# Microwave and Ultrasound Based Methods in Sludge Treatment: A Review
## 6 Conclusions  
Microwave and ultrasound-based treatment processes of sludge have been widely investigated in the past few decades. Amongst the advanced treatment methods, these physical procedures present promising and effective alternatives when applying them to different kinds of sludge. Studies report that standalone microwave irradiation can increase the organic content of the soluble phase (i.e., SCOD) due to certain thermal and athermal effects, which lead to better anaerobic digestibility. The amount of increment somewhat oscillates between the different scientific studies, however, all of them show an average result of around 10-20%. AD can also be enhanced via microwave irradiation, studies show that the level of increase can reach up to 135%. The effects of microwave-alkaline combined were found to be additive according to numerous studies. In regards to the SCOD/TCOD ratio, combining microwaves with alkali, the growth can reach 30% or more, while the disintegration degree can be increased up to 66%. Biogas yield can be significantly improved as well, a study concludes that the overall increase exceeded 228%. Application of microwave-H\({}_{2}\)O\({}_{2}\) treatment to sludge also show promising results, studies report that the extent of soluble COD can be doubled when using this type of combination (from 5000 mg/L to 10,000 mg/L). Ultrasonic treatment of sludge also results in the solubilization of organic particles, which is beneficial for the following anaerobic digestion. In the case of US treatment, the solubilization of COD is mostly due to the disintegration of extracellular polymeric substances. A study reports that by using standalone ultrasonic treatment, the methane content in the produced biogas almost reached 72%. However it has been shown that the individual use of ultrasound in terms of energy efficiency is usually inadequate, and therefore it should be combined with other mostly chemical treatments. Combining ultrasound with alkaline treatment, the solubilization of certain organic molecules can be significantly enhanced, up to 67% in the case of proteins. It can be also used for increasing the efficiency of AD, a study shows that the ultrasound--NaOH combination improved the biogas yield by 21%, and in automatized reactors, the daily methane production rate can reach 0.15 m\({}^{3}\) CH\({}_{4}\)/m\({}^{3}\) reactor. Combining US with H\({}_{2}\)O\({}_{2}\) can be applied to increase sludge reduction rate, as well as improving SCOD, and DD--the difference in SCOD reached 3662.78 mg/L when applying 15 min of US treatment with an initial pH of 11, and DD exceeded almost 29% during the same conditions. Although microwave and ultrasonic treatments have verified advantageous effect on biodegradability and improve the biogas yield from sludge, further researches are needed to investigate their applicability in technologies focusing on the non-energetic utilization of sludge. According to the circular economy concepts another important area of research is the microwave and ultrasound-assisted processes to extract the valuable non-organic compounds of municipal and industrial sludge. Detailed analysis of dielectric properties of different originated sludge can help to achieve higher heating efficiency of microwave-assisted processes. Development of industry scale microwave or ultrasonic