# Biogas Production Enhancement through Chicken Manure Co-Digestion with Pig Fat
## 1 Introduction
It is also a quest for a regulatory arrangement on conserving the environment in the area of concern and selecting decent technology for waste handling with all the essential standards for effective operation. So far, there are many available techniques for degradable waste transformation to energy, which we could assort into groups according to the standard method of technology: the group of thermal conversion (incineration, gasification, pyrolysis, etc. ), the group of biochemical conversion (anaerobic digestion (AD), ethanol fermentation, etc. ), the group of chemical and mechanical treatment, and the group of new trends of WtE technologies [9]. What exact technology we choose depends on our needs, the possibility of implementing that technology, local regulations for that technology, and WtE conversion's effectiveness. So far, processed fat could be used as a renewable fuel [8], but burning that kind of matter does not help to meet the Green energy targets of the European Union [19] and Paris agreement [13] because flue gas contaminates the environment with nitrogen oxides and solid particles. According to the literature [20; 21], livestock by-products, mainly generated from the meat processing industry, are sustainable feedstock for the synthesis of biodiesel production [20; 21]. Other authors [22] also have noted that biodiesel could be made from broadly available animal fat wastes (also called lipid-rich wastes [22]) instead of vegetable oils. This way is a more promising and sustainable method, allowing elimination of the need for crops in the debate of food vs. fuel simultaneously. Still, in the EU, there is a gap between the centralised and industry-level facilities targeting lipid-rich wastes. As a consequence, these substrates are often disposed of uncontrolled instead of being re-used for WtE conversion to biogas under anaerobic digestion. It should be emphasised that the production of biogas from lipids is increasing in popularity among anaerobic digestion studies, and biogas production could be considered as the appropriate way for producing bioenergy from anaerobic digestion in the frame of modern bioeconomy, too [23]. So far, AD is a well-known technology and an effective method for dealing with animal manure. This technology is a good measure for organic waste utilisation, pollution and greenhouse gas emission reduction, renewable energy production, and digestate as valuable fertiliser used in agriculture [24]. As was mentioned, the fermentation of animal and chicken manure is considered an appropriate option for the management of such waste. However, the high organic nitrogen content (30 g\(\cdot\)kg\({}^{-1}\) fresh weight) and low carbon-to-nitrogen (C/N) ratio (7:1) [25] is a major drawback that inhibits the anaerobic process. In such cases, even at a low organic load, the ammonia concentration can exceed the inhibition threshold and cause process interruption [25]. In this case, improving the C/N ratio with carbon-rich additives can help to optimise co-digestion. The latest studies show that animal manure can be well co-digested with other biodegradable organic materials such as fat waste, producing renewable fuel-biogas and the organic fertiliser-digestate. It is predicted that adding other materials can increase the total solids' (TS) concentration, causing difficulties in pumping and mixing the digester substrates [26]. Depending on the TS content of the feedstock, anaerobic digestion can be categorised into wet anaerobic digestion with a TS content <10%, semi-dry anaerobic digestion with a TS content ranging from 10 to 20%, and dry anaerobic digestion with a TS content \(\geq\)20% [24]. The quality of feedstock conversion to biogas characterises the efficiency of AD conversion systems. According to the literature [27], the AD conversion efficiency is generally high at low organic loads (OLRs), because microorganisms access nutrients well. Still, too low OLR can cause low activity of microorganisms due to inadequate provision of nutrients for their metabolism, and conversion efficiency drops.