## 2 Pretreatment Methods for Solids Reduction
### Mechanical Disintegration
#### 2.1.2 Microwave Pretreatment  
Microwave pretreatment has been applied as an alternative to thermal pretreatment due to its lower consumption of energy and lesser reaction times. The production of heat through conventional heating is susceptible to excessive heat losses due to energy transmission which does not occur in heating using microwave treatment because it is an internal heat mechanism (activates water molecules causing heating). Faster heating, compact size of the instrument, and improved reduction in pathogens over traditional heating have made this technology applicable in decomposition of organic pollutants, sterilization of medical waste, water disinfection, and inactivation of pathogens [21]. Microwave pretreatment uses a set microwave frequency that ranges between 0.3 GHz to 300 GHz (home ovens use 2.45 GHz) which falls between infrared and radio waves [22]. Thus, the commonly used frequency is generally at 2450 MHz level with power ratings in the 700 W to 1000 W range to disrupt cells within the sludge. The power and treatment time dictate the level of energy applied on the sludge. Microwave can also have both thermal and non-thermal positive effects on sludges [15]. The thermal effect includes breaking of the cells by friction and molecular rotation caused by the increased interaction of electric field. The use of microwave-based pretreatment also is reported to increase the solubilization of organic matter within the sludge which aids in the reduction of solids after digestion. Organic matter such as intracellular matter can be exposed by the lysing of cell membranes and denaturalization of membrane proteins. High temperature can lead to boiling leading to the formation of bubbles which create high pressure and stress on the cell walls resulting in the rupturing of cell walls/membranes. The non-thermal effect occurs due to rapid change in dipole orientation of cell membranes which breaks the hydrogen bonds between molecules leading to cell membrane rupture [12]. Internal resistance to rotation converts microwave energy to heat through bond decomposition and repositioning [22]. Non-thermal effects in microwave irradiation are difficult to monitor since the heating mechanism is different than conventional heating. [PERSON] et al. pretreated primary and secondary sludge mixtures for testing within mesophilic, thermophilic, and temperature-phased AD systems using SRTs of 5, 10, 15, and 20 days. SRT did not show a significant increase in VS removal which was about 48% for the mesophilic two-stage system and about 51% for the thermophilic two stage-system [23]. Similarly, [PERSON] et al. treated sludge at microwave energy of 336 KJ/kg,power of 800 W, and the feeding of thickened sludge weekly using an HRT of 20 days [24]. Their results indicate the achieving an increased solubilization of sludge as observed via the sludge sCOD increasing by 214% over the untreated sludge [25]. A study by [PERSON] et al. on the solubilization of sludge organic matter composed of primary and secondary sewage sludge, using microwave specific energies up to 20,000 J/g TS, and power outputs in the 400 to 700 W range, showed solubilization of the whole sludge organic matter over a digestion period of 90 days [26] A 30% increase in sCOD was observed at 400 W and specific energy of 30,000 J/kg TS. When using a higher energy, 700 W at a specific energy of 10,000 J/kg, the resulting TS increased the sCOD concentration by 67%; thus, indicating a more effective pretreatment at the higher power but at a lower applied specific energy [26] Reports indicate that microwave pretreatment does not require as much capital investment as ultrasonic treatment, consumes less energy compared to ultrasonic pretreatment, thus, also keeping O&M costs down. Unfortunately, microwave pretreatment has no field demonstrations performed to date as it has only been applied at the laboratory scale to date [15].