Why Would a Farmer Choose Anaerobic Digestion?
Waste biomass from farming and food-industries, such as slurries and the organic waste from food processing can present a considerable waste disposal problem. Release of large quantities of organic wastes into local water courses or seepage into ground water results in pollution of water courses and water supply. To prevent this occurring, the biomass needs to be treated and suitably stored and Anaerobic Digestion is a great way to achieve this.
Biogas is produced by the natural microbial degradation under anaerobic conditions. This technique has been applied to agricultural wastes world-wide on a small scale. Treating the waste by AD gives some environmental benefits, and in addition to AD also results in energy, in the form of biogas. Biogas is an inflammable gas, which can easily be used to generate electricity and heat. The potential benefits are many from on-farm and local use of fossil fuels and provision of energy within rural communities.
Some commentators have said that in addition to energy production, there is considerable potential for Anaerobic Digestion to assist in centrally managing the distribution of plant nutrients in manures, together with minimising biosecurity risks (ie pathogen kill).
Where is Anaerobic Digestion Most Popular?
In countries such as Denmark, Sweden, Germany, Austria and Switzerland there are many examples of co-digestion of farm manure with other suitable organic wastes in prefabricated digestors. There is a large interest in this option in these countries and in other countries both inside and outside the EU.
The most successful digesters utilize all energy produced. The most efficient use of biogas is direct heating. If biogas is burned in an electric generator, the heat produced by the engine must be harnessed and used in order to achieve optimum efficiency. The electricity produced can be used on site or sold into the power grid.
What Happens to the Sludge After the AD Process?
Once the digestion process is complete, the digested manure, or effluent, is released from the digester and stored until it is land applied. The digestion process breaks down many of the solids contained in the manure. As a result, the effluent causes less stress on pumps and agitation equipment than manure does. Also, effluent is homogeneous and may not require agitation prior to land application.
Many agri-food AD systems are located on farms. Farm-based AD systems work well with liquid manure. AD systems provide a valuable manure treatment option, since most other economically effective manure treatment systems (such as composting) require solid materials with dry matter greater than 30%.
What are the Most Popular AD Types of Plants?
Biogas from biomass has historically been used in Asia as a fuel for household uses such as cooking. Denmark and Germany have many modern digesters operating on farms and in central locations using materials such as manure, energy crops, and food-based products and byproducts. These systems typically use biogas to produce electricity and heat.
Anaerobic digestion (AD) plants can be on-farm units, designed to deal with manures and other organic materials produced at farm level. Alternatively, AD plants can be designed as centralised units to deal with products from a number of farms, along with co-digestion of organic materials from other industries.
AD plants can include mechanical separation of fibrous solids from the digestate that, after further processing, can give a value added product, such as compost or pellets (fertiliser or combustible fuel).
How Would I recognise an Anaerobic Digestion Plant if I Saw One?
Anaerobic Digesters themselves are completely sealed vessels which are covered and sealed. Most are insulated and heated, and the digester is an immediatelely recognisable feature at most large biogas plants as a large tall, usually vertical sided, circular tank, with an access ladder and a number of pipes leading to the top where there is also usually a pedestrian access way and handrail.
Biogas plant digesters of this type can usually be spotted very easily when you see them by the characteristic pairing of the digester with the biogas storage tank which acts like a traditional town gas ‘gasometer’, and is usually elegantly curved or “globe” shaped. Many visibly expand and contract to accommodate the volume of biogas present, the result is that most of them become significant landmark features.
Showing posts with label composting. Show all posts
Showing posts with label composting. Show all posts
Saturday, February 13, 2010
Monday, December 28, 2009
Mechanical Biological Treatment Process with AD Explained
The New Civil Engineer magazine has published (19 November edition 2009) a refreshingly down to earth description of the process which will soon start to mechanically and biologically treat a part of Manchester’s residual (black bag) Municipal Solid Waste (MSW). The following was stated by Peter Harvey in his role as the Business Director of Enpure (Process Engineering). Enpure is the encumbent EPC Contractor, in the Greater Manchester Waste Disposal Authority’s huge city-wide PFI waste management contract.
This multi-billion contract was finally let around about last Easter after extended delays, and not least the problems caused by the credit crunch last Autumn. Under this Contract, Enpure is the EPC Contractor which will provide processing facilities for the PPP Consortium Laing Viridor to operate two MBT Facilities to be built at Reliance Street and Bredbury, under a £57 million Contract.
(In MBT there is always a choice to be made between composting to reduce the activity of the organic material before it goes to landfill, which is of dubious merit, or Anaerobic Digestion which does a better job but requires more investment and some risk of not repaying that investment. - Added by your Blogmaster)
"There are a number of ways that you can biologically treat that [smaller fraction], composting for example, or anaerobic digestion which can then create methane for electricity production. That particular approach [anaerobic digestion] is what we have on our two MBT projects," says Peter Harvey.
The process begins at the materials recovery facility (MRF) whereby black bag [mixed residual] waste (MSW) goes through a very coarse shredder that reduces waste to 250mm to 300mm pieces. At that point the waste is screened and material of less than 80mm (the organic fraction) goes off for anaerobic digestion.
But first it needs further separation. "If we didn't clean it up then grit, pieces of plastic and bits of rubble could cause blockages so we have a very sophisticated wet separation process called the hydro-pulper".
It is this pulper which has not been used before in the UK. It comes from German firm BTA International. "It is a vessel with a high speed impeller in the centre that will transfer any organic material, paper, cardboard, food and organic matter into a sludge, which we can then pass through a screen and into the digesters," says Phil Harvey.
Just under 5MW of electricity will be generated by the Reliance Street and Bredbury facilities to fuel the MBT process and feed back into the grid. "Between these two facilities there is enough to power 10,000 homes from the biogas alone.
As for the larger components, these are sorted for recycling in the case of metals and plastics, or for sending on as Refuse Derived Fuel (RDF) to Runcorn [for incineration].
The fraction above 80mm is then put through a density separation process so that we can eliminate any hardcore material from that size fraction.
Then there is metals removal. After this it is shredded to 30mm ready for RDF production.
We also have an air knife which blows off plastics into the RDF as well.
At Reliance Street the treatment process will handle 100,000t of waste per annum, but only 63,000t of this will go through the MBT plant. The rest, larger particles over 45mm, is sorted out and sent off for recycling.
"The only real difference between the two sites is the degree of sorting that happens before the wet preparation stage," says Peter Harvey.
Of course with plants handling so much waste material and especially the Anaerobic Digestion present, there has clearly and quite understandably been much concern about odours.
Because Reliance Street is right in the city it uses a sophisticated odour control process called regenerative thermal oxidation. The odour is collected via ducts and blown into the treatment system which uses heat in the presence of a catalyst to thermally oxidise the odorous compounds in the air. The heat from the treated air is captured using silicon tiles.
"Bredbury has a more conventional odour system, we operate the buildings under negative pressure so that if there are any leaks they go into the building rather than out and all that air is treated with biofilters," says Peter Harvey.
This multi-billion contract was finally let around about last Easter after extended delays, and not least the problems caused by the credit crunch last Autumn. Under this Contract, Enpure is the EPC Contractor which will provide processing facilities for the PPP Consortium Laing Viridor to operate two MBT Facilities to be built at Reliance Street and Bredbury, under a £57 million Contract.
(In MBT there is always a choice to be made between composting to reduce the activity of the organic material before it goes to landfill, which is of dubious merit, or Anaerobic Digestion which does a better job but requires more investment and some risk of not repaying that investment. - Added by your Blogmaster)
"There are a number of ways that you can biologically treat that [smaller fraction], composting for example, or anaerobic digestion which can then create methane for electricity production. That particular approach [anaerobic digestion] is what we have on our two MBT projects," says Peter Harvey.
The process begins at the materials recovery facility (MRF) whereby black bag [mixed residual] waste (MSW) goes through a very coarse shredder that reduces waste to 250mm to 300mm pieces. At that point the waste is screened and material of less than 80mm (the organic fraction) goes off for anaerobic digestion.
But first it needs further separation. "If we didn't clean it up then grit, pieces of plastic and bits of rubble could cause blockages so we have a very sophisticated wet separation process called the hydro-pulper".
It is this pulper which has not been used before in the UK. It comes from German firm BTA International. "It is a vessel with a high speed impeller in the centre that will transfer any organic material, paper, cardboard, food and organic matter into a sludge, which we can then pass through a screen and into the digesters," says Phil Harvey.
Just under 5MW of electricity will be generated by the Reliance Street and Bredbury facilities to fuel the MBT process and feed back into the grid. "Between these two facilities there is enough to power 10,000 homes from the biogas alone.
As for the larger components, these are sorted for recycling in the case of metals and plastics, or for sending on as Refuse Derived Fuel (RDF) to Runcorn [for incineration].
The fraction above 80mm is then put through a density separation process so that we can eliminate any hardcore material from that size fraction.
Then there is metals removal. After this it is shredded to 30mm ready for RDF production.
We also have an air knife which blows off plastics into the RDF as well.
At Reliance Street the treatment process will handle 100,000t of waste per annum, but only 63,000t of this will go through the MBT plant. The rest, larger particles over 45mm, is sorted out and sent off for recycling.
"The only real difference between the two sites is the degree of sorting that happens before the wet preparation stage," says Peter Harvey.
Of course with plants handling so much waste material and especially the Anaerobic Digestion present, there has clearly and quite understandably been much concern about odours.
Because Reliance Street is right in the city it uses a sophisticated odour control process called regenerative thermal oxidation. The odour is collected via ducts and blown into the treatment system which uses heat in the presence of a catalyst to thermally oxidise the odorous compounds in the air. The heat from the treated air is captured using silicon tiles.
"Bredbury has a more conventional odour system, we operate the buildings under negative pressure so that if there are any leaks they go into the building rather than out and all that air is treated with biofilters," says Peter Harvey.
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