Sludge Is Power
Utilizing microturbines to produce heat and energy from wastewater
treatment plant sludge
Diane McDilda, Distributed Energy, March/April 2009.

"Wastewater treatment plants are looking for ways to utilize the biogas
they produce to generate their own power and heat while reducing their
methane emissions. Possibly creating a closed loop, where both the
offsite electricity and methane emissions become obsolete.

Methane is generated in the anaerobic digester, a solids management
system that reduces both the volume and toxicity of sludge. Sludge
originates in the plant’s primary and secondary clarifiers, where
microorganisms consume bacteria under aerobic conditions and then settle
out, respectively. Waste-activated sludge from the secondary clarifier
is a slurry, at 2–4% solids, and requires additional treatment or disposal.

According to a November 2008 study by BCC Research, the North American
and international market for sludge treatment is growing. One method of
treating waste-activated sludge is anaerobic digestion, where sludge is
processed into methane, carbon dioxide (CO2), and nutrient-laden solids
that can be suitable as soil amendments.

The EPA estimates that in 2006, wastewater treatment plants produced 4%
of the anthropogenic, human-made, methane emissions, generating the
equivalent of 23.9 teragrams of carbon dioxide (Tg CO2 Eq) or, in terms
of emissions, 23.9 million metric tons of CO2. This level has remained
relatively constant since 1990. With respect to methane produced as part
of managing waste in the US, wastewater treatment plants fall between
landfills that create 125.7 Tg CO2 Eq, and composting that generates 1.6
Tg CO2 Eq annually.

With a heating value of 1,000 Btu per cubic foot (Btu/cf) alone, or
approximately 600 Btu/cf when mixed with the other components of biogas,
biogas can easily be flared to destroy the methane. But flaring doesn’t
utilize the potential energy. By employing technologies such as
microturbines, plants can harness the energy of biogas while reducing
carbon emissions.

Microturbines join a cast of other technologies that make use of the
thermal and energy benefits of methane, including boilers, reciprocating
engines, and fuel cells, with fuel cells and microturbines being the
most avant-garde."

Dr. Ann C. Wilkie                          Tel: (352)392-8699
Soil and Water Science Department          Fax: (352)392-7008
University of Florida-IFAS
P.O. Box 110960                         E-mail: [log in to unmask]
Gainesville, FL 32611-0960
Campus location: Environmental Microbiology Laboratory (Bldg. 246).
BioEnergy and Sustainable Technology Society