The Effect of Mercury on the Activated Sludge Process
PAUL D. ZUGGER, Sanitary Engineer
Michigan Water Resources Commission
Lansing, Michigan
MRINGANK.A M. GHOSH    Associate Professor
Department of Civil Engineering
University of Maine
Orono, Maine
INTRODUCTION
The problem of mercury pollution came to focus when a recent Canadian report
indicated the presence of high concentrations of mercury residues found in the fish taken
from Lake St. Clair. Later investigations in this country revealed that mercury pollution of
environment was widespread. Spurred into action by these findings, regulatory agencies
suddenly prohibited the sale of fish containing undesirable amounts of mercury.
Reportedly, more than 80,000 tons of mercury was consumed in the U.S. in the last
century (1). The major contributor of mercury to the environment is the chlorine-alkali
industry, the largest commercial user of the metal in this country. Losses of mercury of up to
0.2 pounds per ton of chlorine produced have been reported (2). The second largest
consumptive use of mercury is in the manufacture of electrical apparatus (3). Mercury is
also used rather extensively in the production of fungicides, plastics, petrochemicals and
photographic chemicals.
It is conceivable that many of these industries are currently treating their wastes
separately, discharging treated effluents into receiving bodies of water, or are
contemplating joint treatment with municipal wastes using conventional biological
treatment processes. Recently, a survey was conducted in Michigan to determine the level of
mercury in raw wastewaters entering municipal treatment plants (Table I). With normal
mercury concentrations s high as 10/ig/l, slug doses of a much greater magnitude may enter
treatment plants. It is yet to be determined whether different chemical species of mercury
have any deleterious effect on the biotic environment. A recent literature survey on the
subject of mercury pollution revealed that there is a decided lack of published information
on the role of mercury in various biological processes. In particular, very little quantitative
information is available on the effect of slug doses of mercury on aerobic biological
wastewater treatment processes.
This paper is concerned with the effects of slug doses of mercury on aerobic batch
cultures of microorganisms similar to those found in the activated sludge treatment process.
TOXICITY OF MERCURY
Mercury is one of the most toxic of all heavy metals. Table II lists the 96-hour median
tolerance limit (TLm) in fish for certain heavy metals (5). The threshold concentration of
mercury (Hg++), i.e.rthe concentration at which metabolic inhibition becomes apparent, in
certain aquatic organisms was reported to be 0.03 mg/1 and the 96-hour TLm was 2.0 mg/1.
The concentrations of Hg++ from HgCl2 required to kill sticklebacks and guppies have
been found to be 0.008 and 0.02 mg/1, respectively (7). Anderson reported that the threshold
concentration of Hg++ for immobilization of Daphnia magna in Lake Erie water at 25 C
was less than 0.006 mg/1 (8). Comparing these toxic concentrations for Hg++ with those of
other heavy metals listed in Table II, mercury seems to be much more toxic than other heavy
metals. The concentration for complete inhibition of nerve excitability in humans has been
reported to be 68 mg/1 as HgCl2 (6).
792