The effect of air-induced velocity gradient and dissolved oxygen on bioflocculation in the trickling filter/solids contact process

Abstract

The trickling filter/solids contact process (TF/SC) was developed in the late 1970s to improve the final effluent quality of existing trickling filters, to be able to meet stricter effluent EPA requirements. Although the TF/SC process has been successful in achieving this goal, it is still not fully understood, and there is limited information regarding the factors affecting the flocculation phenomena occurring in the solids contact chamber (SCC), such as the effect of bubble size and dissolved oxygen on bioflocculation efficiency. To better understand the kinetics of bioflocculation in a continuous flow solids contact chamber, a long-term experimental program was conducted using a TF/SC pilot plant constructed at the Marrero, Louisiana wastewater treatment plant. This program, developed from January 1998 through October 2000, included a detailed investigation of the kinetics of the bioflocculation process taking place in the SCC. This paper deals with the effect of air-induced velocity gradient on bioflocculation. Analysis of the observed data demonstrates that excellent final effluent quality can be obtained with a wide range of air-induced velocity gradients ( G) and with dissolved oxygen levels as low as 0.5 mg/l. However, from the point of view of SCC design, a value of G=25/s, achieved using a fine-bubble aeration system, and dissolved oxygen (DO) levels between 1.0 and 1.4 mg/l, are recommended to consistently produce a very good final effluent. This study also demonstrates that additional mechanical flocculation units would not be needed if a fine-bubble aeration system were used in the SCC and the floc structure could be preserved while transferring the floc from the SCC to the secondary sedimentation tank. However, considering that floc breakup frequently occurs during mixed liquor transfer from the SCC to the secondary clarifier, flocculator center wells should be used as a precaution in final clarification units to re-flocculate the mixed liquor.