The   Effects of Recirculation on the
Performance of Trickling Filter Models
G. J. KEHRBERGER, Senior Research Engineer
Hydro Science Inc.
Leonia, New Jersey
A. W. BUSCH, Professor of Environmental Engineering
Rice University
Houston, Texas
INTRODUCTION
The trickling filter, a waste treatment system which utilizes a fixed bed reactor to biologically remove organics from municipal and industrial wastes, consists
of a bed of rocks, slag, clinkers or synthetic material with microorganisms grown
and maintained on the surface of the media. Wastewater is distributed over the
surface of the media by a fixed or rotary distributor. Atmospheric oxygen is transferred to the microbial mass through the liquid film. The liquid film thickness varies
because of periodic dosing, and thus liquid phase resistance to oxygen transport
varies. Hydrodynamically, the system may be represented as a liquid flowing by
gravity over a surface, as shown in Figure 1.
Many formulations have been developed to predict the phenomena occurring
within the fixed bed reactor. Classical representations of the efficiency of a filter,
(e.g., NRC formula (1), Rankin formula (2),Velz formula (3))were derived from full
scale domestic treatment facilities. These formulations were based primarily on
organic loading, and implied that recirculation increased system efficiency. However,
studies by Howland (4) and Schulze (5) in which hydraulic loading was considered
as a principal parameter, showed that effluent quality was not improved with
recirculation.
Curve fitting techniques employing regression analysis to fit a mathematical
model to field data were developed by Eckenfelder (6) and Galler and Gotaas (7).
Eckenfelder reported that plant efficiency is increased by recycling the effluent
around a filter, and that the overall effect of recirculation was only to dilute the
waste.
Galler and Gotaas, who used data from 322 treatment plants throughout the
country, indicated that although the hydraulic load applied to a filter had an insignificant effect on efficiency of the system, recirculation improved the performance of a filter.
An experimental investigation employing a synthetic medium, Dowpac, and
the formulation developed by Howland (4) was conducted by Germain (8). Germain concluded that recirculation had no effect on system efficiency.
A mechanistic approach to the development of trickling filter equations was
initially undertaken by Atkinson, et al. (9). The fixed bed system was modeled as a
film flow reactor, in which soluble organic compounds flowed over a vertical flat plate
under laminar and turbulent conditions with a biochemical reaction occurring
throughout the liquid phase. Their results predicted that recirculation improved the
effluent quality.
Additional film flow investigations examining the effect of recirculation were
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