# Microwave and Ultrasound Based Methods in Sludge Treatment: A Review
## 2 Efficiency Indicators  
It is evidently clear that to characterize a treatment or pre-treatment process in terms of its effectiveness, a specific indicator (or indicators) should be determined which allows the comparison of the different methods. In sludge (pre-) treatment methods, depending on the approach, several of these indicators can be defined, however, the scientific literature and research articles do not always use them consequentially, and thus the comparison of the different processes has its limitations. In environmental sciences, chemical oxygen demand (COD) is an indicative measure of the amount of oxygen that is needed to oxidize the organic matter content of a given sample. Typically, COD is given in mgO\({}_{2}\)/L. COD measurement and the determination of COD level is widely used in wastewater and sludge treatment, for the reason that the measurement is rather quick (especially when compared to the biochemical oxygen demand, BOD determination), and gives accurate information about the organic content of a wastewater or sludge sample [10]. When comparing different processes in terms of organic matter elimination, the reduction rate of COD can give a precise insight about how efficient a given method is, i.e., the more it can reduce the COD level, the more effective it is. \"Pure\" COD, however, is usually too general and sometimes does not provide enough information about the applied treatment method. Therefore it is favourable to split it into two different variants, the soluble chemical oxygen demand (SCOD) and the total chemical oxygen demand (TCOD). SCOD, as its name suggest, contains only those organic materials that can be found in the soluble phase, whilst TCOD contains all organic content (as well as in the soluble and in the solid phase). The ratio of these two (SCOD/TCOD) can accurately indicate what proportion of the total COD can be found in the soluble phase, and as its extent correlates with anaerobic digestibility [11], this ratio is commonly used in the scientific literature when comparing different methods or processes in sludge treatment. Biochemical oxygen demand (BOD) determination serves a similar purpose as COD, however, both the method and its value differ from that of chemical oxygen demand. BOD also measures the amount of oxygen needed to break down organic compounds, although the determination process is based on microbial activity, i.e., how much oxygen do specific aerobic microorganisms need to oxidize the organic matter of the sample [12]. In contrast to COD measurement, the applied bacteria cannot essentially utilize every type of organic molecule in a wastewater or sludge sample, therefore BOD provides information about the organic load of wastewater, applicable wastewater purification technologies and the organic matter removal efficiency of biological wastewater treatment, for instance. BOD is also used as a key pollutant indicator in wastewater discharge standards and limits in many countries. Contrary to chemical oxygen demand (COD), BOD is a sensitive parameter for the presence of toxic components, therefore can provide information about the bioavailability/biodegradability of substrates in water and wastewater. Consideringthe high variability of industrial wastewater, the determination of the change of BOD influenced by the wastewater characteristics, and the operational parameters of wastewater treatment technologies--can provide useful information for the practice, as well. BOD is usually used as an efficiency indicator when the goal is to determine how a certain process can affect the biological usability of a given sludge sample. Disintegration degree (DD) is one of the most common indicators in sludge treatment. DD is calculated as the ratio of SCOD increase caused by the analysed disintegration method in relation to the SCOD increase caused by the chemical disintegration [13].