# What Advanced Treatments Can Be Used to Minimize the Production of Sewage Sludge in WWTPs?
## 6 Conclusions  
This paper presents a review of the various treatments applicable for the minimization of SS. Only advanced technologies and treatments are reported. This work is based on about 130 papers, reviews, books and conference proceedings. A description of the process, the advantages, the drawbacks and the results of the literature are presented for each type of technology. Both the interventions on the sludge line and those on the water line are shown. In order to minimize the SS production, the results of the bibliometric analysis show the increased interest in the combination of different types of treatments and technologies and the reduction of the study of treatments exclusively of a chemical nature. Among the biological treatments, while MMBR and TMBR allow us to obtain encouraging results, the BP must be studied even before possible intensive application at a real scale. In order to minimize the SS, thermal treatments have gained interest in recent years. However, while combustion and co-combustion of SS are already applied at the real scale, pyrolysis and gasification are currently under study and development. Finally, electro-osmosis is arousing a growing interest. The results are encouraging, but the maturity of the technology is not yet developed, and other studies are needed before a real-scale application. Assuming the application of proposed processes in the sludge line in WWTPs with a capacity higher than 100,000 PE, this would significantly reduce the costs related to the different disposal options currently adopted in Europe.  
Figure 5: Comparison between the total costs of the different SS disposal options in Europe in 2015, before and after the hypothetical application of minimization techniques in the plants with more than 100,000 PE. A range of SS disposal costs, between a minimum and maximum value, was determined considering the high variability within the different EU countries.  
**Author Contributions:** [PERSON] contributed to the drafting of the sections related to the different available advanced processes. [PERSON] contributed to the drafting of the summary and discussion (Sections 4 and 5). [PERSON] and [PERSON] supervised the study and the paper drafting. All authors have read and approved the final manuscript.  
**Funding:** This research received no external funding.  
**Acknowledgments:** The authors are grateful to Pavia Acque S.c.a.r.l., ASMia s.r.l. (ASM Mortara S.p.a. Group) and ASM Pavia S.p.a. for providing technical assistance.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Nomenclature**  
\begin{tabular}{l l} AeGSS & Aerobic granular sludge systems \\ AnGSS & Anaerobic granular sludge systems \\ AOP & Advanced oxidation process \\ BP & Biological predation \\ CAS & Conventional activate sludge \\ COD & Chemical oxygen demand \\ DM & Dry matter \\ DOC & Dissolved oxygen carbon \\ GSS & Granular sludge systems \\ HRT & Hydraulic retention time \\ MBR & Membrane biological reactor \\ MLSS & Mixed liquor suspended solids \\ MMBR & Mesophilic membrane bioreactor \\ SBBGR & Sequencing batch biofilter granular reactor \\ SBR & Sequencing batch reactor \\ SRR & Sludge reduction ratio \\ SRT & Sludge retention time \\ SS & Sewage sludge \\ SSR & Sludge solubilization rate \\ TCH & Thermochemical hydrolysis \\ TKN & Total kjeldahl nitrogen \\ TMBR & Thermophilic membrane bioreactor \\ TS & Total solids \\ TSS & Total suspended solids \\ VFAs & Volatile fatty acids \\ VS & Volatile solids \\ VSS & Volatile suspended solids \\ WO & Wet oxidation \\ WW & Wastewater \\ WWTP & Wastewater treatment plant \\ \end{tabular}