High temperature pyrolysis of sewage sludge as a sustainable process alternative to anaerobic digestion for energy recovery

Wastewater is a mixture of pollutants that should be treated in wastewater treatment plants (WWTPs). The wastewater is considered as an energy source in terms of the organic matter it contains. While the WWTPs generate an amount of sewage sludge as a by-product, the energy in the wastewater is transferred into the sewage sludge. Sludge is considered as a renewable energy source as the other recycable wastes with the potential to contribute to the energy needs for both the developed and developing economies worldwide. Beside the biochemical technologies, thermal technologies are initiated to be used for recovering the stored energy of the sludge and for minimizing the environmental impacts at the same time. Pyrolysis is one of the most advantageous methods in terms of applicability as a sustainable process for energy recovery from sewage sludge. In this study, sewage sludge obtained from the drying unit of an extended aeration activated sludge system with 6 % moisture content and 65 % organic content was subjected to high temperature pyrolysis process. Pyrolysis temperature and rotation speed were investigated as the most important operational parameters to determine the diversity of the end products (biochar, tar, and syngas). The experiments were conducted on eight different runs (Run 1-Run 8) at three different temperatures (450 °C, 600 °C, and 850 °C) at a rotation speed of 10 Hz, 40 Hz and 60 Hz to represent the low, medium and high temperature pyrolysis conditions at batch mode. The effect of temperature and rotation speed on sludge pyrolysis was analyzed in terms of the mass balance, syngas composition, energy balance and energy recovery in the system. Comparative evaluation between pyrolysis process and anaerobic digestion was assessed in terms of energy recovery potential of both systems. The results showed that apart from the biochar, a potential of 59-71 % could be recovered in the form of tar and syngas in the pyrolysis experiments. The best condition in terms of energy generation was selected at high temperature and high rotation speed (850 °C, 60 Hz) as of 71 % energy recovery. It was observed that high temperature and high rotation speed provide a more efficient degradation of organics into syngas thus increasing the production of syngas. Syngas generated has a calorific value of 3520 kcal/m3 that represents an energy potential of almost 0.5 m3 natural gas at high temperature and high rotation speed. The high temperature pyrolysis produced a syngas of approximately 0.64 m3/kg organic sludge. The energy generation was calculated as approximately 3560 kcal/kg organic sludge at the best condition of high temperature and high rotation speed pyrolysis whereas a generation of 2070 kcal was theorotically calculated in anaerobic digestion for the same amount of sludge. For ensuring the operational conditions in both pyrolysis and anaerobic digestion systems, a certain amount of energy generated is utilized for internal energy requirement, yielding a net production of 3030 kcal/kg organic sludge for pyrolysis whereas 1700 kcal/kg organic sludge and 2600 kcal/kg organic sludge for mesophilic and thermophilic anaerobic digestion, respectively. It can be concluded that the pyrolysis process eliminated the limitations of anaerobic digestion in terms of organic matter destruction and energy generation. In response to the severe constraints applied to the treatment of sludge and the soaring prices of fossil fuels, the pyrolysis of sewage sludge has been attracting extensive attention in recent years as an economic and environmentally acceptable route for energy recovery. Besides the benefits of pyrolysis technology such as less pollutant than conventional methods, generation of syngas can be used for steam and electricity production for energy recovery in WWTP.