In-vessel co-composting of horse stable bedding waste and blood meal at different C/N ratios:...

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In-vessel co-composting of horse stable bedding waste and

blood meal at different C/N ratios: process evaluation

Journal: Environmental Technology

Manuscript ID: TENT-AWT-2011-1346.R1

Manuscript Type: Advance Waste Treatment Technology

Date Submitted by the Author: n/a

Complete List of Authors: Selvam, A; Hong Kong Baptist University,

Keywords: Composting, In-vessel, Abattoir blood meal, Horse stable bedding waste, C/N ratio

URL: http:/mc.manuscriptcentral.com/tent

Environmental Technology

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IN-VESSEL CO-COMPOSTING OF HORSE STABLE BEDDING

WASTE AND BLOOD MEAL AT DIFFERENT C/N RATIOS:

PROCESS EVALUATION

Jonathan W.C. Wong a,*

, Ammaiyappan Selvam a, Zhenyong Zhao

a, Obuli. P. Karthikeyan

a, Shuk Man Yu

b, Alex C.W. Law

c and Patricia C.P. Chung

d

Affiliations:

a Sino-Forest Applied Research Centre for Pearl River Delta Environment, Hong Kong Baptist

University, Kowloon Tong, Hong Kong SAR, P.R. China.

b Electrical and Mechanical Services Department, Hong Kong SAR, P.R. China

c Jardine Engineering Corporation Ltd, Hong Kong SAR, P.R. China

d New Green Environmental Science Ltd, Hong Kong SAR, P.R. China

Corresponding author

* Corresponding author. Tel: +852 34117056, Fax: +852 34112095, E-mail:

[email protected] (J.W.C. Wong)

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ABSTRACT

Abattoir blood meal is rich in nitrogen and its potential as a co-composting material for

horse stable bedding waste (HSB) was evaluated at two C/N ratios, 32 (LBM, low blood

meal) and 16 (HBM, high blood meal), to improve the nutrient contents of the final

compost. The mix was composted for 7 days in a 10 tonne/day in-vessel composter and

cured aerobically. After 56 days of composting, ammoniacal-N, CO2 evolution rate and

C/N ratio of both LBM and HBM were within the guideline values; however, delayed

decomposition and lower seed germination index were observed with HBM. Besides,

HBM resulted in 84% loss of the initial ammoniacal-N. Almost similar organic

decomposition, 62.4% and 59.6% with LBM and HBM, respectively, were achieved.

However, a stable compost product can be obtained within 6-7 weeks with LBM;

whereas >8 weeks were required for HBM composting. Therefore, co-composting at the

C/N ratio of 32 is recommended to achieve odour-free and faster composting.

Keywords: Composting, In-vessel, Abattoir blood meal, Horse stable bedding waste,

C/N ratio

1. Introduction

Straw and other materials such as newspaper cuttings, peat, saw dust, hemp and linen are

commonly used as bedding materials to improve the hygiene of the horse stables [1]; of

these, straw pellet was reported to be more effective in controlling the ammonia emission

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than other materials [2]. Disposal of the bedding wastes, containing the horse manure

and urine, is also an integral and important component of equine husbandry. Like other

manures, pathogens in the manure can be a potential problem when disposing these

wastes. Effective control of pathogens during the thermophilic stage makes composting

as a safe disposal method for these bedding materials. The advantage of composting is

the formation of an organically stabilized and pathogen free end product that can be used

for organic farming. Since a lot of straw is commonly used to absorb the urine, we

expect that the bedding waste will have a higher carbon/nitrogen (C/N) ratio than the

optimum range of 25-30. C/N ratio is one of the important feedstock properties affecting

the efficiency of the decomposition as well as compost quality. Besides, the uneven

distribution of nitrogen components in horse stable bedding waste (HSB) makes it

unsuitable for composting [3]. Therefore, to obtain an effective composting mix, the

HSB should be mixed with materials having high nitrogen (N) content.

Abattoir blood meal (BM), a by-product from animal slaughtering houses, is rich in N

source comprising mainly of globular proteins that are easily hydrolysable [4]. This BM

can be a potential source of N to co-compost with the HSB so that effective and fast

composting of HSB can be achieved. Use of BM for composting purpose along with

HSB has not been attempted and/or reported earlier. Blood meal is readily soluble due to

the mandatory heat treatment with steam vapour at 133 °C for 20 min to eliminate all

human and animal transmissible diseases. Thermal treatment causes the partial

hydrolysis of complex proteins and the release of amino acids and polypeptides that are

promptly mineralized to ammonium [5-6]. With the easily hydrolysable nature,

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composting BM alone may result in nitrogen loss and the emission of ammonia.

However, successful co-composting of BM with HSB would solve their disposal

problems as well as complete the nutrient cycling through compost production.

Among the different composting methods, in-vessel composting has advantages like less

space requirement and rapid degradation of organic matter [7]. Therefore, the present

study aimed at investigating the feasibility of using BM as a co-composting material for

the HSB composting. Besides, the investigation was conducted at different mixing

ratios resulting in different initial C/N ratios (16 and 32) so as to reveal the effect of

initial C/N ratios on the process efficiency, nitrogen transformation and product quality

in a 10 tonne/day (tpd) pilot-scale in-vessel composter. The results obtained have the

potential to be applied in commercial scale composters.

2. Materials and methods

2.1. Composting mix preparation and composting process

HSB and BM were collected from Beas River Jockey Club and Sheung Shiu abattoir

blood meal collection point, Hong Kong, respectively. HSB consisted of horse manure

and the straw as the bedding material. Selected properties of the HSB and the BM are

presented in Table 1. The HSB was mixed with BM at 39:1 (low blood meal treatment,

LBM) and 9:1 (high blood meal treatment, HBM) (HSB:BM, w/w fresh weight basis) to

obtain the C/N ratios of 32.2 and 15.7, respectively. The HSB was conveyed into a screw

mixer and shredded into small strips of 5 to 10 cm in length before adding an appropriate

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quantity of BM using a bin lifter. The weight and moisture content of composting

feedstock were determined and recorded for the calculation of the mass balance. The mix

was then loaded into an in-vessel composter through conveyer belt with a feeding rate of

10 tpd. Temperature profile of the composting mass inside the composter was

continuously monitored using temperature sensors installed at eight different locations

inside the drum. The composting mix was fed to the composter continuously while the

partially mature compost was discharged from the other end of the composter. The

premature compost discharged from the composting vessel was further cured in negative

aerated bunkers of size 9 m x 5 m x 2.5 m for a period of 49 days. The curing piles were

moisturized daily and turned over once a week, while temperatures of the curing piles

were logged continuously through a computer control systems. For each treatment,

feeding was performed for about two weeks and between each treatment, a blank run was

made to avoid any cross contamination. Once the piles matured, the sampling and

analysis were discontinued.

2.2. Sampling and chemical analyses

Since the in-vessel composting is a continuous process, duplicate samples of the initial

composting mixes and premature compost discharged from the in-vessel composter were

collected daily over three consecutive days to compensate for the heterogeneity of the

waste materials. Hence the reported results are means and standard deviation of six

samples. For those from the curing bunkers, compost samples were collected periodically

for the analyses of different parameters to assess the stability and maturity; and the values

reported are mean ± SD of three replicates. All samples were analyzed for pH, electrical

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conductivity (EC), total organic carbon (TOC), total Kjeldahl nitrogen (TKN),

ammoniacal nitrogen (NH4+-N) and carbon dioxide (CO2) evolution rate as per the Test

Methods for the Examination of Composts and Composting (TMECC) [8]; and the seed

germination index as per the Hong Kong Organic Resource Centre (HKORC) [9]. The

final matured compost was also analysed for the heavy metals and pathogens as per

TMECC [8].

3. Results and discussion

Physicochemical properties of the HSB and BM used in this study are presented in

Table 1. The C/N ratio of the HSB was higher than the optimum range of 25-30;

although this level would not affect the composting seriously, it may delay the

composting process. Besides, a lower bulk density may allow the mass to dry quicker

due to the very high free air space. Therefore, mixing it with a suitable co-composting

material, which can hold some moisture and provide higher surface area for the growth

of microbes, would increase the rate of composting. In this case, HSB mixed with BM

can provide an ideal surface for the microbes to act upon and accelerate the

decomposition.

3.1. Temperature profile of in-vessel composting and curing processes

Within two days, the temperature of the composting mass reached the peak values (zone

3) of 67.1 ºC and 66.6 ºC for LBM and HBM, respectively, and gradually decreased to

62.2 ºC and 59.6 ºC at zone 8, respectively (Figure 1a), indicating that thermophilic

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condition could be established easily in the composting mass. Temperature of LBM was

consistently higher than HBM during the in-vessel composting period. The average

in-vessel composting temperatures were 64 oC and 62

oC for both LBM and HBM

respectively, which was high enough to kill the pathogens. During the initial stages of

curing, high temperatures were observed in both trials indicating the active

decomposition (Figure 1b). However, there was an initial delay in the increase in

temperature for HBM; and after 4 weeks of curing, temperature of HBM was higher

than that of LBM treatment indicating the delayed decomposition with HBM implying

that the LBM mix can reach maturity faster than the HBM mix. Nevertheless, the pile

temperatures gradually declined to ambient temperature after 6-7 weeks of curing. The

gradual decline in temperature during the curing phase was a well-known phenomenon

due to the exhaustion of the readily available organic matter [7].

3.2. Changes in pH and electrical conductivity

The pH of the composting mass increased during the first week from 7.24 and 7.42 to

8.24 and 8.81 in the LBM and HBM treatments, respectively, because of the release of

ammonia and its subsequent solubility in the aqueous phase (Fig. 2a). After day 7, the

pH decreased gradually to 7.8 and 8.1 in LBM and HBM, respectively. During the

whole composting process, the pH of HBM treatment was consistently higher than that

of LBM. Despite the differences in the C/N ratio of the two treatments, the differences

in pH between HBM and LBM were not significant indicating that pH may not be a

good maturity index of the composting progress [10]. The initial EC of the composting

mix were 4.5 and 4.9 mS/cm for LBM and HBM treatments, respectively, due to the

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high EC of the blood meal used. Initially, there was a reduction in EC for both

treatments and the reduction was higher with HBM than that with LBM treatment (Fig.

2b). The reduction was probably due to the loss of NH4+-N from the composting mass

[11]. From the second week onwards, i.e., after the in-vessel composting, the EC

increased gradually until the end of the experiment with comparatively higher values for

HBM than that of LBM due simply to the loss of mass [11]. However, after 6 weeks, the

EC slightly declined with HBM treatment and the difference between the two treatments

was marginal after 8 weeks.

3.3. Changes in carbon, nitrogen and C/N ratio

Changes in TOC, TKN, NH4+-N and solid C/N ratio are presented in Fig. 3. As expected,

the TOC contents decreased rapidly during the in-vessel composting period and

gradually thereafter during the curing stages (Fig. 3a). For the HBM treatment, the TOC

decreased from 44.1% to 37.6% in 56 days, accounting for about 23.4% loss. In contrast,

the TOC of the LBM treatment decreased from 44.8% to 37.4% in 56 days accounting

for about 26.3% indicating a higher decomposition of the composting mass similar to

the results of total organic matter (results not shown), although the difference was

marginal. However, the reduction in TOM was about 9.5% higher for LBM treatment

indicating an initial C/N ratio of 32 would be more suitable than the C/N ratio of 16.

Besides, there was a delay in the organic decomposition in HBM treatment implying

that a longer curing time would be required for HBM. Since the difference between the

decomposition of the two treatments was marginal, a higher ratio can also be used

which would allow a higher utilization of blood meal; however other parameters such as

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nitrogen loss should also be considered in such attempts.

The TKN contents of HBM was significantly higher than the LBM simply due to the

nitrogen content of BM, which led to a higher initial loss of nitrogen from 2.86% on day

0 to 1.79% in the first 7 days (Fig. 3b). After 2 weeks of composting, the TKN contents

increased slightly in both treatments with HBM increasing to 2.28% and LBM to 1.91%

after 56 days of composting. The overall increase in TKN at the end of the composting

period could be attributed to the reduction in compost mass. The initial loss of N might

be due to the solubility of N in BM in the form of NH4+-N which would be easily

volatile at the high pH experienced in the early stage of composting because of the shift

of the ammonia–ammonium ion equilibrium [12]. The NH4+-N concentrations in the

compost mass sharply decreased from 5100 and 2577 mg/kg in the initial composting

mix (day 0) to 811 and 601 mg/kg on day 7 for HBM and LBM treatments, respectively

(Fig. 3c). Factors such as low C/N ratio [13-14], pH > 7 [15], thermophilic temperature

[16], particle size [17], and low available oxygen content [18] are associated with higher

N loss during composting. This reduction accounted for about 84% and 76% for HBM

and LBM treatments, respectively; indicating odour emission will be of concern

especially for HBM. Nevertheless, the ammonia concentrations of the two treatments,

after 56 days, were well within the standard limit of the organic composts [8-9,19],

indicating that 8 week period is enough for the composting process.

High nitrogen loss during the first two weeks of composting resulted in an increase of

the C/N ratio from 15.7 to 24.0 in the HBM treatment (Fig. 3d). After that, the C/N ratio

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slowly declined to 16.6 on day 56. In contrast, C/N ratio of the LBM treatment showed

a gradual decline from 32.2 on day 0 to 19.6 on day 56. In both the treatments, C/N

ratios of mature compost were less than the guideline value of 25 for the organic

composts [9]. Since the organic decomposition did not differ significantly between the

two treatments, the change in the nitrogen profile would be the major factor influencing

the C/N profile. In the present study, the increase in C/N ratio for HBM was due to

higher N loss through volatilization than the rate of C degradation.

Mineralization of the organic matter coupled with increase in N content resulting from

the loss of dry weight led to the decreasing C/N ratios as reported earlier [20-21]. Zhu

[22] composted swine manure with rice straw at C/N ratios of 20 and 25 using aerated

systems in composting bins and found that the C/N ratio of 20 resulted in ~8% higher

nitrogen loss. Further, when composting manure (cattle manure + pig manure, 10:1) and

straw with an initial C/N ratio of 25.3 and 22.1, the C/N ratios increased in the drum

composter to 26.0 and 24.2 before declining [23-24]. These results indicate that when

the C/N ratio was lower than 25, it tend to increase initially before decline during

subsequent composting, implying N loss can be expected with lower initial C/N ratio

and its suitable adjustment is an effective way to reduce the N loss.

3.4. Maturity evaluation

3.4.1. Seed germination index

Seed germination index gradually increased to about 80% in about 6 and 8 weeks in

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LBM and HBM composting, respectively (Fig. 4). The low seed germination index

matched well with the release of ammonium and possibly organic acids, which were

removed by the microbes in a later stage of composting. After 56 days of composting,

seed germination index increased to 109% and 82% in LBM and HBM composts,

respectively. The GI value of ≥80% is considered an indicator for stable compost [9]

and ≥90% is indicative of ‘very stable’ product [8]. Therefore, very stable compost

could be achieved in LBM composting within 7-8 weeks; whereas for HBM, longer

curing period may be necessary. Results from the composting of other organic wastes

such as pig manure [22], food waste [25] and sludge [26] also indicated that compost

maturity can be achieved faster if the initial C/N ratio is within the optimum range, i.e.

25-30, for composting.

3.5. Final compost quality

The nutrient and maturity properties of the final compost products from both LBM and

HBM after 56 days are presented in Table 2. Among the different components analyzed,

NH4+-N content was higher in HBM (261 mg/kg) than in LBM (177 mg/kg); but still

within the required standard value of 500 mg/kg [8]. The CO2 evolution rates were

almost similar in both the treatments and within the regulatory limits of ≤ 2 gC/kg

VS/day. pH values were in neutral to alkaline range and hence suitable for land

application and the C/N ratios were less than 20 meeting the requirement for maturity

from both trials. Seed germination index was ≥ 80% for both treatments after 56 days of

composting; however, a shorter maturation period was observed for LBM. The N, P and

K contents of the HBM compost were higher than that of LBM compost indicating the

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possibility of using higher mix ratio with the purpose of fast turnover of the blood meal;

however, the nitrogen loss and ammonia emission must be considered. Further, the

heavy metal contents, as presented in Table 3, were well within the Compost and Soil

Conditioner Quality Standards for Organic Farming [9] and CCME [27]. Besides,

Salmonella and fecal coliforms were within the guideline values indicating that the

thermophilic conditions achieved in the in-vessel composter were sufficient to remove

the pathogens.

3.6. Mass balance

Based on the difference in the composting mass, the organic reduction was calculated to

assess the mass balance. The mass reduction during the in-vessel composting period

was about 57% and 45% on wet weight basis and 44% and 40% on dry weight basis for

LBM and HBM, respectively. The loss on fresh weight basis was higher than on dry

weight basis and the differences were due to the loss of moisture during composting.

The mass balance results confirmed the higher decomposition in LBM than HBM. The

overall reduction during the composting process was almost similar after 56 days,

62.4% and 59.6% on dry weight basis for the LBM and HBM, respectively. Similar

decomposition with other feedstock such as sewage sludge was achieved with similar

composting system before [26]; indicating the efficiency of the in-vessel composter.

4. Conclusions

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The results revealed that abattoir blood meal is a suitable co-composting material to

compost horse manure straw bedding waste. Overall decomposition with low (C/N 32)

and high (C/N 16) blood meal content was almost similar (62.4-59.6%) indicating that

both the mixing ratios could reach maturity but LBM provided a better composting

condition and shorter maturation period compared with HBM treatment. In addition,

nitrogen loss/ammonia emission was significantly higher with HBM. In conclusion,

co-composting horse manure straw bedding waste with abattoir blood meal at a mixing

ratio of 39:1 (C/N ratio of 32) is recommended to achieve effective composting process

and reduce ammonia emission.

Acknowledgements

We thank the Hong Kong Environmental Protection Department and Electrical and

Mechanical Service Department for the permission in using the data generated from the

consultancy work for this publication.

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Cat. No. PN1341, 2005.

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Figure captions

Figure 1. Temperature profiles of the in-vessel composter (a) and curing piles (b) during

co-composting of horse stable straw bedding waste with blood meal.

Figure 2. Changes in pH (a) and electrical conductivity (b) of the composting mass

during the co-composting of horse stable straw bedding waste with blood

meal.

Figure 3. Changes in total organic carbon (a), total Kjeldahl nitrogen (b) ammoniacal

nitrogen (c) and C/N ratio (d) of the composting mass during co-composting

of horse stable straw bedding waste with blood meal.

Figure 4. Changes in seed germination index during co-composting of horse stable

straw bedding waste with blood meal.

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Table 1. Selected physicochemical properties of horse stable straw bedding waste and

abattoir blood meal used in the present study.

Parameter Horse stable straw bedding

waste Blood meal

Moisture content (%) 63.4 ± 0.1a 62.8 ± 2.0

pH 7.14 ± 0.03 8.52 ± 0.17

Total organic matter (%) 91.3 ± 0.2 98.1 ± 0.2

Total organic carbon (%) 45.6 ± 0.2 35.5 ± 1.9

Total nitrogen (%) 1.10 ± 0.13 13.12 ± 1.01

Total phosphorus (%) 0.024 ± 0.007 0.07 ± 0.002

Total potassium (%) 0.06 ± 0.01 0.24 ± 0.008

C:N ratio 37.8 ± 0.4 2.7 ± 0.3

Bulk density (t/m3) 0.45 ± 0.07 0.64 ± 0.14

a: Mean ± standard deviation (n=6)

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Table 2. Maturity and nutrient properties of the compost obtained from co-composting

of horse stable straw bedding waste with abattoir blood meal.

Parameters

Standard values LBM

(C/N – 32)

HBM

(C/N – 16) HKORCa

TMECCb/

others

Ammoniacal-N (mg/kg dw) ≤ = 700 75-500 177 ± 92d 261 ± 57

CO2 evolution rate (g C/kg

VS/day) ≤ = 2 2-4 1.76 ± 0.18 1.75 ± 0.20

C:N ratio ≤ 25 ≤ 25 19.8 ± 0.3 16.6 ± 1.3

pH Value 5.5 - 8.5 7.78 ± 0.16 8.11 ± 0.03

Organic matter (% dw) > 20 >40c 83.8 ± 2.4 85.8 ± 0.5

Seed germination index (%) ≥ 80 80-90 109 ± 14 82 ± 6

Total nitrogen (as N % dw) - 1.91 ± 0.04 2.28 ± 0.17

Total phosphorous (as P2O5 %

dw)

- 0.74 ± 0.07 1.06 ± 0.06

Total potassium (as K2O % dw) - 1.84 ± 0.06 1.85 ± 0.11

Total N, P, K (% dw) ≥ 4 - 4.47 ± 0.15 5.19 ± 0.28

a- HKORC [9], Compost and Compost and Soil Conditioner Quality Standards.

b- TMECC [8] standards for ‘stable’ compost quality.

c- CCME [27], Guidelines for compost quality.

d- Mean ± Standard deviation (n=3)

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Table 3. Heavy metals and pathogen contents of the compost obtained from

co-composting of horse stable straw bedding waste with abattoir blood meal.

Parameters Standard values LBM

(C/N – 32)

HBM

(C/N – 16) HKORC

a CCME

b

Heavy metals

Arsenic (mg/kg) ≤ 10 ≤ 13 1.0 ± 0.2c 1.5 ± 0.2

Cadmium (mg/kg) ≤ 1 ≤ 3 <0.2 <0.2

Chromium (mg/kg) ≤ 100 ≤ 210 3.5 ± 1.0 2.5 ± 0.4

Copper (mg/kg) ≤ 300 ≤ 400 79 ± 13 54 ± 11

Lead (mg/kg) ≤ 100 ≤ 150 2.0 ± 1.4 2.5 ± 2.1

Mercury (mg/kg) ≤ 1 ≤ 0.8 <0.05 <0.05

Nickel (mg/kg) ≤ 50 ≤ 62 3.0 ± 0.4 2.0 ± 0.3

Selenium (mg/kg) ≤ 1.5 ≤ 2 <1 <1

Zinc (mg/kg) ≤ 600 ≤ 700 98 ± 6 95 ± 9

Pathogens

Salmonella sp

(MPN/ 4 g) ≤ 3 ≤ 3 < 3 < 3

Fecal coliforms

(MPN/ g) ≤ 1000 ≤ 1000 529 ± 30 463 ± 16

a- HKORC [9], Compost and Soil Conditioner Quality Standards for Organic

Farming.

b- CCME [27], Guidelines for Grade A compost quality.

c- Mean ± Standard deviation (n=3)

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In-vessel temperature zone

1 2 3 4 5 6 7 8

Te

mp

era

ture

(o

C)

50

55

60

65

70

75

LBM : C/N - 32

HBM : C/N - 16

Days

10 20 30 40 50

Te

mp

era

ture

(o

C)

20

40

60

80 LBM : C/N - 32

HBM : C/N - 16

(a)

(b)

Figure 1.

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pH

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5 LBM : C/N - 32

HBM : C/N - 16

Days

0 10 20 30 40 50 60

Ele

ctr

ica

l c

on

du

cti

vit

y

(mS

cm

-1)

2

3

4

5

6

7

(a)

(b)

Figure 2.

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To

tal o

rga

nic

carb

on

(%

)

32

36

40

44

48LBM : C/N - 32

HBM : C/N - 16

To

tal

Kje

lda

hl n

itro

gen

(%

)

1.0

1.5

2.0

2.5

3.0

Days

0 10 20 30 40 50

C/N

rati

o

15

20

25

30

35

(a) (b)

Days

0 10 20 30 40 50

NH

4

+ (m

g/k

g)

0

2000

4000

6000

(d)(c)

Figure 3.

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Days

0 10 20 30 40 50 60

Ge

rmin

ati

on

in

de

x (

%)

0

20

40

60

80

100

120

140

LBM : C/N - 32

HBM : C/N - 16

Figure 4.

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In-vessel co-composting of horse stable bedding waste and blood meal at different C/N ratios:...

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