The size of digesters is dependent on the mass of organic matter to be processed into gas and liquid fertiliser.
All mixtures of feedstocks, e.g.. cattle manure and poultry manure mixtures, have optimum loading rates and these need to be determined in an accurate analysis prior to the digester being built.
For a 'rule of thumb' figure use a loading rate of 6kg dry matter per day per cubic meter of digester.
To estimate the range of possible sizes for most high solids digesters, use a retention time (average residence time in the digester ) of 15 days +/- 7.
10 tonnes of animal manure at 10% dry matter (90% water) per day. Note a tonne of water weighs exactly 1000kg and has a volume of 1 cubic meter by definition.
Qu. What size digester?
Answer 1. Loading rate Estimate
The total dry matter fed to the digester per day is 10 tonnes x 0.1 = 1 tonne per day dry matter.
Since 1 tonne = 1000kg, and a typical daily loading rate for a high solids digester is 6kg dry matter per cubic meter of digester , the total volume of digester I need is 1000 / 6 = 166 cubic meters.
Answer 2.
The liquid volume is 10 cubic meters per day and a typical residence time is 15 days +/-7
Therefore the space required in the digester (the hydraulic volume) is most likely to be about
10 cubic meters x 15 = 150 cubic meters.
Comment.
These are very approximate figures but we can say that the digester is likely to have a volume of about 150 - 166 cubic meters. From experience we can put further limits on the error and using a retention time of 8 - 22 days (15 +/- 7) will give the size of digester for most wastes likely to be encountered including most industrial waste waters thus the digester will be in the range of 80 - 220 cubic meters but most likely to be about 150 cubic meters for optimum economy and we would budget on this size prior to getting a full survey of the site and wastes for detailed costing.
How much gas and electricity is produced?
The gas and electricity production is determined by the efficiency of the digester - ie how much dry matter is converted into biogas. For most systems estimate about 50%. For some old organic wastes which may have been subjected to composting e.g. broiler poultry manure, the gas production may be reduced by 2/3 giving a digester efficiency of 16% conversion of dry matter. Oily wastes are sometimes very digestible and almost 100% of the fat and oil may be converted to gas in a suitable digester. The picture for mixed wastes is rarely simple and you should consult Practically Green for further analysis. However, the following table may be of assistance.
Biogas and electricity production from 1 tonne of fresh waste (old wastes yield much less) |
Feedstock |
No. of animals to produce 1 tonne per day |
Dry Matter Content % |
Biogas Yield m3/tonne feedstock |
Approximate electrical value kwh |
| Cattle Slurry | 20-40 | 12 | 25 | 42.5 |
| Pig Slurry | 250 -300 | 9 (no rainwater) | 26 | 44.2 |
| Laying Hen Litter | 8,000 - 9,000 | 30 | 90 -150 | 153 - 255 |
| Broiler Manure | 10,000-15,000 | 60 | 50 -100 | 85 - 170 |
| Food Processing Waste | - | 15 | 46 | 78.2 |
In most systems with electrical generation, the engine will produce about 2kWh of hot water for each 1.7kWh of electricity produced. Half of the hot water will be needed to heat the digester.
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