WREC 1996 Anaerobic Digestion of Household Organic Waste to Produce Biogas
Mir-Akbar Hessami, Sky Christensen and Robert Gani Department of Mechanical Engineering
Monash University Clayton, Victoria 3168
Australia
ABSTRACT
Biogas may be readily obtained by the anaerobic digestion of organic waste. If communities and towns could harness the energy which is contained in the organic waste which they presently dispose of in landfills or compost, this fuel could supplement or completely satisfy their heat energy requirements.
Biogas production from household organic waste is rare because existing digesters are not suitable for small- scale applications. This paper describes a digester design which can make this waste to energy conversion process possible. The proposed digester is inexpensive to construct and maintain, simple to operate and highly efficient. This is made possible by an integration of modem industrial digestion processes, and low- technology digestion concepts as seen in rural Chinese and Indian digesters.
KEYWORDS
Anaerobic digestion; household organic waste; appropriate technology; biogas; waste management.
HARNESSING THE ENERGY IN HOUSEHOLD ORGANIC WASTE (HOW)
HOW is a frequently wasted renewable source of energy. It is composed of the food scraps and garden waste which most householders dispose of in landfills or, at best, compost; only few harness the energy which organic waste contains despite being a readily available resource, and its conversion to energy (ie, biogas) through anaerobic digestion being relatively straightforward. Biogas is a flammable gas containing upward of 50% methane which can be burnt to produce heat energy. In addition to providing clean renewable energy, anaerobic digestion of HOW also produces a residue which is an excellent organic fertiliser that can be used in farming to reduce the amount of waste normally disposed of in landfills.
Statistical data from a suburb of Melbourne, Australia showed that over 76% of HOW was putrescible, or potentially treatable using anaerobic digestion (Christensen, 1995). These figures showed that, on average, each person produced 3.7 kg of digestible material per week, which could be converted to biogas with a heat energy of 0.32 MJ per person weekly as explained later.
THE BIOLOGY OF ANAEROBIC DIGESTION
Anaerobic digestion involves the decomposition of organic matter by micro-organisms in an oxygen-free environment. It results in two by-products: biogas (approximately 60% C& and 40% CQ) and a digested organic sludge. This decomposition takes place in three stages:
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WREC 1996 . Hydrolysis whereby the insoluble solids are broken down into soluble monomers; ?? Acid formation whereupon the soluble monomers are converted to volatile fatty acids; and ?? Methane formation in which the acids are converted to biogas and a residue or sludge.
The optimum temperatures for anaerobic digestion are either 35 “C or 55 “C depending on the application and its operating conditions. The optimum pH and C:N ratio are around 5-8 and 25: 1 - 30: 1, respectively.
‘IRE ANAEROBIC DIGESTER
Anaerobic digestion of HOW takes place in an airtight vessel (the digester) which also makes possible the regulation of the temperature in the vessel and allows for the collection of the generated biogas. Therefore, by selecting an appropriate digester, the yield for a given application can be improved. There are two basic types to consider:
Mixer tank II Gas outlet
“Low-tech” Rural Digesters
Anaerobic digestion of agricultural and human waste is common in India and China. They use digesters which operate on simple principles, have simple construction and are inexpensive to build. Such digesters generally consist of a below-ground digestion vessel which has brick or masonry walls. It incorporates a mixing tank in which the waste to be digested is diluted with water to form a slurry
Figure 1: Chinese agricultural digester.
before entering the digester. Waste is then fed to the digester periodically. Digested sludge is displaced into an outlet tank. The generated biogas is stored in the upper section of the digester vessel. This design, while being simple in construction and operation, is less effective than modern digesters in extracting the energy from the waste.
Modern Digesters
Very efficient digesters have been developed for industrial applications over the last few years. However, many of these digesters are unsuitable for treating HOW because of its high solids content. The ‘high solids” process is one of the better modern processes for producing biogas from HOW because its operation does not suffer from a feed containing a high solids content.
“High solids” digestion involves feeding the waste into the digester vessel without diluting it with as much water as other processes require typically in the 3-8% solids content range prior to digestion, High solids digesters operate well with a slurry which contains IO-25% total solids content, Unfortunately, such digesters are often prohibitively expensive. Hence the need for the design of a new digester type described below:
LOW-COST COMMUNITY-BASED DIGESTER CONCEPT
Both “low-tech” and modern digesters have attributes which would be desirable in a low-cost community- based digester. For example, low-tech digesters are simple and inexpensive but inefficient, while modem digesters are highly efficient but too expensive to be viable for community-based operation. Therefore, designing a vessel which incorporates the low-tech design and modem digestion processes can produce a digester suitable to small-scale applications mentioned earlier.
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ProDosed Desipn
If a “low-tech” digester was operated as a “high solids” digester, its energy production efficiency should increase, Higher efficiency combined with the existing low-tech digester attributes of low cost and simplicity would produce a digester ideally suited to the application considered in this paper.
Such a digester could be based on the design of the Chinese digester shown in Figure 1 after making the following alterations: (1) Because a high solids feed is more viscous than the feed usually used in low-tech digesters, an effective mixing mechanism must be incorporated to ensure that the digester contents are homogeneous. (2) A heater (in the form of a copper coil through which a heating fluid can pass) placed in the digester might be necessary depending on the climate. (3) An external gas storage might also be needed since storage in the upper part of the digester can result in pressure variations in the biogas supply.
SDecifications
A survey of 962 households (in a suburb of Melbourne) using waste disposal bins of 240-litre capacity showed that on weekly basis some 13 tonnes of putrescible waste (about 76% of the total waste disposed) was discarded. The average number of people per household was three. Assuming that 80% of the waste generated was digestible (Christensen, 1995) each person generated about 3.6 kg of digestible waste every week. For a community of 500 people, the available digestible HOW is about 260 kg per day. Assuming that the waste has a density of around 900 kg/m3 due to its high moisture content, the total volume of the raw digestible material per day would be 0.29 m3.
Digester volume. For a digester with a 20-day hydraulic retention time (Vermeulen et al., 1993) and the above volumetric loading rate, the working volume of a high solids digester would be 5.7 m3. Allowing for some variation in the loading rate and also for some gas storage, a digester volume of 9.0 m3 can be considered for the final design.
Biorzas and Fertiliser Production. The high efficiency of industrial digesters is to a large extent due to the relatively high concentration of total solids (TS) in the digester vessel. Although a high concentration of solids in the digester requires more energy when mixing the contents of the vessel, research has shown that with a TS content of around 20% the benefits in terms of biogas production outweigh the higher energy costs.
Therefore, for the purpose of this design, the digester is assumed to operate at that concentration. As reported by Mata-Alvarez et al. (1993) 1.4 m3 of biogas per m3 of active digester volume per day can be expected under such conditions. Therefore, the total biogas output from such a digester would be around 8.0 m3/day with an energy content of about 160 MI/day (or 45 kWh/day) assuming a caloritic value of 20 MJ/m3. The fertiliser output would be the same as the daily total volatile solids (VS) input which can be found if we
assume that the waste has a 80% volatile solids content. Therefore, the fertiliser output on dry basis would be about 42 kg/day
Details of Dieester Design. After considering a number of different options, it was decided that it will be advantageous to build a cylindrical, above ground digester from brick and concrete covered with adequate insulation to regulate the temperature in the vessel. The digester’s specifications are: Digester volume: 9.0 m3. Digester diameter: 2.2 m. Digester height: 2.4 m. Floor: IOO-mm thick concrete insulated with R = 1.7 m’K/W Styrofoam. Walls: double brick insulated with R = 3.0 m*K/W fibreglass batts.
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Roof: 50-mm thick rigid Styrofoam sandwiched between 0.8~mm sheet steel. HOW inlet: 150~mm diameter pipe located near the top of the wall. Material outlet: loo-mm diameter pipe with control valve located in the wall, near the floor. Biogas extraction port: 50-mm diameter pipe with control valve located in the roof. Mixing: centrally-located mechanical stirrer which can move in the angular and vertical directions. Temperature control: 20-m long heating coil made of 19-mm diameter copper pipe located within the digester. In order to keep the contents of the digester at the optimum temperature of 35°C at all times, for the worst condition during winter, hot water at 60°C and a maximum flow rate of 8L/min is required.
Digester Cost. The total cost of the material needed to build such a digester is estimated to be around A$3,000 (Christensen, 1995). It is assumed that the labour for the construction of the digester is provided by the members of the community using the digester. However, if funds are available, the labour cost of the construction of such a simple digester should not exceed A%2,000. This total cost of A$5,000 is in addition to the cost of the necessary facilities (a) to pre-treat the waste by cutting it into small sizes so that a high surface area to volume ratio is obtained, (b) to remove the impurities from the biogas for better combustion, (c) to provide hot water to maintain the digester contents at the required temperature, and (d) to post-process the digested sludge to produce useable fertiliser. Excluding these additional costs, the one-off $5,000 cost of building the digester when amortised over its 20-year life span amounts to an annual expense of $250. This can be offset by the biogas production of 8.0 m3/day (160 MJ/day) which if used to replace natural gas for a domestic application would amount to an annual saving of around $390 assuming a rate of A$0.671/MJ.
Other Benefits. The other benefits which cannot be easily quantified are: (a) production of about 15 tonnes of organic fertiliser annually, (b) diversion of about 117 tonnes of waste per year from landfills, (c) harnessing of the biogas from the digester as opposed to the emission of methane from landfills to the atmosphere, and (d) the absence of unpleasant odours and sights. It should be noted that these benefits can be obtained by building a single digester of the type explained in this paper. However, by carefully designing an incentive package preferably funded by the government, and by educating the community about the non-tangible benefits of such projects, the global environmental advantages of building such simple, community-based, low cost anaerobic digesters would be enormous.
REFERENCES
Christensen, S. (1995). Appropriate Technology Metboa for the Anaerobic Digestion of Organic Household Waste. Department of Mechanical Engineering, Monash University, Clayton, Victoria, Australia.
Vermeulen, J., Huysmans, A., Crespo, M., Van Lierde, A., De Rycke, A., and Verstraete, W. (1993). Processing of Biowaste by Anaerobic Composting to Plant Growth Substrates. Water Science and Technology, 27(2), pp 109-l 19.
Mata-Alvarez, J., Cecchi, F., Pavan, P. and Bassetti, A. (1993). Semi-dry Thermophilic Anaerobic Digestion of Fresh and Pre-Composted Organic Fraction of Municipal Solid Waste (MSW): Digester Performance, Water Science and Technology, 27(2), pp 87-96.
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