Vegetable waste supplemented with human or animal excreta as substrate for biogas production

Phan Thi Thu Hien, T R Preston¹, Vo Lam² and Duong Nguyen Khang³

Department of Biotechnology, Nong Lam University, Ho Chi Minh City, Vietnam
behien177@gmail.com
¹ CIPAV, Carrera 25 No 6-62 Cali, Colombia
² An Giang University, Vietnam
³ Center for Research and Technology Transfer, Nong Lam University, Ho Chi Minh Ciiy, Vietnam.

Abstract

The objective of this study was to determine the effect of co-digestion with vegetable waste on biogas production from excreta from cattle, pigs and humans. There were seven treatments arranged in a completely randomized design with three replicates. The treatments were initial loading rates (DM basis) of vegetable waste (V) supplemented with human (H), cattle (C) or pig (P) excreta. V2H1: 2 parts vegetable waste with 1 part human feces; V2C1: 2 parts vegetable waste with 1 part cow manure; V2P1: 2 parts vegetable waste with 1 part pig manure; V1H2: 1 part vegetable waste with 1 part human feces; V1C2: 1 part vegetable waste with 2 parts cow manure; V1P2:  1 part vegetable waste with 2 parts pig manure. The pH of the contents of the biodigesters, and the gas production, were measured daily until the end of the experiment after 60 days. The content of methane in the gas was measured in samples taken after 14, 21, 28, 35, 42, 49 and 56 days.

Gas production over the 60 day period was 5 times higher on the combination of one part vegetable waste and two parts cattle or pig manure, compared with the lowest value on the substrate of vegetable waste alone. However, these results did not reflect the potential gas production from the substrates as there were major differences in the pH of the fermenting substrates over the whole incubation period (higher values on the treatments with cattle and pig manure and lowest value on the vegetable waste as sole substrate). Overall,  there was a negative relationship between the pH of the digesta medium and the average daily gas production. On all substrates the pH dropped rapidly during the first 5 to 7 days of incubation; however,  the extent of the fall was smaller, and the recovery more rapid, when cattle manure was the companion substrate compared with pig manure on which the pH recovered more quickly than with human feces. On the 100% vegetable waste substrate, the drop in pH was more pronounced, and the recovery much slower, the pH not begining to rise until some 50 days after the start of the incubation. The trends for methane content of the gas were similar to those recorded for total gas production, with highest values for substrates richest in manure and lowest values for treatments with higher proportions of vegetable waste. The relative values for pH of the digesta and methane in the gas were positively correlated. Subsequent research with food waste as substrate should be directed to systems of management of the biodigester (ie: ensuring a slow build up of the substrate concentration in the first 14 to 21 days of the incubation) or inclusion of buffering agents such as sodium bicarbonate.

Keywords: anaerobic digestion, cow manure, human feces, pig manure

Introduction

Currently, renewable energy is a major area for research and development, especially from the huge waste stream (Deublein et al 2008).  According to FAO statistics (2011), some 1.3 billion tonnes of food waste are discharged each year. This is not only a loss of valuable food; it is also a source of greenhouse gas emissions when it decomposes.

Anaerobic digestion in a biodigester is a way to utilize organic waste efficiently, producing not only a combustible gas (methane) but also functioning as a waste disposal system. This is specially relevant in the case of human wastes (Vipul Vaid et al 2013). According to Trinh Thi Thanh (2004), the C:N ratio in human excreta ranges from 2.9 to 10, much lower than the optimum range for biogas production which is from 20 to 30 (Yadvika et al 2004; EPA 2012). Manure from cattle and pigs, with a C:N ratio of the order of 10, is also lacking in carbon to ensure efficient biodigestion. By contrast, the C:N ratio ranges from an average of 13 in vegetable wastes to over 30 in fruit wastes (Asquer et al 2013). Thus adding vegetable and fruit waste to human or animal excreta can be expected to have a positive impact on yield of biogas (Tanimu et al 2014).

Another important issue concerns the site for making effective use of human wastes, both from food and excreta. In Europe, this is mainly done in centralized facilities (Croxatto Vega et al 2014), which entails a considerable cost in energy for transporting the waste, while pollution results from temporary storage of the food waste at household level.

We believe that an alternative approach is to develop biogas systems that can be operated at household level, where the waste is generated, and which do not require animal excreta as substrate (eg: Inthapanya et al 2013). This is facilitated in tropical latitudes where average air temperatures in the 30-40ºC range are adequate for the mesophilic biodigestion process.

The present study examined different proportions of vegetable waste in combination with excreta from humans, pigs and cattle as feedstock in laboratory scale batch biodigesters.

Materials and methods

Location and duration

This experiment was conducted in the biogas laboratory, Nong Lam University, Ho Chi Minh city, Vietnam, from March to August 2014.

Treatments and experimental design

Seven treatments were arranged in a complete randomized design with three replicates. The treatments were initial loading rates (DM basis) of vegetable waste (V) supplemented with human (H), cattle (C) or pig (P) excreta:

V: vegetable waste

V2H1: 2 parts vegetable waste with 1 part human feces

V2C1: 2 parts vegetable waste with 1 part cow manure

V2P1: 2 parts vegetable waste with 1 part pig manure

V1H2: 1 part vegetable waste with 1 part human feces

V1C2: 1 part vegetable waste with 2 parts cow manure

V1P2:  1 part vegetable waste with 2 parts pig manure

Procedure

Experimental biodigesters

The experiment was done in laboratory scale biodigesters (Photo 1), made from recycled polypropylene water bottles,  based on the design developed by Inthapanya et al (2013).  The total volume of each biodigester was 5 liters. The liquid volume was fixed at 4 liters. The biodigesters were operated on a "batch" basis with the initial loading rate set at 160g of  DM in a total liquid volume of 4 liters (4% DM concentration).  Gas was collected by water displacement using 1 liter bottles , with the bottoms removed, and calibrated at 50ml intervals.These were suspended in 5 liter bottles with the tops removed and filled with water (Photos 1 and 2).

Photo 1.  Individual biodigester showing gas collection by displacement	Photo 2. Arrangement of the biodigesters on the floor of the laboratory

Substrates

The vegetable wastes were collected from the market area of Village National University, 6 Quarter, Linh Trung Ward, Thu Duc District, Ho Chi Minh City. They included vegetable and fruit waste : sweet potato buds, bitter melon, convolvulus, cabbage, chayote, gourd, cauliflower, yam, zucchini, bok choi, okra, potato, pumpkin, taro, carrot, lettuce, radish, watercress, smooth luffa, amaranth, perilla plant, katuk, mango, guava and  pineapple. Human feces were collected from housing districts at 254/16C, Noi Hoa 1 Quarter, Binh An Ward, Di rom town, Binh Duong province. Cow and pig manure were collected from the research farm in Nong Lam University, Ho Chi Minh City. The waste vegetables were cut to a size of 1-2 cm, put in a polyethylene bag and tied, then left for anaerobic incubation for 2 days. They were then mixed with a small amount of leftover rice collected from households. Finally, human feces, cattle and pig manure and biodigester effluent (from a biodigester charged with cow manure) were added depending on each treatment (Table 1).

Photo 3. The mixed vegetable wastes after chopping (top-left ); placed in the polyethylene bag (top-righright theleft-over rice (bottom-left); all mixed together (bottom-right)

Data collection and measurements

The vegetable wastes and excreta/manure were analyzed for DM, nitrogen and crude fiber (AOAC 1995)  before adding them to the biodigesters. The C:N ratio was estimated assuming carbon was 40% of the ash-free DM.  The pH (Testr Eco hand-held meter) of the contents of the biodigesters, and the gas production, were measured daily until the end of the experiment (60 days). The content of methane (Gasboard Gas Analyser) in the gas was measured in gas samples taken after 14, 21, 28, 35, 42, 49 and 56 days. 

Statistical analysis

The data were entered and stored in an Excel spreadsheet (Microsoft Excel for Windows, © 2010 Microsoft Corporation), and analyzed by the General Linear Model (GLM) option for repeated measures in the ANOVA Software of  Minitab Version 16 for Microsoft Windows (Minitab 2010).  Sources of variation were: treatments, days and error.

Results and Discussion

Composition of the substrates

There were major differences in the composition of the substrates (Table 2). Nitrogen was very high in human feces; ash was very low in vegetable-fruit waste and very high in pig manure, probably due to comtamination with sand from the floor of the pig pen. The C:N ratios were especially low in human feces and only approached recommended levels (EPA 2012) of 20/30:1 in the case of vegegtable-fruit waste and cow manure.

Gas production and pH

There were major differences in gas production over the 60 period with 5 times higher values on the substrate combination of one part vegetable waste and two parts cattle or pig manure, compared with the lowest value on the substrate of vegetable waste alone (Table 3). However, these results are misleading as there were major differences in the pH over the whole incubation period (higher values on the treatments with cattle and pig manure and lowest value on the vegetable waste as sole substrate). Overall,  there was a negative relationship between the pH of the digesta medium and the average daily gas production (Figure 1).

Figure 1. Relationship between average pH of the digesta medium and average daily gas production

Examination of the trends in pH over time with high (Figure 2) and low (Figure 3) ratios of vegetable waste to manure, reveals the apparent reason for the low gas production for the treatments with high concentrations of vegetable wastes. On all substrates the pH dropped rapidly during the first 5 to 7 days of incubation; however,  the extent of the fall was smaller, and the recovery more rapid, when cattle manure was the companion substrate compared with pig manure on which the pH recovered more quickly than on treatments with human feces; on the 100% vegetable waste treatment, the drop in pH was most pronounced, and the recovery much slower, the pH not begining to rise until some 50 days after the start of the incubation.

Figure 2. Trends in pH during 60 days of incubation of high ratio of vegetable waste to manure	Figure 3. Trends in pH during 60 days of incubation of low ratio of vegetable waste to manure

This result is consistent with the study of Inthapanya et al (2013) using fruit peel waste to produce biogas. These authors showed that in a batch digester charged with fruit waste, the pH after 5 days had fallen from initial values of 6.5 - 7.0 to  a pH of 4.1 with papaya peel, 4.3 with banana peel and 3.7 with orange peel. Similar results were recorded in decomposing orange peels  where the pH value was 4.45 after 8 weeks (Ojikutu Abimbola et al  2014).

Methane in the gas and pH

The trends for methane content of the gas were similar to those recorded for total gas production (Table 2), with highest values for substrates richest in manure and lowest values for the treatments rich in vegetable waste (Figures 4 and 5). The quantities of gas produced on the vegetable waste as sole substrate were too small to permit determination of methane. As for gas production, the values for pH of the digesta and methane in the gas were strongly correlated (Figure 6).

Figure 4. Mean values for methane content of the gas at 14 day intervals during 60 days of incubation of high ratio of vegetable waste to manure	Figure 5. Mean values for methane content of the gas at 14 day intervals during 60 days of incubation of low ratio of vegetable waste to manure

Figure 6. Relationship between pH of the digesta medium and the methane content of the gas

According to Ozturk (2013), if the C:N ratio is too high, the process of biodigestion is limited by N availability and the resulted acidification retards methanogenesis activity; and if it is too low, ammonia may be produced in quantities large enough to be toxic to the bacterial population. However, our results show that the lowering of the pH at the initiation of the incubation occurred on all treatments and was not related to the C:N ratio which would have been highest on the all vegetable waste substrate. The critical issue was the rate of recovery of the pH and this would seem to have been determined by the buffering capacity of the medium  and not the C:N ratio per se which would have been similar on all the treatments having either 1/3 or 2/3 of the substrate as vegetable waste. In each of these two groups of treatments, the substrates with cow manure were associated with higher pH, and higher gas production and higher methane content than the treatments of similar C:N ratio, but with the N source derived from pig manure or human feces.

Subsequent research with food waste as substrate should be directed to systems of management of the biodigester (ie: ensuring a slow build up of the substrate concentration) or inclusion of buffering agents such as sodium bicarbonate.

Conclusions

• Gas production in a laboratory batch digester over a 60 day period was 5 times higher on the combination of one part vegetable waste and two parts cattle or pig manure, compared with the lowest value on the substrate of vegetable waste alone.
  
  
• The pH of the fermenting substrates over the whole incubation period was higher on the treatments with cattle and pig manure and lowest on the vegetable waste as sole substrate.
  
  
• There was a negative relationship between the pH of the digesta medium and the average daily gas production.
  
  
• Methane content of the gas showed similar trends to those recorded for total gas production, with highest values for substrates richest in manure and lowest values for the treatments with high proportions of vegetable waste.
  
  
• pH of the digesta and methane in the gas were positively correlated.
  
  
• Subsequent research with food waste as substrate should be directed to ensuring a slow build up of the substrate concentration and/or inclusion of buffering agents such as sodium bicarbonate.

References

Asquer C, Pistis A and Scano E A 2013 Characterization of fruit and vegetable waste as single substrate for the anaerobic digestion. Environmental Engineering and Management Journal 12(11): 89-92.

Croxatto Vega G A, ten Hoeve M, Birkved M, Sommer S G and Bruun S 2014 Choosing co-substrates to supplement biogas production from animal slurry – A life cycle assessment of the environmental consequences. Choosing co-substrates to supplement biogas production from animal slurry – A life cycle assessment of the environmental consequences. Bioresource Technology, Volume 171 In Progress http://www.sciencedirect.com/science/article/pii/S0960852414012152

Deublein D and Steinhauser A 2008 Biogas from waste and renewable resources, An Introduction, Wiley-. VCH, pp.123 – 125 .

EPA 2012 Increasing Anaerobic Digester Performance with Codigestion. http://www.epa.gov/agstar/documents/codigestion.pdf

Inthapanya S, Chathaokalor D, Phongpanith S, Leng R A and Preston T R 2013 Fermentation of fruit waste in an anaerobic batch digester. Livestock Research for Rural Development 25 (9). http://www.lrrd.org/lrrd25/9/sang25155.html

FAO 2011 Global food losses and food waste. Dusseldorf, Germany: Interpack. http://www.fao.org/save-food/savefood/en/

Minitab 2010 Minitab Statistical Software, Release 16.1.1 for Window (Minitab Incorporated. State College PA, USA)

Ojikutu Abimbola O and Osokoya Olumide O 2014 Evaluation of Biogas Production from Food Waste. The International Journal of Engineering and Science (IJES) 3(1): 1-7.

Ozturk B 2013 Evaluation of Biogas Production Yields of Different Waste Materials. Earth Science Research 2(1):165-174.

Tanimu M I, Mohd Ghazi T I, Harun M R and Idris A 2014 Effect of Feed Loading on Biogas Methane Production in Batch Mesophilic Anaerobic Digesters Treating Food Waste. International Journal of Chemical and Environmental Engineering 5(1): 39-44.

Trinh Thi Thanh, Tran Yen and Dong Kim Loan 2004 Environmental technology syllabus. 2nd Printing, publisher of the Hanoi National University, Hanoi, 147 pages.

Vipul Vaid and Shivangi Garg 2013 Food as Fuel: Prospects of Biogas Generation from Food Waste. International Journal of Agriculture and Food Science Technology (IJAFST) 4(2): 68-71. http://www.ripublication.com/ijafst_spl/ijafstv4n2spl_14.pdf

Yadvika , Santosh , Sreekrishnan T R , Kohli S and Rana V 2004 Enhancement of biogas production from solid substrates using different techniques - a review. Bioresource Technology 95 (1): 1-10 http://www.scribd.com/doc/111620135/Enhancement-of-Biogas-Production-From-Solid-Substrates-Using-Different-Techniques

Received 8 September 2014; Accepted 23 September 2014; Published 3 October 2014

Go to top