Trickling Filter Versus Activated Sludge.
When to Select Each Process
DON F. KINCANNON, Associate Professor
JOSEPH H. SHERRARD, Assistant Professor
School of Civil Engineering
Oklahoma State University
Stillwater, Oklahoma 74074
INTRODUCTION
Secondary treatment of wastewaters containing biodegradable organic matter usually
requires a choice between the trickling filter process or the activated sludge process. A
rational procedure to follow to select either of these processes for a proposed wastewater
treatment project appears to be needed because standard textbooks either ignore proposing
a selection method or only list advantages and disadvantages of each process in vague
terms. Typical historical comparisons between these two processes yield such concepts as
trickling filters: a) require more land area, b) are more costly to construct, c) may cause fly
nuisance and odor problems, and d) are less sensitive to shock loads. Activated sludge
processes, however, are credited with: a) the ability to produce a higher quality effluent, b) a
quicker response to control measures, c) being difficult to operate and therefore unstable
and unreliable, and d) a production of sludge in excess to that found from trickling filters.
Advantages and disadvantages, such as those listed, may or may not describe
adequately either of the two processes because of the wide assortment of modifications
found within a single process, i.e., high-rate versus low-rate activated sludge and high-rate
versus low-rate trickling filters. At best, therefore, previous comparisons have given the
design engineer a poor basis for comparing the two processes.
The purpose of this paper will be an attempt to offer an alternate and more reasonable
basis for comparison of these two biological processes. To accomplish this objective a
comparison of process similarities and differences will be presented, a discussion of process
modifications will be given, a comparison of modifications will be made, and a criteria for
process selection will be established.
PROCESS SIMILARITIES AND DIFFERENCES
To establish a criteria for choice between the two treatment processes requires a
knowledge of the manner in which wastewater is treated in each process and how the
process can be operated to optimize desired results. Basically, both processes are aerobic in
nature and removal of organic compounds from solution occurs as a result of microbial
metabolic activities. Carbon compounds are both incorporated into cellular tissue and
oxidized to metabolic end products.While the trickling filter process relies on microbial
growth on media surfaces, the activated sludge process required the formation of flocculated
microorganisms held in suspension in an aeration basin.
Differences between these two processes result due to physical parameters. Besides the
difference between microorganisms held in suspension or attached to media surface,
oxygen is supplied by mechanical or diffused air aeration to the activated sludge process
and is supplied by natural air convection currents to the trickling filter. An   additional
difference is the operation of recycling. While microorganisms are recycled back to the
aeration basin in the activated sludge process, clarified effluent is returned to the trickling
filters.
A summary of differences and similarities are listed in Table I. Typical process flow
sheets for each process are shown in Figure 1. As shown, the flow diagram for the trickling
filter process is composed of towers of plastic media (as opposed to rocks) because of the
growing trend toward use of plastic media.
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