Design Parameters and Performance of
Biological Systems for Textile Plant Effluents
WILLIAM R. DOMEY, Environmental Engineer
Chas. T. Main, Inc.
Boston, Massachusetts 02199
INTRODUCTION
Biological oxidation is not only the most widely used treatment process for effluents
from textile dyeing and finishing operations, but is also one of the most economical and
potentially capable methods. Considering the wastewater flows usually encountered, along
with land availability, the aerated lagoon or extended aeration modification of activated
sludge have been used, with the former method predominant in the southern portions of the
country. Recent trends of the industry to use chemicals more resistant to degradation, in an
effort to produce more stable dyes has sometimes resulted in the deterioration of
performance of such systems, particularly from a standpoint of BOD and color removal,
which often were quite successfully achieved using conventional design criteria. Based on
current trends, the principle pollutants can be considered to be such parameters as COD,
BOD, color (dye), acids (sulfuric, acetic, formic, oxalic, etc.), alkalies, oxidizing agents and
reducing compounds, sulfides, detergents, heavy metals (chromium and zinc), carriers
(chlorobenzenes, ortho-phenylphenol, bi-phenyl, etc.), oil, and fibres. In order to upgrade
existing facilities, and to design new systems, some evaluation must be made of the process,
with regards to its capability to meet today's and the future's effluent standards. As a first
step, design procedures should be such that biological treatment will be able to operate at
maximum capability. This requires the development of kinetic parameters, to the extent
that design methods using mathematical models might be utilized. The associated color
removal with the process must also be evaluated. A study has been made to determine such
criteria, using data developed by both measurements and observations on full-scale field
systems, as well as continuous flow laboratory scale biological reactors. In the latter,
wastewater samples were composited from actual textile plant operations.
These observations showed that where the land area is available, the extended aeration
system would be the method of choice. This is because of the minimal amount of solids
handling required, the improved color removal, and the increased oxidation of reducing
compounds, such as sulfides, that may be present, which would interfere with the dissolved
oxygen in the effluent and would also increase the chlorine demand. A flow diagram of the
"typical" extended aeration system for textile effluents is shown in Figure 1. An additional
advantage of the extended aeration system, particularly when a complete mixing flow
regime is utilized, is that it dampens the impact of the variations in flow, waste strength, pH,
and color, that are predominant in the textile industry.
BOD — COD REMOVAL MODEL
For the purpose of design, as well as evaluation of existing field scale systems and
laboratory scale systems for parameter development, the use of the models proposed by
Lawrence and McCarty (1) were used, a brief description of which follows.
Extended Aeration System
Figure 2 shows a schematic diagram of a complete mix aeration tank followed by a
clarifier with sludge recycling. Since the basic concept of design is to achieve organic
removal with a minimum of sludge production, no provision is included for wasting of
sludge.
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