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Effect of HRT and Slurry Concentration on BiogasProduction in Cattle Dung Based AnaerobicBioreactorsYadvika , T.R. Sreekrishnan , S. Santosh & S. KohliPublished online: 11 May 2010.

To cite this article: Yadvika , T.R. Sreekrishnan , S. Santosh & S. Kohli (2007) Effect of HRT and Slurry Concentrationon Biogas Production in Cattle Dung Based Anaerobic Bioreactors, Environmental Technology, 28:4, 433-442, DOI:10.1080/09593332808618804

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433

Environmental Technology, Vol. 28. pp 433-442© Selper Ltd., 2007

EFFECT OF HRT AND SLURRY CONCENTRATION ONBIOGAS PRODUCTION IN CATTLE DUNG BASED

ANAEROBIC BIOREACTORS

YADVIKA1, T.R. SREEKRISHNAN*2, S. SANTOSH1 AND S. KOHLI3

1Centre for Rural Development &Technology, I.I.T., Delhi 110016, India2Department of Biochemical Engg. & Biotechnology, I.I.T. Delhi 110016, India

3Department of Mechanical Engineering, I.I.T. Delhi 110016, India

(Received 24 February 2006; Accepted 6 December 2006)

ABSTRACT

The effect of three different cattle dung slurry concentrations (1:1, 1:4 and 1:9) at three different HRTs of 20, 30 and 40 dayswas studied at pilot scale for one year. The results showed that both biogas yield and methane content of gas obtained in 1:4and 1:9 slurry concentrations were significantly higher than those at 1:1 concentration for a given HRT. This was observedfor all the three HRTs studied. At 1:1 and 1:4 slurry concentrations, methane yield was found to increase with HRT.However, at higher dilution of 1:9, increase in HRT from 30 to 40 days resulted in decrease in methane yield.

Keywords: Hydraulic retention time, slurry concentration, methane yield, animal dung, anaerobic digestion

INTRODUCTION

Hydraulic retention time (HRT) and slurry

concentration are two important parameters in the operation

of anaerobic reactors. Anaerobic fermentation of cattle dung

slurry being a slow process, a large HRT of 30-50 days is used

in conventional biogas plants in tropical countries like India

while in countries with colder climate, it may go up to 100

days [1]. While biogas yield (gas production per unit of input)

generally improves with increase in HRT, the gas produced

per unit of the reactor volume tends to decrease at higher

HRTs [2]. Shorter retention time, on the other hand, is likely

to face the risk of washout of the active bacterial population

and hence may affect the stability of the process. However,

researchers have been able to obtain stable methanogenesis

with cattle dung slurry in lab scale reactors at HRTs as low as

6-10 days [3]. From the literature it appears that in larger size

plants shorter HRTs have not been investigated. In field scale

plants operated with daily feed, the effective HRT is generally

lower than the design value of HRT due to short-circuiting of

slurry between the inlet and the outlet [1], making designers

more skeptical of using smaller HRTs in these plants.

However, the pay-off in terms of reduction in volume may be

substantial and thus biogas production with variation of HRT

needs further investigation, particularly in larger plants.

Slurry concentration is another parameter whose role in

cattle dung based reactors is not very clear. Generally cattle

dung slurry of 1:1 concentration (one part water mixed with

one part dung, by weight), which has total solids

concentration of 7-9%, is considered to be best for

conventional biogas plants in India [4]. Baserga [5] reports

that the anaerobic digestion of cattle dung was unstable

below a total solids concentration of 7% while solids

concentration of 10% caused an overloading of the reactor.

However a few studies carried out with dilute slurry (TS

<7%) suggest otherwise. Liao and Lo [3] report higher biogas

yield in a lab scale study carried out with diluted cattle dung

slurry (3% VS) as compared to undiluted one (5.8% VS). Some

field level observations also showed similar results [6-7]. Lo et

al. [8-11] carried out experiments with screened cattle dung

slurry and reported higher gas production as compared to

unscreened slurry (1:1 dung: water). The authors have not

found any more literature on dilute cattle dung slurry

without screening.

In view of the above, the present work focuses on the

effect of HRT as well as slurry concentration, on the

performance of cattle dung based anaerobic bioreactors.

MATERIALS AND METHODS

Experiments were carried out for one full year in pilot

scale reactors of floating drum type having 250 litres of cattle

dung slurry subjected to ambient conditions. These reactors

were constructed using HDPE tanks of 300-litre capacity, the

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detailed design of which is given elsewhere [12-14]. Plate 1

shows the layout of the experimental facility. Nine reactors

were operated with three different slurry concentrations, each

with three different HRTs. The nomenclature used for

different reactors is given in Table 1. The slurry concentrations

(cattle dung: water; by weight) used were 1:1, 1:4 and 1:9. The

HRT values used were 20, 30 and 40 days. Since all the

reactors were of fixed volume, the quantity of daily feed of the

slurry in different reactors varied in accordance with the HRT

at which it was being operated. Along with initial charging of

fresh dung slurry, 10% of digested slurry from an operational

biogas plant was added as inoculum. After 10-15 days of

acclimatization period, regular feeding of the slurry with the

desired concentration and quantity was commenced. The

slurry was prepared by thoroughly mixing the fresh dung

with water manually with the help of sticks and breaking the

lumps with hands to ensure that the slurry fed to the digester

was homogenous. There was no external mixing of the reactor

contents during the course of the digestion. However, the gas

bubbles produced in the system do provide some level of

mixing while they rise through the reactor contents to the gas

collection region.

Plate 1. Layout of the experimental facility.

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Table 1. Operational parameters of the reactors used.

Reactors

Slurry concentration

(Dung: Water)

Total volume of

slurry in the reactor (l)

Daily quantity of dung

fed in each reactor (kg)

HRT

(d)

R1 1:1 250 6.250 20

R2 1:1 250 3.850 30

R3 1:1 250 3.125 40

R4 1:4 250 2.500 20

R5 1:4 250 1.440 30

R6 1:4 250 1.250 40

R7 1:9 250 1.250 20

R8 1:9 250 0.720 30

R9 1:9 250 0.625 40

Biogas production in each reactor was measured daily

using a wet type gas flowmeter. A temperature probe was

used for checking the temperature inside the pilot reactors.

pH of the outlet slurry was measured daily using pH

indicator papers.

Sample Collection and Analysis

Samples of influent and effluent slurry were collected

twice a month for analyzing total solids (TS), volatile solids

(VS), chemical oxygen demand (COD), volatile fatty acids

(VFAs) and pH. Gas samples were also taken on alternate

days for determining the methane content. All the parameters

(i.e. pH, TS, VS) were determined according to the standard

procedures [15]. COD of cattle dung slurry samples was

determined according to the modified method developed by

Yadvika et al. [16].

RESULTS

While various parameters were measured during the

experiments, the focus of this work was on biogas (and

methane) production at different HRT and slurry

concentration. Other parameters have been presented briefly.

The biogas production data have been presented in terms of

weekly average of daily biogas yield, which is taken as biogas

produced per day per kg of fresh dung (before dilution) fed

per day and methane yield taken as methane produced per

day per kg of fresh dung (before dilution) fed per day.

Characterization of Substrate

Fresh cow dung, which was mixed with water in

varying concentrations, was analysed periodically (before

dilution). The TS was found to vary in the range of 150-170 g

kg-1, while the suspended solids were in the range of 142-154 g

kg-1. The total volatile solids were found to be 130-150 g kg-1

and the COD varied from 160-180 g kg-1.

1:1 Slurry Concentration

The temperature variation in all the nine reactors is

shown in Figure 1. The experiments were carried out in Delhi

where the ambient temperature can vary between 450C in

summer to 20C in winter. Hence the reactor temperature is

found to have a large seasonal variation. On the other hand,

the methane percentage in biogas was found to be 50-60% and

did not vary much with temperature as shown in Figure 2.

The weekly average of daily biogas and methane yields are

shown in Figures 3(a) and (b) respectively. It can be seen from

the above figures that the biogas and methane production

increased with the ambient temperature.

It can be seen from the figures that at a reactor

temperature of 38.60C, highest weekly average biogas yield

was 33 l kg-1 in the reactor with 40 d HRT while the highest

weekly average biogas yield in 30 d and 20 d reactors was 25 l

kg-1 and 18 l kg-1 respectively. The highest weekly average

methane yield with the three HRTs was 18 l kg-1, 14 l kg-1 and

11 l kg-1 respectively. The daily highest biogas yields are 35 l

kg-1, 27 l kg-1 and 20 l kg-1 in 40 d, 30 d and 20 d reactors

respectively.

1:4 Slurry Concentration

The weekly average of daily methane yields from

reactors with 1:4 slurry concentration operating at three

different HRTs is shown in Figure 4. The biogas yield

followed a similar trend but the values were higher than those

at 1:1 slurry concentration. In this case also, the biogas and

methane production increased with the ambient temperature.

However methane content of the biogas varied between 55-

69% which is higher than that obtained with 1:1 slurry

concentration (Figure 2).

It was found that maximum weekly average of daily

biogas yield was 54 l kg-1 (daily highest 65 l kg-1) in the reactor

with 40 d HRT at 35.50C, which is nearly 63% higher than the

highest value with 1:1 slurry concentration. The highest

weekly yields in reactors with 30 d and 20 d HRTs were

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Figure 1. Variation in weekly average of temperature in the reactors.

Figure 2. Variation in methane content of biogas.

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(a)

(b)

Figure 3. Weekly average biogas yield (a) and methane yield (b) from reactors with 1:1 slurry concentration.

Figure 4. Weekly average methane yield from reactors with 1:4 slurry concentration.

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35 l kg-1 (daily highest 38 l kg-1) and 30 l kg-1 (daily highest 32 l

kg-1) respectively, which are also higher as compared to

corresponding values for 1:1 slurry concentration. Due to

higher methane content, the highest methane yield with 1:4

slurry concentration was 83% higher than that with 1:1 slurry

concentration (Figure 4).

As in the case of 1:1 slurry concentration, here also the

average methane yield obtained at 20 d HRT was almost half

of the yield obtained at 40 d HRT. Thus, at 1:4 concentration

as well, reactor with 40 d HRT performed better than the

reactors with 30 and 20 d HRT.

1:9 Slurry Concentration

The weekly average of daily methane yields from

reactors with 1:9 slurry concentration operating at three

different HRTs is shown in Figure 5. It is noteworthy from

Figure 2 that the methane content of the gas for this

concentration was found to be 60-79%, which is much higher

than the value of 55-60% normally obtained with 1:1 slurry

concentration [1]. For this dilution, highest weekly biogas

yield was 89 l kg-1, 59 l kg-1 and 39 l kg-1 in reactor with 30 d,

40 d and 20 d HRT, respectively, when the temperature was

36.60C. The daily highest yields were 97 l kg-1, 66 l kg-1, 42 l

kg-1 in reactors with 30 d, 40 d and 20 d HRT respectively. The

corresponding weekly methane yield was 47 l kg-1, 41 l kg-1

and 25 l kg-1 (Figure 5). The daily highest methane yield

obtained with this slurry concentration at 30 d HRT was 3

times the highest value obtained with conventionally used 1:1

slurry at 40 days HRT.

DISCUSSION

On the basis of the performance in different seasons, the

reactors at different slurry concentrations and different HRTs

have been arranged in descending order of their methane

yield in Table 2. It can be seen from the table that the reactor

with 1:9 concentration and 30 d HRT showed the highest

methane yield followed by the reactor with 1:4 slurry

concentration and 40 d HRT. On the other hand, the reactor

with 1:1 slurry concentration and 20 d HRT showed the

minimum yield. Figure 6 gives the direct comparison of

performance with different slurry concentrations at 30 d HRT.

Here, it may be pointed out that the potential of fresh cattle

dung to generate methane can be determined from the COD

of the fresh dung. For one kg of COD removed, 0.35 m3 (at

STP) of methane can be produced [17]. Thus, the fresh cattle

dung can produce 56-63 l kg-1 of methane if all of its COD is

removed and get converted to methane. Figure 5 shows

weekly highest methane yield of 47 l kg-1, which is close to the

limiting value obtained from the COD of the fresh dung.

The substantial increase in methane yield with 1:9

slurry concentration as compared to 1:1 slurry concentration

at 30 d HRT but poorer performance of 1:9 and 40 d HRT can

be understood by a close look at the mechanism of methane

production in cattle dung slurry. In cattle dung, percentage of

Figure 5. Weekly average methane yield from reactors with 1:9 slurry concentration.

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Table 2. Comparative performance of reactors at different HRT and slurry concentrations.

Period of Averaging

Whole yearSummer

(March-August)

Winter

(September-February)

Reactor

Biogas

yield

(l kg-1)

Methane

yield

(l kg-1)

Reactor

Biogas

yield

(l kg-1)

Methane

yield

(l kg-1)

Reactor

Biogas

yield

(l kg-1)

Methane

yield

(l kg-1)

R8 (30d)

(1:9)

40.56 27.58 R8 (30d)

(1:9)

61.17 41.28 R8 (30d)

(1:9)

19.96 13.88

R6 (40d)

(1:4)27.26 16.89

R6 (40d)

(1:4)37.88 23.34

R6 (40d)

(1:4)16.63 10.44

R9 (40d)

(1:9)23.02 13.70

R9 (40d)

(1:9)33.65 19.60

R9 (40d)

(1:9)12.40 7.80

R5 (30d)

(1:4)19.16 11.43

R5 (30d)

(1:4)27.13 15.99

R5 (30d)

(1:4)11.19 6.86

R2 (30d)

(1:1)17.21 9.64

R3 (40d)

(1:1)25.29 13.85

R2 (30d)

(1:1)11.71 6.61

R3 (40d)

(1:1)17.37 9.56

R2 (30d)

(1:1)22.71 12.67

R3 (40d)

(1:1)9.45 5.27

R4 (20d)

(1:4)14.58 8.70

R4 (20d)

(1:4)21.09 12.55

R4 (20d)

(1:4)8.08 4.84

R7 (20d)

(1:9)12.56 8.12

R7 (20d)

(1:9)18.67 12.01

R7 (20d)

(1:9)6.45 4.23

R1 (20d)

(1:1)10.02 5.58

R1 (20d)

(1:1)13.94 7.77

R1 (20d)

(1:1)6.09 3.40

Figure 6. Weekly average methane yield from reactors at 30 d HRT.

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suspended solids is very high, the digestion of which first

requires hydrolysis. In thick slurry such as that with 1:1 slurry

concentration, rate of hydrolysis is low due to the fact that

most of the organic material present in the slurry is in the

solid phase and hydrolysis of this solid substrate by the

hydrolyzing bacteria takes place only on the particle surface.

This limits the rate of hydrolysis and thereby the

solubilization of the substrate. Hydrolysis is, therefore, rate

limiting for the overall anaerobic fermentation of thick slurry.

When the slurry is diluted, rate of hydrolysis increases first.

However, at very high dilution, hydrolysis rate does not

increase any further. Instead, in this condition the dependence

of rate of different reactions on concentration of substrate has

a greater effect, resulting in lower gas production rate at the

combination of high dilution and high HRT, as in the reactor

with 1:9 slurry and 40 d HRT.

It is also noteworthy that in summer, with 1:1 slurry

concentration, average biogas and methane yields obtained in

the reactor with 30 d HRT were around 92% of the average

yields obtained in the reactor with 40 d HRT. This shows that

increase in HRT from 30 to 40 d gives only marginal

advantage in a place like Delhi.

Variation in other Quantities

It is noteworthy that all the reactors remained stable

even at high loading rates or low HRTs. The pH remained

around 7.5-8 and the concentration of VFA remained below

2 g l-1 in all reactors as desired. The methane concentration in

the biogas was not affected significantly due to the reduction

in HRT for the same slurry concentration. The solids and

COD data were non-conclusive as samples representative of

the reactor slurry could not be collected due to settling of

solids at the bottom of the reactors. Therefore, for these

parameters data has not been presented here.

Comparison with Literature

Table 3 gives a comparison of the data obtained in the

present work from pilot scale reactors to that from the lab,

pilot and field scale experiments of some other researchers

carried out with cattle dung slurry. The table shows that the

trends obtained in the present work of increase in biogas and

methane production per unit of volatile solids added with

increase in HRT is reflected in other data as well.

It can be seen from this table that average methane

yield obtained in the present work at 20 d HRT was almost

half of the average methane yield obtained at 40 d HRT.

Almost similar results were obtained by Boodo et al. [18].

They also conducted pilot scale experiments (200 l capacity

reactors), but the concentration of slurry was different.

Mohanrao [19] observed somewhat less biogas production at

higher HRT of 24 d as compared to the present work, though

the concentration of slurry was the same in both cases. It can

Table 3. Comparative performance of different reactors.

Reference Loading Rate

(g VS l-1 d-1 )

HRT (d) %VS in

the Feed

Average Biogas/Methane

Production

5.58 12 6.98 0.1672 l BG g-1VS

2.79 25 6.98 0.2605 l BG g-1VS

1.95 36 6.98 0.3128 l BG g-1VS

1.39 50 6.98 0.3122 l BG g-1VS

Singh et al. [2]

1.12 62 6.98 0.3149 l BG g-1VS

Liao and Lo [3] 3.45 10 3.6 0.053 l CH4 g-1VS

Liao and Lo [3] 6.10 10 5.8 0.040 l CH4 g-1VS

Mohanrao [19] - 24 5.8 0.158 l BG g-1VS

2.78 10 2.72 0.106 l BG g-1VS

(0.053 l CH4 g-1VS)

1.61 20 3.19 0.174 l BG g-1VS

(0.087 l CH4 g-1VS)

Boodo et al. [18]*

0.78 40 3.27 0.303 l BG g-1VS

(0.1515 l CH4 g-1VS)

2.93 20 5.8 0.170 l BG g-1VS

(0.085 l CH4 g-1VS)

1.93 30 5.8 0.196 l BG g-1VS

(0.109 l CH4 g-1VS)

Present Work

1.45 40 5.8 0.306 l BG g-1VS

(0.163 l CH4 g-1VS)Note: BG – Biogas, VS - Volatile Solids*Methane % has been assumed as 50% of the biogas obtained.

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also be observed from Table 3 that Singh et al. [2] observed no

significant improvement in gas production beyond an HRT of

36 d. At HRT of 36 d they observed similar gas yield as

obtained in the present work at 40 d HRT. However their

study was a field level study. At slurry concentration of 5.8%,

gas yield observed by Liao and Lo [3] at 10 d HRT is almost

half of the yield obtained in the present study at 20 d HRT

which shows that as the HRT is reduced gas yield also

decreases.

CONCLUSIONS

Experiments over one year were conducted with cattle

dung based pilot scale reactors with different concentrations

of slurry and different HRT. The work has brought to light the

following important facts.

• The methane yields obtained in 1:4 and 1:9 slurry

concentrations were significantly higher than those at

1:1 concentration for a given HRT due to higher biogas

production rate as well as higher methane content. This

was observed for all three HRTs studied, i.e. 20, 30 and

40 d.

• With 1:9 slurry at 30 d HRT, methane yield was

approximately three times higher than that from

reactors with 1:1 slurry concentration and 40 d HRT.

Even highest biogas yield with 1:9 slurry was found to

be 89 l kg-1 of dung as compared to the conventional

figure of 40 l kg-1 with 1:1 slurry. This is a very

significant result since thousands of biogas plants being

operated all across the country use 1:1 slurry

concentration.

• Increase in HRT beyond 30 d at 1:9 slurry concentration

was not found to be advantageous.

The above results indicate a complex dynamics of the process

with a delicate balance between the effects of slurry

concentration and HRT. These results indicate the possibility

of identifying an optimal combination of slurry concentration

and HRT for best performance and cost effectiveness.

ACKNOWLEDGEMENT

The authors are grateful to MNES, Government of India

for financial support through a sponsored project and CSIR,

India for providing fellowship to Dr. Yadvika.

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