In the Anaerobic Digestion Plant Design section we provide the following articles:
An Introduction to Anaerobic Digestion Plant Design
The first stage of any AD Plant design is to define the primary aim, or purpose which an AD Plant is intended to satisfy.
Setting the Brief for an Anaerobic Digestion Project in Municipal Solid Waste Treatment
The main purpose for Anaerobic Digestion in MSW Treatment may be, for example, to reduce the bulk (or “mass”) of the material.
The new generation of MSW anaerobic digesters are being designed with the further additional design featuress:-
- Biogas biofuel production; which can be burnt for electricity production etc
- Sanitisation; due the AD Process’ ability to operate at raised temperatures (usually using the biogas produced by the process) during the reaction, to ensure compliance with Animal By-Product Regulations (ABPR)
- Producing a digestate comprising a fibrous material, which can be spread on land, co-disposed with compost, or incinerated, and a liquor which (with care) can be used as a liquid fertiliser, but often must be further treated.
Anaerobic Digestion Design
This is due to the fact that the term “biogas” suggests that the plant’s primary purpose will be to produce biogas (methane), and there are many anaerobic digestion plants both in use today and historically that have not been installed primarily to produce and utilise biogas (methane for electricity generation, biofuel for transport vehicles, LPG for cooking and heating use etc,).
Anaerobic digestion design also encompasses plants which are primarily designed to:
- Treat an effluent (as in industrial effluent treatment) to a quality which will allow it to be discharged to a sewer or to a watercourse according to the requirements of the site owner and the regulatory authorities;
- Treat the secondary (sludge) by-product from a water treatment process to reduce volume, sanitise, and permit final disposal (eg to land); as in the digestion of sludge created during the popular (aerobic) activated sludge sewage treatment process;
- Treat Solid Waste (as in Municipal Solid Waste (MSW)) to primarily help meet local government targets (especially within the European Union (EU)) where strict targets must be achieved by member states to divert the overall tonnage of MSW away from landfill, and also to reduce still further the amount of organic/biodegradable content within the waste which is sent to landfill.
It is only since about 2003 in Europe, but possibly a century earlier in China, that the creation of biogas has been a viable function of anaerobic digestion plants. The reason that AD biogas has now become a valued commodity is that fossil fuel sources of methane gas have risen so much in price that (with some encouragement from government grants and tax breaks) biogas methane can be cheaper.
Biogas is, when scrubbed and pressurised, equivalent in composition and broad calorific value to the fossil fuel known as Liquid Natural Gas (LNG). and at lower pressure can double for Natural Gas in town and city gas supply pipelines.
Just as the primary purpose, and scale, of Anaerobic Digestion Plants is very diverse so are the processes which have been developed to meet these challenges. Many processes are similar in principle, but do vary substantially according to the primary purpose and environmental, plus economic and political drivers.
An example of an Agricultural Anaerobic Digestion Plant Design
Slide 26, above, appears to have been produced by Entec GMBh, with the following design data given on the same slide:
- Substrates: cattle manure, corn
- Input: 801 Mg/week (fresh)
- TSS = 6,6 kg/Mg
- VSS = 5,3 kg/Mg
- Reactor volume: 2500 m³
- Detention time: 20 d
- Volume Load:
- Br = 2,5 kg VSS/m³d
- Electric Power: 40655 kWh/week
- Thermal Power: 55435 kWh/week