Sustainable Footprint

On Farm Anaerobic Digesters

Anaerobic Digestion is one of the more promising biological technologies for sustainable waste management. It has the potential to turn a large and worsening agro-headache into a growing opportunity for sustainable farming. It can extract useful biogas energy and high quality fertilizer from manure and other agro waste products while reducing the air and water pollution and emissions of greenhouse gases from a farming operation. Anaerobic Digestion harnesses natural living biological processes to maintain the natural carbon cycle and extract useful energy and fertilizers from the waste streams. Many types of farming could use it.

Current Agro Practices Waste Energy and Nutrients, Pollute and Contribute to Global Warming
Currently the byproducts of many farming operations are poorly and only partially utilized.Is is a cause of pollution instead. Furthermore the scale of this pollution is very large. A single large scale farm can generate the waste equivalent of a small city.

Farming operations produce byproducts such as manure, animal bedding, feed waste, and runoff that in many cases contribute to pollution of waterways, bad smell, attract rodents and flies and can pose a public health risk. Anyone who has ever been in the vicinity of a farm knows the smell.

On most animal farming operations, the animals are crowded into small areas and their manure and urine are funneled into massive waste lagoons. The animal waste cesspits can and often do break, leak or overflow, sending dangerous microbes, including Salmonella, E. coli, Cryptosporidium and others plus nitrate into waterways and aquifers. They also emit toxic gases such as ammonia, hydrogen sulfide and methane into the air, polluting the atmosphere and contributing to global warming.

One of the current practices is to spray the manure onto land, as fertilizer — these “sprayfields” can bring still more of these harmful substances into our air and water.
This way of doing things is not an example of sustainable agriculture and is very damaging for the environment as well as wasteful of energy and fertilizer. ‘Anaerobic digesters’ can help mitigate this problem while producing renewable energy as well.

What Is an On Farm Anaerobic Digestion System?
A farm-based anaerobic digester (AD) —or biodigester— is essentially an air tight sealed, and heated container, located on the farm, in which organic waste materials are broken down to produce biogas, effluent and solid remnants. They are shaped like silos, troughs, or basins, placed underground or on the surface. The biogas produced by the AD is approximately 60% methane, most of the rest is CO2.

AD systems are either batch type

An aerobic digestor on a farm in Scotland

systems, which are simpler to build and maintain and are more suitable for most on farm scenarios, or they are continuous types in which organic material is constantly or regularly fed.

All biodigesters share the following basic components:
* A pre-mixing area or tank, which mixes the feedstock into a properly dilute slurry;
* A digester vessel or series of specialized vessels designed to each provide optimum environments for the specialized microorganisms required for each stage of the complex digestion process;
* A system for collecting, storing and using the biogas and
* A system for storing and using the effluent and solid residue.

What is Anaerobic Digestion?
Anaerobic digestion is a biological process similar to composting. Like composting it relies on microorganisms to decompose agricultural waste such as manure into effluent and biogas. Anaerobic bacteria are some of the oldest forms of life on earth. They evolved before the photosynthesis of green plants released large quantities of oxygen into the atmosphere. Anaerobic bacteria break down or “digest” organic material in the absence of oxygen and produce “biogas” as a waste product.

Anaerobic decomposition is a process that occurs naturally in swamps, water-logged soils and other anaerobic environments such as the guts of animals. It can be managed and promoted in a “digester”, which is basically an airtight tank to process manure and other waste. It is a complex process, occurring in three basic stages as the result of the activity of a variety of different types of microorganisms.

Initially, a group of microorganisms converts organic material to a form that a second group of organisms utilizes to form organic acids. Finally methane-producing (methanogenic) anaerobic bacteria utilize these organic acids to complete the decomposition process.

The Many Benefits of On Farm Anaerobic Digesters
The primary benefits of anaerobic digestion are: on farm nutrient recycling, waste treatment, and odor control. In addition biogas is produced and can be used to provide energy and heat for on farm use or for selling surplus electricity.
Each of these beneficial outcomes contributes towards making the farming operation more sustainable. It promotes sustainability by significantly increasing the energy efficiency of the operation; retaining nutrients that are currently largely being lost and re-using them on the farm. It helps reducing the environmental impact of the farm and prevents spreading disease. It promotes re-use and useful extraction of energy and nutrient content from the waste stream. The farm can significantly reduce its reliance upon external energy and fertilizer and feed inputs.

Effluent from Digesters Is a Lot More Beneficial than it Sounds
The material drawn from the AD often called sludge, or effluent, is a mix of solids suspended in a thick liquid. It is rich in nutrients (ammonia, phosphorus, potassium, and more than a dozen trace elements) and is an excellent soil conditioner.
The solid residue produced in the digester is a stable, odorless, and (depending on the retention time and operating temperature) a largely pathogen-free fertilizer, which can be stored much more easily than raw manure and does not attract flies and rodents.

In the process of anaerobic digestion, the organic nitrogen in the manure is largely converted to ammonium, which is available and taken up by plants.The ease of storage allows the fertilizer to be applied onto fields when they are most needed.
It is important to note that toxic compounds (pesticides, etc.) in the digester feedstock material will become concentrated in this effluent and in the solid co-products; so farmers must take care to minimize pesticides or other toxic compounds.
The separated, digested solids contain most of the phosphorus in the manure, and operators may choose to export those solids or continue to apply them to their land, depending on their available acreage. The solid digested manure co-products may also be used for animal bedding, or composted. These fibrous solids can also be used to fabricate low grade fiber boards.

Less Smell, Less Pollution, less Global Warming and Fewer Pathogens

Anaerobic digesters allow a greater portion of smelly organic acids to be converted to biogas, resulting in less odor-causing compounds than would be present in manure in a typical liquid storage system. The overpowering odor of animal raising farms is one of the more vexing forms of pollution that they cause. Bioreactors can go a long way to eliminating these noxious odors that can lay over the land for miles around.

The potential for pollution of ground and surface water sources is also reduced. The digester extracts much of the nitrogen, phosphorous and potassium nutrient load out of the effluent and by doing so greatly reduces the potential for this type of water pollution –a major issue causing eutrophication of lakes, rivers and waterways and dead zones in the river fan zones in the oceans – for example the famous and growing dead zone at the mouth of the Mississippi river delta. The heat in the reactor significantly reduces the number of pathogens existing in the effluent effectively acting to sterilize it to a pretty high degree.

Anaerobic digesters also work to reduce the greenhouse gas emissions in a farming operation and in this manner increase the sustainability of the practice. Methane is a powerful greenhouse gas that remains in the atmosphere for approximately 9 to 15 years. By capturing methane and converting it to heat or electricity, methane digesters reduce greenhouse gas emissions from manure of farms.

Bioreactors Can Generate Electricity
Anaerobic digestion reduces the overall energy throughput of farms by combining manure with other high energy waste streams, that might otherwise be landfilled, and processing them. The produced biogas can be used on the farm to generate electricity as well as heat required for the anaerobic digesters themselves or can be used wherever heat is needed on the farm. This both saves energy on the farm and produces electricity for on farm use and a surplus that can be sold onto the grid.
The biogas can be stored so that electric generation can be timed for periods of peak demand. Many farms have the potential to create sufficient energy to run their operations (power and heat) and also provide power onto the local grid.

Anaerobic Digesters can be financed
Financial institutions are generally ready to finance projects like this because they reduce operations costs and mitigate risk exposure. It is a win win situation. Several programs exist to promote renewalble energy projects.

Conclusion
Systems to make better use of manure do not have the same sex appeal as say solar power or wind energy, but they are important enough to promote a green sustainable agriculture. Reducing the energy throughput of farming systems is going to be as important and as vital to building a green economy.

The electric car or the solar array may capture the lion’s share of public attention, but the humble bioreactor may be as important although unsung.

Of course it is really really hard to sex up a song singing the praises of digesting manure, but it is a song… a message… that is important to get out there. A sustainable green economy will require us to become much better at how we handle our crap…. literally.

Source: Sustainable Farming (adapted)

see also 'pig manure'

Other link about the system:
energy savers

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