Toxicity of Chlorinated Municipal Wastes to Fish:
An Evaluation Based on Michigan Field Data
ROBERT BASCH
Bureau of Water Management
Water Resources Commission
Michigan Department of Natural Resources
Lansing, Michigan
INTRODUCTION
Chlorine is widely used to disinfect the effluent of municipal wastewater treatment
plants. It is probable that many more municipalities will be required to disinfect to meet
bacteriological standards for water quality (1). Michigan municipal wastewater treatment
plants have been required to continuously chlorinate their effluents since 1967. Recent
research by the EPA National Water Quality Laboratory in Duluth (2) and the Michigan
Water Resources Commission (3) have shown the urgent need for field work on the effect of
chlorinated municipal wastes on aquatic life. The present study was an effort to assess,
under field conditions, the effect of these wastes on fish.
Chlorine present in municipal wastewater treatment plant discharges is almost always
combined with ammonia, ammonium hydroxide or ammonium ions to form mono-, di-, or
trichloramine. It also combines with amino acids, proteinaceous and other organic matter
producing compounds with low disinfecting power. Chlorine also combines with sulfites,
sulfides, nitrites and ferrous or manganous ions to form compounds with no germicidal
power (4). Various researchers have investigated the comparative toxicities of free chlorine
and chloramines and found both forms extremely toxic to fish. Zillich (3) and Basch, et al
(5) have reviewed much of the available toxicity literature. Field investigations of the effects
of chlorine have been noticeably lacking. Tsai (6,7), conducted field surveys of the fish
fauna in three Maryland streams receiving chlorinated wastes. He found that the number of
species and fish abundance decreased drastically in the area immediately below chlorinated
sewage outfalls. Downstream from these outfalls, in organically enriched areas, the fish
community composition changed although there were no changes in species diversity
indices calculated. He also reported that during their spawning season upstream migrations
of white catfish and white perch were blocked by the municipal wastewater treatment plant
effluents concentrated in the area immediately below the sewage outfalls. Our
investigations were patterned after Wuerthele (8,9) in which he held fathead minnows in a
series of live boxes above and below a Michigan treatment plant outfall. He found nearly
complete mortality four miles downstream versus almost complete survival above the
discharge. Interpretation of results from this study was complicated by the presence of
industrial wastes in the municipal waste and an industrial discharge upstream from the
outfall.
METHODS
Studies were conducted at four different municipal wastewater treatment plants. Each
study included a chlorinated and a non-chlorinated phase, in which separate sets offish
were exposed in cages in the river above and below the outfall. The fish held below the
outfall were subjected to chlorinated wastes for 96-hours (120 hours in one study) and
mortalities noted after 48 and 96 hours. The treatment plant stopped chlorinating, new fish
were introduced and exposed for a similar period to the non-chlorinated wastes. During
both the exposures, chemical and bacteriological samples were taken to monitor possible
causes of mortality other than chlorine. Since no other toxic materials were observed
entering the plant or present in the river, differences in survival between the two exposures
were assumed to be due to the chlorinated waste compounds.
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