Anaerobic digestion occurs naturally, in the absence of oxygen, as bacteria break down organic materials and produce biogas. The process reduces the amount of material and produces biogas, which can be used as an energy source.

There are two main types of anaerobic digestion called thermophilic and mesophilic. The primary difference between them is the temperatures reached in the process. Thermophilic processes reach temperatures of up to 600C and mesophilic normally runs at about 35-400C. The system chosen will largely depend on the feedstock to be processed. In this project, we are dealing with 'high solid materials', such as food waste mixture specifically the citrus fruits waste. Therefore, the process tend to be processed at a thermophilic temperature using the batch system.

The process of anaerobic digestion provides a source of renewable energy. The food waste is broken down to produce biogas (a mixture of methane and carbon dioxide), that can be used to generate electricity and heat to power on-site equipment and the excess electricity can be exported to the National Grid. A further by-product of the process is biofertiliser, which is rich in nutrients such as nitrogen, phosphorus and other elements required for healthy plant growth and fertile soil.

 

 

 

Anaerobic digestion occurs naturally, in the absence of oxygen, as bacteria break down organic materials and produce biogas. The process reduces the amount of material and produces biogas, which can be used as an energy source. The movement of adding food waste to anaerobic digesters has many benefits, including:

• Climate Change Mitigation – Food waste in landfills generates methane, a potent greenhouse gas. Diverting food waste from landfills to wastewater treatment facilities allows for the capture of the methane, which can be used as an energy source. In addition to decreased methane emissions at landfills, there are greenhouse gas emissions reductions due to the energy offsets provided by using an on-site, renewable source of energy.

• Economic Benefits – Wastewater treatment facilities can expect to see cost savings from incorporating food waste into anaerobic digesters. These include reduced energy costs due to production of on-site power and tipping fee for accepting the food waste.

• Diversion Opportunities – Anaerobic digestion offer the opportunity to divert all kind of food waste to produce the maximum amount of biogas. In addition, the slurry after each batch can be reprocess to produce more biogas.

 

 

 

3.3.1    Economy

First, Anaerobic digestion of food waste eliminate the need to transport waste to an established waste disposal facility. Utilizing the waste onsite saves upfront costs of transportation and disposal and also generates financial gains with the byproducts produced. The process creates a viable energy source that can be used for heating and electricity for homes or the facility itself. This biomass energy can be a net zero cost and offset electrical costs; provide new forms of energy to areas that might not have access to traditional electrical energy sources; or generate profit if the energy produced can be sold. Additionally, the AD systems often generate commodities such as fertilizer, which can be used at the site or sold for additional income. Creating natural, organic fertilizer in itself saves costs for these facilities, which use the fertilizers on their crops. In developing countries the biogas produced can also be used to run a cook stove, which saves a family from having to purchase alternative fuels for cooking.

• Reduces transport and disposal costs

• Reduces energy costs (for equipment, lighting, heating)

• Reduces costs from organic fertilizer use

• Increases income from sale of energy

• Increases income from sale of fertilizer

• Increases revenue from larger crop production (fertilizer use)

 

3.3.2    Social

The electricity produced from biogas becomes a tremendous social benefit to communities. In rural areas, anaerobic digestion give residents energy independence and make them self-sufficient. Providing an independent energy source gives residents more reliable access to electricity, improves quality of life, and promotes economic growth. Anaerobic digestion change the way communities heat their homes, providing a constant heat source so people do not have to look for wood or other fuels. Another social benefit is the creation of jobs. Learning how to operate an anaerobic digestion system and perform routine and heating source operations and maintenance checks also improves laborer skills and can help establish the local expertise needed for additional biomass electricity systems.

• Provides access to electricity and heating source

• Creates jobs

• Increases skilled labor force

• Improves sanitation and public health

 

3.3.3    Environment

Anaerobic digestion systems reduces water and air pollution, and improves human health and the environment. Biogas produced by anaerobic digesters can replace the use of fuel gas and oil. Harnessing methane lowers greenhouse gas emissions, improving air quality. The removal of manure from these facilities also improves water quality by reducing runoff and controlling pests, decreasing eutrophication and the potential for water pollution in local waterways. Additionally, utilizing manure decreases the time it sits at the farm contributing to odor and pest issues. Another benefit is from the use of the final digestate product (both liquid and solid), which is used as fertilizer at many of the facilities. This organic fertilizer reduces waste byproducts and the need for chemically-enhanced fertilizer, as well as helps improve crop production. Several countries that use AD systems report a reduction in the use of firewood for heating and cooking. The replacement of firewood with biogas-powered electricity and heating helps prevent deforestation, especially in remote populations that count on forestry as a source of exports.

• Reduces GHG emissions and improves air quality

• Improves water quality by lowering eutrophication effects

• Decreases odor

• Lessens pest issues

• Replaces chemical fertilizer with organic fertilizer

• Decreases use of firewood, helping to combat deforestation

 

 

 

Biocatalysts such as enzymes and microorganisms are the catalyst to speed up the reaction of anaerobic digestion (D., G., & Grilc, V., 2012). In most cases biomass is made up of large organic compounds. In order for the microorganisms in anaerobic digesters to access the chemical energy potential of the organic material, the organic matter macromolecular chains must first be broken down into their smaller constituent parts. These constituent parts or monomers such as sugars are readily available to microorganisms for further processing. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore hydrolysis of high molecular weight molecules is the necessary first step in anaerobic digestion. It may be enhanced by mechanical, thermal or chemical pretreatment of the waste. Hydrolysis step can be merely biological (using hydrolytic microorganisms) or combined: bio-chemical (using extracellular enzymes), chemical (using catalytic reactions) as well as physical (using thermal energy and pressure) in nature.

Aerobic microbes require oxygen and organic nutrients to work and grow. Nutrients are provided by the organic material in wastewater and oxygen is usually delivered by pumping air into the treatment tank. The nutrients and oxygen cause aerobic microbes to multiply and their increased number speeds up the digestion process. Anaerobic microbes are bacteria and microorganisms that function in the absence of oxygen. These microbes break down organic contaminants more slowly than aerobic microbes. Anaerobic microbes produce methane, carbon dioxide and more anaerobic microbes. Food waste is treated with anaerobic microbes before undergoing treatment with aerobic microbes.