This research was to determine the effects of hydraulic shock loads by intermittent feeding on the removal efficiency of a small prefabricated real-sized septic tank/anaerobic filter unit. Besides the ‘control’ 24-hour continuous runs the other twice a day feeding periods covered 16, 12 and 8 hours per day, with the tank's retention time varying from 22.5 to 90 hours. It was demonstrated that the variation of the hydraulic feeding patterns ranging from 24-hour continuous feed to 8 h per day intermittent feed did not affect the performance of the system much. The percentage of BOD reduction was decreased from 85 to 82 and 81 percent for the feed times of 24, 16 and 8 hours per day, respectively.

The tank capacity played a relatively major role in determining the removal efficiency of the process. The efficiency of BOD and SS reduction dwindled from 82.2 to 68.9 and 56.0% with the corresponding reduction in tank sizes from 90 to 45 and 22.5 hours retention, respectively. In any event, the unit could not satisfactorily remove nutrients. A retention period of not less than 48 hours is recommended if the Thai effluent standards are to be met. Equations for predicting the BOD removal capability were also developed.

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