Pathogens and Public Health Concerns with Composting
By Chris Cronin
Pathogens
A human pathogen is any organism than can
cause disease. These substances are bacteria,
viruses, parasites, fungi, and toxins. There are two
different types of pathogens: primary pathogens and
secondary pathogens. Primary pathogens are those
that infect healthy humans, and secondary pathogens
infect debilitated individuals.
There have been numerous studies on pathogen
content in the composting process. There have been
some minor incidents of human affliction involving
pathogens as a result of the use of raw manures, or as
a result of contaminated drinking water.
For instance, in San Jose, California literally
hundreds of people were affected by a nearby
composting yard. This case illustrates the importance
of carefully siting compost facilities with adequate
setbacks from residential areas. One study,
presented at a BioCycle conference recommended
two miles isolation distance from residential and high
travel areas.
Composting in a building reduces the risk of
exposure, but it is less expensive to compost in an
open air facility. Determining when to turn the
compost at an open air facility requires more care
because Aspergillus -- and several other mold
spores -- become airborne when the material is
turned over to promote decay.
Most studies have shown Aspergillus and other
fungi to have higher concentrations at the composting
site, but without leading to any infections. The fact is
that we are exposed to the same mold spores every
day, in most bedrooms, bathrooms, and all kinds of
dust. To reduce exposure to sensitive individuals,
compost facility operators look for maximum isolation
distances, proper wind direction, and adequate
moisture to reduce dust.
Class A and Class B
The mere presence of pathogens does not mean
that infections will follow. There must be sufficient
quantity to cause infection. The level of pathogens in
treated waste materials is an issue addressed by the
EPA’s 503 regulations, which were designed
primarily for digested human sludge and septage
(biosolids). There are two designated classes for
biosolids with respect to pathogens: Class A and
Class B. If pathogens are below detectable levels,
the biosolids meet the Class A designation.
Biosolids are designated Class B if pathogens are
detectable but have been reduced to levels that do
not pose a threat to public health and the
environment as long as actions are taken to prevent
exposure to the biosolids after their use or disposal.
Indicators
Since pathogens are not easily detected or
cultured, methods have been developed which
detect the presence of “indicator” organisms. If
these organisms are absent, the probability of the
existence of bacterial pathogens is minimal. The
total coliform group is the most inclusive indicator
classification. For a class A compost, which can be
given or sold to the general public, the density of
fecal coliform in compost must be less than 1000
Most Probable Number (MPN) per gram of total
solids (dry weight basis) at the time of use or
disposal. For a class B compost, the density of
fecal coliform must be less than 2000 MPN per
gram of total solids.
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Reducing Pathogens
In order to reach these standards, there are
various methods. For Class B these are called
Processes to Significantly Reduce Pathogens
(PSRP). There are five possible processes: aerobic
digestion, air drying, anaerobic digestion, lime
stabilization, and composting. To reach class A
regulations, there are six alternatives to achieve the
“Process to Further Reduce Pathogens” (PFRP).
They must be thermally treated, treated in a high
pH-high temperature process, treated to reduce
enteric viruses and viable helminth ova, or another
process equivalent to a PFRP.
to rely on external carbon sources. Thus an active
microflora in the soil or compost will often prevent
disease since the pathogens are outcompeted.
Specific suppression, on the other hand, is
usually explained by one or a few organisms. They
induce systemic resistance in the plant to specific
pathogens, much like a vaccination. With specific
suppression, the causal agent can be clearly
transferred from one soil to another. Recently, Ohio
State University researchers demonstrated that the
beneficial microbes in compost and other
decomposing organic matter can activate certain
disease-resistance systems in plants.
Pathogens in Food
As far as food wastes are concerned, several
organisms are generally associated with food borne
illness and they are responsible for more than 90% of
recorded food borne illnesses: Salmonella, Listeria,
E.coli, Staph.Aureus, and Aflatoxin.
Vegetable food waste is extremely low in
pathogens, and is often made up of mostly water.
Meats on the other hand can have pathogens.
Surprisingly, unground beef and pork do not usually
contain many pathogens. On the other hand, poultry
like chicken and turkey is very likely to have
contaminants. Much of this has to do with the
processing and packaging.
With poultry, conditions often occur for
salmonella. At times the poultry’s stomach is
punctured and this causes leakage onto the intestinal
wall. This usually results in contamination. In fact,
most poultry should be considered “precontaminated” upon purchase. Because most
pathogens are killed off in the cooking process,
pathogens from food waste generally only make it to
compost sites in the form of uncooked or exposed
spoiled food.
Possible Regrowth
Regrowth in composts that were not fully
stabilized has been documented. Thus a compost
could have met processing requirements and
standards for E. coli or Salmonella, but could
subsequently have significant bacterial levels if
regrowth occurs after testing. This speaks to an
important ingredient in the neutralization of
pathogens; the compost pile must be heated
consistently. An inconsistent compost heating
system leads to better chances that pathogens will
survive, or regrow. Maturity is important for
applications involving food crops or human
exposure
Eliminating Pathogens
The scientific community is mostly in agreement
about the reduction of pathogens through composting,
though it is not exactly clear how. Studies have found
that through the composting process, organisms are
exchanging genetic material. When this occurs, the
pathogens often get ‘gene-traded’ out of existence.
In cases where the pathogens survive the process,
they seem to have lost their pathogenetic make-up
and are no longer harmful.
“Pathogenic Health Aspects Of Land Application,”
BioCycle. September 1998, p.62-67.
“Aspergillus Fumigatus & Composting,” Composting
Council Fact Sheet.
USEPA. 1994. “A Plain English Guide to the EPA Part
503 Biosolids Rule.
“Food Product Laboratories,” Food Products Lab., Inc.
Center for Sustaining Agriculture & Natural Resources
Disease Suppression in Compost
Pathologists describe two different types of
disease suppression in compost and soil. General
suppression is due to many different organisms that
either compete with pathogens for nutrients and/or
produce general antibiotics that reduce pathogen
survival and growth. This type of suppression is
effective on those pathogens that are small in size,
which results in small nutrient reserves and the need
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