Evaluation of Evaporation as Suggested by Compost Facility Requirements Guideline

“A windrow composting operation with 3 m deep and 7.6 m wide piles and 1.5 m aisles averages about

195 kg of degradable organic matter per square meter. This provides about 2840 MJoule of evaporation

energy per square meter, which has the potential to evaporate 1125 mm of water per square meter.

Twenty five millimeters (25 mm) of rain on the active composting area at this facility produces about

37,000 litres per hectare of polluted runoff”

Is this true? The model that predicted this does not appear to be available on the web, so let us see if we

can reconstruct it based on theory.

1. What is the average depth of compost on this site? If we assume a 1 hectare site with 10

windrows as described above, with 3 m along each side and 6 m along each end, we have 10

windrows that are 92 m long. Based on the above dimensions and using a windrow shape factor

of 0.66, we get an average of 1.37 m or 4.5 ft.

2. How much degradable organic matter can we expect in this 1.37 m depth? If we assume a bulk

density of 700 kg per cubic meter, and a moisture content of 65%, we have a total of 336 kg of

dry material per square meter. If we assume that 90% of that is organic matter (10% ash), of

which 50% is biodegradable, we come up with a total of 134 kg of degradable organic material.

How does this compare to the data used in one of the sites in Washington state that refers to this

model? (http://www.cwc.org/organics/organic_htms/cm002rpt.htm)

The site is described as having 24 loader turned windrows that were approximately 30 feet at the base,

150 feet long, and 12 feet high on a 3 acre pad area. Assuming the similar edge requirements of 3 m

along each side and 6 m along each end, we have a total area of 1.5 ha, which is slightly larger than the

1.21 ha (3 acres) reported. Based on this, we have an average depth of 1.6 cubic meters per square

meter of area. If we use the 1.2 ha (3 acres) as reported, the compost windrows are piled within 1.5 m

of each of the 4 sides of the composting pad. In this case, we have an average depth of 2 meters of

compost on the site. At this point, I am not quite sure how to interpret the data, as the paper estimates

a total of 15,000 yds on site, but the calculation of volume based on the windrow sizes yields 31,680 yds

(23,760 cubic meters).

The assumptions used to calculate biodegradable organic matter include 90% volatile solids, 45%

degradable fraction, 50% moisture and 600 lb/yd bulk density (363 kg per cubic meter). Based on the

assumed 2 meter average depth of compost (31,680 yds on site), the amount of degradable organic

material is 148 kg.

Both my own reconstruction of the model used in this scenario based on my own assumptions, and the

model calculated from a data set where the model was used, significantly underestimates the amount of

biodegradable organic material per square meter.

3. How do we calculate the 2840 MJoule of evaporation energy per square meter? If we assume

that all of the biodegradable carbon is a carbohydrate such as cellulose, we obtain the following

equation: C6H12O6 + 6O2 = 6CO2+ 6 H2O, (carbohydrate 342.34 g/mole). The heat of reaction of

glucose (the components of cellulose) is -2822 KJ/mole

(http://faculty.ncc.edu/LinkClick.aspx?fileticket=FJO362tolqA%3D&tabid=1939 ), or 15.7 MJ/kg.

Multiplying 15.7 MJ/kg of biodegradable solids by 195 kg of biodegradable solids per square

equals 3062 MJ/square meter, which is close to the 2840 estimated in this scenario.

4. How much water can 2840 MJ of energy per square meter evaporate (this is assuming that all

the available energy will be used for evaporation). The heat of vaporization of water at 25 C is

2260 kJ/kg (http://en.wikipedia.org/wiki/Enthalpy_of_vaporization). This means that the

estimated 195 kg of biodegradable organic matter has enough energy to vaporize 1265 L of

water which corresponds to 1265 mm of rain. Because the energy of vaporization is

temperature dependent, the actual amount of water vaporized at 5 or 10 C will be slightly less

than the 1265 L estimated, so 1125 mm of rain is a reasonable estimate.

5. How many days does it need to rain 25 mm to vaporize the 1265 L of water per square meter?

If we divide the 1265 mm of rain by 25 mm per day, we get 50 days or 7 weeks of composting

required to degrade the 50 % of biodegradable carbon. This would assume a fast composting

process, which is not likely to be achieved with a passively aerated windrow process of the size

described in this scenario.

How does this compare to the data used in site in Washington state?

(http://www.cwc.org/organics/organic_htms/cm002rpt.htm). The assumptions used were 6,500 BTU of

energy released per lb of degraded organics, and 1,010 BTU required evaporate 1 lb of water. If we

convert this comparable units, we obtain 1246 L of water evaporated per square meter, which is similar

to the number that we had achieved.

In summary, all of the available energy released during organic decomposition of the organic matter is

used to vaporize precipitation. The amount of energy actually available may be up to 25% overestimated

in the scenario, because the actual amount of biodegradable energy per square meter is overestimated.

The next question is whether all of the energy really is used to evaporate the water? There are two

important requirements. The first is that all of the precipitation needs to come in contact with the

energy released from the decomposing carbon so that it can be vaporized. The second requirement is

that there can be no other losses of energy from the decomposing energy such as heat loss.

6. How can we ensure that all precipitation gets to the sites of energy release in the composting

mass so that all of the water can be vaporized? In the case of the windrows in this example, the

precipitation is going to fall on the windrow itself as well as on the impermeable surface

between the windrows. There may also be some runoff down the side of the windrow due to

the shape of the windrow (Compost Facility Requirements Guideline). This suggests that up to

50% of the precipitation will not come in contact with energy being released from decomposing

organic material. The one solution is to orient the windrows across the slope, so that the

precipitation can pool between the windrows, and gradually become absorbed into the windrow

as the heat energy vaporizes the water in the windrow. While this sounds like a great idea, the

reality is that the Compost Facility Requirements Guideline suggests that the windrows be

oriented up and down the slope to allow leachate and runoff between the piles rather than

through them. We can safely assume that a minimum of 50% of the precipitation will run off

the site before it has a chance to evaporate after coming in contact with energy released by

decomposing carbon.

7. How are we going to ensure that all of the heat energy is going to be used to vaporize moisture

and will not be lost by other means? The Cornell Compost Engineering website

(http://compost.css.cornell.edu/physics.html) suggests that heat in a compost pile can be lost

via conduction, convection and radiation.

“Through conduction, energy is transferred from atom to atom by direct contact; at the edges of

a compost pile, conduction causes heat loss to the surrounding air molecules.

Convection refers to transfer of heat by movement of a fluid such as air or water. When compost

gets hot, warm air rises within the system, and the resulting convective currents cause a steady

but slow movement of heated air upwards through the compost and out the top. In addition to

this natural convection, some composting systems use "forced convection" driven by blowers or

fans. Much of the energy transfer is in the form of latent heat -- the energy required to

evaporate water. You can sometimes see steamy water vapor rising from hot compost piles or

windrows.

The third mechanism for heat loss, radiation, refers to electromagnetic waves like those that you

feel when standing in the sunlight or near a warm fire. Similarly, the warmth generated in a

compost pile radiates out into the cooler surrounding air. The smaller the bioreactor or compost

pile, the greater the surface area-to-volume ratio, and therefore the larger the degree of heat

loss to conduction and radiation. Insulation helps to reduce these losses in small compost

bioreactors.”

In one research paper using an in-vessel system, the conductive heat loss was calculated to be

30% and the convective heat loss was measured at 61%

(http://engrwww.usask.ca/oldsite/societies/csae/protectedpapers/c0516.pdf).

Based on the above, not all of the energy from the composting material will be released as

water vapor into the air. The amount of energy released as water vapor is likely to be

significantly lower in a passively aerated pile than in a forced aerated pile.

8. Evaporation of water depends on saturation vapor pressure which is affected by temperature

and airflow above the composting material.

There is a good summary of evaporation in a paper by Martin Jensen

http://ccc.atmos.colostate.edu/ET_Workshop/ET_Jensen/ET_water_surf.pdf). He states:

“(Brutsaert (1982) credits Dalton’s 1802 paper (cited by Brutsaert 1982, p. 31) as a major event

in the development of evaporation theory. He expressed Dalton’s results in present day notation

as E =f(ū)(eo – ea) where E is the rate of evaporation expressed as rate per unit time, ū is mean

wind speed, eo is the saturation vapor pressure at the temperature of the water surface and ea is

the vapor pressure of the air.”

The vapor pressure gradient is dependent primarily on the temperature and the turbulence

(mixing of the air)

(http://www.colorado.edu/geography/courses/geog_4371_f04/handouts/9Water.html)

Energy, temperature, humidity and wind are described as the four universal factors affecting

evaporation. In a compost facility, the energy is primarily the heat energy,

The temperature influences the capacity of air to hold water as the saturation vapour pressure

(E), increases with increasing air temperature

E - e (actual vapour pressure) = saturation deficit, which is the amount additional water vapour

that air can hold at a given temperature

(http://uregina.ca/~sauchyn/geog327/et.html).

The effect of temperature on evaporation potential can best be described in the following

diagram, where the moisture content of air is primarily affected by temperature. This has the

greatest influence on how much moisture will actually be removed from the composting

process.

This information is based on the psychometric curve, which is produced from the following

equation

log10PVS=a/Ta+b PVS = saturation water vapor pressure, mm Hg

PV = RHAIR (PVS)

a = constant equal to -2238 for water

b= constant equal to 8.896 for water

Ta= absolute temperature oK

PV = actual water vapor pressure, mm Hg

RHAIR = relative humidity, fraction of saturation vapor pressure

In summary, the energy created by the composting process is not all going to be used for moisture

removal because not all of precipitation comes in contact with the energy created by the composting

process, and a significant amount of the energy is lost as conductive heat loss. The process of

evaporation is much more complex than simply accounting for the energy in the compost. It must also

consider temperature, humidity and air movement through and around the composting material.

How does the estimate of leachate in the scenario described at the beginning of this document (37,000

L) compare to the actual leachate collected in the Washington state example?

(http://www.cwc.org/organics/organic_htms/cm002rpt.htm).

This document reports a rainfall of 56.92 inches of rain from October 1998 through May 1999, with a

total of 2.8 million gallons collected. In a subset of data used in the model, 33.5 inches of rain fell from

December 1998 through February 1999. During this period, 1.65 gallons of water were removed from

the site.

Assuming the same 3 acre site, the amount of precipitation collected was 60.6% of the amount of

precipitation that fell during both time periods. This is much different than the 15% predicted by the

model and suggested in the Compost Facility Requirements Guideline.

In the Washington example, a total of 39.4% of the precipitation was not collected, which was a total of

4065 cubic meters in a three month period. Was this evaporated or is there an alternate explanation?

We have to include the amount of moisture that can be absorbed into the composting process. Lets use

the example from Washington State again. The chart below shows 4 scenarios that are based on the

data from the Washington document. In summary, the 4065 cubic meters of precipitation could all have

been absorbed into the composting material.

Case 1 uses the reported 24 windrows that were 12 ft high, 30 ft wide and 150 ft long. The bulk density

was reported to be 600 lb/cubic yard, and the moisture content 50%. Addition of 4065 cubic meters of

unaccounted for precipitation increased the moisture content of the composting material from 50 to

66%. Most outdoor composting windrows have a significantly higher moisture content than 66%.

Case 2 uses the reported 15,000 yds of material on site. In this case, the volume of material is 50% of

that calculated by estimating the windrow volumes, and the addition of the 4065 cubic meters of

unaccounted for precipitation increased the moisture content of the composting material from 50 to

75%, which is saturated.

Case 1 Case 2 Case 3 Case 4

Material on site (m3) 23,760 11,250 23,760 23,760

Moisture Content 50% 50% 55% 65%

Bulk Density (kg/m3) 363 363 650 700

Weight of material (t) 8625 4084 15,444 16,632

Weight of water (t) 4312 2042 8494 10811

Weight of dry matter (t) 4312 2042 6950 5821

Addition of moisture (t) 4065 4065 4065 4065

Final moisture content 66% 75% 64% 72%

Case 1 - Volume in 24 windrows, bulk density and moisture as per Washington data

Case 2 - same as 1 above, except total volume is 15,000 yds as per Washington data

Case 3 - Volume in 24 windrows, bulk density increased, moisture 50%

Case 4 - Volume in 24 windrows, bulk density increased, moisture 50%

Case 3 and 4 use similar windrow volumes and bulk densities that are more likely to be measured in

British Columbia. Case 3 begins with a moisture content of 55% and a bulk density of 650 kg per cubic

meter, and Case 4 begins with a moisture content of 65% and a bulk density of 700 kg per cubic meter.

With Case 3, the addition of the 4065 cubic meters of unaccounted for precipitation increases the

moisture content to 64%. With Case 4, the final moisture content increases to 72%.

All of these are realistic scenarios. What we don’t know is the actual starting moisture content of

material in outdoor windrows during the winter. Unless the composting material begins with an aerated

indoor process, it is not likely that the compost moisture content will be much below saturation

(between 70 and 75%).

In summary, the Compost Facility Requirements Guideline overestimated the amount of energy

produced during composting by approximately 25%, assuming 50% degradation of the 90% volatile

solids. It was not correct to assume that all of this energy could be used to evaporate precipitation

because a significant portion of the precipitation that falls does not come in contact with the energy

produced from the composting process, and a significant amount of the heat is lost via conduction.

Evaporation loss is highly dependent on temperature, humidity and air movement above the

composting material.