Viscous heating technology can destroy disease-causing microorganisms with no additional heat input. A laboratory-scale unit was constructed and tested with a simulant, and viscous heating achieved temperatures as high as 190°C. This study discusses additional variables – length and spacing – that are important to process design and optimization. The viscosity (μ) was described as a function of shear rate (γ̇); μ = 140 Pa s for t = 0 s and μ = 32*(γ̇)−0.6 Pa s for t > 0 s. The advantages of viscous heating to sanitize fecal mass are presented. The results show temperature gradient is more sensitive to changes in gap spacing than reactor length. For high throughput, the viscous heater length must be increased to provide fluid sufficient residence time to achieve the desired effluent temperature.