# Digestate Quality Originating from Kitchen Waste
## 5 The Challenge of Microplastics' Determination  
The problem of biodegradable waste management is largely due to the lack of regulations and guidelines related to their sorting. Thus, quality control of the bioreactor feedstock becomes more demanding, and the process itself becomes less controllable. Not all sorting technologies incorporate tools for identifying biodegradable plastics, nor for detecting the presence of microplastics, especially if smaller than 2 mm, resulting in the presence of microplastics after the mechanical biological treatment process, in compost or digestate [12; 57; 58; 59]. One of the reasons for this situation is certainly the costs, and the other is technical difficulties regarding the applicable method of identifying bioplastics, particularly in real time.  
There are four major means of identifying microplastics below 2 mm: microscopic, spectroscopic, thermoanalytical, chemical methods, or a combination thereof. These methods have high efficiency in detecting microplastics. FTIR spectroscopy reaches detection limits of down to 20 \(\upmu\)m, while Raman microspectroscopy can identify particles down to 1 \(\upmu\)m in size [57; 60]. Pyrolysis GC-MS allows the plastic types of particles > 100 \(\upmu\)m to be identified [61], from complex matrices and mixtures [62], while the ATR-FTIR was proven to be efficient for the identification of larger particles (>500 \(\upmu\)m) [63; 64]. To analyse smaller particles, ATR-FTIR coupled with a microscope (\(\upmu\)-ATR-FTIR) is recommended [65].  
The methods used to identify microplastics are effective, but costly, especially regarding process scale-up. In addition, the identification of microplastics is used more often to assess microplastic release from fabrics [66] to identify the presence in water samples [67], beach sand [68], marine bottom sediments [69], or aquatic organisms [70], so in relatively clean matrices. In turn, the identification of microplastics in waste management processes is not a common practice, especially as a continuous operation. In the case of an increase in the share of bioplastics in biowaste, the challenge will be to improve the methods of cleaning the feedstock from plastic impurities. It is necessary to consider either efficient identification and further separation or conscious leaving of selected compostable materials susceptible to AD decomposition in a specific plant and process. Studies show that various biodegradable materials, e.g., polylactic acid (PLA) and poly-hydroxy -alkanoate (PHA), have different degradation times and methane-generating potential [21; 71]; therefore, the decision to leave these compounds in the feedstock should depend on individually selected bioreactor operating conditions and eventual type of pre-treatment used.  
On the other hand, a complex matrix of a digestate, and the uncertainty regarding the amount of microplastics, should be considered before either digestate is rejected or not from agriculture applications. The reliability of analytical methods in the case of complex matrices is lower, and therefore the decision on the quality requires more care, whether it refers to the input or output materials.