Evaluación de la obtención de energía y subproductos a partir de un esquema básico de biorefinería centrado en co-digestión anaerobia

Abstract

Environmental impacts are generated in Colombia as a result of the different activities carried out in agricultural sector. The amount of organic matter present in the waste poses an important affectation to water bodies and soils due to inadequate management. This biomass, agricultural and livestock, has an energy potential that can be recovered in different types of energy: electric, liquid, gaseous or calorific. In addition, the production of energy under a biological process, such as anaerobic digestion, has as a parallel benefit a treatment of this residual biomass. Also , the biorefinery scheme favours the recovery of by-products that add value to the treated biomass and to this type of schemes.

This study evaluated the use of three organic residues from coffee crops (coffee mucilage), cocoa crops (cocoa mucilage) and pig farms (pig manure). The evaluation focused on obtaining methane through anaerobic co-digestion and quantification of by-products in the effluent. Additionally, co-digestion favours the synergy of substrates to obtain the greatest amount of biogas, taking into account the composition of each substrate such as, for example, the nutrients and specific buffer capacity of manure and the sugars and microorganisms provided by coffee and cocoa mucilage. Therefore, a previously defined residue mixture was worked on through preliminary trials. The study was carried out in a pilot plant with semi-continuous reactors, where three substrate concentrations (8 g COD/l, 16 g COD/l and 32 g COD/l) were evaluated. The reactors was followed the amount and composition of biogas, and physicochemical parameters such as pH, alkalinity, volatile fatty acids, oxide reduction potential, chemical oxygen demand, metals, metabolites, ammoniacal nitrogen and phosphorus.

In all the cases the base of the substrates was the manure, which provided a stable pH varying between 6.3 and 7.5, due to the buffer capacity that this substrate has. On the other hand, the maximum biogas production rate was 494 ml CH4/g COD under the concentration of 8g COD/l with substrate/seed (S/X) ratio of 0.5. In this case, the average biogas composition was 56.6% methane, which is equivalent to a calorific value of 13.782MJ/m3. In terms of metabolites, acetic acid, propionic acid, lactic acid and furfural acid were identified, which is usually used at an industrial level. While the nutrient content was 992 mg/l nitrogen and 270 mg/l phosphorus, from this can be made a recovery through the process for obtaining struvite. These exploitation currents give viability to these systems for rural applications. In this way, there are alternatives to consolidate other types of production dynamics from residual biomass.