Upgrading of anaerobic digestion of waste activated sludge by temperature-phased process with recycle

• A mesophilic and a temperature-phased digestion with recycle were constructed.
• Thermophilic-mesophilic and hyper-thermophilic-mesophilic conditions were operated.
• The upgrading process enhanced the hydrolysis of waste activated sludge.
• The two-stage thermophilic-mesophilic condition achieved more process improvement.

Thermophilic (55 °C)-mesophilic(35 °C) and hyper-thermophilic(70 °C)-mesophilic TPAD-R (temperature-phased anaerobic digestion with recycle) were conducted to compare and evaluate the operation performance to treat WAS (waste activated sludge) with the MD (conventional mesophilic anaerobic digestion). TPAD-R was based on the TPAD (temperature-phased anaerobic digestion), and introduced a recycle system from the end mesophilic stage to the front stage. Thermophilic-mesophilic TPAD-R produced more biogas 0.99 L/g VS (volatile solids) reduced/d than MD 0.83 L/g VS reduced/d. In thermophilic-mesophilic TPAD-R 35.7% and 18.7% of WAS was converted to methane in the thermophilic stage and in the mesophilic stage, respectively, according to a COD balance. Solids reduction was improved to a similar extent in thermophilic-mesophilic TPAD-R and hyper-thermophilic-mesophilic TPAD-R, which was 10% higher than that in MD (approximately 40% of VS reduction). Hydrolysis, acidogenesis and methanogenesis analysis indicated that thermophilic and hyper-thermophilic stage accelerated the hydrolysis rate of TPAD-R, 0.053 gCOD/gVS/d and 0.040 gCOD/gVS/d, respectively, compared to about 0.025 gCOD/gVS/d in MD. In addition, thermophilic-mesophilic TPAD-R achieved more surplus energy than hyper-thermophilic-mesophilic TPAD-R and MD.