Monitoramento de parâmetros operacionais de um biodigestor modelo lagoa coberta alimentado com resíduo de soro de queijo

Abstract

The milk production chain has important socioeconomic relevance for Brazil, with approximately 1.3 million producers distributed in 99% of the municipalities in the national territory. Among the products derived from milk, cheese is highlighted, which produced 775,000 tons in 2019. During the cheese manufacturing process, a large volume of whey is generated (9 liters of whey for each kilo of cheese produced), which has a high organic load and high degradability that can be treated by means of anaerobic digestion (AD). Based on that, this study aimed to evaluate the main physical-chemical parameters of a biodigester, fed with residues from a dairy industry, to identify possible flaws in the process and propose improvements for the production of biogas. Therefore, the physical-chemical parameters such as pH, redox potential (ORP), electrical conductivity (EC), temperature (T), dissolved oxygen (DO), alkalinities (partial (PA), intermediate (IA) and total (TA)), chemical oxygen demand (COD), series of solids (total (TS), fixed (FS) and volatile (VS)), free ammonia (FA), volatile fatty acids (VFA), crude protein, total organic carbon (TOC) and total Kjeldahl nitrogen (TKN) were evaluated in the liquid substrate of the inlet, interior and outlet of the biodigester. The composition of the generated biogas was analyzed using gas chromatography. The substrate had pH value of 5.5, C:N ratio of 151:1, temperature variations greater than 1 ºC d-1, dissolved oxygen concentrations of 1.4 mg L-1, COD removal in the range of 0 to 35%, CO2 and CH4 production ranging between 23 and 42% and 16 and 20%, respectively. Aiming to understand the parameters that most contributed to the low pollutant load removals, statistical models were developed to estimate the COD concentration using the Pearson correlation. The models that presented the highest values of the correlation coefficient (R2) were composed of a larger number of variables. Variables pH, FS and FA contributed significantly to the removal of COD, while T, TS and VS contributed to its increase. Based on the parameters evaluated, it can be seen that the biogas generated in the biodigester during the study does not have sufficient concentration of CH4 to be used in boilers to generate thermal energy. Although the cheese whey residue has potential for biogas production through AD due to its high VS content, the instability of the operational parameters seen in the analyzed biodigester were the main factors for the low COD removal and low biogas quality generated. Thus, it is suggested that the processes of co-digestion of cheese whey residue with animal manure (poultry, cattle and/or swine) and pH correction with 50% caustic soda in the Entry point substrate be used, in order to provide the conditions necessary for the AD process and consequent stability of the system.