Manuscript_HKIE Environmental Competition_Chow Jun Kang-20140319 (1)

7/22/2019 Manuscript_HKIE Environmental Competition_Chow Jun Kang-20140319 (1)

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1

Quality of Compost and GHG Emissions of Food Waste Composting1

for Four Types of Food Waste Generated at HKUST2

3

Jun Kang Chowa*4

a UG student (His Supervisor is Ir Prof Irene Lo)5

Department of Civil and Environmental Engineering,6

The Hong Kong University of Science and Technology,7

Clear Water Bay, Hong Kong8

* Email: [email protected]; Tel: 852-943320999

10

ABSTRACT: The burgeoning of municipal solid waste (MSW), particularly food11

waste, has drawn great attention from people in Hong Kong. Of the approximately12

9,000 tonnes of MSW that is thrown away at landfills every day, about 40% are made13

up of putrescibles, which mainly consist of food waste. To manage such a huge14

amount of food waste, alternatives, such as composting, anaerobic digestion, fish feed15

production, etc., have emerged. Composting is the bioconversion of biodegradable16

materials into humus, producing carbon dioxide, water and other organic, inorganic17

materials as by-products. In this study, the influences of different types of food waste18

and the operating conditions of composting on the quality of compost produced were19

investigated. A food waste decomposer with a daily maximum input capacity of 2 kg20

was used as the study system. Food waste was collected from four types of restaurants21

in The Hong Kong University of Science and Technology. The Compost and Soil22

Conditioner Quality Standards 2005 published by Hong Kong Organic Resource23

Centre (HKORC) was used as the standard to evaluate the compost characteristics24such as compost maturity, compost quality, seed germination index and nutrient25

content. In addition, a life cycle assessment (LCA) approach was used to evaluate the26

environmental impacts and greenhouse gas (GHG) emissions associated to food waste27

composting.28

29

30

Keywords31

Composting; Food waste; Life cycle assessment; Greenhouse gas emissions32


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Abbreviations133

1. Introduction34

Food waste is any waste, whether raw, cooked, edible and associated with35

inedible parts generated during food production, distribution, storage, meal36

preparation or consumption of meals (HKEB, 2014). Roughly one-third of the food37

produced in the world for human consumption is wasted (FAO, 2011). In a society38

that is producing an ever-increasing amount of food waste, Hong Kong is39

experiencing a major challenge on the management of food waste. The putrescible,40

which was mainly made up of food waste (around 90%), by wet mass percentage of41

total municipal solid waste (MSW) was 41.7% at year 2012 as shown in Figure 142

(HKEPD, 2012).43

1CLP, China Light & Power; EF, Emission Factor; GHG, Greenhouse Gas; GWP, Global Warming

Potential; HKORC, Hong Kong Organic Resource Centre; HKSAR, Hong Kong Special

Administrative Region; HKUST, Hong Kong University of Science and Technology; IPCC,

Intergovernmental Panel on Climate Change; KBPCP, Kowloon Bay Pilot Composting Plant; LCA,

Life Cycle Assessment; MSW, Municipal Solid Waste; OWTF, Organic Waste Treatment Facility;

VOA, Volatile Organic Acid

Figure 1 Composition of MSW in Hong Kong at year 2012


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Presently Hong Kong relies solely on landfills for waste disposal.44

Approximately 9,000 tonnes of unrecoverable MSW are discarded in the landfills45

every day. However, Hong Kong is experiencing a serious shortage of MSW disposal46

sites. The current three strategic landfills, namely South East New Territories (SENT),47

North East New Territories (NENT), and West New Territories (WENT) are expected48

to reach their maximum capacities in 2015, 2017, and 2019 respectively (HKEPD,49

2013a). In addition, landfills potentially lead to negative environmental impact such50

as leachate contamination, greenhouse gas (GHG) emissions, and space limitation51

(Slater and Frederickson, 2001; Norbu et al., 2005). Consequently, the government of52

Hong Kong Special Administrative Region (HKSAR) aims to reduce the amount of53

food waste that goes to landfills by at least 40% by 2022, which is about a reduction54

of 500,000 tonnes per year (HKEB, 2014). To initiate the work of collecting and55

treating food waste in Hong Kong, Kowloon Bay Pilot Composting Plant (KBPCP)56

was developed in the middle 2008 (HKEPD, 2013a). Also, Organic Waste Treatment57

Facility (OWTF) is being constructed to recycle organic waste (mostly food waste)58

from commercial and industrial sectors, thereby minimizing the requirement for59

landfill disposal (HKEPD, 2013b). In short, the emergence of alternatives in food60

waste management is the result of increased environmental awareness and serious61

shortage of waste disposal sites.62

As mentioned above, food waste accounts for almost 40% of the composition of63

MSW being disposed of at landfills in Hong Kong. Landfilling of food waste in Hong64

Kong is not a sustainable approach to treat food waste due to land scarcity, high cost65


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and limited capacity for waste infrastructures. Currently, the food waste disposal in66

Hong Kong is equivalent to throwing away the weight of approximately 25067

double-decker buses every day (HKEB, 2014). In addition, two-thirds of the food68

waste (around 2,500 tonnes) produced in year 2011 was originated from households.69

Hence, reducing the quantity of food waste is critical to Hong Kong in achieving the70

overall waste reduction target by 2022, particularly by cutting down the amount of71

food waste from households. With regard to this challenge, composting can72

potentially be a great alternative to solve the problem of food waste management.73

Composting is defined as the aerobic biological degradation and stabilization of74

organic substrates in conditions that allow development of thermophilic temperatures,75

as a result of biologically produced heat, to produce a final product that is stable, free76

of pathogen, and beneficial to land (Haug, 1993). Thermophilic refers to the77

condition where heat is developed for the growth of microorganisms to break down78

the organic waste and its typical range is about 45 - 65C as shown in Figure 2.79

Figure 2 Variation of temperature during composting


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However, the smaller systems used for household composting are not likely to get80

as hot as compost in large piles or windrows as they are usually associated with well81

aeration and ventilation system (Trautmann, 1998). The technology adopted in this82

project is in-vessel composting where food waste is composted in a reactor under83

uniform condition of moisture and temperature. To produce high quality of compost,84

the following initial conditions of food waste and operating conditions should be85

achieved:86

a) Food waste of carbon (C) to nitrogen (N) ratio of 25-30: C provides energy for87

cellular growth whereas N is the source of protein synthesis.88

b) Continuous oxygen supply: to promote aerobic composting.89

c) Moisture content of 50-60%: excessive moisture content blocks the pore space90

and triggers the onset of anaerobic composting.91

d) Addition of amendments: to maintain appropriate moisture content and increase92

the quantity of biodegradable materials of the compost pile.93

The composting process is a biological breakdown of organic matter associated94

with GHG emissions (Brown et al., 2008). According to the Fourth Assessment95

Report of the IPCC, the most of the observed increase in global average temperature96

since the mid-twentieth century is very likely (i.e, >90% probability) due to the97

observed increase in anthropogenic greenhouse gases concentration (IPCC, 2007). In98

conjunction with this, the HKSAR Government has set a target to reduce carbon99

intensity by 50-60% by 2020 from the 2005 level for its own region (HKEB, 2010).100

The waste management sector accounted for approximately 3-5% of total101


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anthropogenic GHG emissions at a global scale in 2005 (UNEP, 2009). In Hong Kong,102

the maximum, minimum, and annual average shares of GHG emissions from the103

waste sector from 1990 to 2006 were 5.9%, 3.2%, and 4.5% respectively, which was104

the third largest sector after electricity generation and transportation (HKEB, 2010).105

Therefore, the GHG accounting for food waste composting should be investigated in106

order to evaluate its environmental impacts.107

In short, this project aims to investigate how different types of food waste and108

operating conditions in affecting the quality of compost produced, and to evaluate the109

GHG emissions associated to food waste composting using LCA approach.110

111

2. Material and methods112

2.1 Investigation of the quality of compost produced113

2.1.1 Setup of the experimental facility114

The food waste decomposer model CF-100, named Earth System Automatic115

Composter was used as the subject of study. This experimental composter was located116

at the pantry at ground floor of Undergraduate Hall III, Hong Kong University of117

Science and Technology (HKUST) as shown in the Figure 3.118

All parts of the decomposer including leveling glides, central machine body and119

air pipes were assembled. The enzyme pack sawdust (amendment), enzyme120

(microorganism) and sugar (food) was added into the storage tank. In order to activate121

the enzyme, 2000 cm3 of clean water was then poured into the decomposer to mix122

well all the compost materials. This process took about one day to complete.123


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Meanwhile, a power meter was installed to record the electricity consumed by the124

decomposer. After one day, food waste was regularly added into the decomposer to125

start composting.126

127

2.1.2 Preparation of composting materials128

Food waste (post-consumer food waste) was collected from four types of129

restaurants at the HKUST as depicted in Figure 4. They were labelled as samples 1, 2,130

3 and 4 with respect to the food waste collected from the G/F Chinese Restaurant131

(Chinese cuisine), LG7 Canteen (Chinese fast food), LG5 McDonalds (western fast132

food) and 1/F Cafeteria (western cuisine).133

Food waste decomposer

Air pipe

Power meter

Apparatus for pre-treatment of food waste

Figure 3 Setup of apparatus of food waste composting project


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Each kind of food waste was collected over a 3-week period. Although food134

waste was collected from different types of restaurant, they were still categorized as135

food waste from the school canteens, whereby the components in food waste collected136

were similar, which mainly consisted of rice, bread, vegetables, etc. as shown in137

Figure 5. Pre-treatment of food waste was carried out to remove the oil and salt138

content in the food waste. Before adding into the decomposer, food waste was cut into139

3cm to ease the decomposition process. The maximum amount of food waste added140

was 2 kg in order to avoid odour problem.141

Figure 4 Location of food waste collected: (a) G/F Chinese restaurant

b LG7 Canteen c LG5 McDonalds and d 1/F Cafeteria

(c) (d)

(a) (b)


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142

2.1.3 Monitoring of the composting process143

Monitoring process was mainly relied on the control panel located on the top of144

decomposer as depicted in Figure 6.145

(c

(a)

Figure 5 Food waste collected from (a) G/F Chinese restaurant (b) LG7 Canteen

(c) LG5 McDonalds and (d) 1/F Cafeteria

(b)

(c) (d)

TEMPERATURE

DISPLAY

HEATING LIGHT

POWER

LAMP

EXTRACTOR

FAN

ALARM

Figure 6 Control panel of the food waste decomposer


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The control panel shows the functions of aeration, rotating mixers and146

temperature of the system. An alarm is activated when the mixer is not rotating or147

there is hard material in the decomposer. The temperature of the composting system148

was kept at the range of optimum operating temperature, 36 50C, and monitored149

regularly in order to ensure aerobic decomposition had taken place. Aeration was150

checked by placing the operators hand on the outlet of the air pipe to ensure air151

emissions. Also, the level of soil was observed. If the soil level did not drop after two152

to three days with no food input, a check had to be made to ensure the aeration system,153

temperature of the system and particles size of food waste were optimal to ensure154

effective composting.155

156

2.1.4 Samples Curing157

For each type of food waste, 3 samples were extracted. To ensure the158

homogenous properties of the compost produced, only the second and third samples159

were used for laboratory testing. The extraction of compost took place at each 10-day160

interval. Food waste was added in the first 7 days to provide composting materials. It161

was followed by 2 days of no input to ensure complete decomposition of food waste162

and the compost was extracted on 10th day. The soil storage tank was adjusted to163

appropriate level to extract the compost. About 2.02.5 kg of the compost was taken164

out from the decomposer. After extracting, the compost was placed at a ventilated165

area for curing, with no direct exposure to the rain droplets and insects. Compost was166

allowed to cure for 3 weeks to ensure complete stabilization and maturation.167


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2.1.5 Determination of compost properties168

The Compost and Soil Conditioner Quality Standard 2005 was used as the169

guideline to determine the characteristics of four compost samples produced, namely170

compost maturity, compost quality, seed germination index and nutrient content171

(HKORC, 2005). The definitions of the parameters are explained as follows:172

a) Compost maturity: to measure the degree or level of completeness of composting.173

b) Compost quality: to measure the quality of compost applied for the purpose of174

farming and agricultural.175

c) Seed germination index: to indicate whether the compost is inhibitory to plant.176

d) Nutrient content: to measure the nutrient N, phosphorus (P), and potassium (K)177

content of compost in order to classify it as organic fertilizer or soil conditioner.178

Table 1 shows the characteristics of compost to be assessed as established by179

HKORC. All the laboratory tests were carried out by ALS Laboratory Group. Based180

on the laboratory test result obtained, the characteristics of compost was analyzed to181

classify the compost as organic fertilizer or soil conditioner depending on the criteria182

given in the standard as shown in Table 2.183


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Table 1 Standard of tests conducted to assess the quality of compost (HKORC, 2005)184

185

Table 2 Classification of compost based on its characteristics (HKORC, 2005)186


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2.2. LCA study of composting187

In studying the life cycle assessment of a composting process, ISO standards188

(ISO 14040, 2006a; ISO 14044, 2006b) were used as the guidelines. The goal and189

scope definition were first determined. The main objective of this LCA study was to190

evaluate the environmental impacts of food waste composting by accounting its GHG191

emissions.192

Climate change was selected as the impact category and global warming potential193

(GWP) with a time horizon of 100-year time was used in this study. CO2-equivalent194

(CO2e) emitted by a kilogram of treated food waste (wet basis) was chosen as the195

category indicator (functional unit). In a global warming (GW) context, GHGs are196

released from composting process due to degradation of organic waste, and energy197

used by the decomposer for turning and managing the waste. Although there are more198

than 1000 types of GHGs, only six of them are classified as major GHG emissions by199

the Kyoto Protocols, namely carbon dioxide (CO2), nitrous oxide (N2O), methane200

(CH4), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur201

hexafluoride (SF6) (UNFCCC, 1997). According to Intergovernmental Panel on202

Climate Change (IPCC), composting is an aerobic process and a large fraction of the203

degradable organic carbon (DOC) in the material is converted into CO2. However,204

CO2 released from composting is biogenic, which is accounted for as part of the205

natural carbon cycle with land-used and therefore, it is not considered as an206

anthropogenic emission (IPCC, 2006). CH4 is formed in the anaerobic sections of207

compost, but is oxidized to a large extent in the aerobic sections of the compost (IPCC,208


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2006). Since the fully aerated condition is developed in the decomposer, CH4was not209

considered as the GHG emission in this project. N2O is a stable gas which is released210

from microbial cells during nitrification and denitrification (Firestone and Davidson,211

1989). It is found that N2O is mainly produced in the curing phase, later stage of212

composting process, when the readily available C has been consumed, thereby it was213

considered as GHG emission in this project. Table 3 summarizes the consideration of214

each GHG in this food waste composting.215

216

Table 3 Consideration of each type of GHG in food waste composting217

GHGTo be considered in

this project (/)Explanation

Carbon dioxide Biogenic

Methane Oxidized under aerobic condition

Nitrous oxide Produced in nitrification and

denitrification processes

Hydrofluorocarbons Insignificant amount

Perfluorocarbons Insignificant amount

Sulphur hexafluoride Insignificant amount

218

On the basis of the GHG emissions estimates for each individual process, the219

general equation for calculating the net GHG emissions from food waste composting220

is shown in Equation 1.221

GHGCOMPOSTING= GHGEC+ GHGN2OE (1)222

Where GHGCOMPOSTING= net GHG emissions from food waste composting, GHGEC=223


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GHG emissions from electricity consumption of decomposer, GHGN2OE = GHG224

emissions from N2O.225

The system boundary of this LCA study was from the stage of food waste added226

into the decomposer to the end of curing of compost. The scope of study included the227

GHG emissions associated to the electricity consumption by food waste decomposer228

and the direct GHG emissions of food waste composting. The avoided GHG229

emissions of food waste composting from substitution of inorganic fertilizer was not230

considered as the compost produced was not suitable to be used as organic fertilizer.231

In order to estimate the CO2-equivalent produced by the electricity consumption of232

decomposer, the Sustainability Report 2012 published by China Light and Power233

(CLP) was referred. The electricity consumption emission factor (EF) used was 0.58234

kg CO2e/kWh with the fuel composition of 49.1% of coal, 19.2% of gas and 31.7% of235

nuclear (CLP, 2013). The modelling of estimation of CO2e emission during food236

waste composting was based on the assumption that food waste was treated237

completely in 14 days and electricity was equally consumed in decomposing the food238

waste. The general equation of calculating the GHG emissions from electricity239

consumption by decomposer is shown in Equation 2.240

GHGEC= EFelectricity EC (2)241

Where GHGEC = GHG emissions from electricity consumption of decomposer;242

EFelectricity = Electricity consumption emission factor provided by CLP; EC =243

Electricity consumption to treat one kg of food waste (wet basis).244

Regarding the N2O emission, it was assumed that the initial C:N ratio of food245


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waste was between 22-30, and its corresponding conservation nitrogen was estimated246

between 85.2% and 99.5%, with an average value of 92.4% (Washington State247

University, 2013). N2O emission was estimated to 0.5% of its initial total nitrogen248

content during food waste composting (Paul and Dubey, 2013). The CO2e emission249

due to N2O emission was estimated as shown in equation 3.250

GHGN2OE = (NFINAL 100/ N%CONSER.) EFN2O GWPN2O (3)251

Where GHGN2OE = GHG emissions of N2O; NFINAL = Final nitrogen content in252

compost; N%CONSER. = Percentage of nitrogen conservation; EFN2O= Emission factor253

of N2O; GWPN2O= Global warming potential of 298 (for 100-year time horizon).254

255

3. Results and discussion256

3.1. Results of laboratory test for the compost samples257

The summary of laboratory test done by ALS laboratory group for each compost258

sample is shown in Table 4 and 5. Four compost samples passed most of the criteria259

required, except the volatile organic acid (VOA) concentration, pH, moisture content260

and total nutrient content. Due to the failure in passing these parameters, these four261

compost samples were not suitable to be used as organic fertilizers and soil262

conditioners.263


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Table 4 Summary of the result of laboratory test for the compost samples of four types of food waste264

Parameter Standard Unit

Sample ID

1 2 3 4

G/F

Chinese

Restaurant

LG7 CanteenLG5

McDonalds1/F Cafeteria

Compost MaturityGroup A

Ammonia conc. as N 700 mg/kg dw 5mm) 5 % dw < 0.1 < 0.1 < 0.1 < 0.1

Man-made foreign matters (impurities) 0.5 % dw < 0.1 < 0.1 < 0.1 < 0.1

Heavy metal

a

Arsenic 10 mg/kg dw < 1 1 < 1 < 1

Cadmium 1 mg/kg dw < 1 < 1 < 1 < 1

Chromium 100 mg/kg dw 1 < 1 < 1 < 1

Copper 300 mg/kg dw 4 4 3 2

Mercury 1 mg/kg dw < 1 < 1 < 1 < 1


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Nickel 50 mg/kg dw < 1 < 1 < 1 < 1

Lead 100 mg/kg dw < 1 < 1 < 1 < 1

Selenium 1.5 mg/kg dw < 1 < 1 < 1 < 1

Zinc 600 mg/kg dw 22 22 20 20

Physiochemical Properties

pH 5.58.5 N/A 4.6 4.7 4.9 4.9

Organic matter > 20 % dw 90.7 89.8 94.6 95.3

Moisture 2535 % 14.2 13.6 12.1 11.0Pathogen

Salmonella sp. 3 MPN/4g < 3 < 3 < 3 < 3

E. Coli 1000 MPN/g < 3 < 3 < 3 < 3

Seed Germination Index 80 % 100 100 100 100

Nutrient Content

Nitrogen as N % dw 2.760 2.390 2.210 2.370

Phosphorus as P2O5 % dw 0.459 0.328 0.344 0.346

Potassium as K2O % dw 0.587 0.571 0.374 0.417

Total 4 % dw 3.806 3.289 2.928 3.133aLimit used is for the purpose of organic farming, which the most critical compared to that of general agricultural use and non-agricultural use.bValue with grey shading indicates the parameter failed to satisfy the standard.


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Table 5 Summary of classification of the compost samples based on the standard265

proposed by HKORC266

Sample CompostMaturity

a

CompostQuality

b

SeedGermination

Index

NutrientContent

c

Classification asFertilizer/Soil

Conditionerd

1 Not qualified

2 Not qualified

3 Not qualified

4

Not qualifiedaCompost is labelled as good quality compost if it passes the compost maturity test.bAll compost samples failed to pass the compost quality due to acidic pH and low in

moisture content.cAll compost samples failed to achieve the lower limit of total nutrient content of 4%

dw.dGood quality compost is classified as fertilizer if it passes all the test, and is

classified as soil conditioner if it passes at least the tests of compost maturity, compost

quality and seed germination index. Immature compost is classified as fertilizer if it

passes compost quality, seed germination index and nutrient content tests, and isclassified as soil conditioner if it passes compost quality and seed germination tests.


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It is interesting to note that the NH3 concentration for both samples 3 and 4267

(western food waste) was higher than that for both samples 1 and 2 (Chinese food268

waste). Based on the observation throughout the food waste collection, Chinese food269

waste consisted of more rice and vegetables whereas western food waste made up of270

more bread and meats. Table 6 shows the nutrient information of food (serving size of271

100 g) provided by the Hong Kong Centre of Food Safety (HKCFS). It can be seen272

that both types of food consist of similar amount of carbohydrate (source of C) but273

typical western dishes consist of more protein (source of N). It was suspected that274

when available C was fully utilized without stabilizing all of the N, N lost in the form275

of NH3, resulting in higher amount of NH3in sample 3 and 4.276

277

Table 6 Nutritional information of typical food in Hong Kong278

Nutrient Content Chinese Rice Dishes Western Dishes

(Sandwich and burger)

Carbohydrates 1633.88 g 17.4448.19 g

Protein 3.208.40 g 10.3618.31 g

279

Similar observation was found in ammonia:nitrate (NO3-

) ratio and the final C:N280

ratio. The values for both parameters were higher for western food waste sample. This281

was due to the higher amount of NH3released in sample 3 and 4, resulting in higher282

NH3:NO3- ratio. When more NH3 was released, this indicated that less N was283

available the final compost, causing a higher level of C:N ratio.284

Next, VOA concentrations in the four compost samples were higher than the limit285


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proposed. VOA is the by-product of composting formed from the breakdown of fat286

and it degrades during the curing phase. This suggests that the degradation of VOA287

takes longer time than the curing time used, which is at least 3 weeks. According to288

DeVleeschauwer et al. (1981), a great amount of VOAs was found in fresh compost289

but only small amounts of acetic acid in five-month-old compost. Manios et al. (1987)290

found that organic acids decreased with composting time and germination of lettuce291

increased. It took 80 to 180 days for the phytotoxic effect (presence of organic acids)292

to disappear. Also, the pH of four compost samples ranged 4.6 4.9, which were out293

of the acceptable range (5.58.5) proposed. The compost produced was acidic, rather294

than neutral as required in the standard. The acidic property of composts was295

evidently related to the amount of VOAs.296

With regards to the moisture content of compost, values between 11.0 and 14.2%297

were obtained in four compost samples, which were lower than the optimum range298

proposed. Dry compost possibly causes dust and leads to the difficulty in mobilizing299

nutrients into the soil when applying on lands (Stofella & Kahn, 2001).300

The result showed that cress seeds germinated well in all compost. It showed that301

although the VOA was higher than the standard proposed, the compost was ready to302

be utilized. Keeling et al. (1994) used immature MSW compost, which contained303

600028000 ppm of acetic acids to test the phytotoxicity effect of plants. The species304

affected cress (Lepidium sativumL.) and lettuce (Lactuca sativaL.), Compare to the305

literature review, the acetic acid concentration of compost samples was much lower,306

which was ranged between 1240 2180 ppm. This infers that the inhibition of cress307


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and lettuce seed germination possibly occurs at higher concentrations of acetic acid.308

Based on the results, the total nutrients, constituted by nutrient N, P and K, of309

four compost samples were less than 4% of dry weight. The failure of meeting the310

standard was suspected due to the limitation of choices and variation of food waste in311

the school canteens, causing less available nutrient was obtained. The total nutrient312

content for four samples ranged 2.928 3.806% dw, with sample 3 (collected from313

LG 5 McDonalds) consisted of the least amount of nutrients. This result was314

consistent with the findings of fast food was associated with a less nutritious diet and315

poorer food choices (French et al., 2001).316

317

3.2 GHG emissions from food waste composting318

The calculated net GHG emissions of producing compost samples from 4319

different types of food waste is depicted in Figure 7. The net GHG emissions of four320

compost samples ranged 1.600-1.658 kg CO2e/kg food waste. Based on the result, the321

CO2e emissions was mainly contributed by the electricity consumption by322

decomposer, which attributed about 97.5% of the net GHG emissions, whereas N2O323

emission contributed nly around 2.5%.324


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The average electricity consumption for one kg of treated food waste (wet basis)325

was 2.697 2.781 kWh and the corresponding amount of CO2e emissions ranged326

1.5641.613 kg CO2e/kg food waste. Comparing to other studies (van Harren, 2009;327

Daskalopoulos et al., 1998), it is noticed that the capacity of food waste decomposer328

affects its GHG emissions as shown in Table 7. It can be explained by the concept of329

1.613 (97.30%)

0.045 (2.70%)(a)

1.564 (97.58%)

0.039 (2.42%)(b)

1.564 (97.76%)

0.036 (2.24%)(c)

1.607 (97.66%)

0.038 (2.34%)(d)

Electricity consumption (kg CO2e/kg food waste treated)

N2O emission (kg CO2e/kg food waste treated)

Figure7 Summary of GHG emissions of compost samples produced from food waste at (a) G/F

Restaurant, (b) LG7 Canteen, (c) LG5 McDonalds and (d) 1/F Cafeteria


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economies of scale, whereby works are done more efficiently with the increasing of330

scale of operation. Engineers have made a crude estimate of changing the size of331

equipment will change the capital cost by the 0.6 power of the capacity ratio the332

point six power rule (Moore, 1959). This applies to the amount of GHG emissions333

from the electricity consumption of decomposer. The larger the capacity of334

decomposer, the greater amount of food waste to be treated, the lower the electricity335

consumption per kilogram of food waste.336

337

Table 7 Comparison of electricity consumption of different composting system338

Journal/Project Amount of input

(kg of organic

waste/day)

Electricity consumption

(kWh/kg of organic

waste)

Sample 1 (G/F Chinese Restaurant) 1.04 1.613

Sample 2 (LG7 Canteen) 1.06 1.564

Sample 3 (LG5 McDonalds) 1.04 1.564

Sample 4 (1/F Cafeteria) 1.00 1.607

van Harren (2009) 6840 0.055

Daskalopoulos et al. (1998) 27300 0.035

339


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The estimated amount of CO2e from food waste composting contributed by340

emission of nitrous oxide ranged 0.036-0.045 CO2e/kg food waste treated. Although341

the characterization of N2O to CO2e is high with a GWP of 298, it contributes342

insignificantly to the GHG emissions as only a small amount of N2O is emitted in343

aerobic composting. However, this is not the comprehensive picture. Additional N 2O344

production is probable from the reactive nitrogen contained in the compost. Also,345

compost is typically being applied to land where the reactive nitrogen is free to346

nitrifying and denitrifying processes. Consequently, actual N2O losses may be greater347

than the estimated value.348

Due to the failure of classifying compost as organic fertilizer or soil conditioner,349

the avoided emissions of substituting the inorganic fertilizers was not considered in350

this study. Estimated value of 0.014-0.028 kg CO2e/kg of food waste is avoided if the351

compost produced is able to be applied to substitute the inorganic fertilizer (Smith et352

al., 2001). Also, CH4 emission was not considered in this LCA study as it was353

assumed that the constant aeration provided suppressed the CH4formation. Awareness354

should be given in monitoring the composting process to assess whether it is carried355

under aerobic or anaerobic condition. If composting was found to be done under356

anaerobic conditions, CH4 released should be considered in the GHG emissions.357

358

4. Conclusions359

Despite the well-controlled condition of composting provided by the food waste360

decomposer, the four compost samples were not suitable to be used organic fertilizers361


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or soil conditioners according to the standard proposed by HKORC. This was mainly362

due to the insufficient time in curing period, causing the incomplete degradation of363

VOA, thereby giving rise to the acidic pH. Significant difference was observed in the364

parameters of NH3 concentration, NH3:NO3- ratio and C:N ratio. This was because365

western food waste consisted of more protein. As a result, more N was released when366

C became the limiting factor. Regarding the LCA study of food waste composting,367

electricity consumption by food waste decomposer was the main contributor of GHG368

emissions, which was about 97.5%. Based on the findings of CO2e emissions from369

electricity consumption by food waste decomposer, it showed that the limitation of370

capacity (amount of food waste input) resulted in higher CO2e emissions per kilogram371

of food waste treated. Further comprehensive measurements ought to be carried out372

that take account of the activities contributing to GHG emissions and reduction in373

food waste composting.374

375

Acknowledgement376

The author thankfully acknowledges the financial support and invaluable377

assistance from Associate Engineers Ltd. Company.378

379

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