## 2 Pretreatment Methods for Solids Reduction
### Integrated Pretreatment
#### 2.5.2 Thermal-Chemical Pretreatment  
High temperature helps in abiotic hydrolysis of complex organic sludge constituents resulting in the sludge components becoming more soluble. This mechanism is often enhanced via the addition of a chemical(s) which breaks down cell membranes making the intracellular material more available for digestion. This integrated pretreatment can be applied using either low or high temperature additions such as 70 to 90 \({}^{\circ}\)C for low temperature treatment and 115 to 170 \({}^{\circ}\)C for high temperature treatment. The chemical pretreatment component can be the addition of an alkali, ozone, or other oxidation agent, such as hydrogen peroxide. The integration of thermal methods with chemical addition reduces the consumption of chemicals by up to six times over chemical addition alone, while achieving improved solubilization at the higher temperatures compared to chemical pretreatment alone. The use of thermal methods in combination with an alkali has been extensively studied as an integrated pretreatment system [34; 40; 61]. Acid has also been used in conjunction with thermal treatment; however, integrated alkaline/thermal pretreatment systems have shown the maximum VSS reductions. Low temperature addition coupled with chemical pretreatment requires a higher amount of chemical reagent to be added but is less energy intensive to achieve an improved sludge solubilization over chemical addition alone. A study by [PERSON] et al. on the pretreatment of WAS over a temperature range between 39 and 200 \({}^{\circ}\)C and alkaline treatment dosing NaOH over different concentrations reported an optimal treatment condition for WAS pretreatment of 70 \({}^{\circ}\)C and 0.04 M NaOH mixed for one hour. This pretreatment strategy reduced sludge production post-AD by increasing the sCOD by 34% when compared to the untreated WAS but only increased the sludge degradation by 2% when compared to thermally treated sludge at 70 \({}^{\circ}\)C. This reduction resulted in the lowering of annual biosolids disposal costs by 21% [61]. [PERSON] et al. using WAS achieved 26% reduction for VSS and 200% for sCOD by using alkali dosing of 0.05 g NaOH/g TS that was mixed/reacted for 24 h followed by thermal treatment step using a temperature of 70 \({}^{\circ}\)C that was then reacted for either 2 h and 9 h under mixed conditions [34]. A suspended solids (SS) reduction of 21% was achieved for alkaline addition with 2 h of thermal treatment, while a 68% SS removal was achieved for alkaline with 9 h of thermal treatment. The control which was alkalized for 24 h achieved a 2% SS reduction. The increase in SS reduction suggests more availability of substance for methane production during AD [34]. Biogas production, measured at the end of 30 days of digestion, was 45 mL for the control and 329 mL for the alkalized and thermally treated sludge for 9 h. The pretreated AD system had an average methane content of 64%. The increase in biogas production in the alkalized and thermally pretreated sludge suggests that the integrated use of alkali and thermal treatment makes more substrate available for anaerobic digestion via the increased sCOD. Research by [PERSON] et al. used a thermal-chemical process via hydrogen peroxide dosing at an applied dose of 2 g H\({}_{2}\)O\({}_{2}\)/g influent VSS and the heating to a temperature of 90 \({}^{\circ}\)C as the thermal pretreatment step followed by AD using a SRT of 30 days [62; 63]. The sludge mixture was composed of primary sludge and WAS at a 1:1 mass ratio. The achieved VSS removal for the control, which is untreated, and for the integrated pretreatments prior to AD were 48% and 75%, respectively. The single thermal pretreatment of 90 \({}^{\circ}\)C resulted in removing 49% of the sludge VSS with the control showing no removal. The volume of methane production per gram of VSS removed for the control and the integrated pretreated sludge mixtures were 650 and 293 mL, respectively.