Digestion tanks can produce biogas and save energy in sewage plants

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    A study from the Fraunhofer Institute in Germany has found that small sewage plants can also produce biogas to generate electricity.
    Sewage plants remove organic matter from wastewater. The accumulating sludge then decays and biogas is generated as a by-product. However, according to the researchers, only 1,156 of the 10,200 sewage plants in Germany have a digestion tank.
    While high-rate digestion with microfiltration to remove accumulated sludge and produce biogas is standard in state-of-the-art large sewage plants, it is not so common in small plants.
    Smaller operations often feel that the cost of a new digestion tank is not worth it. Instead, they enrich the sludge with oxygen in the existing activation basin to stabilize it.
    “Activation basins require a lot of electricity. At the same time, enormous energy potential is lost since no biogas is produced,” says Brigitte Kempter-Regel of the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB in Stuttgart. “A sewage plant eats up more electricity in the municipalities than their hospitals.”
    “Based on a sewage plant for 28,000 inhabitants, we calculate that the plant can reduce its annual waste management costs from 225,000 euros [US $318,000] by as much as 170,000 euros if sludge is decayed in a high-rate digestion unit with microfiltration, as opposed to treating it aerobically,” Kempter-Regel added.
    A process observing this was developed at IGB and it is said to be much more effective than conventional digestion. Instead of the usual 30 to 50 days, sludge only remains in the tower for five to seven days. Around 60% of the organic matter is converted into biogas.
    The biogas can then be used to operate the plant which, according to the case study, would cut energy costs by at least 70,000 euros ($99,000) annually.
    High-rate digestion also results in less residual sludge that needs disposal, which could save the operator another 100,000 euros.

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