Energy from water purification: what it is and how it is produced

Wastewater treatment plants from civil or industrial discharges can become real energy producers.
Through the purification of these effluents and the subsequent fermentation process of the resulting sludge, biogas can be obtained, from which in turn renewable electricity and heat can be produced.

Renewable energy, what does it mean?

We speak of renewable energy to indicate the type of energy production derived from all natural resources considered inexhaustible because they regenerate over time respecting the cycles of nature. This is why they are considered economically sustainable.
Renewable energy is produced from natural sources that have the characteristic of regenerating in a short time: when the rate of regeneration of a resource is equal to or greater than its rate of use, we speak of sustainable energy sources. In addition to their inexhaustibility, one of the indisputable advantages of renewables is the possibility of producing electricity without polluting emissions. Emissions that are released into the atmosphere from non-renewable sources, which are destined to be exhausted and therefore unavailable in the future. Renewable energies are solar, wind, geothermal, hydroelectric, ocean and biomass fermentation processes.

How is energy produced from wastewater treatment?

In traditional water treatment plants, wastewater undergoes several processes of “filtration” and sedimentation until an organic material is obtained, which is called activated sludge. The final water purification process is a completely natural operation. Thanks to the bacteria present in this residue, the sludge is literally eaten. The inert part is expelled and sediments at the bottom of the tank. This properly treated sediment is then sent to landfill. The purified water is recycled for industrial use or irrigation.
But the digestion process of the bacteria generated our biogas!

Biogas is a natural gas consisting mainly of methane, carbon dioxide and other elements such as oxygen, hydrogen and nitrogen. This type of gas is obtained through the anaerobic fermentation of organic substances called biomasses such as, for example, organic waste (FORSU, Organic Fraction of Municipal Solid Waste) and the biodegradable part of industrial and municipal waste.
The biogas produced in water treatment plants as a result of anaerobic digestion of sewage sludge is used in modern plants as fuel for cogeneration systems, thus producing both electricity and heat.

IBT case studies on cogeneration plants in our cities’ sewage treatment plants

IBT Group, thanks to its long-standing partnership with Capstone Green Energy, the Californian company and the world’s only manufacturer of energy systems with oil-free gas turbines, offers biogas-powered cogeneration solutions for water treatment plants, which guarantee high efficiency, near-zero emissions and low maintenance. With Capstone technology, turbogenerators can also operate in the presence of biogas with low or no methane content. Examples include the Trento Nord water treatment plant, developed with a system of three 30 kWe Capstone oil-free turbines fed with biogas from anaerobic digestion, or the Cossato Spolina water treatment plant with a 200 kWe Capstone turbine. In this type of plant, turbine technology is particularly effective because it operates perfectly even in the presence of biogas containing low percentages of methane.
Another particularly interesting case study is that of the municipal wastewater treatment plant of the Municipality of Rovereto, which used the first biogas cogeneration system obtained from both sludge and OFMSW.
It is therefore a wet fermentation system (FORSU) and a sludge treatment system that in an integrated form feeds the biogas produced by two 200 kWe Capstone oil-free micro-turbines.
The electricity generated through this process is used for self-consumption by the purification plant itself, thus achieving a high energy yield with a significant reduction in electricity consumption.
The thermal energy produced by the cogeneration process is divided between the heating of the office building, the anaerobic digesters in the FORSU shed and the sludge drying process in the adjacent plant.