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A NEW high-rate digestion process with microfiltration should make digestion tanks in water treatment affordable for small- and medium sized units, Germany’s Fraunhofer Institute says.

At present, most small and medium-sized sewage plants baulk at the cost of installing a digestion tank to remove the accumulated sludge and turn it into biogas, even though the technology is state-of-the-art in large sewage plants. Case in point: only 1156 of Germany’s 10,000-plus sewage plants have a digestion tank, Fraunhofer Institute says. Instead, most these plants stabilise sludge in activation basins with the help of oxygen.

“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 do.”

Kempter-Regel carried out a cost-benefit-study which, she says, shows that investing in a digestion tank is money well spent. “Based on a sewage plant for 28,000 inhabitants, we calculate that the plant can reduce its annual waste management costs from €225,000 ($322,000) by as much as €170,000 if sludge is decayed in a high-rate digestion unit with microfiltration, as opposed to treating it aerobically,” she says.

Together with her colleagues at IGB, Kempter-Regel developed a novel, ultra-fast digestion process, in which sludge only remains in the tower for five to seven days, rather than 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 spoil is approximately a third more than in the traditional digestion process. The biogas obtained can be used to operate the plant, which, in the case study, would cut energy costs by at least €70,000/y. In addition, high-rate digestion produces less residual sludge needing disposal, saving the operator another €100,000/y, says Kempter-Regel.