Sludge pre-recycle control of dynamic enhanced biological phosphorus removal system: an application of on-line fuzzy controller

This study presents a specific process control strategy, i.e. sludge pre-recycle control, for a dynamic enhanced biological phosphorus removal (EBPR) system by integratedly considering two factors: (1) appropriately controlling F/M ratio in the anaerobic zone and (2) eliminating hydraulic shock in the secondary clarifier. Fuzzy logic inference is also introduced while constructing an on-line controller owing to the ill-defined characteristics of the dynamic enhanced biological phosphorus removal system. For comparison, continuous-flow pilot plant experiments with automatic monitoring and real-time control facilities are performed using two control strategies: sludge pre-recycle control and conventional (F/M)an control. Experimental results indicate that similar effluent ranges of dissolved COD and dissolved phosphate can be obtained from the two strategies due to their similar (F/M)an ratios. However, reliability analysis reveals that sludge pre-recycle control improves the effluent total SS concentration significantly more than conventional (F/M)an control because of the reduction of hydraulic loading in the clarifier. Therefore, sludge pre-recycle control is highly desired because of its ability to reduce the hydraulic loading in the secondary clarifier at peak loading period and, at the same time, maintain a similar (F/M)an ratio in the biological reactor as conventional (F/M)an control. Moreover, statistical correlation of effluent qualities indicates that suspended COD and suspended phosphorus contents are proportional to total SS. Based on these findings, we can infer that when in the operation stage, the effluent total SS control is of critical consideration to system performance. This inference verifies the effectiveness of sludge pre-recycle control by introducing the hydraulic loading factor into the control strategy.