Anaerobic Digestion - A sustainable opportunity

This is the author-manuscript version of this work - accessed from http://eprints.qut.edu.au Senadeera, Wijitha and Harris, Paul and Burge, Andrew and Fielke, John (2004) Anaerobic digestion - a sustainable opportunity. In Proceedings Biennial Conference of the Society for Engineering in Agriculture, Dubbo, Australia. Copyright 2004 (please consult author)

Anaerobic Digestion – A sustainable opportunity Wijitha Senadeera1, Paul Harris2, Andrew Burge1 and John Fielke1

1. Agricultural Machinery Research and Design Centre, University of South Australia, Mawson Lakes SA 5095, Australia 2. Department of Agronomy and Farming Systems, Adelaide University, Roseworthy SA 5371, Australia

ABSTRACT The anaerobic digestion process is one of the established technologies for sustainable processing of residues and wastes in the agro-food industry. It can be either used to treat biodegradable wastes or produce saleable products with economical value. This is a natural process where complex organic matter is broken into simpler substances by microorganisms under airless conditions. Anaerobic microorganisms digest the organic matter in the absence of oxygen, to produce methane, carbon dioxide and solid residue. The digestion process itself takes place in a digester and there are several types and designs available depending on feed wastes, climate and other conditions.

In Australia there are many agro-food based industries producing different wastes, which are creating problems in handling and disposing. Anaerobic digestion is a sustainable opportunity for these problems.

In this paper, the application of anaerobic digestion process for the sustainability in the agrofood industry is discussed. Also relevant industries are identified and opportunities with them are shown.

INTRODUCTION There are many types of wastes produced from municipal, industrial and agricultural operations. Sustainable technologies are needed for efficient and cost-effective waste management practices. Anaerobic digestion is one of the available technologies that will treat wastes and convert them into usable products of economic value in a sustainable manner.

Anaerobic digestion is a natural process occurring in the absence of oxygen. Anaerobic bacteria will ferment biodegradable matter and convert it into usable products. One of the main abilities of anaerobic digestion is the conversion of organic matter to energy rich biogas that can be used as a fuel. Additional benefits of anaerobic digestion are the conservation of fertilizer value of the feed material, pathogen reduction, odour reduction, resource recovery, and mitigation of green house gases of environmental concern. Installation of anaerobic digesters, are an attractive sustainable alternative to current waste management practices and recovers outputs of economic interest (Harris, 1999). Figure 1 shows a schematic of the process.

BIO GAS

FOOD/FARM

DIGESTION

WASTES

SOLIDS LIQUOR

FIGURE 1 Schematic of the digestion process

Waste materials for the anaerobic digestion A recent literature review on anaerobic digestion revealed that there are many sources of waste available in Australia from different agro/food industries in large quantities suitable for anaerobic digestion to produce energy and by products. Treating these wastes to produce biogas while recycling fertiliser, nutrients and possibly water creates a cycle of sustainability. Some of the possible waste materials in SA and anaerobic treatment capability are given in Table 1 and Table 2 respectively.

TABLE 1 Examples of suitable industries for anaerobic digestion Agricultural waste

Agro-industrial waste

Municipal waste

Dairy

Dairy Processing

Sewage sludge

Farm

Fish processing

Municipal solid waste

Beef cattle

Meat processing

Green/botanical waste

Pig

Vegetable processing

Chicken

Wine processing

Horse

Olive processing

Sheep

Breweries

TABLE 2 Anaerobic treatment capacity of some waste material Type of waste material

COD reduction (%)

Sugar beet

75

Potato processing

84

Vegetable canning

60~80

Dairy processing

80

Slaughter houses

55

Distillery waste

45~65

Waste water

90~95

Olive waste

85~93 source: Sreekrishnan and Ali (1999)

The main product of anaerobic digestion is the biogas. After the gas extraction (energy), the effluent comes as the byproduct of the system as slurry. This is a pathogen free stabilized product that could be used to improve soil fertility and enhance crop production or as an animal feed. Energy content of the biogas can be converted to mechanical energy or heat energy. A stoichiometric equation was proposed to estimate methane production from substrate, Ham and Barlaz (1989);

⎛ a b 3 ⎞ ⎛n a b 3 ⎞ ⎛n a b 3 ⎞ Cn Ha Ob Nc + ⎜ n − − + c⎟ H2 O = ⎜ − + + c⎟ C O2 + ⎜ + − − c⎟ CH4 ⎝ 4 2 4 ⎠ ⎝2 8 4 8 ⎠ ⎝2 8 4 8 ⎠ The composition of the most substrates can be found in ‘PHYLLIS-the composition of biomass and waste’ at ‘www.ecn.nl/phyllis’. Biogas yield depends on the characteristics of the waste and conditions. Some properties of the biogas are given in Table 3.

TABLE 3 Properties of Biogas 1 m3 of biogas generates 1.5 KWh 1 m3 of biogas equals 1 lb of LPG 1 m3 of biogas 0.52 l of diesel 1 m3 of biogas 0.54 l of petrol Calorific value ~ 24 MJ/m3 Contains 65% CH4 & 35% CO2 Source: Horst, W. D. (2000)

Based on the biogas yields found in literature for waste materials yearly estimate of biogas production for some materials in the South Australian region are given in given in Table 4.

TABLE 4 Biogas production in South Australia Waste

Yearly estimate biogas production in

Average biogas 3

SA (million m3)

material

yield per day (m )

Cattle

0.37

20 (0.07 PJ)

Pig

0.18

30 (0.11 PJ)

Poultry

0.011

15 (0.05 PJ)

Olive waste

3.5 (0.01PJ)

(values given in the brackets are primary energy production calculated based on gas volumes)

Primary energy consumption in South Australia by Biogas, predicted by Australian Bureau of Agricultural and resource Economics (ABARE) could be met by the biogas generation potential from available wastes in the region.

Anaerobic Digesters This is a physical structure known as a biogas plant, anaerobic reactor or bio-reactor. The important function of the anaerobic digester is to provide anaerobic conditions (air and water tight). The way in which the digestion is carried out depends mainly on the type of the feed. It can be carried out as a batch process, a continuous process or as a multi-stage process.

There are various types of digester designs available for various wastes, ranging from simple digesters to bioreactors. The anaerobic digester could be a small farm digester treating only farm wastes or a larger scale Centralised Anaerobic Digester (CAD) for a farming community. The process is same for both, but operation and marketing of products are more complex in CAD. Figure 2 shows schematics of some commonly available digester systems. Different digester systems were developed to cater for different wastes overcoming drawbacks in the simple digesters.

BIO GAS

EFFLUENT

INFLUENT

MIXING

Standard digester

BIO GAS

EFFLUENT INFLUENT

M IXING

RETURN SLUDGE

Anaerobic contact process BIO GAS B IO G A S

EFFLUENT

EFFLUENT

P A C K IN G

SLUDGE BLANKET

IN F L U E N T

Anaerobic Filter

INFLUENT

Upflow Anaerobic sludge blanket

FIGURE 2 Schematics of Digester Systems

Research at University of South Australia To improve biogas adoption in South Australia a facility for small-scale trials and pilot scale testing is needed, to evaluate work done overseas and encourage commercial uptake of this technology. A grant has been awarded for fabrication and optimization of a small-scale digester to suit South Australian climatic conditions and their wastes. One of the industries willing to adopt this technology is Olive industry.

Expected outcomes of the project: 1. A detailed evaluation and quantification of wastes in South Australian region suitable for anaerobic digestion 2. Design and construction of a suitable small scale facility for testing different wastes and treatment methods 3. Detailed evaluation and quantification of environmental benefits 4. Flow diagrams and preliminary facilities depicting the conceptual layout of possible pilot plants

CONCLUSION Anaerobic digestion has number of potential benefits. Methane is the main component of the biogas (produce local, renewable energy). Burning it reduces the methane emission to the atmosphere and this also replace the use of fossil fuel. This can reduce the farm waste odour, reduce the risk of pollution by stabilizing and controlling effluent (reduce volume of waste going to landfill and reduce BOD of waste water). Reduce the need of inorganic fertilizers as effluent contains nutrients. This can be a part of an effective integrated waste management plan for farmers.

ACKNOWLEDGEMENT The Authors wish to thank the Agricultural Machinery Research and Design Centre of the University of South Australia for the financial supports of this presentation at the Biennial Conference of the Society of Engineers in Agriculture (2004) in Dubbo, Australia.

REFERENCES

1. ABARE – Primary energy consumption in Australia, by fuel, by state (2004) 2. Ham, R. K. and Barlaz, M. A. (1989). Measurement and Prediction of landfill gas quality and quantity. Sanitary lanfilng: Process, technology and environment Impact. Eds T. H. ChristensonR. Cossu and R. Stegmann, Academic Press Ltd., London, UK, 155-66. 3. Harris, P. (1999). The role of Anaerobic Digestion in an Integrated Biosystem, In Proceedings of the National Workshop on Wastewater treatment and Integrated Aquaculture, Eds S. Kumar, SARDI Aquatic Sciences, 182-185.

4. Horst, W. D. (2000). Biofuel Generation, Proceedings In Form 2000, National workshop on Integrated Food Production and Resource Management: 101-105.

5. Shrikanthan, T. R. and Ali, M. (1999). New developments in Bioreactor Design for Biomethanation Process, Bio Energy, 3 (4):