Demanding targets have been set by the EU Landfill Directive (1999/31/EC) in order to reduce the amount of biodegradable waste being sent to landfill. These targets are: by 2010, to reduce the amount of landfilled biodegradable municipal waste to 75% of that arising in 1995; by 2013, to reduce the amount to 50% of that arising in 1995; and by 2020, to reduce the amount to 35% of that arising in 1995
This is because biodegradable waste (green waste, food waste, paper/card waste) that is sent to landfill will eventually go through the natural process of anaerobic digestion. This process can have a negative impact on the environment as it can; pollute soils, groundwater, and release greenhouse gases methane (CH4) and carbon dioxide (CO2) into the atmosphere.
With this in mind, biological treatments of organic wastes are increasing in importance as an option to reduce water and soil pollution, as well as the level of greenhouse gas emissions released into the atmosphere, which in turn contribute to climate change.
Anaerobic Digestion
Anaerobic digestion is a biological process that controls greenhouse gas emissions by using them to generate energy and can be defined as a naturally occurring process where anaerobic bacteria breakdown organic material, in the absence of oxygen to produce biogas. Anaerobic digestion can occur at three different temperature ranges. These ranges are the psychrophilic range (15oC – 25oC), the mesophilic range (30oC – 38oC), and the thermophylic range (50oC – 60oC). To find out more detailed information on the anaerobic digestion process please visit: http://www.energysavingwarehouse.co.uk/news/60/20/Waste-Treatment-Technologies-Anaerobic-Digestion.html
Biogas
Biogas is made up of approximately 60% methane and 30-40% carbon dioxide. A large range of bio-waste materials can be used to produce biogas including all types of livestock waste, energy crops such as corn silage and off-farm organic waste materials such as cooking oil and grease. Kitchen garbage and paper are other types of organic material than can also be used to produce biogas. Any waste material that is not utilized in the anaerobic digestion process is used by farmers as fertilizer.
Biogas produced from anaerobic digestion can be used in many ways, one of which being a natural gas substitute. More commonly it is used to fuel boilers to produce heat (hot water and steam), or to fuel generators in combined heat and power (CHP) applications to generate electricity, as well as heat [1]. The amount of biogas electricity produced per tonne of waste can vary considerable amounts depending on certain factors. These factors include the feedstock of the waste being digested, the speed in which biogas is being produced, and the quality of the equipment being used to convert biogas into electricity. Biogas being used to fuel boilers requires very little pre-treatment as boilers are less sensitive to hydrogen. Biogas being used as fuel or a substitute for natural gas requires more pre-treatment. This treatment includes: the removal of hydrogen sulphide (a corrosive gas); moisture removal; pressurization to boost gas pressure; and removing carbon dioxide to increase the calorific value of the biogas.
Digesters
Anaerobic digesters are used to produce biogas. One reason for this is because when anaerobic digestion occurs in an enclosed vessel or anaerobic digester, biogas can be collected and used as fuel in a generator to produce electricity and heat. However, the main practical reason is because they provide the heat and anaerobic conditions bacteria require to break down organic matter in order to produce biogas.
Co-Digestion of Waste Materials
Because biogas plants are difficult to run with economically profitable results if the process is based only in livestock manure, co-digestion strategies are widely applied in order to enhance the methane production in agricultural biogas plants [2]. Co-digestion, in anaerobic digestion is the combined treatment of several wastes at the same time. Anaerobic co-digestion of organic fraction of municipal solid wastes (OFMSW) and food wastes with organic matter increases the production of biogas. It also allows for digestion of poorly biodegradable wastes, which cannot be digested alone (e.g. fat or protein wastes), when mixing with other more degradable wastes.
References:
[1] DEFRA, 2007. Advanced Biological Treatment of Municipal Solid Waste. http://www.defra.gov.uk/environment/waste/wip/newtech/pdf/abt.pdf
[2] Neves, L., Oliveira, R., Alves, M.M., 2008. Co-digestion of cow manure, food waste and intermittent input of fat. Bioresource Technology 100.
