The effect of recirculation on biological nitrification in rotating biological contactors (RBCs) was examined in an on-site pilot-scale RBC system which simulated full-scale plant RBC operations at the Longmont, Colorado, Wastewater Treatment Plant (LWWTP). Two LWWTP wastewater streams were used as pilot RBC influent. One wastewater stream, designated the “fresh” influent, was first stage trickling filter effluent. The second stream, designated the “recirculation” influent, was clarified secondary effluent. Three recirculation ratios, defined as the ratio of the recirculation influent to the fresh influent, were examined: 0 (no recirculation), 1 (50% of the flow is recirculation), and 3 (75% of the flow is recirculation). Each recirculation ratio was studied at three hydraulic loading rates.

Recirculation improved nitrification at all hydraulic loading rates. Improved nitrification with recirculation was due to the dilution of influent biodegradable organic carbon (BOD5) which occurs as a result of mixing secondary plant effluent with first stage effluent. An unexpected and operationally significant result was that extremely low concentrations of influent organic carbon did not improve nitrification.

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