# The Potential of the Biodigester as a Useful Tool in Coffee Farms
## 1 Introduction  
Coffee production in Colombia during 2019 reached 14.8 million bags (60 kg/bags), which was the fifth consecutive year of producing over 14 million bags. These statistics are due to the production of 21.4 bags per hectare, representing an exponential increase of 88% in 9 years. For the same year, the country had a total of 853,700 cultivated hectares of coffee, concentrated mostly in regions such as Tolima, Huila, Cauca, and Caldas [1]. Tolima is the country's third largest coffee producer, with a national contribution between 11 and 12.5% [2]. Out of a total of 47 municipalities in the Tolima regions, 38 are coffee growers, representing 29% of the department's main crops by area planted [2]. This activity brings to the economy more than 60,000 households that account for 68% of rural populations, in which 96% of farmers have coffee plantations of under 2 hectares [3]. This fact could complicate the treatment and disposal of waste and wastewater produced in coffee processing. The traditional methodology used for coffee processing is wet fermentation (see Figure 1), in which the cherry pit (bean) is extracted from the pulp, then dried and prepared for export and roasting. The method is called the wet process because water is the primary means of both moving the coffee through the process and facilitating the extraction of the seed. This process is particularly important for the grower because it affects the aroma and flavor of coffee. Moreover, the wet process generates solid wastes and wastewater with a high content of organic matter [4]. The solid waste accounts for 60% of the fresh fruit and is composed of: pulp (67%), mucilage (27%), and husk (6%), while wastewater is divided into two main forms: the pulping water with a high content of quickly fermenting sugar (honey water) and the wastewater generated from processing applications [5]. Thus, the characteristics of honey water from coffee processing are thatit has an extremely low pH (4 to 4.5) and a Biochemical Oxygen Demand (BOD) of up to 20,000 mg/L as well as a Chemical Oxygen Demand (COD) of up to 50,000 mg/L, which make it difficult to treat by biological means [5]. These residues are often discharged into rivers or surrounding agricultural land without previous treatment because the farmer does not have the necessary knowledge or resources to protect the environment [6; 7]. [PERSON] et al. [6] evaluated the impact of effluents from traditional wet coffee processing plants on the downstream water quality in Ethiopia. The organic content of the effluents was considerably high with values of 7200 mg/L and 871 mg/L for COD and BOD, respectively. This high pollution level caused a depletion of dissolved oxygen to levels of 0.25 mg/L in the effluents and the presence of nutrients invoked a large risk of eutrophication. As a result, the polluting potential of the wet coffee process from the coffee farm is enormous. Thus, to comply with the environmental regulations and achieve the restoration of the environment, it is necessary to find an economical and easily adaptable technology for the treatment of honey water. Literature reviews indicate several treatment methods for coffee wastewater. These include physicochemical processes, such as adsorption, advanced oxidation, and biological processes such as the use of an up-flow anaerobic sludge blanket (UASB) reactor [8; 9]. However, on a small-scale in rural areas, the treatment of wastewater from coffee processing requires a simple, efficient, and cost-effective system. Moreover, several authors have demonstrated that the anaerobic digestion of coffee wastewater could be an attractive solution in rural areas for the production of renewable energy in the form of biogas [10; 11; 12]. In this regard, Centro Nacional de Investigacion del Cafe (CENICAFE) developed a Modular Anaerobic Treatment System (MATS) for coffee farms in the Colombian mountains [11].