Codigestão anaeróbia do lodo obtido em abatedouros de frangos e batata-doce: valorização energética e agronômica

Abstract

The generation of float sludge from the slaughter of poultry has increased due to the growing productivity of the sector, especially in Southern Brazil. Anaerobic digestion is a technology that can combine the stabilization of such waste with the generation of a clean and renewable source of energy and a nutrient-rich agricultural input, biogas, and biofertilizer. However, the low C/N ratio of the sludge, as well as the lipid and protein contents, may have an adverse effect on the microorganisms involved in the process, impairing their overall performance. Thus, the objective of this work was to study the anaerobic co-digestion of broiler sludge with increasing amounts of sweet potato as a carbon supplement in batch and semicontinuous trials, exploring the energy and agronomic potential of by-products, stability, and efficiency of the process. The batch assay was conducted in 6.0 L reactors, with 4.5% total solids and room temperature. Six treatments were evaluated in triplicate, which consisted of different proportions of sludge and sweet potato (100:0, 80:20, 60:40, 40:60, 20:80 and 0:100). The experiment was conducted until the production of biogas ceased. The semi-continuous assay was conducted in tubular reactors of 60.0 L at mesophilic temperature, with a 25-day hydraulic retention time, daily feed containing 4.5% (2.4 L, of which 60% was a digestate recycle and 40% water). In nine reactors and four periods, seven treatments with three replicates were evaluated, varying the amounts of sweet potato and mud (80:20, 70:30, 60:40, 50:50, 40:60, 30:70, and 20:80). Based on the data obtained in the semi-continuous trial, real-scale scenarios were defined in order to evaluate the economic-financial viability of co-digestion technology between sludge and sweet potato. The batch of the mixture 80% sludge and 20% sweet potato, in addition to accelerating the process, also presented the largest (p<0.05) potentials for the production of biogas (0.731 m³ kg SV-1) and methane (0.503 m³ kg VS-1), and the highest removal efficiencies of organic material (solids, lipids and sugars, p<0.05). In addition, it provided alkalinity to the system (≈ 6.0 CaCO3 L-1), maintaining it stable (AV/AT ratio of 0.11 at the end of the process). The treatments with greater proportions of sludge than of sweet potato resulted in biofertilizers with higher contents of nutrients and free of phytotoxicity as long as the electrical conductivity is maintained sufficiently low by means of dilutions and, therefore, they have greater agronomic value. The multivariate analysis showed that sludge mixtures with proportions greater than 40% of sweet potato in batch assay are quite similar due to the accumulation of organic acids of short molecular chain, causing inhibition of the process. In the semi-continuous assay, the proportions of sweet potatoes ranging from 60 to 40% have the highest potential methane production (0.33 to 0.34 m³ kg VS-1d-1). Above 60% of sweet potato in the blend, the process is adversely affected by the accumulation of volatile acidity (>4 g L-1), and below 40% sweet potato, the process is partially inhibited by significant acidity accumulation volatile (>10 g L-1) and the high concentration and ammoniacal nitrogen (>2000 mg L-1), probably due to the lipid and protein content of the sludge. All the economic scenarios defined were financially attractive. Finally, it is concluded that anaerobic co-digestion is an interesting alternative for recycling the energy and nutrients contained in the sludge on both environmental and economic perspectives.