## 2 Microplastics and Microfibres in WWTPs
### Microfibres Retained in Sludge
An overview of the incidence of MPs, and specifically microfibres, in sewage sludge worldwide is shown in Table 2. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{2}{c}{**Dry Sludge Generated**} & \multicolumn{3}{c}{**Dry Sludge**} & \multicolumn{3}{c}{**Number of MPFs per Day (kg)**} \\ \cline{3-6}  & **Per Day (kg)** & **(MPs/g)** & **\% MPFs** & **(MPFs/g)** & **Day Retained in Sludge** & **References** \\ \hline \multirow{4}{*}{Australia} & W1: \(9\times 10^{4}\) & W1: 56.5 & W1: 86.5 & W1: 48.9 & W1: 44 \(\times 10^{9}\) & \\  & W2: \(7.1\times 10^{4}\) & W2: 51.2 & W2: 98 & W2: 50.2 & W2: \(3.6\times 10^{9}\) & [63] \\  & W3: \(1\times 10^{5}\) & W3: 48.5 & W3: 82.7 & W3: 40.1 & W3: \(4.0\times 10^{9}\) & \\ \cline{2-6}  & \(1.5\times 10^{6}\)b & 7.91 b & 30.5 & 2.41 b & 3.7 \(\times 10^{6}\) & [64] \\ \hline Canada & \(1.4\times 10^{6}\) & 19.3 & 68.9 & 13.3 & \(1.9\times 10^{10}\) & [65] \\ \hline \multirow{4}{*}{Australia} & - & 240.3 & 17.5 & 42.05 & - & [57] \\  & Part A: 9.2 \(\times\) 10\({}^{3}\) & Part A: 0.72 & Part A: 50 & Part A: 0.36 & \multirow{4}{*}{3.3 \(\times\) 10\({}^{6}\)} & [67] \\  & Part B: 28.8 & Part B: 4 & Part B: 0 & Part B: 0 & & \\ \cline{2-6}  & \(1.3\times 10^{6}\) & 46.3 & 35 & 16.21 & \(2.1\times 10^{10}\) & [71] \\ \cline{2-6}  & - & 22.4 & 21 & 4.70 & - & [73] \\ \cline{2-6}  & \(1.1\times 10^{5}\) & 2.92 & 65 & 1.90 & \(2.1\times 10^{8}\) & [74] \\ \hline \multirow{4}{*}{China} & & W1: 13.4 & W1: 60 & W1: 8.0 & W1: 8.0 & \multirow{4}{*}{-} & [75] \\  & W2: 63.4 & W2: 75 & W2: 47.6 & - & [75] \\ \cline{2-6}  & & W2: 12.1 & W2: 70 & W2: 8.5 & \multirow{4}{*}{-} & [78] \\  & W3: 5.02 & W3: 40 & W3: 2.01 & & \\  & W4: 9.36 & W4: 77 & W4: 7.21 & & \\ \cline{2-6}  & & W1\&W2: 6908.3 & W1\&W2: 17 & W1\&W2: 1174.4 & \\ \cline{2-6}  & - & W3: 2190.4 & W3: 83 & W3: 1818.0 & - & [79] \\  & W4: 234.7 & W4: 35 & W4: 82.2 & & \\ \cline{2-6}  & \(2.0\times 10^{8}\) & 22.7 & 63 & 14.3 & \(2.9\times 10^{12}\) & [142] \\ \cline{2-6}  & \(1\times 10^{6}\) & 4.04 & 18.7 & 0.76 & \(7.6\times 10^{8}\) & [151] \\ \cline{2-6}  & - & 220 & 7.3 & 16.06 & - & [152] \\ \cline{2-6}  & - & 5.55–13.46 & 75.8–88.8 & 4.21–11.95 & - & [153] \\ \hline \multirow{4}{*}{-} & W1: 960.9 & W1: 31 & W1: 297.9 & \multirow{4}{*}{-} & [154] \\  & W2: 550.8 & W2: 40 & W2: 220.3 & - & [154] \\ \cline{2-6}  & \(6.5\times 10^{4}\) & W1: 5.16 & W1: 15 & W1: 0.77 & \multirow{4}{*}{[154]} \\  & W2: 2.19 & W2: 85 & W2: 1.86 & \multirow{4}{*}{1.8 \(\times\) 10\({}^{8}\)} & [155] \\  & W3: 0.25 & W3: 35 & W3: 0.09 & & \\ \hline \multirow{4}{*}{Finland} & \(9.6\times 10^{4}\) & 170.9 & 94.2 & 161 & \(1.6\times 10^{7}\) & [51] \\ \cline{2-6}  & \(6\times 10^{7}\) & 186.7 & - & - & - & [84] \\ \cline{2-6}  & France & \(6.4\times 10^{4}\) & 16.13 & 77.2 & 12.46 & \(8.0\times 10^{8}\) & [85] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Overview of the incidence of MPs, and specifically microfibres, in sewage sludge worldwide. “W” followed by a number refers to different WWTP analysed in the cited reference. As can be seen in Table 2, MP concentration in dry sludge ranges between 0.51 and 6908 MPs/g, with and average value of 320 MPs/g. In general, approximately 53% of these MPs correspond to microfibres, so this is the form most detected in sludge, although fragments, films and pellets can also be found [39]. Additionally, the number of microfibres reported in sewage sludge notably varies (between not detected and 1808 MPFs/g), with the mean being a value of 101 MPFs/g. Considering all of the case studies reported, it is estimated that between 3.3 \(\times\) 10\({}^{6}\) and 6.7 \(\times\) 10\({}^{13}\) MPFs are found in the total daily sludge generated by a WWTP. The search for new technologies for the removal of pollutants, and specifically MPs, from sludge is a key aspect in the foreseeable future to achieve the requirements of regulations and to improve the valorisation of this waste.