Comparative Biodrying Performance of Municipal Solid Waste in the Reactor under Greenhouse and Non-greenhouse Conditions

Journal of Environmental Treatment Techniques

2021, Volume 9, Issue 1, Pages: 211-217

J. Environ. Treat. Tech.

ISSN: 2309-1185

Journal web link: http://www.jett.dormaj.com

https://doi.org/10.47277/JETT/9(1)217

Comparative Biodrying Performance of Municipal

Solid Waste in the Reactor under Greenhouse and

Non-greenhouse Conditions

Katitep Ngamket, Komsilp Wangyao, Sirintornthep Towprayoon*

The Joint Graduate School of Energy and Environment (JGSEE), King Mongkut’s University of Technology Thonburi, Center of Excellence on Energy

Technology and Environment (CEE), PERDO, Ministry of Higher Education, Science, Research and Innovation, Bangkok

Received: 01/09/2020

Accepted: 05/11/2020

Published: 20/03/2021

Abstract

The high moisture content of municipal solid waste yields a lower energy content of solid fuel that affects the thermal conversion

efficiency. Biodrying is an alternative drying method using bio-heat generated by microbial metabolism to reduce the moisture content of

municipal solid waste. This research was conducted in three pilot-scale biodrying reactors, two under greenhouse conditions compared with

one conventional non-greenhouse condition. Two bunkers with greenhouse cladding were connected with aerators, and airflow rates were

set at 0.4 and 0.6 m /(kgwaste·day), respectively. Meanwhile, a passive aeration method was applied to the non-greenhouse bunker. This study

aims to investigate the effect of the greenhouse condition on the biodrying process and assess the performance of the drying process through

different operating conditions. The result shows that the greenhouse mainly affects the air temperature rise in the reactor. The aeration rate

is positively correlated with weight reduction (r = 0.93). At 0.6 m /(kgwaste·day) airflow rate, the treatment can reach a moisture content less

than 30% on average within ten days, while at 0.4 m /(kgwaste·day) airflow rate, it takes 15 days to reduce the moisture content to less than

0%. Biodrying under the greenhouse condition with active aeration potentially achieves desirable moisture content reduction and heating

value increase more efficiently than the common biodrying. However, the airflow rate is a crucial factor in determining the suitable drying

time in biodrying under the greenhouse condition.

Keywords: Biodrying, Greenhouse, Municipal Solid Waste, Refuse Derived-Fuel, Solar Radiation

There are many available drying methods for MSW, such as

Introduction

thermal drying and biodrying. The benefit of thermal drying is the

shorter drying period (2.64±1.44 h) [2]. Meanwhile, the drying

time of biodrying is about 16±7 days [2]. Nonetheless, thermal

drying could have higher maintenance and operation costs for

large-scale drying applications. Biodrying is an alternative drying

method that uses metabolic energy from biodegradation for water

evaporation in MSW or wastewater sludge.

Aeration is a critical concern for heat generation from aerobic

decomposition. The various optimal airflow rate in many

biodrying studies depends on initial moisture content, reactor

type, and feedstock composition. A study from Shao et al. [3]

conducted biodrying lysimeter measurements, using 0.34

The Thai government has been developing the Bio-Circular-

Green Economy (BCG) model since 2019. This model focuses on

socio-economic and environmental improvements to reach

sustainable development goals. The linear economy of the take-

make-use-dispose concept is being reformed into a circular

economy. The development of community-based biomass power

plants using refuse-derived fuel (RDF), which is obtained from

processed municipal solid waste (MSW), is aligned with the BCG

model. Additionally, alternative energy will amount to 30% of all

energy consumption in this country by 2036 [1]. The challenge of

RDF production is that MSW has a high moisture content (MC)

and organic fraction. The MSW characteristics cause low energy

content in RDF, leading to low energy gain from the thermal

conversion process. Therefore, it is necessary to improve the

MSW properties for the efficiency of energy production by

reducing the MC while increasing the heating value (HV), by

adding a drying process to MSW preparation before the heat

conversion process.

m /(kgwaste·day) of airflow rate to dry MSW with initially 74%

MC for 16 days resulting in 30% MC. Research of Tom et al. [4]

set 0.53 m /(kgwaste·day) of airflow rate to reduce MC of MSW

from 61.25 to 30% in approximately 33 days. In the case of

Debicka et al. [5], a study in the industrial-scale biodrying reactor

applied an airflow rate at 1.80–2.16 m /(kgwaste·day) to reduce MC

by 50% within one week (the final MC was less than 22%). The

Corresponding author: Sirintornthep Towprayoon, The Joint Graduate School of Energy and Environment (JGSEE), King Mongkut’s

University of Technology Thonburi (KMUTT), Bangkok, Thailand. E-mail: sirin.jgsee@gmail.com

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2021, Volume 9, Issue 1, Pages: 211-217

advantages of the biodrying method are the decreases in weight,

volume, and MC of MSW. However, this method can be a costly

technology for developing countries.

2.2 MSW preparation

MSW feedstock from the On-nut waste transfer station in

Bangkok, Thailand, was unpacked and mixed until homogenous.

The feedstock was sampled to determine the initial characteristics

prior to entering greenhouse bunkers. The MC measurement of

MSW follows the oven-drying method at 105 °C (ASTM D-3173,

1997), which employs a bomb calorimeter to estimate the heating

value of MSW. Each greenhouse bunker loaded approximately

4,000 kg of MSW. The feedstock in all treatment units was set at

1.2 m height.

Biodrying under greenhouse conditions is appropriate for

developing countries that have high solar radiation intensity. In

Thailand, the average of daily solar radiation intensity is 17.6

MJ/(m ·day) [6]. The drying process under greenhouse conditions

contributes to two main effects: 1) higher air temperature and

relative humidity in the greenhouse is than the external condition

and 2) an improvement in microbial growth and activity [7].

Additionally, the research of biodrying under greenhouse

conditions conducted by Zaman et al. [8] showed that the

2.3 Experimental design

biodrying process can abate CO

emission by 13 times and can

We performed three different conditions: a greenhouse with

produce RDF with an HV up to 6,265 kcal/kg. According to the

meta-analysis by Tun and Juchelkova [2], most biodrying studies

were conducted on a laboratory scale. Particularly in biodrying

under greenhouse conditions, there is no publicised research on a

pilot or industrial scale. To fill in this gap in the literature, this

study aims to compare the effect of greenhouse and non-

greenhouse conditions on the biodrying process on a pilot scale.

The result of this novel design study can be a practical direction

for the improvement and implementation of MSW biodrying

under greenhouse conditions in developing countries.

an aeration rate of 0.4 m /(kg·day) (0.4AR), a greenhouse with an

aeration rate of 0.6 m /(kg·day) (0.6AR), and a passive aeration

condition (CB). The CB was operated under a water-proof

cladding, which is a non-greenhouse condition, to compare to the

biodrying performance under the greenhouse condition. We

assigned passive aeration in the control system, and this system

was simulated as a conventional windrow biodrying. The

experimental period of all conditions was 15 days.

2.4 Data collection and monitoring

In all greenhouse bunkers, four thermocouples were placed

into the core layer of the MSW feedstock (more than 60 cm from

the top of the waste pile) and on the waste pile surface.

Meanwhile, one thermocouple was suspended above the waste

pile inside the greenhouse, and another was set outside the

greenhouse for measuring the ambient temperature. We used type

K thermocouples with a temperature range of -270 to 1,327 °C to

monitor the temperature change during the experiment. A

Graphtec (GL240, Japan) data logger recorded all temperature

data. We installed a pyranometer (RK200-03, China) above the

greenhouse to measure the solar radiation intensity, which crosses

the greenhouses daily pending the experiment. The daily

accumulated intensity values of solar radiation were retained by

the data logger (RK600, China) connected directly to the

Materials and Method

.1 Greenhouse structure and design

Three greenhouse bunkers are located at the On-nut sewage

treatmentplantnear the On-nut waste transferstationin Bangkok,

Thailand. We designedthe squarebunkersto be 3.50 m wide, 4.35

m long, and 2.20 m high. The base for the bunkers was raised to

.25 m to provide inner gutters. These internalgutters allowed for

air pathways and leachate drainage. A centrifuge aerator was

connected at the posterior of each greenhouse; the air was fed

through 6 in (0.15 m) diameter pipes. The ventilation fans at the

superior space inside greenhouses automatically expelled the

exhausted air when the relative humidity inside the greenhouse

exceeded 60%. When relative humidity was less than 60%, the

exhausted air was drained by passive ventilation. The controller

box at the rear of each greenhouse contained a data logger,

humidity sensor, and switching power supply. Metal rooftops

were covered by transparent acrylic material. We set the base

angles of triangle rooftop at 35º to advance high solar radiation

transmissivity [9]. Figure 1 illustrates the structure of the

greenhouse bunker in this experiment.

pyranometer. Dried MSW at the 10 and 15 days was also

sampled to determine the product characteristics during the

drying process. Parameters and analytical methods are the same

as those mentioned in section 2.2.

.5 Assessment of drying performance

All treatments in this experiment were compared based on

various measurements of drying performance. We focused on

weight loss, temperature differences, and the fraction of HV

increase. Those indices are described as follows.

.5.1 Weight loss

Weight loss is related to MC reduction after the drying

process. We weighed the feedstock before and after the

experiment to calculate the percentage weight loss (%wl) in

different operating conditions using Equation 1. This index can

indicate not only the mass reduction but also the fuel cost

improvement for waste transportation.

%wl=(∆w/wi)×100

(1)

Figure 1: The structure of greenhouse bunker

where ∆w is the difference between initial and final MSW weights

kg), and wi refers to the initial MSW weight (kg).

(

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2021, Volume 9, Issue 1, Pages: 211-217

.5.2 Temperature difference

performed by mesophilic and thermophilic microorganisms [10].

In this study, the pattern of temperaturechanges varies depending

on operating conditions. A comparison of the temperature profile

in different MSW layers is shown in Figure 2. The results show

that 0.4AR achieved the highest temperature at the surface layer

(54.53±2.15 °C in daytime and 55.53±2.42 °C at night).

Remarkably, there is a nonsignificant difference between surface

temperatures in CB and 0.6AR on daytime (p = 0.51) despite

0.6AR operating under the greenhouse condition. 0.6AR also

presentsthe lowest average temperatureat the surface MSW layer

(49.68±2.62 °C in daytime and 47.22±2.37 °C at night) because

An important parameter in bioprocessing is temperature, as it

affects the biodegradation and evaporation of water in MSW. The

daily temperature change during the experiment can indicate the

drying behaviour in the system. The difference in operating

conditions can result in heating moist MSW and, moreover, could

affect the waste and ambient temperatures. To distinguish the

temperature differences between the three treatments, the

temperature integration (TI) indicates the accumulated daily

differences between the average waste temperature and the

ambient temperature. The calculation of TI is as follows:

operating under 0.6 m /(kg·day) might release the heat inside the

system by air ventilation.

푇퐼 = ∑^푛_ꢀ_ꢁ₁(푇푊푖 − 푇퐴푖)

(2)

As shown in Figure 2B, CB presents the highest core

temperature (54.69±2.92 °C in daytime and 54.39±2.60 °C at

night) in comparison to the other treatments. It is evident that CB

can hold the heat in the middle of the waste pile since there is no

significant difference between the day and night core

temperatures (p = 0.99). This result differs from the case of the

greenhouse with the aeration system. In the 0.4AR and 0.6AR

treatments, a significant difference occurred between the day and

night core temperatures (p < 0.01). Additionally, there is a

significant difference between core temperatures of 0.4AR and

0.6AR in the daytime (p < 0.05), while the result shows no

significant difference between them at night (p = 0.22).

Remarkably, 0.6AR is the only treatment that shows a

nonsignificant difference between surface and core temperatures

(p = 0.22). Hence, the aeration rate influences the temperature

profile of feedstock, which is an essential factor affecting the

drying process in the system.

where TWi (°C) refers to the average waste temperatures, and

TAi (°C) is the ambient temperature on the experimental day i.

The different types of bunker coverages can cause variations in

the air inside and outside the bunkers. The difference in air

temperature (DA), defined as the accumulated air temperature

difference between inside and outside the treatments, indicates the

effect of the different roof types. The calculation of DA is shown

below.

퐷퐴 = ∑^푛_ꢀ_ꢁ₁(푇퐺푖 − 푇퐴푖)

(3)

where TGi (°C) refers to the air temperature in the system (or

above the waste pile), and TAi (°C) is the ambient temperature on

the experimental day i.

.5.3 Heating value increase fraction

The initial and final HVs were investigated, and the HV

increase was reported in terms of an increasing fraction. The

following equation shows the calculation of the fraction of HV

Figure 2C shows the air temperature inside the greenhouse

among the treatments. There are significant differences in air

temperaturesin the system in the daytime between CB and 0.4AR

(p < 0.01) and between CB and 0.6AR (p < 0.01). On the other

hand, there is a nonsignificant difference in the air temperature

between all treatments at night (p = 0.99). The air temperature

inside the greenhouse fluctuated extensively. These results

indicate that solar radiation causes the air temperature to rise in

the greenhouse condition. In the daytime, the difference in the air

temperatures above the waste piles in CB and 0.4AR is 8.21 °C,

while this difference is 8.75 °C for CB and 0.6AR.

(

HVF) increase:

HVF=∆HV/HVi,

(4)

where ∆HV is the difference between the final and initial

HVs, and HVi is initial HV. The HV for this equation refers to

the lower HV (kcal/kg).

Temperature profiles illustrate the dissimilarity of

temperature change patterns under various operating conditions.

These profiles also show the critical role of aeration and

greenhouse conditions in the biodrying process. Roble-Martinez

et al. [11] conducted a study with laboratory-scale greenhouse

dryers under passive aeration. The greenhouse in this study could

perform drying temperatures between 23–32 °C. Fabian et al. [7]

conducted a similar study with artificial greenhouses under

passive aeration, and the greenhouse achieved drying

temperatures of 29–37 °C within the first four days. Fabian et al.

.6 Statistical analysis

The mean values of the hourly temperature at four monitored

points in each MSW layer were reported. One-way analysis of

variance was used to investigate the statistically significant

differences between comparative treatments. To accept the

alternative hypothesis that refers to a statistically significant

difference between comparative treatments, a significance level

of less than 5% (p < 0.05) was applied. The Pearson correlation

was used to determine the correlation between the two

parameters. The r-value refers to the correlation coefficient that

describes the strength of the relationship between two variables

in the positive or negative direction. The R program for windrows

was used for all statistical analyses.

[7] stated that the waste temperature above 45 °C for the

thermophilic phase was unavailable owing to the low volume of

the waste pile (0.5 m high and 2.5 m long), and the bio-heat was

not sufficient to evaporate water in organic wastes in this case.

Meanwhile, the greenhouses of our study achieved a drying

temperature range of 43.22–57.92 °C in 0.4AR and 42.86–

Results and Discussion

.1 Temperature profiles

Typical aerobic decomposition is able to heat feedstock or

52.49 °C in 0.6AR. The crucial factor was that the volume of the

waste pile in our study was adequate for heat capture inside the

MSW pile. For CB, the drying temperature range was 42.26–

organic substrates by microbial metabolism, which drives the

temperatures above 50 °C, and the organic degradation is

56.94 °C similar to that of 0.4AR. However, the obvious

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2021, Volume 9, Issue 1, Pages: 211-217

distinction is the air temperature in the bunker. The air

temperature under the greenhouse condition was higher than in

the non-greenhouse condition as CB, and the higher air

temperature indicates a higher water absorbability affecting the

water content reduction.

Figure 2: Temperature profiles of all treatments: surface temperatures, (B) core temperatures, and (C) air temperatures inside greenhouses

.2 Solar radiation intensity throughout the drying process

Solar radiation is the determining factor in raising the air

receiving solar energy as well as CB. This result reveals the

importance of airflow rate. Even though 0.6AR operated under

the greenhouse condition, this airflow rate can be a disturbing

factor that causes air temperature loss in the greenhouse.

Nevertheless, there is no relationship between surface

temperatures and solar radiation intensity

temperature inside greenhouse cladding. In this experiment, the

minimum solar radiation was 8.34 MJ/(m ·day), and the

maximum value was 17.70 MJ/(m ·day). The average value was

5.66 MJ/(m ·day). Figure 3 illustrates the relationship between

solar radiation intensity and the air temperatures in the treatments.

There are strong relationships between solar radiation intensity

and air temperature inside the greenhouses in 0.4AR (r = 0.76)

and in 0.6AR (r = 0.63). The result also shows a strong correlation

between both parameters in the case of CB, the non-greenhouse

condition (r = 0.61). The correlation coefficients, or r-values,

indicate that operating under the greenhouse condition with the

low aeration rate (0.4AR) raises the air temperature in the

greenhouse more significantly than the other cases. Meanwhile,

.3 Product characteristics

The product from the drying process is RDF. According to the

American Society for Testing and Materials (ASTM), RDF is

classified into seven groups depending on physical

characteristics. In this study, dried MSW is RDF-1 (in the form of

raw MSW) following the ASTM standard E856-83 (2006) and

requires an additional mechanical process to treat for RDF

utilisation. Considering chemical characteristics, the desirable

properties of RDF based on the market’s needs are: 1) MC is less

.6AR can affect the air temperature inside the bunker when

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2021, Volume 9, Issue 1, Pages: 211-217

than 30% by mass, and 2) LHV is greater than 4,500 kcal/kg [12].

Table 1 shows the product characteristicsin this experimentat the

recommended to achieve desirable RDF characteristics for the

implementation on a commercial scale.

th th

0 and 15 days of the drying process. The result indicates that

all treatmentcan reach an LHV of more than 4,500 kcal/kg within

ten days. Nonetheless, theMC is stilla significant concern for this

experiment. The average MCs in the 0.6AR and CB cases were

less than 30% within ten days. After 15 days, all treatments

reached an average MC of less than 30%. Remarkably, the high

variability of both MC and HV occurred in this experiment

because MSW as feedstock has high heterogeneity.

Homogenising MSW and mixing during the drying process are

3.4 Assessment of the biodrying process in the experiment

This section addresses the assessment of biodrying process

under different conditions. TI and DA were calculated following

Equation 2 and 3, respectively. The accumulated temperature

difference in the form of TI indicates the self-heating capability

under various conditions. Meanwhile, DA can indicate the heat

capture capacity in the bunkers. The result shows a significant

difference of TI between the treatments, and CB presents the

highest TI value (see Table 2).

Figure 3: Relationship between solar radiation intensity and air temperatures in the bunkers

Table 1: Product characteristics

0 days

15 days

10 days

15 days

Initial heating

value (kcal/kg)

Treatment

Initial MC (%)

Final HV

(kcal/kg)

Final MC (%)

(

kcal/kg)

26.24±12.85

33.62±18.49

24.62±10.08

25.88±21.09

22.01±18.07

22.35±19.38

6,070.78±1,057.90

5,774.56±680.70

6,337.15±621.40

6,461.63±1,006.95

7,550.84±265.55

7,079.80±17.55

.4AR

.6AR

68.73±2.21

901.44±270.65

Note: heating value refers to lower heating value (LHV) as received

Table 2: Assessment of biodrying between treatments

Moisture reduction

0 days

Moisture reduction

HVF

TI (°C)

DA (°C)

Weight loss (%wl)

15 days

62.35%

67.98%

67.48%

10 days

5.73

15 days

6.17

607.11

529.66

439.91

23.75

21.26%

24.82%

28.84%

61.82%

51.08%

64.18%

.4AR

.6AR

163.03

173.92

5.41

7.38

5.29

6.85

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2021, Volume 9, Issue 1, Pages: 211-217

Remarkably, the aeration rate is inversely proportional to TI

with active aeration potentially has higher efficiency in MC

reduction and HV increase compared to the non-greenhouse

condition with passive aeration. 0.6AR and CB produced

desirable characteristics of RDF based on the market’s needs

within ten days, while 0.4AR yielded this state after 15 days.

Nevertheless, operation in the CB condition is not recommended

because an open system is non-hygienic for waste management.

The heterogeneity of MSW yielding uncertainty of RDF

characteristics, especially in MC, is a significant concern.

Homogenising and mixing prior to the biodrying process should

be added. However, the balance of energy consumption for the

biodrying process with additional machinery required for

desirable RDF characteristics should be further evaluated.

(

r = -0.93), revealing the role of forced aeration on the heat

capture in the MSW pile. Yuan et al. [13] conducted a study on

the effects of bulking agent addition in biodrying process, and the

aeration rate of their study was set at 0.43 m /(kgwaste·day). In the

case of MSW feedstock, TI was 523.7 °C, which is similar to the

TI in 0.4AR of our study. There is a nonsignificant difference in

DA between 0.4AR and 0.6AR (p = 0.72) despite operating under

different aeration rates. This result indicates that the greenhouse

condition has an effect on heat capture capability of air inside the

system in comparison to the treatment with the non-greenhouse

condition.

When considering weight and moisture reduction, weight loss

is directly weakly proportional to moisture reduction (r = 0.20).

Unlike the meta-analysis study of biodrying from Tun and

Juchelkova [2], this study found that the correlation coefficient

between weight and moisture reduction was 0.80 in developing

countries. This difference in results is due to the undefined MSW

heterogeneity effect on the final MC in our study. However, the

result shows a strong correlation between aeration rate and weight

loss (r = 0.93) indicating the critical role of aeration in MSW

weight reduction. 0.4AR had the least moisture decrease after ten

days, but, by the 15^t^hday of the experiment, 0.4AR yielded the

same MC as 0.6AR. Meanwhile, CB exhibited a constant

moisture reduction rate in both the 10^t^hand 15 days of the

experiment. MC over 30% causes further bioactivity in the waste

pile [14], so 0.4AR possibly continued bio-heat generation until

the end of the biodrying process.

Aknowledgment

This research was financially supported by the Thailand

Research Fund (TRF), Pairojsompongpanich Co. Ltd. (PSP), The

Joint Graduate School of Energy and Environment (JGSEE),

King Mongkut’s University of Technology Thonburi (KMUTT),

and the Center of Excellence on Energy Technology and

Environment (CEE), PERDO, Ministry of Higher Education,

Science, Research and Innovation. We extend our sincere thanks

to Asst. Prof. Dr. Suthum Patumsawad and Assoc. Prof. Dr. Pipat

Chaiwiwatworakul for their advice. Additionally, we most

gratefully acknowledge all cohorts in the waste research team at

JGSEE for their help to facilitate the experiment.

Ethical issue

According to Table 2, the HV increased by more than five

times in all treatments within ten days, and more than six times

within 15 days of the drying process. Remarkably, the HV

Authors are aware of, and comply with, best practice in

publication ethics specifically with regard to authorship

(avoidance of guest authorship), dual submission, manipulation

of figures, competing interests and compliance with policies on

research ethics. Authors adhere to publication requirements that

submitted work is original and has not been published elsewhere

in any language.

increase fraction increased by more than 1.5 from the 10 to the

5 day in the treatments under greenhouse condition with active

aeration (0.4AR and 0.6AR). Meanwhile, this fraction increased

by less than 0.5 times in CB.

Altogether, the results indicate that active aeration is vital for

the biodrying process, and airflow rate affects the suitable drying

time to obtain desirable RDF characteristics. Biodrying under the

greenhouse condition with low active aeration can unlock the

maximum efficiency of moisture reduction and HV increase in

comparison to the control system of CB, which has no significant

change between 10 and 15 days of drying time. Although CB can

reach the RDF standard of the market’s needs within ten days,

operating on the open condition has no sanitation on a waste

management issue. In a study by Malinowski and Wolny-Koladka

Competing interests

The authors declare that there is no conflict of interest that

would prejudice the impartiality of this scientific work.

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