**5. Conclusions**

Waterworks sludge that contains iron works very effectively as an additive to reduce the hydrogen sulfide concentration in biogas production. This has been demonstrated in the 13 full-scale biogas plants surveyed in this project. According to the costs reported for the transport and handling of the sludge, there is potential to save up to 50% compared to a scenario in which these plants use virgin ferric chloride instead. In the manure-based biogas plants, one ton of waterworks sludge with 15% TS is sufficient to reduce the hydrogen sulfide concentration to below 100 ppm in 2–3000 Nm<sup>3</sup> biogas. In the digestion plants that participated in the study, the same amount of sludge was enough for more than twice as much gas—about 8000 Nm3. In both cases, the exact figure depends on the substrate composition and the level at which the hydrogen sulfide concentration is reduced. The content of heavy metals in the waterworks sludge is well below the concentrations used in Avfall Sverige's certification system, SPCR 120, for the reuse of biosolids from solid waste plants. The reuse of 56% of the sludge by replacing virgin ferric chloride with waterworks sludge saves 740 tons of carbon dioxide, corresponding to about 17% of all carbon dioxide emitted by Sydvatten in 2020.

**Author Contributions:** Conceptualization, T.P. and K.M.P.; methodology, T.P., investigation, T.P., K.M.P. and J.Å.; writing, T.P., K.M.P. and J.Å. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Lars Månsson and Irene Bohn are acknowledged for sampling and other technical support in the project.

**Conflicts of Interest:** The authors declare no conflict of interest.
