Performances of coffee husk biochar addition in a lab-scale SBR system for treating low carbon/nitrogen ratio wastewater

In this study, coffee husk was used as input raw material to produce biochar by slow pyrolysis process at 350 °C for 60 min. Coffee husk biochar with the size from 0.2 to 1.0 mm was introduced into sequencing batch reactor (SBR) system to evaluate the granulation process of activated sludge together with chemical oxygen demand (COD) and ammonium removal efficiency. Coffee husk biochar in the experiment acted as a bio carrier to promote the formation of the granular sludge in the system. Synthetic wastewater used in this study had COD/total nitrogen (C/N) ratio was adjusted gradually reduced from the optimal ratio of 20/1 to low C/N ratio of 3/1. In about 3 weeks of continuous operation, the granular sludge was initially formed with small size (less than 1 mm) and then gradually increased in bigger size and was stable in the size of 2–3 mm for long operation time (over 90 days). The treatment efficiency of COD and ammonium in the system was maintained (COD treatment efficiency was 90% and ammonium was nearly 70%). The results indicated that granular sludge formation process also possessed sludge volume index (SVI) reduction dramatically, sludge settled quickly and reduced the amount of sludge discharged. Therefore, using biochar could be a potential effective method to enhance granulation of activated sludge in SBR system for treating low C/N ratio wastewater. Graphical abstract