li et al. ^[29][53] analyzed the changes in the physicochemical property of three MPs, namely, polyamide (PA), polyethylene (PE), and polystyrene (PS) by passing through the wastewater pipeline, grit, and biological aeration tanks, confirming that during all these treatments the surface area and the content of organic group containing O atoms increase, while the glass transition measured by calorimetry decreases; this confirms the oxidation and rupture of the polymeric chains during the treatments. Interestingly, the Cd adsorption capacity of the MPs increases with respect to the virgin materials after both the sulfidation in the pipeline and the biological treatment in aeration tank, but results to decrease after mechanical abrasion in the grit tank, in correspondence to the decrease of the carbonyl index. Huang et al. ^[30][56] investigated the adsorption of Cu, Mn, Pb, and Zn by pristine and artificially aged low-density polyethylene (LDPE), showing that the amount of adsorbed metals increases with the aging time, together with the surface area of the plastic material, reaching values higher than 650 μg m⁻² after 10 h aging. Recently, several authors studied also the effects of MPs on methane and hydrogen production through anaerobic digestion of SeS, showing a strong dependence of the chemical nature of the plastic materials and their particle size and concentration. In general, low concentrations of MPs seem not to affect or slightly increase the methane production, while a highest number of MPs particles leads to the opposite effect ^[31][32][88,89]. For instance, Zhang et al. ^[33][90] highlighted that methane production in anaerobic system was not affected by polystyrene MPs in the case of low concentration (0.2 g L⁻¹), while it can be reduced almost by 20% in presence of the same MPs in higher dosage (0.25 g L⁻¹). Hydrogen production was found to decrease during alkaline anaerobic fermentation of activated waste in presence of PET microplastic, which was found to inhibit hydrolysis, acidogenesis and acetogenesis ^[34][91]. Feng et al. ^[35][92] found that Pd-doped polystyrene nanoplastics (2.36 × 10¹⁰ particles mL⁻¹) were able to reduce the methane production up to 14.29%. Further, aerobic digestion seems to be affected by MPs presence in sewage sludge (SeS) ^[36][93]. A very detailed and exhaustive summary of the recent literature and of the chemical and biological mechanisms evolving during the anaerobic and aerobic processes in presence of MPs can be found in ^[8][37][11,94]. Further, some results regarding the positive or negative effects induced by MPs and/or by the metals adsorbed on MPs as a function of their chemical nature on the evolution of the different reactions appear to be controversial, and more focused studies with common guidelines are needed to clarify these aspects, exploiting the potential ecotoxicity of the plastic wastes.

3. Discussion and Conclusions

To date, there is not a standard method for the extraction and analysis of MPs from SeS, causing difficulties in the direct comparison of the results obtained in the different studies regarding the quantity and quality of MPs in the SeS. Moreover, the effect of both the surface characteristics and the MPs size on the chemical activity and the fate of the particles needs further research work. The quantity of MPs in sludge seems to be mainly a function of two aspects: (i) the configuration of water line treatments in WWTPs and (ii) MPs presence in the influential WW. To date, no clear direct influence of the type of treatments present in the sludge line and MPs found in sludge line can be highlighted. From the analyzed studies, the presence of MPs can affect the properties of the SeS by altering the adsorption capacity of metals and modifying the digestibility of the sludge itself. Further, some results regarding the positive or negative effects induced by MPs and/or by the metals adsorbed on MPs appear to be controversial, and more focused studied with common guidelines are needed to clarify these aspects.

Abbreviations

FTIR	micro-Fourier transform infrared;
LDPE	low-density polyethylene;
MPs	microplastics;
PET	polyethylene terephthalate;
SeS	sewage sludge;
WW	wastewater;
WWTPs	wastewater treatment plants.