## 1 Introduction  
Waste oil has great potential in renewable energy production because of its high energy density and the large amount generated as a result of human activities. Fat, oil, and grease (FOG) in waste streams produced from restaurant kitchens and food industries is a major problem in the sewer systems of urban areas. FOG deposits formed on pipe walls lead to clogging, which can cause flooding, pollution, and other public health problems. Since direct discharge of FOG into sewer lines is no longer permitted, grease traps are generally installed in commercial facilities to remove FOG before it enters sewer lines. The accumulated oily organic waste, including FOG and food scraps, has a high energy potential. Major forms of energy from those oily organic wastes are biodiesel fuel and biogas. Biogas conversion via anaerobic digestion (AD) technology is a better choice when the waste has a difficult-to-extract oil fraction because of emulsion formation or high water content. The average methane yield from grease trap waste is 767.5 \(\pm\) 107 mL/g of volatile solids (VS) added, which is higher than those from other kinds of municipal solid wastes (around 500 mL/g-VS added) [1]. [PERSON] and [PERSON] [2] presented the methane yield from grease trap waste as 845-1050 mL/g-VS added. These very high values are due to the high methane potential of lipids. Several researchers have demonstrated that the AD of oily organic waste is subjected to inhibition caused by long-chain fatty acids (LCFAs) such as oleic acid, stearic acid, and palmitic acid produced from hydrolysis of triglycerides, which is one of the main components of FOG. Since \(\beta\)-oxidation is the rate-limiting step in lipid degradation [3], faster hydrolysis of lipids can lead to LCFA accumulation during the AD of wastes with high oil or fat content. Only a few millimolar of concentration of accumulated LCFAs can strongly inhibit methanogens in an anaerobic process [3; 4; 5; 6] and result in the failure of operation [7]. The toxicity effect is impacted by temperature and the presence of cations such as calcium and magnesium. Hence the management of LCFA toxicity is considered a promising way to overcome poor performance of a digester due to LCFA accumulation [8]. [PERSON] et al. [9] showed decreased levels of inhibition caused by LCFAs with the addition of calcium ions. On the other hand, [PERSON] [10] demonstrated that only 20 min exposure to 7.5 mmol/L lauric acid, which is the lag time for adding calcium, reduced 67% of the original activity of methanogens. Once inhibited by LCFAs, a digester takes a very long time to recover and produce methane [11]. When treating oils organic wastes, digester operators should timely minimize the inhibitory effect; for example, by adding calcium as soon as the digester shows a trend for LCFA accumulation. For such reasons, routine measurements of LCFAs in a digester are required. Determination of LCFA concentrations in anaerobic digesters mostly depends on gas chromatography (GC) [1; 12; 13; 14]. This GC-based method is accurate and reliable but the analytical procedure is complicated and time-consuming. Moreover, GC analysis requires expensive instrumentation. As such, LCFA measurement has not yet been widely adopted in the field of AD; however recent increased interest in the AD of oily waste is likely to create a new need for an alternative method for LCFA analysis that is rapid and easy. Quartz crystal microbalance (QCM) is a widely used sensing device for rapidly detecting target substances in air or water. QCM comprises an AT-cut quartz crystal disc sandwiched between two metal electrodes, which oscillate at a specific frequency when voltage is applied. The frequency is reduced by mass changes on the electrode and allows for monitoring of small mass absorption on the electrode with the use of specific coatings for target substances.