THE COMPOSTING CONUNDRUM: JUST HOW AEROBIC IS
“AEROBIC”?
Geoff Hemm, Waste Minimisation Officer, EnviroWaste Services Limited, Private Bag
42, Nelson
([email protected])
Abstract
The composting process, by definition, is the microbial degradation of organic matter
in the presence of free oxygen, i.e. the process is aerobic. If oxygen is not adequately
available, degradation will still proceed, but will spontaneously and progressively
switch over to an anaerobic pathway with accompanying production of odour,
leachate and phytotoxic compounds.
Successful compost-making depends on correct preparation and blending. Its texture,
moisture content, thoroughness of mixing, Carbon: Nitrogen ratio and aeration are all
important, but pre-eminent amongst them is aeration. Crucially, all these parameters
interact with aeration through influencing oxygen demand and delivery. Ensuring
aerobic conditions is more involved than first meets the eye.
As many compost practitioners have found, aerobic vs. anaerobic conditions are not
absolute alternatives. Batches of composting material can support both; sometimes
there is spatial separation, sometimes they co-exist. There is a continuum from fully
aerobic to fully anaerobic conditions. The art lies in managing the process to keep it
as oxygen rich as possible without wasting energy and without promoting
unnecessary losses of moisture and nitrogen.
There is certainly no consensus on what constitutes an aerobic compost environment.
This paper examines the evidence and concludes a tighter definition of aerobic
conditions is possible. Only by understanding oxygen delivery and uptake can the
process be optimised.
Introduction
Like motherhood and apple pie, composting is widely considered to be wholesome. It
is an activity where tradition, belief, grandpa’s experience, science and the demands
of a warming world all converge. Those involved form a broad church indeed. On the
one side dispassionate scientists focus on unravelling the subtle complexities of the
biochemistry of the process while on the other, new age elements claim it as their
own. Mystics who believe that a handful of yarrow leaves in a compost heap or an
antelope horn buried at full moon imbues compost with special qualities, are alive and
1
well and amongst us still. But no more on that, this analysis focuses on commercial
composting and draws on the research of conventional science. It attempts to shed
light on what is probably the single most important issue – that of maintaining
aerobia.
Carbon Loss and Oxygen Consumption
Composting consists of a complex series of mainly oxidative reactions that degrade a
wide range of organic molecules into simpler forms, ultimately whittling them down
to the constituent carbon to be released as CO2. The vast bulk of material that
confronts the composter consists largely of carbon, but biochemically, it is of varying
availability. The range in biodegradability of carbon is what makes the compost
process so drawn out. Those forms that are easily available are used rapidly (such as
sugars and starches), those that take some serious biochemical disassembly take
longer, leaving the recalcitrant forms of carbon (such as lignin) in the composted
residue.
An examination of the mass balance of carbon over the composting process is
illuminating. Table 1 shows a few figures drawn from the literature to illustrate
typical carbon losses that occur. It is clear that carbon loss varies greatly with
feedstock, composting conditions and time. Researchers express it in different ways,
measuring units based on Dry Weight, Organic Matter (= Volatile Solids), Carbon or
Available (= Biodegradable) Carbon. When the figures for carbon loss are converted
to kg/tonne which are the units dealt with on a commercial scale, the extent of the
process is large.
The oxidation of carbon requires copious amounts of oxygen and reveals what is
basically the other side of the same coin. This chemical or stoichiometric oxygen
demand, lies at the heart of composting and defines the very process. As the feedstock
and the biological processes working within are very varied and complex, a 1:2.66
stoichiometric relationship (1 carbon to 2 oxygen on a per mass basis) is an oversimplification. According to Haug (1993) typical values for decomposing organics are
1.2 to 2.0g O2/g Biodegradable Volatile Solids (BVS) though for highly saturated
organics this may go up to as high as 4.0g O2/g BVS. This is the major process in
terms of chemistry and is the source of the large amounts of energy and metabolic
water released in aerobic composting. Table 2 shows a range of oxygen consumption
measurements. During composting, the dry weight of the initial mass of material
reduces through the oxidation of carbon to CO2 depending on the biodegradability of
the carbon (Preston et al, 1997) At EnviroWaste’s Timaru composting facility, a full
windrow was measured at start as weighing 457 tonnes. When the 58.2% water
content is excluded, the dry weight amounts to 191 tonnes. Best estimates indicate a
weight loss of 32% after 8 weeks active composting which amounts to 61 tonnes.
(The exact weight loss of different batches has yet to be measured accurately.) A loss
of 61 tonnes of carbon through conversion to carbon dioxide (disregarding O2 lost as
NOx which is minor in comparison) stoichiometrically requires 162 tonnes of oxygen.
Expressed as an average daily uptake of oxygen, this is 2.89 tonnes per heap per day
(though it will be greater for the first ten days and less for the last two weeks as per
the well established process dynamics of composting). This is a considerable amount
of oxygen being used. When considering that oxygen makes up only 21% of air by
2
26.50%
30.10%
39.80%
87%
93%
% Dry
Wt
Loss
Org
Matter
Loss (=
VS
Loss)
22%
80%
88%
50%
% Org
Matter
Loss
(=%
VS
Loss)
t = tonne (metric)
VS = Volatile Solids
C = Carbon
DW = Dry Weight
OM = Organic Matter
AC = Available Carbon
BVS = Biodegradable Volatile solids
301
kg/t
268
kg/t
Dry
Wt
Loss
314kg/t
Total
Carbon
34.50%
52.80%
19%
33 kg/t
153kg/t
174
kg/t
154
kg/t
%
Carbon
Loss
Carbon
Loss
Avail
Carbon
Loss
(BVS)
Table 1: Carbon Mass Balance in the Composting Process
75.22%
70%
%
Avail
Carbon
Loss
(BVS)
Wood chip-bed manure, over 99 days
In-vessel foodwaste & sawdust over 41 days
Straw-bed manure, over 99 days
Manure composting over 50 days
Swine manure & straw over 21 days
Manure over 36 days, windrowed
Manure over 120 days, turned 2X/week
Poultry manure, 150 days, turned 2x/wk
Cattle Feedlot manure
Cow dung & Bedding Litter over 197 days
Kraft Paper composting over 18 days
Kraft Paper composting over 31 days
Nature of Trial
Hao et al, 2004
Bari et al, 2000
Hao et al, 2004
Sommer & Dahl, 1999
Epstein, 1997
Epstein, 1997
Epstein, 1997, citing
Inbar
Barrington et al, 2002
Cooperbrand, 2002
Cooperbrand, 2002
Cooperbrand, 2002
Larney et al, 2006
Reference
3
268 DW
752 AC
301 DW
500 OM
700 AC
220 OM
800 OM
880 OM
398 DW
33 of C
only
870 DW
930 DW
kg/tonne
Equivalent
Loss
80g /100g OM over whole process
1.2g to 4.0g /g BVS
1.99g/g BVS
1.2g/g substrate (dry wt)
1.27 kg/kg BVS
1.34 kg/kg DW of substrate
1.28g/kg DM/hr
2.6 to 4.3 mg/gVS/hr
6.1 to 19.6 mg/gVS/hr
21.8 to 51.8 mg/gVS/hr
1 mg/gVS/hr
5 mg/gVS/hr
General rule of thumb
Depending on substrate, range of uptake is:
Sludge composting
Proteinaceous waste with 80% biodegradability
In-vessel foodwaste composting
Foodwaste mix of composition C18H26O10N
Peak uptake in sludge composting
Suboptimal uptake restricted by low aeration
Uptake at mid-range aeration rate
Uptake at high aeration rate
Consumption at 30°C
Consumption at 63°C
OM = Organic Matter
BVS = Biodegradable Volatile Solids
DW = Dry Weight
DM = Dry Matter
VS = Volatile Solids
O2 Consumption
Conditions
O2 demand varies with substrate
O2 demand varies with substrate
O2 demand varies with substrate
Equals 5.17g air/g substrate
Observations
TABLE 2: Oxygen Consumption in Actively Composting Material
Kutzner, 2000
Haug, 1993
Haug, 1993
Preston et al, 1997
Bari et al, 2000
Bari et al, 2000 citing Kayhanian et al
Tremier et al, 2005
Epstein, 1997
Epstein, 1997
Epstein, 1997
U N Environment Programme,
U N Environment Programme,
Reference
4
volume (23.2% by weight), the delivery of oxygen into the composting mass is of
major importance. And this is where aeration comes in.
Aeration
“The aeration system is an important feature of all modern composting systems. The
ability to control aeration is one of the key points of process control and is an
important consideration in process selection” – Haug, 1993.
The primary need for Aeration is to deliver oxygen into the composting mass to fulfil
the stoichiometric demand of the decomposing organics. But depending on the
technology, aeration is also used to remove excess heat and excess water. In sealed
composting vessels, the heat liberated by decomposition can build up to the point
where microbial action is greatly inhibited and a mechanism for heat removal is
necessary. Excess moisture may also be a problem as the biochemical degradation of
the organic material produces large amounts of water, usually of the order of half a
tonne of water per tonne of degraded organic material expressed on a dry weight basis
(Iowa State, DABE). If initial water content is high and if the feedstock contains high
amounts of encapsulated water, the situation is exacerbated. When aeration (with
direct venting of exhaust gases) is used to cool the composting material and reduce
moisture, the rate of air delivery will supply far more oxygen than is biochemically
required (Haug, 1993 & National Engineering Handbook, 2000). In such instances,
monitoring oxygen levels is immaterial. However, with increasingly sophisticated
process controls employing air recirculation and using heat exchangers and
condensers, attention to O2 levels is required to ensure adequate oxygen is provided.
It is enlightening to calculate how much air must be forced into the compost mass to
meet its oxygen demand. An organic mix with a demand of 15 kg O2 /tonne VS/hr
will require an average of 55.5 m³ air/tonne VS/hr. Peak air demands for sludge
composting have been established in the range of 110 to 140 m³ air /tonne/hr
(National Engineering Handbook, 2000). Haug (1993) gives a full exposition of the
mathematics to calculate air demand to fulfil stoichiometric oxygen demand as well as
that for cooling and moisture removal.
Much has been written about aeration: what aids and abets it, and what hinders it.
Many of the factors at work within the compost mass are inter-related, and an upset in
one parameter can have a cascading negative effect on numerous others though the
links may not initially be obvious. The need to pay attention to detail is vital in
commercial composting. Aeration, and the engineering behind providing it, is indeed
vital to the whole process.
Factors Influencing O2 Consumption and Movement
Before oxygen can be used by micro-organisms within the compost mass, aeration
must deliver it into and through the material at a rate at least as fast as it is being used.
It must penetrate throughout the mass into pores and spaces right down to the
molecular level. The more unimpeded, thorough and even the penetration, the more
aerobic the compost will be. Depending on the compost mass, air movement occurs
through convection, diffusion and by forced mass flow.
5
The following factors influence air movement and penetration into the mass:
1. Size reduction of feedstock. Though different technologies are used, ideally this
process should break and fluff-up solid organic material into pieces of differing
sizes with a tearing mechanism. The shear action helps split, bruise and rip
material apart so as to maximise the exposed surface area of the pieces and
produces differing particle sizes, allows the material to remain porous and open to
air penetration when in a heap.
2. Moisture has been well researched and most authors and practitioners agree it
should initially be in the 50 to 65% range, inclusive of encapsulated water. Wetter
than this and the bacteria on the particles of substrate are bathed in much thicker
films of water through which oxygen is unable to diffuse rapidly. The outcome is
oxygen starvation and anaerobia. (Agitated in-vessel systems are designed to
ameliorate this by using forced air flow to drive off excess moisture.) Drier than
this, and the material is unlikely to support sustained degradation as composting
progresses. Re-wetting is then required. Indeed, in many systems, re-wetting is
required as a matter of course to keep material optimally damp. When composting
occurs in outside heaps, the need is to conserve moisture rather than remove it.
3. The C:N ratio. In the developed world, organic wastes are often nitrogen rich
leading to excessively high composting temperatures, higher oxygen demand and
the production of odour. Adding extra available carbon, if possible, is necessary.
If not, exhaust gases are likely to require scrubbing or biofiltration to remove
ammonia.
4. Mixing the shredded feedstock, if necessary. Material to be composted often
consists of a mixture of differing organic wastes such as food residues, garden
greens, sludges and lawn clippings. If too dense, a carbonaceous bulking agent
must be added to open up the mix (and add supplementary carbon). This and the
other components need to be thoroughly and evenly mixed to maintain porosity,
even moisture distribution and consistent nutrient distribution. If the mix is
uneven, patches of wet and/or dry and/or clumped material will develop and
degradation will be uneven and odorous.
5. Maintaining porosity through the composting process is vital to keep sustained
aeration unless an agitated system is used. As the composting process progresses
and the material degrades, the compost mass tends to slump and compact. The free
airspace declines. This impedes aeration (and oxygen supply) and unless the
material is moved and fluffed up again, anaerobic conditions will develop. In
practice, any compaction within the heap can cause localised anaerobia. An
inattentive loader driver may nudge into a heap during its construction with the
front wheels of the machine creating two parallel compacted layers that prevent
horizontal air movement within the heap. Mathsen (2004) discusses at length the
importance of maintaining porosity and avoiding poor aeration caused by
channelling which is the perverse result of over-vigorous aeration.
6. Temperature is one of the most frequently measured parameters to gauge compost
metabolic activity. Numerous workers, citing a wealth of research, aim to manage
compost at lower temperatures than those that are spontaneously attained, to
achieve more rapid and complete degradation. It is not the scope of this paper to
enter the debate, save to observe that in a literature review, Lin (2006) noted that
there is not consensus on optimal composting temperature. He records the
following authors as regarding differing temperatures as optimal: Snell: 45°C,
Jervis & Regan: 60°C, Atchley & Clark: 70°C and Haug: 72°C. Generally high
6
temperatures reflect high metabolic activity which in turn has a high oxygen
demand., though this can be self-terminating.
7. Depending on the composting system used, the shape and size of the composting
heaps has an important bearing on aeration and oxygen delivery. For passively
aerated windrows it has long been common practice to restrict the height and
width of piles so that the physical distance the air has to move within the heaps is
not excessive – the bigger the heaps, the greater the resistance to airflow. When
forced aeration is used, the heaps may be far bigger but then an important
consideration becomes that of oxygen concentration across the heap. If fresh air is
introduced through appertures in the floor, the compost closest to the ground is fed
with air having 21% oxygen. As the air rises within the heap, oxygen is consumed
so that with increasing height, oxygen concentration falls. If the air supply is not
suitably regulated, the base of the heap may be fully aerobic while higher up the
material may be starved of oxygen. It is for this reason that many researchers use
agitated in-vessel units for their work and measure the O2 concentration in the
exhaust air.
8. Even the old chestnut of contamination has an effect on aeration. Plastic bags
form very effective localised barriers to airflow allowing pockets of anaerobic
activity to exist within an overall aerobic environment. While in most
circumstances this will not materially affect compost quality in the end, it may
contribute to noticeable odour if the amount of plastic is significant.
“Aerobic” Conditions – Differing Perceptions
There is not much debate amongst composters when confronted with a wet slimy heap
oozing leachate and smelling bad. Material in this state is clearly anaerobic. At the
other end of the spectrum, a heap of black, moist, friable material with a rich earthy
aroma is most certainly aerobic. The problem arises somewhere in the middle, where
material is degrading well, no leachate is being produced and as long as the heap is
left undisturbed, no environmentally negative characteristics are evident. However,
the moment the heap is opened, what seems to be an innocuous odour, somewhat
sweet, somewhat silage-like, escapes into the air. The operator sees no cause for
concern until the phone starts ringing with irate neighbours complaining bitterly about
the terrible smell. On close examination, such a heap appears aerobic and well aerated
but its olfactory output suggests otherwise. Under circumstances such as these,
composters are not in general agreement. Logically, if one turns to the literature there
must surely be some clear definition of what constitutes aerobic conditions just as
most compost literature agrees on the issue of optimal moisture levels. Table 3
reviews a good cross section of recommended O2 levels in actively decomposing
compost.
7
Minimum O2
Level in
Compost Mass
(%)
Minimum O2
Level in
Exhaust Gases
(%)
5
10
18
5
5
5
Optimum O2
Level in
Compost Mass
(%)
> 10
10 to 14
2 to 5
5
15
19
1% sufficient
16 to 17
>> 5
5
5
6
5
> 12
5 to 15
> 15
12 to 18
10
8 to 12
Reference
Kutzner citing Strom
Kutzner citing
Bidlingmaier
Kutzner citing De
Bertoldi
Cooperband citing Rynk
Nat Eng Handbook, 2000
Jackson, ROU
Grubinger citing Brinton
Pace et al, 1995
Crockett, 2007
Hill, 2007
Richard, Cornell
University
On Farm Composting
Hndbk
Das et al
Wortmann et al, 2006
Key, 2005
Ontario Min of Enviro,
2004
Xi et al, 2005
Table 3: Recommended oxygen levels in actively composting material.
Clearly, there are two broad oxygen level bands that emerge. The lower one, where
the minimum level is recommended at around 5% applies to passively aerated
windrow composting where, ironically, natural processes are barely able to lift this
value to double digits during early phase composting when oxygen demand is high.
The higher band is aimed at more advanced technology where forced aeration is used
and where, broadly speaking, management of the composting process is able to
accelerate the process. At the Redruth composting facility in Timaru, oxygen levels
are measured 1 m below the top of 3 m high covered windrows where forced aeration
introduces an upward stream of fresh air from aeration channels in the floor. If the
oxygen level at the point of measurement is maintained at 8% or higher, the bulk of
the heap is found to be aerobic judging visually and by olfactory perception. When it
is set between 8 and 6% O2, sweetish off odours begin to develop and below 6%,
characteristic anaerobic odours dominate.
Several authors refer to the continuum between aerobic and anaerobic conditions and
some (Grobe, 1998 & Kutzner, 2000) state it is impossible to exclude all anaerobic
activity and claim that aerobic composting will always include at least some anaerobic
elements (Atkinson et al, 1996 & Grubinger). Richard (1996) of the Cornell
University Waste Management Institute has authored an information sheet outlining
how marginally excess moisture in organic material creates a three zone biochemical
8
continuum with an aerobic zone where free oxygen is available, a fermentation zone
where conditions are partially anaerobic and an anaerobic zone where the bacteria are
fully oxygen starved. If the oxygen supply is improved, the material will become
increasingly aerobic, but if it is restricted it will become fully anaerobic with an
accompanying deterioration in odour. Sweet fermentation and silage smells emanate
from the intermediate fermentation state and should be taken as a sign that the oxygen
supply is insufficient.
Some less formal practitioners take the adage “Compost Happens” to heart and accept
the spontaneity and inevitability of decomposition without trying to mange the
process. This has given rise to the concept of “No-Turn” compost which is also more
or less what is practised with the composting of animal mortalities (Keener et al,
1997). The contents of a heap, at least in part, may be quite obnoxious, but if left
undisturbed, the outer layer of the heap acts as a biofilter controlling odour. The
degradation process within will then proceed aerobically or anaerobically (or both)
and run to completion, thereafter gradually reverting to a state of aerobia. All that is
required is plenty of time and benign neglect.
One notable anomaly can be seen in table three in the recommendation of Hill that 1%
O2 is sufficient to supply enough the oxygen to the microflora within the compost
heap based on calculations showing the diffusion of oxygen through water to be a
slow and rate limiting step in its transport to the actively respiring organisms. In
reality, such low levels of oxygen have repeatedly been shown to be anaerobic. This
thesis is therefore rejected. Interestingly, Hill also promotes minimal heap disturbance
and regards anaerobic decomposition as part of composting.
A question often posed is that of aerating to excess. Why bother trying to define and
achieve aerobia, when one can just aerate to excess and let the organisms take as
much oxygen as they need. As already mentioned, when airflow is used primarily to
manage temperature and moisture, oxygen is supplied surplus to requirements. Where
cooling and drying are not needed, excess aeration is wasteful of energy and can
impact negatively on the composting process. Seekins (1999) presents a cogent case
against over-aeration. He points out it leads to channelling and thus poor air
distribution as well as excessive moisture removal to the point where composting
ceases, at least in parts of the heap. Dehydrated compost remains immature and
presents a problem when it gets wet. Moisture re-activates it and can lead to a
resumption of active decomposition. At the Timaru facility, moisture loss with
increasing aeration is clearly evident. Managing heaps is a trade off between
delivering enough air and conserving moisture and in this facility at least, being able
to maintain aerobic conditions with the least aeration possible will greatly benefit site
management.
Another logical enquiry is to look at the possibility of supplying oxygen enriched air
to compost to see if degradation can be enhanced. Suler and Finstein (1977)
experimentally assessed the effect of using oxygen rich air (leaving a 25% O2 residual
in the exhaust), but found it had little effect.
9
Towards Defining Aerobia
It is clear that there is no sharp cut-off line below which conditions are anaerobic and
above which they are aerobic. However, there appears to be a band of partial
anaerobia where fermentation occurs, which is a transition between full aerobia and
full anaerobia. Where this lies in terms of percentage oxygen concentration, appears
not to have been determined. It is likely that it will occur in wet patches in otherwise
well aerated compost or in generally under-aerated compost.
In sophisticated dynamic simulation modelling conducted by Xi et al (2005) and
verified by lab scale trials, maximum feedstock degradation was obtained in the
oxygen range of 8 to 12% in the exhaust gas. This means that the material has access
to air containing between 21 and 8% oxygen.
If fresh air is introduced into the compost mass, then by default the maximum oxygen
level is 21% and the question now becomes what is the minimum level in the exhaust
air that should be maintained? Probably the most illuminating information comes
from the providers of in-vessel composting systems where air is re-circulated and
blended with fresh air rather than feeding in fresh air exclusively. In the case of
Rotocom and WTT, aeration is used to manage temperature and moisture, but oxygen
levels are held at 15% as a minimum (Kroening, 2007).
In on-going work, Crockett (2007) found that to maximise microbial growth it is
necessary to keep the CO2 level in compost (within a vessel) constantly below 2% and
the O2 level above 15%. He then goes on to specify an aeration rate of 3.39 ft³
air/min/yard³ of substrate (which equates to 4.4 m³air/hr/m³ of substrate) which was
experimentally established as necessary to maintain an oxygen concentration of 15%.
Taking into account the literature cited and various oxygen levels recommended, a
broad zone of common ground emerges between 8 and 15%. However, Suler and
Finstein found little benefit between having a residual O2 level of 10 and 18% in the
exhaust gas. It would therefore appear that to be fully aerobic, oxygen levels should
be held at 10% or above over the whole compost mass. This assumes, naturally, that
all other factors have been optimised. Good aeration and oxygen supply will count for
nought if moisture content is excessive or porosity is poor. The rate of aeration will
also vary depending on the feedstock – richer material will require more air to supply
its higher O2 demand.
10
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