Sewage sludge produced in municipal wastewater treatment plants (WWTPs) is stabilized to produce methane/energy and a final stabilized biosolid suitable for land use using anaerobic digestion (AD) process. Fat, oil, and grease (FOG) matter present in the sewage sludge and their products, long chain fatty acids (LCFA), are not monitored qualitatively and quantitatively for their contribution or inhibition to methane production during the process. AD is designed and operated based on average volatile solid (VS) removal criteria. LCFA can be both present in the sewage sludge and produced as intermediate products during the hydrolysis of FOG. A 1.5-year monitoring of the primary sludge (PS) and secondary sludge (SS) fractions was conducted and
evaluated in a timely base in a municipal WWTP. According to the results, the most common and highest presence belonged to palmitate (C16:0) and total LCFA occurred up to 11963 and 927 mg/L in PS and SS, respectively. Detected LCFA species were mostly saturated as laurate, myristate, palmitate, stearate, oleate and elaidate. The level in the mixed sludge feed complied with the inhibitory threshold values neglecting the accumulation in the anaerobic digesters.