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From a holistic perspective, mesophilic or thermophilic anaerobic digestion (AD) is the preferred choice for the reduction of volume and mass of solids to be disposed from a wastewater treatment plant (WWTP).A net energy output compared to alternative treatments is the main factor favoring anaerobic digestion.The efficiency of the process is improved with higher influent concentrations of degradable matter among other factors.A more recent development to improve the process is the application of sludge pretreatments (SPTs) which increase the rate and efficiency of anaerobic digestion.SPTs do not convert nondegradable matter into degradable compounds but by various means allow the degradation potential to be realized.Breakdown of solids to smaller particles to their solubilisation (so-called hydrolysis) are the underlying phenomena responsible for success of pretreatments.Physical processes such as mechanical grinding and heat treatment and chemical treatments such as acidification or alkaline treatment are alternatives that have been explored.Obviously any pretreatment option involves energy and capital inputs.The question is whether the outputs from energy to reduced residual solids and their disposal costs (energy, labor, land) exceeds the values of the additional inputs for the SPT.The performance and potential benefits of various SPT options have been researched over the past 15-25 years.A comprehensive evaluation of available and sufficiently comparable data for all SPTs was performed to compile their energy demand and performance improvements.Organic matter hydrolysis by ultrasound, alkaline, conventional heating, and microwave SPTs prior to AD has been shown to improve biogas production (10#-40#) and reduce solids (20#-50#).A comprehensive mass-balance spreadsheet model was created and used to evaluate these four SPTs.Well-established energy equations and wastewater characteristics were used.The model was checked against other models to validate it.From simulation of a number of operating conditions and flow regimes for WWTPs the utility of SPTs was ascertained.The most influential parameters impacting the energy balance were determined along with the conditions which make SPT favorable.Influent solids concentrations,SPT effluent temperatures, and AD treatment temperatures are the most significant parameters affecting SPT efficacy.Cost gains from heat recoveries at the SPT and mesophilic AD are crucial for practical SPT scenarios to be obtained.Tipping fee and dried solids content variables at the sludge dewatering unit are more important than the hauling distance in contributing to energy efficiency of SPT.Thermophilic AD energy demands are more significant than SPT energy demand increases and thus, thermal SPTs are less costly with thermophilic AD than mesophilic AD.