[USCC] Maturity vs Stability

[Jim McNelly]

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