For most wastes, the size of digesters is largely 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.
However, the amount of biogas derived from a waste depends on many factors including:
In short it is the digester feedstock itself that controls the gas production for most systems, not the digester.
There are two ways of getting to a first estimate of the size of the digester - organic loading rate and hydraulic retention time (RT).
For a 'rule of thumb' figure use a daily loading rate of 6kg dry matter 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 with a limit of +/- 7 days for a wide range of materials.
You have 10 tonnes of animal manure at 10% dry matter (90% water) per day to treat.
Qu. What size digester?
Note a tonne (1000kg) of water has a volume of 1 cubic meter (1000Litres) by definition. There are 220 UK gallons in a cubic meter. For most approximations, liquid food and farm wastes have a density close to that of water.
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. Retention Time Estimate
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 on average wastes and we would budget on this size prior to getting a full survey of the site and wastes for detailed costing. In general, low solids liquid effluents (3 - 8% Dry Matter) and fatty wastes need a lower Retention Time than high solids effluents (8 -20% Dry Matter). Some dry matter figures for various wastes are given in the table below.
PERFORMANCE
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 50% of the dry matter is converted to biogas. 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 to biogas. 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 and 1.7kWh of electricity from each cubic meter of reasonably good quality biogas. Half of the hot water will be needed to heat the digester. If the biogas is used for heating water in a boiler without electricity production, one cubic meter of biogas will produce about 2.5kW of hot water in a 70% efficient boiler.
These figures are for fresh wastes and storing an organic waste for a week in hot weather may reduce the biogas potential by a half. Rain water entering slurry and then into the digester has to be heated inside the digester . Low total solids in the digester feedstock is a major source of heat loss problems in Northern climates.