Particular internal recirculation frequency scope for enhancing denitrifying phosphorus removal in an oxidation ditch

Abstract

This study investigated the influence of the unique internal recirculation characteristics of an oxidation ditch (OD) system, namely, the internal recirculation frequency (IRF) on denitrifying phosphorus removal (DNPR). The ratios of denitrifying polyphosphate-accumulating organisms (DPAOs) to polyphosphate-accumulating organisms (PAOs) under different IRF conditions were measured using a batch experiment. On this basis, the variation of nutrient transformations were studied using the IRF changes by the mass balance method. The results showed that for the OD system that had an anaerobic zone upstream from the circular corridor and set anoxic and aerobic zones along the circular corridor, when the IRF was between 3.4 h−1 and 7.5 h−1, the DPAOs/PAOs ratio reached about 50%. Approximately 20% of the total phosphorus (TP) was removed and over 11% of the total nitrogen (TN) was transformed into nitrogen gas by the DNPR process, and meanwhile the total removal efficiencies of the TP and TN were over 93% and 80%. When the IRF was greater than 11.5 h−1, the TN removal efficiency decreased significantly, and this was not conducive to simultaneous nitrogen and phosphorus removal. The results indicated that OD process would possess a better DNPR potential if the IRF was controlled within the proper scope.

Parameters to characterize the internal recirculation of an oxidation ditch

“…where T is the circulation period, namely the time consumed for the mixed liquor to complete one lap in the circular corridor (h) (Wang et al, 2019b).…”

“…where IRF is the internal recirculation frequency, namely the laps performed by the mixed liquor in the circular corridor during 1 h (h -1 ) (Wang et al, 2019b).…”

“…According to the influent ammonia concentration and the expected effluent nitrate concentration, Argaman (1984) obtained a simplified formula of IRR in OD based on the requirement of nitrified mixed liquor needed to return to the anoxic zone during a traditional denitrification process. The internal recirculation period (T), i.e., the time taken for the mixed liquor to complete one lap in the circular corridor (Wang et al, 2019b), is another parameter that can characterize the internal recirculation of an OD system, and was occasionally used. For instance, based on the oxygen supply and demand balance and the invariable number of aerators, Chen et al ( 2012) studied the effectiveness of a dual DO control technology in OD systems when T varies with the recirculation speed.…”