AN OPEN CHAMBER TECHNIQUE FOR DETERMINATION OF METHANE EMISSION FROM STORED LIVESTOCK MANURE

The feasibility of an open chamber technique for measuring CH4 emissions from farm scale pig slurry (PS) and pig solid manure (PSM) storages was examined. Emission rates were over-estimated considerably when the ratio between the area of the inlet and outlet ports was below 5 and 8 for chambers operated on PS and PSM, respectively. A first-order equation describing the chamber ventilation rate necessary to obtain steady state conditions showed high correlation with experimental data. For ventilation rates of 2-3 cm min-1 steady state was established after 40-54 min. The CH4 emission from PS was controlled by the transfer resistance in the liquid phase leading to a low effect of wind speed. In contrast, the CH4 emission from PSM was controlled by the gas-phase resistance and thus strongly dependent on wind speed. A very high spatial variability in CH4 emissions was found for both PS and PSM storages.-Coefficients of variation for emission rates during one day in March 1992 were 371% and 465% for PS and PSM, respectively. Daily CH4 emission rates from PS ranged from 0.5 to 49.8 g CH4 m-3d-1 during a 19-day period in April-May 1992. The variation depended strongly on storage operations, such as loading and stirring prior to field application. Daily CH4 emission rates from PSM ranged from 17.9 to 92.0 g CH4 m-3 d-1 during a 31-day period in June-July 1992, the rate being correlated with PSM temperatures. Due to the large spatial and temporal variability in the CH4 emission rate from PS and PSM, it is concluded that the accuracy of annual emission estimates is limited, when based on a few seasonal measurements by open chambers. It is recommended that annual CH4 emission estimates as a minimum are based on weekly determinations of CH4 emission rates.