# What Advanced Treatments Can Be Used to Minimize the Production of Sewage Sludge in WWTPs?
## 1 Introduction  
Sewage sludge (SS) is a mixture of organic and inorganic matter and its composition strongly depends on the treatment and on the wastewater origin [1]. It can be composed of primary sludge, derived from primary sedimentation and formed by suspended sedimentary materials, secondary sludge, made up of excess biomass and, tertiary sludge, obtained in the advanced wastewater treatment stages, when nutrient (nitrogen and phosphorus) removal is required [2].  
In recent years, the population growth and consequent urbanization have increased the number of wastewater treatment plants (WWTPs) and the production of SS [3, 4]. Moreover, the application of more stringent requirements for WWTPs effluents quality increases the production, and worsens the quality, of the SS. For instance, in the EU, the implementation of the European Commission Directive 98/15/EC (amending Council Directive 91/271/EEC) led to a strong rise in sludge generation, up to 50% [5], with an estimate of more than 13 million Mg of dry matter in 2020. According to these data, developing urgent and effective approaches to reduce and recover SS becomes a priority [6, 7].  
Particularly, in order to respect the European Directive 2018/851 on waste, the first aspect to take into consideration is the minimization of the waste produced [8] and therefore, sludge [9]. Furthermore, the economic aspect has become important in recent years. Researchers show that the cost of sludge management is approximately 50% of the total running costs of the WWTP with the conventional activate sludge (CAS) process [10, 11]. Moreover, also in the case of the WWTPs, asalready suggested in recent years for drinking water treatment plants [12; 13], some procedures for evaluating the performance of the plants and for defining optimal solutions for management in order to optimize operation and minimize the production of SS should be studied.  
In 2015, 9.7 million MgDM (DM - dry matter) of SS were produced in Europe. More than 60% of the total SS was produced in Germany (around 19%) and the western countries (UK, Italy, Spain and France) (Figure 1). By contrast, eastern countries (such as Poland, Romania, Estonia, Latvia and Lithuania) represent around 12% of SS produced in Europe [14].  
In the last years, in the EU, SS recovery in agricultural use and composting quickly increased from 3 million MgDM in 1995 (42% of total production) to 4.2 million MgDM in 2005 (54% of total production); then, this value increased further (5.2 million MgDM in 2010 to 59% of total production) [14]. The EU issued the Directive 86/278/CEE (recently amended by the EU Decision 2018/853) with the aim of avoiding the application on land of SS that was dangerous for human health. Therefore, every member state has issued a national regulation on this topic [15]. For instance, currently, a high concentration of metals in sludge makes recovery using agriculture impossible [16].  
Considering: (i) the framework of the ever-stricter European regulations regarding the recovery of SS in agriculture, (ii) the EU Directive 2018/851 [8] in which minimization is listed as a priority, (iii) the high costs related to SS disposal, it is clear that techniques for minimizing sludge production have become strictly necessary.  
In this review, only advanced technologies and treatments are reported. The process description, the advantages, the drawbacks and the results of the literature are presented for each type of treatment. This review provides significant information on the technologies still under development.  
Figure 1: Sewage sludge (SS) production in 2015 in Europe [14]. DM: Dry matter. (a): Data referring to 2014. (b): Data referring to 2013. (c): Data referring to 2012. (d): Data referring to 2010. Data from Turkey, Montenegro, Macedonia, Kosovo, Iceland and Norway are not available.