## 2 Pretreatment Methods for Solids Reduction
### Mechanical Disintegration
#### High Pressure Homogenization  
High pressure homogenization (HPH) is a less energy intensive technique compared to ultrasonic treatment that uses pressures up to 10 MPa and homogenizes substrate under strong depressurization to break down the particles in the sludge influent. Pressure and homogenization cycles, which is the number of times the sludge fluid is passed through the homogenizer, are the main factors that determine the effectiveness of this pretreatment method [27] Pressure is applied externally with an air displacement pump that forces fluid to pass through a narrow orifice. The pressure which can be controlled by a homogenizing valve drops instantly causing strong shear forces within the fluid. The intercellular materials are released when the external pressure is greater than the internal resistance pressure which ruptures the cell membrane [28]. The process uses three mechanisms to break down the microbial cells within the sludge: eddies, impingement, and cavitation [27]. The energy applied by the valve is converted into kinetic energy which creates turbulence in the sewage sludge that results in formation of mixing eddies. The high-energy eddies are responsible for the shear forces that break the cells and make intercellular materials available. Impinge stress when applied on a plate creates an impact which results in a liquefied homogenate. The sudden drop in pressure causes forces within the bulk fluid to collapse onto the plate near the exit orifice creating cavitation and shearing. Cavitation is formed near the exit orifice and in spaces between valves in the homogenizer where the fluid pressure drops below the liquid vapor pressure (hence cavitation). The increase in surface area by sludge particle disintegration creates a finer emulsion thus accelerating the hydrolysis step with the AD. This pretreatment method requires an air displacement pump which is relatively easy to use, does not require the use of chemicals, effectively increases solubilization, requires low capital investment, and has lower O&M costs over the other two option discussed thus far which makes it a favorable candidate pretreatment option [29].  
In a study by [PERSON] et al., anaerobic sludge was pretreated with HPH at pressures ranging from 0-50 MPa for one and two homogenization cycles. VS removal increased from 36% to 42% when HPH was applied at 50 MPa for one and two homogenization cycles, respectively. However, at 40 MPa pressure, VS removal was 33% at first homogenization cycle and then increased to 40% after the second homogenization cycle [29]. Sludge disintegration was analyzed with the researchers concluding that as the pressure was increased, the disintegration degree was increased; however, it was not as linearly as it performed during the lower pressure range of 0-30 MPa than it did when the pressure was changed from 40-50 MPa. The disintegration of sludge and increase in VS removal indicated that the pretreatment solubilized sludge matter. A subsequent study by the same team at 80 MPa and four homogenization cycles decreased TSS by 31% and the VSS by 37% during subsequent AD processing. A linear relationship of VSS and TSS indicated 86% of the TSS was contributed by the VSS which means the organic matter was transferred from the sludge solid matrix into the liquid matrix. The increase in homogenization cycles and high pressure resulted in an increase in both VS and TCOD removals [27].