## 2 Pretreatment Methods for Solids Reduction
### Mechanical Disintegration
#### 2.1.1 Ultrasound Pretreatment  
The use of ultrasound produces cavitation in liquid which creates high pressure and temperature pockets necessary to create destruction shear forces on the sludge components. Hydroxyl radicals are also created from water during ultrasonic treatment hence this treatment uses cavitation and chemical reaction mechanisms for cell lysis within the sludge which breaks down cell walls and thus releases cell components, such as DNA [15]. Sonic frequencies in the range of 20-40 kHz have been reported to be optimal for disintegrating the microbial cells for later solids reduction [13]. [PERSON] et al. studied the use of ultrasound pretreatment of sewage sludge within an AD showing a 60% reduction in the required digestion retention time, from retention time of 22 days to 8 days and an increase sludge VS removal by 44%.The biogas produced in the treated WAS for use a 8 day retention time was twice that of the biogas produced without pretreatment with a 22 day retention time [14]. In terms of sonic power needs, a sonication density of 0.51 W/m and sonication intensity of 4.8 W/cm\({}^{2}\) for 15 min resulted in 24.6% increase in sludge VS reduction during AD treatment of WAS [16]. [PERSON] et al. treated WAS prior to AD with 42 kHz of ultrasound for 120 min and achieved a 39% VS enhanced reduction during AD [17]. The effects of 20 kHz ultrasonic treatment of gravity thickened WAS showed an insignificant change in TS and VS removal; however, the applied energy had positive effect on particulate solubilization with an increase in sCOD, which means COD was transferred from particulates to soluble form by increasing ultrasonic specific energy to 8800 kJ/kg TS [18]. This means although there was no effect on mineralization and evaporation phenomena occurring, there was an increase in disintegration degree. [PERSON] et al. pretreated WAS using a 20 kHz ultrasound system with an energy density of 0.5 W/mL for 0 to 100 min. They reported an improvement in sludge COD removal between 30% to 50% as pretreatment time was increased, along with an improvement in VS removal ranging from 4.9% to 29.8% compared to no pretreatment [19]. This study concluded that an increase in ultrasonic treatment time increased the COD removal efficiency, likely due to an increase in sludge compositional soluble fractions. Studies have also indicated that solubilization is directly dependent on the energy applied with higher energy leading to higher disintegration. Pretreatment of WAS prior to AD at 53,000 kJ/kg TSS increased sCOD by 45% according to [PERSON] et al. [16]. An increase in energy density from 0.12 W/mL to 1.5 W/mL for 30 min applied to WAS increased sCOD from 10.78% to 15.11% according to a study by [PERSON] et al. [20] These studies show that higher input energy leads to higher solids disintegration and thus better solids reduction during subsequent AD treatment. Sonication also was observed to increase soluble COD and sludge disintegration with an increase in methane generation during AD of WAS. The overall value of the process must break even in terms of the cost of energy consumed for this process versus biosolids volume reduction thus reduced tipping fees to be economically viable. Additionally, the economic benefit of biogas used as a recovered energetic product should be considered.