## 4 Conclusions  
The implementation of an AcoD strategy requires an in-depth analysis of numerous factors. It should take into account technological, economic and environmental issues. The obtained results indicated that introduction of the highest doses of PL and BSG (R8) to municipal SS led to increased methane production as well as enhanced VRS, which were related to improved composition. On the contrary, in the three-component series supplied with OP and BSG (R6), reduced biogas and methane production as compared to the control was observed. Therein, the highest accumulation of most HMs within AD was also achieved, indicating the importance of selecting an adequate co-substrate. This fact also confirmed the relationship between the content of HMs and process performance. It should be pointed out that the presented results are preliminary, and should be further studied in semi-flow reactors and, finally, on a technical scale. Only such investigations will determine the general and long-term influences of co-substrate applications on process performance. Additionally, subsequent studies should focus on the application of additive/s that would eliminate the influence of the AD inhibitors, in particular in multi-component co-digestion \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Heavy Metal** & **EU Regulation** & **US Regulation** & **This Study** \\ \hline Cd & 20–40 & 85 & 7.6–10.8 \\ Cu & 1000–1750 & 4300 & 1387–2059 \\ Ni & 300–400 & 420 & 61.7–81.4 \\ Pb & 750–1200 & 840 & 18.6–63.8 \\ Zn & 2500–4500 & 7500 & 3689–5530 \\ Hg & 16–25 & 57 & 19.6–37.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The allowable limits of HMs [mg/kg] in the EU and US [78; 79]. with OP. Another pathway that should be investigated is the monitoring of microbial consortia occurring in particular reactors. Conceptualization, A.S. and A.M.; methodology, A.S. and M.L. ; formal analysis, A.S. and A.M.; investigation, A.S. and M.L. ; resources, A.S.; writing--original draft preparation, A.S. and A.M.; writing--review and editing, A.S. and A.M.; visualization, A.S. and M.L. ; supervision, A.M.; project administration, A.S.; funding acquisition A.S., M.L. and A.M. All authors have read and agreed to the published version of the manuscript. The authors are thankful for the financial support provided by the Ministry of Science and Higher Education in Poland [grant numbers FD-20/IS-6/036, FD-20/IS-6/023, FD-20/IS-6/020]. Not applicable. The authors are thankful for the technical support and preparation of elemental analysis methodology provided by [PERSON]. The authors declare no conflict of interest. * [PERSON] et al. (2001) [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Advanced monitoring and control of anaerobic wastewater treatment plants: Software sensors and controllers for an anaerobic digester. _Water Sci. Technol._**2001**, _43_, 175-182. [CrossRef]
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