# Sewage Sludge Quality and Management for Circular Economy Opportunities in Lombardy
## 1 Introduction  
Sewage sludge is a typical by-product of our society. It can be originated from civil or industrial activities. Sewage sludge contains valuable organic matter and can be a source of nutrients to be recovered for soil fertilization and remediation [1]. It also contains pathogens, organic and inorganic pollutants and potentially toxic compounds, such as polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), dioxins, nonylphenols, and other trace pollutants, such as (phyto)pharmaceuticals, personal care products, and microplastics [2; 3; 4], currently so-called \"emerging contaminants\" [5]. Pollutants can be persistent in the environment, accumulating and biomagnifying [6], and may pose hazards to human health and the ecosystems, such as carcinogenicity, teratogenicity, reproductive toxicity and genotoxicity [3; 5]. Moreover, substances can form intermediate products or mixtures resulting in synergistic/additive/antagonistic effects [3]. Consequently, sewage sludge treatment activities focus on improving its quality for recovery opportunities, whilst minimizing the quantities to be treated and disposed of. In line with circular economy principles, the current plan calls for wastewater treatment plants (WWTP) to be turned into water resource recovery facilities [7]. With a view to waste management priorities, which prioritize material recovery, sludge reuse in agriculture potentially represents an optimum solution, due to its valuableproperties: organic carbon content, nitrogen, phosphorus, micronutrients (in particular metals such as nickel and copper) [8; 9; 10]. Nitrogen can be easily available from adequately treated biosolids, decreasing the need for chemical fertilizers [11]. Phosphorus has been brought to attention due to the progressive depletion of mines [12] and is included in the list of critical raw materials [13]. Phosphorous and nitrogen could be recovered from the sludge line of WWTPs by precipitation/crystallization; phosphorus is also extracted from sewage sludge and residual ash of incineration by wet chemical recovery technologies [14]. Recovery of micronutrients has become more relevant these days, with fluctuation in micronutrient deficiency levels in soils being a global phenomenon [15]. Treated sludge is indicated to improve soil properties, such as texture and water-holding capacity, favoring root growth and drought resistance [16; 17; 18]. Moreover, emerging technologies are aimed at recovering volatile fatty acids and polyhydroxyalkanoates from secondary wastewater sludge [19]. Energy recovery from sludge is becoming more and more interesting due to the need for substitute energy sources and for technology development [20], and a viable alternative pathway for sludge recovery. Biogas from anaerobic digestion can be a supply for heating and electricity generation, and it can be upgraded to biomethane for injection into natural gas networks or as transport fuel [21; 22]. Sewage sludge can be dried and converted into energy in cogeneration plants, thermal power plants, waste-to-energy plants, cement kilns, and mono-incineration plants by incineration, pyrolysis, or gasification [23; 24]. Besides thermal processes, which significantly reduce sludge, technologies for sludge minimization are employed in the water and sludge lines. Chemical processes include chemical oxidation (ozonation and wet oxidation); among biological processes, there are membrane biological reactors (meso- and thermophilic), and biological predation and Granular Sludge Systems [25]. Electro-osmosis is growing in interest among pressurized electro-dewatering treatments [25]. Currently, the law still in force at the European level is the Council Directive 86/278/ EEC [26]: it is aimed at regulating the reuse of sludge in agriculture, increasing its recovery while protecting the environment and health, by ensuring that heavy metals in soil and sludge do not exceed set limits.