Energy analysis and numerical evaluation of the decanter centrifuge for wastewater management to allow a sustainable energy planning of the process

The decanter-centrifuge is widely used for dewatering and thickening of civil and industrial sludge. The latest generation decanters, both the bowl motors (main) and the screw (back-drive) are often driven by variable- frequency drives (VFD), with the back-drive able to recover the energy during braking. We created a decanter centrifuge energy model equipped with a braking recovery system during the sludge dewatering process, with the aim of identifying its optimal operating conditions for both energy consumption and product quality. Specific models at 15–20–25 m3 h 1 and various differentials speed (Δn) were used to derive a general model, then validated with experiments at 18–20 m3 h 1. Specific models used to identify the best operating conditions in terms of specific energy (e) and energy recovery (ERec) show that at 15–20–25 m3 h 1 the lowest energy consumptions were 1.88–1.76–1.57 kWh m3, respectively, instead, ERec was 5.88–0.31–12.10 kW respectively, highlighting that a high recovery is not necessarily linked to an increased energy saving. The accuracy of these models was confirmed by high values of correlation coefficients R2 and very low Root Mean Square Errors (RMSE) in each case. The general model, extrapolated from the specific models, makes it possible to predict specific consumptions at different flow rates within the operating range of the decanter. This was validated with an experimental test at 18–20 m3 h 1 with R2 above 97 % and RMSE 2,59E-02 kWh m3. The dry matter content in the cake decreases when the Δn or the feed rate increases. In conclusion, the decanter centrifuge model could represent a useful tool for optimizing the sludge dehydration process.