Monitoring of Stored Dairy Manure: Methane emissions, Physiochemical Properties and Biogas Potential
Although animal manures are traditionally viewed as a valuable resource material, there is also a risk for adverse environmental effect from manure generation and utilization. Research was conducted into impacts of common liquid dairy manure storage practices in Canada on methane emissions as well as nutrient composition changes during storage. Further investigation evaluated liquid dairy manure as feedstock for biogas production along with off-farm organic waste. During 17-month continuous on-farm monitoring of stored liquid dairy manure CH4 fluxes varied diurnally, monthly and seasonally, the variability largely regulated by measured internal manure temperatures, particularly at 2 m depth. Peak fluxes occurred in summer and fall seasons, increasing from July to September, falling in October and November, and remained constantly minimal from December to June. From September 2010 to August 2011, 85% of CH4 emissions occurred in summer and fall while 15% took place during winter and spring. Estimated emissions using IPCC methodology varied between months by 357% below to 88% above measured emissions, but on an annual basis the measured emission were 3.8% higher than the IPCC estimate. Monitoring of changes in manure composition during 12-month manure storage showed temporal changes and high variability in volatile solids (VS), chemical oxygen demand (COD) and volatile fatty acid (VFA) contents. These changes reflect biological activity levels that were a function of location within the tank and depended on meteorological factors such as temperature and precipitation. Nitrogen losses amounted to 0.32%-0.35% TKN per day or about 10% per month. Biochemical methane potential (BMP) assay of liquid Dairy manure (LDM) in co-digestion with poultry manure (SPM) and dissolved air flotation waste (DAF) showed maximum specific CH4 yield (SMY) of 0.40 L g-1 VS from a mixture with about 10% DAF along with 40% VS of LDM and 50% SPM. Observed reduced biogas production at higher levels of DAF and SPM was likely due to high ammonia (NH3) or VFA content. Study results underline that manure storage condition and duration affect emissions of greenhouse gases as well as losses of nutrients from stored manure. Improved manure storage conditions therefore have the potential to enhance value of the manure resource and limit environmental impacts from its management and use.