## 1 Introduction  
Sugar beet leaves (SBL) are a by-product of sugar beet harvesting. They make up an average of 27% of the fresh sugar beet plant [1]. Every year, about 120 million tons of SBL is produced in Europe [2], with an estimated 18 million tons produced in Germany. The nutritional content of SBL has been reported to be low, making it an unlikely source of animal nutrition [2]. Despite the huge availability of this biomass, it has largely remained underutilised without previous utilisation as fodder or biogas substrate. To be used for year-round biogas production, seasonal harvested SBL will need to be preserved so that it can be made available whenever it is needed. One promising way of preserving SBL is by ensiling. Ensiling is a preservation technique for wet biomass. It has been used extensively to preserve animal feed [3] and as a pretreatment technique to improve biogas production by causing a change in the chemical and structural composition of the biomass [4]. Different fermentation pathways can take place during ensiling, depending on the conditions used [5]. These different fermentation pathways determine the effectiveness and further characteristics of the ensiling process. One way of improving the efficiency of ensiling is by the use of additives to control the properties of the substrates to be ensiled [6]. The type of fermentation products formed during ensiling can be influenced by the type of additives used [7]. These fermentation products are important factors that play a key role in the quantity and quality of biogas produced from the silages. Effects of additives on silages are given by (i) conserving the dry matter and energy of the substrate against losses by degradation in the ensiling process itself and in the aerobic phase before feed-out to the digester, (ii) enhancing anaerobic degradability of ensiled materials during digestion, and (iii) enhancing biogas generation from the fermentation products [8]. Additives such as formic acids quickly decrease the pH of a system, thereby inhibiting the activities of undesirable microbes such as Clostridia [9], while additives such as lactic acid bacteria (LAB) accelerate the fermentation of water-soluble carbohydrates (WSC) to lactic acid, which also reduces the pH of the system and in turn prevents undesirable microbial activities [10]. Some other additives such as molasses increase the amount of WSC that are needed for the formation of lactic acids in the system [11]. A large number the additives used for ensiling enhance the conservation of energy and nutritional content of the silage for animal feed [3]. These silages that are fit for use as animal feed may not be optimal for use as a substrate for biogas production [12], although the general requirements of silages for anaerobic digestion (AD) are much lower than those for animal nutrition, regarding aspects such as palatability, content of essential amino or fatty acids, and digestibility. If silages are to be used for AD, the effect of the additives on the kinetics and especially on the biomethane potential (BMP) needs to be considered. While the literature is very rich with regards to additives for use in ensiling wet biomass for animal nutrition, the research on additives for silages that are used for AD is limited [12]. Additives such as lactic acid bacteria enhance the quality of ensiling but have little or no effect on biogas production [6]. Therefore, the purpose of this study was to (i) investigate the effect of a newly developed enzymatic additives on the quality of ensiling and BMP of SBL and (ii) evaluate the kinetics of the BMP of SBL. The study is within the subject of circular economy as it involves using waste materials to produce biomethane which has been reported to have the potential to reduce greenhouse gas emissions [13; 14], thereby providing affordable and clean energy, consistent with the millennium developmental goals of the United Nations.