Anaerobic co-digestion of agri-industrial waste for energy and agricultural valorization purposes

[eng] Anaerobic codigestion (AcoD) is the anaerobic degradation of two or more substrates with complementary properties. It is a waste treatment and recovery alternative that has shown better results than anaerobic digestion (AD). Still a research gap, as most studies have been carried out under mesophilic and thermophilic conditions, due to the misconception that psychrophilia (T ≤20 ºC) is not viable for biomethane production. In areas with these temperature conditions, various agro-industrial activities are carried out, such as the production of dairy products, which generate waste that, in some contexts, is not properly managed and is, therefore, a source of contamination. AD and AcoD studies were carried out using whey CW and CW, by-products of the dairy chain, as substrates. Firstly, the risk of inhibition and methane production at 20 and 35 ºC was evaluated using CW as substrate under four (1, 2, 3, and 4) different inoculum/substrate ratios (ISR). The results showed that the process is viable at 20 ºC, but there is a plausible risk of inhibition, and the process takes longer to stabilise than at 35 ºC. Based on these results, AcoD tests between CW and CM were developed. The mixing ratio (% W/W in VS basis) and ISR at 20 ºC were evaluated. The results showed that the best mixing ratio (65:35 %w/w of CW and CM, respectively) and that with an ISR of 3 (OL 0.6 gVS/L) is the best of each variable for the development of the process. Based on this information, semi-continuous set ups were carried out to evaluate these conditions. Three reactors of 5L total volume were used. Initially, a 1:3 ratio of CM:water was introduced to allow the microbial population to develop and degrade the organic matter. After that, the reactor was loaded to test the previously identified mixture and OL, yields of 0.51 m3CH4/kgVS were found. An 8 m3 reactor was then installed at the Rancho Los Álamos farm in Ubatè. The reactor was initially loaded with 375 kg of CM and 6000 L of water. After three months, the loading was started, maintaining the proportions added to the 5 L reactors evaluated in the previous stage. The scaled-up reactor showed yields of 11447 m3 of biomethane per month, taking into account the available biomass that can be valorised within the farm (19 m3/month of CW and 10.46 t/month of CM). The LCA also showed that by incorporating AcoD into the cheese production process, it is possible to reduce indicators such as climate change, eutrophication and acidification by 23%, 3.84% and 5.26% respectively. The fertiliser potential of digestate produced was evaluated. A field trial was conducted with ryegrass (Lolium perenne) with three urea digestate ratios (100:0 (U), 50:50 (UD), 0:100 (D)) and a control (C) without fertiliser. Over a period of 60 days, the dry matter yield of three cuttings and the nitrogen content in the aerial part of the plant were measured. After the final cutting, other chemical and biological properties of the soil were also analysed. An economic evaluation was also carried out. The results showed that yields in relation to biomass production were similar in the U and UD treatments (4179 and 4282 kg/ha, respectively). Therefore, a 50% reduction in urea use on the farm would be possible. Finally, an economic analysis was carried out considering two scenarios, the current scenario and a scenario of use and valorisation of the above-mentioned quantities and wastes. The economic analysis showed that the scenario with the CoDA system resulted in a positive Net Present Value (NPV) of approximately USD 1,962,124, a Benefit Cost Ratio (BCR) of more than 1 (6.15) and an Internal Rate of Return (IRR) of 7.51%, indicating financial viability over the project horizon. In addition, the Value Added (VA) was consistently USD 148,861.91.