With the rapid urbanization and ballooning population in many cities, it is vital to find a sustainable solution to treating waste. Wastewater plant managers are always looking to optimize the use of by-products from sewage sludge. Renewable energy from wastewater treatment presents a promising opportunity. We discuss the different technologies available to derive energy from sewage sludge.
Mesophilic Anaerobic Digestion
This is the most commonly utilized method of treating sludge. The sludge is broken down by using a diverse bacterial consortium in the absence of oxygen. The digestion of complex proteins and sugars results in simple compounds such as water, carbon dioxide, and methane. The methane rich biogas created can fuel a combined heat power (CHP) plant. In a usual Anaerobic Digestion process, each tonne of dry matter fed will result in 350m3 of biogas (65% methane), which generates up to 820kWh of electrical energy.
Thermal Hydrolysis
Thermal Hydrolysis uses direct steam injection and requires high temperature (130 – 180 degrees Celsius) and pressure (7 – 10 bar) to disrupt and dissolve sludge before transferring it to a conventional digester. It destroys pathogens and breaks down the organic matter resulting in increased methane production. Each tonne of dry matter fed will produce 450m3 of biogas, which would generate up to 1,100kWh of electrical energy. But since Thermal Hydrolysis requires an input of high grade heat, the energy needed for the process outweighs the energy available from a CHP.
Pyrolysis
Sludge pyrolysis is groundbreaking procedure that can convert sludge into useful bioenergy in the form of oil and gas. Sludge goes through a thermochemical treatment and is exposed to high temperature in an oxygen-free atmosphere. This results in the degradation of a complex material into three phases: solid (charcoal, biochar), liquid and non-condensable gases (H2, CH4, CnHm, CO, CO2 and N). This mostly endothermic process ensures high energy content in the by-products. Biochar, a carbon energy-rich solid material, has several applications such as carbon sequestration and soil amendment. Produced gas can be used as sources of heat, steam, and electricity.
Incineration
Incineration is a method that results in the complete conversion of sewage sludge to oxidized by-products such as carbon dioxide, water and ash. It has several benefits such as considerable volume reduction, sterilization and the production of steam that can power turbines to create electricity. However, it also requires high capital and can have negative environmental effects.
All over the world, new technologies are being developed to generate energy from sewage sludge. In Canada for example, an innovative company based in Quebec called XEBEC has developed a suite of technologies to process wastewater and purify biogas for high quality pipeline natural gas and transportation fuel. The technology is based on an energy-self-sufficient system of anaerobic digesters to treat waste. Quebec aims to is prohibit organic material in landfills by 2020, and the province claims that it will soon be able to sell 4 to 5 million cubic meters of biomethane per year prices at around 25–30 cents per cubic meter.
If you are a municipality in Ontario and in need of a biosolids management solution, please feel free to contact us at 1 (877) 479-1388.
Sources:
https://www.asme.org
https://thinkprogress.org