## 6 Environmental Impacts of Anaerobic Digestion of FW with Sewage Sludge  
As waste management plays an important role in environmental engineering, studying its environmental impact appears as an essential part of its feasibility and applicability [264]. Through the years, many tools have been used. However, the most suitable and powerful tool for determining environmental, and even economic and social impact of waste management is Life Cycle Assessment (LCA) [265]. LCA enables comparing and quantifying the impacts of different waste solutions, acting as a support tool for decision making in waste management. If LCA is applied to anaerobic digestion of food waste (and sluges for Waste Water Treatment Plants), the following conclusions can be reached:  
Mostly, all of the environmental impacts of AD are due to climate change. A large part are caused by methane leaks and other accidental emissions during the AD process. As methane is the major component of biogas and it is considered a greenhouse effect gas, these emissions should be taken into account. In order to reduce leaks and their impact, flaring systems and emergency torches are usually incorporated in biogas plants [33; 266; 267]. Other impacts are usually related to different gas leaks that cause acidification and eutrophication [268; 269]. Mainly, because high ammonia or hydrogen sulphide biogas contents, which are important parameters used as control factors in biogas production with food waste as substrate, and digestate uses. Acidifications come from biogas combustion, resulting in nitrogen oxide and sulphur dioxide emissions [13; 270]. Both are considered secondary particulate precursors, resulting in particulate matter formation. On the other hand, eutrophication can be a result of digestate management [271; 272], as it contains a high level of N, and it is usually considered as fertilizer.  
However, when comparing AD with other waste management solutions, it seems to be, if not the best, one of the most preferable environmental solutions. Studies have been carried out [4] when comparing different scenarios and their impacts, showing CoAD treatment as the most viable solution, in spite of its undesirable impacts, as it generates the highest amount of renewable energy in form of biogas turning this disposal method into an energy recovery system. Biogas can be used in different forms: for electricity generation, heat production [14; 273; 274], and even for vehicle use through biogas upgrading [12; 275]. Also, biogas is used in cogeneration, producing heat and electricity simultaneously. Biogas can supply energy in form of electricity to the principle industries, as well as to those close or nearby, excess is often delivered to the public grid. Heat can be used internally to maintain the anaerobic reactor temperature and to provide heat and steam to industries. It can also be connected to district heating systems. Moreover it will be upgraded and injected to the gas grid for household use, or used as fuel, meeting traditional fuel standards through purification processes. Environmental emissions from biogas utilization should also be considered, although methane is the cleaner hydrocarbon due to its lower carbon content.