The overall aim of the dissertation is to validate and apply a monitoring tool for quantification of fugitive methane (CH4) emissions from biological waste treatment facilities including composting and anaerobic digestion (AD) plants. Main emphasis lies on the determination of total emission rates of the plants as well as individual component emissions (e.g. open digestate storage tanks) based on the multi-source reconstruction. The inverse dispersion technique derives emission rates from measured concentrations at a point upwind and downwind from the source combined with meteorological data (3D ultrasonic anemometer) using a dispersion model (LASAT - Lagrangian Simulation of Aerosol-Transport). In this work an open-path tunable diode laser spectroscopy (OP-TDLS) is used to measure CH4 concentration over a path length up to several hundred meters. The applied method can act as a useful tool for quantifying fugitive CH4 emissions from biological waste treatment facilities. The method provides the opportunity to quantify whole plant and component emission. The dissertation offers insight into deriving emission rates from multi-sources (e.g. open digestate tanks) within a real-world industrial setting relevant to a range of systems, not only biogas plants. In addition, methane emissions were quantified for the first time considering different operating conditions (e.g. filling level or agitation of the openly stored digestate, turning of compost windrows) at composting and AD plants, thereby providing valuable information to evaluate plant-specific efficiencies. Key factors in implementing this method include accurate measurement of gas concentrations and meteorological data, representative instrument placement as well as suitable meteorological (e.g. sufficiently strong wind speed) and topographical conditions. In principle, the approach can be applied at other fugitive emission sources, such as landfills or waste water treatment plants, as well as for other gaseous emissions.