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 study now reveals that even small plants can
benefit from this process.
Sewage plants remove organic matter from wastewater. If the
accumulating sludge decays, biogas is generated as a by-product. However, only
1,156 of the 10,200 sewage plants in Germany have a digestion tank.
Smaller operations, especially, baulk 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 Dr.
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”.
In a cost-benefit-study Dr. Kempter-Regel has shown that it
also pays 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.
This process was developed at IGB and is 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 percent 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
euros each year. 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. In addition to high energy prices, budgets are also
being hit hard by increasing waste management costs. The use of residual sludge
in agriculture is controversial, and slurry can no longer be disposed of on
landfills; burning the sludge is a very expensive alternative. So an effective
reduction of sludge through digestion pays off. Even small sewage plants have
already followed the recommendation of the Stuttgart Institute and converted to
the high-rate digestion process.
Original
article
SOURCE: Fraunhofer-Gesellschaft
