Increase in biogas production from predigested aquatic plant using human urine as enhancer

IJRET: International Journal of Research in Engineering and Technology

eISSN: 2319-1163 | pISSN: 2321-7308

INCREASE IN BIOGAS PRODUCTION FROM PREDIGESTED AQUATIC PLANT USING HUMAN URINE AS ENHANCER Tapas Kumar Manna1, Biswajit Mandal2 Professor, Department of Chemical Engineering, Haldia Institute of Technology, West Bengal, India – 721657 Assistant Professor, Department of Chemical Engineering, Haldia Institute of Technology, West Bengal, India – 721657 1 2

Abstract Water hyacinth, an aquatic plant, due to its rapid growth, is easily available and is used for treatment of variety of waste water and is also a good biogas producer, due to its high water content, soft organic matter and a favorable C/N ratio. The aim of the present study is to assess the anaerobic biodegradability of water hyacinth – both thermo-chemical pretreated and untreated.

Human urine is used as an enhancer of the gas production. It increases the gas production rate and decreases the total gas production time as per the following reaction CH3COOH + NH3 CH4 + CO2 + NH3. It also, supplies the nitrogen for bacterial growth. The temperature and human volume is taken as the parameter. Water hyacinth, collected from local pond is cut into pieces and treated with sodium hydroxide and then heated in a pressure cooker for 15 mins. The PH is maintained at 10. Then the material is kept for 7 days in facultative condition. The experiment is then conducted by taking 100 gm of the substrate, the human urine of varying volume and inoculum in a conical flask. The human urine volume and temperature is varied in different experiment. The gas produced is collected in the gas burette by water displacement. It has been observed that, pretreatment reduces the time for hydrolysis step in the conversion of complex water hyacinth organic matter in order to increase the soluble organic constituents. Gas production started within 7 days. With increase in urine volume upto 150 ml, the gas production is increased and beyond which it is decreased due to the accumulation of ammonia. The production rate increases upto 41 oC. The gas production is quantified as first order reaction as per black box model. The reaction rate constant is seen to fit the Arrhenius equation in a better way. The rate expression is seen to be function of both temperature and urine volume.

Keywords: Aquatic Plant, Enhancer, Pre-Digestion, Kinetic, Anaerobic Digestion.

--------------------------------------------------------------------***--------------------------------------------------------------------1. INTRODUCTION There is an increasing demand of bioenergy which is the eco-friendly, alternate and cheap renewable source of energy. Biogas is formed as the byproduct of anaerobic digestion [5] of organic substrates and is mostly composed of methane and carbon di oxide with other trace elements.

Anaerobic fermentation being a slow process, a large hydraulic retention time (HRT) of 30 -50 days is used in conventional biogas production plant leading to large volume of the digester and hence high cost of the system.

Methane is also a source of renewable energy for production of electricity in power plants. Anaerobic digestion of organic matter is the oldest method for disposing the waste.

The anaerobic digestion of animal, agricultural and industrial wastes has been widely studied. Anaerobic digestion is the process in which biodegradable material is broken down by micro-organism. Kalia and Singh (1) showed that the biogas production rate depends on the season and in winter the less production rate is reported compared to summer. The biomethanation process involves hydrolysis, acidogenesis, acetogenesis and methanogenesis in batch digestion, where, the hydrolysis step takes much longer time during conversion of complex organic molecule to the simpler molecule and is the rate limiting step.

Hence there is a need to improve the biogas production yield by reducing the HRT, optimizing the temperature in anaerobic digestion. Optimization of various parameters by retaining the condition for the survival of microorganism is one such way (3).

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(b)

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Urine also increases gas production from cow dung.

Different models have been proposed for biogas production through anaerobic production considering the bacterial growths which are complicated. A simplified model considering the digester as black box is not available.

The aim is to study the effect of predigestion by thermochemical treatment, the effect of temperature on the bio gas production from the predigested substrate and the effect of the human urine on the gas production at different temperatures. The first order kinetic model can be tested.

2. EXPERIMENTAL 2.1 Material and method Water hyacinth is collected from local pond. This is cut into pieces and digested with dilute sodium hydroxide. This is then kept under facultative condition for 7 days. The human urine volume is varied as 100, 120,150 and 200 ml. The temperature varies as 25oC, 35oC, 41oC and 46oC.

Fig- 1: (a) Schematic diagram and (b) snap-shot of the experimental set-up Pretreatment is another way which can increase the methane yield in anaerobic digestion process by breaking down the complex organic structure into simpler organic molecules which are soluble and more susceptible to microbial degradation. The speed up of hydrolysis step increases the possibility of early production of biogas in the predigested substrate.

2.2 Procedure 100 gm of the predigested water hyacinth is taken in a 500 ml conical flask, 50 ml of previously prepared inoculum is added. The calculated amount of human urine is then added to the substrate and sealed. The apparatus is shown in figure 1(a and b). The produced gas is collected in the gas burette by downward displacement of water. The water level in the gas burette is adjusted by aspirator bottle.

Due to high carbon content in plant residues and in the aquatic plant than conventional cow dung as substrate, high methane containing biogas is expected. Water hyacinth with rapid production rateunder favorable condition creates problem in many ways. This is thought to be used as a potential biomass source of biogas due to large availability in all over the world (4).

3. RESULTS AND DISCUSSION Gas production started within5 days and finished in another 4 – 6 days.

3.1 Cumulative gas production

To maintain proper C/ N ratio, in a high carbonaceous material, some calculated nitrogen source can be added.

Laura and Idnani (2) found that the quantity and quality of biogas is increased by addition of small amount of urea.

The predigested water hyacinth is used to study the effect of temperature on the bio gas production. Cumulative gas production at different temperature is shown in fig 2.

350

100 ml urine, 25C 120 ml urine, 25 C

Cumulative gas production, ml

300

150 ml urine, 25 C

250

200 ml urine, 25 C 200

100 ml urine, 35 C 120 ml urine, 35 C

150

150 ml urine , 35 C

100

200 ml urine, 35 C 50

100 ml urine, 41 C

0

120 ml urine , 41 C 0

0.5

1

1.5

Time, day

2

2.5

3

3.5

150 ml urine, 41 C 200 ml urine, 41 C

Fig-2. Cumulative gas production of water hyacinth after predigestion

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In the fig 2, it is seen that, the curves are concave downwards indicating that, the rate of gas production is high initially and then it decreases. With increase in temperature from 25oC to 41oC, the gas production rate increases.

Afterwards, from 41oC to 46oC the gas production decreases and the calculation is discarded in that zone. This increase falls in the mesophilic zone.

Also, the gas production rate increases with increase in urine amount around 150 ml at a fixed temperature. The total gas production is observed to increase upto 200 ml urine, but at a slower rate.

1/P, day/ml

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0.02 0.018 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0

3.2 Fitting the Kinetic Expression

1 dN A  k CA V dt

CA = concentration of A, gm/ml V= volume of the reactant, ml.

The above equation on integration, gives at any time, moles of substrate converted, Δ NA= NAo – NA = NAo [1 – exp (kt)].

So, total volume of gas produced at any moment = Q ΔNA = Q×NAo [1 – exp (- kt)], ml.

Q = volume of gas produced (ml) per gm of water hyacinth converted.

The gas production is quantified by assuming the reaction rate as first order. Accordingly per day gas production (P) equation is written as

Q × N Ao  k Q × N A P  t 1  kt

200 ml urine 2

4

Fig- 3: P vs time plot for predigested water hyacinth at 25oC 0.014

35oC

0.012 0.01 0.008

100 ml urine

0.006

120 ml urine

0.004

150 ml urine

0.002

200 ml urine

0 0

1

2

3

4

Time, day Fig- 4:. P-1vs time plot for water hyacinth at 35oC

(1) t = time, day, NAo = Initial amount of water hyacinth, gm The reaction rate constant (k) is determined by taking the inverse of the above equation as follows

1 1  kt  P Q × N Ao  k 1 1 t Or,   P Q × N Ao  k Q × N Ao

0.014

41oC

0.012 0.01 0.008

100 ml urine

0.006

120 ml urine

0.004

150 ml urine

0.002

(2) So, a plot of

150 ml urine

Time, day

1/P, day/ml



120 ml urine

-1

1/P, day /ml

A

100 ml urine

25oC

0

The rate of consumption of the substrate (A) can be assumed to be first order with respect to the substrate as per black box model and is represented as

r

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1 vs t gives straight line with slope P

1 1 and intercept, I  .

S Q × N Ao Q × N Ao  k Knowing the value of the slope, the rate constant (k) is found out from the intercept as k=S/I (3)

200 ml urine

0 0

2

4

Time, day Fig- 5: P-1vs time plot for water hyacinth at 41oC. The k values determined as per equation 3 is shown in table 1

The k values are determined for different samples by plotting 1/P vs t and finding the slope and intercept and then using the equation 3 for temperatures 25, 35, 41 oC as shown in fig. 3, 4, 5, respectively.

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Table 1: k values determined experimentally Substrate Volume of Temperature, k, day-1 urine, ml K

Water hyacinth

120

150

200

1/T 0

298

0.2

-0.2 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034

308

0.26

-0.4

314

0.3

-0.6

298

0.29

-0.8

308

0.32

314

0.39

298

0.29

308

0.34

314

0.43

298

0.125

308

0.17

314

0.2

ln k

100

-1.6

Fig- 7: Arrhenius plot for water hyacinth for 120 ml urine.

0

-0.4

-0.6

ln k

-0.8

y = -2372.x + 6.353 R² = 0.999

-1

-1.2

150 ml…

ln k

-0.6

y = -2206.x + 6.145 R² = 0.928

-0.8

-1 -1.2 -1.4

Fig-8: Arrhenius plot for water hyacinth for 150 ml urine.

0

1/T

0.00315 0.0032 0.00325 0.0033 0.00335 0.0034 -0.5

-1

ln K

-0.4

1/T

0.00315 0.0032 0.00325 0.0033 0.00335 0.0034 -0.2

1/T

0.00315 0.0032 0.00325 0.0033 0.00335 0.0034 -0.2

y = -2310.x + 6.396 R² = 0.978

-1.4

o

The reaction rate constant (k) increases upto 41 C and then decreases. The temperature dependence on the reaction rate constant is tried to fit the Arrhenius equation k = koe-E/RT, as Arrhenius equation gives the best fit to the k values. ko = frequency factor, E = Activation energy, R = universal gas constant, T = temperature, K. The ko and E/R values are calculated analytically for the temperatures 298 K and 308 K and 314 K . For water hyacinth, the Arrhenius plot is fitted for different urine volume as in fig 6 – 9.

120 ml urine

-1

-1.2

3.3 Determination of the Suitability of the Expression For K

0

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-1.5

200 ml urine y = -2756.x + 7.173 R² = 0.999

-2

-1.4 -1.6 -1.8 Fig- 6: Arrhenius plot for water hyacinth for 100 ml urine.

-2.5

Fig-9: Arrhenius plot for water hyacinth for 200 ml urine.

The frequency factor (ko values) are calculated by taking the inverse of logarithm of the intercept of the linear fit of the ln k data with 1/T for different urine volume.

The slope of the linear fit is (-E/R). The (ko) and the E/R values are seen to vary with urine volumeat a fixed

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temperature and are plotted against urine volume in fig 10and 11 respectively.

700

y = -2.260x + 815.1 R² = 0.870

600 500

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So, the reaction rate constant expression in terms of the urine volume and the temperature is expressed as

k = (- 2.26 Vurine + 815.1) e

-

- 3.331 Vurine + 2706.9 T

(4) The k values calculated by equation 4 ( ktheo) is compared with the experimentally determined k values (kexpt) and is seen a good fit. This is shown in fig 12

400

200

k theo., 1/day

ko

300

100 0 0

50

100

150

200

Urine vol, ml Fig-10: Frequency factor vs urine volume plot

0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

y = 1x R² = 0.93

0

2400

y = -3.331x + 2706.

R² = 0.999

2350

0.4

0.6

k expt.,1/day Fig-12: Comparison of the experimental and theoretical k values

2300

E/R

0.2

2250

The rate law is then written as follows: In terms of the substrate

2200 2150 0

50

100

150

200

r

A

 (- 2.26Vurine + 815.1) e

-

- 3.331Vurine + 2706.9 T

CA

Urine vol, ml Fig-11: E/R vs urine volume plot

According to per day gas production (P) equation is written as

The ko, the frequency factor, is expressed as linear expression in urine volume as ko = – 2.26Vurine + 815.1 and the E/R term is expressed as E/R = – 3.331Vurine + 2706.9

P

Q × N Ao  (- 2.26Vurine + 815.1) e 1  (- 2.26Vurine + 815.1) e

4. CONCLUSIONS The enhancement of biogas production is done by the temperature change and the addition of the human urine as enhancer. The predigestion reduces the initial and the overall gas production time drastically. The gas production is seen to be enhanced as the temperature is increased to the mesophilic zone. Beyond 41oC, the production rate is decreased. In the psychrophilic zone (25oC), the rate of gas

-

-

- 3.331 Vurine + 2706.9 T

- 3.331 Vurine + 2706 T

t

production is less. The Arrhenius equation gives better results over the equations proposed by collision theory or transition state theory. The gas production is enhanced by human urine upto 150 ml. The gas generation fits the first order kinetics very well. The reaction rate constant is expressed as a function of the urine volume and the temperature. An equation for rate is developed in terms of the amount of human urine and temperature.

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REFERENCES [1] Kalia, A.K. and S.P.Singh, 1996” Performance Evaluation of Pragati and KVIC biogas plant in hilly regions. Biogas Forum, 64, 6 -10 [2] Laura, R.D. and M.A. Idnani, 1971,”Increased production of biogas from cowdung by adding other agricultural waste materials,” Journal of Science, Food and agriculture 22:164 – 167.

[3] Lettinga, G., A.F.M., Van Velson, S.W.Hobma, W. De Zeeuw, A. Klapwijk, 1980,” Use of upflow sludge blanket reactor for biological wastewater treatment, especially for anaerobic treatment.” Biotechnol. Bioengg.(4), 674-699.

[4] Shoeb, F. and H.J.Singh,2000, “Kinetic Studies of biogas evolved from water hyacinth,” 2nd International Symposium on new technologies for environmental monitoring and Agro application, 1-12 [5] Yadvika, S., T.R.Srikrishan, S. Kohli, V. Rana,2004, “Enhancement of biogas production from solid substrates using different Techniques,”Bio Resource Technology, Elsvier, pp 1-10

BIOGRAPHIES Prof. Tapas Kumar Manna received the B.Tech. Degree from Calcutta University, Kolkata and both M.Tech. and Ph.D. degrees from IIT Kharagpur in Chemical Engineering; His areas of research are Reaction Engineering, Waste water treatment, Non Newtonian flow and energy technology etc. At present he is working as a Professor in the department of Chemical engineering, Haldia Institute of Technology, Haldia, West Bengal.

Biswajit Mandal is working as assistant professor in Chemical engg. Department at Haldia Institute of Technology for ten years.

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