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Anaerobic digestion of sewage sludge

Sludge is the solid material, primarily organic in nature, which settles out in the residential/municipal waste water treatment process. Although the main focus of this solution is waste water treatment sludge, other organic wastes may also be used as complementary substrates (e.g.: waste from slaughter houses, animal farms, food markets, restaurants, etc.).
Anaerobic digestion of sludge is a process of biodegradation of the organic matter in the absence of oxygen to yield methane, carbon dioxide and water. This technique not only reduces the sludge volume and destroys pathogens but also releases methane gas, which can be used as a renewable energy source.
The biogas can be used for small scale heat/electricity production or can generate revenue through electricity supply to the national grid; through biogas purification it can also be injected into the natural gas grid or purified for use as fuel for transport.
The local government, as wastewater treatment plant operator, is responsible for ensuring that the anaerobic sludge digestion takes place in an environmentally sound way and that the waste to energy conversion process is efficient. Proper planning, implementation and regulation are required to ensure viability of the biogas plant.

Motivation / Relevance

Wastewater treatment plants generate sludge as by-product. The sludge must be adequately treated and disposed of to avoid health and environmental problems since it contains high concentrations of pathogens, organic matter, nutrients, metals and other substances, holding a very offensive odor.

Traditional treatment options for sludge have disadvantages. For example, landfilling can have very high transportation costs and greenhouse gas emissions, and sludge incineration can be very energy intensive due to sludge`s high water content and analytical control and treatment of emissions is necessary. Anaerobic digestion is one of the best ways to treat sludge mainly because it significantly reduces the amount of sludge and is a net energy positive mechanism, as it leads to the production of biogas, which can be used as a source for energy generation. The anaerobic digestion treatment also reduces the pathogens in the resulting residual matter. It contain nutrients which can be used to improve the soil quality (for example, the liquid fraction can be used as as organic fertilizer and the solid fraction as soil filler).

With the increase in population in the urban areas the amount of waste water generated increases, which in turn escalates the need for efficient sewerage systems. As the amount of sewage volume increases, the amount of sludge generated during sewage treatment also increases, with the need to render it harmless. Hence, the anaerobic sludge digestion can be considered of great relevance for the proper waste water management process.

Main impacts

  • Reduced likelihood of soil, air and water pollution and odors, compared to disposal of untreated sludge.
  • Contribute to energy security and conservation of natural resources.
  • Climate change mitigation.
  • Contribute to the economic viability of local farms.

Benefits and Co-Benefits

  • Reduce the volume, and organic and pathogens content of the sludge resulting from wastewater treatment.
  • Renewable energy generation in the form of biogas. Biogas generated can be used as a substitute to fossil fuel energy for heat/electricity generation or as vehicle fuel.
  • Decrease the consumption of fossil fuels, namely in the operation of the wastewater treatment plant.
  • Direct and indirect reduction of greenhouse gas emissions.
  • On a financial and environmental perspective, conversion of residues into potentially saleable products: biogas, soil conditioner, fertilizer.
  • Generate revenue for the local government through the sale of electricity produced from biogas or through purification of biogas to enable injection in the natural gas grid or use as fuel by the transportation sector
  • Generation of employment for operation and maintenance of anaerobic digestion process, and energy processes.
  • Production of compost and organic fertilizer products that recycle nutrients and organic matter back into the local environment: agriculture, landscaping and public works projects with the benefits of erosion control, water absorption, soil nutrient infusion, and reduction of petroleum-based fertilizer use.
  • Reduce need for analytical control of sludge by comparison with other disposal options such as: land application, disposal in water bodies and incineration. (The sludge content in heavy metals, pesticides, and hydrocarbons has to be determined when incineration and land application methods are considered).