A smaller sewage plant in Schwerzen, Germany has has already opted for high-rate digestion. (Fraunhofer IGB)
High-rate digestion with microfiltration is state-of-the-art in large sewage plants. It effectively removes accumulated sludge, and produces biogas to generate energy. A Fraunhofer Institute study reveals that even smaller plants can benefit from this process.

Sewage plants remove organic matter from wastewater, and if the accumulating sludge decays, biogas is generated as a by-product. However, many smaller operations balk at the costs of a new digestion tank. Instead, they enrich the sludge with oxygen in the existing activation basin, and 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, Germany.

Only 1156 of the 10,200 sewage plants in Germany have a digestion tank. In a cost-benefit-study, Kempter-Regel has shown that it also pays for small sewage plants to transfer to more energy-efficient processes – even if they have to invest in a sludge digestion unit. “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 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,” she says.

The Fraunhofer IGB-developed process is 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 percent of the organic matter is converted into biogas, which 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 euros each year. In addition to high energy prices, waste management costs are also rising. The use of residual sludge in agriculture is controversial, and slurry can no longer be disposed of in landfills. Burning the sludge is an expensive alternative. High-rate digestion has the added advantage of producing less residual sludge needing disposal. “This saves the operator another 100,000 euros”, says Kempter-Regel.

(Fraunhofer IGB)