BIOGAS PRODUCTION FROM CO-DIGESTION OF HORSE MANURE AND

WASTE SEWAGE SLUDGE

Elvin Agayev* and Aysenur Ugurlu**

*Hacettepe University Environmental Engineering Department,

Beytepe, Ankara, Turkey, elvinaga@hacettepe.edu.tr

**Hacettepe University Environmental Engineering Department,

Beytepe, Ankara, Turkey, ugurlu@hacettepe.edu.tr

ABSTRACT

The biogas yield of horse manure was investigated in

batch and semi-continous digesters under mesophilic

conditions (35 ±2⁰ C) under three stages. The biogas yields

obtained in the batch studies fed with horse manure after

35 days of the digestion period were 339 ml/gVS, 374

ml/gVS, 370 ml/gVS, 381 ml/gVS for 0,5 %, 1%,2% and

4% TS contents respectively. In the second part of the batch

studies, biogas formation from co-digestion of horse

manure with waste sewage sludge was investigated. The

horse manure (90%) was mixed with waste sewage sludge

(10%) and the biogas production was investigated under

two different starting VS concentrations: %4 and %2. The

biogas yields after 35 days were 410 ml/gVS and 425

ml/gVS respectively. In the third stage, co-digestion of

horse manure (90%) and sewage sludge (%10) mixture was

investigated in a lab scale semi-continuous digester with

the volume of 5 l. Total solids content was %4. The resulted

VS reduction was about 50%.

Keywords: biogas, horse manure, co-digestion, renewable

energy, methane yield.

1 INTRODUCTION

Biomass is a valuable renewable energy source.

Anaerobic digestion of animal wastes for production of

biogas have various benefits, including production of heat,

light, and electricity,transformation of organic waste into

high-quality fertilizer, improvement of hygienic conditions

through reduction of pathogens, and environmental

advantages through protection of soil, water, air, and

woody vegetation [1,2]. The biogas produced can

contribute the greenhouse gas emissions by substituting

fossil fuels. Anaerobic digestion of animal manure has a

great methane production potential. It is reported that

biogas production from animal manure with a hydraulic

retention time of 15-30 days is 50-70% and the

corresponding conversion of VS into methane is 0.20-0.25

m3/kg. The particulate matter present in the animal manure

requires long period for solubilization and hydrolysis which

are the rate limiting steps for their digestion. The other

parameters that affect the methane production from manure

are the hydraulic retention time and solids loading rate. It is

known that longer HRT periods are necessary for a higher

conversion of VS into methane due to the complex organic

materials present in the manure. Temperature effect has

been observed by many researchers [3–5]. However,

concerning the anaerobic digestion at psychrophilic

temperature there is a severe lack of fundamental

knowledge [6]. With respect to retention time, the

fermentation has been report to become more stable with a

higher methane yield and reduction of VS with an

increasing hydraulic residence time (HRT) [3].

Long retention times are required for manure digestion,

not only due to the presence of complex organic

compounds, but also due to the high concentration of

ammonia nitrogen as well as sulphates, which affects

anaerobic decomposition [7].The studies with different

animal manures showed that cattle manure produced 0.28

CH4 m3/m3/d, swine manure 0.64 m3/m3/d. A small scale

study with horse and cattle dung mixture resulted in biogas

production in a range of 0.325-0.75 m3/m

3/d.

Municipal wastewater treatment systems include sludge

treatment units which covers the higher portion of the

operating costs. Anaerobic digestion of sewage sludge is

beneficial with high transformation of organic matter into

biogas which is a valuable energy source.

The objective of this work is to evaluate the biogas

production of horse dung and also to investigate the co-

digestion with waste sewage sludge. The studies were

carried out under lab scale batch and continuous studies.

The system performance was evaluated and analysed with

respect to productivity, biogas and methane yields.

2 METHODS AND MATERIALS

The waste activated sludge was obtained from large scale

wate water treatment plant. The horse manure was obtained

as it was produced without mixed with straw. It was then

solubilised with tap water to desired VS content. The

continuous studies were carried out in 5 l volume fermentor

(Bioflo 200). The pH was automatically measured and

controlled. The system was operated under mesophilic

conditions and heated to 35±2oC by the water jacket around

the reactor. The contents were mixed by a propeller type of

mixer to obtain a homogeneous content and to provide gas

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release. The system was fed in a semi-continuous mode as

200 and 300 ml of the content was displaced with fresh feed

once a day. The retention times for 200 and 300 ml/day

fresh feeding werecorresponding to 22 and 15 days. The

volumetric exchange ratio was about 4-6%. The solids

content of initial horse manure and sewage sludge mixture

was % 4. The total solids (TS) and volatile solids (VS) in

the feed and effluent slurry of the digesters were analysed

using standard methods.

The batch studies were carried out with 500 ml serum

bottles equipped with gas collection and feeding tubes on

the top. The bottles were heated and mixed by the help of a

mechanical stirrer. The gas produced was collected by

water displacement in both systems. The methane gas

percentage was measured by gas chromatography. The total

solids (TS) and volatile solids (VS) in the feed and effluent

slurry of the digesters were analysed using standard

methods. The pH was measured by pH meter.

3 RESULTS

There are roughly five million horses living in stables in

Europe today and 166.753 horses living in Turkey. The

residues from the horses contain solid as well as liquid

portions of waste, typically about 60% solids and 40%. In

this study, biogas yield of horse manure was evaluated in

batch and continous digesters under mesophilic conditions

(35 ⁰ C). Studies were carried out in three stages. At the

first stage, the batch studies were conducted with solid

ratios of 0.5 %, 1.0%, 2.0% and 4.0%. The systems were

fed with horse manure alone. The results obtained in these

stage are summarized in Table 1.

The biogas yields of horse manure after 35 days of the

digestion period for the concentrations of 0,5 %, 1%, 2%

and 4% were 339 ml/gVS, 374 ml/gVS, 370 ml/gVS, 381

ml/gVS respectively. As can be seen, the biogas production

increased with increasing VS content. The daily biogas

production and VS reductions are shown in Figures 1 and 2.

The VS conversion to biogas was about 80 to 90 %.

0

0.5

1

1.5

2

2.5

3

3.5

1 6 11 16 21 26 31

% V

S c

on

te

nt r

ed

uctio

n

Time (days)

% 4 TS

% 2 TS

% 1 TS

% 0,5 TS

Figure 1: VS reductions obtained from the digestion of

horse manure.

Total solid

( %)

0,5 1 2 4

Biogas yield ml/g VS 339 374 370 381

Total biogas production ml 620 1340 2550 5050

Average methane content

of biogas (%)

60 64 63 65

Methane yield ml/g VS 203 239 236 247

Initial pH 7.10 7.25 7.15 7.25

Final pH 7.48 7.55 7.52 7.60

Table.1: Results of anaerobic digestion of horse manure in

batch studies; biogas and methane yields and gas

production.

0

100

200

300

400

500

600

700

800

1 6 11 16 21 26 31

Bio

ga

s p

rod

uct

ion

ml/

da

y

Time (days)

4% TS

2%TS

1%TS

0,5%TS

Figure 2: The rate of daily biogas production obtained from

the digestion of horse manure.

In the second stage, biogas formation from co-digestion of

horse manure with waste sewage sludge was investigated in

batch systems. The horese manure (90%) was mixed with

waste sewage sludge (10%) and the biogas production was

investigated under two different starting concentrations of

%2 and %4 (TS). It was observed that the VS amount is the

significant parameter for anaerobic digestion of horse

manure.

The biogas productivity (not methane yield) was

increased with increased VS loading. The methane yields

after 35 days were 410 ml/gVS and 425 ml/gVS

respectively. These results show that co-digestion of horse

manure with sewage sludge is beneficial. The biogas yields

per g of VS converted were about 10 % higher. The biogas

production was improved when the manure was mixed with

sewage sludge even in amount of 10%. The conversion of

organic matter into biogas was as high as 90 %.

NSTI-Nanotech 2011, www.nsti.org, ISBN 978-1-4398-7138-6 Vol. 3, 2011658

Total solid

content ( %)

2 4

Biogas yield ml/g VS 410 425

Total gas production ml 3130 6575

Methane yield ml/g VS 270 280

Average methane content

of biogas (%) 65 66

Initial pH 7,1 7

Final pH 7, 4 7,5

Table.2: Biogas and methane yields from the co-digestion

of horse manure and sewage sludge in batch studies.

0

0.5

1

1.5

2

2.5

3

3.5

1 6 11 16 21 26 31

VS

Re

du

ctio

n (

%)

Time (days)

%4 manure+sludge

%2 manure+sludge

Figure 3: VS reductions obtained from the co-digestion of

horse manure and sewage sludge.

0

100

200

300

400

500

600

700

800

900

1 6 11 16 21 26 31

Bio

gas

pro

du

ctio

n m

l/d

ay

Time (days)

%4 manure+sludge

%2 manure+sludge

Figure 4: The daily biogas production from the co-digestion

of horse manure and sewage sludge.

0

0.5

1

1.5

2

2.5

3

3.5

1 6 11 16 21 26 31 36 41

Bio

ga

s p

rod

uct

ion

ra

te l

/d

ay

Time (days)

Figure 5: Daily biogas production.

In the third stage, co-digestion of horse manure (90%) and

sewage sludge (%10) mixture was investigated in a lab

scale continuous digester with the volume of 5 l. Volatile

solids ratio was kept as % 4. About 200 and 300 ml of the

content was removed each day and replaced with fresh

horse manure and sewage sludge mixture with 4% VS

content. The corresponding volumetric exchange ratio was

about 5 %. During these studies the methane content of the

biogas produced was between 66 and 68 %. VS reduction

and biogas production was monitored during the

operational period. The daily biogas production is given in

Figure 5 and the corresponding % VS reductions are given

in Figure 6. The VS reductions obtained in the continuous

studies were higher than 50 %. The pH change during this

period was also monitored (Figure 7). The pH was low at

the beginning of the study. The pH control was not applied.

However, it changed between 6.7 to 7.5 after the starting

period.

0

0.5

1

1.5

2

2.5

3

3.5

1 6 11 16 21 26 31 36 41

% V

S r

ed

uct

ion

Time(days)

Figure 7: The change of % VS amounts in the reactor.

NSTI-Nanotech 2011, www.nsti.org, ISBN 978-1-4398-7138-6 Vol. 3, 2011 659

5.8

6

6.2

6.4

6.6

6.8

7

7.2

7.4

7.6

1 6 11 16 21 26 31 36 41

pH

va

lue

Time (days)

Figure 6: Variation of pH values during the co-digestion of

horse manure and sewage sludge.

The methane productions and the VS conversion into

methane was lower in the semi-continuous studies. This is

mainly due to higher organic loading rates applied during

these studies.

4 CONCLUSIONS

The results of this study show that technically it is

feasible to produce biogas from anaerobic digestion of

horse manure. The results showed that horse manure has a

great biogas production potential. The system produced

highest biogas under 4% and %2 of VS feeding. The co-

digestion with sewage sludge was beneficial for

degradation of horse manure, the organic reduction was

about 90 %.

The methane yield and the daily biogas production are

highly dependent on HRT and VS content in the feed. VS

content was found as the more significant factor affecting

the biogas production than HRT.

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