Enhancing Biogas Production of Co-Digested Cattle Manure with Grass Silage from a Local Farm in Landshut, Bavaria, through Chemical and Mechanical Pre-Treatment and Its Impact on Biogas Reactor Hydraulic Retention Time

Cattle manure usually contains a proportion of carbohydrates in the form of organic residues from incompletely digested feed and farm husbandry practices. These are not usually available for biogas production due to the long fermentation time. This paper investigates the optimal application of alkali, NaOH and KOH and mechanical pre-treatments to improve the degradation of the lignocellulosic content and the potential biogas yields from a local farm in Bavaria, Germany. Parameters such as temperature, pH, soluble chemical oxygen demand, organic acids, dry matter and volatile solids were analysed for this purpose. Alkali pre-treatments in 0.2, 0.1 and 0.05 M NaOH concentrations were tested in single mode and combined with shredding in batch experiments. The maximum increment of the soluble chemical oxygen demand during the pre-treatments took place during the first 50 h of experimentation, and it showed an improvement of 10,060.0 ± 8% mg/L s COD after the application of 0.2 M NaOH compared to the untreated substrate, which had an initial value of 2145.0 ± 8% mg/L s COD. Pre-treatments with 0.1 and 0.05 M NaOH concentrations showed similar s COD increments, with an additional 6860.0 ± 8% mg/L s COD and 8505.0 ± 8% mg/L s COD, respectively. The pH values varied strongly after the addition of the pre-treatment chemicals, with a continuous pH of 12 by 0.2 M NaOH during the 7 days of pre-treatment. Batch biogas experiments were done by applying 0.05 M NaOH and 0.05 M KOH pre-treatments in single mode and combined with shredding. The chemically pre-treated substrates showed a faster biogas production with an advantage of 18 days in comparison to the untreated cattle manure by a biogas yield of 350.0 NL/kg VS. All experiments were done under mesophilic conditions.