## 2 Pretreatment Methods for Solids Reduction
### Biological Pretreatment
#### Temperature Phased Anaerobic Digestion (TPAD)  
Typically, AD is implemented in a single, semi-mixed vessel. All biological phases of digestion occur within this single-tanked system without any physical separation. With TPAD, the methanogenesis step is separated from the previous three primary reaction steps (hydrolysis, acidogenesis, and acetogenesis) to increase the efficiency of the digestion and enhance sludge reduction [15]. Most applications involve using a thermophilic digester which operates at temperatures higher than 50 \({}^{\circ}\)C which is placed in series prior to a mesophilic AD reactor that operates at 37 \({}^{\circ}\)C. The higher temperature of thermophilic digester promotes hydrolysis of the feedstock and the integration of the two different temperature ranges increases the syntropy of the acetogens and methanogens [25]. This configuration is reported to utilize low quality thermal energy, increase biogas production, increase solids reduction, and deactivate more pathogens as compared to a typical, single stage digester. TPAD systems are different from two phased anaerobic digestion systems. Thermophilic digestion maintains the methanogenic activity in TPAD while the methanogenic activity is suppressed in the first stage of a two phase AD system [54]. [PERSON] et al. compared TPAD applied toward WAS digestion with single stage mesophilic and thermophilic digesters operated using various HRTs. The input sCOD was higher in the TPAD digester compared to the single staged digesters as well as the achieved solids reduction, which was almost 12 to 15% higher compared to single stage digestion [55]. [PERSON] el al. conducted temperature phased AD of primary sludge (PS) with the organic fraction of municipal solid waste (OFMSW) at various ratios. They obtained a 48% reduction of VS on 100% primary sludge and a 71% reduction of VS on 20% primary sludge [54]. The first phase was a completely mixed thermophilic digestion operated at 55 \({}^{\circ}\)C and was followed by a completely mixed mesophilic digestion operated at 35 \({}^{\circ}\)C. It was concluded that the hydrolysis step was accelerated due to the high temperature which resulted in an increase in solids reduction. A specific hydrolysis rate of 0.3 g particulate COD/g VS was achieved in this system with a maximum methane production of 0.4 L/g VS fed obtained. [PERSON] et al. compared TPAD treated sludge with thermally treated sludge. The first stage of the TPAD was thermophilic digestion 55 \({}^{\circ}\)C followed by 42 \({}^{\circ}\)C which obtained a 23% reduction in VSS. A thermophilic single digestion at 165 \({}^{\circ}\)C resultedin a 22% reduction in VSS [56]. The sCOD increased by 287% for the TPAD while the sCOD for the thermophilic digester increased by 378% compared to AD of untreated sludge. The methane yield of TPAD system and single thermophilic digester was 23% and 20% higher than AD fed the untreated sludge. This study presents that the methane production from a TPAD system operating at temperature range between 42 to 55 \({}^{\circ}\)C for 3 days is comparable to a thermal pretreatment at 165 \({}^{\circ}\)C for 30 min.