Energy efficiency optimisation of wastewater treatment: Study of ATAD

The aim of this paper is to minimise the energy requirement of autothermal thermophilic aerobic digestion (ATAD). To this end, a dynamic ATAD model is presented and assessed. A global sensitivity analysis was performed to identify the operating conditions with the strongest impact on the energy requirements, and thus to choose the most promising optimisation variables. The latter turned out to be the aeration flowrate, the reaction time, and the sludge flowrate. The optimisation problem was formulated following the sequential approach for dynamic optimisation, due to the discontinuous nature of ATAD. The problem was implemented in MATLAB® and solved for two case studies using the eSS algorithm, a global scatter search method that alternates with local algorithms (in our case fmincon) to refine the best solutions. The two selected full-scale case studies include a single-stage and a two-stage system. For the former, a 22% improvement of the energy requirement was achieved after optimisation, and 18% for the latter. Despite its advantages and common use in other fields, optimisation is still relatively rare in wastewater engineering. In the light of the high, rising cost of wastewater treatment, optimisation should become the norm when it comes to design and operation of wastewater treatment plants.

► Development of dynamic ATAD model.
► Model assessment and parameter estimation.
► Sensitivity analysis: aeration and sludge flowrate and reaction time have strongest impact on energy requirement.
► Dynamic optimisation of two ATAD plants with 18 and 22% reduction of energy requirement.