Sludge footprints of municipal treatment plant for the management of net useful energy generation beyond energy neutrality

The paper devised a unified basis for energy recovery processes in municipal treatment plants based on sludge footprints. Emphasis was placed upon innovative energy recovery technologies to test whether energy neutrality could be sustained or even, net useful energy could be produced. In this context, it evaluated first the energy potential of municipal wastewater treatment plant based on sludge footprints: it adopted a unified perspective where energy recovery was considered as an integral part of energy conservation in sludge. Sludge footprints of different activated sludge configurations were first analyzed based on latest concepts of process stoichiometry and modeling. The performance indicators of process modifications were analyzed and interpreted for energy conservation for a wide range of sludge age between of 0.5-50 d. An influent COD level of 500 mg/L was adopted, also defining all significant COD fractions compatible with related characterization studies. The role of primary sedimentation was accounted for where applicable. Calculations showed that 1259 kcal/m(3) (75%) to 379 kcal/m(3) (23%) of COD energy could be preserved in sludge as the sludge age was increased between the range of 0.5-50 d. Conventional activated sludge (CAS) conserved around 35-40% energy, corresponding to 600-650 kcal/m(3). Primary sedimentation boosted the overall energy conservation to 901-990 kcal/m(3), corresponding to 54-59% of the available energy in sewage COD. As expected, energy recovery by means of anaerobic digestion (biogas), remained far below the energy consumption in the treatment plant. This study envisaged syngas production by means of high temperature pyrolysis as a novel alternative instead of sludge digestion. Pyrolysis revealed a potential to yield a net useful energy (positive net energy) of 0.08-0.49 kWh/m(3) for systems operated at sludge ages of 2-10 d without primary sedimentation; much higher net useful energies of 0.33-0.59 kWh/m(3) could be obtained in systems with primary sedimentation operated at sludge ages 2-14 d. The study provided conclusive evidence that the real problem in establishing energy neutrality in treatment systems was the low energy recovery of the conventional sludge digestion and not the level of energy conservation in the activated sludge process; therefore, the step forward should look for more effective energy recovery processes such as high temperature pyrolysis (syngas generation) or similar novel technologies. (C) 2019 Elsevier Ltd. All rights reserved.