*2.1.3 Future prospects of AD application*

 According to the United Nations Sustainability Development Goal of 2030, the use of renewable energy is expected to reach 100% by the year 2050. The seventh goal focuses on the production of affordable and clean energy globally which is environmentally friendly [21, 22]. Renewable energy has gained attention to cater for the ever-increasing use and over-reliance of non-renewable forms of energy. This arises because of the emission of greenhouse gases compelling researchers in the past decades to search for an alternative means of sustainable energy production [17, 23–25]. The reserve for energy has become necessary for global concern in maintaining a sustainable way in lieu of the resources available especially at WWTPs. Aside the protection of the environment, wastewater treatment plants (WWTPs) also serve as a source of generating renewable forms of energy such as biogas. In a WWTP, the dumping of sewage sludge produced as a by-product is a problem of growing significance representing up to 50% of the entire operating costs of all WWTPs [8, 12, 14]. Also, wastewaters with a high content of nitrogen can be treated with the nitrification and denitrification technique form of AD generally known as Anammox [26]. Constructive government policies have shown Germany as being the dominant global biogas energy generation country globally for the future [23, 25]. Latest reports predict that biogas production could increase from 18,244 Gigawatt hours (GWh) in 2012 to 28,265 GWh in 2025, indicating a compound annual growth rate (CAGR) of 3.4%. In 2011, Germany contributed the largest share of the world's cumulative installed capacity as the country accounted for approximately a quarter of the global biogas biogas [23, 25, 27].
