## 2 Pretreatment Methods for Solids Reduction
### Integrated Pretreatment
#### 2.5.3 Mechanical-Chemical Pretreatment  
Mechanical pretreatment on sludge creates shear forces due to the applied pressurization and depressurization which results in the degradation of the cells. The integration of chemical(s) enhances the decomposition process by breaking the cell walls and increasing the solubility of the organic matter within the microbial cells [16]. The use of chemical(s) has shown an additional benefit along with mechanical forces on the hydrolysis step during AD [57; 59; 65; 66]. Ultrasound and ozonation are an example of a mechanical-chemical pretreatment that has the simultaneous advantage of increased solubility of sludge components through the production of radical oxidizer species, enhanced ozone mass transfer, and increased reaction sludge solid surface area via particle disintegration. [PERSON] et al. studied an ultrasound-ozonation pretreatment system applied on WAS and its effect on the particle size of the sludge solids. Their study indicated that reaction temperature, ultrasonic energy density, pH, and O\({}_{3}\) dose directly affected the outcome. An increase in sludge total COD of up to 20% was observed as the treatment time increased from 0 to 60 min and ultrasonic energy density was increased from 0.12 W/mL to 1.50 W/mL at a steady O\({}_{3}\) dosage of 0.6 g/h [20]. The doses of ozone used ranged from 0.05-0.5 g O\({}_{3}\)/g TS and the tested specific energy output of the sonication unit ranged from 40 W to 600 W. However, at higher O\({}_{3}\) doses, an increase in treatment time did not increase biogas production suggesting the potential formation of refractory by-product compounds perhaps due to oxidizer overdosing. The use of ultrasound was reported to produce mechanical shear forces on the extra-cellular polymeric substances while the reaction of the O\({}_{3}\) molecules resulted in increased cell disintegration, and thus, higher solids removal post-AD. [PERSON] et al. pretreated WAS with NaOH and 20 kHz ultrasound, which resulted in sludge hydrolysis of 211 mg/L/minute [16]. They reported that the NaOH breaks the cell walls which releases the intracellular material available for lysis during sonication resulting in a net high solubility of sludge components for later AD treatment. Microwave pretreatment can also be applied with the dosing of a base as an example of the integration of a mechanical/chemical mechanisms as a sludge pretreatment that has shown promising results by increasing the solubilization of sludge components [59; 67]. Alkaline pretreatment solubilizes the cells while MW irradiation increases the temperature through the friction caused by the rotation of polar molecules within the microwave electric field. The increase in temperature is faster and requires less energy compared to conventional heating and results in increased sludge component hydrolysis [12]. A study by [PERSON] and [PERSON] reported that the proteins available by integrated pretreatment using microwave and alkalis is greater than the proteins available by separate pretreatment which suggests the synergistic effect of the two treatments on increased sludge component solubilization [59]. [PERSON] and [PERSON] found that the sCOD to tCOD ratio was increased from 0.005 for the control to 0.37 using a pretreatment run at a pH of 12.5. Their results also showed a VSS reduction of 42% for the control and a 48% VSS reduction for the WAS pretreated with microwaves at a pH of 12 followed by 49 days of AD processing. The biogas produced after digestion (50 days) was also reported with the untreated WAS fed AD producing 65 mL of cumulative methane while the integrated pretreatment fed the same influent that was operated at a pH of 12 produced 80 mL of cumulative methane [12]. High-pressure homogenization (HPH) coupled with base dosing is another integrated treatment that has been shown to improve the solubilization of sludge components through solids degradation and increased bioavailability of the organic materials within sludges. organic material [41; 57].