Heat integration of two-stage autothermal thermophilic aerobic digestion system for reducing the impact of uncertainty

The operation of Autothermal Thermophilic Aerobic Digestion (ATAD) wastewater treatment systems is subjected to daily uncertainties regarding the parameters of the inlet streams, which lead to prolongation of the process and increase the energy consumption. This, together with the heat loss in the environment from the product tank, makes the ATAD system energy inefficient. Heat integration plays an important role in improving the sustainability of ATAD systems. In this study an approach for optimal redesign of heat integrated two-stage ATAD system operating under uncertainties is presented. It includes a two-stage stochastic optimization model for heat integration with an optimization criterion - the annual capital cost for redesign of the heat integrated system and its operating costs. Artificial Neural Network (ANN) models of the bioreactors are used for simulation of the system operation. They are combined with the heat integration model for the obtained with the stochastic approach values of design parameters of the heat integration equipment. The simulation results show a thermal shock reduction of 5–6 °C, an increase of the inlet sludge temperature of 8–10 °C, achievement of close to normal operating temperatures in the ATAD system and reduction of solids in both bioreactors by about 2 wt%. •Heat integration framework for Autothermal Thermophilic Aerobic Digestion system.•Modeling the heat integration process.•Two-stage stochastic optimization problem for annual capital cost minimization is defined.•Simulation of operation of heat integrated ATAD system.•The results show reduction of thermal shock and volatile solids content of the sludge.