Effect of solar energy on composting microorganisms with organic matter in aerobic composting process

V.Ananth1, a* and E.Natarajan2, b

1Institute for Energy Studies, Anna University, Chennai-25, India

2Institute for Energy Studies, Anna University, Chennai-25, India

aananthveekas125@gmail.com, benat123@gmail.com

Abstract

The solar composting process is consisting of composting bin with air spreader, solar thermal

flat-plate collector, flow control valve and electronic monitoring unit. The composting system

is the most efficient method for processing organic waste. However, the composting activity

of microorganisms can be altered by solar irradiance. In this study, the effect of solar

irradiation on composting microorganism was investigated through reducing composting

process period and improving the quality of compost. The viability of microorganisms in

compost soil after solar irradiation has been improved causing by providing optimized

environment to the composting bin. Moreover, it is an aerobic composting process that is

periodically turned over the organic matter for aeration and maintaining most favorable

moisture content level. Where, the experiment has conducted with absence of solar thermal

flat-plate collector, but the system is exposed to the direct solar irradiation. The composting

bin volume has taken as 0.121 m3 and compost materials were prepared by mixing of cabbage

waste, wood ash, sawdust and soil. The process was conducted for three months with

controlled environment rather than natural way of composting process. Finally, the result of

solar composting process has been obtained as NPK composition of 1:2:1 by conducting

proper chemical test. The total nitrogen (N) content is identified as 1639 mg/kg through

ASTM D5233-92, APHA 22nd

Edition, 2012; 4500 N B, C test; the total phosphorous (P)

content is identified as 3534 mg/kg through APHA 22nd

Edition, 2012; 4500 P B, C test; and

the total potassium (K) content has been identified as 1639 mg/kg through APHA 22nd

Edition, 2012; 3500 K B test.

Keywords: Solar flat-plate collector, solar irradiation, microorganisms, aerobic composting

process, moisture content, NPK composition.

Introduction

In general, synthetic fertilizers are used widely for agricultural purposes to increase the crop’s

yield and bringing an agricultural revolution but it may cause to sterilize of soil in order to

use for long term. Whereas, the use of natural fertilizers that can maintain good soil fertility

and nutrient’s level of soil. Now-a-days 5 to 10% of people are only using natural fertilizer

remains using synthetic fertilizer due to lack of availability and complication in

manufacturing of natural fertilizers. Meanwhile, getting yield by use of natural fertilizers will

be low compared with synthetic fertilizers. And also the natural composting process would be

conducted in uncontrolled environment that may lead to long duration (i.e. six months to one

year).

The present paper deals with solar composting process that the organic materials is

decomposed in controlled environment as the bin was maintained at 70 o C - 85

o C in phase-I

is called as high temperature thermophilic process and then bin was to be maintained at 20 -

45 o

C in phase-II is called as mesophilic process after that low temperature thermophilic

process as phase-III with 65 - 70 o

C was followed up to end of composting process. It

reduced composting period of organic material into 30 to 40 days rather than results of other

methods as 2 or 3 months. Moreover, NPK composition of compost was obtained as 1:2:1.

Commonly, the electrical energy is used to provide heat for growth of microorganisms in

artificial fertilizers preparing industries that will also increase cost of the system. Hence, the

use of solar energy may reduce cost of the system and it is emission free.

Loubna El Fels et al. [1] have reviewed that in composting of organic wastes is a bio-

oxidative process involving the mineralization and partial humidification of the organic

matter that leads to stabilize final product with free of phytotoxicity, pathogens and certain

humus properties. During the first phase of composting process the simple organic carbon

compounds are easily mineralized and metabolized by the microorganisms that producing

CO2, NH3, H2O, organic acids and heat. The accumulation of this heat raises the temperature

of the pile. Composting is a spontaneous biological decomposition process of organic

materials in mostly aerobic environment. During the process, the bacteria, fungi and micro

arthropods that will break down the organic materials to stable, usable organic substances

called compost. The composting also implies the volume reduction of the wastes, the

destruction of weed seeds and of pathogenic microorganisms.

Minchul Yoon et al. [2] have reviewed that the intensity and concentrated activity of the

livestock industry generate vast amounts of biodegradable wastes which must be managed

under appropriate disposal practices to avoid a negative impact on the environment such as

odor and gaseous emission, soil and water pollution. Composting process cannot be

considered as a new technology but amongst the waste management strategies which gains

interest of suitable option for manures with economic and environmental profits. Since this

process reduces or eliminates the risk of spreading of pathogens, parasites and weed seeds

associated with manures that leads to a final stabilized product which can be used to improve

and maintain soil quality and fertility.

Kulcu et al. [3] have reviewed that the composting of animal manures has been traditionally

carried out by the farmers after manure collection for better handling and transporting

purposes. Frequently, the wastes are heaped up with little regard to control of the process

conditions (aeration, temperature, ammonia loss, etc.) and rudimentary methodology.

However, as the fertilizer value of animal manures has been always recognized and also

nowadays their composting is seen like as an alternative way of recycling the manures in

farms without enough agricultural land for their direct use as a fertilizer. But, the cost of

composting of animal manures will be considerably higher than the direct utilization of raw

manures.

Miao-miao He et al. [4] have reviewed that the factors affects the composting of animal

manures for production of high quality compost with added agricultural value, focusing on

the nutrient content, organic matter (OM) humification and maturity degree. Complementary

information on safety and environmental aspects related to manure composting is reviewed

by Moral et al. (2009) in this OECD special issue, including the suppressive effect against

phyto-pathogens of compost and recent techniques to determine the OM humidification

process during composting.

Methodology

Energy and Exergy Analysis: In order to write the energy balance equation for each

component of the solar composting process, the following assumptions are made:

1. The system is in a Quasi- steady State.

2. The heat capacity of the flat-plate thermal collector is neglected in comparison

with heat capacity of organic matter in composting bin.

3. There is no temperature stratification in composting bin due to forced mode of

operation.

4. One dimensional heat conduction is a good approximation.

For Solar flat-plate thermal collector The energy balance equation for the solar flat plate thermal collector,

S o la r en e rg y fa llin g H ea t lo ss d u e H ea t lo ss d u e H ea t lo ss d u e

= U sefu l h ea t g a in + + +o n th e rm al co llec to r to co n d u c tio n to co n v ec tio n to rad ia tio n

The following formulas have been used to analyze the performance of solar flat plate thermal

collector system theoretically is given below,

The solar energy falling on solar thermal collector,

s t cQ = I A τ α (1)

The useful heat gain,

u p o iQ = m C (T - T ) (2)

The overall heat loss from the collector,

L L C p m a Q = U A T - T( )

(3)

The overall heat loss coefficient,

l t b eU = U + U + U (4)

The overall heat loss from the collector,

lo s st b e

 Q = q + q + q (5)

The top loss of heat from the collector,

t t p p m aq = U A (T - T ) (6)

The bottom loss of heat from the collector,

b b p p m aq = U A (T - T ) (7)

The edge loss of heat from the collector,

e e p p m a

q = U A (T - T ) (8)

For composting bin

The energy balance equation for the solar composting process,

H eated a ir in p u t to In te rn a l en e rg y s to re d H ea t lo ss to am b ien t H ea t lo ss d u e to c o n vec tio n fro m = + +

th e co m p o s tin g b in in co m p o s tin g b in th ro u g h o u le t a ir to p su rface o f co m p o s ti

n g b in

The following formulas have been used to analyze the performance of solar flat plate

thermal collector system theoretically is given below,

Energy extracted from the solar thermal collector through air,

a ira ir a ir p a ir

Q = m C (Δ T ) (9)

Internal energy stored in the composting bin,

IE c a b v b inQ = m C (Δ T ) (10)

Heat loss due to convection from top surface of the composting bin,

bc o n v s b a

Q = h A (T - T ) (11)

Organic material loss during decomposition,

m P

lo ss

m P

O M (% ) - O M (% )O M = × 1 0 0

O M (% ) × [1 0 0 - O M (% )] (12)

Carbon content in the organic matter,

(1 0 0 - a sh (% )C =

1 .8 (13)

Percentage of total organic carbon in the organic matter,

% v o la tile so lid% o f to ta l o rag an ic ca rb o n =

1 .8 (14)

Percentage of moisture content in the organic matter during decomposition,

d ry w e t

d ry

W eig h t - W eig h t% o f m o is tu re co n ten t =

W eig h t (15)

Percentage of total nitrogen in the organic matter,

1 4 (R -S ) N% o f to ta l n itro g en =

w (16)

Percentage of volatile solid in the organic matter,

% of volatile solid = 100 - % Ash (17)

Percentage of ash content in the organic matter,

in i t ia l f in a l

in it ia l

W e ig h t - W e ig h t% o f a s h c o n te n t =

W e ig h t (18)

Rate of Moisture content removed from the substrate,

m

b w b b m m b m b

b s

1 = (δ -δ ) s /s - 1 + M (1 - f ) - (1 - f )

M

(19)

Nitrogen losses during decomposition from organic matter,

1 2

1 2

X XN - L o ss (% ) = 1 0 0 - 1 0 0

N N

(20)

System description

The solar composting process consists of composting bin, solar thermal flat-plate collector,

thermocouples, electronic monitoring unit and flow control valve etc. The composter bin is

made up of cement with proper dimension as shown in Fig.1. It has pores in the base through

which hot air is passed to inside of the composting bin. The bin’s base is connected to output

of the collector through pipe with proper insulation. Where, the bin is filled up with selected

raw, First of all, spread the mixing of cabbage, wood ash, soil etc., in ratio of 2:1 up to 2 inch

layer in the bottom of composting bin. Afterwards, sprinkle the water over the raw materials.

The above process is repeated up to 1 foot of the composter bin. Finally, one more layer of

bedding material with wood ash and soil is added in ratio of 4:2:1.

Fig.1. Cross sectional view of composter bin

It has solar air collector for providing hot air to the composter. The inlet of the collector is

connected with blower output and outlet is connected to the base of the composter bin. It has

temperature sensing unit such as thermocouple to measure the temperature of bin inside.

Generally, it is located in three various locations of the bin that is top, middle and base. One

end such as probe is kept on the bin and another end is connected with displaying unit.

Fig.2. Schematic representation of solar composting process

Table.1. Raw materials and its quantity

S.No Raw materials Quantity

1 Cabbage 6.0 kg

2 Soil 3.0 kg

3 Water Required quantity

4 Bedding material 0.850 kg

5 Wood ash 0.400

The blower blows air into the air heater which heats up the air to required temperature

depending upon various processes such as higher thermophilic process (80 o

C), mesophilic

process (25 o

C), lower thermophilic process (65 o

C). Then, the heated air is sent to inside of

the composter bin through pores on the base of bin. Here, flow control valve is used to

control the flow of air into inside of composter bin which is operated manually. In general,

three temperature sensors are connected with electronic monitoring unit to monitor the

temperature of composter bin at various locations such that top layer, middle layer and

bottom layer.

Fig.3. Photographical view of Experimental setup of solar composting process

Table. 2. Design parameters of solar composting process

Component Description Value

Flat-plate collector Area (Ac) 0.5 m2

Blower Rated power 1 hp

Composting bin Volume 0.121 m3

Depending upon the temperature of the bin, the flow control valve is opened or closed for

controlling air flow into the composter bin. Because of, the composter bin should be

maintained at various temperature levels in various stages of entire process. In addition to

that, the raw materials are turned periodically to motivate the microorganism’s growth for

entire process. The raw material is turned for three times to provide proper aeration and

providing of optimum moisture content that is 40% of total mass of composting material for

entire process. (i.e. First turning: 10-12 days after filling the bin, Second turning: 10 days

after first turning, Third turning: After 2 months of second turning).

Table.3. various phases of solar composting process

Phases Duration

(days)

Temperature range

(Standard)

(O C)

Low temperature thermophilic

process

10-12 70-85

Mesophilic process 12-27 20-45

High temperature thermophilic

process

27-120 65-70

Table.4. Solar composting process without thermal flat-plate collector

Phases Time(days) Temperature (⁰C)

Low temperature thermophilic process

1 30

2 32

3 33

5 34

7 35

10 36

12 35

Mesophilic process

15 33

22 31

23 30.5

24 30.2

25 29.5

26 29

28 28.8

High temperature thermophilic process

29 28

30 31

45 33

60 34

70 34.5

83 35

90 36

Results and discussion

The experiment on composting process has been performed which is conducted in two ways

such as natural and solar energy influenced composting process. The test was conducted in

Chennai at latitude of 13.0827 o N and longitude of 80.2707

o E in the month of September to

December. The results for a sample collected from the various place of the composting bin

such as bottom, middle and top surfaces have been chosen randomly and are used to plot the

graphs correlating the various parameters of the system.

Fig.4. shows the Temperature vs Time curve. This curve represents that the three various

phase’s temperature levels. In low temperature thermophilic process, the temperature is

maintained at the range of 30 - 36 o C, in mesophilic process about temperature is maintained

at the range of 27 - 36 o C and in high temperature thermophilic process, the temperature is

maintained at the range of 35 - 36 o

C on an average. The temperature range is not fair

compared with standard defined temperature levels. It has also affected the quality of

compost that is NPK composition. But, it is better than natural composting process.

Fig.5. shows the C/N ratio vs Time curve. It represents that the degradation of carbon and

nitrogen during composting process from 20:1 to 10:1. It affects duration of composting

process which purely depending on microorganism’s growth and activity. Here, the solar

irradiation and proper aeration is provided to encourage microorganism’s growth and activity.

Where, the microbial activity was not better due to uncomfortable environment. But, it is fair

than natural composting process. Fig.6. shows Carbon reduction vs Time curve. It reveals the

degradation of carbon contents, where the carbon reduction rate is gradually increased from

the beginning that is reached high as 18% at 50th day but after that, the rate of carbon

degradation is maintained at same rate. It is fair one, but it can be improved further up to 30 -

35% by providing best probable environment to the composting bin.

Fig.7. shows Moisture content vs Time curve. It exposes that the moisture content level in the

composting bin and also it is maintained in the range of 60-65%. But, it is to be maintained in

the range of 40-50% of total mass. Due to high moisture contents, the microorganism may be

destroyed which will delay the decomposition rate and reduce the compost quality. Fig.8.

shows Microorganism growth vs Time curve. It expresses that the microorganism growth

over the period with corresponding temperature. The microorganism growth rate is found to

be high at 15th

day to 20th day; thereafter it is gradually decreased up to the end of this

process. Moreover, the composting bin’s temperature is also playing the major role in it.

Fig.4. Temperature vs Time

Fig.5. C/N ratio vs Time

Fig.6. Carbon reduction vs Time

Fig.7. Moisture content vs Time

Fig.8. Microorganism growth vs Time

Conclusion

The experimental setup of solar composting process consists of composting bin, air spreader

unit, flow control valve, electronic monitoring unit and solar flat-plate collector which is

designed and fabricated. The work includes the study of composting process for a scale unit

of 0.121 m3

of bin size. The NPK growth of the compost is investigated through conducting

proper test on compost. Such a kind of process provides more quality of the compost that is

NPK growth compared to a convention composting process as it conducts at a controlled

environment. The conclusions deduced from the present work are:

1. The period of solar composting process is 3 months without solar thermal flat-

plate collector.

2. The NPK growth on the compost in solar composting process is 1:2:1

3. The volume of the organic matter is finally reduced to half of original volume that

is 0.061 m3 from 0.121 m

3.

4. For the same process, by introducing solar thermal flat-plate collector the

composting period can be considerably reduced and the quality of compost also

can be highly improved.

Phases Sample quantity

(g) N P K ratio

Low temperature thermophilic

process

100 3:3:2

Mesophilic process 100 2:2:1

High temperature thermophilic

process

100 1:2:1

Further work is needed to identify and design an advanced predictive control strategies for

Solar composting process on optimum environment forecast and determine how the process

can be optimized. It shows that need of solar flat-plate collector and thermal storage system

for control strategies to provide better environment to composting microorganism.

The results presented in these papers are very optimistic one. Even though this work has been

carried out for a specific environment, it has the potential that solar composting process may

be used in a compost preparing applications. Since it requires solar energy only, the cost of

solar composting process is inexpensive.

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