FREQUENCY DISTRIBUTIONS OF SECONDARY
AND TERTIARY EFFLUENT PARAMETERS
Joseph A. Fitzpatrick, Assistant Professor
Charles L. Swanson, Research Assistant
Northwestern University
Evanston, Illinois 60201
Wesley O. Pipes, Professor
Department of Biological Sciences
Drexel University
Philadelphia, Pennsylvania  19104
INTRODUCTION
Over the past few years many wastewater treatment plants have been adding some
type of filtration process following secondary treatment in order to achieve additional
removal of suspended solids and BOD.   The designs used for these tertiary wastewater
filters have mostly been adapted from experience with the design of filters for water
treatment plants.   Many questions about how performance is related to design and operation of the filters, performance of the secondary process preceeding the filter, and wastewater characteristics are yet to be answered.
One way to study full-scale filter performance is to compare the quality of a secondary
effluent with a tertiary effluent and the quality of tertiary effluents produced by different filters.   These comparisons should be made on a statistical basis.   This requires a
knowledge of the frequency distributions of effluent parameters.   Knowledge of these
frequency distributions is also essential for the proper design of data collection programs
for survey and enforcement monitoring, and for establishment of effluent limitations.
There are several reports in the literature in which the frequency distributions for
effluent parameters have been characterized to some extent [1-3J.   Hoogerhyde [1] indicated that tertiary filter effluent suspended solids are normally distributed.   Dean and
Forsythe [3] have argued that effluent parameters from a wide variety of treatment
processes should have a log-normal distribution.   There are also other frequency distributions which might adequately characterize effluent parameters.
A recent study at Northwestern University has the objective of examining the performance of several tertiary wastewater filters of different designs operating under various
conditions.   In the first part of the study the distributional properties of the effluent
quality parameters are characterized.   This chapter deals with the distributional properties
of secondary and tertiary process flow, total and soluble five-day biochemical oxygen
demand (BOD,) for secondary and tertiary processes at two different wastewater treatment plants.
DATA COLLECTION
The two plants are the Addison, Illinois North and South Plants operated under
authority of the Village of Addison Department of Public Works.   Secondary treatment
at the South Plant consists of a trickling filter and an activated sludge process in parallel
The combined secondary effluent passes through an equalization basin before entering
the filters.   The tertiary process consists of seven Smith and Loveless dual-media filter
units operated in parallel.   The North Plant has a contact stabilization process followed
by three Smith and Loveless filters.   There are no storage or equalization tanks between
the secondary and tertiary processes at the North Plant.
Filter design data for both plants are shown in Table I.   Average flow to each plant
is approximately 2 mgd.   Average filter flow rate at the North Plant is almost twice
1034