ties are readily removed by a "wetting front" moving
We wondered whether the outer membrane of mi-
through the snowpack.
croorganisms would be destroyed by this process given
We found that relatively little has been published in
that gram-positive bacteria have an inner cell membrane
peer-reviewed journals on the ability of this process to
but no outer membrane and that other microorganisms,
remove organic and inorganic contaminants. Most of
such as viruses, consist only of a nucleic acid (either
the publications that present data (Zapf-Gilje et al. 1986,
DNA or RNA) contained in a protein or lipoprotein coat
Rabinowitz et al. 1988) are based on the thesis work of
and thus do not have a cell membrane. Because of dif-
Zapf-Gilje (1985). Zapf-Gilje (1985) reported that, on
ferences in the cell wall composition and membrane
average, 86% of the N and 65% of the P were concen-
differences between gram-positive and gram-negative
trated in the initial 20% of the snowmelt. He attributed
bacteria, we wondered whether gram-positive bacteria
the lesser degree of concentration of P to its high affin-
would be more resistant to the effects of this treatment.
ity for particulate matter in the snow. For some metals,
We also wondered whether the repeated freezethaw
such as iron and manganese, there was no evidence of
cycling would have any effect on bacterial survival in
any concentration (Rabinowitz et al. 1988). Passage
these systems, as reportedly found by the Ontario Min-
through a soil column is a key element in this treatment
istry of the Environment in its unpublished report.
process (Rabinowitz et al. 1988).
To answer these questions, we conducted a litera-
Delta Engineering describes the removal of biologi-
ture review on the effects of freezing and thawing on
cal contaminants as follows. During snowmaking, "the
bacteria and conducted studies to determine the effect
wastewater is atomized into small droplets, which freeze
of this process on bacteria.
rapidly. The mechanical forces inherent in the freezing
process, combined with the rapid freezing and expan-
LITERATURE REVIEW
sions of the droplets, cause rupture of the outer mem-
brane of the microorganisms, thereby killing them. A
Effects of chilling, freezing, low-temperature
relatively few surviving pathogens held within the
storage, and warming on bacteria
snowpack are either too severely damaged to reproduce,
or are ultimately eliminated through exposure to the
Bacteria can be injured or die as a result of cold
sun's ultraviolet rays. Studies show that SnowfluentTM
shock, freezing, storage at low or subzero temperatures,
disinfects wastewater more effectively than any tradi-
and subsequent warming. Cold shock is caused by sud-
tional treatment methods such as chlorination, ozone,
den chilling without freezing. Studies have shown that
or UV radiation."
cold shock can damage the cytoplasmic (inner) mem-
Again we found relatively little information pub-
brane and DNA of bacteria and can damage the outer
lished in the peer-reviewed literature on the effective-
membrane of gram-negative bacteria (MacLeod and
ness of this type of process on biological contaminants.
Calcott 1976, Mackey 1984). Freezing and thawing has
The literature indicates that there is partial destruction
been shown to damage the cytoplasmic membrane, cell
of some species of bacteria, especially coliforms, but
wall, and DNA (MacLeod and Calcott 1976, Mackey
that bacteria are not completely destroyed by this pro-
1984, Ray 1989). When the cytoplasmic membrane is
cess. Rabinowitz et al. (1988) reported that snowmaking
damaged, low molecular weight materials (such as po-
tassium and magnesium cations [K+, Mg2+], inorganic
reduced total coliform and fecal coliform concentra-
phosphate, and amino acids) are lost from the cell, and
tions by 50%. In Zapf-Gilje's thesis (1985), he reported
there is an increased penetrability of small molecular
work described in an unpublished draft report by the
weight compounds, such as toxic metals, into the cell
Ontario Ministry of Environment (1982) that revealed
(MacLeod and Calcott 1976). Researchers have attrib-
the following. There was more than a 99%, or 2-log
uted death and injury to one or both of these processes.
reduction, in total and fecal coliforms during snow pro-
However, depending upon the species and the surround-
duction, with further reduction of total coliforms in the
ing medium, many of the cells injured by these pro-
snowpack. Other strains of bacteria had higher survival
cesses can self-repair.
rates during snowmaking, but no details were given.
Destruction was attributed to the freezing process because
Cold shock
microorganisms in atomized, unfrozen wastewater were
Both gram-positive and gram-negative bacteria have
not killed while those in atomized, frozen wastewater
been shown to be affected by cold shock (MacLeod
were. Unfortunately, there was no description of the test
and Calcott 1976). Factors that affect the sensitivity of
methods that were used in this unpublished study. Fur-
cells to cold shock include age (cold shock usually oc-
thermore, we could find no information on the survival
curs in cells harvested in the exponential growth but
of any other types of microorganisms of health concern,
not stationary phase), composition of medium in which
such as helminths, viruses, and protozoa.
2
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