Effects of pretreatment strategies on hydraulic retention time of dairy cow manure through anaerobic digestion: Exploratory study

Conventional dairy farming in Poland is associated with environmental challenges, particularly related to inadequate manure management, which may lead to water, soil, and air pollution.Dairy cow manure, typically composed of animal excreta and wheat straw, represents a valuable resource for anaerobic digestion, enabling the production of renewable energy (biogas recovering) and stabilized digestate suitable for soil fertilization/amendment while mitigating greenhouse gas emissions.However, the high lignocellulosic content present at cow manure limits the biodegradability by hindering the hydrolysis stage, thereby reducing process efficiency and prolonging hydraulic retention time.Pretreatment strategies can increase substrate accessibility by promoting the release of fermentable compounds.This study evaluates the effects of mechanical (particle size reduction) and thermal (pasteurization at 70 ºC for one-hour) pretreatments on hydraulic retention time, biogas yield, and methane production during anaerobic digestion of cow manure.Biomethane potential tests were conducted in accordance with DIN 4630:2016 standards.The results demonstrated that all pretreatment strategies increased digestion kinetics, leading to shorter fermentation times.The most significant reduction was observed for pasteurized fresh manure, achieving up to 46% reduction in hydraulic retention time to reach 80% of cumulative methane production, although accompanied by a decrease in methane yield (-8.7%).Mechanical pretreatment showed more balanced performance, with the 5-10 mm particle size achieving the highest biogas (468.0 ± 86.0 m³/Mg OM) and methane yields (265.5 ± 36.0 m³/Mg OM), corresponding to increases of 11% and 6.9%, respectively, compared to untreated cow manure.Generally, the results showed that while pretreatments accelerate biodegradation, optimizing anaerobic digestion performance demands balancing hydraulic retention time reduction and methane production.This exploratory study gives relevant information for improving manure treatment and management and increasing the efficiency of anaerobic digestion units.