SUSTAINABLE RESEARCH METHODOLOGY ON THE EFFECT OF THE REUSE OF BLACK LIQUOR IN THE ALKALINE PRE-TREATMENT OF GARDEN RESIDUES FOR THE PRODUCTION OF BIOGAS

Biogas is an important renewable source of energy, which converts organic material into energy. Lignocellulosic biomass emerges as a strategy to ncrease biogas production through pre-treatments that provide organic matter to the anaerobic environment. Among the chemical pre-treatments available, the alkaline is the most used one because it presents the highest yield in biogas production. However, the effluent (black liquor) it generates is an alkaline byproduct that, if disposed irregularly, can cause environmental problems. In this context, the objective of the present study is to present the methodology employed to evaluate the effect of alkaline pre-treatment with potassium hydroxide solution (KOH) applied to garden waste in the production of biogas and the reuse of black liquor as a new alkaline medium. The pre-treatment was divided into seven subsequent batches, with the first batch consisting of a 5% KOH solution and dry and crushed substrate (garden pruning waste). For the other batch the black liquor from the liquid fraction of the separation process was reused. The Biochemical Methane Potential (BMP) tests were carried out with the fraction sifted in mesophilic conditions (35[degrees] C) for 25 days, following the German standard VDI 4630. The results showed that the second batch had the highest biogas production (620 LN biogas/VS-1 kg) and an efficiency of 30% when compared to the non-pretreated substrate. A positive effect was also observed in the biogas yield after reusing the black liquor twice, presenting an average efficiency of 20%. In this sense, this study demonstrates that the reuse of the remaining black liquor from pre-treatment with KOH is a viable and sustainable technique for pre-treatment of garden waste and contributes to the reduction of costs in real scale.