The Effect of Surface Active Agents
on Substrate Utilization in an
Experimental Activated Sludge System
NINA I. MC CLELLAND, Staff Consultant
National Sanitation Foundation
Ann Arbor, Michigan
K. H. MANCY, Associate Professor
Department of Environmental Chemistry
School of Public Health
The University of Michigan
Ann Arbor, Michigan
INTRODUCTION
Since World War II, the popularity of detergents has increased steadily. One
billion lb were produced for household use in 1963. Current demand is increasing at
an estimated rate of from 3.8 to 5.0 per cent/yr (1).
Although they may be anionic, cationic, or nonionic in character, anionic
surfactants are by far the most important in the field of waste treatment. Their
concentration in untreated domestic waste water is reported to average ten mg/l (2).
Until June 1965, alkylbenzene sulfonate (ABS) was the surface active agent
most commonly used as the active portion of packaged household detergents. Because of its highly branched molecular configuration, ABS degrades relatively slowly
in conventional biological wastewater treatment systems. Linear alkylate sulfonate
(LAS) was developed as a replacement for ABS in response to public demands for
"biodegradable" surfactants.
Pre-market testing of detergent products with linear surface active agents was
limited almost exclusively to biodegradation rates and foaming characteristics (3,4).
A search of the literature failed to provide significant information on how the performance characteristics of conventional biological waste treatment systems would
be affected by their general use.
The purpose of this investigation was to determine: 1) to what extent "soft"
detergents affect the performance characteristics of an experimental activated sludge
system; 2) how this effect compares with that of a "hard" detergent; and, 3) to
provide a conceptual understanding of the mechanism of interference of compounds
with surface active characteristics on this type of treatment process.
Aerobic biological oxidation systems are used extensively for treating domestic
and industrial wastes. In the activated sludge process, a flocculent suspension of
microorganisms is developed in a fluidized, continuous flow reactor system. Organic
pollutants in the wastewater serve as substrates and are transferred to the biomass
matrices. Air is supplied to keep the system aerobic and to provide a dynamic
interface (contact opportunity) across which substrate transfer can occur.
The active biomass is a physiologically mature, heterogeneous population of
bacteria, fungi, protozoa, rotifers, and nematodes. After a defined aeration period,
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