# Sewage Sludge Quality and Management for Circular Economy Opportunities in Lombardy
## 3 Results
### Sludge Quality  
A total of 46 websites were identified and analyzed, and 66 tender processes satisfied the selection criteria. A total of 258 certificates of analysis conducted from 2014 to 2020 on sludge from 102 different WWTPs, across all 12 Lombardy provinces, were identified. The total number of WWTPs in Lombardy in 2019 was 1379, of which 968 (70%) serving \(<\) 2000 equivalent inhabitants (EI) and 411 (30%) serving \(>\) 2000 EI [47]. Collected data were representative of 7.4% of the total number of WWTPs in Lombardy, and of 23% of plants serving \(>\)2000 EI. Most of considered WWTPs (41) have a capacity of 10,000 \(<\) EI \(<\) 50,000; of the remaining, 24 have a capacity of 2000 \(<\) EI \(<\) 10,000, 13 have a capacity of 50,000 \(<\) EI \(<\) 100,000, 12 have a capacity \(>\) 100,000 EI, 3 have a capacity \(<\) 2000 EI; for 9 plants there was no information. Figure 1: Estimated total production of sewage sludge (EWC 190805) in Lombardy, and the percentage variation of quantities compared to the previous year and 2002. Figure 3: Mass balance of the total production and management of sludge (EWC 190805) in Lombardy for the year 2018, by origin and destination. Figure 2: Mass balance of the total production and management of sludge (EWC 190805) in Lombardy for the year 2017, by origin and destination. Data were from analyses carried out in 2014 (6%), 2015 (8%), 2016 (23%), 2017 (24%), 2018 (15%) and 2019 (21%), and 2020 (3%). An influence on the degree of stabilization of the waste by the type of stabilization used was not observed (\(n=78\)). Plants with aerobic treatment (\(n=50\)) produced sludge with an average degree of stabilization of 69% and a median of 71%; plants with anaerobic treatment (\(n=28\)) produced sludge with an average degree of stabilization of 70% and a median of 72%. Comparing mechanical dewatering processes (\(n=152\)), the percentage of dry matter (DM) achieved by means of filter-presses is the highest (25%, \(n=16\)), followed by centrifuges (21%, \(n=95\)) and belt presses (17%, \(n=35\)). Detailed results are reported in Table S1 (Supplementary Materials). Table 1 shows the results of elaborations on normed parameters of sludge produced by WWTPs. The comparison with limits set by the most recent regulations for the recovery in agriculture [36,38] helped to delineate sludge quality in relation to the possibilities for direct or indirect recovery in agriculture. The D.D. 6665/2019 defines two sets of limits for some of the parameters for \"suitable\" and for \"high-quality\" sludge to be reused in agriculture. Table S2 in the Supplementary Material presents the results of elaborations on non-normed parameters reported in the analyses. With reference to limits for sludge \"suitable\" for reuse in agriculture, the average and median values of all regulated heavy metals, trace organic pollutants, agronomic parameters (except the germination index) and of the degree of stabilization complied with the normative limits for recovery in agriculture in the examined analyses and period (Table 1, Figure 4). When considering single values for each parameter, it can be observed that: (a) all analyses satisfied normative limits for lead, cadmium, mercury and chromium VI, total N and total P, PAH, nonylphenols, Bis(2-ethylhexyl)phthalate (DEHP), heavy hydrocarbons and polychlorinated dibenzo-p-dioxins, furans and biphenyls (PCDD/F) + Dioxin-like PCB; (b) non-compliance for one or more heavy metal (total chromium, copper, zinc, nickel, arsenic, beryllium, or selenium) was found in 37 analyses, with the concentrations of total chromium and arsenic being the most frequently over the limits (Figure 4); (c) among agronomic parameters, the percentage of organic carbon (C\({}_{\rm org}\)) respected the limit value in the 96.3% of analyses, with 7 non-compliant values; the limit for the germination index conformed with in 42.7% of analyses; the pH was non-compliant in 2 analyses; (d) only 29.2% of analyses for fecal coliforms and 83% for _Salmonella_ sp.