The effects of hydraulic retention time on the sludge characteristics and effluent quality in an ASSBR

This study was performed to improve the settling performance of the solids in the activated sludge process without using any of the additives that have been used recently by researchers worldwide to tackle this issue. Four different hydraulic retention times (HRTs) have been studied to find their effects on the solids' settling and treatment efficiency in an aerobic suspension sequencing batch reactor (ASSBR). The results obtained from this study showed that increasing the hydraulic retention time from 6 h to 12 h led to improving the COD and nitrate-nitrogen removal efficiency; it was raised from 78% to 94% for COD and from 75% to 97% for nitrate-nitrogen. However, when the HRT increased from 12 h to 18 h, the removal efficiency of the COD and nitrate-nitrogen reduced from 94% - 91 for COD and 97% - 94% for nitrate-nitrogen The removal efficiency of COD and nitrate-nitrogen remained constant when the HRT was further increased from 18 h to 24 h. The solids' settling performance was monitored through sludge volume index (SVI) test and scanning electron microscopy (SEM). The SVI value was decreased from 123 ml/g to 82 ml/g when the HRT was increased from 6 h to 12 h, while it was not affected when the HRT was extended to 18 h and to 24 h. The optimal HRT obtained from this study is 12 h; it was superior for COD and nitrate-nitrogen removal and the solids settled steadily during that range.