[USCC] Maturity vs Stability
David Schellinger
dschellinger at agcenter.lsu.edu
Fri Sep 1 17:43:10 CDT 2006
Jim,
This is an interesting project you have undertaken, and one that may provide
very useful information. My opinion is that you try some methods that are
often ignored but are well established by literature. Conventional testing
for stability can provide erroneous results unless matched with other
parameters (as suggested by Matthew Cotton referencing the maturity index
testing program). It is not difficult to predict parameters as composts
stabilize and mature, but we usually discuss such issues in terms of trends;
temperatures decrease, oxygen demands decrease, ammonia decreases, nitrate
increases, etc. Perhaps a more active testing approach should be
implemented rather than relying on spot checks somewhere during the
composting process. Breaking from analytical tradition, it makes little
sense to me to perform a single or a set of analyses on a single sample or
even replicate samples obtained at a single point in time in an attempt to
assess stability of a biologically diverse and ever changing material like
compost. Within a week the material properties can range from a
microbiologically stabilized condition to an active condition.
I often use daily and/or weekly checks of temperature, pH and electrical
conductivity, and pore space oxygen (in windrows) to observe the trends in
the compost properties over a period of time. The correlations we have
observed between these parameters and another test of microbiological
activity (SOUR) suggest strong relationships between the stability of
composts and these parameters. As implied by Dr. Breitenbeck, a test of
stability may best be accomplished by observing trends in specific
properties that are known to represent stable material over a period of
time.
I agree with Dr. Breitenbeck that nitrate accumulations are a logical
starting point for stability tests in the composting processes, but I note
that nitrification occurs in cycles during composting of some materials just
as do pH, electrical conductivity, or even pore space oxygen or temperature.
During periods of "stabilized" conditions nitrifying bacteria may be active
is greater population numbers than during periods of higher temperatures,
and nitrate concentrations may increase for a period of time. There are
often more than a single period of heating during composting processes which
makes short-term composting processes suspect to incomplete composting
processes. A test kit for nitrate and ammonia concentrations would be
acceptable and useful for field checks of composts if the interferences from
other parameters do not cause inconsistent results (I site some
interferences that cause false readings in the Solvita maturity index test).
A more accurate approach to measuring nitrates and ammonia may be field
laboratory application of nitrate and ammonium ion selective electrodes, but
the interference with nitrate analysis by potassium may be a problem.
Whether using nitrate accumulation, ammonium reduction, pH, electrical
conductivity, or respirometry, observing trends provide more information
about microbiological activity than assuming that a single analysis or even
a group of analyses at a point in time will provide adequate evidence for
compost stability. I have always promoted the idea that compost must be
stable before it can be considered mature.
In reference to reinoculation of biosolids compost by pathogens, survival of
pathogens after composting definitely suggests that the materials can not be
stable because sufficient readily available nutrition and even anaerobic
conditions are present in the materials to support a population of fecal
coliform bacteria, excluding the overwhelming numbers of aerobic
microorganisms that would exist if aerobic conditions were present. Most of
the fecal coliform components can not survive long under aerobic conditions.
This observation would indicate that if fecal coliform bacteria exist in
higher concentrations than desired, the materials will require additional
aerobic processing before considered stable. Interestingly, if nitrates
accumulate in the materials, aerobic conditions may be implied because
nitrification is an aerobic process. In my opinion, however, pore space O2
levels in composting materials do not provide an indication of stability as
much as to show the adequacy or need for aeration in the materials, as does
the level of pH.
When we perform respirometry analyses (TMECC 05.08B) we should incubate
wetted composts (70-85% of the water holding capacity) at mesophilic
conditions (25-28C) for 24 hours under aerobic conditions and at 100%
relative humidity to provide conditions for microbiological activity for a
period prior to testing. This negates moisture level influence on
microbiological activity by allowing "suitable" moisture in all samples for
microbiological growth while ensuring adequate aeration for aerobic
conditions. However, this test alone performed on woody materials may
provide an indication of stability when, if sufficient nutrients are added
to the mix increased microbiological activity would be expected.
If VOCs or organic acids are present in the material, they are quickly
consumed, and microbiological activity should reflect the organic substrate
availability. The presence of organic acids or VOCs can provide evidence of
the lack of stability.
Like you, I believe that stability indicators will differ between composts,
but only due to the chemical and microbilogical population differences
exhibited by the materials. However, trends in the process can be easily
predicted, and analyses over time will provide the best evidence of
stability in materials than any one spot check, even if multiple analyses
are performed. For instance in some composted materials with elevated pH
(manures), temperatures cool, pore space oxygen levels may increase, pH may
start decreasing while soluble salts increase to a maximum level. The trend
for pH over a longer period might be toward a neutral pH, and salts may be
observed to decrease (as materials matured). Ammonia concentrations
decrease while nitrate accumulates. These trends are predictable and when
we observe these trends, possibly within a set interval of confidence to
account for variability in sampling and analysis, over a specified period
(maybe 15 days) we can assume stability in materials. Laboratory analyses
could confirm stability through respirometry or the maturity index testing
if desired.
Dave Schellinger
W. A. Callegari Environmental Center
-----Original Message-----
From: compost-bounces at composter.com [mailto:compost-bounces at composter.com]
On Behalf Of Jim McNelly
Sent: Thursday, August 31, 2006 12:08 PM
To: compost at composter.com
Subject: [USCC] Maturity vs Stability
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Fellow Composters,
I am being asked in one of my projects to propose
a standard of compost stability that a certain
composting process should meet other than the EPA
time and temperature requirements for pathogens
and vectors. This standard would be for a
maximum of 21 days of composting and should not
be a maturity standard for the suitability for
plant growth, but an indicator of how well a
particular composting system is performing.
Regarding the definition of compost maturity, in
the past ten years, great progress has been made
using testing methodologies developed in the
USCC's laboratory handbook, "Test Methods for the
Examination of Composting and Compost
(TMECC). More information can be found at
www.tmecc.org . Compost maturity largely deals
with the suitability of a bionutrient or compost
product to facilitate growth a particular plant
under controlled conditions. Stability, as I
understand it, deals more with regulatory
compliance parameters dealing with nuisances
while protecting public health and the
environment. Heated garbage, for example, may be
pathogen free but it is not "stable".
I am wondering how the issue of stability is
being addressed in actual composting facilities
in operation today. Specifically, I am looking
for indicators of stability required by various
states or used by operators. In wastewater
treatment solids evaluation, volatile solids
reduction is typically used for determining the
change in raw and digested solids, but VSR does
not work as well as a standard for composting as
bulking materials that do not decompose or are
not a part of the putrescible material being
evaluated. Specifically wood chips have a high
level of volatile solids but show little volatile
solids reduction in composting whereas food
products or biosolids show a greater volatile
solids reduction. The two materials commingled
make the VSR analysis usable perhaps for an
operator in evaluating their own process compared
to other time periods in their operation, but are
not reliable when evaluated against other
technologies, other blends or other facilities.
Regarding time and temperature, the basics of
stability begin with the EPA's 503 rules for
pathogens and vectors. While there are many ways
of meeting these rules, composting operations
typically use the time and temperature standards.
PFRP, the Process to Further Reduce Pathogens
Using either the within-vessel composting method
or the static aerated pile composting method, the
temperature of the biosolids is maintained at
55°C or higher for 72 hours. For windrow
composting, the requirement is 15 days over 55°C with a minimum of 5
turnings.
For Vector Attraction Reduction (VAR), one of the
options is aerobically treating the biosolids for
14 days or longer, during which time the
temperature always must be over 40°C and the
average temperature must be maintained at
45°C. This time and temperature standard applies
to windrow and in-vessel systems both. I am not
aware of how a windrow system can document
meeting VAR as it appears to me that there is a
conflict between temperatures exceeding 55C for a
total of 15 24 hour time periods and being
"maintained" at 45C during the same time.
Most windrow systems I am aware of use conduct
quarterly pathogen testing. If testing is used
as alternative, these are the EPA standards:
This alternative states that biosolids are
considered to meet the Class A standards if:
* the density of fecal coliform in the biosolids
must be less than 1,000 MPN per gram total solids (dry-weight)
or
* the density of Salmonella sp. Bacteria in the
biosolids must be less than 3 MPN per 4 grams of total solids (dry-weight).
Regarding the pathogen tests, have seen
composting systems meet PFRP and VAR, only to
become reinoculated with fecal coliforms due to
cross contamination with unprocessed materials,
either from the loader bucket or commingling in
curing piles. Should the ability of a compost
product to resist reinoculation be considered as an indicator of stability?
Forced aeration systems can meet PFRP in three
days and then VAR in an additional 14 days,
suggesting to me that regulatory compliance using
time and temperature requires at least 17 days
plus the time it takes for the composting mass to
reach PFRP temperatures, meaning a minimum of 18
days. Forced aeration alone is typically unable
to bring the temperature of the composting mass
down to 45C and maintain it there without
temperature feedback and heat exchange. In
addition, I have never received an authoritative
answer to the question as to whether the 14 days
of VAR can include the 3 days of PFRP. I would
assume not, based on the term "maintained",
suggesting that three days over 55C is not the
same as "maintaining" the temperature at 45C.
The PFRP and VAR rules tell us little about how
many BTUs are actually oxidized in the composting
process, as a mass retaining heat tells us little
about how much heat is being removed. PFRP and
VAR are at least, in my opinion, minimum
standards to meet, but I think that there should
be additional indicators and standards of stability.
Which brings us back to my original question,
which is concerning alternative standards or
measurements of stability other than the EPA time and temperature standards.
Seed germination rates are commonly used in
determining compost maturity, but can we use them
as indicators of stability as well? If so, which
seeds and under what conditions and compost
sampling protocol? Some compost feedstocks high
in carbon, meaning mostly woody materials, can
have respectable seed germination results in only
three weeks, whereas high nitrogen mixes can take
longer. Seed germination rates are largely
indicative of stability or maturity of a
particular mass being composting relative to its
initial characteristics. I can see this test
having value in certain cases, like the VSR test,
relative to starting parameters, but I question
this approach as an "across the board" indicator of stability.
What about ammonia off-gassing? I have seen its
value in determining maturity, but can it be a
good indicator of stability as well? Measuring
ammonia might be problematic, however, other than
the general observation of whether the mass
stinks or not. I am not aware of a reliable low
cost ammonia indicator test other than the change
in color of pH litmus paper, which is a good
general indicator, but to me, not a sufficient
specific indicator, given the subjectivity of
color change in the paper. Again, if the
feedstock is high in carbon, there is low ammonia
off gassing in the first place.
I suggest that compost mass re-heating is not a
reliable indicator as, like VSR, it is indicative
of the original feedstock more than the process
being used. It can also be affected by reduced
moisture. Mass dried under 35% can often avoid
reheating as there is insufficient moisture for significant microbial
activity.
The fusarium suppression test is a good
indicator, but again only of maturity, not stability.
I was leaning toward 02 consumption and CO2
production indicators, but I ran across a
decomposition system that effectively sterilized
the decomposing mass. Their media showed zero 02
consumption unless it was re-inoculated with
microbes. I think that 02 consumption and or CO2
production values could work if the microbial
population is intact, there were sufficiently
available volatile solids to begin with and there
is adequate moisture. O2 consumption is an
indicator of volatile solids reduction, which can
be measured by heat removal. Has anyone ever
used heat being removed as an indicator of
stability? I think that it might be a good
measurement for temperature feedback - forced
aeration systems where temperature can be
measured in the exhaust pipe related to the
volume of air indicating the degree of decomposition activity going on.
An alternative suggestion for forced aeration
systems might be (VOCs) Volatile Organic
Compounds or methane, CH4 production. Tests from
the South Coast Air Quality Management District
in Southern California showed what appear to he
high levels of VOCs, presumably CH4 released from
windrows. Low VOCs might serve as an indicator
that the aerobic process was working to keep 02
levels up. Perhaps an easier method might be
oxygen sampling within the composting mass to
show levels of 02 over 10% or 15%. But that
would show viability of the aeration process,
perhaps, but is it an indicator of stability? If
windrow technology is actually anaerobic system
with aerobic edges, heated with intermittent
oxygenation after turning, this does not mean
that the process is an inadequate decomposition
process? I don't think so. Windrows achieve
stability and maturity, just in a different way
than forced aeration or at a different
rate. That would be a question more for air
quality officials or the role of composting in
mitigating fugitive methane in the atmospheric warming debate.
Perhaps something about nitrogen conversion and
the relative concentrations of the form of
nitrogen in the composting process. Any suggestions?
I am wondering if there ever can be a standard
compost stability indicator other than time and
temperature given the differences in
decomposition methods and the wide range of
materials to be composted. My thought at the
moment is that we need to define the parameters
of a compost feedstock related to a particular
management strategy and then define a stability
method for that particular set of variables and
then measure how well the particular
decomposition process is working against its baseline standard.
Thanks in advance for any feedback,
Jim~ McNelly
Renewable Carbon Management LLC 320-253-5076
NaturTech, NaturSoil, CompostMan
jim at composter.com
www.composter.com
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